CXCR4 Engagement Research Articles

Targeting CXCR4 and CD47 Receptors: An Overview of New and Old Molecules for a Biological Personalized Anticancer Therapy.

Biological therapy, with its multifaceted applications, has revolutionized the treatment of tumors, mainly due to its ability to exclusively target cancer cells and reduce the adverse effects on normal tissues. This review focuses on the therapies targeting the CXCR4 and CD47 receptors. We surveyed the results of early clinical trials testing compounds classified as nonpeptides, small peptides, CXCR4 antagonists or specific antibodies whose activity reduces or completely blocks the intracellular signaling pathways and cell proliferation. We then examined antibodies and fusion proteins against CD47, the receptor that acts as a “do not eat me” signal to phagocytes escaping immune surveillance. Despite these molecules being tested in early clinical trials, some drawbacks are emerging that impair their use in practice. Finally, we examined the ImmunoGenic Surrender mechanism that involves crosstalk and co-internalization of CXCR4 and CD47 upon engagement of CXCR4 by ligands or other molecules. The favorable effect of such compounds is dual as CD47 surface reduction impact on the immune response adds to the block of CXCR4 proliferative potential. These results suggest that a combination of different therapeutic approaches has more beneficial effects on patients’ survival and may pave the way for new accomplishments in personalized anticancer therapy.

CXCR4 engagement triggers CD47 internalization and antitumor immunization in a mouse model of mesothelioma

Boosting antitumor immunity has emerged as a powerful strategy in cancer treatment. While releasing T‐cell brakes has received most attention, tumor recognition by T cells is a pre‐requisite. Radiotherapy and certain cytotoxic drugs induce the release of damage‐associated molecular patterns, which promote tumor antigen cross‐presentation and T‐cell priming. Antibodies against the “do not eat me” signal CD47 cause macrophage phagocytosis of live tumor cells and drive the emergence of antitumor T cells. Here we show that CXCR4 activation, so far associated only with tumor progression and metastasis, also flags tumor cells to immune recognition. Both CXCL12, the natural CXCR4 ligand, and BoxA, a fragment of HMGB1, promote the release of DAMPs and the internalization of CD47, leading to protective antitumor immunity. We designate as Immunogenic Surrender the process by which CXCR4 turns in tumor cells to macrophages, thereby subjecting a rapidly growing tissue to immunological scrutiny. Importantly, while CXCL12 promotes tumor cell proliferation, BoxA reduces it, and might be exploited for the treatment of malignant mesothelioma and a variety of other tumors.

Control of TLR7-mediated type I IFN signaling in pDCs through CXCR4 engagement-A new target for lupus treatment.

Type I interferons are highly potent cytokines essential for self-protection against tumors and infections. Deregulations of type I interferon signaling are associated with multiple diseases that require novel therapeutic options. Here, we identified the small molecule, IT1t, a previously described CXCR4 ligand, as a highly potent inhibitor of Toll-like receptor 7 (TLR7)-mediated inflammation. IT1t inhibits chemical (R848) and natural (HIV) TLR7-mediated inflammation in purified human plasmacytoid dendritic cells from blood and human tonsils. In a TLR7-dependent lupus-like model, in vivo treatment of mice with IT1t drives drastic reduction of both systemic inflammation and anti-double-stranded DNA autoantibodies and prevents glomerulonephritis. Furthermore, IT1t controls inflammation, including interferon α secretion, in resting and stimulated cells from patients with systemic lupus erythematosus. Our findings highlight a groundbreaking immunoregulatory property of CXCR4 signaling that opens new therapeutic perspectives in inflammatory settings and autoimmune diseases.

SOS1, ARHGEF1, and DOCK2 rho-GEFs Mediate JAK-Dependent LFA-1 Activation by Chemokines.

