key: cord-008530-yni0poh9
authors: Asensio, Valerie C.; Campbell, lain L.
title: Chemokines and viral diseases of the central nervous system
date: 2004-01-07
journal: Adv Virus Res
DOI: 10.1016/s0065-3527(01)56006-6
sha: 
doc_id: 8530
cord_uid: yni0poh9

This chapter discusses chemokines and their receptors in the evolution of viral infectious diseases of the central nervous system (CNS). Infection of the human CNS with many different viruses or infection of the rodent CNS induces vigorous host-inflammatory responses with recruitment of large numbers of leukocytes, particularly T lymphocytes and macrophages. Chemokines coordinate trafficking of peripheral blood leukocytes by stimulating their chemotaxis, adhesion, extravasation, and other effector functions. In view of these properties, research efforts have turned increasingly to the possible involvement of chemokines in regulating both peripheral tissue and CNS leukocyte migration during viral infection. The biological effects of chemokines are mediated via their interaction with receptors belonging to the family of seven transmembrane (7TM)-spanning, G-protein coupled receptors (GPCRs). In the normal mammalian CNS, the number of leukocytes present in the brain is scant. However, these cells are attracted to, and accumulate in, a variety of pathologic states, many involving viral infection. Although leukocyte migration into local tissue compartments, such as the CNS, is a multifactorial process, it has become clear that chemokines are pivotal components of this process, providing a necessary chemotactic signal for leukocyte recruitment.

The central nervous system (CNS) has been viewed as a relatively immune privileged organ. In physiologic states, the CNS contains few, if any, leukocytes, and an absence of professional antigen-presenting cells (APC); it is barren for the expression of key immune accessory molecules such as major histocompatibility molecules (MHCs), and is fortified by an effective blood-brain barrier. Nevertheless, the past decade has witnessed a revision of this concept, as evidenced by the fact that activated T lymphocytes, regardless of their antigen specificity, migrate effectively through the blood-brain barrier (BBB), but also by the fact that the CNS contains numerous resident cells, such as microglia, cerebral endothelial cells, and astrocytes, that can, under the appropriate conditions, acquire the expression of immune accessory molecules and may function as APC (Hickey, 1999; Owens et al., 1994) .

Significantly, in numerous pathological conditions, immune cells are readily recruited to and accumulate in the CNS. Infection of the human CNS with many different viruses (e.g., human T cell leukemia virus type I [HTLV-I]; measles virus, human immunodeficiency virus [HIV] type 1; and herpes simplex virus [HSV] type 2), or of the rodent CNS (e.g., Theiler's murine encephalomyelitis virus [TMEV] , mouse hepatitis virus [MHV] , lymphocytic choriomeningitis virus [LCMV] and vesicular stomatitis virus) induces vigorous host inflammatory responses with recruitment of large numbers of leukocytes, particularly T lymphocytes and macrophages. This host response can be a two-edged sword that, on the one hand, is needed to control and extinguish the invading virus but, on the other, can produce immune pathology and tissue injury resulting in mental and physical debilitation, and often death, of the host organism. Therefore, increasing our knowledge of the mechanisms that control the trafficking of immune cells into the CNS, and knowledge of the subsequent interactions between these cells that contribute to CNS damage, is an important objective.

Our understanding of the factors that govern leukocyte trafficking has been advanced considerably with the discovery of a large superfamily of mostly small proteins (termed chemokines) that function as leukocyte chemoattractants in immunoinflammatory states (for reviews, see Bacon and Oppenheim, 1998; Rollins, 1997; Taub and Oppenheim, 1994) . Chemokines coordinate trafficking of peripheral blood leukocytes by stimulating their chemotaxis, adhesion, extravasation, and other effector functions. In view of these properties, research efforts have turned increasingly to a focus on the possible involvement of chemokines in regulating both peripheral tissue and CNS leukocyte migration during viral infection. However, it is clear that the scope of chemokine involvement in the pathogenesis of host/viral interactions extends well beyond leukocyte migration. With studies on HIV-1 in the vanguard, new roles for chemokines and their receptors in viral infection became apparent with the demonstration that certain classes of chemokine receptors functioned as essential coreceptors for HIV-1 binding and entry into the cell. Yet more recent work has indicated that the genomes of some large viruses, belonging to the herpesvirus and poxvirus families, contain homologs of the mammalian chemokines and their receptors. These homologs may function to modulate local inflammatory responses and favor viral survival and spread. From the host perspective, accumulating evidence indicates that chemokines are plurifunctional molecules that may have a significant impact on the CNS, regulating cellular communication in the developing and the normal adult CNS (Asensio and Campbell, 1999) .

Here, we review the subject of chemokines and their receptors in the evolution of viral infectious diseases of the CNS. As a prelude to the chemokines. ATAC, activation-induced chemokine-related molecule exclusively expressed in CD8 ÷ T lymphocytes; BCA-1/BLC, B cell-attracting chemokine/B-lymphocyte chemoattractant; GRO, growth-regulated oncogene; IL-8, interleukin-8; IP-10, inflammatory protein 10 kD; I-TAC, interferon-inducible T cell alpha-chemoattractant; LIX, LPS-induced CXC chemokine; MCP, monocyte chemotactic protein; Mig, monokine induced by interferon-gamma; MIP, macrophage inflammatory protein; MRP-1, MIPrelated protein 1; PBP, platelet basic protein; PF4, platelet factor 4; RANTES, regulated on activation-normal T cell expressed and secreted; SDF-1, stromal cell-derived factor-1; SLC, secondary lymphoid tissue chemokine; chr., chromosome. main subject of this chapter, our discussion initially will provide the reader with a general background to the rapidly expanding area of chemokines and their receptors and the expression of these molecules in the CNS.

The chemokines are currently divided into 4 families ( Fig. 1) : the CXC or ~ family; the CC or ~ family; the C or 7 family; and the CX3C or 5 family (Zlotnik et al., 1999) . In general, within each chemokine subfamily, the individual members show considerable homology in their 130 VAL]~RIE C. ASENSIO AND IAIN L. CAMPBELL amino acid sequence and often possess overlapping chemoattractant specificity. Chemokines share a common three-dimensional structure including an NH2-terminal loop and three antiparallel ~ sheets that are followed by a C-terminal a helix. The amino acid backbone of chemokines belonging to the CC, CXC, and CX3C families contains four conserved cysteines, while the C family chemokine, lymphotactin, contains only two cysteines, which correspond to the first and third cysteines in the other groups.

Chemokines can be produced by a variety of immune cells such as T lymphocytes, macrophages, and natural killer (NK) cells, but also by organ-specific cell types such as resident glial cells in the CNS (see the discussion below). Depending on the chemokine and its cellular source, the expression of chemokines may be constitutive or inducible, and a new classification of chemokines, defined according to their expression pattern, has been proposed (Mantovani, 1999a; Sallusto et al., 1999a) . The regulation of chemokine expression can be mediated by soluble factors such as inflammatory cytokines, e.g., interleukin (IL)-I, tumor necrosis factor (TNF)-a, interferon (IFN)-7, as well as by microbial products, e.g., bacterial lipopolysaccharide (LPS) or HIV coat protein gp120 (Baggiolini, 1998; Baggiolini et al., 1997; Premack and Schall, 1996; Strieter et al., 1996) . In contrast, counterregulatory cytokines can downregulate the production of chemokines. For example, IL-10 and transforming growth factor (TGF)-~ have been shown to inhibit LPS-induced microglial cell RANTES mRNA expression and protein release (Hu et al., 1999) . The glucocorticoid dexamethasone is also a particularly potent inhibitor for CC and CXC chemokine production (Tobler et al., 1992; Villiger et al., 1992) .

The biological effects of chemokines are mediated via their interaction with receptors belonging to the family of seven transmembrane (7TM)-spanning, G-protein-coupled receptors (GPCRs). All chemokine receptors contain two conserved cysteines, one in the NH2-terminal domain and the other in the third extracellular loop; these are assumed to form disulfide bonds critical for the formation of the ligand binding pocket. In accordance with the chemokine classification, the receptors are divided into four major groups, CXCR, CCR, XCR, and CX3CR (Table I) . Individual chemokine receptors may often be promiscuous, meaning that they are capable of binding several different chemokines, and conversely, individual chemokines can often bind to several different receptors. Interaction between a chemokine and its receptor leads to generation of a coordinated series of signaling events such as mobilization and influx of Ca 2+ and also activation of various signal transduction pathways (Premack and Schall, 1996) . Lymphotactin a Chemokines in boldface type are cross-subfamily ligand-binding receptors. BCA-1/BLC, B cell-attracting chemokine/B-lymphocyte chemoattractant; ELC, Epstein-Barr virus-induced receptor ligand chemokine; ENA-78, epithelial cell-derived neutrophil-activating factor-78; GCP-2, granulocyte chemoattractant protein-2; GRO, growth-regulated oncogene; IL-8, interleukin-8; IP-10, inflammatory protein 10 kD; I-TAC, interferoninducible T cell alpha-chemoattractant; LARC, liver-and activation-related chemokine; LIX, LPS-induced CXC chemokine; MCP, monocyte chemotactic protein; Mig, monokine induced by interferon-gamma; MIP, macrophage inflammatory protein; MRP-1, MIPrelated protein 1; PBP, platelet basic protein; PF4, platelet factor 4; RANTES, regulated on activation-normal T cell expressed and secreted; SDF-1, stromal cell-derived factor-l; SLC, secondary lymphoid tissue chemokine; TECK, thymus-expressed chemokine. protein 10 kDa); and Mig (monokine induced by interferon-y), (see Fig.  1 ). The presence or absence of an ELR motif (glutamic acid-leucinearginine sequence) preceding the first cysteine further subdivides the CXC chemokines into ELR or non-ELR groups. Examples of some common ELR chemokines include IL-8, GRO-a, and MIP-2, while non-ELR chemokines include IP-10, Mig and SDF-1. This grouping not only reflects a structural compartmentalization, but also a functional one in that ELR CXC chemokines are predominantly chemoattractant for polymorphonuclear leukocytes and bind to the CXCR1 and CXCR2 receptors, while non-ELR chemokines are chemoattractant for T and B cells and bind to the CXCR3 or CXCR4 receptors. In addition, members of the CXC chemokine family can either promote or inhibit anglogenesis, depending on whether they are ELR or non-ELR CXC chemokines, respectively. The ELR-chemokines are generally produced during inflammatory responses provoked by bacterial infections. On the other hand, production of the non-ELR chemokines, IP-10/crg-2 and Mig, is commonly associated with viral infections where they likely function to recruit T cells and macrophages that predominate at sites of infection (Amichay et al., 1996; Asensio and Campbell, 1997; Asensio et al., 1999a; Carr et al., 1998; Charles et al., 1999; Fisher et al., 1995; Lahrtz et al., 1997; Nazar et al., 1997; Vanguri and Farber, 1994) . This scenario is borne out by the recent demonstration of pronounced antiviral activity of Mig and IP-10 when it is expressed ectopically by recombinant vaccinia viruses (Mahalingam et al., 1999) . The antiviral actions of Mig and IP-10 in this setting are mediated indirectly and require NK cells and IFN-a, IFN-~, and IFN-y. SDF-1 was originally isolated from bone marrow stromal cells as a pre-B cell growth-stimulating factor. In contrast to IP-10 and Mig, SDF-1 is produced constitutively in many organs (e.g., heart, lung, and CNS) and binds to the CXCR4 receptor. Animals lacking SDF-1 or its receptor invariably die at or soon after birth and have major abnormalities affecting the immune system (an impairment in B lymphocyte development) as well as various organs including the heart and CNS Nagasawa et al., 1996; Tachibana et al., 1998; Zou et al., 1998) . These studies revealed that SDF-1 is a crucial mediator of cellular migration and patterning during organogenesis.

