key: cord-0910746-wdsish2b
authors: Belizaire, Roger; Makar, Robert S
title: Non-Alloimmune Mechanisms of Thrombocytopenia and Refractoriness to Platelet Transfusion
date: 2020-09-17
journal: Transfus Med Rev
DOI: 10.1016/j.tmrv.2020.09.002
sha: 66d2cf7323d700bfb325283f7869ce8005c33d8b
doc_id: 910746
cord_uid: wdsish2b

Refractoriness to platelet transfusion is a common clinical problem encountered by the transfusion medicine specialist. It is well recognized that most causes of refractoriness to platelet transfusion are not a consequence of alloimmunization to human leukocyte, platelet-specific, or ABO antigens, but are a consequence of platelet sequestration and consumption. This review summarizes the clinical factors that result in platelet refractoriness and highlights recent data describing novel biological mechanisms that contribute to this clinical problem.

Platelet refractoriness describes the circumstance in which platelet transfusion fails to yield an adequate increase in the platelet count. Definitions for platelet refractoriness depend upon the timing of the post-transfusion platelet count and the metric used to assess the increase in platelet count (Table 1) . One definition for platelet refractoriness is a corrected count increment less than 5X10 9 /L/m 2 on two sequential occasions after transfusion of ABOcompatible platelets [1] . Although alloimmunization to human leukocytes antigens (HLA) or platelet-specific antigens is a well understood mechanism of platelet refractoriness that can be ameliorated through specific product selection strategies, it is observed in <10% of cases of platelet refractoriness [2] . Thus, the majority of platelet refractoriness is not a result of alloimmunization but is instead a result of clinical factors in the patient [3] . Indeed, in patients with both clinical and alloimmune factors for platelet refractoriness, use of HLA-selected platelet components may not substantially improve the observed post-transfusion platelet increment [4] . The purpose of this review is to summarize the clinical factors associated with platelet refractoriness and highlight recent data describing biological mechanisms that contribute to this phenomenon.

Many different factors related to the patient's clinical state have been implicated as causes of splenomegaly, the proportion of labeled platelets detectable in the peripheral blood decreased while increasing in the spleen. It was estimated that at any given time, 30% of total platelet mass resided in the spleen of normal subjects and 50-90% of platelet mass in patients with splenomegaly [15] . Interestingly, epinephrine infusion increased circulating platelet levels in individuals with spleens but not in asplenic individuals. Furthermore, in patients with splenomegaly, the percent increase in platelet count from baseline was almost double that observed in normal individuals. Thus, platelet sequestration was reversible, and platelets could be mobilized by stress [15] , distinguishing splenomegaly from consumptive processes in which transfused platelets are destroyed. Indeed, in many patients with splenomegaly, satisfactory post-transfusion increments may be observed [9] . However, among the subset of patients with massive splenomegaly resulting in repeatedly inadequate post-transfusion increments, the efficacy of prophylactic platelet transfusion is uncertain.

The systemic inflammatory response and sepsis may prompt platelet consumption and thrombocytopenia through disseminated intravascular coagulation. However, recent work has

A series of studies over the last two decades have reported that platelets express pathogen recognition receptors (PRRs), including Toll-like receptors (TLRs) and NOD-like receptors. Prior to the discovery of PRRs, Davis et al. observed that intravenous injection of purified lipopolysaccharide (LPS) from E. coli was associated with rapid platelet activation and profound thrombocytopenia [16] . More recently, several independent groups showed that the receptor required for LPS-mediated intracellular signaling, TLR4, is expressed on both human and mouse platelets [17] [18] [19] . In mouse models, platelet expression of TLR4 was involved in several LPSinduced platelet phenotypes, including thrombocytopenia, adhesion to fibrinogen, P-selectin expression, and potentiation of thrombin-or collagen-induced aggregation [17, 18, 20] . In addition to TLR4, it has been reported that platelets express TLR2 [18, 19, 21, 22] , TLR7 [23] , TLR9 [24, 25] , and NOD2 [26] , suggesting that platelets are also potentially responsive to a wide range of pathogen-associated molecular patterns. The bacterial cell wall mimetic, Pam3CSK4, stimulated platelet aggregation that was significantly reduced by TLR2 genetic deletion or a TLR2-blocking antibody [22] . Thrombocytopenia that developed in mice treated with the guanosine analog loxoribine or infected with encephalomyocarditis virus required TLR7 [23] . Thon et al. reported that platelets exposed to unmethylated CpG DNA showed an increase in aggregation and P-selectin expression that was reduced in the absence of TLR9 [25] . Finally, NOD2 was essential for the muramyl dipeptide-mediated potentiation of platelet aggregation and ATP release after stimulation with collagen or thrombin [26] .