JAK-dependent activation of the rho module of integrin affinity triggering mediates chemokine-induced leukocyte adhesion. However, the signaling events linking JAKs to rho small GTPase activation by chemokines is still incompletely described. In this study, we show that son of sevenless 1 (SOS1), rho guanine nucleotide exchange factor (GEF)1 (ARHGEF1), and dedicator of cytokinesis (DOCK)2 GEFs mediate CXCL12-induced LFA-1 activation in human primary T lymphocytes. Downregulated expression of SOS1, ARHGEF1, and DOCK2 impairs LFA-1-mediated rapid T lymphocyte adhesion as well as underflow arrest on ICAM-1 induced by CXCL12. Moreover, LFA-1 affinity triggering by CXCL12 is impaired by SOS1, ARHGEF1, and DOCK2 downregulation. Notably, the three GEFs are all critically involved in chemokine-induced RhoA and Rac1 activation, thus suggesting the occurrence of a SOS1 specificity shift in the context of chemokine signaling. Accordingly, SOS1, ARHGEF1, and DOCK2 are tyrosine phosphorylated upon chemokine signaling with timing coherent with rapid LFA-1 affinity activation. Importantly, chemokine-induced tyrosine phosphorylation of these GEFs is fully mediated by JAK protein tyrosine kinases. Unexpectedly, and differently from VAV1, tyrosine phosphorylation of SOS1, ARHGEF1, and DOCK2 is completely inhibited by pertussis toxin pretreatment, thus suggesting different routes of rho-GEF triggering upon CXCR4 engagement. Taken together, these findings reveal a deeper level of complexity in the rho-signaling module, with at least four different rho-GEFs cooperating in the regulation of chemokine-induced integrin activation, possibly suggesting the emergence of stochastic concurrency in signaling mechanisms controlling leukocyte trafficking.

Platelet-derived CXCL12 regulates monocyte function, survival, differentiation into macrophages and foam cells through differential involvement of CXCR4-CXCR7.

Platelets store and release CXCL12 (SDF-1), which governs differentiation of hematopoietic progenitors into either endothelial or macrophage-foam cells. CXCL12 ligates CXCR4 and CXCR7 and regulates monocyte/macrophage functions. This study deciphers the relative contribution of CXCR4–CXCR7 in mediating the effects of platelet-derived CXCL12 on monocyte function, survival, and differentiation. CXCL12 and macrophage migration inhibitory factor (MIF) that ligate CXCR4–CXCR7 induced a dynamic bidirectional trafficking of the receptors, causing CXCR4 internalization and CXCR7 externalization during chemotaxis, thereby influencing relative receptor availability, unlike MCP-1. In vivo we found enhanced accumulation of platelets and platelet-macrophage co-aggregates in peritoneal fluid following induction of peritonitis in mice. The relative surface expression of CXCL12, CXCR4, and CXCR7 among infiltrated monocytes was also enhanced as compared with peripheral blood. Platelet-derived CXCL12 from collagen-adherent platelets and recombinant CXCL12 induced monocyte chemotaxis specifically through CXCR4 engagement. Adhesion of monocytes to immobilized CXCL12 and CXCL12-enriched activated platelet surface under static and dynamic arterial flow conditions were mediated primarily through CXCR7 and were counter-regulated by neutralizing platelet-derived CXCL12. Monocytes and culture-derived-M1–M2 macrophages phagocytosed platelets, with the phagocytic potential of culture-derived-M1 macrophages higher than M2 involving CXCR4–CXCR7 participation. CXCR7 was the primary receptor in promoting monocyte survival as exerted by platelet-derived CXCL12 against BH3-mimetic induced apoptosis (phosphatidylserine exposure, caspase-3 activation, loss of mitochondrial transmembrane potential). In co-culture experiments with platelets, monocytes predominantly differentiated into CD163+ macrophages, which was attenuated upon CXCL12 neutralization and CXCR4/CXCR7 blocking antibodies. Moreover, OxLDL uptake by platelets induced platelet apoptosis, like other platelet agonists TRAP and collagen-related peptide (CRP). CXCL12 facilitated phagocytosis of apoptotic platelets by monocytes and M1–M2 macrophages, also promoted their differentiation into foam cells via CXCR4 and CXCR7. Thus, platelet-derived CXCL12 could regulate monocyte-macrophage functions through differential engagement of CXCR4 and CXCR7, indicating an important role in inflammation at site of platelet accumulation.

MIF promotes B cell chemotaxis through the receptors CXCR4 and CD74 and ZAP-70 signaling.

