Signal transduction due to HIV-1 envelope interactions with chemokine receptors CXCR4 or CCR5

HIV-1 envelope is a neutral antagonist to CXCR4 in T-cells

The chemokine receptor CXCR4 is the principal coreceptor for X4 strains of HIV-1. We show that gp120 is unable to induce interactions between CXCR4 and G-protein in T-cells, but antagonized the agonist effect of SDF-1alpha, the natural ligand for CXCR4. Gp120 had ten times lower affinity for CXCR4 than CD4, implying that a substantial role for cellular CD4 may be to facilitate binding of the viral envelope to CXCR4. Binding of gp120 to CXCR4 was neither regulated by guanine nucleotides, nor affected by divalent cations, was temperature independent and bound to a homogenous population of CXCR4, which is characteristic for an antagonist to a G-protein coupled receptor. In contrast, SDF-1alpha binds to two affinity states of CXCR4 in T-cell membranes, which are modulated by guanine nucleotides. Binding of SDF-1alpha to CXCR4 was highly temperature dependent. Thus, the interaction of CXCR4 with HIV-1 viral envelope and chemokine exhibits fundamental differences.

Viral exploitation and subversion of the immune system through chemokine mimicry

The chemokine superfamily of leukocyte chemoattractants coordinates development and deployment of the immune system by signaling through a family of G protein-coupled receptors. The importance of this system to antimicrobial host defense has been supported by the discovery of numerous herpesviruses and poxviruses that encode chemokine mimics able to block chemokine action. However, specific herpesviruses and lentiviruses can also exploit the immune system through chemokine mimicry, for example, to facilitate viral dissemination or, as in the case of HIV-1, to directly infect leukocyte target cells. The study of viral mimicry of chemokines and chemokine receptors is providing important new concepts in viral immunopathogenesis, new anti-inflammatory drug leads and new targets and concepts for antiviral drug and vaccine development.

Signaling events following chemokine receptor ligation in human dendritic cells at different developmental stages

Responsiveness of dendritic cells (DC) to inflammatory CC chemokines is down-regulated during their maturation. We analyzed the mechanism underlying these events. Cell-surface expression of CC chemokine receptor (CCR)-1, -3 and -5 was increased during differentiation of immature DC (iDC) from monocytes. In contrast, these expressions were decreased during development of iDC into mature DC (mDC) to levels similar to those of monocytes. Transcriptional expression of CCR-1, -3 and -5 was increased during differentiation of iDC from monocytes, while the expression was decreased during development of iDC into mDC. Expression of CCR-7 transcript was detected in mDC, but not in monocytes or iDC. Both monocytes and iDC, but not mDC, migrated in response to inflammatory CC chemokines such as regulated on activation normal T cell expressed and secreted (RANTES)/CCL5, whereas mDC, but not monocytes or iDC, migrated to macrophage inflammatory protein (MIP)-3ss/CCL19. Receptor engagement of monocytes or iDC by RANTES (for CCR-1, -3 and -5) resulted in protein tyrosine phosphorylation events including activation of focal adhesion kinase as well as mitogen-activated protein kinase, whereas this stimulation induced little activation of these molecular events in mDC when compared with monocytes or iDC. On the other hand, stimulation with MIP-3ss (for CCR-7) induced tyrosine phosphorylation events in mDC, but not in monocytes or iDC. These results suggest that the down-regulation of cell-surface expression of CCR and of their downstream signaling events may be involved in the reduced chemotaxis of DC to inflammatory CC chemokines during their maturation.

SHP2 and cbl participate in alpha-chemokine receptor CXCR4-mediated signaling pathways

Stromal cell-derived factor (SDF)-1alpha and its receptor, CXCR4, play an important role in cell migration, embryonic development, and human immunodeficiency virus infection. However, the cellular signaling pathways that mediate these processes are not fully elucidated. We and others have shown that the binding of SDF-1alpha to CXCR4 activates phosphatidylinositol-3 kinase (PI-3 kinase), p44/42 mitogen-associated protein kinase, and the transcription factor nuclear factor-kappaB, and it also enhances the tyrosine phosphorylation and association of proteins involved in the formation of focal adhesions. In this study, we examined the role of phosphatases in CXCR4-mediated signaling pathways. We observed significant inhibition of SDF-1alpha-induced migration by phosphatase inhibitors in CXCR4-transfected pre-B lymphoma L1.2 cells, Jurkat T cells, and peripheral blood lymphocytes. Further studies revealed that SDF-1alpha stimulation induced robust tyrosine phosphorylation in the SH2-containing phosphatase SHP2. SHP2 associated with the CXCR4 receptor and the signaling molecules SHIP, cbl, and fyn. Overexpression of wild-type SHP2 increased SDF-1alpha-induced chemotaxis. Enhanced activation of fyn and lyn kinases and the tyrosine phosphorylation of cbl were also observed. In addition, SDF-1alpha stimulation enhanced the association of cbl with PI-3 kinase, Crk-L, and 14-3-3beta proteins. Our results suggest that CXCR4-mediated signaling is regulated by SHP2 and cbl, which collectively participate in the formation of a multimeric signaling complex.

Dancing to the tune of chemokines

Since their discovery 13 years ago, chemokines have emerged as the most important regulators of leukocyte trafficking. On target cells, chemokines bind to seven-transmembrane-domain receptors that are coupled to heterotrimeric Gi proteins. The common response of all cells to chemokine stimulation is chemotaxis. In addition, leukocyte activation triggers diverse signal transduction cascades; which cascade is triggered depends on the chemokine and receptor engaged. The selective activation of distinct pathways suggests that the receptors couple not only to G proteins but also to additional downstream effectors. This review discusses recent advances in the elucidation of the signal transduction that occurs in proximity to receptors and that leads to the early biochemical events in leukocyte activation.

In vitro induction of HIV-1 replication in resting CD4(+) T cells derived from individuals with undetectable plasma viremia upon stimulation with human T-cell leukemia virus type I

Microbial coinfections have been associated with transient bursts of human immunodeficiency virus (HIV) viremia in patients. In this study we investigated whether human T-cell leukemia virus type I (HTLV-I), another human retrovirus that is prevalent among certain HIV-infected populations, can induce HIV-1 replication in patients who had been successfully treated with highly active antiretroviral therapy. We demonstrate that supernatants from HTLV-I-producing MT-2 cells can induce in vitro replication of HIV-1 from highly purified, resting CD4(+) T cells obtained from individuals with undetectable plasma viremia. Depletion of proinflammatory cytokines from the supernatants reduced, but did not abrogate, the ability to induce HIV-1 replication, indicating that other factors such as HTLV-I Tax or Env also have a role. The HTLV-I-mediated effect does not require productive infection: exposure to heat-inactivated HTLV-I virions, purified Tax protein, or HTLV-I Env glycoprotein also induced expression of HIV-1. Furthermore, we demonstrate that coculture of resting CD4(+) T cells with autologous CD8(+) T cells markedly inhibits the HTLV-I-induced virus replication. Our results suggest that coinfection with HTLV-I may induce viral replication in the latent viral reservoirs; however, CD8(+) T cells may play an important role in controlling the spread of virus upon microbial stimulation.

