CXCR4 WHIM syndrome is a cancer predisposition condition for virus-induced malignancies

Warts, hypogammaglobulinaemia, infections and myelokathexis syndrome (WHIMS) is a rare combined primary immunodeficiency caused by the gain of function of the CXCR4 chemokine receptor. We present the prevalence of cancer in WHIMS patients based on data from the French Severe Chronic Neutropenia Registry and an exhaustive literature review. The median follow-up of the 14 WHIMS 'patients was 28.5 years. A central review and viral evaluation of pathological samples were organized, and we conducted a thorough literature review to identify all reports of WHIMS cases. Six French patients were diagnosed with cancer at a median age of 37.6 years. The 40-year risk of malignancy was 39% (95% confidence interval [CI]: 6%-74%). We observed two human papillomavirus (HPV)-induced vulvar carcinomas, three lymphomas (two Epstein-Barr virus [EBV]-related) and one basal cell carcinoma. Among the 155 WHIMS cases from the literature, 22 cancers were reported in 16 patients, with an overall cancer 40-year risk of 23% (95% CI: 13%-39%). Malignancies included EBV-associated lymphoproliferative disorders and HPV-positive genital and anal cancers as in the French cohort. Worldwide, nine cases of malignancy were associated with HPV and four with EBV. Immunocompromised WHIMS patients appear to be particularly susceptible to developing early malignancy, mainly HPV-induced carcinomas, followed by EBV-related lymphomas.

Optimized protocol for 3D epithelial cultures supporting human papillomavirus replication

Human papillomaviruses (HPVs) are commensal viruses with pathogenic potential. Their life cycle requires the proliferation and differentiation of keratinocytes (KCs) to form pluristratified epithelia. Based on the original organotypic epithelial raft cultures protocol, we provide an updated workflow to optimally generate pluristratified human epithelia supporting the complete HPV replicative life cycle, here called 3D full-thickness epithelial cultures (3Deps). We describe steps for HPV genome preparation, KC transfection, and dermal equivalent preparation. We then detail procedures for 3Deps culture, harvesting, and analysis.

[The cutaneous virome: from virology to personalized medicine]

The virome of the skin, defined as all viruses detected in the skin, represents a significant part of the microbiota. A much more recent discovery than the bacterial flora, the existence of the cutaneous virome has been revealed by recent metagenomic studies. The normal human skin virome is dominated by bacteriophages, Papillomaviridae, whose genomic diversity has proved extraordinary, and Polyomaviridae. Many yet unknown viral genomes within this virome await identification. The composition of the virome of the skin has been shown to be strictly individual and relatively stable over time, resulting from adaptation to everyone's genetics, lifestyle and mechanisms of immunological tolerance finely selected over the course of evolution. Yet little studied, the virome of the skin and all its interactions with other microbiota and the host are attracting growing interest. Indeed, constitutional or acquired alterations in the homeostasis between the commensal virome and the skin, ranging from sub-clinical viral dysbiosis to severe transformation of keratinocytes or adnexal cells, have been observed. These recent observations are stimulating the search for innovative solutions aimed at measuring or even modulating its pathological expression, with a view to personalized medicine.

Severe CD8+ T Lymphopenia in WHIM Syndrome Caused by Selective Sequestration in Primary Immune Organs

Warts, hypogammaglobulinemia, infections, and myelokathexis (WHIM) syndrome is an ultra-rare combined primary immunodeficiency disease caused by heterozygous gain-of-function mutations in the chemokine receptor CXCR4. WHIM patients typically present with recurrent acute infections associated with myelokathexis (severe neutropenia due to bone marrow retention of mature neutrophils). Severe lymphopenia is also common, but the only associated chronic opportunistic pathogen is human papillomavirus and mechanisms are not clearly defined. In this study, we show that WHIM mutations cause more severe CD8 than CD4 lymphopenia in WHIM patients and WHIM model mice. Mechanistic studies in mice revealed selective and WHIM allele dose-dependent accumulation of mature CD8 single-positive cells in thymus in a cell-intrinsic manner due to prolonged intrathymic residence, associated with increased CD8 single-positive thymocyte chemotactic responses in vitro toward the CXCR4 ligand CXCL12. In addition, mature WHIM CD8+ T cells preferentially home to and are retained in the bone marrow in mice in a cell-intrinsic manner. Administration of the specific CXCR4 antagonist AMD3100 (plerixafor) in mice rapidly and transiently corrected T cell lymphopenia and the CD4/CD8 ratio. After lymphocytic choriomeningitis virus infection, we found no difference in memory CD8+ T cell differentiation or viral load between wild-type and WHIM model mice. Thus, lymphopenia in WHIM syndrome may involve severe CXCR4-dependent CD8+ T cell deficiency resulting in part from sequestration in the primary lymphoid organs, thymus, and bone marrow.