Macrophage migration inhibitory factor (MIF) is a proinflammatory cytokine with chemokine-like functions that plays a pivotal role in the pathogenesis of inflammatory diseases by promoting leukocyte recruitment. We showed that MIF promotes the atherogenic recruitment of monocytes and T cells through its receptors CXCR2 and CXCR4. Effects of MIF on B cell recruitment have not been addressed. In this study, we tested the involvement of MIF in B cell chemotaxis and studied the underlying mechanism. We show that MIF promotes primary murine B cell chemotaxis in a dose-dependent manner, comparable to the B cell chemokines CXCL13 and CXCL12. Splenic B cells express CXCR4 and the receptor CD74 but not CXCR2. Inhibition of CXCR4 or CD74 or a genetic deficiency of Cd74 in primary B cells fully abrogated MIF-mediated B cell migration, implying cooperative involvement of both receptors. MIF stimulation of B cells resulted in a rapid increase in intracellular Ca(2+) mobilization and F-actin polymerization. Intriguingly, the tyrosine kinase ZAP-70 was activated upon MIF and CXCL12 treatment in a CXCR4- and CD74-dependent manner. Pharmacological inhibition of ZAP-70 resulted in abrogation of primary B cell migration. Functional involvement of ZAP-70 was confirmed by small interfering RNA-mediated knockdown in Ramos B cell migration. Finally, primary B cells from ZAP-70 gene-deficient mice exhibited ablated transmigration in response to MIF or CXCL12. We conclude that MIF promotes the migration of B cells through a ZAP-70-dependent pathway mediated by cooperative engagement of CXCR4 and CD74. The data also suggest that MIF may contribute to B cell recruitment in vivo (e.g., in B cell-related immune disorders).

The carboxyl terminal trimer of procollagen I induces pro-metastatic changes and vascularization in breast cancer cells xenografts

BackgroundThe COOH terminal peptide of Pro-collagen type I (PICP, also called C3) is chemotactic for endothelial melanoma and breast cancer cells. PICP induces the expression of Metalloproteinases-2 and -9, of Vascular endothelial growth factor and of the chemokine CXCL-12 receptor CXCR4 in MDA MB231 breast carcinoma cells in vitro.MethodsWe used a model of xenografts in BalbC/nude mice obtaining tumors by implanting in contro-lateral subcutaneous positions MDA MB231 cells added or not with purified PICP and studied the earlier phases of tumor development, up to 48 days from implant, by histology, immunostain and in situ hybridization.ResultsAddition of PICP promotes rapid vascularization of the tumors while does not affect mitotic and apoptotic indexes and overall tumor growth. PICP-treated, relative to control tumors, show up-modulation of Vascular endothelial factor, Metalloproteinase-9 and CXCR4, all tumor prognostic genes; they also show down-modulation of the endogenous Metalloproteinase inhibitor, reversion-inducing-cysteine-rich protein with kazal motifs, and a different pattern of modulation of Tissue Inhibitor of Metalloproteinase-2. These changes occur in absence of detectable expression of CXCL-12, up to 38 days, in control and treated tumors.ConclusionPICP has an early promoting effect in the acquisition by the tumors of prometastatic phenotype. PICP may be play a relevant role in the productive interactions between stroma and tumor cells by predisposing the tumor cells to respond to the proliferation stimuli ensuing the activation of signaling by engagement of CXCR4 by cytokines and by fostering their extravasion, due to the induction of increased vascular development.

P52Shc is required for CXCR4-dependent signaling and chemotaxis in T cells

AbstractShcA is an important mediator of Ras/MAPK activation in PTK-regulated pathways triggered by surface receptors. This function is subserved by the constitutively expressed p52-kDa isoform. Besides activating Ras, p52Shc couples the TCR to Rho GTPases, and thereby participates in actin cytoskeleton remodeling in T cells. Here we have addressed the potential involvement of p52Shc in T-cell chemotaxis and the role of the phosphorylatable tyrosine residues, YY239/240 and Y317, in this process. We show that CXCR4 engagement by the homeostatic chemokine, SDF-1α, results in p52Shc phosphorylation and its assembly into a complex that includes Lck, ZAP-70, and Vav. This process was found to be both Lck and Gi dependent. Expression of p52Shc mutants lacking YY239/240 or Y317, or p52Shc deficiency, resulted in a profound impairment in CXCR4 signaling and SDF-1α–dependent chemotaxis, underscoring a crucial role of p52Shc as an early component of the CXCR4 signaling cascade. p52Shc was also found to be required for ligand-dependent CXCR4 internalization independently of tyrosine phosphorylation. Remarkably, CXCR4 engagement promoted phosphorylation of the ζ chain of the TCR/CD3 complex, which was found to be essential for CXCR4 signaling, as well as for SDF-1α–dependent receptor endocytosis and chemotaxis, indicating that CXCR4 signals by transactivating the TCR.