Functional expression of CCR1, CCR3, CCR4, and CXCR4 chemokine receptors on human platelets

Platelets are known to contain platelet factor 4 and β-thromboglobulin, α-chemokines containing the CXC motif, but recent studies extended the range to the β-family characterized by the CC motif, including RANTES and Gro-α. There is also evidence for expression of chemokine receptors CCR4 and CXCR4 in platelets. This study shows that platelets have functional CCR1, CCR3, CCR4, and CXCR4 chemokine receptors. Polymerase chain reaction detected chemokine receptor messenger RNA in platelet RNA. CCR1, CCR3, and especially CCR4 gave strong signals; CXCR1 and CXCR4 were weakly positive. Flow cytometry with specific antibodies showed the presence of a clear signal for CXCR4 and weak signals for CCR1 and CCR3, whereas CXCR1, CXCR2, CXCR3, and CCR5 were all negative. Immunoprecipitation and Western blotting with polyclonal antibodies to cytoplasmic peptides clearly showed the presence of CCR1 and CCR4 in platelets in amounts comparable to monocytes and CCR4 transfected cells, respectively. Chemokines specific for these receptors, including monocyte chemotactic protein 1, macrophage inflammatory peptide 1α, eotaxin, RANTES, TARC, macrophage-derived chemokine, and stromal cell–derived factor 1, activate platelets to give Ca++ signals, aggregation, and release of granule contents. Platelet aggregation was dependent on release of adenosine diphosphate (ADP) and its interaction with platelet ADP receptors. Part, but not all, of the Ca++ signal was due to ADP release feeding back to its receptors. Platelet activation also involved heparan or chondroitin sulfate associated with the platelet surface and was inhibited by cleavage of these glycosaminoglycans or by heparin or low molecular weight heparin. These platelet receptors may be involved in inflammatory or allergic responses or in platelet activation in human immunodeficiency virus infection.

Involvement of protein kinase C in HIV-1 gp120-induced apoptosis in primary endothelium

We previously showed that HIV-1 gp120-induced apoptosis in primary human umbilical vein endothelial cell cultures (HUVEC), through CCR5 and CXCR4. Here, we have found that agonists of protein kinase C (PKC), basic fibroblast growth factor (bFGF), and short exposure to low concentrations of phorbol esters were found to block gp120-induced apoptosis in HUVEC cultures. PKC antagonists, sphingosine, H7, and extended exposure of cultures to high concentrations of phorbol esters were also found to block gp120-induced apoptosis in HUVEC cultures. A significant increase in the total amount of cellular PKC enzymatic activity was observed on exposure of HUVEC to gp120. No increase in total PKC activity was observed on exposure of HUVECs to the natural ligands SDF-1alpha, or regulated-on-activation normal T-expressed and secreted (RANTES) cells, and gp120-induced PKC induction was found to be totally blocked by CXCR4 antibodies and partially blocked by the caspase 3 inhibitor, DEVD-CHO. Alternatively, CXCR4 antibodies and DEVD-CHO totally blocked apoptosis. Finally, gp120-induced effects were found to be insensitive to pertussis toxin. Accumulated evidence suggests PKC involvement at multiple points in the gp120-induced apoptotic pathway; also suggests involvement of the CXCR4 receptor internalization pathway, and potentially suggests different downstream effects of gp120-receptor interactions and natural ligand-receptor interactions.

Functional expression of CCR1, CCR3, CCR4, and CXCR4 chemokine receptors on human platelets

Platelets are known to contain platelet factor 4 and β-thromboglobulin, α-chemokines containing the CXC motif, but recent studies extended the range to the β-family characterized by the CC motif, including RANTES and Gro-α. There is also evidence for expression of chemokine receptors CCR4 and CXCR4 in platelets. This study shows that platelets have functional CCR1, CCR3, CCR4, and CXCR4 chemokine receptors. Polymerase chain reaction detected chemokine receptor messenger RNA in platelet RNA. CCR1, CCR3, and especially CCR4 gave strong signals; CXCR1 and CXCR4 were weakly positive. Flow cytometry with specific antibodies showed the presence of a clear signal for CXCR4 and weak signals for CCR1 and CCR3, whereas CXCR1, CXCR2, CXCR3, and CCR5 were all negative. Immunoprecipitation and Western blotting with polyclonal antibodies to cytoplasmic peptides clearly showed the presence of CCR1 and CCR4 in platelets in amounts comparable to monocytes and CCR4 transfected cells, respectively. Chemokines specific for these receptors, including monocyte chemotactic protein 1, macrophage inflammatory peptide 1α, eotaxin, RANTES, TARC, macrophage-derived chemokine, and stromal cell–derived factor 1, activate platelets to give Ca++ signals, aggregation, and release of granule contents. Platelet aggregation was dependent on release of adenosine diphosphate (ADP) and its interaction with platelet ADP receptors. Part, but not all, of the Ca++ signal was due to ADP release feeding back to its receptors. Platelet activation also involved heparan or chondroitin sulfate associated with the platelet surface and was inhibited by cleavage of these glycosaminoglycans or by heparin or low molecular weight heparin. These platelet receptors may be involved in inflammatory or allergic responses or in platelet activation in human immunodeficiency virus infection.

Combination of CCR5 and CXCR4 inhibitors in therapy of human immunodeficiency virus type 1 infection: in vitro studies of mixed virus infections

We studied the combined anti-human immunodeficiency virus type 1 (HIV-1) effects of a derivative of stroma-derived factor 1beta (SDF-1beta), Met-SDF-1beta, and a modified form of RANTES, aminooxypentane (AOP)-RANTES. The antiviral agents were tested singly or in combination at 95 and 99% virus inhibitory concentrations. Clinical R5 and X4 HIV-1 isolates were used. AOP-RANTES inhibited R5 but not X4 viruses, whereas Met-SDF-1beta had the opposite effect. Combinations of these compounds inhibited mixed infections with R5 and X4 viruses (95 to 99%), whereas single drugs were less inhibitory (32 to 61%). Combinations of R5 and X4 inhibitors are promising and deserve further evaluation.

Multiple nonfunctional alleles of CCR5 are frequent in various human populations

CCR5 is the major coreceptor for macrophage-tropic strains of the human immunodeficiency virus type I (HIV-1). Homozygotes for a 32-base pair (bp) deletion in the coding sequence of the receptor (CCR5Δ32) were found to be highly resistant to viral infection, and CCR5 became, therefore, one of the paradigms illustrating the influence of genetic variability onto individual susceptibility to infectious and other diseases. We investigated the functional consequences of 16 other natural CCR5 mutations described in various human populations. We found that 10 of these variants are efficiently expressed at the cell surface, bind [125I]-MIP-1β with affinities similar to wtCCR5, respond functionally to chemokines, and act as HIV-1 coreceptors. In addition to Δ32, six mutations were characterized by major alterations in their functional response to chemokines, as a consequence of intracellular trapping and poor expression at the cell surface (C101X, FS299), general or specific alteration of ligand binding affinities (C20S, C178R, A29S), or relative inability to mediate receptor activation (L55Q). A29S displayed an unusual pharmacological profile, binding and responding to MCP-2 similarly to wtCCR5, but exhibiting severely impaired binding and functional responses to MIP-1α, MIP-1β, and RANTES. In addition to Δ32, only C101X was totally unable to mediate entry of HIV-1. The fact that nonfunctional CCR5 alleles are relatively frequent in various human populations reinforces the hypothesis of a selective pressure favoring these alleles.

