Reciprocal desensitization of CCR5 and CD4 is mediated by IL-16 and macrophage-inflammatory protein-1 beta, respectively.

The ability of HIV-1 gp120 to inhibit chemokine signaling prompted us to determine whether signaling through CD4 by a natural ligand, IL-16, could alter cellular responsiveness to chemokine stimulation. These studies demonstrate that IL-16/CD4 signaling in T lymphocytes results in a selective loss of macrophage-inflammatory protein (MIP)-1 beta/CCR5-induced chemotaxis. There was no effect on monocyte chemoattractant protein-2/CCR1, -2, or -3-induced chemotaxis. Desensitization of CCR5 by IL-16 required at least 10 min of pretreatment; no modulation of CCR5 expression was observed, nor was MIP-1 beta binding to CCR5 altered. Using murine T cell hybridomas transfected to express native or mutated forms of CD4, it was determined that IL-16/CD4 induces a p56lck-dependent signal that results in desensitization of CCR5. The desensitization process is reciprocal and again selective, as prior CCR5 stimulation, but not CCR1, -2, or -3 stimulation, completely inhibits IL-16/CD4-induced T cell migration. Of interest, while p56lck enzymatic activity is not required for IL-16-induced migration, it was required for desensitization of CCR5. These studies indicate the existence of reciprocal receptor cross-desensitization between CD4 and CCR5 induced by two proinflammatory cytokines and suggest a selective relationship between the two receptors.     

The Toll-like receptor 5 stimulus bacterial flagellin induces maturation and chemokine production in human dendritic cells

Toll-like receptors (TLRs) are pattern recognition receptors that serve an important function in detecting pathogens and initiating inflammatory responses. Upon encounter with foreign Ag, dendritic cells (DCs) go through a maturation process characterized by an increase in surface expression of MHC class II and costimulatory molecules, which leads to initiation of an effective immune response in naive T cells. The innate immune response to bacterial flagellin is mediated by TLR5, which is expressed on human DCs. Therefore, we sought to investigate whether flagellin could induce DC maturation. Immature DCs were cultured in the absence or presence of flagellin and monitored for expression of cell surface maturation markers. Stimulation with flagellin induced increased surface expression of CD83, CD80, CD86, MHC class II, and the lymph node-homing chemokine receptor CCR7. Flagellin stimulated the expression of chemokines active on neutrophils (IL-8/CXC chemokine ligand (CXCL)8, GRO-alpha/CXCL1, GRO-beta/CXCL2, GRO-gamma/CXCL3), monocytes (monocyte chemoattractant protein-1/CC chemokine ligand (CCL)2), and immature DCs (macrophage-inflammatory protein-1 alpha/CCL3, macrophage-inflammatory protein-1 beta/CCL4), but not chemokines active on effector T cells (IFN-inducible protein-10 kDa/CXCL10, monokine induced by IFN-gamma/CXCL9, IFN-inducible T cell alpha chemoattractant/CXCL11). However, stimulating DCs with both flagellin and IFN-inducible protein-10 kDa, monokine induced by IFN-gamma, and IFN-inducible T cell alpha chemoattractant expression, whereas stimulation with IFN-beta or flagellin alone failed to induce these chemokines. In functional assays, flagellin-matured DCs displayed enhanced T cell stimulatory activity with a concomitant decrease in endocytic activity. Finally, DCs isolated from mouse spleens or bone marrows were shown to not express TLR5 and were not responsive to flagellin stimulation. These results demonstrate that flagellin can directly stimulate human but not murine DC maturation, providing an additional mechanism by which motile bacteria can initiate an acquired immune response.     

Chemokine receptor expression and function in CD4+ T lymphocytes with regulatory activity