Reprogramming of connexin landscape fosters fast gap junction intercellular communication in human papillomavirus-infected epithelia

Human papillomaviruses (HPVs) are highly prevalent commensal viruses that require epithelial stratification to complete their replicative cycle. While HPV infections are most often asymptomatic, certain HPV types can cause lesions, that are usually benign. In rare cases, these infections may progress to non-replicative viral cycles associated with high HPV oncogene expression promoting cell transformation, and eventually cancer when not cleared by host responses. While the consequences of HPV-induced transformation on keratinocytes have been extensively explored, the impact of viral replication on epithelial homeostasis remains largely unexplored. Gap junction intercellular communication (GJIC) is critical for stratified epithelium integrity and function. This process is ensured by a family of proteins named connexins (Cxs), including 8 isoforms that are expressed in stratified squamous epithelia. GJIC was reported to be impaired in HPV-transformed cells, which was attributed to the decreased expression of the Cx43 isoform. However, it remains unknown whether and how HPV replication might impact on the expression of Cx isoforms and GJIC in stratified squamous epithelia. To address this question, we have used 3D-epithelial cell cultures (3D-EpCs), the only model supporting the productive HPV life cycle. We report a transcriptional downregulation of most epithelial Cx isoforms except Cx45 in HPV-replicating epithelia. At the protein level, HPV replication results in a reduction of Cx43 expression while that of Cx45 increases and displays a topological shift toward the cell membrane. To quantify GJIC, we pioneered quantitative gap-fluorescence loss in photobleaching (FLIP) assay in 3D-EpCs, which allowed us to show that the reprogramming of Cx landscape in response to HPV replication translates into accelerated GJIC in living epithelia. Supporting the pathophysiological relevance of our observations, the HPV-associated Cx43 and Cx45 expression pattern was confirmed in human cervical biopsies harboring HPV. In conclusion, the reprogramming of Cx expression and distribution in HPV-replicating epithelia fosters accelerated GJIC, which may participate in epithelial homeostasis and host immunosurveillance.

Clinical and Hematologic Effects of Endotoxin in Warts, Hypogammaglobulinemia, Infections, and Myelokathexis Syndrome Model Mice

Warts, hypogammaglobulinemia, infections, and myelokathexis (WHIM) syndrome immunodeficiency is caused by autosomal dominant gain-of-function CXCR4 mutations that promote severe panleukopenia caused by bone marrow retention of mature leukocytes. Consequently, WHIM patients develop recurrent bacterial infections; however, sepsis is uncommon. To study this clinical dichotomy, we challenged WHIM model mice with LPS. The LD₅₀ was similar in WHIM and wild-type (WT) mice, and LPS induced acute lymphopenia in WT mice that was Cxcr4 independent. In contrast, in WHIM mice, LPS did not affect circulating T cell levels, but the B cell levels anomalously increased because of selective, cell-intrinsic, and Cxcr4 WHIM allele-dependent emergence of Cxcr4^high late pre-B cells, a pattern that was phenocopied by Escherichia coli infection. In both WT and WHIM mice, the CXCR4 antagonist AMD3100 rapidly increased circulating lymphocyte levels that then rapidly contracted after subsequent LPS treatment. Thus, LPS-induced lymphopenia is CXCR4 independent, and a WHIM mutation does not increase clinical LPS sensitivity. Anomalous WT Cxcr4-independent, but Cxcr4 WHIM-dependent, promobilizing effects of LPS on late pre-B cell mobilization reveal a distinct signaling pathway for the variant receptor.

Case Report: Altered NK Cell Compartment and Reduced CXCR4 Chemotactic Response of B Lymphocytes in an Immunodeficient Patient With HPV-Related Disease

The study of inborn errors of immunity (IEI) provides unique opportunities to elucidate the microbiome and pathogenic mechanisms related to severe viral infection. Several immunological and genetic anomalies may contribute to the susceptibility to develop Human Papillomavirus (HPV) pathogenesis. They include different acquired immunodeficiencies, EVER1-2 or CIB1 mutations underlying epidermodysplasia verruciformis (EV) syndrome and multiple IEI. Whereas EV syndrome patients are specifically unable to control infections with beta HPV, individuals with IEI show broader infectious and immune phenotypes. The WHIM (warts, hypogammaglobulinemia, infection, and myelokathexis) syndrome caused by gain-of-CXCR4-function mutation manifests by HPV-induced extensive cutaneous warts but also anogenital lesions that eventually progress to dysplasia. Here we report alterations of B and NK cells in a female patient suffering from cutaneous and mucosal HPV-induced lesions due to an as-yet unidentified genetic defect. Despite no detected mutations in CXCR4, B but not NK cells displayed a defective CXCR4-dependent chemotactic response toward CXCL12. In addition, NK cells showed an abnormal distribution with an expanded CD56^bright cell subset and defective cytotoxicity of CD56^dim cells. Our observations extend the clinical and immunological spectrum of IEI associated with selective susceptibility toward HPV pathogenesis, thus providing new insight on the immune control of HPV infection and potential host susceptibility factors.

HPV infection alters vaginal microbiome through down-regulating host mucosal innate peptides used by Lactobacilli as amino acid sources

Despite the high prevalence of both cervico-vaginal human papillomavirus (HPV) infection and bacterial vaginosis (BV) worldwide, their causal relationship remains unclear. While BV has been presumed to be a risk factor for HPV acquisition and related carcinogenesis for a long time, here, supported by both a large retrospective follow-up study (n = 6,085) and extensive in vivo data using the K14-HPV16 transgenic mouse model, we report a novel blueprint in which the opposite association also exists. Mechanistically, by interacting with several core members (NEMO, CK1 and β-TrCP) of both NF-κB and Wnt/β-catenin signaling pathways, we show that HPV E7 oncoprotein greatly inhibits host defense peptide expression. Physiologically secreted by the squamous mucosa lining the lower female genital tract, we demonstrate that some of these latter are fundamental factors governing host-microbial interactions. More specifically, several innate molecules down-regulated in case of HPV infection are hydrolyzed, internalized and used by the predominant Lactobacillus species as amino acid source sustaining their growth/survival. Collectively, this study reveals a new viral immune evasion strategy which, by its persistent/negative impact on lactic acid bacteria, ultimately causes the dysbiosis of vaginal microbiota.

Biallelic CXCR2 loss-of-function mutations define a distinct congenital neutropenia entity

Not available.