Multiple nonfunctional alleles of CCR5 are frequent in various human populations

CCR5 is the major coreceptor for macrophage-tropic strains of the human immunodeficiency virus type I (HIV-1). Homozygotes for a 32-base pair (bp) deletion in the coding sequence of the receptor (CCR5Δ32) were found to be highly resistant to viral infection, and CCR5 became, therefore, one of the paradigms illustrating the influence of genetic variability onto individual susceptibility to infectious and other diseases. We investigated the functional consequences of 16 other natural CCR5 mutations described in various human populations. We found that 10 of these variants are efficiently expressed at the cell surface, bind [125I]-MIP-1β with affinities similar to wtCCR5, respond functionally to chemokines, and act as HIV-1 coreceptors. In addition to Δ32, six mutations were characterized by major alterations in their functional response to chemokines, as a consequence of intracellular trapping and poor expression at the cell surface (C101X, FS299), general or specific alteration of ligand binding affinities (C20S, C178R, A29S), or relative inability to mediate receptor activation (L55Q). A29S displayed an unusual pharmacological profile, binding and responding to MCP-2 similarly to wtCCR5, but exhibiting severely impaired binding and functional responses to MIP-1α, MIP-1β, and RANTES. In addition to Δ32, only C101X was totally unable to mediate entry of HIV-1. The fact that nonfunctional CCR5 alleles are relatively frequent in various human populations reinforces the hypothesis of a selective pressure favoring these alleles.

Macrophage-tropic HIV induces and exploits dendritic cell chemotaxis

Immature dendritic cells (iDCs) express the CC chemokine receptor (CCR)5, which promotes chemotaxis toward the CC chemokines regulated on activation, normal T cell expressed and secreted (RANTES), macrophage inflammatory protein (MIP)-1alpha, and MIP-1beta. By contrast, mature DCs downregulate CCR5 but upregulate CXC chemokine receptor (CXCR)4, and as a result exhibit enhanced chemotaxis toward stromal cell-derived factor (SDF)-1alpha. CCR5 and CXCR4 also function as coreceptors for macrophage-tropic (M-tropic) and T cell-tropic (T-tropic) human immunodeficiency virus (HIV)-1, respectively. Here, we demonstrate chemotaxis of iDCs toward M-tropic (R5) but not T-tropic (X4) HIV-1. Furthermore, preexposure to M-tropic HIV-1 or its recombinant envelope protein prevents migration toward CCR5 ligands. The migration of iDCs toward M-tropic HIV-1 may enhance formation of DC-T cell syncytia, thus promoting viral production and destruction of both DC and T helper lymphocytes. Therefore, disturbance of DC chemotaxis by HIV-1 is likely to contribute to immunosuppression in primary infection and AIDS. In addition, migration of iDCs toward HIV-1 may aid the capture of R5 HIV-1 virions by the abundant DC cell surface protein DC-specific intercellular adhesion molecule (ICAM)3-grabbing nonintegrin (DC-SIGN). HIV-1 bound to DC cell-specific DC-SIGN retains the ability to infect replication-permissive T cells in trans for several days. Consequently, recruitment of DC by HIV-1 could combine with the ability of DC-SIGN to capture and transmit the virus to T cells, and so facilitate dissemination of virus within an infected individual.

Identification of residues of CXCR4 critical for human immunodeficiency virus coreceptor and chemokine receptor activities

CXCR4 is a G-coupled receptor for the stromal cell-derived factor (SDF-1) chemokine, and a CD4-associated human immunodeficiency virus type 1 (HIV-1) coreceptor. These functions were studied in a panel of CXCR4 mutants bearing deletions in the NH(2)-terminal extracellular domain (NT) or substitutions in the NT, the extracellular loops (ECL), or the transmembrane domains (TMs). The coreceptor activity of CXCR4 was markedly impaired by mutations of two Tyr residues in NT (Y7A/Y12A) or at a single Asp residue in ECL2 (D193A), ECL3 (D262A), or TMII (D97N). These acidic residues could engage electrostatical interactions with basic residues of the HIV-1 envelope protein gp120, known to contribute to the selectivity for CXCR4. The ability of CXCR4 mutants to bind SDF-1 and mediate cell signal was consistent with the two-site model of chemokine-receptor interaction. Site I involved in SDF-1 binding but not signaling was located in NT with particular importance of Glu(14) and/or Glu(15) and Tyr(21). Residues required for both SDF-1 binding and signaling, and thus probably part of site II, were identified in ECL2 (Asp(187)), TMII (Asp(97)), and TMVII (Glu(288)). The first residues () of NT also seem required for SDF-1 binding and signaling. A deletion in the third intracellular loop abolished signaling, probably by disrupting the coupling with G proteins. The identification of CXCR4 residues involved in the interaction with both SDF-1 and HIV-1 may account for the signaling activity of gp120 and has implications for the development of antiviral compounds.

Immediate and neurotoxic effects of HIV protein gp120 act through CXCR4 receptor

Primary rat hippocampal neurones show pronounced elevations of intracellular calcium within minutes of exposure to the HIV coat protein gp120. Culture of hippocampal neurones with gp120 causes significant neurotoxicity. We find that the peptide VSLSYRCPCRFF, a competitive inhibitor of the CXCR4 chemokine receptor, markedly inhibits toxicity and eliminates the acute calcium elevation. CXCR4 receptors are thought to signal to the Gi/Go family of trimeric GTP binding proteins. Pretreatment of hippocampal neurones with pertussis toxin to inactivate Gi/Go proteins markedly reduced gp120 neurotoxicity. These results indicate that both short and long term effects of gp120 are the result of activation of the CXCR4 receptor.

Absence of marginal zone B cells in Pyk-2-deficient mice defines their role in the humoral response

The lymphoid organs contain specialized microanatomic structures composed of lymphoid, myeloid and stromal cells that are vital to the generation of an effective adaptive immune response. Although the existence of these specialized structures has been known for over a century, the developmental signals that generate them and the specific roles of these structures in the immune response have remained largely elusive. Because of their position adjacent to the marginal sinuses, marginal zone B (MZB) cells are amongst the first population of cells seen by blood born antigens and are presumed to have a critical role in host defense against bacterial pathogens. Here we demonstrate that a deficiency of the tyrosine kinase (Pyk-2) results in a cell autonomous defect of MZB cell production. In response to repetitive polysaccharide antigens (T-independent type II (TI-II)) Pyk-2-deficient mice displayed marked suppression of IgM, IgG3 and IgG2a production. Furthermore, complement receptor engagement proved necessary for the specific targeting of polysaccharide antigens to MZB cells. These results suggest how innate immune responses mediated through complement coupling are translated into an adaptive response by MZB cells, and provide a potential mechanism for the T cell independence of humoral responses to polysaccharide antigens.

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.

Circulating levels and ex vivo production of beta-chemokines, interferon gamma, and interleukin 2 in advanced human immunodeficiency virus type 1 infection: the effect of protease inhibitor therapy

Cytokines and beta-chemokines play an important role in the complex interaction between HIV-1 and the immune system. We studied platelet-free plasma (PFP) levels and ex vivo production of cytokines and beta-chemokines at different HIV disease stages and the influence of potent protease inhibitor therapy on their production in late-stage patients. Mitogen-induced production of MIP-1alpha, MIP-1beta, and RANTES by PBMCs was higher in HIV-infected patients than in HIV-seronegative controls. Patients with late-stage HIV infection (CD4+ cells <50/microl) showed a higher production of MIP-1alpha and RANTES and lower plasma levels of IL-2 compared with HIV-positive patients at the intermediate stage (CD4+ cells >150/microl). Pretreatment RANTES production correlated negatively with CD4+ and CD8+ cell counts; also, MIP-1alpha production was inversely correlated with CD4+ cell counts. Among patients with a CD4+ cell count <50/microl, RANTES production before protease inhibitor treatment was inversely correlated with viral load. Late-stage patients with IL-2 production higher than 50 pg/ml before treatment showed a more impressive increase in CD4+ cell counts after protease inhibitor therapy. The production of MIP-1alpha, MIP-1beta, RANTES, and IFN-gamma was markedly reduced at 8 weeks and partially restored at 24 weeks after beginning protease inhibitor therapy. PFP levels of RANTES showed a concurrent decrease. Patients with more advanced HIV infection show a higher production of inflammatory cytokines, which is reduced by protease inhibitor therapy. Residual late-stage IL-2 producers may represent a subset of patients with a higher potential for immunologic reconstitution.