J o u r n a l P r e -p r o o f Journal Pre-proof Despite these data, the role of PRR signaling in platelet activation remains controversial. Several studies did not observe a significant change in platelet aggregation or P-selectin expression after treatment with agonists of TLR2, TLR4, or TLR9 [19, [27] [28] [29] [30] [31] . In addition, platelet-specific deletion of the MyD88 signaling adaptor, which is required for signaling downstream of most TLRs, had a minimal effect on platelet counts and P-selectin expression in mouse models of S. pneumoniae and K. pneumoniae sepsis, implying that platelet TLR signaling may have a limited role in the platelet activation and thrombocytopenia observed in systemic bacterial infections [30, 32] . Whether engagement of TLRs modulates platelet biology via "non-canonical" mechanisms [28] remains an open question and deserves further investigation. However, the existence of such mechanisms is suggested by multiple studies in which binding of TLR4 by LPS alters platelet function in a TLR-dependent manner, including mitochondrial function [28] , fibrinogen binding [17] , neutrophil binding [17, 27] , and cytokine secretion [29] .

In addition to recognition of bacterial molecules via PRRs, platelet binding to whole bacterial pathogens has been widely described [33] . Platelet-bacterium interactions can promote platelet activation and aggregation through multiple mechanisms, including indirect binding to a plasma protein that is subsequently bound by a platelet receptor or direct bacterial protein binding to a platelet receptor. For example, S. aureus ClfA and FnbpA/B proteins both bind to fibronectin and fibrinogen, which facilitates interactions with the GPIIb/IIIa receptor on platelets [34] ; J o u r n a l P r e -p r o o f another S. aureus surface protein, IsdB, promotes platelet-bacterium interaction via direct binding to glycoprotein (GP) IIb/IIIa [35] .

Numerous other platelet-bacterium interactions involving staphylococcal and streptococcal proteins have been characterized and are presumed to play important roles in infective endocarditis and other pathologic states [36] . Platelet activation driven by bacterial interactions with GPIIb/IIIa and GPIb receptors most likely contributes to platelet aggregation and consumption in sepsis. Interestingly, H. pylori binding to von Willebrand factor (vWF) promotes GPIb-mediated interactions with platelets [37] , resulting in platelet activation, aggregation, and thrombocytopenia in a subset of patients. Remarkably, antibiotic therapy for H. pylori that effectively resolves infection also normalizes platelet counts [38] [39] [40] [41] , suggesting that plateletbacterium interaction alone could be sufficient to cause thrombocytopenia [42] .

In response to infection, neutrophils release neutrophil extracellular traps (NETs), which are composed of DNA, histones, and granule proteins [43] . NET release is a host defense mechanism employed by neutrophils to trap and kill bacteria, however exuberant NET formation or reduced NET clearance are implicated in numerous pathologic processes including autoimmunity, thrombosis, ischemia-reperfusion injury, and cancer progression [44] [45] [46] .

Platelets appear to facilitate NET formation and to bind NETs. Infusion of LPS in mice caused thrombocytopenia and pulmonary sequestration of platelets through a mechanism that required both neutrophils and platelet TLR4 [17] . LPS was subsequently shown to bind platelet J o u r n a l P r e -p r o o f TLR4, leading to platelet activation and binding to neutrophils. The interaction between activated platelets and neutrophils resulted in neutrophil activation, degranulation, and NET formation. Plasma from severely septic, thrombocytopenic patients also stimulated NET release via TLR4-dependent platelet-neutrophil interactions [27] . Platelets may stimulate NET release via signaling between platelet P-selectin and neutrophil PSGL-1 [47] . Platelets can bind directly to histone/DNA complexes within NETs or via plasma proteins that decorate NETs [46] .

NET binding results in platelet adhesion, activation, and aggregation in vitro [48] . Histone H3 and H4 are able to directly bind to platelet TLR2 and TLR4 to mediate platelet aggregation [48, 49] ; platelet aggregation requires both histone-induced calcium signaling to activate GPIIb/IIIa and fibrinogen crosslinking of histone-coated platelets [49] . The pathophysiologic relevance of these findings is suggested by murine models in which NETs promote deep venous thrombosis [50, 51] and histone infusion induces profound thrombocytopenia in vivo [49] .

In addition to NETs, cell injury and death are other sources of circulating DNA and histones.

High levels of circulating histones may be observed in patients with sepsis [52] [53] [54] or following trauma [55] and may contribute to end-organ injury [52, 54, 55] . In a murine model, infusion of a sublethal dose of purified histones resulted in pulmonary injury characterized by accumulation of neutrophils and platelet-rich thrombi in the alveolar microvessels, vacuolation of endothelial and epithelial cells, intra-alveolar hemorrhage with accumulation of fibrin as well as platelet-rich microthrombi, and deposition of fibrin and collagen in the interalveolar septae [52] . A relationship between circulating histones and thrombocytopenia in the clinical setting is suggested by the observation that high levels of plasma histones detected in critically J o u r n a l P r e -p r o o f ill patients was associated with subsequent development of moderate to severe thrombocytopenia [56] .