CXCR4 engagement promotes dendritic cell survival and maturation

It has been reported that human monocyte derived-dendritic cells (DCs) express CXCR4, responsible for chemotaxis to CXCL12. However, it remains unknown whether CXCR4 is involved in other functions of DCs. Initially, we found that CXCR4 was expressed on bone marrow-derived DCs (BMDCs). The addition of specific CXCR4 antagonist, 4-F-Benzoyl-TN14003, to the culture of mouse BMDCs decreased their number, especially the mature subset of them. The similar effect was found on the number of Langerhans cells (LCs) but not keratinocytes among epidermal cell suspensions. Since LCs are incapable of proliferating in vitro, these results indicate that CXCR4 engagement is important for not only maturation but also survival of DCs. Consistently, the dinitrobenzene sulfonic acid-induced, antigen-specific in vitro proliferation of previously sensitized lymph node cells was enhanced by CXCL12, and suppressed by CXCR4 antagonist. These findings suggest that CXCL12–CXCR4 engagement enhances DC maturation and survival to initiate acquired immune response.

CXCR4 chemokine receptor engagement modifies integrin dependent adhesion of renal carcinoma cells

The mechanisms leading to renal cell carcinoma (RCC) metastasis are incompletely understood. Although evidence shows that the chemokine receptor CXCR4 and its ligand CXCL12 may regulate tumor dissemination, their role in RCC is not clearly defined. We examined CXCR4 expression and functionality on RCC cell lines, and explored CXCL12-triggered tumor adhesion to human endothelium (HUVEC) or extracellular matrix proteins. Functional CXCR4 was expressed on A498 tumor cells, enabling them to migrate towards a CXCL12 gradient. CXCR4 engagement by CXCL12 induced elevated cell adhesion to HUVEC, to immobilized fibronectin, laminin or collagen. Anti-CXCR4 antibodies or CXCR4 knock down by siRNA applied prior to CXCL12 stimulation impaired CXCL12-triggered tumor adhesion. However, blocking CXCR4 subsequent to CXCL12 stimulation did not. This pointed to an indirect control of tumor cell adhesion by CXCR4. In fact, CXCR4 engagement by CXCL12 also induced alterations of receptors of the integrin family, notably alpha3, alpha5, beta1 and beta3 subunits, and blocking beta1 integrins with a function-blocking antibody prevented CXCL12-induced A498 adhesion. Focal adhesion kinase (total and activated) and integrin-linked kinase significantly increased in CXCL12-treated A498 cells, accompanied by a distinct up-regulation of ERK1/2, JNK and p38 phosphorylation. Therefore, CXCR4 may be crucial in controlling adhesion of A498 cells via cross talking with integrin receptors. These data show that CXCR4 receptors contribute to RCC dissemination and may provide a novel link between CXCR4 chemokine receptor expression and integrin triggered RCC adhesion to the vascular wall and subendothelial matrix components.

MMP-9 in B-cell chronic lymphocytic leukemia is up-regulated by α4β1 integrin or CXCR4 engagement via distinct signaling pathways, localizes to podosomes, and is involved in cell invasion and migration

AbstractB-cell chronic lymphocytic leukemia (B-CLL) progression is determined by malignant cell extravasation and lymphoid tissue infiltration. We have studied the role and regulation of matrix metalloproteinase-9 (MMP-9) in B-CLL cell migration and invasion. Adhesion of B-CLL cells to the fibronectin fragment FN-H89, VCAM-1, or TNF-α–activated human umbilical vein endothelial cells (HUVECs) up-regulated MMP-9 production, measured by gelatin zymography. This effect was mediated by α4β1 integrin and required PI3-K/Akt signaling. The chemokine CXCL12 also up-regulated MMP-9, independently of α4β1 and involving ERK1/2 but not Akt activity. Accordingly, α4β1 engagement activated the PI3-K/Akt/NF-κB pathway, while CXCL12/CXCR4 interaction activated ERK1/2/c-Fos signaling. Anti–MMP-9 antibodies, the MMP-9 inhibitor TIMP-1, or transfection with 3 different MMP-9 siRNAs significantly blocked migration through Matrigel or HUVECs. Cell-associated MMP-9 was mainly at the membrane and contained the proactive and mature forms. Moreover, B-CLL cells formed podosomes upon adhesion to FN-H89, VCAM-1, or fibronectin; MMP-9 localized to podosomes in a PI3-K–dependent manner and degraded a fibronectin/gelatin matrix. Our results are the first to show that MMP-9 is physiologically regulated by α4β1 integrin and CXCL12 and plays a key role in cell invasion and transendothelial migration, thus contributing to B-CLL progression. MMP-9 could therefore constitute a target for treatment of this malignancy.