To date, 5 CXC chemokine receptors have been described and their ligands identified (see Table I ) (Baggiolini et al., 1997; Mackay, 1997; Moser et al., 1998) . Most of the early characterization of the CXC chemokine receptors was done with the IL-8 receptors CXCR1 (also known as IL-8RA) and CXCR2 (also known as IL-8RB). CXCR2 is expressed on a variety of cells including T cells, monocytes, melanoma cells, synovial cells, neutrophils, and myeloid precursor cells. Expression of CXCR1 is restricted largely to myeloid cells and neutrophils. While CXCR2 binds all the ELR-chemokines, CXCR1 binds only IL-8. Mice lacking CXCR2 show normal development of neutrophils but have decreased numbers of myeloid progenitors (Broxmeyer et al., 1996; Cacalano et al., 1994) . CXCR2-deficient mice do not exhibit any other overt pathologic abnormalities. CXCR3 is the receptor for several of the non-ELR CXC chemokines, IP-10, Mig and I-TAC. Surprisingly, some CC chemokines such as eotaxin, MCP-4 (Weng et al., 1998) , and 6Ckine (also called exodus 2 or SLC) also bind to CXCR3. Through this interaction, eotaxin does not activate the CXCR3 receptor but effectively blocks IP-10 binding and prevents receptor activation. Thus, these CC chemokines may serve as natural CXCR3 antagonists highlighting complexity in the regulation of chemokine actions. Human CXCR3 has been shown to be produced specifically by IL-2-activated CD4 ÷ T cells, B cells, and NK cells. CD8 ÷ T cell and NK cell clones have also been shown to express the CXCR3 receptor . This suggests non-ELR chemokines such as IP-10 may exert a selective influence on Thl and cytotoxic lymphocyte and NK cell recruitment, which may be important in the development of innate and adaptive immune responses to viral infections. CXCR4 and CXCR5 bind the other non-ELR chemokines SDF-1 and BCA, respectively. CXCR4, also known as LESTR or fusin, serves as a major coreceptor for lymphotropic strains of HIV-1 (Deng et al., 1996; Feng et al., 1996) and is expressed in a variety of tissues including brain, heart, liver, and colon and in nonhematopoietic cells (Lavi et al., 1997; Ohtani et al., 1998) . CXCR4 mRNA expression is also found in murine lymphocytes, macrophages, neutrophils, and glial cells . As indicated above, disruption of the CXCR4 gene causes defects in vascular development, hematopoiesis, cardiogenesis, and neurogenesis of the cerebellum. CXCR5, originally described as BLR-1 (Burkitt's lymphoma receptor-l), is highly related to the IL-8 receptors (Dobner et al., 1992) . CXCR5 mRNA is expressed by B cell lymphomas and by granule and Purkinje cells of the cerebellum. Therefore, in addition to regulation of recirculating mature B lymphocytes (Forster et al., 1996; Forster et al., 1994; Kaiser et al., 1993) , this receptor may be involved in some CNS functions. However, mice lacking CXCR5 show significant phenotypic changes in lymphoid organs but not in the brain, suggesting that this receptor, unlike CXCR4, does not play an obligatory role in CNS development (Forster et al., 1996) .

The CC or ~-chemokines contain the largest number of members and are so named because the NH2-terminal cysteines are adjacent to each other. Individual CC chemokines may exert their actions on multiple leukocyte subtypes, including monocytes, basophils, eosinophils, T cells, dendritic cells, and natural killer cells. Well-known members of this family include MCP-1, MIP-la, C10, eotaxin, and RANTES. A more comprehensive list of the members of this family can be found in Fig. 1 . One of the most studied chemokines of this family is MCP-1, which attracts T cells and monocytes but not neutrophils (Carr et al., 1994; Matsushima et al., 1989) . Mice with targeted disruption of the MCP-1 gene have defects in monocyte/macrophage recruitment in a variety of inflammatory and immunological models. Conversely, transgenic mice with expression of MCP-1 targeted to pancreatic islets (Grewal et al., 1997) or oligodendrocytes in the brain (Fuentes et al., 1995) show an accumulation of monocytes at these sites. Interestingly, these accumulating monocytes showed little evidence of activation, indicating that MCP-1 is an effective monocyte chemoattractant but lacks the ability to further activate these cells. Another well-studied CC chemokine is MIP-la, which induces migration of monocytes, T lymphocytes, and eosinophils (Baggiolini et al., 1994) . Mice with a targeted disruption of MIP-la show no obvious hematopoietic abnormalities; however, they are resistant to coxsackievirus-induced myocarditis and clear influenza at a delayed rate (Cook et al., 1995) .

To date, 9 CC receptors (CCR1-CCR9) have been identified (Table  I) . CCR1 was originally designated as an MIP-la/RANTES receptor (Gao et al., 1993; Gao and Murphy, 1995; Neote et al., 1993) and was later shown to also bind MCP-2 and MCP-3 (Ben- Baruch et al., 1995) . The CCR1 receptor is mainly found on circulating mononuclear cells. Mice lacking CCR1 (receptor for MIP-la, MIP-5, MCP-2, MCP-3, and RANTES) have a phenotype similar to that of MIP-la deficient mice with a defect in inflammatory responses to microbial challenge such as coxsackie B and influenza viruses (Gao et al., 1997) . In addition, these animals have impaired trafficking of subsets of myeloid progenitor cells. The primary receptor for MCP-1, CCR2, is expressed on monocytes, myeloid precursor cells, activated T lymphocytes, and B lymphocytes but not on neutrophils (Bonecchi et al., 1999; Frade et al., 1997; Myers et al., 1995) . Mice with targeted disruption of the CCR2 receptor have defects in monocyte/macrophage recruitment in a variety of inflammatory and immunological models (Boring et al., 1998; Kurihara et al., 1997; Kuziel et al., 1997) . CCR3, which is the eotaxin receptor, is prominently expressed by eosinophils and is involved in the recruitment of these cells during allergic reactions. CCR4 is the main receptor for RANTES and MIP-la, while CCR5, expressed on monocytes/macrophages, T cells, and granulocyte precursors (Alkhatib et al., 1996; Deng et al., 1996) , binds MIP-la, MIP-1~, and RANTES. The receptors CCR6 and CCR7, originally identified as orphan receptors, bind MIP-3a, and MIP-3~ and 6Ckine respectively Yoshida et al., 1997) . CCR8 binds 1-309 (Goya et al., 1998) , TARC, and MIP-I~ and is expressed in the thymus (Zingoni et al., 1998) . CCR9, also designated as GPR-9-6, was shown to specifically bind TECK (Zaballos et al., 1999) .

Compartmentalization of CC chemokine receptor expression is emerging as an important regulatory component during the adaptive immune response. CCR5, the receptor for RANTES, MIP-I~, and MIP-1~ is expressed by memory and activated T cells but not by naive T cells (Bleul et al., 1997) . Moreover, CCR5 is preferentially expressed by human Thl cells, in contrast to CCR4 and CCR3, which are found predominantly on Th2 cells (Bonecchi et al., 1998; Qin et al., 1998) . Concomitant with their differential chemokine receptor expression, Thl and Th2 cells selectively migrate in response to the corresponding chemokine ligand. More recently, Sallusto et al. showed that memory T cells can be distinguished with respect to their effector function in peripheral organs by their expression of the chemokine receptor CCR7 (Sallusto et al., 1999b) . CCR7 ÷ memory T cells retain lymph node homing receptors and lack immediate effector function, whereas CCR7memory T cells express receptors for migration to inflamed tissues and possess effector function. CCR8, like CCR4 (Bonecchi et al., 1998) , is preferentially expressed by activated Th2 cells. CCR8 may be important in lymphocyte development as well as in Th2-immune responses. Polarized Th2 cells have been shown to differentially express the chemokine receptors, CCR8 (Zingoni et al., 1998) , CCR4 (Bonecchi et al., 1998; Sallusto et al., 1999a) and CCR3 (Sallusto et al., 1997) . In the murine system, a recent study showed that Thl and Th2 cells expressed comparable levels of CCR1, CCR2, and CCR4 mRNA. Similar to human T helper cells, murine Thl cells preferentially expressed CCR7 and CCR5, whereas Th2 cells expressed more CCR3 (Randolph et al., 1999) . In conclusion, depending on their antigenic stimulation and/or polarization toward Thl versus Th2 subsets, chemokine receptors are differentially expressed, which, in addition to their characteristic cytokine profiles, may thus further distinguish these cells (Mantovani, 1999b ).

Lymphotactin, the lone member of the C-chemokine family, is produced by activated mouse T cells (Kelner et al., 1994) . Lymphotactin is unique because (1) it has only two cysteines, the second and the fourth, of the four cysteines conserved in the CXC, CC, and CX3C chemokines; and (2) the C-terminal sequence is much longer than those of other chemokines. Lymphotactin, and the subsequently reported single motif cysteine (SMC) chemokine-1 and the activation-induced, T-cell-derived, and chemokine-related molecule (ATAC), are all identical (Muller et al., 1995; Yoshida et al., 1995) . Lymphotactin expression on activation is extremely rapid, which suggests that it has a role in the early phases of an inflammatory response. When lymphotactin is injected into peritoneum, it causes an influx of T lymphocytes and NK cells by 24h (Hedrick et al., 1997) . Yoshida et al. (1998) identified an orphan receptor, GPR5 (Heiber et al., 1995) , as being a high-affinity functional receptor for lymphotactin this receptor was renamed XCR1.

The single member of the CX3C or 8 family, named fractalkine, is also unique with a novel arrangement of three amino acids separating the first two conserved cysteines. Furthermore, fractalkine exists in both soluble and membrane-bound forms (Bazan et al., 1997; Pan et al., 1997) . This chemokine is found on activated endothelial cells and is tethered to the membrane by a mucinlike stalk that suggests a novel role for this chemokine in juxtacrine signaling. The interaction of endothelial-cell-expressed fractalkine with its receptor, CX3CR1, on leukocytes mediates the initial capture, firm adhesion, and activation of circulating leukocytes independently of integrin or adhesion molecule involvement (Fong et al., 1998) . Thus, fractalkine not only induces the migration of leukocytes but also facilitates their adhesion directly to the endothelium at sites of inflammation. Fractalkine has been shown to foster the migration of monocytes, T cells, and NK cells (Hedrick et al., 1997; Kelner et al., 1994) . The receptor for fractalkine was originally cloned from a rat brainstem cDNA library and named RBSll (Harrison et al., 1994) . Subsequently, the human receptor was identified as V28 (Raport et al., 1995) , or CMKBLR1, which is highly expressed in the brain and in neutrophils, monocytes/macrophages, and T lymphocytes (Combadiere et al., 1995) . Imai et al. (1997) identified this molecule as being the fractalkine receptor (CX3CR1) that is expressed mainly by NK cells and monocytes and to a lesser extent in CD8 ÷ T cells.

Many of the large DNA viruses are infamous for their ability to undermine host immunity. Two genres of viruses, the herpesviruses and poxviruses, were recently shown to contain open reading frames (ORFs) that encode homologs of mammalian chemokines and chemokine receptors (for detailed reviews, see Ahuja et al., 1994; Dairaghi et al., 1998; Lalani et al., 2000; Murphy, 1994; Smith et al., 1997) .

Kaposi's sarcoma-associated herpes virus (KSHV) encodes three CC-like chemokines, vMIP-I, vMIP-II, and vMIP-III. The vMIP-I and vMIP-II show 60% sequence identity to each other and 43% and 52% identity to MIP-la, while vMIP-III is more distantly related to its viral and mammalian counterparts (Stine et al., 1999) . Of the three viral chemokines, the function of vMIP-II is the best characterized. Receptor binding studies suggest vMIP-II is a ligand for CCR3 (Boshoff et al., 1997) . In addition to being a potent inhibitor of HIV infection mediated principally through this receptor (Boshoffet al., 1997), vMIP-II is able to mobilize calcium and stimulate the chemotaxis of eosinophils where CCR3 is predominantly expressed (Boshoffet al., 1997) . In contrast to this agonist action on CCR3, vMIP-II also binds with a high degree of affinity to a number of other CC (CCR1, 2, 5) and CXC chemokine receptors (CXCR4), where it acts as an antagonist (Kledal et al., 1997) , as well as to CX3CR1 . Human monocyte chemotaxis induced by RANTES, MIP-I(~, and MIP-I[~ is inhibited by vMIP-II, suggesting that this viral mediator may be a broad-spectrum chemokine antagonist. Recently, vMIP-I was shown to selectively bind to CCR8 (a CC chemokine receptor associated with Th2 lymphocytes), acting as an agonist and stimulating Ca 2+ mobilization on human T cells . Interestingly, vMIP-II also binds to CCR8; however, this is associated with antagonistic actions . Kaposi's sarcoma is an angioproliferative disorder, so it is interesting that in addition to deviation of the host chemokine response, vMIP-I and vMIP-II both induce angiogenesis in a chick chorioallantoic assay (Boshoff et al., 1997) . Thus these viral chemokines might contribute directly to the pathogenesis of the proliferative angiopathy in Kaposi's sarcoma.