Apoptosis comprises an ordered series of biochemical and morphologic changes that ultimately result in cell death [57] . Though nuclear condensation is one of the original hallmark features of apoptosis [58] , it is now well-established that platelets, which lack a nucleus, can undergo apoptosis [59] . In addition, several lines of evidence indicate that apoptosis plays a significant role in determining platelet lifespan. In this context, it is possible that clinical factors (e.g., drugs, inflammation, infection) could affect the lifespan of endogenously-produced and transfused platelets via accelerated apoptosis.

Oltersdorf et al. reported a reduction in platelet counts in mice treated with ABT-737, a small molecule antagonist targeting the antiapoptotic proteins BCL2, BCL2L1 (aka BCL-xL), and BCL2L2

(aka BCL-w) [60] ; similarly, patients with lymphoid malignancies who received a related compound with improved oral bioavailability, ABT-263, also developed dose-limiting thrombocytopenia [61, 62] . One would predict that ABT-737 and ABT-263 also affect transfused platelets, though there are currently no published data on platelet lifespan after transfusion in mice or humans treated with either of these drugs.

Using a genome-wide screening approach in mice, Mason et al. identified Bcl2l1 as a key determinant of platelet lifespan [63] . Moreover, in vitro analyses demonstrated that both J o u r n a l P r e -p r o o f human and mouse platelets express BCL2, BCL2L1, and BCL2L2, suggesting that inhibition of antiapoptotic pathways promotes platelet death and removal from the circulation [64] [65] [66] .

Genetic ablation or targeted inhibition of BCL2L1 alone was sufficient to recapitulate the thrombocytopenic phenotype observed with ABT-737, demonstrating that the activity of BCL2L1 is essential to prevent platelet apoptosis [64, 67, 68] . Interestingly, human platelets exposed to E. coli or S. aureus showed rapid BCL2L1 degradation, mitochondrial depolarization, and cytoplasmic condensation, indicating that bacteria or bacterial products have the capacity to induce platelet apoptosis [69] ; it is likely that both endogenously-produced and transfused platelets would be affected by this pro-apoptotic mechanism.

Inhibition of protein kinase A (PKA) may promote apoptosis in platelets. A murine model in which PKA was knocked out only in the megakaryocytic lineage demonstrated that platelets in mice homozygous for the deletion had a significantly shortened life span and the mice themselves were thrombocytopenic. Interestingly, apoptosis was observed in platelets from thrombocytopenic patients with immune thrombocytopenic purpura (ITP), diabetes, and sepsis and was accompanied by markedly reduced PKA activity. Reduced PKA activity and apoptosis was detected in normal platelets following incubation in the plasma from patients with ITP or diabetes [70] . Antibodies directed against glycoprotein Ib⍺ (GPIb⍺) can induce platelet apoptosis in vitro, which can be prevented by genetic or chemical inhibition of the AKT, a serine/threonine kinase that functions upstream of PKA [71] . Thus humoral factors may activate apoptotic pathways in platelets, shortening their lifespan and resulting in thrombocytopenia.

Examination of mice lacking proapoptotic proteins also supports a role for apoptosis in the regulation of platelet lifespan. Combined loss of Bak and Bax, which drive apoptosis via mitochondrial permeabilization, was associated with a significant increase in platelet number and survival in circulation [72] . In addition, thrombocytopenia in the setting of BCL2L1 loss or inhibition by ABT-737 was completely rescued in Bak/Bax double knockout mice, indicating that the equilibrium between pro-and antiapoptotic proteins is a key determinant of platelet lifespan [63, 64, 72] . Interestingly, platelet numbers and survival were minimally affected in mice lacking upstream activator(s) of BAK and BAX, including BAD, BBC3 (aka PUMA), BID, and BIM [64, 73, 74] ; the absence of a robust platelet phenotype in these mice could reflect functional redundancy among these proteins or the presence of a novel BAK/BAX activation pathway.

Platelets also undergo phenotypic changes consistent with apoptosis, including mitochondrial depolarization, cytochrome c release, membrane blebbing and phosphatidylserine (PS) exposure [63, 66, 75] . In nucleated cells, exposed PS is recognized by several receptors and secreted proteins, serving as a canonical "eat me" signal that promotes cell clearance via phagocytosis [76] . Notably, PS exposure on platelets occurs after activation with physiologic agonists and promotes the procoagulant function of platelets by facilitating the assembly of tenase and prothrombinase complexes [77, 78] . The precise role of PS exposure in platelet clearance is uncertain, though there is evidence from patients with acute myocardial infarction, bacteremia, dengue fever, and essential thrombocythemia suggesting that increased PS exposure is associated with platelet phagocytosis by neutrophils, macrophages, and endothelial J o u r n a l P r e -p r o o f cells [79] [80] [81] [82] . Activation of Akt by antibodies against GPIb⍺ results in exposure of PS on platelets and their phagocytosis by macrophages in the liver; conversely, inhibition of Akt signaling or prevention of PS exposure rescues platelets from phagocytosis [71] . Interestingly, platelet PS exposure due to activation and apoptosis appear to occur via distinct molecular mechanisms [75, 83] ; whether thrombocytopenia and platelet transfusion refractoriness in critically ill patients are predominantly linked to one or both of these mechanisms remains to be determined.