Beta-chemokine receptor CCR5 signals through SHP1, SHP2, and Syk

The beta-chemokine receptor CCR5 has been shown to modulate cell migration, proliferation, and immune functions and to serve as a co-receptor for the human immunodeficiency virus. We and others have shown that CCR5 activates related adhesion focal tyrosine kinase (RAFTK)/Pyk2/CAK-beta. In this study, we further characterize the signaling molecules activated by CCR5 upon binding to its cognate ligand, macrophage inflammatory protein-1beta (MIP1beta). We observed enhanced tyrosine phosphorylation of the phosphatases SHP1 and SHP2 upon MIP1beta stimulation of CCR5 L1.2 transfectants and T-cells derived from peripheral blood mononuclear cells. Furthermore, we observed that SHP1 associated with RAFTK. However, using a dominant-negative phosphatase-binding mutant of RAFTK (RAFTK(m906)), we found that RAFTK does not mediate SHP1 or SHP2 phosphorylation. SHP1 and SHP2 also associated with the adaptor protein Grb2 and the Src-related kinase Syk. Pretreatment of CCR5 L1.2 transfectants or T-cells with the phosphatase inhibitor orthovanadate markedly abolished MIP1beta-induced chemotaxis. Syk was also activated upon MIP1beta stimulation of CCR5 L1.2 transfectants or T-cells and associated with RAFTK. Overexpression of a dominant-negative Src-binding mutant of RAFTK (RAFTK(m402)) significantly attenuated Syk activation, whereas overexpression of wild-type RAFTK enhanced Syk activity, indicating that RAFTK acts upstream of CCR5-mediated Syk activation. Taken together, these results suggest that MIP1beta stimulation mediated by CCR5 induces the formation of a signaling complex consisting of RAFTK, Syk, SHP1, and Grb2.

gp120 induces cell death in human neuroblastoma cells through the CXCR4 and CCR5 chemokine receptors

To infect target cells, the human immunodeficiency virus (HIV) type I (HIV-1) must engage not only the well-known CD4 molecule, but it also requires one of several recently described coreceptors. In particular, the CXCR4 (LESTR/fusin) receptor allows fusion and entry of T-tropic strains of HIV, whereas CCR5 is the major coreceptor used by primary HIV-1 strains that infect macrophages and CD4(+) T-helper cells (M-tropic viruses). In addition, the alpha chemokine SDF1alpha and the beta chemokines MIP1alpha, MIP1beta, and RANTES, natural ligands of CXCR4 and CCR5, respectively, are potent soluble inhibitors of HIV infection by blocking the binding between the viral envelope glycoprotein gp120 and the coreceptors. Approximately two-thirds of individuals with acquired immunodeficiency syndrome (AIDS) show neurologic complications, which are referred to a syndrome called AIDS dementia complex or HIV-1-associated cognitive/motor complex. The HIV-1 coat glycoprotein gp120 has been proposed as the major etiologic agent for neuronal damage, mediating both direct and indirect effects on the CNS. Furthermore, recent findings showing the presence of chemokine receptors on the surface of different cell types resident in the CNS raise the possibility that the association of gp120 with these receptors may contribute to the pathogenesis of neurological dysfunction. Here, we address the possible role of alpha and beta chemokines in inhibiting gp120-mediated neurotoxicity using the human neuroblastoma CHP100 cell line as an experimental model. We have previously shown that, in CHP100 cells, picomolar concentrations of gp120 produce a significant increase in cell death, which seems to proceed through a Ca(2+) - and NMDA receptor-dependent cascade. In this study, we gained insight into the mechanism(s) of neurotoxicity elicited by the viral glycoprotein. We found that CHP100 cells constitutively express both CXCR4 and CCR5 receptors and that stimulation with phorbol 12-myristate 13-acetate down-regulates their expression, thus preventing gp120-induced cell death. Furthermore, all the natural ligands of these receptors exerted protective effects against gp120-mediated neuronal damage, although with different efficiencies. These findings, together with our previous reports, suggest that the neuronal injury observed in HIV-1 infection could be due to direct (or indirect) interactions between the viral protein gp120 and chemokine and/or NMDA receptors.

HIV-1 gp120 and chemokines activate ion channels in primary macrophages through CCR5 and CXCR4 stimulation

HIV type 1 (HIV-1) uses the chemokine receptors CCR5 and CXCR4 as coreceptors for entry into target cells. Here we show that the HIV-1 envelope gp120 (Env) activates multiple ionic signaling responses in primary human macrophages, which are important targets for HIV-1 in vivo. Env from both CCR5-dependent JRFL (R5) and CXCR4-dependent IIIB (X4) HIV-1 opened calcium-activated potassium (K(Ca)), chloride, and calcium-permeant nonselective cation channels in macrophages. These signals were mediated by CCR5 and CXCR4 because macrophages lacking CCR5 failed to respond to JRFL and an inhibitor of CXCR4 blocked ion current activation by IIIB. MIP-1beta and SDF-1alpha, chemokine ligands for CCR5 and CXCR4, respectively, also activated K(Ca) and Cl(-) currents in macrophages, but nonselective cation channel activation was unique to gp120. Intracellular Ca(2+) levels were also elevated by gp120. The patterns of activation mediated by CCR5 and CXCR4 were qualitatively similar but quantitatively distinct, as R5 Env activated the K(Ca) current more frequently, elicited Cl(-) currents that were approximately 2-fold greater in amplitude, and elevated intracellular Ca(+2) to higher peak and steady-state levels. Env from R5 and X4 primary isolates evoked similar current responses as the corresponding prototype strains. Thus, the interaction of HIV-1 gp120 with CCR5 or CXCR4 evokes complex and distinct signaling responses in primary macrophages, and gp120-evoked signals differ from those activated by the coreceptors' chemokine ligands. Intracellular signaling responses of macrophages to HIV-1 may modulate postentry steps of infection and cell functions apart from infection.

Stromal cell-derived factor-1α stimulates tyrosine phosphorylation of multiple focal adhesion proteins and induces migration of hematopoietic progenitor cells: roles of phosphoinositide-3 kinase and protein kinase C

The stromal cell-derived factor-1 (SDF-1) is an alpha chemokine that binds to the CXCR4 receptor. Knock-out studies in mice demonstrate that this ligand-receptor pair is essential in hematopoiesis. One function of SDF-1 appears to be the regulation of migration of hematopoietic progenitor cells. We previously characterized signal transduction pathways induced by SDF-1 in human hematopoietic progenitors and found tyrosine phosphorylation of focal adhesion components, including the related adhesion focal tyrosine kinase (RAFTK), the adaptor molecule p130 Cas, and the cytoskeletal protein paxillin. To better understand the functional role of signaling molecules connecting the CXCR4 receptor to the process of hematopoietic migration, we studied SDF-1–mediated pathways in a model hematopoietic progenitor cell line (CTS), as well as in primary human bone marrow CD34+cells. We observed that several other focal adhesion components, including focal adhesion kinase (FAK) and the adaptor molecules Crk and Crk-L, are phosphorylated on SDF-1 stimulation. Using a series of specific small molecule inhibitors, both protein kinase C (PKC) and phosphoinositide-3 kinase (PI-3K) appeared to be required for SDF-1–mediated phosphorylation of focal adhesion proteins and the migration of both CTS and primary marrow CD34+ cells, whereas the mitogen-activated protein kinases ERK-1 and -2 were not. These studies further delineate the molecular pathways mediating hematopoietic progenitor migration and response to an essential chemokine, SDF-1.

