key: cord-0037795-8w733jzn
authors: Maedler-Kron, Chelsea; Marcus, Victoria A.; Michel, René P.
title: Hematopoietic Stem Cell Transplantation
date: 2016-09-16
journal: Pathology of Transplantation
DOI: 10.1007/978-3-319-29683-8_10
sha: fcaf761f98a534a755e251dbb88315f44b47b082
doc_id: 37795
cord_uid: 8w733jzn

Hematopoietic stem cell transplantation offers patients the only prospect of long-term survival for a substantial variety of otherwise incurable malignant neoplasms as well as selected non-neoplastic conditions. This therapeutic modality, however, carries significant risks and is associated with a wide range of complications affecting not only the bone marrow but many organ systems, in particular by graft-versus-host disease. Patients before, during, and following stem cell transplantation thus require close monitoring and the pathologist is frequently implicated in this process. The aim of this chapter is to provide the pathologist with the tools required to interpret biopsies from these patients at all stages of the transplantation process. Following an overview of hematopoietic stem cell transplantation and graft-versus-host disease, the chapter will focus on the histopathologic findings of graft-versus-host disease and other complications in several organs and tissue including bone marrow, skin, lung, kidney, gastrointestinal tract and liver, with emphasis on the diagnostic criteria and approaches to graft-versus-host disease.

Hematopoietic stem cell transplantation (HSCT), defi ned as the process of infusing hematopoietic stem and progenitor cells intravenously to reestablish normal hematopoiesis in states of marrow failure, is an important strategy to treat a variety of malignant and non-malignant conditions. Of note, the term HSCT has supplanted the term bone marrow transplantation (BMT) as hematopoietic stem cells can be obtained from several sources other than bone marrow, e.g., peripheral blood and umbilical cord blood [ 1 -4 ] . By defi nition these cells are of hematopoietic origin, to differentiate them from other types of stem cells derived from bone marrow such as endothelial progenitors used in revascularization therapy of the myocardium, limbs and others sites [ 5 ] .

The concept of utilizing bone marrow as a form of therapy dates back as far as the Middle Ages but even attempts in the early and midtwentieth century achieved only a modicum of success. An important driving force for the development of this therapy came in part from the observation of the marked myelosuppressive and other toxic effects of radiation in survivors of the nuclear bomb attacks in Hiroshima and Nagasaki [ 6 ] . Although attempts at allogeneic BMT in the 1950s and 1960s were fraught with diffi culties, advances in the understanding of the major histocompatibility complex (MHC) and human leukocyte antigens (HLA) as key factors in graft rejection provided the framework for further experimental studies and clinical application of allogeneic HSCT [ 1 , 7 ] . HSCT came of age in the 1970s, when the Seattle group and others reported improved outcomes in the treatment of aplastic anemia and acute leukemia [ 8 -12 ] . Further advancements were reported in the 1980s with the introduction of improved conditioning regimens, potent immunosuppressive drugs and novel antibiotics for viral and fungal infections [ 13 ] .

The early days of autologous HSCT was similarly marked by alternating successes and failures. It was initially used to treat patients with non-Hodgkin lymphoma in the late 1970s followed by selected solid malignancies and Hodgkin lymphoma in the 1980s [ 14 ] . In the earliest protocols, autologous bone marrow was infused. Later approaches utilized hematopoietic cells derived from peripheral blood. In the mid to late 1980s the addition of hematopoietic growth factors such as G-CSF and GM-CSF was an important modifi cation of the technique that signifi cantly increased the yield of stem cells [ 15 , 16 ] . Detailed historical aspects of HSCT can be found in several excellent reviews [ 1 , 7 , 13 , 14 , 17 ] .

Currently, the major categories of HSCT are (1) autologous HSCT in which the stem cells are obtained from the patient's own bone marrow or peripheral blood and re-infused following conditioning, (2) syngeneic HSCT , where donor cells are from an identical twin, and (3) allogeneic HSCT where the donor cells come from another donor. The choice of autologous vs. allogeneic depends on the availability of a donor and the underlying condition. In general, allogeneic HSCT is indicated when the bone marrow or peripheral blood harbors a signifi cant tumor burden. However, each type has specifi c indications, limitations, and complications.

Following poor outcomes in the 1960s and early 1970s and initial enthusiasm for allogeneic HSCT, autologous HSCT (auto-HSCT) regained clinical favor in the late 1970s in reaction, in part to the shortcomings of allogeneic HSCT, namely graft-versus-host disease (GvHD) and to the diffi culty of fi nding suitable donors [ 18 , 19 ] . Auto-HSCT facilitates the prompt reconstitution of a markedly depleted or ablated marrow following very aggressive chemotherapy and radiotherapy intended to eradicate hematologic and nonhematologic malignancies.

The general principles on which autologous HSCT are founded include (1) the sensitivity of the malignancy to chemotherapy, specifi cally the ability to ablate the marrow with aggressive salvage therapy, (2) the timing of the transplant, i.e., as part of the primary therapy vs. fi rst or second relapse, and (3) the specifi c biological characteristics of the malignancy, e.g., cytogenetic subgroups in acute myeloid leukemia [ 19 ] .

Current indications for autologous HSCT include (1) refractory or relapsed Hodgkin and aggressive non-Hodgkin lymphoma and germ cell tumors, (2) plasma cell myeloma after induction therapy, (3) mantle cell lymphoma in fi rst remission, and (4) acute myeloid leukemia in second remission if an allogeneic bone marrow donor is unavailable. Less common indications include low-grade B-cell lymphomas, breast cancer, and autoimmune diseases [ 19 ] . In the overwhelming majority of patients, peripheral blood is the source of the stem cells [ 20 ] .

The advantages of autologous over allogeneic transplantation include the absence of graft-versus-host and the reduced intensity of the conditioning regimens. The drawbacks are the potential inability to harvest the patient's stem cells, the relapse of the original malignancy (by contamination of the donor cells), the general toxicity of the conditioning process, the absence of the graft-versus-malignancy response, as well as the late development of secondary malignancies such as myelodysplastic syndrome and acute myeloid leukemia.

Allogeneic HSCT is used for both hematopoietic malignancies and non-malignant conditions, and as salvage therapy in patients not responding to standard chemotherapy or radiotherapy, or following failure of autologous HSCT. The general principles of allogeneic HSCT include: (1) it enables the rescue of patients following administration of potentially myeloablative doses of chemotherapy and radiation in aggressive diseases or following the development of resistance to conventional therapeutic doses, and (2) it exerts a signifi cant graft-versus-malignancy response that potentiates eradication of the malignancy and prevention of recurrence. Data from the Center for International Blood and Marrow Transplant Research ( CIBMTR ) reported a total of 8860 allogeneic transplants in the USA in 2010. The most frequent indications were acute myeloid leukemia (about onethird of cases), acute lymphoblastic leukemia and myeloproliferative/myelodysplastic disorders (each about 10-15 %), followed by smaller percentages of patients with non-Hodgkin lymphomas, chronic myeloid leukemias, other leukemias, aplastic anemia and other non-malignant conditions, plasma cell myeloma and Hodgkin lymphoma [ 20 ] .

The procedure entails a conditioning regimen of chemotherapy, immunosuppressant drugs, and/or radiation followed by infusion of donor hematopoietic stem cells. Immunosuppressive therapy is subsequently administered to prevent or treat GvHD. The conditioning regimen serves two purposes: to eliminate malignant cells and to diminish the recipient's immune response and thereby prevent the rejection of the donor hematopoietic cells [ 21 ] . Currently, reduced intensity conditioning regimens have expanded the pool of recipients, especially older patients who poorly tolerated the associated toxicity, and lowered transplant-related mortality, although this advantage is offset by higher relapse rates [ 21 ] . Further details pertaining to procedures and other therapeutic measures are available in several detailed publications [ 1 , 19 , 21 , 22 ] .

The source of stem cells between 2005 and 2010, according to the CIBMTR database, in patients over the age of 20, was peripheral blood (80 %), bone marrow (15 %), and cord blood in 5 % of cases. In pediatric patients and young adults up to age 20, the distribution was 25 %, 50 %, and 25 %, respectively. Although analysis of the advantages and drawbacks of each of these sources is dependent on multiple variables and beyond the scope of this chapter, in general peripheral blood stem cells appear to engraft at the highest rate and are associated with a better survival in some disorders, but produce higher rates and severity of GvHD [ 21 ] .

The causes of mortality in patients undergoing HSCT reported by the CIBMTR vary with the primary source of the stem cells: in autologous HSCT, 72 % of deaths are due to the primary disease, 7 % to infection; when HLA-identical siblings are donors, 49 % die from the primary disease, 16 % from GvHD, and 13 % from infection whereas with unrelated donors the respective percentages are 37, 18, and 18 % [ 20 ] .

The pathologist plays an important role in the management of patients both prior to and following HSCT. These include (1) the pre-transplant assessment or confi rmation of the primary disease (malignant or non-neoplastic) and/or the disease status of remission or relapse, (2) the posttransplant evaluation for potential complications and possible recurrence of the original disease.

The pre-transplant evaluation is discussed in detail in standard texts of hematopathology including the WHO classifi cation of tumors of the hematopoietic and lymphoid tissues [ 23 -25 ] . Highlights of important points include fi rstly the evaluation of the bone marrow , ensuring the precise typing of the myeloproliferative, myelodysplastic or leukemic disorder and assessing the amount of residual disease, if any. For the lymphoma patients, the bone marrow is evaluated to ensure absence of disease prior to autologous HSCT. In the setting of aplastic anemia, mimics such as myelodysplastic syndrome must be ruled out. Also important is assessing the presence of any fi brosis in the marrow that may delay engraftment [ 26 , 27 ] . Secondly, the possibility of preexisting hepatic disease should be evaluated in selected patients as it may portend the development of sinusoidal obstructive syndrome (previously termed veno-occlusive disease) and related entities discussed in detail below [ 27 ] .

The role of the pathologist is particularly important in the posttransplant setting , especially in the assessment of complications. There are myriad side effects related to radiation and chemotherapy administered as part of the conditioning process. These occur most frequently in the bone marrow, but other organs such as the liver, heart, and lungs may be affected; these complications occur early on after transplantation. The pathologist also has a critical role in diagnosing other complications such as immunologically mediated graft rejection and graftversus-host disease later on post-HSCT. Looking for possible infections posttransplant must always be part of the assessment and differential diagnosis of any biopsy performed to assess organ dysfunction following HSCT. Infections should be sought for diligently with appropriate histochemical and immunohistochemical stains, and be correlated with microbiological fi ndings [ 28 -30 ] . Also important is the assessment of potential reactions to the panoply of drugs the patients usually receive. Lastly, the pathologist is vital in the diagnosis of posttransplant recurrence of the primary malignancy and in the detection of " de novo " neoplasms; on the topic of de novo posttransplant malignancies , the reader is referred to chapter "Transplantation and Malignancy".

The remainder of the chapter will focus on GvHD and other organ-specifi c complications following HSCT.

The diagnosis of acute GvHD is primarily a clinical diagnosis that is established by the Gluckenberg grading scheme and a severity index proposed by the International Bone Marrow Transplant Registry (IBMTR). It incorporates a number of signs and symptoms including the body surface area for the severity of cutaneous involvement, the level of serum bilirubin as a measure of hepatic involvement, and the daily volume of diarrhea and the presence or absence of abdominal pain and ileus, refl ecting gastrointestinal (GI) disease [ 36 , 37 ] .

Common histopathologic constituents of GvHD in the different sites center on individual cellular apoptosis and ulceration in labile (intestine and skin) or stable (liver) tissues, variable cellular atypia and a generally minor mononuclear infl ammatory infi ltrate. These alterations are nonspecifi c and can be manifestations of radiation and chemotherapy-induced injury, drug reactions, and infection and therefore require careful clinical correlation as well as diligence in excluding an infectious etiology. The histopathologic criteria for the different organs are discussed in detail below. Interestingly, acute GvHD of the lung is less common than cutaneous, GI or hepatic GvHD [ 38 ] .

According to the NIH consensus group, chronic GvHD can occur at any time posttransplant, and includes two subtypes, (1) classic chronic GvHD , without evidence of acute GvHD, and (2) "overlap syndrome" in which features of both acute and chronic GvHD are present [ 32 ] . The latter was re-evaluated and clarifi ed in the NIH 2014 consensus project and "overlap syndrome" is now defi ned as the occurrence of one or more acute manifestations of GvHD in a patient with chronic GvHD. The acute component can occur concurrently with or following the initial diagnosis of chronic GvHD [ 33 ] . The incidence of chronic GvHD varies greatly, from less than 10 % to 80 % depending on a wide range of factors [ 32 , 39 ] . The principal features of chronic GvHD mimic several autoimmune and immunologic disorders, including scleroderma, Sjögren syndrome, primary biliary cirrhosis, bronchiolitis obliterans, immune cytopenias, and chronic immunodefi ciency.