O-and N-linked gycans decorating platelet glycoproteins, particularly GPIb⍺, terminate in sialic acid residues [84] . Removal of sialic acids, or desialylation, exposes β-galactose moieties to which the sialic acids were linked. Exposed β-galactose on the surface of the platelet can lead to its binding and clearance from the blood via the Ashwell-Morrell receptor (AMR, also known as the asialoglycoprotein receptor) on hepatocytes and Kupffer cells [85] [86] [87] . Desialylation occurs as platelets age and may be a mechanism for removal of senescent platelets [86, 88] . In a murine model of S. pneumoniae sepsis, marked thrombocytopenia results not from disseminated intravascular coagulation but is instead the result of platelet desialylation by the bacterial NanA neuraminidase, leading to platelet clearance by the AMR [89, 90] . Similarly, Jansen et al. recently reported that binding of influenza virus to platelet sialoglycans was associated with platelet desialylation by the viral neuraminidase [91] , providing a possible mechanistic explanation for the thrombocytopenia observed in influenza-infected patients [92] .

Interestingly, platelet activation also leads to desialylation. In mice, translocation and surface expression of endogenous platelet lysosomal neuraminidase occurs after platelet activation with antibodies directed against GPIb⍺, leading to platelet desialylation. Desialylation results in platelet clearance and thrombocytopenia in an Fc receptor (FcR)-independent mechanism that depends upon the AMR [93] . In patients with immune thrombocytopenic purpura, detection of autoantibodies targeting GPIb⍺ predicts refractoriness to therapies that inhibit clearance by the FcR (i.e., steroids and intravenous immunoglobulin), suggesting that platelet activation, desialylation, and AMR-mediated platelet clearance may be a key driver of thrombocytopenia in these patients [94, 95] . In this context, increased platelet activation observed in SARS-CoV2 infection may underlie the thrombocytopenia and thromboembolic complications observed in patients with severe COVID-19 [96] [97] [98] , though platelet sialylation in COVID-19 patients has yet to be examined. Binding of soluble vWF to platelet GPIb⍺ under shear stress also results in platelet signaling, activation, desialylation and clearance. The mechanism of platelet activation involves shear stress-induced unfolding of a mechanosensory domain of GPIb⍺ that occurs when vWF binds, leading to platelet signaling and activation [99] . This mechanism may explain why increased binding of vWF to platelets, as in type 2B von Willebrand disease or following administration of ristocetin, results in thrombocytopenia [84] . Interestingly, platelet desialylation resulting from a marked increase in vWF binding to platelets is thought to mediate the thrombocytopenia observed in patients with acute dengue infection [100] .

The clinical relevance of desialylation as a mechanism of thrombocytopenia in critically ill patients is supported by a recent prospective study [101] . Patients meeting the clinical J o u r n a l P r e -p r o o f definitions of sepsis, severe sepsis, and septic shock were stratified by the presence of thrombocytopenia, defined as a platelet count <100X10 9 /L. The degree of platelet desialylation was compared between septic patients with and without thrombocytopenia and was found to be significantly greater among the group with thrombocytopenia. The patients enrolled in this study who met clinical criteria for severe sepsis with a platelet count ≤50X10 9 /L were further enrolled in a clinical trial in which they were randomized to receive standard antimicrobial therapy versus antimicrobial therapy combined with oseltamivir, a neuraminidase inhibitor that has clinical utility for the treatment and prevention of influenza. In comparison with patients receiving standard of care, a larger proportion of patients receiving oseltamivir increased their platelet count to at least 100X10 9 /L during the trial. Additionally, patients in the oseltamivir arm had a shorter duration of thrombocytopenia and received fewer platelet transfusions.

Nevertheless, there was no impact of treatment with oseltamivir on overall 28-day mortality [101] .

Under healthy, steady-state conditions, platelet interactions with endothelial cells are limited by the endothelial glycocalyx, which serves the dual purposes of electrostatic repulsion and masking of platelet adhesion receptors [102] . Endothelial nitric oxide, prostacyclin, and CD39 ecto-ATPase activity also regulate platelet-endothelium interactions by inhibiting the surface expression of adhesion receptors on both platelets and endothelial cells [103] . Impairment of the endothelial mechanisms that prevent platelet adhesion occurs in numerous pathologic states, including ischemia [104] , chronic kidney disease [105] , hyperglycemia [106] , J o u r n a l P r e -p r o o f dyslipidemia [107] , trauma [108] , inflammation [109] , and sepsis [110] ; it has also been suggested that platelet-endothelial interactions can initiate the development of atherosclerotic lesions [111] [112] . These pathologic states are often associated with inflammatory cytokine production that leads to endothelial activation [113] [114] [115] , which is marked by E-selectin, Pselectin, and vWF exposure on the endothelial luminal surface [116] . Endothelial P-selectin engages P-selectin glycoprotein ligand-1 (PSGL-1) or GPIb on platelets to support platelet rolling along the endothelium [117] [118] [119] [120] [121] ; platelet GPIb binding to endothelial vWF also enables platelet rolling [122, 123] . With additional inflammation and platelet activation, fibrinogen can facilitate firm adhesion of platelets to the endothelial surface by bridging platelet integrin IIb3 with endothelial intercellular adhesion molecule-1 (ICAM-1) or V3 [124] . Finally, activated platelets can promote endothelial cell activation [125, 126] , indicating that platelets have the capacity to initiate and sustain platelet-endothelial interactions.