We have investigated the chemokine receptor expression and migratory behavior of a new subset of nickel-specific skin-homing regulatory CD4(+) T cells (Th(IL-10)) releasing high levels of IL-10, low IFN-gamma, and undetectable IL-4. These cells inhibit in a IL-10-dependent manner the capacity of dendritic cells to activate nickel-specific Tc1 and Th1 lymphocytes. RNase protection assay and FACS analysis revealed the expression of a vast repertoire of chemokine receptors on resting Th(IL-10), including the Th1-associated CXCR3 and CCR5, and the Th2-associated CCR3, CCR4, and CCR8, the latter at higher levels compared with Th2 cells. The most active chemokines for resting Th(IL-10), in terms of calcium mobilization and in vitro migration, were in order of potency: CCL2 (monocyte chemoattractant protein-1, CCR2 ligand), CCL4 (macrophage-inflammatory protein-1beta, CCR5 ligand), CCL3 (macrophage-inflammatory protein-1alpha, CCR1/5 ligand), CCL17 (thymus and activation-regulated chemokine, CCR4 ligand), CCL1 (I-309, CCR8 ligand), CXCL12 (stromal-derived factor-1, CXCR4), and CCL11 (eotaxin, CCR3 ligand). Consistent with receptor expression down-regulation, activated Th(IL-10) exhibited a reduced or absent response to most chemokines, but retained a significant migratory capacity to I-309, monocyte chemoattractant protein-1, and thymus and activation-regulated chemokine. I-309, which was ineffective on Th1 lymphocytes, attracted more efficiently Th(IL-10) than Th2 cells. I-309 and CCR8 mRNAs were not detected in unaffected skin and were up-regulated at the skin site of nickel-allergic reaction, with an earlier expression kinetics compared with IL-10 and IL-4. Results indicate that skin-homing regulatory Th(IL-10) lymphocytes coexpress functional Th1- and Th2-associated chemokine receptors, and that CCR8/I-309-driven recruitment of both resting and activated Th(IL-10) cells may be critically involved in the regulation of Th1-mediated skin allergic disorders.     

HIV-1 Nef induces dendritic cell differentiation: a possible mechanism of uninfected CD4(+) T cell activation

Human immunodeficiency virus (HIV)-1 Nef protein is an essential modulator of AIDS pathogenesis and we have previously demonstrated that rNef enters uninfected human monocytes and induces T cells bystander activation, up-regulating IL-15 production. Since dendritic cells (DCs) play a central role in HIV-1 primary infection we investigated whether rNef affects DCs phenotypic and functional maturation in order to define its role in the immunopathogenesis of AIDS. We found that rNef up-regulates the expression on immature DCs of surface molecules known to be critical for their APC function. These molecules include CD1a, HLA-DR, CD40, CD83, CXCR4, and to a lower extent CD80 and CD86. On the other hand, rNef down-regulates surface expression of HLA-ABC and mannose receptor. The functional consequence of rNef treatment of immature DCs is a decrease in their endocytic and phagocytic activities and an increase in cytokine (IL-1beta, IL-12, IL-15, TNF-alpha) and chemokine (MIP-1alpha, MIP-1beta, IL-8) production as well as in their stimulatory capacity. These results indicate that rNef induces a coordinate series of phenotypic and functional changes promoting DC differentiation and making them more competent APCs. Indeed, Nef induces CD4(+) T cell bystander activation by a novel mechanism involving DCs, thus promoting virus dissemination.     

TNF-alpha-induced secretion of C-C chemokines modulates C-C chemokine receptor 5 expression on peripheral blood lymphocytes

Peripheral blood lymphocytes express CCR5, a chemokine receptor for immune cell migration and calcium signaling that serves as an important coreceptor for the HIV. After in vitro stimulation, CCR5 expression is dramatically increased on mature T lymphocytes, especially on the CD45RO+ memory subset. In this study, we report that TNF-alpha delays the surface expression of CCR5 on PBLs after activation and diminishes CCR5 irrespective of its initial level. Functional loss of CCR5 is reflected in a decreased capability of the treated cells to migrate and signal calcium after MIP-1beta stimulation. The effect is mediated via the p80 type II TNF receptor (TNFR2), which induces NF-kappaB among other factors, leading to an enhanced secretion of the chemokines macrophage-inflammatory protein-1alpha, macrophage-inflammatory protein-1beta, and RANTES. Expression of these chemokines directly down-regulates CCR5. These findings reveal a new regulatory mechanism utilized by activated peripheral T cells to modulate their chemotaxis and potentially other functions mediated by CCR5, including the infection of T lymphocytes by macrophage-tropic HIV strains.     

Mediators of innate immunity that target immature, but not mature, dendritic cells induce antitumor immunity when genetically fused with nonimmunogenic tumor antigens.

Chemokine receptors are differentially expressed on immature and mature dendritic cells (DC). Herein, we demonstrate for the first time that murine antimicrobial peptides beta-defensins 2 and 3 bind murine CCR6, similarly to inflammatory chemokine macrophage-inflammatory protein 3alpha, and they chemoattract bone marrow-derived immature, but not mature DC. Using various chemokines or defensins fused with nonimmunogenic tumor Ags, we studied their capacity to delivery Ags to subsets of immune cells to elicit antitumor immunity. We demonstrate that DNA immunizations with fusion constructs with beta-defensin 2 or inflammatory chemokines that target immature DC, but not homeostatic chemokines secondary lymphoid tissue chemokine, CCL21, or stromal cell-derived factor 1, CXCL12, which chemoattract mature DC, elicit humoral, protective, and therapeutic immunity against two different syngeneic lymphomas.     