The clinical signs and symptoms of chronic GvHD are categorized as (1) "diagnostic" signs and symptoms, denoting manifestations that establish the diagnosis of chronic GvHD without a need for other criteria, (2) "distinctive," denoting manifestations not usually found in acute GvHD but considered insuffi cient to establish an unequivocal diagnosis alone, (3) "other" features, i.e., unclassifi ed manifestations of chronic GvHD indicating unusual, controversial, or nonspecifi c features of chronic GvHD that cannot be used in isolation to confi rm the diagnosis, and (4) "common features," i.e., signs and symptoms encountered in both acute and chronic GvHD [ 32 , 33 ] .

According to the NIH consensus project [ 32 , 33 ] , the diagnosis of chronic GvHD requires (1) distinction from acute GvHD, (2) evidence of at least one "diagnostic" sign of chronic GvHD or of at least one "distinctive" feature with confi rmation by biopsy or other appropriate test, and (3) elimination of other disorders that mimic GvHD. Once established, a numeric grading scheme of 0-3 is applied, taking into account the patient's performance status and severity of involvement of the principal organ systems, specifi cally skin, mouth, eyes, GI tract, liver, lungs, joints and fascia, and genital tract. By combining the organ-specifi c scores, a global assessment of GvHD severity (mild, moderate, or severe) is established. In general, systemic therapy is reserved for patients with moderate to severe chronic GvHD [ 32 ] .

The histopathologic diagnosis of chronic GvHD is challenging for several reasons [ 40 , 41 ] : (1) the precise histopathologic diagnosis can be confounded by the effects of the conditioning regimens, drugs, infections, and may be altered by immunosuppressive therapy initiated prior to the diagnostic biopsy; (2) uniform strict diagnostic criteria have not been established for all the organ systems; (3) the current histologic grading schemes have not been validated in prospective studies; (4) the distinction between the late forms of acute GvHD and chronic GvHD can be difficult; (5) the degree of infl ammation or of epithelial damage is not an accurate predictor of response to therapy; (6) sampling issues, and timing of the biopsy as the disease progresses may also hamper interpretation. In general because the prevalence of chronic GvHD in patients with stem cell transplants is high, the positive predictive value of a biopsy showing GvHD is high, whereas the negative predictive value is low. The relatively high rate of false negatives may be explained by sampling issues or tissue adequacy or by the fact the alterations can be focal in distribution, or altered by concomitant therapy [ 41 ] . Nevertheless, despite these shortcomings, biopsies serve a useful purpose in the assessment of patients after HSCT, particularly when the fi ndings are combined with careful clinical correlation.

The indications for biopsy in patients following HSCT as outlined by the NIH consensus guidelines [ 32 , 41 ] are to confi rm the diagnosis of GvHD when (1) only the distinctive clinical features of GvHD are present, (2) other diagnoses are being considered, (3) clinical fi ndings are confi ned to internal organs, or in general (4) in the presence of atypical clinical features, or when infection or drug toxicity are in the differential diagnosis. The criteria for the histopathologic diagnosis of GvHD in the individual tissues and organs are outlined in the balance of this chapter.

In the 2014 NIH consensus project [ 41 ] , the recommended categories for fi nal histologic diagnosis have been simplifi ed from the original four groups in the 2006 publication [ 40 ] to three: (1) "not GvHD," (2) "possible GvHD," with evidence of GvHD but with other possible causes, and (3) "likely GvHD" that combines the previous categories of "consistent with" and "unequivocal," in which the diagnosis is either clear, or most likely, or confi rmed by a subsequent biopsy or by the lack of a signifi cant competing differen-tial diagnosis. These histopathologic fi ndings should always be correlated with the clinical, laboratory, and imaging data and the pathology report should incorporate this recommended terminology.

In evaluating the bone marrow in a patient preor post-HSCT, the general principles applicable to the practice of hematopathology apply. Firstly there needs to be a clear knowledge of the original disease. Secondly, it is critical to perform a careful morphological examination of the blood smear, the clot and smear of the bone marrow aspirate, and the marrow core biopsy. Of note, the smear of the marrow aspirate is particularly valuable in the evaluation of residual blasts. Aside from morphology, histochemical, immunohistochemical, fl ow cytometric, genetic and cytogenetic testing, as well as molecular data and other laboratory parameters (e.g., serology and microbiological data) may be useful. Of paramount importance, all of these pathologic and laboratory parameters need to be correlated with the clinical features and status of the patient.

Following HSCT the bone marrow undergoes a series of temporally predictable morphologic alterations. There can be some variation on account of patient-related or disease-specifi c factors along with the choice of conditioning regimen and source of the stem cells [ 42 , 43 ] . The conditioning regimen typically produces complete ablation of the marrow that persists into the fi rst week following HSCT . Histologically, this is characterized by a markedly hypocellular marrow with aplasia of myeloid, erythroid and megakaryocytic series, loss of adipocytes, fi brinoid necrosis composed of acellular granular eosinophilic material, a protein-rich edema, dilated sinuses, and variable numbers of residual stromal cells such as lymphocytes, plasma cells, hemosiderinladen macrophages and histiocytes including small lipogranulomata (Fig. 1 ) [ 44 -47 ] . During week 2 , the marrow remains hypocellular but exhibits early regeneration of stromal adipocytes and islands of left-shifted erythroid (megaloblastic) elements. This is followed by the appearance of centrally placed myeloid colonies, i.e., non-paratrabecular in distribution (and in the setting of HSCT, not an indication of myelodysplasia). There is restoration of megakaryocytic elements usually arranged in clusters and displaying dysplastic or hypolobated nuclei. There is frequently a mild increase in reticulin fi bers (Fig. 2 ) . Of note, the centrally located early regenerating erythroid colonies are often associated with activated macrophages (so-called pseudo-Gaucher cells) that are thought to promote recovery of erythropoiesis [ 48 ] . By week 3 , megakaryocytes are present and the expansion and maturation of all three lineages proceed, so that by the end of the fi rst month , the marrow cellularity should reach 50-100 % of normal with near complete resolution of necrosis and reticulin deposition. By 2-3 months , complete regeneration with normal trilineage hematopoiesis is established (Fig. 3 ) . Other fi ndings that are best appreciated on smear preparations include B-cell precursors (hematogones), dysplastic changes with ring sideroblasts and pseudo Pelger-Huët change in neutrophils [ 45 , 46 , 49 , 50 ] . Hematogones are more generally prominent in children than adults.

There are three principal bone marrow complications of HSCT to consider [ 43 , 45 , 46 ] . The fi rst complication is the persistence of a markedly hypocellular or aplastic marrow. The marrow Persistence of a hypocellular/aplastic marrow may result from failure or delay in engraftment. A markedly fi brotic marrow pre-transplant may delay engraftment. Alternatively, this complication may be due to GvHD in the marrow as a consequence of alloreactive donor T-cells. Of note, the marrow is much less frequently involved by GvHD than the other organs as discussed below. The manifestations of GvHD in the marrow are not very specifi c. Apart from the reduced myeloid, erythroid, and megakaryocytic elements, their progenitor cells are characteristically decreased, as are fi broblastic progenitors and stromal cells, refl ecting a derangement of the recipient's microenvironment important in sustaining the donor cells (Fig. 4 ) [ 51 ] . Another feature of GvHD, particularly chronic, can be an increase in marrow T-cells [ 43 ] .

Secondly, infections in the marrow after HSCT, although uncommon, should be borne in mind. Viral infections, particularly cytomegalovirus (CMV ), HHV6 and EBV have been implicated in graft failure due to suppression of stem cells. As noted previously, epithelioid granulomas can be a component of the engraftment process but their presence, especially if conspicuous should also prompt a diligent search for potential infectious agents with appropriate histochemical stains (Fig. 5 ) . Hemophagocytosis can be a manifestation of an infectious incident, particularly viral or parasitic such as toxoplasmosis [ 43 , 52 ] .

The third complication is persistence or relapse of the primary malignancy. As detailed above, this complication is the principal cause of mortality (Figs. 6 and 7 ). Stringent assessment of the marrow for overt or minimal residual disease (MRD ) is therefore mandatory after HSCT. Likewise knowledge of the chimeric status of the transplant in the setting of allogeneic HSCT is essential. For example, complete chimerism (i.e., 100 % of the cells in the marrow are of donor origin) is a marker of complete remission. Mixed chimerism (with 5-90 % of cells of donor origin and the balance from the recipient) raises the possibility of relapse. The presence of a small number of malignant cells, particularly leukemic cells, indicates a failure of the conditioning regimen. Therefore it is imperative to evaluate for MRD and to differentiate it from normal hematopoiesis in the host. A detailed discussion of the topics of MRD and chimerism in HSCT is beyond the scope of this book and chapter but is provided by several excellent sources [ 45 , 53 -56 ] .

The skin is the most frequently involved organ in GvHD and may reveal itself as acute or as chronic GvHD, the latter 4-6 months following the transplant, although, as indicated above, the distinction between the two is somewhat arbitrary [ 57 ] . The pathogenesis of skin involvement, as with other organs, implicates the mature donor T-cells infused with the graft, and indeed GvHD is mitigated by T-cell depletion of the transplants. Donor cytotoxic T-cells have been shown to target the host endothelial cells, resulting in a substantial loss of microvessels, thereby promoting the dermal fi brosis characteristic of chronic GvHD [ 58 ] .

The clinical cutaneous manifestations of acute GvHD are succinctly incorporated by the staging scheme [ 57 , 59 ] : stage 0-absence of a GvHDrelated rash; stage 1-maculopapular rash involving <25 % of the surface area of the body; stage 2-maculopapular rash or erythema involving ≥25-50 % of surface area with associated symptoms such as pruritus; stage 3-generalized erythroderma, or papular, maculopapular, or vesicular eruptions with bullae or desquamation involving over 50 % of the surface area, and stage 4-generalized exfoliative, bullous, or ulcerative dermatitis. These skin manifestations should be corroborated with extracutaneous manifestations of acute GvHD such as hyperbilirubinemia and diarrhea.

The clinical features of chronic GvHD are similarly illustrated by the scoring scheme of the NIH [ 32 , 60 ] . These are divided into (1) "diagnostic" (suffi cient to establish the diagnosis) such as poikiloderma, lichen planus-like features, sclerotic features, morphea-like features and lichen sclerosus-like features; (2) "distinctive" features (alone are insuffi cient to establish the diagnosis) such as depigmentation for the skin; (3) "other" features (part of the GvHD spectrum if the diagnosis is confi rmed by other means) such as impairment of sweating, ichthyosis, keratosis pilaris, hypo-or hyperpigmentation; and (4) "common" (fi ndings of both acute and chronic GvHD) including erythema, maculopapular rash, and pruritus. In addition changes in the nails or scalp and body hair may be encountered and can aid in the diagnosis. These cutaneous fi ndings can then be incorporated into a scoring system ranging from 0 to 3: 0 or absence of fi ndings, 1 (mild) involvement of <19 % body surface area by the disease, without sclerotic features, 2 (moderate) involvement of 19-50 % of body surface, or involvement with superfi cial sclerotic features, and 3 (severe) involvement of >50 % body surface, or deep sclerotic features, or impaired mobility, ulceration or severe pruritus [ 60 ] . These site-specifi c scores are then incorporated into a global scoring system to assess the overall severity of GvHD. A more detailed description of the clinical features is available in a number of publications [ 57 , 61 , 62 ] .

As discussed by Hymes et al. [ 57 ] , the histopathologic changes in the skin biopsy serve primarily to corroborate the clinical suspicion of GvHD but do not alter its grading or staging. Moreover, the fi ndings are nonspecifi c and cannot be confidently distinguished from drug reaction (including the conditioning regimen), or infectious dermatitides. The value of the skin biopsy in the diagnosis of GvHD remains controversial [ 27 ] . Kuykendall and Smoller [ 63 ] argue against the use of skin biopsy in the fi rst 3 months following the transplant while Firoz et al. [ 64 ] advocate its use only in patients where the probability of GvHD is low.