It is likely that enhanced platelet-endothelial interaction contributes directly to thrombocytopenia in critically ill patients. Gawaz et al. observed a significant increase in platelet-endothelial interaction in vitro when normal donor platelets were treated with plasma from septic patients compared to plasma from healthy individuals [127] . ADAMTS13 (a disintegrin-like and metalloprotease with thrombospondin type I repeats 13) activity, which is required for the cleavage of vWF that releases platelets from endothelial interaction, is reduced in a significant fraction of critically ill, thrombocytopenic adults and children [128] . Notably, these patients are distinct from patients with thrombotic thrombocytopenic purpura (TTP): they typically have higher platelet counts and measurable ADAMTS13 activity (11-40% of J o u r n a l P r e -p r o o f normal); they do not harbor the autoantibodies that directly inhibit ADAMTS13 function, which are pathognomonic for TTP; and they do not respond to plasma exchange [129] [130] [131] . Not surprisingly, ADAMTS13 deficiency associated with critical illness and thrombocytopenia has been detected in the setting of sepsis with consumptive coagulopathy [130, [132] [133] [134] [135] [136] [137] [138] [139] [140] [141] [142] [143] ;

thrombocytopenic patients with non-infectious systemic inflammation have also been described to have below normal ADAMTS13 activity, though levels are typically higher in these patients compared to those with sepsis [132, 134] .

Splenic sequestration and disease processes that result in increased platelet consumption can result in refractoriness to platelet transfusion. Myriad mechanisms have been defined that potentially explain why thrombocytopenia and refractoriness to platelet transfusion are reproducibly observed in the setting of infection and inflammation. Understanding these pathophysiologic processes may lead to novel therapeutic interventions in the future, such as the use of neuraminidase inhibitors in patients with sepsis [101] . However, readers of this review are most probably confronted with the dilemma of what to offer now to the severely thrombocytopenic patient who is bleeding and unresponsive to platelet transfusion. On this question, there is little clinical guidance. After exonerating antibody-mediated clearance as a cause of platelet refractoriness, there is no product selection strategy available that will temper other mechanisms that significantly reduce platelet lifespan in the circulation. Attempting to exceed a defined platelet count threshold, which is itself anecdotally determined, with repeated platelet transfusions is unproven as a therapeutic intervention to treat bleeding, risks J o u r n a l P r e -p r o o f volume overload in the patient and contributes to local, regional, and national platelet shortages. Treatment decisions should be guided by careful assessment of the patient and the nature of the patient's bleeding. Where possible, local bleeding should be addressed through local measures, such as packing for a nosebleed, rather than platelet transfusion. Additionally, consideration should be given to administration of antifibrinolytic agents such as epsilon aminocaproic acid or tranexamic acid [7, 144] ; multiple clinical case series suggest benefit in bleeding patients with thrombocytopenia [145] [146] [147] . Randomized controlled trials using tranexamic acid in diverse clinical settings have demonstrated safety and observed no to minimal risk of thrombosis [148] [149] [150] . Prophylactic use of tranexamic acid in patients with hematologic malignancies receiving chemotherapy or stem cell transplant is currently being studied in a double-blind randomized controlled trial, the TREATT Trial [151] . In conclusion, platelet transfusion refractoriness due to clinical factors associated with critical illness are often unavoidable and unmodifiable, representing a significant therapeutic challenge and opportunity. As in many areas of transfusion medicine, well-designed clinical studies are needed to inform treatment decisions in this setting.