Dendritic cells express CCR7 and migrate in response to CCL19 (MIP-3beta) after exposure to Helicobacter pylori

Helicobacter pylori infection induces chronic inflammation in the gastric mucosa with a marked increase in the number of lymphoid follicles consisting of infiltrating B and T cells, neutrophils, dendritic cells (DC) and macrophages. It has been suggested that an accumulation of mature DC in the tissue, resulting from a failure of DC to migrate to lymph nodes, may contribute to this chronic inflammation. Migration of DC to lymph nodes is regulated by chemokine receptor CCR7, expressed on mature DC, and the CCR7 ligands CCL19 and CCL21. In this study we analysed the maturation, in vitro migration and cytokine production of human DC after stimulation with live H. pylori. For comparison, DC responses to non-pathogenic Escherichia coli bacteria were also evaluated. Stimulation with H. pylori induced maturation of DC, i.e. up-regulation of the chemokine receptors CCR7 and CXCR4 and the maturation markers HLA-DR, CD80 and CD86. The H. pylori-stimulated DC also induced CD4(+) T-cell proliferation. DC stimulated with H. pylori secreted significantly more interleukin (IL)-12 compared to DC stimulated with E. coli, while E. coli-stimulated DC secreted more IL-10. Despite low surface expression of CCR7 protein following stimulation with H. pylori compared to E. coli, the DC migrated equally well towards CCL19 after stimulation with both bacteria. Thus, we could not detect any failure in the migration of H. pylori stimulated DC in vitro that may contribute to chronic gastritis in vivo, and our results suggest that H. pylori induces maturation and migration of DC to lymph nodes where they promote T cell responses.     

Up-regulation of CCR5 and CCR6 on distinct subpopulations of antigen-activated CD4+ T lymphocytes.

Following infection, naive T cells are activated in the secondary lymphoid tissue, but then need to move to the infected tissue in the periphery to mediate their effector functions. The acquisition of inflammatory chemokine receptors, such as CCR5 and CCR6, may contribute to the efficient relocation of activated T cells to inflamed sites in the periphery. In keeping with this idea, the present study has demonstrated that CCR5 and CCR6 are up-regulated on CD4+ T cells upon activation in the MLR. The observed increase in expression correlated well with the acquisition of an activated/memory phenotype and was largely (CCR5) or completely (CCR6) separated temporally from the initiation of cell division. In contrast, the regulation of two other chemokine receptors, CXCR3 and CXCR4, occurred in close parallel with the cell division process. Increased mRNA levels are likely to contribute to the enhanced surface expression of CCR5 and CCR6, but in the case of CCR6, translocation of intracellular stores of protein to the cell surface may be an additional mechanism of regulation. The up-regulation of CCR5 was more extensive than that of CCR6, as only approximately half the activated CCR5+ T cells coexpressed CCR6. The increased expression of CCR5 resulted in enhanced chemotaxis toward the CCR5 ligand macrophage-inflammatory protein-1beta/CCL4, but up-regulation of CCR6 did not result in altered chemotactic responsiveness to macrophage-inflammatory protein-3alpha/CCL20, suggesting an alternative function for this receptor.     

IL-10 induces CCR6 expression during Langerhans cell development while IL-4 and IFN-gamma suppress it.

Immune responses are initiated by dendritic cells (DC) that form a network comprising different populations. In particular, Langerhans cells (LC) appear as a unique population of cells colonizing epithelial surfaces. We have recently shown that macrophage-inflammatory protein-3alpha/CCL20, a chemokine secreted by epithelial cells, induces the selective migration of LC among DC populations. In this study, we investigated the effects of cytokines on the expression of the CCL20 receptor, CCR6, during differentiation of LC. We found that both IL-4 and IFN-gamma blocked the expression of CCR6 and CCL20 responsiveness at different stages of LC development. The effect of IL-4 was reversible and most likely due to the transient blockade of LC differentiation. In contrast, IFN-gamma-induced CCR6 loss was irreversible and was concomitant to the induction of DC maturation. When other cytokines involved in DC and T cell differentiation were tested, we found that IL-10, unlike IL-4 and IFN-gamma, maintained CCR6 expression. The effect of IL-10 was reversible and upon IL-10 withdrawn, CCR6 was lost concomitantly to final LC differentiation. In addition, IL-10 induced the expression of CCR6 and responsiveness to CCL20 in differentiated monocytes that preserve their ability to differentiate into mature DC. Finally, TGF-beta, which induces LC differentiation, did not alter early CCR6 expression, but triggered its irreversible down-regulation, in parallel to terminal LC differentiation. Taken together, these results suggest that the recruitment of LC at epithelial surface might be suppressed during Th1 and Th2 immune responses, and amplified during regulatory immune responses involving IL-10 and TGF-beta.     