The histopathologic fi ndings in acute or active GvHD are centered on the dermal-epidermal junction and include (1) dyskeratotic apoptosis in the basal layer or lower malphigian (spinosum) layer of the epidermis, the outer root sheath of hair follicle or in the most superfi cial (intraepidermal) portion of the sweat gland duct, (2) lichenoid lymphocytic infl ammation in the dermis parallel to the epidermis with or without an associated lymphocytic satellitosis (i.e., lymphocytes surrounding the apoptotic cells in the epidermis or at the dermal-epidermal junction), and (3) vacuolar change in the basal layer [ 40 ] (Figs. 8 and 9 ). Of all these criteria, apoptosis is the key histopathologic fi nding required to fulfi ll the "minimal criteria" of the NIH Pathology Working Group.

Although there are no recent specifi c pathologic grading schemes for reporting the severity of the cutaneous changes in acute GvHD, Shulman [ 27 ] and others [ 65 ] advocate using the older scheme of Lerner et al. [ 66 ] . According to this 1974 grading system, grade I is characterized by a superfi cial perivascular dermatitis with vacuolization of the epidermal basal cells; grade II is an interface dermatitis with scattered apoptotic or dyskeratotic keratinocytes in the basal or lower malphigian layers with or without closely associated lymphocytes (i.e., lymphocytic satellitosis); grade III reveals extensive apoptosis with damage to the basal layer (nuclear irregularities and atypia), formation of suprabasal bullae, and degeneration of the reticular dermis; in grade IV, the epidermis is lost, with ulceration. There are a number of published variations of this grading scheme [ 62 , 67 , 68 ] . These criteria can be used to estimate the severity of acute GvHD and be incorporated into the pathology consultation report. The pathologist should bear in mind that the histopathologic grade does not necessarily correlate well with the overall clinical severity of the acute GvHD, particularly early after trans-plantation [ 65 , 69 ] . Serial biopsies may be useful to consolidate the diagnosis.

A variant of GvHD termed " eczematoid GvHD " was described in ten patients by Creamer et al. [ 70 ] . This condition develops 2-18 months post- showing several dyskeratotic apoptotic bodies, sparse lymphocytic infi ltrate in dermis and lower epidermis, and mild fi brosis in dermis; also note satellitosis in ( c ) ( arrow ). High power of sweat gland ( d ) and hair follicle ( e ) with lymphocytic infi ltrate and apoptotic bodies transplant and presents initially as dermatitis, but progresses to an erythrodermic process. Biopsies of eczematoid GvHD show the histopathologic features of GvHD enunciated above with, in addition, spongiosis, parakeratosis and a sparse perivascular dermal infi ltrate. Interestingly multiple biopsies may be required before the diagnosis can be confi rmed [ 71 ] . Eczematoid GvHD usually requires aggressive immunosuppressive therapy.

The histopathologic fi ndings of chronic GvHD , although distinct from acute or active GvHD, may follow from repeated episodes of acute GvHD or even overlap or occur concurrently. These features also transform over time and are altered by therapeutic interventions. The proper assessment of chronic GvHD requires a full-thickness skin biopsy to evaluate alterations in the dermis, sweat glands, subcutaneous fat, and fascia. The fi ndings can be classifi ed into four principal groupings [ 27 , 41 , 72 ] . The earliest or lichen planus-like changes are characterized by epidermal acanthosis, orthorkeratosis, hypergranulosis with epidermal-dermal lichenoid changes (infl ammation, apoptosis and vacuolization of keratinocytes, shortened rete ridges), infl ammation of eccrine glands with or without a component of panniculitis . The second phase comprises the sclerotic manifestations (lichen sclerosus-like) , usually but not always preceded by the lichen planus-like changes. There is progressive deposition of collagen in the deeper layers, resulting in thickening of the papillary and/or reticular dermis, with or without panniculitis. The third group, morpheic changes, manifests as localized dyspigmented lesions and is characterized by sclerosis in the lower reticular dermis or along the lower dermal-hypodermal border, with or without involvement of the epidermis and appendages. In the fasciitis pattern there is thickening of the fascial septa by infl ammation and fi brosis, with or without panniculitis. Typically the epidermis is not involved.

The presence of early lichen planus-like features together with concurrent features of acute GvHD are most likely to progress to the more advanced forms of chronic GvHD than those with chronic changes alone [ 73 ] .

The poor specifi city of the cutaneous alterations in acute GvHD makes it diffi cult to distinguish from other causes of posttransplant rashes such as viral infections, drug reactions, and conditioning regimens and from the engraftment syndrome associated with lymphocyte recovery [ 74 ] . For this reason we, like others, are reluctant to make a diagnosis of acute GvHD a priori in the early posttransplant period [ 63 ] . Sviland et al. [ 75 ] emphasize the lack of specifi city of skin biopsy The principal diffi culty is distinguishing acute GvHD from a drug reaction because these patients typically receive multiple medications. As emphasized by Shulman et al. [ 27 ] , rendering a diagnosis of acute GvHD requires a careful search for apoptotic keratinocytes in the basal layer of the epidermis including at the tips of the rete ridges and the corresponding areas of the hair follicles, as well as a relatively sparse infl ammatory infi ltrate at the dermal-epidermal junction (Fig. 10 ) . In contrast drug reactions are characterized by an eczema-like pattern with spongiosis, few if any apoptotic cells, and more numerous lymphocytes. However, on occasion, a severe drug reaction resulting in a Steven-Johnson syndrome with formation of bullae or denudation may render the differentiation from GvHD diffi cult or impossible. Although eosinophils are typically touted as hallmarks of a drug reaction, they may be present also in GvHD [ 76 ] . Of interest, however, Weaver and Bergfeld [ 77 ] found that they could rule out a diagnosis of acute GvHD when they found more than 16 eosinophils per 10 high-power fi elds.

Radiation and chemotherapy effects can be diffi cult to differentiate from acute GvHD. The pathologist should exercise caution in interpreting biopsies in patients who have received these as part of their preconditioning regimens a short time prior to the biopsy [ 78 , 79 ] . A related entity is the skin eruption of " lymphocyte recovery " that develops 14-21 days following the administration of cytoreductive therapy and coinciding with the normalization of lymphocyte counts. It occurs in the absence of HSCT and produces a maculopapular rash. The histologic fi ndings of lymphocyte recovery in the skin resemble mild GvHD, with mild epidermal changes consisting of variable intercellular edema, exocytosis of lymphocytes, and rare dyskeratotic keratinocytes that can mimic the effects of cyclosporine [ 67 , 80 ] . Mechanistically the effects of cyclosporine, the cutaneous eruption of lymphocyte recovery, and GvHD in the setting of autologous HSCT are likely related to this same phenomenon [ 81 ] .

Infections , particularly viral infections (Herpes simplex and zoster ) can elicit skin rashes. In the setting of HSCT they are more common in other sites like the gastrointestinal tract, liver, and lung than the skin. Infectious cutaneous lesions after transplant are recognized by their characteristic histopathologic fi ndings and should be correlated with clinical, serological, PCR, and microbiological studies [ 27 , 71 ] . Other examples of infectious complication due to human herpes virus 6 and toxoplasmosis have been reported [ 82 , 83 ] .

In summary , the role of the skin biopsy in the diagnosis of GvHD remains controversial, largely on account of the lack of specifi city of fi ndings. However, a number of points deserve emphasis. Firstly, a biopsy in the fi rst 3 weeks following transplant should be avoided except for clinical indications like herpetic infection. Secondly, the presence of numerous epidermal apoptotic bodies and of lymphocyte satellitosis, particularly in skin appendages, raises the distinct possibility of GvHD. Thirdly, serial skin biopsies may be required to confi rm the diagnosis. Fourthly, the diagnosis of GvHD by cutaneous biopsy may be enhanced by the presence of extracutaneous fi ndings such as hyperbilirubinemia and diarrhea [ 84 ] . Finally, initiation of therapy for GvHD should not be delayed pending the results of a biopsy or if the histopathologic fi ndings are not diagnostic [ 85 ] .

Since the introduction of HSCT in the early 1970s, pulmonary complications have been registered [ 86 ] and remain a signifi cant cause of morbidity and mortality. A study in the early 1980s [ 87 ] found that 41 % of 525 patients receiving allogeneic stem cell transplants developed nonbacterial pneumonias. The majority were CMV-related (85 cases, untreatable at the time) or idiopathic (64 cases) and the mortality rates ranged up to 91 % in CMV pneumonitis. The histopathologic fi ndings in idiopathic pneumonia consisted of an interstitial mononuclear infi ltrate without identifi able organisms by histological or microbiological analyses.

Currently, lung injury occurs in 25-50 % of patients with HSCT. It is highest in allogeneic HSCT, and carries a mortality rate of about 50 %. Non-infectious pulmonary complications of HSCT have now supplanted infections due in part to antimicrobial prophylaxis and more effective therapeutic agents [ 88 ] . Of the non-infectious lesions, idiopathic pneumonia syndrome (IPS ) is a particularly devastating and complex complication. Clinically, the vast majority of patients with non-infectious pulmonary complications present with variable respiratory symptoms and signs (fever, cough, dyspnea, hypoxemia), and localized or diffuse infi ltrates on pulmonary imaging.

A number of issues related to the pulmonary complications of HSCT should be highlighted. The current nomenclature is confusing, with overlapping entities, poorly defi ned syndromes of imprecise or multifactorial etiology and/or pathogenesis, and lack of clear defi nitions, all of which render the interpretation of the literature and the precise assignment of a specifi c pathological diagnosis challenging. Secondly, there is a paucity of literature describing the pathological fi ndings, particularly the acute ones. The pathological fi ndings are nonspecifi c and refl ect patterns of injury rather than explicit or etiologic entities. Examples include diffuse alveolar damage (DAD), interstitial or organizing pneumonias and airway infl ammation, and airway fi brosis amongst others. Although many of the complications may be related to the HSCT per se, lung injury can be compounded by other factors such as the conditioning regimen, infection, aspiration injury, and drug-related toxicity. Finally, the diagnosis of pulmonary GvHD is one of exclusion and an infectious process should always be considered in the differential diagnosis of clinical dysfunction.

Despite the diminished incidence of infectious pulmonary complications after HSCT, these remain at the forefront when a patient presents with respiratory symptoms and/or an infi ltrate on imaging studies. The spectrum of radiologic changes includes localized consolidation(s) or multifocal or diffuse alveolar and/or interstitial infi ltrates. The lung is at risk for both nosocomial and community-acquired infections on account of the perpetual contact with the external environment. Moreover, as the patients are immunocom-promised, the normal morphologic patterns of response to infection may be altered, diminished, or even nonspecifi c. The lung is also at risk for multiple concurrent injuries such as infection, GvHD, and/or drug reaction and the constellation of morphologic fi ndings might consist of variable acute and chronic infl ammation, DAD, alveolar hemorrhage, or interstitial infl ammatory infi ltrates with possible superimposed granulation tissue and fi brosis. Finally, it should be noted that the pathologist faces additional challenges due to the limited tissue samples that are submitted e.g., from bronchoalveolar lavage (BAL ), endoscopic or CT-guided biopsies, transbronchial biopsies (TBBx) or video-assisted thoracoscopic (VATS) biopsies. The liberal use of levelled sections, histochemical, immunohistochemical and molecular testing in these clinical situations is imperative along with awareness of the radiologic fi ndings and communication with the treating clinicians.

Of the different investigative methods available, the simplest and least invasive is BAL, or thoracocentesis in the presence of a pleural effusion. Cytospin-or liquid-based cytologic preparations can be used to evaluate for infectious organisms or malignancy [ 89 ] . The fi beroptic TBBx yields suffi cient tissue samples to evaluate for patterns of injury and infectious agents. The VATS biopsy is generally reserved for patients who have failed conventional therapies or are suspected of having more than one etiology to account for the radiologic alterations. Regardless of the specimen obtained, a thorough search for infectious agents such as fungal infections including Pneumocystis jiroveci (PJP), Candida and Aspergillus species, and viruses such as CMV, Herpes simplex and Varicella zoster, as well as more uncommon organisms such as Toxoplasma gondii should be performed [ 90 ] . Gram-positive and gram-negative bacterial infections remain the most common causes of infection and tissue or fl uid Gram stains, microbiologic cultures and molecular sequencing are helpful. Mycobacterial infections are much rarer and usually represent reactivation episodes rather than de novo infections. We routinely perform silverbased stains such as the Gomori-methanamine silver (GMS ) stain on BAL samples and tissue sections to exclude PJP or other fungal organisms. Immunohistochemical staining for viruses and Toxoplasm gondii. may be performed routinely or as indicated [ 91 ] . These fi ndings should be correlated with data from the microbiology laboratory (including immunofl uorescence staining for viruses, polymerase chain reaction for viruses, fungi, and organisms such as Chlamydia and Mycoplasma ), imaging studies and from the clinical context.