The authors have disclosed no conflicts of interest. J o u r n a l P r e -p r o o f 

Trial to Reduce Alloimmunization to Platelets Study. Leukocyte reduction and ultraviolet B irradiation of platelets to prevent alloimmunization and refractoriness to platelet transfusions

Clinical and laboratory correlates of platelet alloimmunization and refractoriness in the PLADO trial

Relative importance of immune and non-immune causes of platelet refractoriness

Utilization of crossmatched or HLA-matched platelets for patients refractory to platelet transfusion

Refractoriness to platelet transfusion

New paradigms in the management of alloimmune refractoriness to platelet transfusions

Platelet refractoriness--practical approaches and ongoing dilemmas in patient management

Clinical factors influencing the efficacy of pooled platelet transfusions

Platelet transfusions administered to patients with splenomegaly

Clinical and laboratory factors associated with platelet transfusion refractoriness: a case-control study

Influence of antibiotics on posttransfusion platelet increment

Factors affecting posttransfusion platelet increments, platelet refractoriness, and platelet transfusion intervals in thrombocytopenic patients

Effect of anticoagulant and ABO incompatibility on recovery of transfused human platelets

Pooling of platelets in the spleen: role in the pathogenesis of "hypersplenic" thrombocytopenia

Immediate effects of intravenous endotoxin on serotonin concentrations and blood platelets

Platelets express functional Toll-like receptor-4

Platelet Toll-like receptor expression modulates lipopolysaccharide-J o u r n a l P r e -p r o o f Journal Pre-proof induced thrombocytopenia and tumor necrosis factor-alpha production in vivo

Agonists of toll-like receptor (TLR)2 and TLR4 are unable to modulate platelet activation by adenosine diphosphate and platelet activating factor

Lipopolysaccharide stimulates platelet secretion and potentiates platelet aggregation via TLR4/MyD88 and the cGMP-dependent protein kinase pathway

Novel phosphatidylethanolamine derivatives accumulate in circulation in hyperlipidemic ApoE-/-mice and activate platelets via TLR2

Stimulation of Toll-like receptor 2 in human platelets induces a thromboinflammatory response through activation of phosphoinositide 3-kinase

Platelet-TLR7 mediates host survival and platelet count during viral infection in the absence of platelet-dependent thrombosis

Engagement of platelet toll-like receptor 9 by novel endogenous ligands promotes platelet hyperreactivity and thrombosis

T granules in human platelets function in TLR9 organization and signaling

Nucleotide-binding oligomerization domain 2 receptor is expressed in platelets and enhances platelet activation and thrombosis

Platelet TLR4 activates neutrophil extracellular traps to ensnare bacteria in septic blood

Van Der Poll. Platelet Toll-like receptor expression and activation induced by lipopolysaccharide and sepsis

Toll-like receptor 4 ligand can differentially modulate the release of cytokines by human platelets

Toll-Like Receptor Signalling Is Not Involved in Platelet Response to Streptococcus pneumoniae In Vitro or In Vivo

Mechanisms involved in the antiplatelet activity of Escherichia coli lipopolysaccharide in human platelets

The role of platelet MyD88 in host response during gram-negative sepsis

The interaction of bacterial pathogens with platelets

Multiple mechanisms for the activation of human platelet aggregation by Staphylococcus aureus: roles for the clumping factors ClfA and ClfB, the serine-aspartate repeat protein SdrE and protein A

Direct interaction of iron-regulated surface determinant IsdB of Staphylococcus aureus with the GPIIb/IIIa receptor on platelets

Platelets and the innate immune system: mechanisms of bacterial-induced platelet activation

Helicobacter pylori binds von Willebrand factor and interacts with GPIb to induce platelet aggregation

Platelet count response to H. pylori treatment in patients with immune thrombocytopenic purpura with and without H. pylori infection: a systematic review

Effect of Helicobacter pylori eradication on platelet recovery in Japanese patients with chronic idiopathic thrombocytopenic purpura and secondary autoimmune thrombocytopenic purpura

Effect of eradication of Helicobacter pylori on platelet recovery in patients with chronic idiopathic thrombocytopenic purpura: a controlled trial

Effect of Helicobacter pylori eradication on platelet recovery in patients with chronic idiopathic thrombocytopenic purpura

P-selectin-dependent platelet aggregation and apoptosis may explain the decrease in platelet count during Helicobacter pylori infection

Neutrophil extracellular traps kill bacteria

Neutrophils and NETs in modulating acute and chronic inflammation

Peptidylarginine deiminase 4: a nuclear button triggering neutrophil extracellular traps in inflammatory diseases and aging

Extracellular DNA NET-Works With Dire Consequences for Health

P-selectin promotes neutrophil extracellular trap formation in mice

Extracellular DNA traps promote thrombosis

Histones induce rapid and profound thrombocytopenia in mice

Monocytes, neutrophils, and platelets cooperate to initiate and propagate venous thrombosis in mice in vivo

Cooperative PSGL-1 and CXCR2 signaling in neutrophils promotes deep vein thrombosis in mice

Extracellular histones are major mediators of death in sepsis

Impact of plasma histones in human sepsis and their contribution to cellular injury and inflammation

Circulating Histones Are Major Mediators of Cardiac Injury in Patients With Sepsis

Circulating histones are mediators of trauma-associated lung injury

Histone-Associated Thrombocytopenia in Patients Who Are Critically Ill

Apoptosis: a review of programmed cell death

Apoptosis: a basic biological phenomenon with wideranging implications in tissue kinetics