In addition to these chemokine homologs, herpesviruses also contain open reading frames (ORFs) that share significant sequence conservation with the chemokine receptors. KSHV and Herpesvirus saimiri (HVS) contain ORF 74 in HSV, whose sequence is most similar to the human IL-8 receptors CXCR1 and CXCR2 (Ahuja and Murphy, 1993) . Like CXCR1 and CXCR2, ORF 74 binds IL-8 as well as the other ELR CXC chemokines, GRO-(z and PF-4 (Ahuja and Murphy, 1993) . However unlike CXCR1 and CXCR2, ORF 74 can signal in a constitutive agonist-independent manner and stimulate the proliferation of kidney fibroblasts (Arvanitakis et al., 1997) . In addition to the stimulation of proliferation of nontransformed cells, ORF 74 signaling may also lead to cellular transformation and tumorigenicity (Bais et al., 1998) . Thus, it would appear that ORF 74 may contribute to the development of the transformed cell phenotype found with KSHV-infected cells in Kaposi's sarcoma lesions.

Cytomegalovirus (CMV) is a common opportunistic pathogen of immunocompromised hosts in whom the virus exhibits tropism for leukocytes. Human and murine CMV possess genes that encode a variety of chemokine homologs (MacDonald et al., 1997) . The murine CMV chemokine 1 and 2 (MCK-1 and MCK-2) peptides induce calcium signaling and adherence in peritoneal macrophages (Saederup et al., 1999) . The human CMV gene UL146 encodes a protein designated as vCXC-1 and provides the first example of a virus-encoded CXC chemokine (Penfold et al., 1999) . The vCXC-1 glycoprotein shows high-affinity binding to the CXCR1 and CXCR2 chemokine receptors, inducing calcium mobilization and chemotaxis of neutrophils. Thus, the common property of the CMV chemokine homologs is to function as chemokine agonists. This in turn may promote leukocyte migration to sites of infection and may be responsible for more efficient dissemination of CMV in the host. In support of this notion, MCK-1/MCK-2 mutant CMV displays markedly reduced peak levels of monocyte-associated viremia in experimentally infected mice (Saederup et al., 1999) .

A gene encoding a viral homolog of CCR1 was found within the genome of the human CMV. This gene, ORF US28, of human cytomegalovirus (HCMV) was shown to bind MIP-la,-~, MCP-1, RANTES , and MCP-3 (Bodaghi et al., 1998) with higher affinity than their cognate receptors. Thus, it is attractive to speculate that HCMV-infected cells expressing US28 may be used for sequestration of chemokines from the extracellular milieu. US28 can also serve as a coreceptor for HIV infection of CD4 T cells and may facilitate an interplay between HIV and HCMV (Pleskoffet al., 1997) .

The poxvirus molluscum contagiosum virus (MCV) infects humans, causing papules of the skin associated with virtually no immune response to the virus in the skin lesion. The genome of MCV contains an ORF that encodes a protein (MC148R) that is a CC chemokine homolog (Senkevich et al., 1996) . MC148R has homology to MIP-la/[3 but does not induce chemotaxis and antagonizes MIP-la-induced chemotaxis (Krathwohl et al., 1997) . Furthermore, MC148R inhibits the leukocyte response to several CC and CXC chemokines (Damon et al., 1998) , indicating that it can act as a broad-spectrum chemokine antagonist. Therefore, a primary function of this virus-encoded chemokine homolog might be to provide an immune evasion strategy that acts to limit the recruitment of effector leukocytes to MCVinfected epidermal cells.

In conclusion, unquestionably these large DNA viruses encode functional homologs of chemokines and chemokine receptors. Although the precise role of the viral chemokine and chemokine receptor homologs (discussed above) in infectious processes remains to be determined, they clearly could contribute to the strategies employed by these viruses to subvert host immunity. The examples discussed here indicate that these strategies are diverse and, depending on the virus, range from wholesale suppression of leukocyte trafficking--providing immune evasion--to promoting selective leukocyte chemotaxis fostering the more efficient infection and dissemination of virus via its preferred target host cell.

A concerted research effort has been mounted recently to ascertain the role of chemokines in immunoinflammatory disorders of the CNS, such as multiple sclerosis (MS). As a result, it has become clear that most cells resident in the CNS, including neurons, astrocytes, and microglia, can synthesize and secrete various classes of chemokines. Importantly, these same cells are endowed with a variety of different chemokine receptors and therefore have the potential to respond to chemokines in their local environment. As recently speculated by us (Asensio and Campbell, 1999) and by others (Mennicken et al., 1999) , it is likely the CNS has its own chemokine ligand/receptor network, the function of which could extend well beyond the regulation of leukocyte trafficking in antiviral and other neuroimmune responses (see Fig. 3 ).

At present, the genes for only three chemokines, MCP-1, SDF-1, and fractalkine, are known to be expressed constitutively in the CNS. Evidence suggests MCP-1 is expressed in the cortex and hippocampus of the rat brain during development as well as in the adult animal (Pousset, 1994) . More recently, MCP-1 expression at the protein level was shown in the human cerebellum, medulla oblongata, and pons (Meng et al., 1999) . However, the function of constitutively expressed MCP-1 in the brain is unknown.

The SDF-1 gene is found under physiological conditions in many organs including murine (Tashiro et al., 1993) and human (Shirozu et al., 1995) brain. SDF-1 has two isoforms, SDF-la and SDF-I~, that are produced by alternative splicing from a single gene (Tashiro et al., 1993) . SDF-la RNA is present at high levels in cultured rat astrocytes, at low levels in neurons, and is not detectable in microglia, while SDF-1~ RNAis found at low levels in all these cell types . Tanabe et al. (1997b) showed that SDF-la was able to induce migration of microglial cells but not astrocytes, even though the SDF-1 receptor, CXCR4, was expressed on both cell types. Furthermore, human SDFla was able to induce calcium flux in cultured astrocytes (Bajetto et al., 1999) . It was hypothesized that astrocyte-secreted SDF-1 could facilitate interneural cell communication . This is certainly the case during CNS development, since, as noted above, knocking out CXCR4 results in a major defect in granule cell migration and cerebellar development Zou et al., 1998) .

The CX3C chemokine fractalkine is also found constitutively in the brain (Bazan et al., 1997; Pan et al., 1997) . Fractalkine mRNA is expressed at high levels by neurons in the olfactory bulb, cerebral cortex hippocampus, caudate putamen, and nucleus accumbens Nishiyori et al., 1998; Schwaeble et al., 1998) . In vitro studies have confirmed that fractalkine is constitutively expressed by neurons and, further, can be induced in astrocytes by TNF-a and IL-I~ (Maciejewski-Lenoir et al., 1999) . In a model of peripheral nerve injury, Harrison et al. have shown that fractalkine and its receptor were highly upregulated in the brain, with motor neurons being the primary source of the chemokine. Fractalkine treatment of primary cultured rat microglial cells induces vigorous increases in intracellular Ca 2+ levels and chemotaxis, which are inhibited by an antibody against the fractalkine (CX3CR1) receptor . Although the precise function of fractalkine in the CNS awaits clarification, the localization of the fractalkine receptor on microglia and the expression of its ligand by neurons clearly establish a spatial framework that could facilitate juxtacrine communication and migration between these cells (see Fig. 3 ) Nishiyori et al., 1998) .

A majority of chemokines are not detectable in the CNS under physiological conditions, but are present during diverse pathologic states including, as discussed below, viral diseases. It is beyond the scope of this chapter to detail all of the findings in these different pathologic states and the reader is encouraged to consult many excellent recent reviews dealing with this subject (Asensio and Campbell, 1999; Glabinski and Ransohoff, 1999; Karpus and Ransohoff, 1998; Mennicken et al., 1999; Ransohoff, 1997; Ransohoff and Tani, 1998) . It is clear from most, if not all, cases examined that glial cells represent an important source for the localized production of specific chemokines. For example, in MS, expression of IP-10 (Balashov et al., 1999; Sorensen et al., 1999) and MCP-1 (Simpson et al., 1998; Van Der Voorn et al., 1999) is prominent in astrocytes while MIP-I(~ expression is strongly associated with macrophage/microglia (Balashov et al., 1999) . In support of these clinicopathologic findings, studies in vitro demonstrate that expression of various CC and CXC chemokines can be induced in astrocytes and microglia by proinflammatory cytokines and LPS. Although differential expression of MCP-1 by astrocytes, and of MIP-I(z by microglia, was reported for murine glial cells (Hayashi et al., 1995) , this was not the case with human microglia, with expression of both these chemokines being induced in these cells by LPS (McManus et al., 1998) . Similarly, expression of IP-10 by murine astrocytes and microglia can be induced by IFN-~/or LPS (Luo et al., 1998; Majumder et al., 1998; Ren et al., 1998; Vanguri, 1995) . In contrast to the glial cells, few inducible chemokines have been reported to be expressed by neuronal cells either in vitro or in vivo.

How does CNS chemokine expression contribute to the pathogenesis of neuroinflammatory disease states? Clearly, one key role involves recruitment of leukocytes to the brain. Differences in the chemokine gene expression patterns can be found in different neuroinflammatory diseases and correlate with a bias toward the involvement of particular leukocytes. In experimental and clinical bacterial meningoencephalitis, where CNS lesions consist predominantly of neutrophils and monocytes, there is dominant cerebral production of the neutrophil and monocyte attractant chemokines IL-8, MIP-2, and GROa, and MCP-1, MIP-I~, and MIP-I~, respectively (Lahrtz et al., 1998; Spanaus et al., 1997; Sprenger et al., 1996) . In contrast, as discussed below, in viral meningoencephalitis, mostly lymphocytes and monocytes are found in the CNS, in parallel with the cerebral expression of the chemokine genes encoding IP-10, MCP-1, and RANTES, which are effective lymphocyte and monocyte chemoattractants (Asensio and Campbell, 1997; Lahrtz et al., 1997) . Transgenic studies show that individual chemokines can have a specific chemoattractant "signature." Thus, CNS expression of either the GROa genes (Tani et al., 1996) or the MCP-1 genes (Fuentes et al., 1995) under the control of the myelin basic protein (MBP) promoter induces robust leukocyte infiltration of the CNS that is composed predominantly of either neutrophils or monocytes, respectively. Studies with intracerebral microinjection also demonstrate that the acute presence of chemokines in the CNS leads to robust and cell-specific leukocyte recruitment. For example, MIP-2 is more potent than IL-8, IP-10, or MCP-1 in stimulating polymorphonu-

VAL]~RIE C. ASENSIO AND IAIN L. CAMPBELL clear leukocyte recruitment to the brain (Bell et al., 1996) , while the CC chemokine C10, when injected into the lateral ventricle, is able to induce a recruitment of macrophages, but not T cells (Asensio et al., 1999b) . Finally, neutralization of the chemokine MIP-la, which is expressed in EAE, is associated with a reduction in disease severity and in CNS inflammation (Karpus et al., 1995) . In all, these studies indicate that the qualitative makeup of chemokine production in the CNS during disease can dictate the recruitment of selected leukocytes to the CNS, and that targeting these molecules may provide an effective therapeutic approach to suppress CNS inflammation.