There are a variety of non-infectious complications that can develop after HSCT and these are summarized in Table 1 . A useful clinical and radiologic algorithm for evaluating pulmonary complications after HSCT was formulated by Haddad [ 92 ] .

Delayed pulmonary toxicity syndrome (DPTS ) has been linked closely to the conditioning regimen for HSCT rather than the transplant per se. Whether it is a component of idiopathic pulmonary syndrome (IPS) or a distinct entity is controversial but the 2010 ATS guidelines exclude DPTS from the strictly defi ned IPS [ 91 , 93 ] . It occurs in up to two-thirds of patients receiving high-dose chemotherapy prior to autologous HSCT, typically at a median time interval of 45 days posttransplant. Fortunately most patients respond well to corticosteroids.

The pathologic fi ndings , are similar to the toxic effects of high-dose chemotherapy in immunocompetent patients, and comprise an acute alveolar injury pattern with nonspecifi c interstitial pneumonitis with vascular injury, alveolar edema, alveolocapillary septal thickening with fi brosis, proliferating atypical type-II pneumocytes and increased alveolar macrophages, i.e., similar to a mild form of DAD [ 94 ] . 

Peri-engraftment respiratory distress syndrome (PERDS ) is defi ned as pulmonary dysfunction occurring within 5 days of the neutrophil count returning to normal after HSCT. It accounts for one-third of cases of "idiopathic pneumonia syndrome" after allogeneic HSCT. PERDS is currently included in the spectrum of IPIS according to the 2010 ATS guidelines but its pathogenesis is distinct from the alloimmune response and from GvHD. PERDS occurs after both autologous and allogeneic HSCT [ 95 , 96 ] . The pulmonary injury is mediated primarily by the newly engrafted neutrophils intensifying the insult of the conditioning regimen. Like DPTS, PERDS responds well to corticosteroids, and has a better prognosis than IPS. The pathological fi ndings of PERDS are nonspecifi c manifesting predominantly as DAD or diffuse alveolar hemorrhage (DAH ) [ 91 , 97 ] . Clinicopathological correlation is required to establish this diagnosis.

The term "idiopathic pneumonia syndrome" (IPS) was coined at the 1991 NHLBI workshop [ 98 ] and is defi ned thus: "diffuse lung injury occurring after marrow transplant for which an infectious etiology is not identifi ed." In 2010, the American Thoracic Society (ATS ) refi ned IPS as an "idiopathic syndrome of pneumopathy after HSCT, with evidence of widespread alveolar injury and in which infectious etiologies, cardiac dysfunction, acute renal failure, or iatrogenic fl uid overload have been excluded" [ 91 ] . The manifestations and outcome differ signifi cantly between allogeneic and autologous transplants. In allogeneic recipients the median onset is about 19 days. The incidence in this group has fallen from about 12 % to 2-3 % largely because of less aggressive conditioning regimens but mortality rates remain as high as 60-80 % overall and reach over 95 % in patients needing mechanical ventilation. The incidence of IPS is lower in autologous HSCT patients and the median time of onset is 63 days. These patients generally respond well to corticosteroids with a better prognosis than the allogeneic group. A variety of risk factors for IPS have been identifi ed [ 91 ] . The principal etiopathogenetic mechanisms implicated revolve in part around the toxic conditioning regimens (total body irradiation (TBI) and high-dose chemotherapy), but primarily around immune mechanisms involving donor T-cells and cytokines. TNFα targets endothelial cells causing increased vascular permeability and also damages epithelial cells producing cellular apoptosis and inducing proliferation of type-II pneumocytes. Indeed the proposed therapy of IPS currently under evaluation in clinical trials advocates blocking TNFα signalling with etanercept [ 91 , 99 ] . Therefore, in the absence of infection, IPS is best regarded primarily as the pulmonary manifestation of acute GvHD.

The pathologic fi ndings in the clinical spectrum of IPS are likewise varied and nonspecifi c. Firstly , a small number (5-10 %) develop DAH as an early manifestation and clinical complication (1-4 weeks). The diagnosis of DAH is made by progressively increasing bloody return or the presence of >20 % hemosiderin-laden macrophages on BAL [ 100 , 101 ] . Open or transbronchial lung biopsies are seldom performed. Autopsy examination reveals hemorrhage in at least 30 % of alveoli, with the absence of infection-related changes (Fig. 11 ) [ 102 ] . Other fi ndings include a vasculopathy of small muscular arteries with medial hyperplasia and intimal thickening, myxoid mural change, and thrombotic microangiopathy [ 103 ] . Of note, the distinction between IPS and DAH may be diffi cult and in actuality they probably overlap, both clinically and pathologically. The mortality rate in DAH is high despite corticosteroid therapy [ 104 ] .

A second pattern reported in IPS is DAD with its clinical correlate, the acute respiratory distress syndrome (ARDS ). The lungs are heavy, consolidated, and edematous with or without hemorrhagic areas. Microscopically, they display alveolar epithelial damage, a protein-rich alveolar fl uid with airspace hyaline membranes in the exudative phase (Fig. 12a, b ) . In the organizational stages, interstitial and alveolar granulation tissue plugs and variable proliferative interstitial fi brosis ensue (Fig. 12c, d ) . The pattern of DAD also characterizes acute interstitial pneumonia (AIP); parenthetically, the 2010 ATS guidelines include AIP in the clinical spectrum of IPS. However the clear association of the IPS with HSCT suggests that the idiopathic AIP should be excluded from IPS [ 105 ] .

A third pattern of injury in IPS is lymphocytic bronchitis/bronchiolitis and interstitial pneumonitis , and rarely lymphoid interstitial pneumonia (LIP) [ 106 ] . Xu et al. [ 107 ] describe a similar pattern under the term of "chronic interstitial pneumonia." Light microscopy shows a perivascular and interstitial mononuclear infl ammatory infi ltrate composed of lymphocytes and plasma cells with direct infi ltration and apoptosis of the bronchial and bronchiolar epithelium; the latter may display hyperplastic, dysplastic or squamous metaplastic changes. In addition a lymphocytic perivasculitis, particularly a perivenulitis, may also be present. These alterations closely resemble those described in cutaneous and gastrointestinal GvHD, are rather specifi c and can assist in distinguishing pulmonary GvHD from pulmonary infections [ 107 ] . This form of pulmonary GvHD can progress to bronchiolitis obliterans (BO) described below.

Cryptogenic organizing pneumonia (COP ), unlike bronchiolitis obliterans discussed below, is associated with both acute and chronic GvHD in the setting of HSCT. COP should be distinguished from BO on account of its different clinicopathologic presentation (dry cough, fever, mild restrictive pattern on lung function testing, and variable infi ltrates on imaging), response to corticosteroids and a more favorable outcome [ 108 ] . The pathological fi ndings resemble those in the non-transplant setting, consisting of distinctive fi bromyxoid plugs within in the lumens of the small airways and extending into alveolar ducts and sacs. These are associated with a chronic interstitial infl ammatory infi ltrate [ 40 , 91 , 105 ] . In the context of HSCT, however, there may also be changes more specifi c to GvHD such as increased mononuclear cells, infi ltration of lymphocytes in the small airway epithelium that may display apoptosis and atypia, and a vasculitis (Fig. 13 ) [ 107 ] .

Bronchiolitis obliterans (BO ), also named bronchiolitis obliterans syndrome (BOS) or obliterative bronchiolitis (OB) is the hallmark of ( a ) Extensive alveolar hemorrhage and ( b ) mixed hemorrhage with hyaline membranes characteristic of DAD pulmonary chronic GvHD, and is included (loosely) in the spectrum of IPS according to the 2010 ATS defi nition. According to the 2014 NIH Consensus guidelines BO diagnosed by lung biopsy is classifi ed as a "diagnostic" feature of chronic GvHD provided distinctive clinical signs or symptoms are also present in another organ. Alternatively, the diagnosis of BOS can be rendered by the presence of an obstructive pattern on pulmonary function tests (PFTs) [ 109 ] . Indeed, screening PFTs are now recommended at day 100 after transplantation or at other appropriate intervals as warranted [ 33 ] . The incidence of BO ranges from 2 to 30 %, although recent data suggests it now approaches the lower end of this spectrum (2.8 % of 2087 patients at 5 years after transplant in the study by Nakaseko et al. [ 110 ] ). The onset varies from a few months to more than 10 years after HSCT, but generally most patients present 7-15 months after HSCT [ 91 ] . Symptoms include dyspnea, cough, wheezing and an obstructive pattern is found on PFTs.

The pathologic fi ndings in BO resemble closely those of BO in chronic lung allograft rejection in lung transplant recipients (see chapter "Lung Transplantation"). The lesions consist of dense eccentric or concentric subepithelial collagenous fi brosis causing partial or complete obliteration of small airways, with variable numbers of lymphocytes and foam cells and associated with fl attening, hyperplasia or squamous metaplasia of the epithelium [ 27 , 40 , 106 , 111 ] . 

This rare pulmonary disorder is characterized by the diffuse accumulations of surfactant proteins and lipids within the alveolar spaces. It is caused by defective surfactant clearance by alveolar macrophages resulting in bilateral alveolar infi ltrates on chest radiographs. It is considered in the clinical differential diagnosis of IPS but is not generally included under this umbrella term. Pulmonary (1) genetic or congenital forms due to mutations in surfactant proteins or granulocytemacrophage-colony-stimulating factor (GM-CSF) receptor genes; (2) autoimmune (formerly called primary or idiopathic PAP) (~90 % of cases), wherein anti-GM-CSF antibodies block the activation of alveolar macrophages and (3) secondary forms of PAP most frequently due to toxic exposures, chronic infections (e.g., PJP and other fungi, Nocardia, mycobacteria, CMV, and other viruses), immune defi ciencies or hematological disorders [ 112 , 113 ] . The latter category includes a small number of cases of PAP that have been reported following HSCT in which the putative pathogenesis may involve a combination of the depletion and dysfunction of macrophages related to the profound cytopenia and/or to anti-GM-CSF antibodies generated as part of the immune disparity between donor and recipient [ 114 -116 ] .

The diagnosis of PAP generally relies on examination of BAL fl uid, of a transbronchial biopsy or of a VATS biopsy. The BAL fl uid is frequently milky brown and microscopically reveals a paucicellular, amorphous or fi nely granular PAS-positive material, cell debris, and scattered macrophages (Fig. 14 ) . There is minimal if any cellular infl ammatory infi ltrate and histochemical stains, in particular for PJP, should be negative. In tissue sections, the airspaces are fi lled and expanded by the granular eosinophilic material with a minimal interstitial infl ammatory component.

Pulmonary hypertension is a rare complication of HSCT and can be classifi ed into pulmonary arterial hypertension and pulmonary veno-occlusive disease (to distinguish it from hepatic venoocclusive disease/ sinusoidal obstruction syndrome (SOS ) discussed below). Both entities share predominant endothelial dysfunction and damage as part of their pathogenesis.

Pulmonary arterial hypertension (PAH ) is even rarer than pulmonary veno-occlusive disease. Although PAH may be attributable in part to the conditioning regimen, there are recognized cases that are associated with GvHD [ 117 , 118 ] . Patients present with mild respiratory symptoms and have evidence of elevated pulmonary arterial pressures. Lung biopsies show hyperplastic intimal thickening, although plexiform lesions have not been described (Fig. 15 ) .

Pulmonary veno-occlusive disease (PVOD ) , unlike its hepatic analog, is a very rare complication of HSCT, and is defi ned as postcapillary pulmonary venular and venous obstruction leading to pulmonary vascular congestion, pulmonary hypertension, and right ventricular heart failure. Although PVOD was fi rst described in 1934 by Hora et al. [ 119 ] , its occurrence in HSCT was fi rst reported only in 1984 by Troussard [ 120 ] . PVOD shares pathogenetic mechanisms with PAH, specifi cally the role of chemotherapy and radiation used in conditioning, and the immune-mediated damage. To date, most published cases of PVOD develop in allogeneic rather than autologous HSCT. The pathologic fi ndings in PVOD, often discovered only at autopsy, include irregular and often eccentric intimal fi brosis with formation of luminal septa, recanalization, and with variable obstruction of pulmonary venules and small veins, less frequently of larger veins (Fig. 15c, d, e ) . Secondary congestion of capillaries, arterial intimal fi brosis, and medial hypertrophy may ensue, manifesting clinically as pulmonary edema and pulmonary hypertension. Plexiform lesions are typically absent, but interstitial fi brosis and hemosiderinladen macrophages within airspaces may be seen.