The role of apoptosis in megakaryocytes and platelets

An inhibitor of Bcl-2 family proteins induces regression of solid tumours

Substantial susceptibility of chronic lymphocytic leukemia to BCL2 inhibition: results of a phase I study of navitoclax in patients with relapsed or refractory disease

Navitoclax, a targeted high-affinity inhibitor of BCL-2, in lymphoid malignancies: a phase 1 dose-escalation study of safety, pharmacokinetics, pharmacodynamics, and antitumour activity

Programmed anuclear cell death delimits platelet life span

BH3-only activator proteins Bid and Bim are dispensable for Bak/Bax-dependent thrombocyte apoptosis induced by Bcl-xL deficiency: molecular requisites for the mitochondrial pathway to apoptosis in platelets

Alterations in Bcl-2/Bax protein levels in platelets form part of an ionomycin-induced process that resembles apoptosis

Bcl-2 family proteins are essential for platelet survival

Discovery of a Potent and Selective BCL-XL Inhibitor with in Vivo Activity

Conditional deletion of the Bcl-x gene from erythroid cells results in hemolytic anemia and profound splenomegaly

Bacteria differentially induce degradation of Bcl-xL, a survival protein, by human platelets

Protein kinase A determines platelet life span and survival by regulating apoptosis

Akt-mediated platelet apoptosis and its therapeutic implications in immune thrombocytopenia

Megakaryocytes possess a functional intrinsic apoptosis pathway that must be restrained to survive and produce platelets

Loss of PUMA (BBC3) does not prevent thrombocytopenia caused by the loss of BCL-XL (BCL2L1)

Individual and overlapping roles of BH3-only proteins Bim and Bad in apoptosis of lymphocytes and platelets and in suppression of thymic lymphoma development

Two distinct pathways regulate platelet phosphatidylserine exposure and procoagulant function

An Apoptotic 'Eat Me' Signal: Phosphatidylserine Exposure

Generation of prothrombin-converting activity and the exposure of phosphatidylserine at the outer surface of platelets

TMEM16F forms a Ca2+-activated cation channel required for lipid scrambling in platelets during blood coagulation

Phagocytosis by endothelial cells inhibits procoagulant activity of platelets of essential thrombocythemia in vitro

Phosphatidylserine-mediated platelet clearance by endothelium decreases platelet aggregates and procoagulant activity in sepsis

Neutrophils phagocytose activated platelets in vivo: a phosphatidylserine, P-selectin, and {beta}2 integrin-dependent cell clearance program

Platelet activation determines the severity of thrombocytopenia in dengue infection

Both TMEM16F-dependent and TMEM16F-independent pathways contribute to phosphatidylserine exposure in platelet apoptosis and platelet activation

Mechanisms of platelet clearance and translation to improve platelet storage

Dual roles for hepatic lectin receptors in the clearance of chilled platelets

Role of sialic acid for platelet life span: exposure of beta-galactose results in the rapid clearance of platelets from the circulation by asialoglycoprotein receptor-expressing liver macrophages and hepatocytes

Sialylation on O-glycans protects platelets from clearance by liver Kupffer cells

The loss of sialic acid and its prevention in stored human platelets

The Ashwell receptor mitigates the lethal coagulopathy of sepsis

Inducing host protection in pneumococcal sepsis by preactivation of the Ashwell-Morell receptor

Influenza-induced thrombocytopenia is dependent on the subtype and sialoglycan receptor and increases with virus pathogenicity

Pandemic Influenza. Hospitalized patients with 2009 H1N1 influenza in the United States

Desialylation is a mechanism of Fc-independent platelet clearance and a therapeutic target in immune thrombocytopenia

Relative efficacy of steroid therapy in immune thrombocytopenia mediated by anti-platelet GPIIbIIIa versus GPIbalpha antibodies

Association of autoantibody specificity and response to intravenous immunoglobulin G therapy in immune thrombocytopenia: a multicenter cohort study

Platelet activation and platelet-J o u r n a l P r e -p r o o f Journal Pre-proof monocyte aggregates formation trigger tissue factor expression in severe COVID-19 patients

The association between severe COVID-19 and low platelet count: evidence from 31 observational studies involving 7613 participants

COVID-19 and its implications for thrombosis and anticoagulation

Platelet clearance via shear-induced unfolding of a membrane mechanoreceptor

Desialylation of platelets induced by Von Willebrand Factor is a novel mechanism of platelet clearance in dengue

Platelet desialylation is a novel mechanism and a therapeutic target in thrombocytopenia during sepsis: an open-label, multicenter, randomized controlled trial

The endothelial glycocalyx: composition, functions, and visualization

Endogenous mechanisms of inhibition of platelet function

Shedding of the endothelial glycocalyx in patients undergoing major vascular surgery with global and regional ischemia

Endothelial dysfunction

Endothelial dysfunction in diabetes

Oxidized lipoproteins degrade the endothelial surface layer : implications for platelet-endothelial cell adhesion