HIV-1 Envelope Triggers Polyclonal Ig Class Switch Recombination through a CD40-Independent Mechanism Involving BAFF and C-Type Lectin Receptors

Switching from IgM to IgG and IgA is essential for antiviral immunity and requires engagement of CD40 on B cells by CD40L on CD4(+) T cells. HIV-1 is thought to impair CD40-dependent production of protective IgG and IgA by inducing progressive loss of CD4(+) T cells. Paradoxically, this humoral immunodeficiency is associated with B cell hyperactivation and increased production of nonprotective IgG and IgA that are either nonspecific or specific for HIV-1 envelope glycoproteins, including gp120. Nonspecific and gp120-specific IgG and IgA are sensitive to antiretroviral therapy and remain sustained in infected individuals with very few CD4(+) T cells. One interpretation is that some HIV-1 Ags elicit IgG and IgA class switch DNA recombination (CSR) in a CD40-independent fashion. We show that a subset of B cells binds gp120 through mannose C-type lectin receptors (MCLRs). In the presence of gp120, MCLR-expressing B cells up-regulate the CSR-inducing enzyme, activation-induced cytidine deaminase, and undergo CSR from IgM to IgG and IgA. CSR is further enhanced by IL-4 or IL-10, whereas Ab secretion requires a B cell-activating factor of the TNF family. This CD40L-related molecule is produced by monocytes upon CD4, CCR5, and CXCR4 engagement by gp120 and cooperates with IL-4 and IL-10 to up-regulate MCLRs on B cells. Thus, gp120 may elicit polyclonal IgG and IgA responses by linking the innate and adaptive immune systems through the B cell-activating factor of the TNF family. Chronic activation of B cells through this CD40-independent pathway could impair protective T cell-dependent Ab responses by inducing immune exhaustion.

Coreceptor Dependent Signaling in Individual Primary Resting CD4+ T-cells Mediated by Low Levels of HIV Binding

In order to enter into the target cell, HIV requires functional contact with CD4 and CCR5 or CXCR4. The last two are G-protein coupled receptors that when activated with chemokines, or HIV envelope can initiate a wide range of biological responses, including Ca2+ mobilization, cytoskeletal rearrangements and cell migration. To determine the specificity of X4-tropic gp120-mediated signaling through CXCR4, we have chosen a microscopybased approach to observe the response at the level of individual cells providing greater sensitivity. Target cells were able to activate a signaling cascade in response to both monomeric recombinant gp120 and virion-bound trimeric gp120. C2+ elevation was a direct measurement of CXCR4 engagement because it was dependent on the tropism of the envelope, engagement of CD4, and sensitive to the CXCR4 antagonist AMD-3100. Signaling required much lower levels of envelope when virion associated. Imaging analysis allowed the correlation of the pattern of virion-mediated C2+ fluxing with the exact number of viral particles bound to cells. This analysis revealed that an average of four virions, and as few as two virions associating with a primary resting T cell could mediate C2+ mobilization. The ability of several virions to stimulate signaling in primary resting T cells is physiologically relevant and has important implications for AIDS pathogenesis. Funded in part by DHHS NO1-CO-12400 and RO1AI052051. from 2005 International Meeting of The Institute of Human Virology Baltimore, USA, 29 August – 2 September 2005

CXCR4 engagement is required for HIV-1–induced L-selectin shedding

AbstractThe chemokine receptor, CXCR4, serves as the primary coreceptor for entry of T-cell tropic human immunodeficiency virus (HIV). Binding of either the CXC-chemokine, stromal-derived factor 1α (SDF-1α), or a CXCR4 antagonist, AMD3100, to CXCR4 inhibits infection of CD4+ T cells by T-tropic HIV-1, although only SDF-1α triggers T-cell signaling cascades. We have previously demonstrated that ligation of CD4 by T-cell tropic HIV-1 NL4-3 induces metalloproteinase-dependent L-selectin (CD62L) shedding on resting CD4+ T cells. However, the role of CXCR4 in HIV-induced L-selectin shedding is unclear. Here, we show that L-selectin shedding induced by HIV-1 NL4-3 is completely reversed by AMD3100, but not SDF-1α, although SDF-1α alone does not induce L-selectin shedding. These results indicate that engagement of both CD4 and CXCR4 is required for HIV-induced shedding of L-selectin on primary resting CD4+ T cells.