This rare complication (4 % of 324 pediatric patients in the series of Smith et al. [ 121 ] ) occurs following allogeneic HSCT. Most patients are under 18 years of age and present with fever, cough, and chest pain, and display multiple peripheral nodular lesions rather than infi ltrates on imaging studies. Their pathogenetic association with acute GvHD is supported by the non-occurrence in autologous HSCT and by an association with GvHD in other organs. Pulmonary cytolytic thrombi (PCT) is associated with a greater incidence of GvHD and a lower rate of relapse of leukemia. In general, patients respond to corticosteroids and the prognosis is very good [ 121 , 122 ] . Open lung biopsy is required to establish the diagnosis and to rule out an Aspergillus infection: VATS biopsy typically shows a nodular lesion composed of areas of hemorrhagic infarction associated with occlusion of small and medium-sized arteries and veins by deeply basophilic amorphous granular or fi brillar thrombus material [ 123 , 124 ] . The cells within the thrombi are monocytes, primarily of donor origin [ 125 ] . 

Although the kidney is not generally recognized as a primary target in HSCT, chronic renal disorders develop in 20-60 % of patients following There is also evidence of pulmonary veno-occlusive disease with organized thrombus in medium vein HSCT. The etiopathogenesis is likely multifactorial and includes the conditioning process, episodes of GvHD, acute renal injury, infections, and drug reactions. Renal biopsies often exhibit several simultaneous pathological fi ndings [ 126 , 127 ] . The principal indications for renal biopsy are nephrotic-range proteinuria and/or persistently elevated creatinine levels. Glomerulopathies constitute the majority of renal dysfunction following HSCT. In descending order of frequency these consist of membranous glomerulonephritis, minimal-change disease, membranoproliferative glomerulonephritis, and focal segmental glomerulosclerosis. Although it has been suggested that membranous glomerulonephritis is a glomerular expression of GvHD [ 128 ] , its occurrence in autologous HSCT as well as in allogeneic HSCT appears to cast some doubt on that notion. Nonetheless transplant-related immune dysfunction probably still plays a role in the pathogenesis of renal dysfunction and implicates in particular B-cells [ 127 ] .

Another common complication is thrombotic microangiopathy (TMA ) , manifested clinically as hemolytic-uremic syndrome. It is a consequence of endothelial injury by factors like radiation, chemotherapy, ischemia, and sepsis amongst others. An autopsy study found TMA in over 50 % of the 26 HSCT patients [ 129 ] . A diagnosis of TMA conveys a poor prognosis. Other less specifi c renal fi ndings following HSCT include acute renal injury (acute tubular necrosis), polyoma virus nephropathy, interstitial nephritis, calcineurin inhibitor toxicity and recurrent disease, particularly of amyloid and myeloma-related changes [ 126 , 127 ] .

Cytoreductive and myeloablative therapies including radiation and chemotherapy used in conditioning regimens can cause GI tract injury. Symptoms including nausea and vomiting emerge within 15 days of transplantation and can persist for up to 3 weeks [ 130 -132 ] . It is now thought that this initial mucosal injury contributes to the development and enhancement of acute GvHD.

The gastrointestinal (GI) tract is the second most common site of involvement by GvHD, often concurrently with skin and liver involvement [ 133 , 134 ] . However, the incidence of isolated GI GvHD is increasing [ 135 ] . Although enhanced pre-transplant therapy is linked to more severe GvHD, there is no difference in the severity or incidence of GvHD following peripheral blood vs. BMT [ 132 ] .

It is believed that conditioning therapies upregulate the release of cytokines such as tumor necrosis factor-α and interleukins IL1 and IL7 that in turn increase antigen presentation from host to donor T-cells through HLAs [ 35 , 136 ] . Additionally, GI mucosal damage further exacerbates the infl ammatory process by releasing endotoxins that amplify and propagate the "cytokine storm" [ 137 ] . Epithelial stem cells, residing in intestinal and gastric crypts are preferentially targeted by donor cytotoxic lymphocytes. Eosinophils and granulocytes are also recruited in GvHD [ 138 ] .

Acute GI GvHD usually manifests within the fi rst few days to as many as 100 days after HSCT [ 139 ] . As previously noted there can be an overlap between late onset acute and chronic GvHD [ 140 ] . Overlap syndromes have been linked to reduced intensity conditioning regimens [ 141 ] . Clinically it is important to establish a diagnosis of acute GI GvHD as it guides therapy and is associated with a poorer prognosis, particularly the higher grades of GI GvHD [ 141 ] . In addition to the assessment of GI-related signs and symptoms such as diarrhea, abdominal pain and ileus, dysfunction and alterations in other organ systems should be considered [ 142 ] .

The clinical features of acute GI GvHD are outlined in Table 2 .

The decision for upper or lower endoscopic evaluation is guided in part by the type and localization of symptoms. Although newer imaging techniques have been explored for the assessment of GvHD, endoscopy with biopsy remains critical in the correlation with clinical symptoms [ 143 ] . The classic endoscopic fi ndings include mucosal edema, sloughing, erythema, erosion, and ulceration [ 144 -148 ] . Studies have shown that the degree of injury viewed on endoscopy, however, does not always correlate with histopathologic severity [ 149 ] .

It should be emphasized that the diagnosis of GI GvHD requires close clinicopathologic correlation because the histologic features are nonspecifi c. Acute GI GvHD occurs throughout the GI tract, but is least common in the esophagus [ 150 ] . The classic fi nding in acute GvHD is the apoptotic body within the epithelium . Apoptotic bodies can manifest as hyperchromatic karyorrhectic nuclear debris within a large clear halo (so-called "exploding crypts") or as a shrunken cell with a condensed (pyknotic) nucleus and eosinophilic cytoplasm [ 41 ] (Figs. 16 and 17 ) . In the esophagus , apoptotic bodies are in the form of densely eosinophilic dyskeratotic keratinocytes; there may also be a lichenoid interface infl ammatory infi ltrate, sloughing, and other nonspecifi c changes in the squamous mucosa (see Table 3 ). Elsewhere in the GI tract, the apoptotic bodies are within glandular or crypt epithelium, forming intracytoplasmic vacuoles fi lled with karyorrhectic nuclear debris. In gastric body biopsies , these apoptotic bodies are usually found at the neck of glands where they interface with the gastric pit. In antral biopsies , apoptotic bodies are most often seen in the deep glands [ 151 ] . Focally enhanced gastritis, defi ned as a mixed lymphohistiocytic infi ltrate, in some cases with neutrophils, surrounding a small group of foveolae or glands without apoptosis is associated with but not diagnostic of GvHD [ 152 ] . In the small intestine, colon, and rectum , the deep crypt cells show apoptosis. When these apoptotic bodies are particularly prominent, the term "exploding crypts" has been used [ 153 ] . Other fi ndings in more severe cases include dilated crypts, luminal debris, crypt abscesses, crypt destruction, mucin (Figs. 18 and 19 ). The salient fi ndings are summarized in Table 3 .

There is controversy around the defi nition of the minimal histological changes suffi cient to establish the diagnosis of acute GvHD. Currently, the consensus favors a minimum of >1-2 apoptotic bodies per biopsy piece as the minimum diagnostic criterion because it increases sensitivity albeit with some loss of specifi city [ 41 , 154 ] (Fig. 18 ) . Unfortunately, due to the patchiness of injury along the GI tract, the rate of false negative error in diagnosis remains a concern. It has been suggested that at least eight and up to 20 serial sections should be reviewed to minimize the risk of missing infrequent apoptotic bodies [ 41 ] . Multiple biopsies along the GI tract may also improve the diagnostic yield. Currently, several histologic grading systems have been proposed. These schema, such as the Lerner [ 66 ] and the Sale modifi cation [ 27 ] serve as useful descriptors but do not necessarily refl ect clinical severity; all four grades may be associated with severe symptoms and extensive endoscopic involvement. 

Although there has been recent interest in the use of caspase-3 as an apoptotic marker [ 155 ] , it is not used in routine clinical practice. Immunohistochemistry for CMV as well as histochemical stains for Helicobacter may be useful for excluding infectious causes.

Given the wide range and lack of specifi city of the histologic fi ndings in acute GvHD, there is a considerable differential diagnosis. Conditioning regimens damage the mucosa but typically resolve within 20 days after transplantation. Drugs, including NSAIDs [ 156 ] , mycophenolate mofetil (MMF), ticlopidine [ 157 ] , proton-pump inhibitors [ 158 ] , 5-fl uorouracil, cyclosporine A, and phosphate bowel preparations [ 159 ] can initiate apoptosis and GvHD-like changes. In particular, MMF-related toxicity causes ulcerative esophagitis, reactive gastropathy, and colonic architectural disarray; in the bowel it leads to increased apoptosis, crypt dilatation with accumulation of luminal debris [ 160 ] . However, unlike acute GvHD where neuroendocrine cells are spared, MMF therapy leads to loss of neuroendocrine cell nests [ 41 ] . Refl ux esophagitis may also need to be excluded. Infl ammatory bowel disease and celiac disease should be considered in the setting of chronic architectural disarray and in the appropriate clinical context [ 161 , 162 ] . Infectious causes of GI symptoms must also be ruled out. The histologic changes should be corroborated by serologic testing, cultures, and immunohistochemical stains. CMV colitis mimics GvHD; more importantly both can coexist [ 151 , 163 ] . Cryptosporidium has also been associated with increased epithelial apoptosis [ 164 ] . Other infectious agents to consider include adenovirus, Clostridium diffi cile, campylobacter, H. pylori as well as HSV, HIV, candida, rotavirus, and norovirus [ 165 -168 ] .

As discussed previously, there can be signifi cant overlap between acute and chronic GvHD.

Symptoms typically manifest between 3 months and 2 years, although two-thirds of patients present within the fi rst 12 months following transplant [ 169 ] . There is no time limit on the presentation of chronic GvHD.

Chronic GI GvHD is thought to represent an endstage alloimmune and autoimmune disorder initiated by donor T-cells. The pathogenesis involves insults to both the recipients' histocompatibility antigens and antigens common to both donor and recipient.

Unlike acute GvHD, chronic GI GvHD not uncommonly involves the esophagus [ 150 ] . The key features of chronic GvHD are esophageal webs, strictures or concentric rings seen on endoscopy or barium contrast studies. According to the latest NIH consensus document, identifi cation of esophageal webs is the only universally accepted criterion to diagnose chronic GI GvHD [ 41 ] . Clinically, there is considerable overlap in the symptoms of both acute and chronic GvHD such as nausea, vomiting, weight loss, and diarrhea. Wasting syndrome may also be seen in chronic GvHD although it is likely a multifactorial process [ 33 ] . Chronic GvHD may also be associated with pancreatic atrophy and exocrine insuffi ciency [ 170 ] .

Like the clinical presentation of chronic GvHD, there is morphologic overlap with acute GvHD. As with the clinical presentation, the histologic fi ndings of chronic GI GvHD resemble those of acute GvHD affecting the gastrointestinal tract; their distinction cannot be made on histology alone. In addition, chronic GvHD may show changes of long-standing injury including marked architectural disarray, crypt loss, cystic glands, and areas of atrophy admixed with signs of regeneration. Chronic GvHD may show fewer apoptotic bodies and a more pronounced lymphoplasmacytic infi ltrate than acute GvHD. Paneth cell metaplasia as well as fi brosis of the lamina propria have also been described. It is important to bear in mind that

As indicated above, GvHD was historically classifi ed as acute or chronic by using an arbitrary cutoff of 100 days; however, it is now recognized that overlap exists and the distinction between hepatic acute and chronic GvHD is made on clinical grounds [ 32 , 171 ] . Studies show that liver involvement is present in 30 % and 35-50 % of patients with acute and chronic onset of symptoms, respectively [ 32 , 40 , 171 ] , but it can occur in as many as 85 % of patients following BMT [ 172 , 173 ] .