Stromal cell-derived factor-1α stimulates tyrosine phosphorylation of multiple focal adhesion proteins and induces migration of hematopoietic progenitor cells: roles of phosphoinositide-3 kinase and protein kinase C

The stromal cell-derived factor-1 (SDF-1) is an alpha chemokine that binds to the CXCR4 receptor. Knock-out studies in mice demonstrate that this ligand-receptor pair is essential in hematopoiesis. One function of SDF-1 appears to be the regulation of migration of hematopoietic progenitor cells. We previously characterized signal transduction pathways induced by SDF-1 in human hematopoietic progenitors and found tyrosine phosphorylation of focal adhesion components, including the related adhesion focal tyrosine kinase (RAFTK), the adaptor molecule p130 Cas, and the cytoskeletal protein paxillin. To better understand the functional role of signaling molecules connecting the CXCR4 receptor to the process of hematopoietic migration, we studied SDF-1–mediated pathways in a model hematopoietic progenitor cell line (CTS), as well as in primary human bone marrow CD34+cells. We observed that several other focal adhesion components, including focal adhesion kinase (FAK) and the adaptor molecules Crk and Crk-L, are phosphorylated on SDF-1 stimulation. Using a series of specific small molecule inhibitors, both protein kinase C (PKC) and phosphoinositide-3 kinase (PI-3K) appeared to be required for SDF-1–mediated phosphorylation of focal adhesion proteins and the migration of both CTS and primary marrow CD34+ cells, whereas the mitogen-activated protein kinases ERK-1 and -2 were not. These studies further delineate the molecular pathways mediating hematopoietic progenitor migration and response to an essential chemokine, SDF-1.

Chemokine signaling in inflammation

The events that lead to an inflammatory response are characterized by recognition of the site of injury by inflammatory cells, specific recruitment of subpopulations of leukocytes into tissue, removal of the offending agent and "debridement" of the injured cells/tissue, and repair of the site of injury with attempts to reestablish normal parenchymal, stromal, and extracellular matrix relationship. The molecular regulation of this complex physiologic process involves the interaction between cell surface, extracellular matrix, and soluble mediators, such as chemokines. Chemokine activities are mediated through G-protein coupled receptors. This is the largest known family of cell-surface receptors, which mediate transmission of stimuli as diverse as hormones, peptides, glycopeptides, and chemokines. In this review, we will focus on the signaling pathways involved in the production and function of chemokines as they relate to the inflammatory response.

Caspase-dependent apoptosis of cells expressing the chemokine receptor CXCR4 is induced by cell membrane-associated human immunodeficiency virus type 1 envelope glycoprotein (gp120)

Human immunodeficiency virus type 1 (HIV-1) envelope glycoproteins interact with CD4 and chemokine receptors on T cells to deliver signals that trigger either activation, anergy, or apoptosis. However, the molecular mechanisms driving these responses remain poorly understood. In this study we demonstrate that apoptosis is induced upon HIV-1 envelope binding to the chemokine receptor CXCR4. Cells expressing a mutant form of CXCR4 with a C-terminal deletion were also sensitive to HIV-1 envelope-mediated apoptosis, indicating that the cytoplasmic tail of CXCR4 is not required to induce the apoptotic pathway. The specificity of this process was analyzed using several inhibitors of gp120-CD4-CXCR4 interaction. Monoclonal antibodies directed against the gp120-binding site on CD4 (ST4) and against CXCR4 (MAB173) prevented the apoptotic signal in a dose-dependent manner. The cell death program was also inhibited by SDF-1alpha, the natural ligand of CXCR4, and by suramin, a G protein inhibitor that binds with a high affinity to the V3 loop of HIV-1 gp120 envelope protein. These results highlight the role played by gp120-binding on CXCR4 to trigger programmed cell death. Next, we investigated the intracellular signal involved in gp120-induced apoptosis. This cell death program was insensitive to pertussis toxin and did not involve activation of the stress- and apoptosis-related MAP kinases p38(MAPK) and SAPK/JNK but was inhibited by a broad spectrum caspase inhibitor (z-VAD.fmk) and a relatively selective inhibitor of caspase 3 (z-DEVD.fmk). Altogether, our results demonstrate that HIV induces a caspase-dependent apoptotic signaling pathway through CXCR4.

Interleukin (IL)-7 induces rapid activation of Pyk2, which is bound to Janus kinase 1 and IL-7Ralpha

Interleukin-7 (IL-7) receptor signaling begins with activation of the Janus tyrosine kinases Jak1 and Jak3, which are associated with the receptor complex. To identify potential targets of these kinases, we examined Pyk2 (a member of the focal adhesion kinase family) using an IL-7-dependent murine thymocyte line, D1. We demonstrate that stimulation of D1 (or normal pro-T) cells by IL-7 rapidly increased tyrosine phosphorylation and enzymatic activity of Pyk2, with kinetics slightly lagging that of Jak1 and Jak3 phosphorylation. Conversely, IL-7 withdrawal resulted in a marked decrease of Pyk2 phosphorylation. Pyk2 was found to be physically associated with Jak1 prior to IL-7 stimulation and to increase its association with IL-7Ralpha chain following IL-7 stimulation. Pyk2 appeared to be involved in cell survival, because antisense Pyk2 accelerated the cell death process. Activation of Pyk2 via the muscarinic and nicotinic receptors using carbachol or via intracellular Ca(2+) rise using ionomycin/phorbol myristate acetate promoted survival in the absence of IL-7. These data support a role for Pyk2 in coupling Jak signaling to the trophic response.

Fibroblast growth factor modulates HIV coreceptor CXCR4 expression by neural cells. HNRC Group

Recent studies suggest that the chemokine receptor CXCR4 may be involved in mediating the neurodegenerative process in the brains of patients with acquired immunodeficiency disease (AIDS). In this context, we hypothesize that neurotrophic factors, such as fibroblast growth factor (FGF), might protect against human immunodeficiency virus (HIV)-mediated neurotoxicity via regulating the expression of CXCR4 in neural cells. For this purpose, levels of CXCR4 were determined in neuronal and glial cell lines after FGF1 and 2 treatment. In addition, levels of CXCR4 immunoreactivity were associated with levels of FGF1 immunoreactivity in the brains of HIV-positive patients. These studies showed that neuronal CXCR4 levels decreased in a dose-dependent manner after exposure to FGF. Conversely, glial CXCR4 was increased in a dose-dependent manner after FGF2 treatment. These effects were dependent on the FGF receptor tyrosine kinase signaling pathway, because FGF-induced effects on CXCR4 were blocked by the tyrosine kinase inhibitor, 5'-deoxy-5'methylthioadenosine, or by anti-FGF receptor antibody. Stromal cell-derived factor-1, the ligand for CXCR4, and HIV gp120 neurotoxicity was attenuated by FGF1 in a dose-dependent manner in vitro, further supporting physiological relevance. In the brains of AIDS patients, the levels of neural CXCR4 immunoreactivity were inversely associated with FGF levels. Taken together, these results support the possibility that the neuroactive effects of FGF in HIV encephalitis might be mediated through regulation of the expression of CXCR4.