IFN-producing cells respond to CXCR3 ligands in the presence of CXCL12 and secrete inflammatory chemokines upon activation.

Human natural IFN-producing cells (IPC) circulate in the blood and cluster in chronically inflamed lymph nodes around high endothelial venules (HEV). Although L-selectin, CXCR4, and CCR7 are recognized as critical IPC homing mediators, the role of CXCR3 is unclear, since IPC do not respond to CXCR3 ligands in vitro. In this study, we show that migration of murine and human IPC to CXCR3 ligands in vitro requires engagement of CXCR4 by CXCL12. We also demonstrate that CXCL12 is present in human HEV in vivo. Moreover, after interaction with pathogenic stimuli, murine and human IPC secrete high levels of inflammatory chemokines. Thus, IPC migration into inflamed lymph nodes may be initially mediated by L-selectin, CXCL12, and CXCR3 ligands. Upon pathogen encounter, IPC positioning within the lymph node may be further directed by CCR7 and IPC secretion of inflammatory chemokines may attract other IPC, promoting cluster formation in lymph nodes.

HIV-1 gp41 six-helix bundle formation occurs rapidly after the engagement of gp120 by CXCR4 in the HIV-1 Env-mediated fusion process.

The onset of cell fusion mediated by HIV-1 IIIB Env is preceded by a lag phase of 15-20 min. Fusion mediated by the CD4-independent HIV-1 Env 8x, which is capable of interacting directly with CXCR4, proceeds with a greatly reduced lag phase. We probed the intermediate steps during the lag phase in HIV-1 IIIB Env-mediated fusion with Leu3-a, an inhibitor of attachment of gp120 to CD4, AMD3100, an inhibitor of attachment of gp120 to CXCR4, and C34, a synthetic peptide that interferes with the transition of gp41 to the fusion active state. Inhibitions of fusion as a function of time of addition of C34 and of AMD3100 were equivalent, indicating that engagement of gp120 by CXCR4 and formation of the gp41 six-helix bundle follow similar kinetics. The initial steps in fusion mediated by the CD4-independent Env 8x are too rapid for these inhibitors to interfere with. However, when 8x Env-expressing cells were incubated with target cells at 25 degrees C in the presence of AMD3100 or C34, prior to incubation at 37 degrees C, these inhibitors were capable of inhibiting 8x Env-mediated fusion. To further examine engagement of gp120 by CXCR4 and exposure of binding sites for C34, we have reversibly arrested the fusion reaction at 37 degrees C by adding cytochalasin B to the medium. We show that CXCR4 engagement and six-helix bundle formation only occur after the release of the cytochalasin arrest, indicating that a high degree of cooperativity is required to trigger the initial steps in HIV-1 Env-mediated fusion.

Human immunodeficiency virus type 1 (HIV-1)-induced GRO-alpha production stimulates HIV-1 replication in macrophages and T lymphocytes.

We examined the early effects of infection by CCR5-using (R5 human immunodeficiency virus [HIV]) and CXCR4-using (X4 HIV) strains of HIV type 1 (HIV-1) on chemokine production by primary human monocyte-derived macrophages (MDM). While R5 HIV, but not X4 HIV, replicated in MDM, we found that the production of the C-X-C chemokine growth-regulated oncogene alpha (GRO-alpha) was markedly stimulated by X4 HIV and, to a much lesser extent, by R5 HIV. HIV-1 gp120 engagement of CXCR4 initiated the stimulation of GRO-alpha production, an effect blocked by antibodies to CXCR4. GRO-alpha then fed back and stimulated HIV-1 replication in both MDM and lymphocytes, and antibodies that neutralize GRO-alpha or CXCR2 (the receptor for GRO-alpha) markedly reduced viral replication in MDM and peripheral blood mononuclear cells. Therefore, activation of MDM by HIV-1 gp120 engagement of CXCR4 initiates an autocrine-paracrine loop that may be important in disease progression after the emergence of X4 HIV.