Regardless of chronicity, hepatic GvHD is a process driven by immune-mediated destruction of the small bile ducts and ductules. The role of the pathologist is twofold: fi rstly, to distinguish GvHD from other causes of liver dysfunction such as drug toxicity and infection, especially in cases where there are atypical or confounding clinical features [ 174 ] , and secondly, to identify the minimal criteria for active GvHD [ 40 ] . Owing to the patchy and irregular distribution of bile ductal involvement, liver biopsies for GvHD have a sensitivity of 66 % and specifi city of 91 % [ 172 ] . Thus, in cases with normal histologic fi ndings, clinicians may elect to treat patients for hepatic GvHD on the basis of clinical fi ndings alone. Other confounding factors in the diagnosis of hepatic GvHD are listed in Table 4 . There is little evidence to support the use of a liver biopsy as a screening tool or to assess response to therapy for GvHD [ 40 ] . However, the extent of bile duct damage and infl ammation does correlate with liver function tests, and in some cases with outcomes and survival [ 41 ] .

Risk factors for the development of hepatic GvHD include older recipient age, female donor cells into a male recipient, use of peripheral blood stem cells and previous episodes of GvHD [ 175 ] . Patients can present without symptoms or signs, with painless hepatomegaly alone or with multiorgan involvement. Laboratory tests show an increase in serum bilirubin (both direct and indirect), in alkaline phosphatase and the transaminases. Clinical and laboratory parameters utilized for the diagnosis of hepatic GvHD are in Table 5 .

Unfortunately, there is no consensus currently for the minimal histological criteria for the diagnosis of hepatic GvHD [ 41 ] . Classically, acute hepatic GvHD manifests as a cholestatic hepatitis with jaundice and elevated bilirubin and alkaline phosphatase levels. Concurrent GI or skin manifestations are usually present: indeed patients rarely present with isolated hepatic involvement [ 176 ] . Clinically, liver dysfunction in acute hepatic GvHD typically develops within 25-100 days following transplant. [ 40 , 171 , 242 , 243 ] ALT alanine aminotransferase; AST aspartate aminotransferase. See Ref. [ 32 ] The pathologic fi ndings in acute hepatic GvHD are dominated by bile duct injury [ 66 , 132 ] . Table 6 highlights the common histopathologic fi ndings from very early acute to late-chronic GvHD. The minimal histopathologic criteria required for acute or active GvHD include the presence of dysmorphic or attenuated "withered" bile ducts (Fig. 20 ) which may or may not occur in association with cholestasis, lobular, or portal infl ammation (Fig. 21 ) [ 41 ] . These classic morphologic fi ndings may be altered by the type and duration of immunosuppression, the use of ursodeoxycholic acid and any underlying liver disease. At a minimum, evidence of bile duct damage should be present. Lymphocytic portal infi ltrate, lobular infi ltrate, and prominent necroinfl ammatory activity can also be seen at any stage, most notably after cessation of immunosuppressive therapy [ 27 ] . In small biopsy specimens or cases where tissue was acquired early in disease progression, these fi ndings may be minor or even absent [ 177 , 178 ] . Very early changes are nonspecifi c and overlap with a variety of other etiologies [ 172 , 179 ] . The diagnosis of chronic hepatic GvHD requires distinctive manifestations in at least one other organ system such as the GI tract or skin [ 32 ] . The histopathologic features of chronic hepatic GvHD can also overlap with those of acute GvHD and the transition between the two may be subtle. The presence of marked biliary ductopenia and portal fi brosis suggest chronicity [ 132 ] but still a potentially reversible process [ 180 ] . In summary, the current criteria for chronic hepatic GvHD include ductopenia, portal fi brosis, chronic cholestasis but these are not specifi c for the disease (Fig. 22 ) . Prolonged GvHD gives rise to a more pronounced cholestatic pattern in which canalicular bile plugs, pigment-laden macrophages and hepatocyte ballooning degeneration can be seen. It is not uncommon to observe also bile ductular proliferation (ductular reaction). Chronic hepatic GvHD rarely progresses to frank cirrhosis and this fi nding warrants extensive clinical and pathologic workup for other etiologies such as hepatitis C viral infection [ 181 -184 ] . Parenthetically, other than the presence of bile ductular proliferation, the changes of GvHD in the liver closely resemble those found in chronic rejection of the liver allograft (see chapter "Liver Transplantation").

Histologic grading schemes for hepatic GvHD have been described but have shown poor correlation between clinical outcome and the severity of liver injury on histology (including bile duct injury and portal infl ammation) [ 66 , 185 ] . Moreover, none have been prospectively validated [ 40 ] . Due to their weak prognostic and predictive power, current guidelines do not recommend their use [ 41 ] .

The diagnosis of GvHD usually can be made on H&E-stained slides. Masson's trichrome stain highlights portal fi brosis, and the loss of bile ducts can also be highlighted by immunohistochemical (IHC) staining for cytokeratin 7 or cytokeratin 19. Bile ductal cells can show the same senescence-related changes that are seen in chronic liver allograft rejection; these can be highlighted by p21 IHC staining [ 186 ] . However p21 staining, determination of the Th17/T regula-tory cell ratio or C4d immunostaining have not been fully validated for routine clinical use [ 187 , 188 ] . IHC or in situ hybridization for CMV, herpes simplex virus (HSV), adenovirus, varicella zoster virus, human herpesvirus-6 and Epstein-Barr virus (EBV) can be useful to exclude infectious causes of hepatic dysfunction.

The differential diagnosis of hepatic GvHD is broad and is dependent on the timing of presentation and the level of liver enzymatic abnormalities. Table 7 lists the important entities to consider in the setting of HSCT and abnormal liver function. Notably, CMV, adenovirus, and herpes simplex virus I/II should be ruled out as potentially treatable causes. Drugs, including cyclosporine, can cause a mild increase in bilirubin by altering bile transport [ 189 ] . As there is no defi nitive histologic pattern of this drugrelated etiology, discontinuation of the drug(s), if possible, is helpful in establishing the cause. Sepsis can cause a cholestatic pattern of injury, resulting, in some cases in cholangitis lenta [ 190 ] . In the setting of hepatic GvHD and severe concomitant gastrointestinal GvHD, a severe bile ductular reaction with dilated ductules fi lled with bile pigment and fi brosis can occur and morphologically overlap with cholangitis lenta. In cases where viral hepatitis is suspected, the qualitative assessment of bile duct injury may be useful. Hepatitis C virus tends to incite focal rather than diffuse bile duct injury and has prominent lymphoid aggregates. 

Iron overload is now recognized as an important contributor to both hepatic and pulmonary disorders post-HSCT [ 191 -193 ] . Iron overload can arise from repeated blood transfusions and by increased intestinal iron absorption secondary to ineffective hematopoiesis. Many patients, due to the nature of their underlying disease, present with a 1.25 fold increase in iron stores [ 194 , 195 ] . This can increase the risk for systemic opportunistic infections and has been associated with a reduced overall survival [ 196 -198 ] . Iron overload has also been implicated in the early development of SOS following HSCT [ 192 ] . A serum ferritin level over 1000 ng/mL is considered an adverse prognostic factor [ 199 ] . Prussian blue staining reveals increased iron in parenchymal cells (i.e., hepatocytes) and variably in mesenchymal cells (i.e., macrophages and endothelial cells) (Fig. 23 ) [ 200 ] . The pathologist should indicate the degree or "grade" of iron deposition and qualify its location (parenchymal vs. mesenchymal vs. mixed). The grade of iron in the liver can be assessed using either the semi-quantitative Scheuer system or more recent modifi cations [ 201 -203 ] . Identifying iron overload in patient post-HSCT is particularly important because phlebotomy and chelation therapies can improve survival [ 193 ] . Hepatic iron overload can be superimposed on opportunistic infections such as Listeria monocytogenes , Mucormycosis , Yersinia enterocolitica , and non-cholera Vibrio species [ 195 , 204 , 205 ] .

Introduction SOS [ 206 ] , previously termed veno-occlusive disease of the liver (VOD) [ 207 ] results from cytotoxic liver injury. It is characterized by obliterative venulitis of the terminal hepatic venules. SOS is linked to myeloablative regimens for HSCT, cyclophosphamide, and TBI. The incidence of SOS varies, with centers reporting rates as low as 5.3 % to upwards of 54 %, depending on the type and dose of various chemotherapeutic agents and the use of TBI [ 208 -212 ] . Recent studies have suggested that the rate of SOS has dropped dramatically due to lower doses of TBI, the use of fl udarabine and the utilization of prophylactic therapy in patients considered at increased risk [ 212 ] . SOS is associated with increased morbidity and mortality, and severe SOS confers a mortality rate surpassing 90 % [ 210 , 211 ] .

SOS usually manifests within 10-21 days posttransplant [ 213 ] but can occur up to 50 days [ 214 ] . SOS severity can be graded by different sets of clinical parameters. One approach classifi es SOS as mild when resolution occurs in the Fig. 23 Hepatic iron overload. High power H&E ( a ) and iron ( b ) stains demonstrating increased iron in hepatocytes and in macrophages. This patient had received multiple blood transfusions absence of treatment, moderate when resolution occurs after therapeutic intervention, and severe when patients show signs and symptoms of SOSrelated liver disease that do not resolve before the 100th day post-HSCT or which cause death [ 208 ] . Most patients succumb to renal, cardiorespiratory, and multiorgan failure [ 211 ] . Another scheme for severity of SOS is based on modifications of clinical parameters used in the Modifi ed Seattle and Baltimore criteria detailed below [ 215 ] .

The classic presentation of SOS includes painful hepatomegaly, jaundice, and rapid weight gain due to fl uid retention (ascites) [ 216 ] . The Modifi ed Seattle [ 217 ] and the Baltimore criteria [ 218 ] both factor in hepatomegaly, weight gain, and bilirubin levels but differ in their diagnostic weighting (see Table 8 ). Moreover, the presence of portal hypertension, renal and pulmonary dysfunction as well as refractory thrombocytopenia provides additional support for the diagnosis of SOS [ 212 ] . Currently there are no laboratory parameters with diagnostic specifi city or prognostic signifi cance.

In most cases the diagnosis of SOS is a clinical diagnosis and a liver biopsy is reserved for inconclusive or atypical presentations. Several studies have advocated a transvenous approach, that allows for simultaneous tissue biopsy sampling and hemodynamic measurements of free and wedged hepatic venous pressures (and therefore the hepatic venous pressure gradient), and provides the most accurate diagnosis of SOS [ 219 , 220 ] . A hepatic venous pressure gradient >10 mmHg is highly specifi c and sensitive for the diagnosis of SOS in the appropriate clinical setting (e.g., provided there is no cirrhosis).

Factors that increase risk for the development of SOS in HSCT patients include advanced age, a history of prior liver injury (such as viral hepatitis, steatohepatitis, iron overload), systemic infl ammation, infection and long-term use of antibiotics [ 221 , 222 ] .

The pathogenesis of SOS and the sequence of events have been established in rodent models [ 206 ] and are described below. The initiating event is sinusoidal endothelial cell injury; endothelial cells appear to be more susceptible to cytotoxic injury than hepatocytes [ 222 , 223 ] . By light microscopy, very early in the course of the disease (24-48 h) the earliest manifestations are gradual subtle injury of sinusoidal endothelial cells and sinusoidal hemorrhage [ 206 ] . Between 3 and 5 days, there was marked sinusoidal and central venous endothelial damage with sloughing and downstream embolization into the sinusoids and subsequent obstruction, associated with predominantly centrilobular hemorrhage and coagulative necrosis of hepatocytes (Fig. 24 ) [ 224 ] . Livers at days 6-7 show deposition of subendothelial and adventitial fi brosis mediated by proliferation of stellate cells and of subendothelial fi broblasts. By day 8, either resolution ensues or the hepatic injury persists as variably severe late SOS.

The histologic hallmark of SOS is luminal obliteration of central (terminal hepatic) veins (Fig. 25 ) . Table 9 highlights the principal histopathologic fi ndings and differential diagnoses are enumerated in Table 10 .

In the appropriate clinical context the majority of cases of SOS can be established by the presence of obliterative venulitis and fi brosis using routine Weight gain ( >5 % from pre-transplant weight) Bilirubin > 34.2 μmol/L (2 mg/dL) Ascites H&E-stained slides in combination with Masson's trichrome and/or reticulin stains. It has been suggested that sinusoidal capillarization defi ned as the ultrastructural transformation of endothelial sinusoidal cells into cells that more closely resemble capillary vessels can be highlighted by CD34 positivity and may be contributed to the hepatic dysfunction of SOS [ 225 , 226 ] . However, immunostains for CD34 are not used routinely in the diagnosis of SOS.