Multi-Tissue Characterization of GILZ Expression in Dendritic Cell Subsets at Steady State and in Inflammatory Contexts

Dendritic cells (DCs) are key players in the control of tolerance and immunity. Glucocorticoids (GCs) are known to regulate DC function by promoting their tolerogenic differentiation through the induction of inhibitory ligands, cytokines, and enzymes. The GC-induced effects in DCs were shown to critically depend on increased expression of the Glucocorticoid-Induced Leucine Zipper protein (GILZ). GILZ expression levels were further shown to control antigen-presenting cell function, as well as T-cell priming capacity of DCs. However, the pattern of GILZ expression in DC subsets across tissues remains poorly described, as well as the modulation of its expression levels in different pathological settings. To fill in this knowledge gap, we conducted an exhaustive analysis of GILZ relative expression levels in DC subsets from various tissues using multiparametric flow cytometry. This study was performed at steady state, in the context of acute as well as chronic skin inflammation, and in a model of cancer. Our results show the heterogeneity of GILZ expression among DC subsets as well as the complexity of its modulation, that varies in a cell subset- and context-specific manner. Considering the contribution of GILZ in the control of DC functions and its potential as an immune checkpoint in cancer settings, these results are of high relevance for optimal GILZ targeting in therapeutic strategies.

The CXCL12/CXCR4/ACKR3 Axis in the Tumor Microenvironment: Signaling, Crosstalk, and Therapeutic Targeting

Elevated expression of the chemokine receptors CXCR4 and ACKR3 and of their cognate ligand CXCL12 is detected in a wide range of tumors and the tumor microenvironment (TME). Yet, the molecular mechanisms by which the CXCL12/CXCR4/ACKR3 axis contributes to the pathogenesis are complex and not fully understood. To dissect the role of this axis in cancer, we discuss its ability to impinge on canonical and less conventional signaling networks in different cancer cell types; its bidirectional crosstalk, notably with receptor tyrosine kinase (RTK) and other factors present in the TME; and the infiltration of immune cells that supporttumor progression. We discuss current and emerging avenues that target the CXCL12/CXCR4/ACKR3 axis. Coordinately targeting both RTKs and CXCR4/ACKR3 and/or CXCL12 is an attractive approach to consider in multitargeted cancer therapies. In addition, inhibiting infiltrating immune cells or reactivating the immune system along with modulating the CXCL12/CXCR4/ACKR3 axis in the TME has therapeutic promise.

Differential activity and selectivity of N-terminal modified CXCL12 chemokines at the CXCR4 and ACKR3 receptors

Chemokines play critical roles in numerous physiologic and pathologic processes through their action on seven-transmembrane (TM) receptors. The N-terminal domain of chemokines, which is a key determinant of signaling via its binding within a pocket formed by receptors' TM helices, can be the target of proteolytic processing. An illustrative case of this regulatory mechanism is the natural processing of CXCL12 that generates chemokine variants lacking the first two N-terminal residues. Whereas such truncated variants behave as antagonists of CXCR4, the canonical G protein-coupled receptor of CXCL12, they are agonists of the atypical chemokine receptor 3 (ACKR3/CXCR7), suggesting the implication of different structural determinants in the complexes formed between CXCL12 and its two receptors. Recent analyses have suggested that the CXCL12 N-terminus first engages the TM helices of ACKR3 followed by the receptor N-terminus wrapping around the chemokine core. Here we investigated the first stage of ACKR3-CXCL12 interactions by comparing the activity of substituted or N-terminally truncated variants of CXCL12 toward CXCR4 and ACKR3. We showed that modification of the first two N-terminal residues of the chemokine (K1R or P2G) does not alter the ability of CXCL12 to activate ACKR3. Our results also identified the K1R variant as a G protein-biased agonist of CXCR4. Comparative molecular dynamics simulations of the complexes formed by ACKR3 either with CXCL12 or with the P2G variant identified interactions between the N-terminal 2-4 residues of CXCL12 and a pocket formed by receptor's TM helices 2, 6, and 7 as critical determinants for ACKR3 activation.

Identification by high-throughput sequencing of HPV variants and quasispecies that are untypeable by linear reverse blotting assay in cervical specimens

The linear reverse blotting assays are valid methods for accurate human papillomavirus (HPV) typing required to manage women at risk of developing cervical cancer. However, some samples showed a positive signal in HPV lines but failed to display a positive signal in subsequent typing lines (designated as HPV-X), which indicate that certain types were not available on the respective typing blots. The aim of this study is to elucidate the types or variants of HPV through the high-throughput sequencing (HTS) of 54 ASCUS cervical samples in which the viruses remained untypeable with INNO LiPA HPV^® assays. Low-risk (LR)-HPV types (HPV6, 30, 42, 62, 67, 72, 74, 81, 83, 84, 87, 89, 90 and 114), high-risk (HR)-HPV35 and possibly (p)HR-HPV73 were detected among HPV-X. Individual multiple infections (two to seven types) were detected in 40.7% of samples. Twenty-two specimens contained variants characterised by 2–10 changes. HPV30 reached the maximal number of 17 variants with relative abundance inferior or equal to 2.7%. The presence of L1 quasispecies explains why linear reverse blotting assays fail when variants compete or do not match the specific probes. Further studies are needed to measure the LR-HPV quasispecies dynamics and its role during persistent infection.

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Types which linear reverse blotting assays are unable to identify are mainly multiple LR-HPV.

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Deep sequencing of L1 permits to identify minority variants in 41% of these samples.

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Understanding of LR-HPV quasispecies dynamics during infection is awaited.