Reduced chemokine and chemokine receptor expression in spinal cords of TCR BV8S2 transgenic mice protected against experimental autoimmune encephalomyelitis with BV8S2 protein

The perivascular transmigration and accumulation of macrophages and T lymphocytes in the CNS of mice with experimental autoimmune encephalomyelitis (EAE) may be partly regulated by low m.w. chemotactic cytokines. Using the RNase protection assay and ELISA, we quantified expression of chemokines and chemokine receptors in the spinal cord (SC), brain, and lymph nodes of BV8S2 transgenic mice that developed or were protected from EAE by vaccination with BV8S2 protein. In paralyzed control mice, the SC had increased cellular infiltration and strong expression of the chemokines RANTES, IFN-inducible 10-kDa protein, and monocyte chemoattractant protein-1 and the cognate chemokine receptors CCR1, CCR2, and CCR5, with lower expression of macrophage-inflammatory protein (MIP)-1alpha, MIP-1beta, and MIP-2; whereas brain had less infiltration and a lower expression of a different pattern of chemokines and receptors. In TCR-protected mice, there was a decrease in the number of inflammatory cells in both SC and brain. In SC, the reduced cellular infiltrate afforded by TCR vaccination was commensurate with profoundly reduced expression of chemokines and their cognate chemokine receptors. In brain, however, TCR vaccination did not produce significant changes in chemokine expression but resulted in an increased expression of CCR3 and CCR4 usually associated with Th2 cells. In contrast to CNS, lymph nodes of protected mice had a significant increase in expression of MIP-2 and MIP-1beta but no change in expression of chemokine receptors. These results demonstrate that TCR vaccination results in selective reduction of inflammatory chemokines and chemokine receptors in SC, the target organ most affected during EAE.     

Semimature stage: a checkpoint in a dendritic cell maturation program that allows for functional reversion after signal-regulatory protein-alpha ligation and maturation signals

CD47 on live cells actively engages signal-regulatory protein-alpha (SIRP-alpha) on phagocytes and delivers a negative signal that prevents their elimination. We evaluated the biological consequences of SIRP-alpha ligation on the dendritic cell (DC) response to maturation signals and the potential interplay with the IL-10/IL-10R inhibitory pathway. At first, CD47/SIRP-alpha allowed the generation of mature migratory DCs not producing IL-12, IFN-gamma-inducible protein-10, and CCL19. Rather, they secreted neutrophils attracting chemokine CXCL5 and IL-1beta, reflecting a partial block in functional DC maturation. Afterward, semimature DCs functionally regressed in an IL-10-independent fashion toward cells that retrieved the cardinal features of immature DCs: re-expression of CCR5, loss of DC-lysosome-associated membrane protein, high endocytosis, and impaired allostimulatory functions. The global gene expression profile of IL-10 and SIRP-alpha-ligated DC demonstrated two distinct molecular pathways. IL-10R and SIRP-alpha expression were reciprocally down-regulated by CD47 and IL-10, respectively. These results emphasize that the SIRP-alpha pathway might be part of the molecular machinery used by the DC to dampen or resolve an inflammatory response in an IL-10-independent manner.     

Toxoplasma gondii: comparison of human CD34+ and monocyte-derived dendritic cells after parasite infection

Human dendritic cells (DC) obtained in vitro from CD34(+) progenitors (CD34-DC) or blood monocytes (mo-DC) are different DC which may be used in a model of T. gondii infection. We compared the survival, infection rate and cell surface receptor expression of both DC types after living T. gondii tachyzoite infection. CD34-DC appeared less resistant to the parasite than mo-DC. At 48h post-infection, chemokine receptors responsible for DC homing and migration were absent in mo-DC, while down regulation of CCR6 and up regulation of CCR7 was observed in CD34-DC. This result, suggesting migration ability of CD34-DC, was confirmed by in vitro migration experiments against different chemokines. Tachyzoite supernatant, used as chemokine, attracted immature CD34-DC as observed by MIP3alpha, while MIP3beta, as expected, attracted mature CD34-DC. Under similar conditions, no significant difference was noticed between mature or immature mo-DC. These data indicated that CD34-DC represent an alternative model that allows migration assay of infected DC by T. gondii.     