Nodular regenerative hyperplasia (NRH ) in HSCT is thought to result from hepatic vascular injury in the setting of SOS or it may represent a manifestation of chemotherapy-induced drug injury. It arises as a specifi c adaptive response to changes in blood fl ow secondary to a wide variety of insults [ 227 ] . Histologically, it is characterized by partial or complete transformation of the normal liver parenchyma into hepatocellular nodules less than 3 mm in diameter. These nodules contain regenerating hepatocytes more than one cell wide, and are not associated with periportal or perisinusoidal fi brosis or septa. The regenerating nodules cause compression of adjacent structures, leading to hepatocyte atrophy and sinusoidal dilatation, best highlighted with a reticulin stain. NRH is often asymptomatic but may be complicated by portal hypertension, bleeding varices, ascites, and splenomegaly [ 228 ] .

The principal types of HSCT are autologous and allogeneic: the choice is dependent on the primary disease, the availability of a suitable donor, and recipient-related issues. Autologous HSCT has the advantage of near absence of graft-versus-host (GvHD) but the drawback of increased disease recurrence. Allogeneic HSCT, important in the therapy of many hematological conditions, has the advantage of an important graft-versusmalignancy effect but the complication of GvHD. GvHD is currently classifi ed as acute and chronic primarily on clinical features rather than the arbitrary cutoff of 100 days. The histopathologic fi ndings of GvHD are often nonspecifi c and require careful clinical correlation and preferably confi rmation by multiorgan involvement. The principal bone marrow complications following HSCT are persistence of hypocellularity, Fig. 24 Sinusoidal obstruction syndrome in liver biopsy from a patient following chemotherapy. Note hemorrhage, sinusoidal congestion, and the atrophic hepatocytes infection, and persistence or relapse of the primary malignancy. The key features of cutaneous acute GvHD are apoptosis in the lower malphigian epithelium and basal layers of appendages, satellitosis, and lichenoid infl ammation.

There are several forms of chronic GvHD in the skin: the pathologic features overlap with those of acute GvHD but also display variable infl ammation and fi brosis of deeper layers of dermis and subcutaneous tissue. Pulmonary complications are important determinants in the outcome of patients following HSCT.

Infections remain an important consideration and should be carefully evaluated in all cytologic and histopathologic specimens. Most of the acute non-infectious complications are categorized under the umbrella term of "idiopathic pneumonia syndrome." It likely represents a manifestation of GvHD. The pathologic manifestations include several entities characterized pathologically by variable pulmonary edema and/or hemorrhage, DAD and interstitial or organizing pneumonias. The subacute and chronic forms of GvHD are COP and bronchiolitis obliterans, respectively. The latter is the most characteristic and has a poor prognosis. Pulmonary alveolar proteinosis, pulmonary hypertension, and pulmonary cytolytic thrombi are occasionally encountered after HSCT. The principal renal complications following HSCT are acute tubular necrosis, thrombotic microangiopathy and glomerulopathy; the relationship of the latter two to GvHD remains unsettled. The gastrointestinal tract is the second most common site of involvement by GvHD. The principal pathologic fi nding in acute GvHD is crypt or gastric gland epithelial cell apoptosis. Differential diagnostic considerations include infection (e.g., CMV) and drug-related alterations. The pathologic fi ndings of GvHD in the liver closely resemble those of chronic liver allograft rejection and include dysmorphic or attenuated "withered" bile ducts, ductopenia (in advanced stages), variable cholestasis, and lobular, and/or portal infl ammation. Other hepatic alterations following HSCT that should be included in the pathology report include the presence of iron overload and sinusoidal obstruction syndrome. 

Principles and overview of allogeneic hematopoietic stem cell transplantation

Bone marrow transplantation

Translating stem and progenitor cell biology to the clinic: barriers and opportunities

Overview of hematopoietic stem cell transplantation

Therapeutic stem and progenitor cell transplantation for organ vascularization and regeneration

Clinical symptoms of radiation sickness, time to onset and duration of symptoms among Hiroshima survivors in the lethal and median lethal ranges of radiation

A history of allogeneic hematopoietic cell transplantation

Bone-marrow transplantation (fi rst of two parts)

One hundred patients with acute leukemia treated by chemotherapy, total body irradiation, and allogeneic marrow transplantation

Marrow transplantation for acute nonlymphoblastic leukemia in fi rst remission

Current status of bone marrow transplantation for aplastic anemia and acute leukemia

Bone-marrow transplantation (second of two parts)

Hematopoietic stem cell transplantation in clinical practice

The history of autologous hematopoietic cell transplantation

Effect of recombinant human granulocyte-macrophage colony-stimulating factor on chemotherapy-induced myelosuppression

Granulocyte-macrophage colony stimulating factor expands the circulating haemopoietic progenitor cell compartment in man

Historic landmarks in clinical transplantation: conclusions from the consensus conference at the University of California

Autologous bone-marrow transplantation in relapsed adult acute leukaemia

The principles and overview of autologous hematopoietic stem cell transplantation

Current use and outcome of hematopoietic stem cell transplantation: CIBMTR Summary Slides

Indications and outcome of allogeneic hematopoietic cell transplantation for hematologic malignancies in adults

Allogeneic stem cell transplantation

WHO classifi cation of tumours of haematopoietic and lymphoid tissues

Knowles neoplastic hematopathology

Relevance of marrow fi brosis in bone marrow transplantation: a retrospective analysis of engraftment

Pathology of hematopoietic cell transplantation

Viral infections in hematopoietic stem cell transplant recipients

Allogeneic stem cell transplantation. Contemporary hematology

Diagnostic pathology of infectious disease

Allogeneic stem cell transplantation. Contemporary hematology

National Institutes of Health consensus development project on criteria for clinical trials in chronic graft-versus-host disease: I. Diagnosis and staging working group report

National Institutes of Health consensus development project on criteria for clinical trials in chronic graft-versus-host disease: I. The 2014 diagnosis and staging working group report

Chronic graft-versushost syndrome in man. A long-term clinicopathologic study of 20 Seattle patients

Hematopoietic stem cell transplantation in clinical practice

IBMTR severity index for grading acute graft-versus-host disease: retrospective comparison with Glucksberg grade

Allogeneic stem cell transplantation. Contemporary hematology

Graft versus host-associated pulmonary disease and other idiopathic pulmonary complications after hematopoietic stem cell transplant

Allogeneic stem cell transplantation. Contemporary hematology

Histopathologic diagnosis of chronic graft-versus-host disease: National Institutes of Health Consensus Development Project on criteria for clinical trials in chronic graft-versus-host disease: II

NIH consensus development project on criteria for clinical trials in chronic graftversus-host disease: II. The

Haemopoietic regrowth after chemotherapy for acute leukaemia: an immunohistochemical study of bone marrow trephine biopsy specimens

Morphology of the bone marrow after stem cell transplantation

Bone marrow biopsy changes following chemotherapy for acute leukemia

Evaluation of the bone marrow after therapy

Post-therapy bone marrow fi ndings

Bone marrow stroma and bone disorders

Macrophages and their subpopulations following allogenic bone marrow transplantation for chronic myeloid leukaemia

Early reconstitution of haematopoiesis after allogeneic bone marrow transplantation: a prospective histopathological study of bone marrow biopsy specimens

Severe hematopoietic alterations in vitro, in bone marrow transplant recipients who develop graft-versus-host disease

Infectious complications in patients undergoing unrelated donor bone marrow transplantation: experience from a single institution

Minimal residual disease

Signifi cance of chimerism in hematopoietic stem cell transplantation: new variations on an old theme

Hemopoietic chimerism following stem cell transplantation

Chimerism patterns of longterm stable mixed chimeras posthematopoietic stem cell transplantation in patients with nonmalignant diseases: follow-up of long-term stable mixed chimerism patients

Graft-versushost disease: part I. Pathogenesis and clinical manifestations of graft-versus-host disease

Endothelial injury mediated by cytotoxic T lymphocytes and loss of microvessels in chronic graft versus host disease

consensus conference on acute GVHD grading

National Institutes of Health chronic graft-versus-host disease staging in severely affected patients: organ and global scoring correlate with established indicators of disease severity and prognosis

Cutaneous manifestations of chronic graftversus-host disease

The spongiotic reaction pattern

Lack of specifi city in skin biopsy specimens to assess for acute graftversus-host disease in initial 3 weeks after bonemarrow transplantation

Role of skin biopsy to confi rm suspected acute graft-vs-host disease: results of decision analysis

Value of skin biopsies in assessing prognosis and progression of acute graft-versus-host disease

Histopathology of graft-vs.-host reaction (GvHR) in human recipients of marrow from HL-A-matched sibling donors

Acute cutaneous eruptions after marrow ablation: roses by other names?

Reappraisal of histologic features of the acute cutaneous graft-versus-host reaction based on an allogeneic rodent model

Clinical signifi cance of skin biopsies in the diagnosis and management of graft-vs-host disease in early postallogeneic bone marrow transplantation

Eczematoid graft-vs-host disease: a novel form of chronic cutaneous graft-vs-host disease and its response to psoralen UV-A therapy

Cutaneous infections and infestations-histological patterns

Toward biomarkers for chronic graft-versus-host disease: National Institutes of Health consensus development project on criteria for clinical trials in chronic graft-versus-host disease: III

Histological evaluation of acute mucocutaneous graft-versus-host disease in nonmyeloablative hematologic stem cell transplants with an observation predicting an increased risk of progression to chronic graft-versus-host disease

Engraftment syndrome: a common cause for rash and fever following autologous hematopoietic stem cell transplantation for multiple sclerosis

Histological features of skin and rectal biopsy specimens after autologous and allogeneic bone marrow transplantation

Tissue eosinophils and the perils of using skin biopsy specimens to distinguish between drug hypersensitivity and cutaneous graft-versus-host disease

Quantitative analysis of eosinophils in acute graft-versus-host disease compared with drug hypersensitivity reactions

Subacute radiation dermatitis: a histologic imitator of acute cutaneous graft-versus-host disease

The skin biopsy in the diagnosis of acute graft-versus-host disease in man

The lichenoid reaction pattern ('interface dermatitis'). In: Patterson JW, Hosler GA, editors. Weedon's skin pathology

Eruption of lymphocyte recovery or autologous graft-versus-host disease?

Human Herpesvirus-6 cytopathic inclusions: an exceptional and recognizable fi nding on skin biopsy during HHV6 reactivation after autologous stem-cell transplantation

Cutaneous toxoplasmosis histologically mimicking graft-versus-host disease

Clinical differentiation of acute cutaneous graft-versus-host disease from drug hypersensitivity reactions

The "drug vs graft-vs-host disease" conundrum gets tougher, but there is an answer: the challenge to dermatologists

Interstitial pneumonia and cytomegalovirus infection as complications of human marrow transplantation

Nonbacterial pneumonia after allogeneic marrow transplantation-a review of 10 years experience

Bronchiolitis obliterans and other late onset non-infectious pulmonary complications in hematopoietic stem cell transplantation

The yield and safety of thoracentesis in hematopoietic stem cell transplantation recipients

Complications after hematopoietic stem cell transplantation

An offi cial American Thoracic Society research statement: noninfectious lung injury after hematopoietic stem cell transplantation: idiopathic pneumonia syndrome

Stem cell transplantation and lung dysfunction

Delayed pulmonary toxicity syndrome following high-dose chemotherapy and bone marrow transplantation for breast cancer

Pulmonary drug toxicity in patients with primary breast cancer treated with high-dose combination chemotherapy and autologous bone marrow transplantation

Noninfectious pneumonitis after blood and marrow transplant

Engraftment syndrome after nonmyeloablative allogeneic hematopoietic stem cell transplantation: incidence and effects on survival

Peri-engraftment respiratory distress syndrome during autologous hematopoietic stem cell transplantation

Idiopathic pneumonia syndrome after bone marrow transplantation

TNF-receptor inhibitor therapy for the treatment of children with idiopathic pneumonia syndrome. A joint Pediatric Blood and Marrow Transplant Consortium and Children's Oncology Group study (ASCT0521)

Diffuse alveolar hemorrhage in hematopoietic stem cell transplant recipients

Alveolar hemorrhage. Diagnostic criteria and results in 194 immunocompromised hosts

Diffuse alveolar hemorrhage in allogeneic bone marrow transplantation. A postmortem study

Diffuse alveolar hemorrhage associated with microangiopathy after allogeneic bone-marrow transplantation