Atypical Chemokine Receptor 3 (ACKR3): A Comprehensive Overview of its Expression and Potential Roles in the Immune System

Atypical chemokine receptor 3 (ACKR3), previously known as C-X-C chemokine receptor type 7 (CXCR7), has emerged as a key player in several biologic processes, particularly during development. Its CXCL11 and CXCL12 scavenging activity and atypical signaling properties, together with a new array of other nonchemokine ligands, have established ACKR3 as a main regulator of physiologic processes at steady state and during inflammation. Here, we present a comprehensive review of ACKR3 expression in mammalian tissues in search of a possible connection with the receptor function. Besides the reported roles of ACKR3 during development, we discuss the potential contribution of ACKR3 to the function of the immune system, focusing on the myeloid lineage.

A Neutrophil Timer Coordinates Immune Defense and Vascular Protection

Neutrophils eliminate pathogens efficiently but can inflict severe damage to the host if they over-activate within blood vessels. It is unclear how immunity solves the dilemma of mounting an efficient anti-microbial defense while preserving vascular health. Here, we identify a neutrophil-intrinsic program that enabled both. The gene Bmal1 regulated expression of the chemokine CXCL2 to induce chemokine receptor CXCR2-dependent diurnal changes in the transcriptional and migratory properties of circulating neutrophils. These diurnal alterations, referred to as neutrophil aging, were antagonized by CXCR4 (C-X-C chemokine receptor type 4) and regulated the outer topology of neutrophils to favor homeostatic egress from blood vessels at night, resulting in boosted anti-microbial activity in tissues. Mice engineered for constitutive neutrophil aging became resistant to infection, but the persistence of intravascular aged neutrophils predisposed them to thrombo-inflammation and death. Thus, diurnal compartmentalization of neutrophils, driven by an internal timer, coordinates immune defense and vascular protection.

Natural history of GATA2 deficiency in a survey of 79 French and Belgian patients

Heterozygous germline GATA2 mutations strongly predispose to leukemia, immunodeficiency, and/or lymphoedema. We describe a series of 79 patients (53 families) diagnosed since 2011, made up of all patients in France and Belgium, with a follow up of 2249 patients/years. Median age at first clinical symptoms was 18.6 years (range, 0-61 years). Severe infectious diseases (mycobacteria, fungus, and human papilloma virus) and hematologic malignancies were the most common first manifestations. The probability of remaining symptom-free was 8% at 40 years old. Among the 53 probands, 24 had missense mutations including 4 recurrent alleles, 21 had nonsense or frameshift mutations, 4 had a whole-gene deletion, 2 had splice defects, and 2 patients had complex mutations. There were significantly more cases of leukemia in patients with missense mutations (n=14 of 34) than in patients with nonsense or frameshift mutations (n=2 of 28). We also identify new features of the disease: acute lymphoblastic leukemia, juvenile myelomonocytic leukemia, fatal progressive multifocal leukoencephalopathy related to the JC virus, and immune/inflammatory diseases. A revised International Prognostic Scoring System (IPSS) score allowed a distinction to be made between a stable disease and hematologic transformation. Chemotherapy is of limited efficacy, and has a high toxicity with severe infectious complications. As the mortality rate is high in our cohort (up to 35% at the age of 40), hematopoietic stem cell transplantation (HSCT) remains the best choice of treatment to avoid severe infectious and/or hematologic complications. The timing of HSCT remains difficult to determine, but the earlier it is performed, the better the outcome.

CXCR4-Specific Nanobodies as Potential Therapeutics for WHIM syndrome

WHIM syndrome is a rare congenital immunodeficiency disease, named after its main clinical manifestations: warts, hypogammaglobulinemia, infections, and myelokathexis, which refers to abnormal accumulation of mature neutrophils in the bone marrow. The disease is primarily caused by C-terminal truncation mutations of the chemokine receptor CXCR4, giving these CXCR4-WHIM mutants a gain of function in response to their ligand CXCL12. Considering the broad functions of CXCR4 in maintaining leukocyte homeostasis, patients are panleukopenic and display altered immune responses, likely as a consequence of impairment in the differentiation and trafficking of leukocytes. Treatment of WHIM patients currently consists of symptom relief, leading to unsatisfactory clinical responses. As an alternative and potentially more effective approach, we tested the potency and efficacy of CXCR4-specific nanobodies on inhibiting CXCR4-WHIM mutants. Nanobodies are therapeutic proteins based on the smallest functional fragments of heavy chain antibodies. They combine the advantages of small-molecule drugs and antibody-based therapeutics due to their relative small size, high stability, and high affinity. We compared the potential of monovalent and bivalent CXCR4-specific nanobodies to inhibit CXCL12-induced CXCR4-WHIM-mediated signaling with the small-molecule clinical candidate AMD3100. The CXCR4-targeting nanobodies displace CXCL12 binding and bind CXCR4-wild type and CXCR4-WHIM (R334X/S338X) mutants and with (sub-) nanomolar affinities. The nanobodies' epitope was mapped to extracellular loop 2 of CXCR4, overlapping with the binding site of CXCL12. Monovalent, and in particular bivalent, nanobodies were more potent than AMD3100 in reducing CXCL12-mediated G protein activation. In addition, CXCR4-WHIM-dependent calcium flux and wound healing of human papillomavirus-immortalized cell lines in response to CXCL12 was effectively inhibited by the nanobodies. Based on these in vitro results, we conclude that CXCR4 nanobodies hold significant potential as alternative therapeutics for CXCR4-associated diseases such as WHIM syndrome.