Inhibition of CD3/CD28-mediated activation of the MEK/ERK signaling pathway represses replication of X4 but not R5 human immunodeficiency virus type 1 in peripheral blood CD4(+) T lymphocytes

Binding of human immunodeficiency virus type 1 (HIV-1) to CD4 receptors induces multiple cellular signaling pathways, including the MEK/ERK cascade. While the interaction of X4 HIV-1 with CXCR4 does not seem to activate this pathway, viruses using CCR5 for entry efficiently activate MEK/ERK kinases (W. Popik, J. E. Hesselgesser, and P. M. Pitha, J. Virol. 72:6406-6413, 1998; W. Popik and P. M. Pitha, Virology 252:210-217, 1998). Since the importance of MEK/ERK in the initial steps of viral replication is poorly understood, we have examined the role of MEK/ERK signaling in the CD3- and CD28 (CD3/CD28)-mediated activation of HIV-1 replication in resting peripheral blood CD4(+) T lymphocytes infected with X4 or R5 HIV-1. We have found that the MEK/ERK inhibitor U0126 selectively inhibited CD3/CD28-stimulated replication of X4 HIV-1, while it did not affect the replication of R5 HIV-1. Inhibition of the CD3/CD28-stimulated MEK/ERK pathway did not affect the formation of the early proviral transcripts in cells infected with either X4 or R5 HIV-1, indicating that virus reverse transcription is not affected in the absence of MEK/ERK signaling. In contrast, the levels of nuclear provirus in cells infected with X4 HIV-1, detected by the formation of circular proviral DNA, was significantly lower in cells stimulated in the presence of MEK/ERK inhibitor than in the absence of the inhibitor. However, in cells infected with R5 HIV-1, the inhibition of the MEK/ERK pathway did not affect nuclear localization of the proviral DNA. These data suggest that the nuclear import of X4, but not R5, HIV-1 is dependent on a CD3/CD28-stimulated MEK/ERK pathway.

Chemically induced infection of CD4-negative HeLa cells with HIV-1

Infection with human immunodeficiency virus type-1 (HIV-1) requires the presence of a CD4 molecule and chemokine receptors such as CXCR4 or CCR5 on the surface of target cells. However, it is still not clear how the virus enters the cells. Although CD4 was initially identified as the primary receptor for HIV-1, the expression of CD4 or one of the chemokine receptors alone is not sufficient to render susceptibility to infection with the virus. To ascertain whether or not adsorption of the virus needs charge-to-charge interaction between viral envelope and host cell membrane protein(s) and if binding alone promotes penetration of the virus into the cells, we have developed a chemically induced infection system targeting a CD4-negative and CXCR4-positive HeLa cell clone (N7 HeLa) which is usually not susceptible to infection with the LAI strain of HIV-1. Use of a poly-L-lysine (PLL)-coated culture plate to enhance the attachment of the virus to the cells made N7 HeLa cells infectable with HIV-1 at very low efficiency. PLL alone cannot fully substitute for the function of the CD4 molecule. However, trypsin-treated viruses, which have largely lost infectivity to CD4-positive MT-4 cells that are highly susceptible to HIV-1 infection, enhanced infectivity against N7 HeLa cells when the PLL-coated plate was used. These results provide evidence that infection with HIV-1 requires both high binding affinity between viruses and cells, and then needs a modification of the viral envelope such as cleavage of gp120/160 to enhance the infection, probably resulting in exposure of the hydrophobic fusion domain of gp41. HIV-1 infection of N7 HeLa cells was also enhanced by treatment with low pH, 12-O-tetradecanoylphorbol-13-acetate (TPA) and some factor(s) from the MT-4 cell culture supernatant. Not only tight viral adsorption with cleavage of the viral envelope but also some activated status of the cells may be required for sufficient HIV-1 infection in this artificial condition.

Lipopolysaccharide induces actin reorganization and tyrosine phosphorylation of Pyk2 and paxillin in monocytes and macrophages

The bacterial endotoxin LPS is a potent stimulator of monocyte and macrophage activation and induces adhesion of monocytes. Morphological changes in response to LPS have not been characterized in detail, however, nor have the signaling pathways mediating LPS-induced adhesion been elucidated. We have found that LPS rapidly induced adhesion and spreading of peripheral blood monocytes, and that this was inhibited by the Src family kinase inhibitor PP1 and the phosphatidylinositide 3-kinase inhibitor LY294002. LPS also stimulated actin reorganization, leading to the formation of filopodia, lamellipodia, and membrane ruffles in Bac1 mouse macrophages. Proline-rich tyrosine kinase 2 (Pyk2), a tyrosine kinase related to focal adhesion kinase, and paxillin, a cytoskeletal protein that interacts with Pyk2, were both tyrosine phosphorylated in response to LPS in monocytes and macrophages. Both tyrosine phosphorylation events were inhibited by PP1 and LY294002. Adhesion also stimulated tyrosine phosphorylation of Pyk2 and paxillin in monocytes, and this was further enhanced by LPS. Finally, Pyk2 and paxillin colocalized within membrane ruffles in LPS-stimulated cells. These results indicate that LPS stimulation of monocytes and macrophages results in rapid morphological changes and suggest that Pyk2 and/or paxillin play a role in this response.

HIV-1 envelope induces activation of caspase-3 and cleavage of focal adhesion kinase in primary human CD4(+) T cells

Binding of HIV type 1 (HIV-1) envelope glycoproteins to the surface of a CD4(+) T cell transduces intracellular signals through the primary envelope receptor, CD4, and a coreceptor, either CCR5 or CXCR4. Furthermore, envelope-CD4(+) cell interactions increase rates of apoptosis in peripheral blood mononuclear cells (PBMCs). We demonstrate that in primary T lymphocytes, recombinant HIV-1 envelope proteins induce the activation of caspase-3 and caspase-6, which belong to a family of cysteine proteases that, upon activation, promote programmed cell death. Envelope-mediated activation of caspase-3 and caspase-6 depended on envelope-CD4 receptor interactions; CCR5-utilizing as well as CXCR4-utilizing envelopes elicited this response. Focal adhesion kinase (FAK) is a substrate of both caspase-3 and caspase-6, and inactivation of FAK by these caspases promotes apoptosis. En-velope treatment of lymphocytes led to the cleavage of FAK in a manner consistent with caspase-mediated cleavage.

The CXCR4 antagonist AMD3100 efficiently inhibits cell-surface-expressed human immunodeficiency virus type 1 envelope-induced apoptosis

Infection by human immunodeficiency virus type 1 (HIV-1) has been associated with increased cell death by apoptosis in infected and uninfected cells. The envelope glycoprotein complex ([gp120/gp41](n)) of X4 HIV-1 isolates is involved in both infected and uninfected cell death via its interaction with cellular receptors CD4 and CXCR4. We studied the effect of the blockade of CXCR4 receptors by the agonist stromal derived factor (SDF-1alpha) and the antagonist bicyclam AMD3100 on apoptotic cell death of CD4(+) cells in different models of HIV infection. In HIV-infected CEM or SUP-T1 cultures, AMD3100 showed antiapoptotic activity even when added 24 h after infection. In contrast, other antiviral agents, such as zidovudine, failed to block apoptosis under these conditions. The antiapoptotic activity of AMD3100 was also studied in coculture of peripheral blood mononuclear cells or CD4(+) cell lines with chronically infected H9/IIIB cells. AMD3100 was found to inhibit both syncytium formation and apoptosis induction with 50% inhibitory concentrations ranging from 0.009 to 0.24 microg/ml, depending on the cell type. When compared to SDF-1alpha, AMD3100 showed higher inhibitory potency in all cell lines tested. Our data indicate that the bicyclam AMD3100 not only inhibits HIV replication but also efficiently blocks cell-surface-expressed HIV-1 envelope-induced apoptosis in uninfected cells.