The CC and CXC receptors are expressed on a number of different cell types and their expression is often regulated by exogenous and endogenous stimuli. Numerous in vitro and in vivo studies show that different neural cells express a variety of chemokine receptors under normal conditions and could therefore facilitate a direct interaction of these cells with chemokines. Neurons have prominent expression of many chemokine receptors including CXCR2, CXCR4, CCR1, CCR4, CCR5, CCR9/10, CX3CR1, and Duffy antigen-related chemokine (DARC) (Horuk et al., 1997; Klein et al., 1999; Lavi et al., 1998; Meucci et al., 1998; Sanders et al., 1998; Vallat et al., 1998) . Immunohistochemical staining of the human brain revealed CXCR1 is not detectable, whereas CXCR2 is expressed at high levels by subsets of projection neurons in different regions of the brain including the hippocampus, dentate nucleus, pontine nuclei, locus coeruleus, and paraventricular nucleus, and also in the spinal cord (Horuk et al., 1996; Horuk et al., 1997) . DARC is expressed by subsets of endothelial cells and Purkinje cells in the cerebellum, suggesting that this enigmatic receptor that lacks signaling function may have multiple roles in normal and pathological physiology, perhaps as a chemokine clearance receptor (Horuk et al., 1996; Horuk et al., 1997) . Functional CCR3, CCR5, and CXCR4 receptors, with different patterns of distribution, were demonstrated on cultured fetal human and macaque neurons (Klein et al., 1999) . Expression of CXCR4 has also been detected by immunohistochemical staining on subpopulations of neurons in the normal adult brain (Lavi et al., 1997; Sanders et al., 1998; Vallat et al., 1998) , while in the rat, levels of the CXCR4 receptor RNA are higher on specific neurons, which include cerebellar Purkinje cells, hippocampal hilar neurons, and cerebral cortical neurons (Wong et al., 1996) . A clear theme emerging from all these studies is that neurons express a variety of chemokine receptors and that this expression in the CNS exhibits marked regional and cellular diversity, suggesting that chemokines could have differential effects on different neurons.

Other than neurons, glial cells also express a variety of chemokine receptors. The fractalkine receptor CX3CR1 is found at high levels in rodent and human brain and shows strong localization to microglia (Combadiere et al., 1998; Harrison et al., 1998; Imai et al., 1997) . The fact that fractalkine is expressed by neurons (as noted above), and its receptor by microglia, suggests a possible important role for this chemokine and its receptor in communication between neurons and microglia. A similar scenario may follow with SDF-1, whose receptor, CXCR4, in addition to expression by neurons, is found on astrocytes (Heesen et al., 1996) and on microglial cells (Tanabe et al., 1997b) . Finally, the CC chemokine receptors CCR1, CCR5, and CCR3 have been shown to be expressed by astrocytes and/or microglial cells both in vitro and in vivo (Boddeke et al., 1999; He et al., 1997; Lavi et al., 1998; Tanabe et al., 1997a; Vallat et al., 1998; Westmoreland et al., 1998; Xia et al., 1998) .

The expression of many of these chemokine receptors in the CNS is not static and can be dynamically regulated by various stimuli. For example, in a model of excitotoxic brain injury induced by intracerebral injection of NMDA in the rat, marked upregulation of CCR5 expression was observed by microglia and neurons (Galasso et al., 1998) . In brain from Alzheimer's cases, plaque-associated reactive microglia also show increased CCR5 expression . Expression of the fractalkine receptor CX3CR1 is increased on perineural microglia following facial nerve axotomy in the rat . Boddeke et al., have shown, by RT-PCR (reverse transcription-polymerase chain rear) analysis, that CCR1, CCR2, and CCR5 expression in cultured rat microglial cells is significantly upregulated after LPS treatment (Boddeke et al., 1999; Spleiss et al., 1998) . The pathophysiological significance of altered CNS chemokine receptor expression in disease states remains to be determined. However, it might be speculated that this provides yet another level of regulation in the already complex cellular communication processes mediated by the chemokines.

CNS infection, with a number of different classes of viruses, can provoke vigorous inflammatory responses with subsequent recruitment of large numbers of leukocytes (for a detailed discussion of this topic, see Chapter 6 in this volume). In view of their functional properties, increasing interest has focused on the possible involvement of chemokines in regulating CNS leukocyte migration during viral infection. However, as discussed below, studies of HIV-1 have revealed that chemokine involvement in the pathogenesis of host-virus interactions extends well beyond leukocyte migration.

The lentivirus HIV-1 and its simian counterpart, simian immunodeficiency virus (SIV), both infect the CNS and are responsible for causing neurologic disease. In the case of HIV, a vast amount of research effort focused on this virus led to the discovery of new chemokine receptors that proved to be critical for understanding the process of HIV-1 entry into cells (Bleul et al., 1996; Feng et al., 1996; Oberlin et al., 1996) . The interactions between this virus and the host CNS have also been studied extensively, and more recently the focus has turned to the chemokines and their receptors. As discussed below, the findings point to a significant role for the chemokines and the chemokine receptors in HIV-1 neuropathogenesis.

Although CD4 was identified initially as a primary receptor for HIV-1 binding and entry, it was clear this could not account for many of the characteristics of HIV-1 infectivity, such as the existence of different strains of HIV-1 with selective tropism for either T lymphocytes (Ttropic) or monocytes (M-tropic). A clue to the identity of other possible cofactors for HIV-1 entry into cells came with the discovery that the chemokines MIP-la, MIP-I~, and RANTES produced by CD8 ÷ T cells could suppress HIV-1 infection of cultured cells (Cocchi et al., 1995) . A seminal advance then came when it was shown that in addition to CD4, CXCR4 was required for T-tropic HIV-1 strains to infect host cells (Bleul et al., 1996; Feng et al., 1996; Oberlin et al., 1996) . The identification of CCR5 as a receptor for MIP-la, MIP-I~, and RANTES soon led to the demonstration that CCR5 served as a prominent coreceptor for cell entry by M-tropic strains ofHIV-1 (Alkhatib et al., 1996; Choe et al., 1996; Deng et al., 1996; Doranz et al., 1996; Dragic et al., 1996) . The factors that govern HIV-1 entry into cells are clearly very complex, and while CXCR4 and CCR5 are the major coreceptors for Ttropic and M-tropic HIV-1 strains, a large number of other chemokine receptors including CCR2b, CCR3, and CCR8, the US28 chemokine receptor homolog encoded by cytomegalovirus, as well as the orphan chemokine receptors gprl, gpr15/BOB, and STRL33/BONZO, can function as coreceptors for HIV-1 infection (Deng et al., 1997; Heiber et al., 1996; Liao et al., 1997; Littman, 1998; Marchese et al., 1994; Pleskoffet al., 1997) . The importance of the chemokine receptors for HIV-1 infection in vivo is illustrated in the case of individuals that are homozygous for a 32 base pair mutation in the CCR5 gene and who have marked resistance toward HIV infection (Biti et al., 1997; O'Brien et al., 1997; Theodorou et al., 1997) . In addition, it has been reported that individuals with mutations in the CXCR4 and CCR2 genes also exhibit a retarded progression of HIV-l-associated disease .

A significant number of patients with HIV-1 infection suffer from a variety of neurological problems known collectively as HIV-l-associated cognitive and motor disorder, or "neuroAIDS" (see Chapter 18 in this volume). In its severest form, neuroAIDS can be responsible for subcortical dementia, memory impairment, and motor disorder. Although the neuropathology of neuroAIDS is often variable, typical features include formation of multinucleated giant cells, diffuse gliosis, myelin pallor, and increased blood-brain barrier permeability. Neurodegeneration, with disrupted synaptodendritic organization and loss of neurons, is observed in many but not all cases (Everall et al., 1993) . The cause of neuroAIDS and the mechanisms that lead to brain injury are not well understood, although a combination of both host-and HIV-derived toxic factors is likely to be involved (Gendelman et al., 1997) . It is clear, however, that productive HIV-1 infection of the CNS is largely restricted to macrophage/microglia and infrequently to neurons or astrocytes. Infection of the CNS is thought to result from the entry of HIV-l-infected lymphocytes or macrophages--the subsequent presence of HIV-1 proteins in microglia and multinucleated giant cells suggests spread of the virus from the infiltrating infected leukocytes.

As discussed above, resident cells of the CNS are well endowed with a variety of different chemokine receptors. The levels and type of coreceptot expression are likely to be pivotal determinants of HIV-1 tropism in the brain. The coreceptors for M-tropic variants of HIV-1, CCR5, CCR3, and CXCR4 are expressed by microglia (He et al., 1997; Lavi et al., 1997; Vallat et al., 1998) . Studies with primary cultures of human microglia have shown that CCR5 and CCR3 can serve as coreceptors with CD4 for HIV-1 infection, while CXCR4 is relatively ineffective in this capacity. Microglia have very low surface-expressed CD4 (Buttini et al., 1998) and this may thus preclude the efficient use of CXCR4 as a coreceptor. HIV-1 infection of microglial cultures can be inhibited by the cognate ligands MIP-I~ (CCR5) and eotaxin (CCR3), further verifying the key role of the microglial-expressed CC-chemokine receptors for HIV-1 entry into cells.

Interestingly, the use of both CCR3 and CCR5 for HIV-1 infection of microglia contrasts with blood-derived macrophages in which CCR5, but not CCR3, functions as a coreceptor (Alkhatib et al., 1996; Deng et al., 1996; Wu et al., 1997) . This suggests that M-tropic HIV-1 variants may arise in the brain that have distinct coreceptor requirements. In support of this, HIV-1 in the brain is typically M-tropic (Korber et al., 1994; Power et al., 1994) . Moreover, He and colleagues (1997) demonstrated that YU2 and JRL Env proteins that are cloned directly from HIV-l-infected brain use CCR5 or CCR3, together with CD4, for virus entry into transfected cells.

A further determinant of HIV-1 entry and infection in the CNS may be the chemokines themselves. Increased levels of MIP-la, MIP-I~, RANTES, MCP-1, IP-10, and SDF-1 have been reported in the brain or the CSF of individuals with neuroAIDS (Conant et al., 1998; Kelder et al., 1998; Kolb et al., 1999; Letendre et al., 1999; Sanders et al., 1998; Schmidtmayerova et al., 1996; Zheng et al., 1999b) . HIV-1 may directly regulate the expression of these chemokines. Thus, infection of cultured microglia and/or astrocytes markedly increases the expression of MIP-la and MIP-I~ (Schmidtmayerova et al., 1996) , while expression of SDF-1 RNA decreases in HIV-l-infected microglia but increases in similarly infected astrocytes (Zheng et al., 1999b) . In recent unpublished studies, we have observed induction of IP-10 gene expression in astrocytes in the brain of transgenic mice with astrocyte-targeted expression of HIVgpl20. These transgenic mice exhibit similar neurodegenerative and other pathologic changes (e.g., astrocytosis) to neu-roAIDS (Toggas et al., 1994) --our findings suggest that a further action of HIVgpl20 is the induction of IP-10. The CNS expression of IP-10 and other chemokines, such as MCP-1, correlates with viral load and increases with the progression of brain injury, indicating a possible role for these chemokines in the pathogenesis of neuroAIDS (Kelder et al., 1998; Kolb et al., 1999) . Other than their potential contribution to brain injury (discussed in more detail below), the increased presence of these chemokines in the CNS during HIV-1 infection may promote, via their chemoattractant activity, the further recruitment and accumulation of T lymphocytes and macrophages in the brain. In support of this idea, HIV-Tat, protein-induced MCP-1 expression by astrocytes can stimulate the transmigration of monocytes across an in vitro model of the human BBB (Weiss et al., 1999) . Finally, since MIP-la and RANTES can effectively inhibit CCR5 coreceptor-mediated cell entry by HIV-1 (Cocchi et al., 1996; Dragic et al., 1996) , expression of these and other chemokines in the CNS in neu-roAIDS might limit the spread of HIV-1 by antagonizing infection of target microglia.

The wide distribution of chemokine receptors in the normal and HIV-infected human brain, as well as the increased expression of their cognate ligands, not only provides an avenue for HIV entry into the brain via the microglia, but might also contribute to the neuronal injury and loss that ensue. As indicated above, current evidence supports the idea that neuronal injury and death in neuroAIDS are consequences of indirect mechanisms involving a variety of host-derived molecules for example, cytokines and excitotoxic amino acids or HIVencoded factors such as gpl20; it is probable that the host and viral products work in combination. Loss of neurons, astrocytes, and microglia through apoptosis appears to be prominent in neuroAIDS (Adle-Biassette et al., 1995; Gelbard et al., 1995; Shiet al., 1996) and can be induced by HIV-1 infection of these cells in vitro (Shi et al., 1996) . Analysis of a panel of diverse HIV-1 primary isolates or strains revealed that, compared with blood-derived or T-tropic HIV-1 isolates, brain-derived M-tropic HIV-1 isolates are ineffective at inducing neuronal and astrocyte apoptosis (Ohagen et al., 1999; Zheng et al., 1999b) . Thus, the primary determinant of neurodegeneration may not be the M-tropic HIV forms constituting the major CNS reservoir of HIV-1, but rather, it resides with blood-derived viruses that are prominent during later stages of disease. Furthermore, the Env V3 region is the major determinant of neuronal apoptosis and also determines chemokine coreceptor usage for infection (Ohagen et al., 1999) , implicating these viral coreceptors in the effector pathways that mediate neuronal damage during neuroAIDS.