Lipopolysaccharide inhibits HIV-1 infection of monocyte- derived macrophages through direct and sustained down-regulation of CC chemokine receptor 5

It is now well established that HIV-1 requires interactions with both CD4 and a chemokine receptor on the host cell surface for efficient infection. The expression of the CCR5 chemokine receptor in human macrophages facilitates HIV-1 entry into these cells, which are considered important in HIV pathogenesis not only as viral reservoirs but also as modulators of altered inflammatory function in HIV disease and AIDS. LPS, a principal constituent of Gram-negative bacterial cell walls, is a potent stimulator of macrophages and has been shown to inhibit HIV infection in this population. We now present evidence that one mechanism by which LPS mediates its inhibitory effect on HIV-1 infection is through a direct and unusually sustained down-regulation of cell-surface CCR5 expression. This LPS-mediated down-regulation of CCR5 expression was independent of de novo protein synthesis and differed from the rapid turnover of these chemokine receptors observed in response to two natural ligands, macrophage-inflammatory protein-1alpha and -1beta. LPS did not act by down-regulating CCR5 mRNA (mRNA levels actually increased slightly after LPS treatment) or by enhancing the degradation of internalized receptor. Rather, the observed failure of LPS-treated macrophages to rapidly restore CCR5 expression at the cell-surface appeared to result from altered recycling of chemokine receptors. Taken together, our results suggest a novel pathway of CCR5 recycling in LPS-stimulated human macrophages that might be targeted to control HIV-1 infection.     

Histamine polarizes human dendritic cells into Th2 cell-promoting effector dendritic cells

Allergic disorders are characterized by allergen-specific Th2-biased responses. Signals controlling Th2 cell polarization, especially those acting by polarizing dendritic cells (DC) into Th2-promoting DC (DC2), are not well known. Histamine, a mediator released by allergen-stimulated mast cells from allergic subjects, has been reported to activate human immature DC. We have therefore tested whether histamine affects DC polarization. We report here that histamine inhibits LPS-induced IL-12 production and polarizes uncommitted maturing DC into effector DC2. DC matured in the presence of histamine fail to produce IL-12 upon subsequent stimulation and prime Th2 responses, even in presence of IFN-gamma, a potent DC1-driving factor. All these effects are mediated through both H1 and H2 receptors. These data show that histamine is a potent DC2-polarizing factor and provide evidence for a novel mechanism that explains the initiation and maintenance of a predominant Th2 response in allergic disorders.     

uPA/uPAR system is active in immature dendritic cells derived from CD14+CD34+ precursors and is down-regulated upon maturation

We recently described a subset of peripheral CD14+CD34+ cells able to migrate across endothelial cell monolayers and differentiate into immunostimulatory dendritic cells (DC). In this paper we show that immature DC derived from CD14+CD34+ precursors are also capable of reverse transendothelial migration and extracellular matrix (ECM) invasion using the urokinase plasminogen activator receptor (uPAR). We found that these cells respond to macrophage-inflammatory protein (MIP)-1alpha, enhancing their ability to invade ECM and supporting the idea that immature DC are selectively recruited at the site of inflammation to expand the pool of APCs. Interestingly, MIP-1alpha was also capable of preventing the decreased matrix invasion observed by blocking uPAR, suggesting that the uPA/uPAR system and MIP-1alpha cooperate in driving immature DC migration through the subendothelial matrix. Upon exposure to maturating stimuli, such as TNF-alpha, CD14+CD34+-derived DC enhance their APC function and decrease the capacity of invading ECM; these changes are accompanied by altered expression and function of uPAR. Moreover, mature DC shift their sensitivity from MIP-1alpha to MIP-3beta, enhancing their transendothelial migration capability in response to the latter chemokine. Our data support the hypothesis that bloodborne DC can move through ECM toward the site of pathogen entry where they differentiate into fully mature APCs with their motility and function regulated by microenvironmental stimuli, including MIP-1alpha, MIP-3beta, and TNF-alpha.     