Differential signalling of the chemokine receptor CXCR4 by stromal cell-derived factor 1 and the HIV glycoprotein in rat neurons and astrocytes

CXCR4 is the Gi protein-linked seven-transmembrane receptor for the alpha chemokine stromal cell-derived factor 1 (SDF-1), a chemoattractant for lymphocytes. This receptor is highly conserved between human and rodent. CXCR4 is also a coreceptor for entry of human immunodeficiency virus (HIV) in T cells and is expressed in the CNS. To investigate how these CXCR4 ligands influence CNS development and/or function, we have examined the expression and signalling of this chemokine receptor in rat neurons and astrocytes in vitro. CXCR4 transcripts and protein are synthesized by both cell types and in E15 brain neuronal progenitors. In these progenitors, SDF-1, but not gp120 (the HIV glycoprotein), induced activation of extracellular signal regulated kinases (ERKs) 1/2 and a dose-dependent chemotactic response. This chemotaxis was inhibited by Pertussis toxin, which uncouples Gi proteins and the bicyclam AMD3100, a highly selective CXCR4 antagonist, as well as by an inhibitor of the MAP kinase pathway. In differentiated neurons, both SDF-1 and the glycoprotein of HIV, gp120, triggered activation of ERKs with similar kinetics. These effects were significantly inhibited by Pertussis toxin and the CXCR4 antagonist. Rat astrocytes also responded to SDF-1 signalling by phosphorylation of ERKs but, in contrast to cortical neurons, no kinase activation was induced by gp120. Thus neurons and astrocytes can respond differently to signalling by SDF-1 and/or gp120. As SDF-1 triggers directed migration of neuronal progenitors, this alpha chemokine may play a role in cortex development. In differentiated neurons, both natural and viral ligands of CXCR4 activate ERKs and may therefore influence neuronal function.

Differential patterns of interaction between HIV type 1 and HTLV type I in monocyte-derived macrophages cultured in vitro: implications for in vivo coinfection with HIV type 1 and HTLV type I

The interaction between human immunodeficiency virus type 1 (HIV-1) and human T cell leukemia-lymphoma virus type I (HTLV-I) has generated substantial interest. However, there is disagreement on the in vivo consequences of the double infection. We investigated the interactions between HIV-1 and HTLV-I in monocyte-derived macrophages cultured in vitro. For study, the T cell-tropic strain IIIB and the macrophagetropic strain Ada-M of HIV-1 were used. The HTLV-I was prepared from the supernatants of the virus-producing MT-2 cell line. We found that coinfection of macrophages with T cell-tropic HIV-1 and HTLV-I significantly enhanced HIV-1 replication, whereas double infection of the cells with macrophage-tropic HIV-1 and HTLV-I resulted in marked upregulation of HTLV-I production. Stimulatory interactions between HIV-1 and HTLV-I were mediated by their trans-acting proteins. Results of study on nuclear translocation of proviral DNA showed that the tax gene product of HTLV-I was able to facilitate the nuclear import of the reverse-transcribed HIV-1(IIIB) DNA. In contrast, the HIV-1 Tat protein did not increase the intranuclear trafficking of HTLV-I DNA, which suggests another mechanism for HTLV-I enhancement by the tat gene product. In conclusion, this study provides possible mechanisms whereby coinfection of an individual with HIV-1 and HTLV-I may influence the clinical outcome of double infection.

The CXC Chemokine Stromal Cell-Derived Factor Activates a Gi-Coupled Phosphoinositide 3-Kinase in T Lymphocytes

The cellular effects of stromal cell-derived factor-1 (SDF-1) are mediated primarily by binding to the CXC chemokine receptor-4. We report in this study that SDF-1 and its peptide analogues induce a concentration- and time-dependent accumulation of phosphatidylinositol-(3,4,5)-trisphosphate (PtdIns(3,4,5)P3) in Jurkat cells. This SDF-1-stimulated generation of D-3 phosphoinositide lipids was inhibited by pretreatment of the cells with an SDF-1 peptide antagonist or an anti-CXCR4 Ab. In addition, the phosphoinositide 3 (PI 3)-kinase inhibitors wortmannin and LY294002, as well as the Gi protein inhibitor pertussis toxin, also inhibited the SDF-1-stimulated accumulation of PtdIns(3,4,5)P3. The effects of SDF-1 on D-3 phosphoinositide lipid accumulation correlated well with activation of the known PI 3-kinase effector protein kinase B, which was also inhibited by wortmannin and pertussis toxin. Concentrations of PI 3-kinase inhibitors, sufficient to inhibit PtdIns(3,4,5)P3 accumulation, also inhibited chemotaxis of Jurkat and peripheral blood-derived T lymphocytes in response to SDF-1. In contrast, SDF-1-stimulated actin polymerization was only partially inhibited by PI 3-kinase inhibitors, suggesting that while chemotaxis is fully dependent on PI 3-kinase activation, actin polymerization requires additional biochemical inputs. Finally, SDF-1-stimulated extracellular signal-related kinase (ERK)-1/2 mitogen-activated protein kinase activation was inhibited by PI 3-kinase inhibitors. In addition, the mitogen-activated protein/ERK kinase inhibitor PD098059 partially attenuated chemotaxis in response to SDF-1. Hence, it appears that ERK1/2 activation is dependent on PI 3-kinase activation, and both biochemical events are involved in the regulation of SDF-1-stimulated chemotaxis.

Use of a gp120 binding assay to dissect the requirements and kinetics of human immunodeficiency virus fusion events

Binding of the extracellular subunit of human immunodeficiency type 1 (HIV-1) envelope (Env) glycoprotein (gp120) to CD4 triggers the induction or exposure of a highly conserved coreceptor binding site in gp120 that helps mediate membrane fusion. Characterizing the structural features involved in gp120-coreceptor binding and the conditions under which binding occurs is important for understanding the fusion process, the evolution of pathogenic strains in vivo, the identification of novel anti-HIV compounds, and the development of HIV vaccines that utilize triggered structures of Env. Here we use the kinetics of interaction between CCR5 and gp120 to understand temporal and structural changes that occur during viral fusion. Using saturation binding and homologous competition analysis, we estimated the K(d) of interaction between CCR5 and gp120 from the macrophage tropic HIV-1 strain JRFL to be 4 nM. Unlike Env-mediated fusion, gp120 binding to CCR5 did not require divalent cations or elevated temperatures. Binding was not significantly affected by the pH of binding, G-protein coupling of CCR5, or partial gp120 deglycosylation. Oligomeric, uncleaved JRFL gp140 failed to bind CCR5 despite its ability to bind CD4 and monoclonal antibody 17b, suggesting that the uncleaved ectodomain of gp41 interferes with full exposure of the chemokine receptor binding site. Exposure of the chemokine receptor binding site on gp120 could be induced rapidly by CD4, but exposure of this site was lost upon CD4 dissociation from gp120, indicating that the conformational changes in gp120 induced by CD4 binding are fully reversible. The functional gp120-soluble CD4 complex was remarkably stable over time and temperature ranges, offering the possibility that complexes in which the highly conserved coreceptor binding site in gp120 is exposed can be used for vaccine development.