As mentioned above, the blood-derived, T-tropic HIV-1 isolates rely primarily on CXCR4 as a coreceptor, whereas the brain-derived, Mtropic isolates use CCR5 and CCR3. In view of the congruity between the induction of neuronal apoptosis and chemokine coreceptor usage, and the fact that neurons express CXCR4 (as noted above), many recent studies have focused on the possible role of CXCR4 in mediating brain injury in neuroAIDS. Hesselgesser and colleagues showed that signaling by CXCR4, following the binding of T-tropic HIVgpl20, activates apoptotic cell death in human hNT neuronal cells (Hesselgesser et al., 1998) . This does not appear to reflect an unusual property of hNT cells, which have a transformed phenotype, as induction of apoptotic cell death is also induced in rat hippocampal neurons exposed to HIVgpl20 (Meucci et al., 1998) . In these latter experiments, pretreatment of the neurons with the cognate ligand for CXCR4, SDF-1, partially protected against gpl20-induced neuronal apoptosis, suggesting that the effects of the viral product are mediated, in part, by CXCR4 binding. More recently, purified virions from T-tropic, but not M-tropic, HIV isolates were shown to induce CD4 independent neuronal signaling and apopto-sis that could be blocked by antibodies to CXCR4 (Zheng et al., 1999a) . In these studies, the purified T-tropic HIVgpl20 protein proved to be far less effective in affecting neuronal signaling and cell death than the whole virions. Since the effects of the virions could be blocked by antibodies to gpl20 and gp41, the findings indicate that the envelope proteins in their native state are more potent mediators than their recombinant or purified derivatives. In all, the implication from these studies, which utilized either human neuronal cell lines or highly purified rodent or human primary neuronal cultures, is that direct binding of HIV gpl20 to the CXCR4 found on neurons activates apoptotic cell death in these cells. Contrary to this notion, in mixed brain-cell cultures derived from embryonic rat cerebrocortex, gpl20-induced neuronal apoptosis was completely blocked by the tripeptide TKP, which blocks macrophages microglial activation (Kaul and Lipton, 1999) . This blockade by TKP was specific for gpl20 as SDF-l-induced neuronal apoptosis (as noted below) was unaffected by the peptide. Interestingly, gpl20induced neuronal apoptosis in this model system was also blocked by the CC chemokines RANTES and MIP-I~, which bind to CCR5, suggesting there may be regulatory cross talk between the CCR5 and CXCR4 receptors. However, the conclusion from this study is that gpl20-induced neuronal apoptosis depends predominantly on an indirect pathway via activation of chemokine receptors on macrophages microglia. Many possible explanations exist as to why there are differences between these studies with respect to the observed effects of gpl20's being conveyed by direct or indirect pathways. These include differences in the species and types of neuronal or brain-cell cultures used and the types, form, and levels ofgpl20 added to the cultures.

While the question of whether the induction of neuronal apoptosis by gpl20 is consequent to interaction directly with the CXCR4 receptor on neurons or occurs indirectly, following activation of the macrophage/microglia, is unresolved, there is general agreement that HIVgpl20 binding to CXCR4 can regulate a number of cellular signaling pathways (Kaul and Lipton, 1999; Meucci et al., 1998; Zheng et al., 1999a; Zheng et al., 1999b) . These include inhibition of cyclic AMP; activation of phosphoinositol (PI) hydrolysis, which is partially prevented by antibody to CXCR4 (Zheng et al., 1999a; Zheng et al., 1999b) ; activation of p38 mitogen-activated kinase (p38 MAPK) (Kaul and Lipton, 1999) ; and increased Ca 2+ mobilization (Meucci et al., 1998) . HIV gpl20 neuronal apoptosis can be inhibited by inhibitors of p38 MAPK (Kaul and Lipton, 1999) and by calcium/calmodulin-dependent protein kinase II, protein kinase A, and protein kinase C (Zheng et al., 1999a) , linking neuronal apoptosis, in part, to these signal transduction pathways.

The preceding discussion indicates that HIV gpl20 neurotoxicity likely involves, in part, the chemokine receptor CXCR4, raising the possibility that HIV-1 subverts a key physiological cell death pathway. As we noted above, SDF-1/CXCR4 signaling is crucial for normal cerebellar neuronal migration and patterning during CNS development. SDF-1 is expressed constitutively in the adult CNS predominantly by astrocytes and to a lesser degree by neurons , raising the likelihood that this chemokine may regulate physiological processes in the adult brain. In support of this, SDF-la regulates a number of signal transduction pathways in neurons, causing decreased cyclic AMP, increased PI hydrolysis, activation of p38MAPK, and increased Ca 2+ mobilization (Kaul and Lipton, 1999; Meucci et al., 1998; Zheng et al., 1999a) . Functionally, exposure of hippocampal neurons to SDF-1 results in increased synaptic transmission, which is blocked by antibodies to CXCR4 (Zheng et al., 1999a) . The similar effects of SDF-1 and HIV gpl20 in regulating CXCR4 signaling raise the question as to whether SDF-1 might also induce neuronal apoptosis. Here, the findings have been conflicting. Four studies revealed that SDF-1 promoted increased neuronal apoptosis (Hesselgesser et al., 1998; Kaul and Lipton, 1999; Zheng et al., 1999b) and activation of a key cell death gene, caspase 3 (Zheng et al., 1999a) . In contrast to these findings, Meucci and colleagues reported that, following treatment with SDF-1, there was marked protection of neurons from culture-and HIV gpl20-induced apoptosis (Meucci et al., 1998) . The reason for these discrepancies is not known but again likely reflect differences in cell preparations and culture conditions.

Similar to HIV, SIV infection of macaques can lead to the development of an AIDS-associated encephalitis that is virtually indistinguishable from neuroAIDS (Raghavan et al., 1999; Sasseville and Lackner, 1997) . Both T-tropic and M-tropic SIV variants have been isolated and, like neuroAIDS, macrophage/microglia constitute the primary infected target cell population in the brain of SIV-infected monkeys. The host immune system plays a major role in the infection of the CNS with SIV, where perivascular macrophages are continuously replaced via recruitment from the circulating monocyte pool through the blood-brain barrier (Lane et al., 1996) . Thus, infected T cells and monocytes likely enter the CNS parenchyma during the asymptomatic phase of infection.

In addition to leukocyte and endothelial adhesion molecules, chemokines could be critical components involved in SIV infection of the brain. Using immunohistochemical staining to investigate chemokine expression in the encephalitic brain of SIV-infected macaques, Sasseville and coworkers reported upregulation in the expression of a number of chemokines, including MIP-la, MIP-I~, RANTES, MCP-3, and IP-10, by vascular endothelium and/or perivascular mononuclear cells . In contrast to neuro AIDS, MCP-1 was not elevated in the SIV-encephalitic monkey brain. With the exception of MCP-1, expression of these proinflammatory chemokines in the SIV-infected brain is similar to that found in the human brain in neuroAIDS (as noted above). However, like neu-roAIDS, the in vivo function of these chemokines in SIV-encephalitis remains unknown; it is reasonable speculation however, to suggest that leukocyte recruitment to the SIV-infected CNS may be directed by these chemokines. In a follow-up study by this group, examining chemokine receptor expression in SIV-AIDS encephalitis, the chemokine receptors CCR3, CCR5, CXCR3, and CXCR4 were found to be expressed by inflammatory cells within perivascular lesions (Westmoreland et al., 1998) . In addition, expression of CCR3, CCR5, and CXCR4 was detected on subpopulations of large hippocampal and neocortical pyramidal neurons and on glial cells in both the normal and the SIV-encephalitic brain. Again, these findings show an overlap with those reported for the normal and the HIV-infected brain in humans (as noted above). M-tropic strains of SIV utilize CCR5 as a coreceptor with CD4 for infection of macrophages Deng et al., 1997) . Expression of CCR5 is enriched on microglial cells in the macaque brain and may therefore provide the molecular framework for infection by SIV in the monkey CNS.

Overall, the preceding discussion highlights the overlapping expression patterns and potentially similar roles for the chemokines and their receptors in the pathogenesis of SIV-encephalitis and neu-roAIDS. This idea is further underscored by a recent study in which chemokine receptor expression and signaling were examined in cultured neurons derived from either macaque or human brains (Klein et al., 1999) . Thus, both the qualitative pattern of expression and the functional behavior of the chemokine receptors, CCR3, CCR5, and CXCR4, in the two species are remarkably alike.

In humans, multiple sclerosis is the most common and the bestknown CNS demyelinating disorder. MS is thought to be an autoim-mune T-cell-mediated disease that is targeted at the myelin sheath. The etiology of MS is unknown (Hailer, 1999) , although at one time or another, different viruses have been implicated in its pathogenesis, including Epstein-Barr virus (EBV), measles virus, and recently, HHV-6. The human T lymphotropic virus type I (HTLV-1) infection of the CNS can cause a progressive inflammatory demyelinating disease and shares many similarities with MS. In view of a possible viral role in MS, research has focused on several experimental viral models associated with the development of neuroimmune responses and primary demyelination that show many clinicopathologic similarities with MS. Among the best-known and most widely studied of these experimental models are Theiler's murine encephalomyelitis virus (TMEV)-and murine hepatitis virus (MHV)-induced demyelinating disease.

HTLV-1 is the etiologic agent for adult T cell leukemia (ATL) and for HAM/TSP, which shows similar features to primary progressive MS (Calabresi et al., 1999) . HAM-TSP is a chronic progressive neurological disorder characterized by a perivascular demyelination and axonal degeneration, along with the presence of inflammatory infiltrating cells such as macrophages and CD8 ÷ T cells in the damaged areas (Moore et al., 1989; Umehara et al., 1993; Wu et al., 1993) . These infiltrating cells produce several inflammatory mediators including the chemokine MCP-1 (Umehara et al., 1993) . In addition, HTLV-1infected T cell lines produce a number of chemokines including SDF-1, RANTES, MIP-la, and MIP-I~ (Arai et al., 1998; Baba et al., 1996; Mendez et al., 1997) . HAM/TSP patients have a high frequency of CD8 ÷ CTL in their peripheral blood and cerebrospinal fluid (CSF). Biddison and coworkers have shown that HTLV-I-specific CD8 ÷ CTL clones secrete MIP-la, MIP-I~, and IL-16 (Biddison et al., 1997) . Thus, these HTLV-1 CTLs may be an important source of chemokines in HAM/TSP, where they might contribute to the pathogenesis of this disorder. Consistent with this, elevated MIP-la protein was found in the CSF in two of three patients with HAM/TSP (Miyagishi et al., 1995) . A further clinical study showed that MCP-1 was detectable on perivascular inflammatory cells and the vascular endothelium in active-chronic lesions of spinal cords of HAM/TSP patients (Umehara et al., 1996) .

MHV belongs to the coronaviruses, a ubiquitous group of positivestranded RNA viral pathogens of man and animals associated with a variety of respiratory, gastrointestinal, and neurological disorders.

VAL]~RIE C. ASENSIO AND IAIN L. CAMPBELL CNS infection of susceptible murine hosts with neuro-adapted strains of MHV leads to a robust initial recruitment of leukocytes to the brain that follows an early peak of viral replication. This acute encephalomyelitis phase, with its resultant tissue injury, often leads to death of the host. However, in surviving animals, further viral replication is controlled by the host response and a chronic mononuclear cell inflammation, which occurs in the brain and spinal cord, is linked to a progressive demyelinating disease (Buchmeier and Lane, 1999; Lane and Buchmeier, 1997; Weiner, 1973) . The mechanisms of demyelination in this model are not clear, although there is a requirement for a competent immune response.