Diffuse alveolar hemorrhage and infection-associated alveolar hemorrhage following hematopoietic stem cell transplantation: related and high-risk clinical syndromes

statement: update of the international multidisciplinary classifi cation of the idiopathic interstitial pneumonias

The histological spectrum of pulmonary graft-versus-host disease in bone marrow transplant recipients

Histologic fi ndings in lung biopsies in patients with suspected graft-versus-host disease

Bronchiolitis obliterans syndrome (BOS), bronchiolitis obliterans organizing pneumonia (BOOP), and other late-onset noninfectious pulmonary complications following allogeneic hematopoietic stem cell transplantation

Diagnosis and treatment of pulmonary chronic GVHD: report from the consensus conference on clinical practice in chronic GVHD

Incidence, risk factors and outcomes of bronchiolitis obliterans after allogeneic stem cell transplantation

Biopsy-verifi ed bronchiolitis obliterans and other noninfectious lung pathologies after allogeneic hematopoietic stem cell transplantation

Pulmonary alveolar proteinosis

Pulmonary alveolar proteinosis

Secondary pulmonary alveolar proteinosis after unrelated cord blood hematopoietic cell transplantation

Pulmonary complications in adult blood and marrow transplant recipients: autopsy fi ndings

Pulmonary alveolar proteinosis following allogeneic hematopoietic cell transplantation

Late-onset pulmonary arterial hypertension in association with graft-versus-host disease after allogeneic stem-cell transplantation

Pulmonary arterial hypertension after childhood cancer therapy and bone marrow transplantation

Zur histologie der klinischen "primaren Pulmonalsklerose

Pulmonary veno-occlusive disease after bone marrow transplantation

Lower leukemia relapse in pediatric patients with pulmonary cytolytic thrombi following allogeneic transplant

Pulmonary nodular lesions in bone marrow transplant recipients: impact of histologic diagnosis on patient management and prognosis

Pulmonary cytolytic thrombi: a previously unrecognized complication of bone marrow transplantation

Pulmonary cytolytic thrombi: unusual complication of hematopoietic stem cell transplantation

Pulmonary cytolytic thrombi after allogeneic hematopoietic cell transplantation: a further histologic description

Spectrum of renal pathology in hematopoietic cell transplantation: a series of 20 patients and review of the literature

Renal pathology in hematopoietic cell transplantation recipients

Nephrotic syndrome after hematopoietic cell transplantation: do glomerular lesions represent renal graft-versus-host disease?

Renal pathology at autopsy in patients who died after hematopoietic stem cell transplantation

Defi ning the intensity of conditioning regimens: working defi nitions

Gastrointestinal and hepatic complications of hematopoietic stem cell transplantation

Hematopoietic stem cell transplantation: graft versus host disease and pathology of gastrointestinal tract, liver, and lung

Graft-vs.-host disease

Risk factors for acute graftversus-host disease after allogeneic blood stem cell transplantation

Increasingly frequent diagnosis of acute gastrointestinal graftversus-host disease after allogeneic hematopoietic cell transplantation

Total body irradiation and acute graftversus-host disease: the role of gastrointestinal damage and infl ammatory cytokines

The primacy of the gastrointestinal tract as a target organ of acute graft-versus-host disease: rationale for the use of cytokine shields in allogeneic bone marrow transplantation

Prognostic value of apoptotic cells and infi ltrating neutrophils in graftversus-host disease of the gastrointestinal tract in humans: TNF and Fas expression

Graftversus-host disease

Chronic graft-versus-host disease-implementation of the National Institutes of Health consensus criteria for clinical trials

Gastrointestinal emergencies in patients with acute intestinal graft-versus-host disease

The clinical spectrum of acute graft-versus-host disease

Clinical molecular imaging in intestinal graft-versus-host disease: mapping of disease activity, prediction, and monitoring of treatment effi ciency by positron emission tomography

Prospective endoscopic evaluation for gastrointestinal graft-versus-host disease: determination of the best diagnostic approach

Endoscopic evaluation and histological fi ndings in graft-versus-host disease

Endoscopic diagnosis of pediatric acute gastrointestinal graft-versus-host disease

Endoscopic and histologic diagnosis of intestinal graft-versushost disease after marrow transplantation

Usefulness of magnifying endoscopic evaluation of the terminal ileum for a patient with graft-versus-host disease after allogeneic hematopoietic stem cell transplantation

Endoscopic fi ndings predict the histologic diagnosis in gastrointestinal graft-versus-host disease

Endoscopic diagnosis of gastrointestinal graftversus-host disease

Gastric graft-versus-host disease: a blinded histologic study

The clinical signifi cance of focally enhanced gastritis

Gastrointestinal graft-versus-host disease in man. A clinicopathologic study of the rectal biopsy

Mycophenolic acid (Cellcept and Myofortic) induced injury of the upper GI tract

Detection of apoptoses in gastro-intestinal graftversus-host disease and cytomegalovirus colitis by a commercially available antibody against

Drug-related pathological lesions of the intestinal tract

Ticlopidine induced colitis: a histopathological study including apoptosis

Selective induction of apoptosis with proton pump inhibitor in gastric cancer cells

Colorectal infl ammation and increased cell proliferation associated with oral sodium phosphate bowel preparation solution

Mycophenolate mofetil-related gastrointestinal mucosal injury: variable injury patterns, including graft-versus-host disease-like changes

Importance of apoptosis in the histopathology of drug related lesions in the large intestine

Increased small intestinal apoptosis in coeliac disease

Impact of cytomegalovirus gastrointestinal disease on the clinical outcomes in patients with gastrointestinal graft-versus-host disease in the era of preemptive therapy

Intestinal epithelial cell apoptosis following Cryptosporidium parvum infection

Mechanism of Clostridium diffi cile toxin A-induced apoptosis in T84 cells

Induction of gastric epithelial apoptosis by Helicobacter pylori

Increase in proliferation and apoptosis of gastric epithelial cells early in the natural history of Helicobacter pylori infection

Helicobacter pylori infection and graft-versus-host disease

Chronic graft-versus-host disease: clinical manifestations and therapy

Pancreatic atrophy in patients with chronic graft-versus-host disease

Evaluation of NIH consensus criteria for classifi cation of late acute and chronic GVHD

A coded histologic study of hepatic graft-versus-host disease after human bone marrow transplantation

Liver disease after human marrow transplantation

Clinical importance of confi rming or excluding the diagnosis of chronic graft-versus-host disease

Pathogenesis and management of graft-versus-host disease

Phase III study comparing methotrexate and tacrolimus (prograf, FK506) with methotrexate and cyclosporine for graft-versus-host disease prophylaxis after HLAidentical sibling bone marrow transplantation

Intestinal and hepatic complications of human bone marrow transplantation. Part II

Intestinal and hepatic complications of human bone marrow transplantation

The pathology of bone marrow transplantation

Chronic graftversus-host disease of the liver: presentation as an acute hepatitis

Autoantibodies in human chronic graft-versus-host disease after hematopoietic cell transplantation

Hepatitis viruses and hematopoietic cell transplantation: a guide to patient and donor management

Hepatitis C virus infection and bone marrow transplantation: a cohort study with 10-year follow-up

Cirrhosis of the liver in long-term marrow transplant survivors

Histologic features of the liver biopsy predict the clinical outcome for patients with graft-versus-host disease of the liver

Replicative senescence of biliary epithelial cells precedes bile duct loss in chronic liver allograft rejection: increased expression of p21(WAF1/Cip1) as a disease marker and the infl uence of immunosuppressive drugs

Increased Th17/Treg ratio in chronic liver GVHD

Frequent expression of C4d in hepatic graft-versus-host disease: potential clue for diagnosis and distinguishing acute and chronic form

Phase I/II trial of cyclosporine as a chemotherapy-resistance modifi er in acute leukemia

Sepsis-induced cholestasis

Risk factors for fungal pulmonary infections in hematopoietic stem cell transplantation recipients: the role of iron overload

Iron overload: predictor of adverse outcome in hematopoietic stem cell transplantation

Iron overload in patients undergoing hematopoietic stem cell transplantation

Pathology of the liver in leukaemia and lymphoma. A study of 110 autopsies

Iron overload in hematopoietic cell transplantation

Severe hemosiderosis post allogenic bone marrow transplantation

Role of non-transferrin bound iron in iron overload and liver dysfunction in long term survivors of acute leukaemia and bone marrow transplantation

Oxygen radicals and human disease

Iron overload in allogeneic hematopoietic cell transplantation outcome: a meta-analysis

Iron overload in allogeneic hematopoietic stem cell transplant recipients

Assessment of liver iron content in 271 patients: a reevaluation of direct and indirect methods

Pathology of hepatic iron overload

Differentiation between heterozygotes and homozygotes in genetic hemochromatosis by means of a histological hepatic iron index: a study of 192 cases

High prevalence of iron overload in adult allogeneic hematopoietic cell transplant survivors

Ironoverload after autologous hematopoietic cell transplantation

Characterization of a reproducible rat model of hepatic veno-occlusive disease

Veno-occlusive disease of liver with nonportal type of cirrhosis, occurring in Jamaica

Veno-occlusive disease of the liver and multiorgan failure after bone marrow transplantation: a cohort study of 355 patients

An analysis of hepatic venocclusive disease and centrilobular hepatic degeneration following bone marrow transplantation

Incidence and outcome of hepatic veno-occlusive disease after blood or marrow transplantation: a prospective cohort study of the European Group for Blood and Marrow Transplantation. European Group for Blood and Marrow Transplantation Chronic Leukemia Working Party

Hepatic veno-occlusive disease following stem cell transplantation: incidence, clinical course, and outcome

Hepatobiliary complications of hematopoietic cell transplantation, 40 years on

BCSH/BSBMT guideline: diagnosis and management of veno-occlusive disease (sinusoidal obstruction syndrome) following haematopoietic stem cell transplantation

Veno-occlusive disease of the liver after busulfan, melphalan, and thiotepa conditioning therapy: incidence, risk factors, and outcome

How I, treat sinusoidal obstruction syndrome

Venocclusive disease of the liver after bone marrow transplantation: diagnosis, incidence, and predisposing factors

Hepatic venoocclusive disease liver toxicity syndrome after bone marrow transplantation

Venoocclusive disease of the liver following bone marrow transplantation

Transjugular liver biopsy in BMT

Utility of transvenous liver biopsies and wedged hepatic venous pressure measurements in sixty marrow transplant recipients

How can we reduce hepatic veno-occlusive disease-related deaths after allogeneic stem cell transplantation?

Hepatic veno-occlusive disease (sinusoidal obstruction syndrome) after hematopoietic stem cell transplantation

Review article: updates in the pathogenesis and therapy of hepatic sinusoidal obstruction syndrome

Vascular disorders of the liver

Liver injury due to chemotherapy-induced sinusoidal obstruction syndrome is associated with sinusoidal capillarization

Capillarization of hepatic sinusoids in man

Micronodular transformation (nodular regenerative hyperplasia) of the liver: a report of 64 cases among 2,500 autopsies and a new classifi cation of benign hepatocellular nodules

Nodular regenerative hyperplasia: not all nodules are created equal

Late pulmonary complications after allogeneic hematopoietic stem cell transplantation: diagnosis, monitoring, prevention, and treatment

Pulmonary veno-occlusive disease following hematopoietic stem cell transplantation: a rare model of endothelial dysfunction

Bronchiolitis obliterans syndrome epidemiology after allogeneic hematopoietic cell transplantation

Bronchiolitis obliterans syndrome after allogeneic hematopoietic stem cell transplantation-an increasingly recognized manifestation of chronic graft-versus-host disease

Lung injury following hematopoietic cell transplantation

Association between acute and chronic graft-versus-host disease and bronchiolitis obliterans organizing pneumonia in recipients of hematopoietic stem cell transplants

Bronchiolitis obliterans in bone-marrow transplantation and its relationship to chronic graftv-host disease and low serum IgG

Lymphoid interstitial pneumonia after allogeneic bone marrow transplantation. A possible manifestation of chronic graft-versus-host disease

Bronchiolitis obliterans following haematopoietic stem cell transplantation

Late pulmonary, cardiovascular, and renal complications after hematopoietic stem cell transplantation and recommended screening practices

Pathology of graft-versushost disease in the gastrointestinal tract

Global and organ-specifi c chronic graft-versushost disease severity according to the 2005 NIH Consensus Criteria

Graft-versus-host disease

Liver disease is a major cause of mortality following allogeneic bone-marrow transplantation

Imatinibinduced immune hepatitis: case report and literature review