Peptide T blocks GP120/CCR5 chemokine receptor-mediated chemotaxis

We previously reported that certain short gp120 V2 region peptides homologous to vasaoactive intestinal peptide (VIP), such as "peptide T," were potent inhibitors of gp120 binding, infectivity, and neurotoxicity. The present study shows that synthetic V2-region-derived peptides have potent intrinsic chemotaxis agonist activity for human monocytes and also act as antagonists of high-affinity (0.1 pM) gp120-mediated monocyte chemotaxis. Selectivity is shown in that peptide T is more potent at suppressing M-tropic than T-tropic gp120 chemotaxis. Peptide T was also able to suppress monocyte chemotaxis to MIP-1beta, a chemokine with selectivity for CCR5 chemokine receptors, while chemotaxis of the more promiscuous ligand RANTES was not inhibited, nor was chemotaxis mediated by SDF-1alpha. In order to determine if peptide T mediated its gp120 antagonistic effects via modulation of CCR5 receptors, RANTES chemotaxis was studied using a CCR5 receptor-transfected HOS cell line. In this case, RANTES chemotaxis was potently inhibited by V2-region-derived short peptides. Peptide T also partially suppressed (125)I-MIP1-beta binding to human monocytes, suggesting action at a subset of MIP1-beta receptors. The V2 region of gp120 thus contains a potent receptor binding domain and synthetic peptides derived from this region modulate CCR5 chemokine receptor chemotactic signaling caused by either gp120 or chemokine ligands. The results have therapeutic implications and may explain recent clinical improvements, in that HIV/gp120 actions at CCR5 receptors, such as occur in the brain or early infection, would be susceptible to peptide T inhibition.

Trafficking of the HIV coreceptor CXCR4. Role of arrestins and identification of residues in the c-terminal tail that mediate receptor internalization

The G protein-coupled chemokine receptor CXCR4 serves as the primary coreceptor for entry of T-cell tropic human immunodeficiency virus. CXCR4 undergoes tonic internalization as well as internalization in response to stimulation with phorbol esters and ligand (SDF-1alpha). We investigated the trafficking of this receptor, and we attempted to define the residues of CXCR4 that were critical for receptor internalization. In both COS-1 and HEK-293 cells transiently overexpressing CXCR4, SDF-1alpha and phorbol esters (PMA) promoted rapid internalization of cell surface receptors as assessed by both enzyme-linked immunosorbent assay and immunofluorescence analysis. Expression of GRK2 and/or arrestins promoted modest additional CXCR4 internalization in response to both PMA and SDF. Both PMA- and SDF-mediated CXCR4 internalization was inhibited by coexpression of dominant negative mutants of dynamin-1 and arrestin-3. Arrestin was also recruited to the plasma membrane and appeared to colocalize with internalized receptors in response to SDF but not PMA. We then evaluated the ability of CXCR4 receptors containing mutations of serines and threonines, as well as a dileucine motif, within the C-terminal tail to be internalized and phosphorylated in response to either PMA or SDF-1alpha. This analysis showed that multiple residues within the CXCR4 C-terminal tail appear to mediate both PMA- and SDF-1alpha-mediated receptor internalization. The ability of coexpressed GRK2 and arrestins to promote internalization of the CXCR4 mutants revealed distinct differences between respective mutants and suggested that the integrity of the dileucine motif (Ile-328 and Leu-329) and serines 324, 325, 338, and 339 are critical for receptor internalization.

Signaling Through CD43 Induces Natural Killer Cell Activation, Chemokine Release, and PYK-2 Activation

Natural killer (NK) cell activation is the result of a balance between positive and negative signals triggered by specific membrane receptors. We report here the activation of NK cells induced through the transmembrane glycoprotein CD43 (leukosialin, sialophorin). Engagement of CD43 by specific antibodies stimulated the secretion of the chemokines RANTES, macrophage inflammatory protein (MIP)-1, and MIP-1β, which was prevented by treatment of cells with the specific tyrosine kinase inhibitor genistein. Furthermore, signaling through CD43 increased the cytotoxic activity of NK cells and stimulated an increase in the tyrosine kinase activity in antiphosphotyrosine immune complexes of NK cell lysates. PYK-2 was identified among the tyrosine kinase proteins that become activated. Hence, PYK-2 activation was observed after 20 minutes of CD43 stimulation, reached a maximum after 45 to 60 minutes, and decreased to almost basal levels after 120 minutes of treatment. Together, these results demonstrate the role of CD43 as an activation molecule able to transduce positive activation signals in NK cells, including the regulation of chemokine synthesis, killing activity, and tyrosine kinase activation.

The moving target: mechanisms of HIV persistence during primary infection

The vigorous host immune responses detected during primary HIV infection fail to prevent the establishment of chronic infection because HIV has evolved multiple strategies to make these responses ineffective. Here, Hugo Soudeyns and Giuseppe Pantaleo discuss the mechanisms that allow HIV to persist in the host.

AIDS and the brain: is there a chemokine connection?

Many HIV-1-positive individuals suffer from a variety of neurological problems known collectively as the HIV-1-related cognitive-motor complex. However, the molecular mechanisms that underlie HIV-1-induced neuropathology are unclear. They might include a combination of indirect effects, which result from the release of neurotoxins from activated astrocytes and microglia, and the direct effects of HIV-1-related proteins, such as gp120, on neurons. As the interaction of gp120 with immune cells has been shown to require the participation of chemokine receptors, this article explores the possibility that such receptors participate in the events underlying HIV-1-induced neuropathology. It is now clear that many types of cell in the brain possess chemokine receptors, including microglia, glia and neurons, and the interaction of gp120 with neuronal chemokine receptors initiates apoptotic death of neurons in vitro. Such effects might be modified by the actions of chemokines that act at these same receptors. However, the importance of this direct interaction with neurons in vivo and its relevance in the pathogenesis of AIDS-related dementia still needs to be established. Furthermore, the existence of chemokine receptors on neurons suggests that chemokines might regulate neuronal functions physiologically.

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

Chemokine receptors pivotal for human immunodeficiency virus type 1 (HIV-1) infection in lymphocytes and macrophages (CCR3, CCR5, and CXCR4) are expressed on neural cells (microglia, astrocytes, and/or neurons). It is these cells which are damaged during progressive HIV-1 infection of the central nervous system. We theorize that viral coreceptors could effect neural cell damage during HIV-1-associated dementia (HAD) without simultaneously affecting viral replication. To these ends, we studied the ability of diverse viral strains to affect intracellular signaling and apoptosis of neurons, astrocytes, and monocyte-derived macrophages. Inhibition of cyclic AMP, activation of inositol 1,4,5-trisphosphate, and apoptosis were induced by diverse HIV-1 strains, principally in neurons. Virions from T-cell-tropic (T-tropic) strains (MN, IIIB, and Lai) produced the most significant alterations in signaling of neurons and astrocytes. The HIV-1 envelope glycoprotein, gp120, induced markedly less neural damage than purified virions. Macrophage-tropic (M-tropic) strains (ADA, JR-FL, Bal, MS-CSF, and DJV) produced the least neural damage, while 89.6, a dual-tropic HIV-1 strain, elicited intermediate neural cell damage. All T-tropic strain-mediated neuronal impairments were blocked by the CXCR4 antibody, 12G5. In contrast, the M-tropic strains were only partially blocked by 12G5. CXCR4-mediated neuronal apoptosis was confirmed in pure populations of rat cerebellar granule neurons and was blocked by HA1004, an inhibitor of calcium/calmodulin-dependent protein kinase II, protein kinase A, and protein kinase C. Taken together, these results suggest that progeny HIV-1 virions can influence neuronal signal transduction and apoptosis. This process occurs, in part, through CXCR4 and is independent of CD4 binding. T-tropic viruses that traffic in and out of the brain during progressive HIV-1 disease may play an important role in HAD neuropathogenesis.