Lymphoid differentiation of hematopoietic stem cells requires efficient Cxcr4 desensitization

The CXCL12/CXCR4 signaling exerts a dominant role in promoting hematopoietic stem and progenitor cell (HSPC) retention and quiescence in bone marrow. Gain-of-function CXCR4 mutations that affect homologous desensitization of the receptor have been reported in the WHIM Syndrome (WS), a rare immunodeficiency characterized by lymphopenia. The mechanisms underpinning this remain obscure. Using a mouse model with a naturally occurring WS-linked gain-of-function Cxcr4 mutation, we explored the possibility that the lymphopenia in WS arises from defects at the HSPC level. We reported that Cxcr4 desensitization is required for quiescence/cycling balance of murine short-term hematopoietic stem cells and their differentiation into multipotent and downstream lymphoid-biased progenitors. Alteration in Cxcr4 desensitization resulted in decrease of circulating HSPCs in five patients with WS. This was also evidenced in WS mice and mirrored by accumulation of HSPCs in the spleen, where we observed enhanced extramedullary hematopoiesis. Therefore, efficient Cxcr4 desensitization is critical for lymphoid differentiation of HSPCs, and its impairment is a key mechanism underpinning the lymphopenia observed in mice and likely in WS patients.

Glucocorticoid-Induced Leucine Zipper Protein Controls Macropinocytosis in Dendritic Cells

Ag sampling is a key process in dendritic cell (DC) biology. DCs use constitutive macropinocytosis, receptor-mediated endocytosis, and phagocytosis to capture exogenous Ags for presentation to T cells. We investigated the mechanisms that regulate Ag uptake by DCs in the steady-state and after a short-term LPS exposure in vitro and in vivo. We show that the glucocorticoid-induced leucine zipper protein (GILZ), already known to regulate effector versus regulatory T cell activation by DCs, selectively limits macropinocytosis, but not receptor-mediated phagocytosis, in immature and recently activated DCs. In vivo, the GILZ-mediated inhibition of Ag uptake is restricted to the CD8α⁺ DC subset, which expresses the highest GILZ level among splenic DC subsets. In recently activated DCs, we further establish that GILZ limits p38 MAPK phosphorylation, providing a possible mechanism for GILZ-mediated macropinocytosis control. Finally, our results demonstrate that the modulation of Ag uptake by GILZ does not result in altered Ag presentation to CD4 T cells but impacts the efficiency of cross-presentation to CD8 T cells. Altogether, our results identify GILZ as an endogenous inhibitor of macropinocytosis in DCs, the action of which contributes to the fine-tuning of Ag cross-presentation.

CXCR4 identifies transitional bone marrow premonocytes that replenish the mature monocyte pool for peripheral responses

It is well established that Ly6C^hi monocytes develop from common monocyte progenitors (cMoPs) and reside in the bone marrow (BM) until they are mobilized into the circulation. In our study, we found that BM Ly6C^hi monocytes are not a homogenous population, as current data would suggest. Using computational analysis approaches to interpret multidimensional datasets, we demonstrate that BM Ly6C^hi monocytes consist of two distinct subpopulations (CXCR4^hi and CXCR4^lo subpopulations) in both mice and humans. Transcriptome studies and in vivo assays revealed functional differences between the two subpopulations. Notably, the CXCR4^hi subset proliferates and is immobilized in the BM for the replenishment of functionally mature CXCR4^lo monocytes. We propose that the CXCR4^hi subset represents a transitional premonocyte population, and that this sequential step of maturation from cMoPs serves to maintain a stable pool of BM monocytes. Additionally, reduced CXCR4 expression on monocytes, upon their exit into the circulation, does not reflect its diminished role in monocyte biology. Specifically, CXCR4 regulates monocyte peripheral cellular activities by governing their circadian oscillations and pulmonary margination, which contributes toward lung injury and sepsis mortality. Together, our study demonstrates the multifaceted role of CXCR4 in defining BM monocyte heterogeneity and in regulating their function in peripheral tissues.

Interaction of chemokine receptor CXCR4 in monomeric and dimeric state with its endogenous ligand CXCL12: coarse-grained simulations identify differences

Despite the recent resolutions of the crystal structure of the chemokine receptor CXCR4 in complex with small antagonists or viral chemokine, a description at the molecular level of the interactions between the full-length CXCR4 and its endogenous ligand, the chemokine CXCL12, in relationship with the receptor recognition and activation, is not yet completely elucidated. Moreover, since CXCR4 is able to form dimers, the question of whether the CXCR4-CXCL12 complex has a 1:1 or 2:1 preferential stoichiometry is still an open question. We present here results of coarse-grained protein-protein docking and molecular dynamics simulations of CXCL12 in association with CXCR4 in monomeric and dimeric states. Our proposed models for the 1:1 and 2:1 CXCR4-CXCL12 quaternary structures are consistent with recognition and activation motifs of both partners provided by the available site-directed mutagenesis data. Notably, we observed that in the 2:1 complex, the chemokine N-terminus makes more steady contacts with the receptor residues critical for binding and activation than in the 1:1 structure, suggesting that the 2:1 stoichiometry would favor the receptor signaling activity with respect to the 1:1 association.