A high admission syndecan-1 level, a marker of endothelial glycocalyx degradation, is associated with inflammation, protein C depletion, fibrinolysis, and increased mortality in trauma patients

Tumor necrosis factor-alpha inhibition protects against endotoxin-induced endothelial glycocalyx perturbation

Coagulopathy, catecholamines, and biomarkers of endothelial damage in experimental human endotoxemia and in patients with severe sepsis: a prospective study

A critical role of platelet adhesion in the initiation of atherosclerotic lesion formation

Circulating activated platelets exacerbate atherosclerosis in mice deficient in apolipoprotein E

Interleukin 1 acts on cultured human vascular endothelium to increase the adhesion of polymorphonuclear leukocytes, monocytes, and related leukocyte cell lines

Stimulation of the adherence of neutrophils to umbilical vein endothelium by human recombinant tumor necrosis factor

Overlapping patterns of activation of human endothelial cells by interleukin 1, tumor necrosis factor, and immune interferon

The vessel wall and its interactions

P-Selectin glycoprotein ligand 1 (PSGL-1) is expressed on platelets and can mediate platelet-endothelial interactions in vivo

Platelets roll on stimulated endothelium in vivo: an interaction mediated by endothelial P-selectin

Plateletendothelial interactions in inflamed mesenteric venules

Platelet-endothelial cell interactions during ischemia/reperfusion: the role of Pselectin

The glycoprotein Ib-IX-V complex is a platelet counterreceptor for P-selectin

Platelets adhere to and translocate on von Willebrand factor presented by endothelium in stimulated veins

Initiation of platelet adhesion by arrest onto fibrinogen or translocation on von Willebrand factor

Fibrinogen deposition at the postischemic vessel wall promotes platelet adhesion during ischemia-reperfusion in vivo

Activated platelets induce Weibel-Palade-body secretion and leukocyte rolling in vivo: role of P-selectin

Activated platelets induce monocyte chemotactic protein-1 secretion and surface expression of intercellular adhesion molecule-1 on endothelial cells

Platelet function in septic multiple organ dysfunction syndrome

ADAMTS 13 in non-thrombotic thrombocytopaenic purpura conditions

Impact of severe ADAMTS13 deficiency on clinical presentation and outcomes in patients with thrombotic microangiopathies: the experience of the Harvard TMA Research Collaborative

Clinical features and outcomes in patients with thrombotic microangiopathy not associated with severe ADAMTS13 deficiency

Treatment with or without plasma exchange for patients with acquired thrombotic microangiopathy not associated with severe ADAMTS13 deficiency: a propensity score-matched study

ADAMTS-13 in Critically Ill Patients With Septic Syndromes and Noninfectious Systemic Inflammatory Response Syndrome

Increased Von Willebrand factor, decreased ADAMTS13 and thrombocytopenia in melioidosis

Inflammation-associated ADAMTS13 deficiency promotes formation of ultra-large von Willebrand factor

Reduced ADAMTS13 in children with severe meningococcal sepsis is associated with severity and outcome

ADAMTS13 deficiency with elevated levels of ultra-large and active von Willebrand factor in P. falciparum and P. vivax malaria

ADAMTS-13, von Willebrand factor and related parameters in severe sepsis and septic shock

Deficiency of ADAMTS-13 in pediatric patients with severe sepsis and impact on in-hospital mortality

Severe Plasmodium falciparum malaria is associated with circulating ultra-large von Willebrand multimers and ADAMTS13 inhibition

Decreased ADAMTS 13 Activity is Associated With Disease Severity and Outcome in Pediatric Severe Sepsis

Severe malaria is associated with a deficiency of von Willebrand factor cleaving protease

Decreased ADAMTS-13 (A disintegrin-like and metalloprotease with thrombospondin type 1 repeats) is associated with a poor prognosis in sepsis-induced organ failure

Severe secondary deficiency of von Willebrand factor-cleaving protease (ADAMTS13) in patients with sepsis-induced disseminated intravascular coagulation: its correlation with development of renal failure

How I do it: platelet support for refractory patients

Use in control of hemorrhage in patients with amegakaryocytic thrombocytopenia

Epsilon aminocaproic acid prevents bleeding in severely thrombocytopenic patients with hematological malignancies

Aminocaproic acid use in hospitalized patients with hematological malignancy: a case series

Effects of tranexamic acid on death, vascular occlusive events, and blood transfusion in trauma patients with significant haemorrhage (CRASH-2): a randomised

Effect of early tranexamic acid administration on mortality, hysterectomy, and other morbidities in women with post-partum haemorrhage (WOMAN): an international, randomised, double-blind, placebo-controlled trial

Tranexamic acid for hyperacute primary IntraCerebral Haemorrhage (TICH-2): an international randomised

The TREATT Trial (TRial to EvaluAte Tranexamic acid therapy in Thrombocytopenia): safety and efficacy of tranexamic acid in patients with haematological malignancies with severe thrombocytopenia: study protocol for a double-blind randomised controlled trial