In a study of the kinetics and histological localization of chemokine gene expression in the brain and spinal cord of MHV-infected mice, Lane and colleagues showed that a number of a-and ~-chemokines were expressed during acute or chronic stages of MHV-infection (Lane et al., 1998) . Expression of the transcripts for IP-10, MIP-2, MCP-1, MCP-3, MIP-1~, and RANTES overlapped with the occurrence of acute viral encephalomyelitis, being present by day 3 postinfection and peaking at day 7 postinfection. During the chronic demyelinating phase of the disease, both IP-10 and RANTES transcripts remained elevated. IP-10 RNA colocalized with viral RNA and was present in astrocytes and microglia associated with demyelinating lesions (Lane et al., 1998) . With the exception of RANTES, a similar pattern of chemokine expression was observed with cultured astrocytes infected with active, but not UV-inactive, MHV, indicating that viral replication in these cells can directly stimulate chemokine gene expression. Therefore, in MHV infection of the brain, early viral-induced chemokine gene expression by resident CNS cells such as astrocytes might promote the recruitment of T lymphocytes and macrophages and contribute to the maintenance of the chronic inflammatory response leading to demyelination.

Further studies in the MHV model using CD4 KO and CD8 KO mice show an important role for CD4 ÷ T-cells in the pathogenesis of inflammatory disease and demyelination (Lane et al., 2000) . Thus, MHVinfected CD4 KO mice have a marked reduction in the number of activated macrophage/microglia within their brains and spinal cords and significantly less demyelination. Concomitant with the reduction in CNS inflammatory disease in the MHV-infected CD4 KO mice, lower levels of RANTES, but not other chemokine transcripts and protein were found indicating that CD4 ÷ T-cells represent one major source of RANTES in the CNS during MHV encephalomyelitis. Administration of RANTES neutralizing antisera to MHV-infected mice is associated with a significant reduction in macrophage infiltration and demyelination compared to control mice (Lane et al., 2000) . These data clearly indicate that CD4 + T-cells have a pivotal role in accelerating CNS inflammation and demyelination within infected mice possibly by regulating RANTES expression.

TMEV-induced demyelinating disease is characterized by CNS mononuclear cell infiltration resulting in a chronic CD4 + T-cell-mediated demyelinating disease (for more discussion, see Chapter 7 in this volume). A study of the expression of CC and CXC chemokines in the CNS throughout the disease course showed that expression of C10, IP-10, MCP-1, MIP-la, MIP-I~, and RANTES mRNA transcripts overlapped with the development of disease (Hoffman et al., 1999) . Expression of MIP-I~ and MIP-I~ protein was also detectable, being present in the spinal cord before the onset of disease and persisting throughout disease progression. These findings are somewhat reminiscent of those reported for MHV encephalomyelitis (discussed above) and reiterate the theme that chemokine expression in these viralinduced immune-mediated demyelinating diseases is a complex process that is clearly associated with disease progression. Identifying the key chemokine protagonists in TMEV-induced demyelinating disease awaits clarification.

Meningoencephalitis represents one of the most devastating consequences of viral infection of the CNS, in which a concerted immunoinflammatory response by the host produces severe injury to the brain, leading to debilitating neurological disease and often death. In the viral meningoencephalitides, the specific mechanisms underlying the localization, extravasation, and activation of immune cells in the CNS and the subsequent interactions between these cells are not well understood.

Studies from our own laboratory examined chemokine gene expression in lymphocytic choriomeningitis (LCM) that was induced by intracranial inoculation of mice with LCMV (Asensio and Campbell, 1997; Asensio et al., 1999a) . Following inoculation, LCMV infects and rapidly replicates in the meninges, the choroid plexus, and the ependymal membranes lining the ventricles. In this very well characterized model, immunocompetent adult mice infected with LCMV develop an acute monophasic disease characterized by the presence of infiltrating mononuclear cells in these same regions of the brain, and this leads to convulsive seizures and death by 6-8 days later (Buchmeier et al., 1980; Doherty et al., 1990) . Infiltrating cells are predominantly T lymphocytes as well as macrophages. MHC class-I-restricted anti-LCMV CD8 ÷ cytotoxic lymphocytes (CTLs), in addition to clearance of the virus, are also the primary effectors of LCM (Buchmeier et al., 1980; Doherty et al., 1990) . Immune-compromised mice that are unable, or fail, to mount anti-LCMV CD8 ÷ CTL responses do not develop immune pathology in the brain and survive.

We hypothesized that chemokines may be important regulatory signals for the cerebral recruitment and extravasation ofleukocytes in LCM (Asensio and Campbell, 1997; Asensio et al., 1999a) . In examining this, we observed that the pattern of chemokine gene expression in LCM is dynamic and complex, with often overlapping expression of a number of different subclasses of chemokine genes. Thus, by day 3 postinfection the expression of a number of chemokine genes is evident, including C10, MCP-3, MIP-I~, MCP-1, CRG-2/IP-10, and RANTES. By day 6 postinfection the expression of all these chemokine genes increases markedly and the expression of the lymphotactin gene is also evident in the brain. A qualitatively similar but markedly decreased level of chemokine gene expression is observed in the brain of LCMV-infected athymic mice. A similar pattern of cerebral chemokine gene expression is also found in LCMV-infected IFN-y KO mice, although the levels of expression in the absence of IFN-y are about 50% of those in LCMV-infected wild-type controls. In both euthymic and athymic mice, expression of IP-10 was predominant at both early and late time points after infection and preceded detectable increases in proinflammatory and interferon cytokine gene expression and CNS leukocyte recruitment in euthymic mice. In all, FIG 2. The CXC chemokine IP-10 is expressed at high levels in the brain, following infection by many viruses. In this example mice were infected intracranially with LCMV, and the viral nucleoprotein (NP) and IP-10 RNA expression in the brain were analyzed by in situ hybridization. High expression of IP-10 RNA was apparent by day 3 (A) postinfection and increased further by day 6 (B) postinfection. Regional expression of IP-10 RNA, on the whole, overlapped with sites of viral infection, as indicated by the expression of the LCMV-NP gene. However, parenchymal expression of the IP-10 gene was also found in the absence of local LCMV-NP expression. By using dual-label analysis to identify the cellular localization of IP-10 RNA expression, these parenchymal cells were identified as GFAP-positive astrocytes (C; arrows). From Asensio, V. C., Kincaid, C., and Campbell, I. L. Chemokines and the inflammatory response to viral infection in the 156 VALl~RIE C. ASENSIO AND IAIN L. CAMPBELL these observations, together with the finding that CRG-2/IP-10, a prominently expressed chemokine gene in many different CNS viral infections, is expressed by resident CNS cells including astrocytes (see Fig. 2 ), suggest that activation ofchemokine gene expression may be a direct, early, and localized host response to LCMV infection. These findings are consistent with the proposed involvement of chemokines as key signaling molecules for the subsequent migration of leukocytes to the CNS, following LCMV infection. However, formal demonstration of the precise chemotactic, and possibly of other functions of chemokines in LCM, presently awaits verification.

Herpes simplex virus (HSV) is a common etiologic agent of acute focal encephalitis. Pathologically, in HSV encephalitis, leukocytes infiltrate the subarachnoid space as a hallmark of meningitis and encephalitis, while CSF analysis typically reveals a predominantly lymphocytic pleocytosis. To evaluate the possible contribution of chemokines to HSV encephalitis, Rosler and coworkers examined the spectrum, quantity, and time course of CSF chemokines in three patients with proven HSV type 1 encephalitis (HSE-1) (Rosler et al., 1998) . High chemokine levels were present in the CSF of all HSE-1 patients. Peak levels occurred at the time of admission, with MCP-1 being greater than either MIP-la or RANTES. IL-8 levels were elevated at 4 to 8 hours after admission. By contrast, plasma chemokine levels were considerably lower than CSF levels, pointing to the localized nature of the CNS chemokine response in these HSE-1 patients. A comparison of MCP-1 levels with clinical status revealed a high reciprocal correlation. This single clinical study implicates chemokines, particularly MCP-1, in the pathogenesis of HSE-1 and suggests these mediators may be useful indicators to determine the stage and severity of riSE-1.

Experimental studies of HSV-1 in mice, following ocular infection, highlight the rapid and sustained upregulation of a number of chemokine genes including GRO-a, MIP-I~, MIP-2, MCP-1, IP-10, and RANTES (Carr et al., 1998; Thomas et al., 1998) . HSV stromal keratitis, associated with productive infection in the eye, results in significant accumulation of PMN, which may be driven by the presence in particular of the CXC chemokines GRO-a and MIP-2 (Thomas et al., 1998) .

A clinical survey of chemokine expression in CSF in patients with viral meningitis, due to infection with paramyxoviruses or enteroviruses, implicated IP-10 and MCP-1 as important contributory factors in the accumulation of activated T cells and monocytes in the CNS (Lahrtz et al., 1997) . Importantly, this study formally demonstrated that the IP-10 and MCP-1 levels in CSF correlated with leukocyte chemotactic activity. MCP-1 was identified as an important chemoattractant for PBMCs while the combination of MCP-1 and IP-10 was required for migration of activated T cells. This study clearly establishes a direct link between the CNS expression of the chemokines IP-10 and MCP-1 and CNS leukocytosis in viral meningitis.

In experimental studies, the paramyxovirus Newcastle disease virus (NDV) induces IP-10 gene expression in infected astrocytes and microglia detectable by 3 hours postinfection (Vanguri and Farber, 1994) . UV-inactivated NDV proved to be as effective as live virus in inducing IP-10 gene expression and was not blocked by the protein synthesis inhibitor cycloheximide, indicating that the IP-10 gene functions as an immediate early response gene, following NDV infection. Similar to NDV, measles virus (MV) is a paramyxovirus that induces IP-10 gene expression in human glioblastoma cells (Nazar et al., 1997) . The promoter requirements for the induction of IP-10 gene expression are similar for IFN-~ and MV; however, these two stimuli use different combinations of DNA binding factors, with STAT 1 or NF~B being more important in the direct induction of IP-10 by IFN-~ or MV, respectively. While it is presently unknown whether these paramyxoviruses induce the expression of other chemokine genes, their potent ability to directly induce IP-10 expression by glial cells may constitute a significant CNS source of production of this chemokine during viral meningitis.

Mouse adenovirus-type 1 (MAV-1) is a double-stranded DNA virus that causes a fatal hemorrhagic encephalopathy in C57B1/6 mice within 4-6 days (Guida et al., 1995) , along with infection of cerebrovascular endothelial cells. Chemokine gene expression was investigated and prominent induction of IP-10 was observed in the spleen and CNS of susceptible mice, whereas MCP-1 and MIP-2, respectively, were found in the spleen and brain of resistant mice (Charles et al., 1999) . Vascular endothelium and CNS glia were identified as sites of IP-10 mRNA expression in susceptible animals.

In the normal mammalian CNS, the number of leukocytes present in the brain is scant. However, these cells are attracted to, and accu-158 VALI~RIE C. ASENSIO AND IAIN L. CAMPBELL mulate in, a variety of pathologic states, many involving viral infection. Although leukocyte migration into local tissue compartments such as the CNS is a multifactorial process, it has become clear that chemokines are pivotal components of this process, providing a necessary chemotactic signal for leukocyte recruitment. Generation of this signal in CNS viral infection may involve localized production of proinflammatory chemokines by cells intrinsic to the brain, including the astrocytes and microglia. The activated glia have the potential to produce a spectrum of CC and CXC chemokines that may vary, depending on the nature of the invading pathogen. This in turn will determine the qualitative makeup of the leukocytes recruited to the brain. It is important to note, however, that how a chemokine that is secreted by a parenchymally located glial cell is "sensed" by circulating peripheral leukocytes is unclear at this time.

Roles beyond leukocyte chemoattraction are implied for the chemokines and their receptors. Studies of HIV highlight the dynamic relationship between chemokine receptor usage and tropism for different isolates of HIV. On the other hand, the fact that chemokine receptor ligands are effective inhibitors of HIV entry into target cells suggests their local production may serve as a host response to limit further spread of the infectious agent. Some chemokines such as IP-10 are documented as having direct antiviral functions. Whether this is important in CNS defense against viral infection where IP-10 is highly expressed, and whether any other chemokines have an antiviral function, remain open questions.

A further consequence of the localized production of chemokines in the brain is likely the direct modulation of CNS cell function. Various classes of chemokine receptors are expressed by CNS cells including neurons and the glia. In many respects the CNS possesses its own chemokine network permitting autocrine and paracrine regulation of cellular activity. Chemokines are implicated in many physiological functions including development, cell growth, angiogenesis, and cellular migration. Perturbation of the CNS chemokine network, resulting from viral infection, could therefore either be beneficial by promoting reparative processes or detrimental by contributing to tissue injury and loss.