Neutropenic Mice Provide Insight into the Role of Skin-Infiltrating Neutrophils in the Host Protective Immunity against Filarial Infective Larvae

Our knowledge and control of the pathogenesis induced by the filariae remain limited due to experimental obstacles presented by parasitic nematode biology and the lack of selective prophylactic or curative drugs. Here we thought to investigate the role of neutrophils in the host innate immune response to the infection caused by the Litomosoides sigmodontis murine model of human filariasis using mice harboring a gain-of-function mutation of the chemokine receptor CXCR4 and characterized by a profound blood neutropenia (Cxcr4^+/1013). We provided manifold evidence emphasizing the major role of neutrophils in the control of the early stages of infection occurring in the skin. Firstly, we uncovered that the filarial parasitic success was dramatically decreased in Cxcr4^+/1013 mice upon subcutaneous delivery of the infective stages of filariae (infective larvae, L3). This protection was linked to a larger number of neutrophils constitutively present in the skin of the mutant mice herein characterized as compared to wild type (wt) mice. Indeed, the parasitic success in Cxcr4^+/1013 mice was normalized either upon depleting neutrophils, including the pool in the skin, or bypassing the skin via the intravenous infection of L3. Second, extending these observations to wt mice we found that subcutaneous delivery of L3 elicited an increase of neutrophils in the skin. Finally, living L3 larvae were able to promote in both wt and mutant mice, an oxidative burst response and the release of neutrophil extracellular traps (NET). This response of neutrophils, which is adapted to the large size of the L3 infective stages, likely directly contributes to the anti-parasitic strategies implemented by the host. Collectively, our results are demonstrating the contribution of neutrophils in early anti-filarial host responses through their capacity to undertake different anti-filarial strategies such as oxidative burst, degranulation and NETosis.

Filariases are chronic debilitating diseases caused by parasitic nematodes affecting more than 150 million people worldwide. None of the current drugs are selective, neither able to eliminate the parasites nor to prevent new infections once the drug pressure has waned. Therefore, blocking the entry and the migration of the infective larvae (L3) could be an efficient way to control the infection. In the present study we investigated the early interaction between the host and the L. sigmodontis murine filariasis with a focus on the neutrophils in the innate host responses. We uncovered a key role of neutrophils in the control of infection provided by the CXCR4-gain-of-function mice (Cxcr4^+/1013) that display a blood neutropenia as well as an accumulation of skin-infiltrating neutrophils. Overall, we reveal that in the early phase of filariasis, i.e. after L3 are delivered into the skin and before they reach their site for reproduction, neutrophils are critical elements of the host innate protective response arsenal. A better understanding of their indirect and/or effector role(s) may provide mechanistic clues to host factors implicated in parasitic nematode entry and potentially lead to the identification of new drug targets.

Altered chemotactic response to CXCL12 in patients carrying GATA2 mutations

GATA2 deficiency-formerly described as MonoMAC syndrome; dendritic cells, monocytes, B cells, and natural killer cell deficiency; familial myelodysplastic syndrome/acute myeloid leukemia; or Emberger syndrome-encompasses a range of hematologic and nonhematologic anomalies, mainly characterized by monocytopenia, B lymphopenia, natural killer cell cytopenia, neutropenia, immunodeficiency, and a high risk of developing acute myeloid leukemia. Herein, we present 7 patients with GATA2 deficiency recruited into the French Severe Chronic Neutropenia Registry, which enrolls patients with all kinds of congenital neutropenia. We performed extended immunophenotyping of their whole blood lymphocyte populations, together with the analysis of their chemotactic responses. Lymphopenia was recorded for B and CD4(+) T cells in 6 patients. Although only 3 patients displayed natural killer cell cytopenia, the CD56(bright) natural killer subpopulation was nearly absent in all 7 patients. Natural killer cells from 6 patients showed decreased CXCL12/CXCR4-dependent chemotaxis, whereas other lymphocytes, and most significantly B lymphocytes, displayed enhanced CXCL12-induced chemotaxis compared with healthy volunteers. Surface expression of CXCR4 was significantly diminished in the patients' natural killer cells, although the total expression of the receptor was found to be equivalent to that of natural killer cells from healthy individual controls. Together, these data reveal that GATA2 deficiency is associated with impaired membrane expression and chemotactic dysfunctions of CXCR4. These dysfunctions may contribute to the physiopathology of this deficiency by affecting the normal distribution of lymphocytes and thus potentially affecting the susceptibility of patients to associated infections.

An atypical addition to the chemokine receptor nomenclature: IUPHAR Review 15

Chemokines and their receptors are essential regulators of in vivo leukocyte migration and, some years ago, a systematic nomenclature system was developed for the chemokine receptor family. Chemokine receptor biology and biochemistry was recently extensively reviewed. In this review, we also highlighted a new component to the nomenclature system that incorporates receptors previously known as 'scavenging', or 'decoy', chemokine receptors on the basis of their lack of classical signalling responses to ligand binding and their general ability to scavenge, or sequester, their cognate chemokine ligands. These molecules are now collectively referred to as 'atypical chemokine receptors', or ACKRs, and play fundamental roles in regulating in vivo responses to chemokines. This commentary highlights this new addition to the chemokine receptor nomenclature system and provides brief information on the four receptors currently covered by this nomenclature.