Anatomic localization of immature and mature dendritic cells in an ectopic lymphoid organ: correlation with selective chemokine expression in rheumatoid synovium

It remains to be clarified whether dendritic cells (DC) reach the rheumatoid arthritis (RA) synovium, considered an ectopic lymphoid organ, as mature cells or undergo local maturation. We characterized by immunohistochemistry the DC subsets and used tonsils as a control. Immature and mature DC were defined by CD1a and DC-lysosome-associated membrane protein/CD83 expression, respectively. Immature DC were mainly detected in the lining layer in RA synovium. Mature DC were exclusively detected in the lymphocytic infiltrates. The DC-lysosome-associated membrane protein/CD1a ratio was 1.1 in RA synovium and 5.3 in tonsils, suggesting the relative accumulation of immature DC in RA synovium. We then focused on the expression of CCL20/CCR6 and CCL19/CCR7, CCL21/CCR7 chemokine/receptor complex, which control immature and mature DC migration respectively. A close association was observed between CCL20-producing cells and CD1a(+) cells, suggesting the contribution of CCL20 to CCR6(+) cell homing. Conversely, CCL21 and CCL19 expression was only detected in perivascular infiltrates. The association among CCL19/21-producing cells, CCR7 expression, and mature DC accumulation is in line with the roles of these chemokines in mature CCR7(+) DC homing to lymphocytic infiltrates. The role of DC in disease initiation and perpetuation makes chemokines involved in DC migration a potential therapeutic target.     

Mature dendritic cells express functional thrombin receptors triggering chemotaxis and CCL18/pulmonary and activation-regulated chemokine induction

Protease-activated receptors (PARs) are a family of G protein-coupled receptors that are activated by serine protease-mediated proteolytic cleavage of their extracellular domain. We have previously characterized the expression and function of PARs in human monocytes and macrophages, yet information about PARs in dendritic cells (DC) is scarce. Monocyte-derived immature DC do not express PARs. Upon maturation with LPS, but not with TNF-alpha or CD40 ligand, DC express PAR1 and PAR3, but not PAR2 or PAR4. Stimulation of DC with the serine protease thrombin or PAR1-activating peptide elicits actin polymerization and concentration-dependent chemotactic responses in LPS-, but not in TNF-alpha-matured DC. The thrombin-induced migration is a true chemotaxis with only negligible chemokinesis. Stimulation of PARs with thrombin or the respective receptor-activating peptides activates ERK1/2 and Rho kinase as well as subsequent phosphorylation of the regulatory myosin L chain 2. The ERK1/2- and Rho kinase 1-mediated phosphorylation of myosin L chain 2 was indispensable for the PAR-mediated chemotaxis as shown by pharmacological inhibitors. Additionally, thrombin stimulated the Rho-dependent release of the CC chemokine CCL18/pulmonary and activation-regulated chemokine, which induces chemotaxis of lymphocytes and immature DC as well as fibroblast proliferation. The colocalization of CD83(+) DC with CCL18 in human atherosclerotic plaques revealed by immunofluorescence microscopy combined with the presence of functionally active thrombin receptors on mature DC point to a previously unrecognized functional role of thrombin in DC biology. The thrombin-induced stimulation of mature DC may be of particular relevance in atherosclerotic lesions, which harbor all components of this novel mechanism.     

The primate lentiviral receptor Bonzo/STRL33 is coordinately regulated with CCR5 and its expression pattern is conserved between human and mouse

Chemokines play necessary and important roles in regulating the trafficking of lymphocytes to intra- or interlymphoid tissues as well as to sites of inflammation. The complex migratory patterns of lymphoid lineage cells is governed by subset-specific expression of chemokine receptors and their access to specific ligands. Several chemokine receptors and chemokine receptor-like orphan receptors also serve, in conjunction with CD4, as coreceptors for infection by human and simian immunodeficiency viruses (HIV and SIV). Here we show that the expression pattern of Bonzo/STRL33, an orphan SIV/HIV coreceptor, is highly restricted to the memory subset of T cells and is up-regulated upon stimulation of these cells with IL-2 or IL-15. Both the pattern and the regulation of Bonzo expression closely paralleled that of CC family chemokine receptors CCR5 or CCR6 and inversely correlated with CXCR4 expression. However, in striking contrast to CCR5, Bonzo expression was not down-modulated by PMA or mitogen stimulation of T cells. Targeted replacement of the Bonzo gene with a gene encoding green fluorescent protein in mice revealed that the expression and cytokine regulation of mouse Bonzo are comparable to those of its human counterpart. The similar expression and regulation patterns of Bonzo and the HIV coreceptor CCR5 may have implications for understanding the role of HIV/SIV receptors in viral evolution and pathogenesis.