In conclusion, it is now apparent that, like their proinflammatory cytokine counterparts, the chemokines are important plurifunctional mediators in the host response to viral infection of the CNS (see Fig. 3 and color section). As such, a greater understanding of the neurocellular and viral targets and functions of the chemokines holds the promise of revealing new molecular focal points for therapeutic inter- [For color reproduction, see color section.] Chemokines are potentially multifunctional effectors in CNS viral infections. In this schema, initial infection of the CNS with a virus results in the localized production of chemokines by reactive astrocytes and microglia. Some chemokines such as SDF-1 and fractalkine are produced constitutively in the CNS and their production may also be altered by viral infection. Increased chemokine production may then promote the further recruitment and extravasation of antiviral immune effector cells from the periphery. Chemokine receptors are widely expressed by neuronal and glial cells and additional functions of the chemokines are likely. These may include direct antiviral actions (for example, by IP-10), and intercellular communication (for example, neuronally derived fractalkine can stimulate microglia). In addition, chemokines may exert detrimental actions; for example, SDF-1 can impair neuronal activity and stimulate apoptosis of these cells. M, macrophage; Tc, T lymphocyte; N, neutrophil. vention that could more effectively control CNS viral infections and prevent tissue injury. ACKNOWLEDGMENTS Val6rie Asensio is a postdoctoral fellow of the National Multiple Sclerosis Society. The authors' studies were supported by NIH grants MH 50426; MH 47680 and NS 36979 to ILC. This chapter is publication No. 13255-NP, from the Scripps Research Institute.

Neuronal apoptosis in HIV infection in adults

Chemokine receptors and molecular mimicry

Molecular piracy of mammalian interleukin-8 receptor type B by herpesvirus saimiri

CC CKR5: a RANTES, MIP-la, MIP-I~ receptor as a fusion cofactor for macrophage-Tropic HIV-1

Genes for chemokines MuMig and Crg-2 induced in protozoan and viral infections in response to IFN-7 with patterns of tissue expression that suggest nonredundant roles in vivo

Human T-cell leukemia virus type 1 Tax protein induces the expression of lymphocyte chemoattractant SDF-1/PBSF

Human herpesvirus KSHV encodes a constitutively active G-protein coupled receptor linked to cell proliferation

Chemokine gene expression in the brains of mice with lymphocytic choriomeningitis

Chemokines and their receptors in the CNS: directing cellular communication

Chemokines and the inflammatory response to viral infection in the central nervous system with a focus on lymphocytic choriomeningitis virus

C10 is a novel chemokine expressed in experimental inflammatory demyelinating disorders that promotes the recruitment of macrophages to the central nervous system

Constitutive expression of various chemokine genes in human T-cell lines infected with human T-cell leukemia virus type 1: role of the viral transactivator Tax

Chemokines in disease models and pathogenesis

Chemokines and leukocyte traffic

Interleukin-8 and related chemotactic cytokines-CXC and CC chemokines

Human chemokines: an update

G-protein-coupled receptor of Kaposi's sarcoma-associated herpesvirus is a viral oncogene and angiogenesis activator

Expression of chemokine receptors in the rat brain

CCR5(+) and CXCR3(+) T cells are increased in multiple sclerosis and their ligands MIP-lalpha and IP-10 are expressed in demyelinating brain lesions

A new class of membrane-bound chemokine with a CX3C motif

Overriding the brain's intrinsic resistance to leukocyte recruitment with intraparenchymal injections of recombinant chemokines

Monocyte chemotactic protein-3 (MCP3) interacts with multiple leukocyte receptors. C-C CKR1, a receptor for macrophage inflammatory protein-1 alpha/Rantes, is also a functional receptor for MCP3

Identification of the CC chemokines TARC and macrophage inflammatory protein-1 beta as novel functional ligands for the CCR8 receptor

Human T cell leukemia virus type I (HTLV-I)-specific CD8+ CTL clones from patients with HTLV-I-associated neurologic disease secrete proinflammatory cytokines, chemokines, and matrix metalloproteinase

HIV-1 infection in an individual homozygous for the CCR5 deletion allele

The lymphocyte chemoattractant SDF-1 is a ligand tbr LESTR/fusin and blocks HIV-1 entry

The HIV coreceptors CXCR4 and CCR5 are differentially expressed and regulated on human T lymphocytes

Chemokine sequestration by viral chemoreceptors as a novel viral escape strategy: withdrawal of chemokines from the environment of cytomegalovirus-infected cells

Cultured rat microglia express functional beta-chemokine receptors

Differential expression of chemokine receptors and chemotactic responsiveness of type 1 T helper cells (Thls) and Th2s

Upregulation of CCR1 and CCR3 and induction of chemotaxis to CC chemokines by IFNgamma in human neutrophils

Decreased lesion formation in CCR2-/-mice reveals a role for chemokines in the initiation of atherosclerosis

Angiogenic and HIV-inhibitory functions of KSHV-encoded chemokines

Involvement of interleukin (IL) 8 receptor in negative regulation of myeloid progenitor cells in vivo: evidence from mice lacking the murine IL-8 receptor homologue

Viral-induced neurodegenerative disease

The virology and immunobiology of lymphocytic choriomeningitis virus infection

Novel role of human CD4 molecule identified in neurodegeneration

Neutrophil and B cell expansion in mice that lack the murine IL-8 receptor homolog

Chemokines in chronic progressive neurological diseases: HTLV-1 associated myelopathy and multiple sclerosis

Cytokine and chemokine production in HSV-1 latently infected trigeminal ganglion cell cultures: effects of hyperthermic stress

Monocyte chemoattractant protein 1 acts as a T-lymphocyte chemoattractant

Differential chemokine induction by the mouse adenovirus type-1 in the central nervous system susceptible and resistant strains of mice

Inactivation of HIV-1 chemokine co-receptor CXCR-4 by a novel intrakine stategy

In vivo inhibition of CC and CX3C chemokine-induced leukocyte infiltration and attenuation of glomerulonephritis in Wistar-Kyoto (WKY) rats by vMIP-II

The beta-chemokine receptors CCR3 and CCR5 facilitate infection by primary HIV-1 isolates

Identification of RANTES, MIP-1 alpha, and MIP-1 beta as the major HIV-suppressive factors produced by CD8+ T cells

The V3 domain of the HIV-1 gpl20 envelope glycoprotein is critical for chemokinemediated blockade of infection

Cloning, chromosomal localization, and RNA expression of a human ~ chemokine receptor-like gene

Gene cloning, RNA distribution, and functional expression of mCX3CR1, a mouse chemotactic receptor for the CX3C chemokine fractalldne

Induction of monocyte chemoattractant protein-1 in HIV-1 Tat-stimulated astrocytes and elevation in AIDS dementia

Requirement of MIP-la for an inflammatory response to viral infection

HHV8-encoded vMIP-I selectively engages chemokine receptor CCR8. Agonist and antagonist profiles of viral chemokines

Abduction of chemokine elements by Herpesviruses

Broad spectrum chemokine antagonistic activity of a human poxvirus chemokine homolog

Identification of a major co-receptor for primary isolates of HIV-1

Expression cloning of new receptors used by simian and human immunodeficiency viruses

Differentiation-specific expression of a novel G protein-coupled receptor from Burkitt's lymphoma

Dissection of an inflammatory process induced by CD8+ T-cells

A dual-tropic primary HIV-1 isolate that uses fusin and the beta-chemokine receptors CKR-5, CKR-3, and CKR-2b as fusion cofactors

HIV-1 entry into CD4+ cells is mediated by the chemokine receptor CC-CKR-5

A review of neuronal damage in human immunodeficiency virus infection: its assessment, possible mechanism and relationship to dementia

Two orphan seven-transmembrane segment receptors which are expressed in CD4-positive cells support simian immunodeficiency virus infection

HIV-1 entry cofactor: functional cDNA cloning of a seven-transmembrane, G protein-coupled receptor

Regulatory mechanisms of MuRantes and CRG-2 chemokine gene induction in central nervous system glial cells by virus

Fractalkine and CX3CR1 mediate a novel mechanism of leukocyte capture, firm adhesion, and activation under physiologic flow

A putative chemokine receptor, BLR1, directs B cell migration to defined lymphoid organs and specific anatomic compartments of the spleen

Selective expression of the murine homologue of the G-protein-coupled receptor BLR1 in B cell differentiation, B cell neoplasia and defined areas of the cerebellum

Characterization of the CCR2 chemokine receptor: functional CCR2 receptor expression in B cells

Controlled recruitment of monocytes and macrophages to specific organs through transgenic expression of monocyte chemoattractant protein-1

Excitotoxic brain injury stimulates expression of the chemokine receptor CCR5 in neonatal rats

Impaired host defense, hematopoiesis, granulomateus inflammation and type 1-type 2 cytokine balance in mice lacking CC chemokine receptor 1

Structure and functional expression of the human macrophage inflammatory protein 1 alpha/RANTES receptor

Human cytomegalovirus open reading frame US28 encodes a functional ~ chemokine receptor

Cloning and differential tissue-specific expression of three mouse ~ chemokine receptor-like genes, including the gene for a functional macrophage inflammatory protein-la receptor

Apoptotic neurons in brains from paediatric patients with HIV-1 encephalitis and progressive encephalopathy

The neuropathogenesis of the AIDS dementia complex

Chemokines and chemokine receptors in CNS pathology

Identification of CCR8 as the specific receptor for the human ~-chemokine 1-309: cloning and molecular characterization of murine CCR8 as the receptor for TCA-3

Transgeuic monocyte chemoattractant protein-1 (MCP-1) in pancreatic islets produces monocyte-rich insulitis without diabetes: abrogation by a second transgene expressing systemic MCP-1

Mouse adenovirus type I causes a fatal hemorrhagic encephalomyelitis in adult C57BL/6 but not BALB/c mice

The distinction blurs between an autoimmune versus microbial hypothesis in multiple sclerosis

cDNA cloning of a G-protein-coupled receptor expressed in rat spinal cord and brain related to ehemokine receptors

Role for neuronally derived i~actalkine in mediating interactions between neurons and CX3CRl-expressing microglia

Production and function of monocyte chemoattraetant protein-1 and other ~-chemokines in murine glial cells

CCR3 and CCR5 are co-receptors for HIV-1 infection of microglia

Lymphotactin is produced by NK cells and attracts both NK cells and T cells in vivo

Cloning of the mouse fusin gene, homologue to a human HIV-1 co-factor

Isolation of three novel human genes encoding G protein-coupled receptors

A novel human gene encoding a G-protein-coupled receptor (GPR15) is located on chromosome 3

Neuronal apoptosis induced by HIV-1 gpl20 and the chemokine SDF-1 alpha is mediated by the chemokine receptor CXCR4

Leukocyte traffic in the central nervous system: the participants and their roles

Central nervous system chemokine expression during Theiler's virus-induced demyelinating disease

The Duffy antigen receptor for chemokines: structural analysis and expression in the brain

Expression of chemokine receptors by subsets of neurons in the central nervous system

Inhibition of microglial cell RANTES production by IL-10 and TGF-beta

Identification and molecular characterization of fractalkine receptor CX3CR1, which mediates both leukocyte migration and adhesion

The G protein-coupled receptor BLR1 is involved in murine B cell differentiation and is also expressed in neuronal tissues

An important role for the chemokine macrophage inflammatory protein-lalpha in the pathogenesis of the T cell-mediated autoimmune disease, experimental autoimmune encephalomyelitis

Chemokine regulation of experimental autoimmune encephalomyelitis: temporal and spatial expression patterns govern disease patbogenesis

Chemokines and activated macrophages in HIV gpl20-induced neuronal apoptosis

Beta-chemokines MCP-1 and RANTES are selectively increased in cerebrospinal fluid of patients with human immunodeficiency virus-associated dementia

Lymphotactin: a cytokine that represents a new class of chemokine

A broad-spectrum chemokine antagonist encoded by Kaposi's sarcoma-associated herpesvirus

Chemokine receptor expression and signaling in macaque and human fetal neurons and astrocytes: implications for the neuropathogenesis of AIDS