MIF interacts with CXCR7 to promote receptor internalization, ERK1/2 and ZAP-70 signaling, and lymphocyte chemotaxis

Macrophage migration-inhibitory factor (MIF) is a pleiotropic cytokine with chemokine-like functions and is a mediator in numerous inflammatory conditions. Depending on the context, MIF signals through 1 or more of its receptors cluster of differentiation (CD)74, CXC-motif chemokine receptor (CXCR)2, and CXCR4. In addition, heteromeric receptor complexes have been identified. We characterized the atypical chemokine receptor CXCR7 as a novel receptor for MIF. MIF promoted human CXCR7 internalization up to 40%, peaking at 50-400 nM and 30 min, but CXCR7 internalization by MIF was not dependent on CXCR4. Yet, by coimmunoprecipitation, fluorescence microscopy, and a proximity ligation assay, CXCR7 was found to engage in MIF receptor complexes with CXCR4 and CD74, both after ectopic overexpression and in endogenous conditions in a human B-cell line. Receptor competition binding and coimmunoprecipitation studies combined with sulfo-SBED-biotin-transfer provided evidence for a direct interaction between MIF and CXCR7. Finally, we demonstrated MIF/CXCR7-mediated functional responses. Blockade of CXCR7 suppressed MIF-mediated ERK- and zeta-chain-associated protein kinase (ZAP)-70 activation (from 2.1- to 1.2-fold and from 2.5- to 1.6-fold, respectively) and fully abrogated primary murine B-cell chemotaxis triggered by MIF, but not by CXCL12. B cells from Cxcr7(-/-) mice exhibited an ablated transmigration response to MIF, indicating that CXCR7 is essential for MIF-promoted B-cell migration. Our findings provide biochemical and functional evidence that MIF is an alternative ligand of CXCR7 and suggest a functional role of the MIF-CXCR7 axis in B-lymphocyte migration.

Chromothriptic cure of WHIM syndrome

Chromothripsis is a catastrophic cellular event recently described in cancer in which chromosomes undergo massive deletion and rearrangement. Here, we report a case in which chromothripsis spontaneously cured a patient with WHIM syndrome, an autosomal dominant combined immunodeficiency disease caused by gain-of-function mutation of the chemokine receptor CXCR4. In this patient, deletion of the disease allele, CXCR4(R334X), as well as 163 other genes from one copy of chromosome 2 occurred in a hematopoietic stem cell (HSC) that repopulated the myeloid but not the lymphoid lineage. In competitive mouse bone marrow (BM) transplantation experiments, Cxcr4 haploinsufficiency was sufficient to confer a strong long-term engraftment advantage of donor BM over BM from either wild-type or WHIM syndrome model mice, suggesting a potential mechanism for the patient's cure. Our findings suggest that partial inactivation of CXCR4 may have general utility as a strategy to promote HSC engraftment in transplantation.

A Broad G Protein-Coupled Receptor Internalization Assay that Combines SNAP-Tag Labeling, Diffusion-Enhanced Resonance Energy Transfer, and a Highly Emissive Terbium Cryptate

Although G protein-coupled receptor (GPCR) internalization has long been considered as a major aspect of the desensitization process that tunes ligand responsiveness, internalization is also involved in receptor resensitization and signaling, as well as the ligand scavenging function of some atypical receptors. Internalization thus contributes to the diversity of GPCR-dependent signaling, and its dynamics and quantification in living cells has generated considerable interest. We developed a robust and sensitive assay to follow and quantify ligand-induced and constitutive-induced GPCR internalization but also receptor recycling in living cells. This assay is based on diffusion-enhanced resonance energy transfer (DERET) between cell surface GPCRs labeled with a luminescent terbium cryptate donor and a fluorescein acceptor present in the culture medium. GPCR internalization results in a quantifiable reduction of energy transfer. This method yields a high signal-to-noise ratio due to time-resolved measurements. For various GPCRs belonging to different classes, we demonstrated that constitutive and ligand-induced internalization could be monitored as a function of time and ligand concentration, thus allowing accurate quantitative determination of kinetics of receptor internalization but also half-maximal effective or inhibitory concentrations of compounds. In addition to its selectivity and sensitivity, we provided evidence that DERET-based internalization assay is particularly suitable for characterizing biased ligands. Furthermore, the determination of a Z'-factor value of 0.45 indicates the quality and suitability of DERET-based internalization assay for high-throughput screening (HTS) of compounds that may modulate GPCRs internalization.

The relevance of the chemokine receptor ACKR3/CXCR7 on CXCL12-mediated effects in cancers with a focus on virus-related cancers

Recent studies have highlighted the importance of understanding the molecular determinants of CXCL12-mediated effects in cancers. Once previously thought to interact exclusively with CXCR4, CXCL12 also binds with high affinity to CXCR7 (recently renamed ACKR3), which belongs to an atypical chemokine receptor family whose members fail to activate Gαi proteins but interact with β-arrestins. In addition to its capacity to control CXCL12 bioavailability, ACKR3 can either enhance or dampen CXCR4-mediated signaling and activity. In light of the most recent findings, we have examined the role of ACKR3 in cancer, including a subset of virus-related cancers.

Neutrophil mobilization via plerixafor-mediated CXCR4 inhibition arises from lung demargination and blockade of neutrophil homing to the bone marrow

The CXCR4 antagonist plerixafor augments frequency of circulating neutrophils via release from the lung and prevents neutrophil homing to the bone marrow.

Blood neutrophil homeostasis is essential for successful host defense against invading pathogens. Circulating neutrophil counts are positively regulated by CXCR2 signaling and negatively regulated by the CXCR4–CXCL12 axis. In particular, G-CSF, a known CXCR2 signaler, and plerixafor, a CXCR4 antagonist, have both been shown to correct neutropenia in human patients. G-CSF directly induces neutrophil mobilization from the bone marrow (BM) into the blood, but the mechanisms underlying plerixafor-induced neutrophilia remain poorly defined. Using a combination of intravital multiphoton microscopy, genetically modified mice and novel in vivo homing assays, we demonstrate that G-CSF and plerixafor work through distinct mechanisms. In contrast to G-CSF, CXCR4 inhibition via plerixafor does not result in neutrophil mobilization from the BM. Instead, plerixafor augments the frequency of circulating neutrophils through their release from the marginated pool present in the lung, while simultaneously preventing neutrophil return to the BM. Our study demonstrates for the first time that drastic changes in blood neutrophils can originate from alternative reservoirs other than the BM, while implicating a role for CXCR4–CXCL12 interactions in regulating lung neutrophil margination. Collectively, our data provides valuable insights into the fundamental regulation of neutrophil homeostasis, which may lead to the development of improved treatment regimens for neutropenic patients.