Identification of a T cell chemotactic factor in the cerebmspinal fluid of HIV-l-infected individuals as interferon-7 inducible protein 10

Genetic differences between blood-and brain-derived viral sequences from human immnnodeficiency virus type 1-infected patients: evidence of conserved elements in the V3 region of the envelope protein of brain-derived sequences

Functional characterization of the C-C chemokine-like molecules encoded by molluscum contagiosum virus types 1 and 2

Defects in macrophage recruitment and host defense in mice lacking the CCR2 chemokine receptor

Severe reduction in leukocyte adhesion and monocyte extravasation in mice deficient in CC chemokine receptor 2

Chemotactic activity on mononuclear cells in the cerebrospinal fluid of patients with viral meningitis is mediated by interferon-gamma inducible protein-10 and monocyte chemotactic protein-1

Chemokines and chemotaxis of leukocytes in infectious meningitis

Modulating chemokines: more lessons from viruses

Neuroinvasion of simian immunodeficiency virus coincides with increased numbers of perivascular macrophages/microglia and intrathecal immune activation

Dynamic regulation of a-and ~-chemokine expression in the central nervous system during mouse hepatitis virus-induced demyelinating disease

Murine coronavirus infection: a paradigm for virus-induced demyelinating disease

A central role for CD4+ T-cells and RANTES in virus-induced central nervous system inflammation and demyelination

Chemokine receptors in the human brain and their relationship to HIV infection

CXCR-4 (Fusin), a co-receptor for the type 1 human immunodeficiency virus (HIV-1), is expressed in the human brain in a variety of cell types, including microglia and neurons

Cerebrospinal fluid beta chemokine concentrations in neurocognitively impaired individuals infected with human immunodeficiency virus type 1

A novel chemokine receptor-like protein, functions as a fusion cofactor for both macrophage-tropic and T cell line-tropic HIV-1

Chemokine receptors: keys to AIDS pathogenesis?

Chemokine receptor specific for IP10 and Mig: structure, function, and expression in activated T-lymphocytes

Abnormalities in monocyte recruitment and cytokine expression in monocyte chemoattractant protein 1-deficient mice

The CXC-chemokine, H174: expression in the central nervous system

Impaired B-lymphopoiesis, myelopoiesis, and derailed cerebellar neuron migration in CXCR4-and SDF-l-deficient mice

Late expression of a beta chemokine homolog by murine cytomegalovirus

Characterization of fractalkine in rat brain cells: migratory and activation signals for CX3CR-l-expressing microglia

Chemokines: what chemokine is that?

The interferon-inducible chemokines MuMig and Crg-2 exhibit antiviral activity in vivo

Regulation of human IP-10 gene expression in astrocytoma cells by inflammatory cytokines

The chemokine system: redundancy for robust outputs

Chemokines. Introduction and overview

Cloning of human genes encoding novel G protein-coupled receptors

Purification and characterization of a novel monocyte chemotactic and activating factor produced by a human myelomonocytic cell line

MCP-1, MCP-2 and MCP-3 expression in multiple sclerosis lesions: an immunohistochemical and in situ hybridization study

Astrocyte-specific expression of human T-cell lymphotropic virus type 1 (HTLV-1) Tax: induction of tumor necrosis factor alpha and susceptibility to lysis by CD8+ HTLV-l-specific cytotoxic T cells

Developmental expression of monocyte chemoattractant protein-1 in the human cerebellum and brainstem

Chemokines and chemokine receptors in the CNS: a possible role in neuroinflammation and patterning

Chemokines regulate hippocampal neuronal signaling and gp120 neurotoxicity

The role of CCR5 and CCR2 polymorphisms in HIV-1 transmission and disease progression

Macrophage inflammatory protein-la in the cerebrospinal fluid of patients with multiple sclerosis and other inflammatory neurological diseases

Tropical spastic paraparesis: a model of virus-induced, cytotexic T-cell-mediated demyelination?

Lymphocyte responses to chemokines

Cloning of ATAC, an activation-induced, chemokine-related molecule exclusively expressed in CD8+ T lymphocytes

Molecular piracy of chemokine receptors by herpesviruses. Infectious Agents Dis

Signal transduction and ligand specificity of the human monocyte chemoattractant protein-1 receptor in transfected embryonic kidney cells

Defects of B-cell lymphopoiesis and bone-marrow myelopoiesis in mice lacking the CXC chemokine PBSF/SDF-1

A novel CXC chemokine PBSF/SDF-1 and its receptor CXCR4: their functions in development, hematopoiesis and HIV infection

Induction of IP-10 chemokine promoter by measles virus: comparison with interferon-gamma shows the use of the same response element but with differential DNA-protein binding profiles

Molecular cloning, functional expression, and signaling characteristics of a C-C chemokine receptor

Localization of fractalkine and CX3CR1 mRNAs in rat brain: does fractalkine play a role in signaling from neuron to microglia?

HIV-1 infection in a man homozygous for CCR5 delta 32

The CXC chemokine SDF-1 is the ligand for LESTR/fusin and prevents infection by T-cell-line-adapted HIV-1

Apoptesis induced by infection of primary brain cultures with diverse human immunodeficiency virus type I isolates: evidence for a role of the envelope

Expression of stromal cell-derived factor-1 and CXCR4 chemokine receptor mRNAs in cultured rat glial and neuronal cells

Inflammatory cytokines in the brain: does the CNS shape immune responses?

Neurotactin, a membrane-anchored chemokine upregulated in brain inflammation

Cytomegalovirus encodes a potent alpha chemokine

Identification of a chemokine receptor encoded by human cytomegalovirus as a cofacter for HIV-1 entry

Developmental expression of cytokine genes in the cortex and hippocampus of the rat central nervous system

Demented and non-demented patients with AIDS differ in brainderived human immunodeficiency virus type 1 envelope sequences

Chemokine receptors: gateways to inflammation and infection

The chemokine receptors CXCR3 and CCR5 mark subsets of T cells associated with certain inflammatory reactions

Morphological correlates of neurological dyshmction in macaques infected with neurovirulent simian immunodeficiency virus

The role of CCR7 in T(H)I and T(H)2 cell localization and delivery of B cell help in vivo

Chemokines in neurological disease models: correlation between chemokine expression patterns and inflammatory pathology

Do chemokines mediate leukocyte recruitment in post-traumatic CNS inflammation?

The orphan G-protein-coupled recepter-encoding gene V28 is closely related to genes for chemokine receptors and is expressed in lymphoid and neural tissues

Lipopolysaccharide-induced expression of IP-10 mRNA in rat brain and in cultured rat astroeytes and mieroglia. Molec

Time course of chemokines in the cerebrospinal fluid and serum during herpes simplex type i encephalitis

Cytomegalovirus-encoded beta chemokine promotes monocyte-associated viremia in the host

Chemokines and chemokine receptors in T-cell priming and Thl/Th2-mediated responses

Two subsets of memory T lymphocytes with distinct homing potentials and effecter functions

Selective expression of the eotaxin receptor CCR3 by human T helper 2 cells

Chemokines and receptors in HIV encephalitis

Neuropathogenesis of simian immunodeflcieney virus infection in macaque monkeys

Chemokine expression in simian immunodefieieney virus-induced AIDS encephalitis

Human immunodeflciency virus type I infection alters chemokine beta peptide expression in human monocytes--implications for recruitment of leukocytes into brain and lymph nodes

Neuronal expression of fractalkine in the presence and absence of inflammation

Genome sequence of a human tumorigenic poxvirus: prediction of specific host response-evasion genes

Apoptosis induced by HIV-1 infection of the central nervous system

Structure and chromosomal localization of the human stromal cell-derived factor 1 (SDF1) gene

Expression of monocyte chemoattractant protein-1 and other ~-chemokines by resident glia and inflammatory cells in multiple sclerosis lesions

Vaccinia virus immune evasion

Expression of specific chemokines and chemokine receptors in the central nervous system of multiple sclerosis patients

The C-C chemokine 6Ckine binds the CXC chemokine receptor CXCR3

The viral chemokine macrophage inflammatory protein-II is a selective Th2 chemoattractant

C-X-C and C-C chemokines are expressed in the cerebrospinal fluid in bacterial meningitis and mediate chemotactic activity on peripheral blood-derived polymorphonuclear and mononuclear cells in vitro

Cloning of rat HIV-l-chemokine coreceptor CKR5 from microglia and upregulation of its mRNA in ischemic and endotoxinemic rat brain

Chemokines in the cerebrospinal fluid patients with meningitis

Virally encoded chemokines and chemokine receptors: genetic embezzlement of host DNA

The role of chemokines in models of human disease

The chemokine receptor CXCR4 is essential for vascularization of the gastrointestinal tract

Murine astrocytes express a fimctional chemokine receptor

Functional expression of the CXCchemokine receptor-4/fusin on mouse microglial cells and astrocytes

Neutrophil infiltration, glial reaction, and neurological disease in transgenic mice expressing the chemokine N51/KC in oligodendrocytes

Signal sequence trap: a cloning strategy for secreted proteins and type I membrane proteins

Chemokines, inflammation and the immune system

HIV-1 infection in an individual homozygous for CCR5 delta 32

Herpes simplex virus replicationinduced expression of chemokines and proinflammatory cytokines in the eye: implications in herpetic stromal keratitis

Glucocorticoids downregulate gene expression of GM-CSF, NAP-1/IL-8, and IL-6, but not of M-CSF in human fibroblasts

Central nervous system damage produced by expression of the HIV-1 coat protein gpl20 in transgenic mice

Immunocytochemical analysis of the cellular infiltrate in the spinal cord lesions in HTLV-I-associated myelopathy

Expression of adhesion molecules and monocyte chemoattractant protein-1 (MCP-1) in the spinal cord lesions in HTLV-I-associated myelopathy

Localization of HIV-1 co-receptors CCR5 and CXCR4 in the brain of children with AIDS

Expression of MCP-1 by reactive astrocytes in demyelinating multiple sclerosis lesions

Interferon---inducible genes in primary glial cells of the central nervous system: comparisons of astrocytes with microglia and Lewis with brown Norway rats

IFN and virus-inducible expression of an immediate early gene, crg-2/IP-lO, and a delayed gene, I-Ao~ in astrocytes and microglia

Molecular cloning, functional characterization and mRNA expression analysis of the murine chemokine receptor CCR6 and its specific ligand MIP-3a

Monocyte chemoattractant protein-1 (MCP-1) expression in human articular cartilage. Induction by peptide regulatory factors and differential effects of dexamethasone and retinoic acid

Pathogenesis of demyelination induced by a mouse hepatitis virus (JHM virus)

HIV-1 Tat induces monocyte chemoattractant protein-l-mediated monocyte transmigration across a model of the human blood-brain barrier and up-regulates CCR5 expression on human monocytes

Binding and functional properties of recombinant and endogenous CXCR3 chemokine receptors

Chemokine receptor expression on resident and inflammatory cells in the brain of macaques with simian immunodeficiency virus encephalitis

Rat LCRI: cloning and cellular distribution of a putative chemokine receptor in brain

Neuroaxonal dystrophy in HTLV-l-associated myelopathy/tropical spastic paraparesis: neuropathologic and neuroimmunologic correlations

CCR5 levels and expression pattern correlate with infectability by macrophage-tropic HIV-1, in vitro

Immunohistochemical study of the beta-chemokine receptors CCR3 and CCR5 and their ligands in normal and Alzheimer's disease brains

Molecular cloning of a novel human CC chemokine EBIl-ligand chemokine that is a specific functional ligand for EBI1, CCR7

Identification of single C motif-lflymphotactin receptor XCR1

Molecular cloning of a novel C-)~ or ~/-type chemokine, SCM-1

Cutting edge: identification of the orphan chemokine receptor GPR-9-6 as CCR9, the receptor for the chemokine TECK

Lymphotropic virions affect chemokine receptor-mediated neural signaling and apoptosis: implications for human immunodeficiency virus type 1-associated dementia

Intracellular CXCR4 signaling, neuronal apoptosis and neuropathogenic mechanisms of HIV-l-associated dementia

The chemokine receptor CCR8 is preferentially expressed in Th2 but not Thl cells

Recent advances in chemokines and chemokine receptors

Function of the chemokine receptor CXCR4 in haematopoiesis and in cerebellar development