β-arrestin-dependent activation of the cofilin pathway is required for the scavenging activity of the atypical chemokine receptor D6

Chemokines promote the recruitment of leukocytes to sites of infection and inflammation by activating conventional heterotrimeric guanine nucleotide-binding protein (G protein)-coupled receptors (GPCRs). Chemokines are also recognized by a set of atypical chemokine receptors (ACRs), which cannot induce directional cell migration but are required for the generation of chemokine gradients in tissues. ACRs are presently considered "silent receptors" because no G protein-dependent signaling activity is observed after their engagement by cognate ligands. We report that engagement of the ACR D6 by its ligands activates a β-arrestin1-dependent, G protein-independent signaling pathway that results in the phosphorylation of the actin-binding protein cofilin through the Rac1-p21-activated kinase 1 (PAK1)-LIM kinase 1 (LIMK1) cascade. This signaling pathway is required for the increased abundance of D6 protein at the cell surface and for its chemokine-scavenging activity. We conclude that D6 is a signaling receptor that exerts its regulatory function on chemokine-mediated responses in inflammation and immunity through a distinct signaling pathway.

Description and outcome of a cohort of 8 patients with WHIM syndrome from the French Severe Chronic Neutropenia Registry

BACKGROUND

WHIM syndrome (WS), a rare congenital neutropenia due to mutations of the CXCR4 chemokine receptor, is associated with Human Papillomavirus (HPV)-induced Warts, Hypogammaglobulinemia, bacterial Infections and Myelokathexis. The long term follow up of eight patients highlights the clinical heterogeneity of this disease as well as the main therapeutic approaches and remaining challenges in the light of the recent development of new CXCR4 inhibitors.

OBJECTIVE

This study aims to describe the natural history of WS based on a French cohort of 8 patients.

METHODS

We have reviewed the clinical, biological and immunological features of patients with WS enrolled into the French Severe Chronic Neutropenia Registry.

RESULTS

We identified four pedigrees with WS comprised of eight patients and one foetus. Estimated incidence for WS was of 0.23 per million births. Median age at the last visit was 29 years. Three pedigrees encompassing seven patients and the fetus displayed autosomal dominant heterozygous mutations of the CXCR4 gene, while one patient presented a wild-type CXCR4 gene. Two subjects exhibited congenital conotruncal heart malformations. In addition to neutropenia and myelokathexis, all patients presented deep monocytopenia and lymphopenia. Seven patients presented repeated bacterial Ears Nose Throat as well as severe bacterial infections that were curable with antibiotics. Four patients with late onset prophylaxis developed chronic obstructive pulmonary disease (COPD). Two patients reported atypical mycobacteria infections which in one case may have been responsible for one patient's death due to liver failure at the age of 40.6 years. HPV-related disease manifested in five subjects and progressed as invasive vulvar carcinoma with a fatal course in one patient at the age of 39.5 years. In addition, two patients developed T cell lymphoma skin cancer and basal cell carcinoma at the age of 38 and 65 years.

CONCLUSIONS

Continuous prophylactic anti-infective measures, when started in early childhood, seem to effectively prevent further bacterial infections and the consequent development of COPD. Long-term follow up is needed to evaluate the effect of early anti-HPV targeted prophylaxis on the development of skin and genital warts.

The chemokine CXCL12 is essential for the clearance of the filaria Litomosoides sigmodontis in resistant mice

Litomosoides sigmodontis is a cause of filarial infection in rodents. Once infective larvae overcome the skin barrier, they enter the lymphatic system and then settle in the pleural cavity, causing soft tissue infection. The outcome of infection depends on the parasite's modulatory ability and also on the immune response of the infected host, which is influenced by its genetic background. The goal of this study was to determine whether host factors such as the chemokine axis CXCL12/CXCR4, which notably participates in the control of immune surveillance, can influence the outcome of the infection. We therefore set up comparative analyses of subcutaneous infection by L. sigmodontis in two inbred mouse strains with different outcomes: one susceptible strain (BALB/c) and one resistant strain (C57BL/6). We showed that rapid parasite clearance was associated with a L. sigmodontis-specific CXCL12-dependent cell response in C57BL/6 mice. CXCL12 was produced mainly by pleural mesothelial cells during infection. Conversely, the delayed parasite clearance in BALB/c mice was neither associated with an increase in CXCL12 levels nor with cell influx into the pleural cavity. Remarkably, interfering with the CXCL12/CXCR4 axis in both strains of mice delayed filarial development, as evidenced by the postponement of the fourth molting process. Furthermore, the in vitro growth of stage 4 filariae was favored by the addition of low amounts of CXCL12. The CXCL12/CXCR4 axis thus appears to have a dual effect on the L. sigmodontis life cycle: by acting as a host-cell restriction factor for infection, and as a growth factor for worms.

Two cases of disseminated Mycobacterium avium infection associated with a new immunodeficiency syndrome related to CXCR4 dysfunctions

Disseminated Mycobacterium avium complex (MAC) infection is a rare but severe disease mostly seen in patients with AIDS. It has been previously described in patients suffering from other kinds of immunodeficiency (e.g. primary immunodeficiency diseases in children or hairy cell leukaemia). We report two cases of disseminated MAC disease in young women with extended granulomatosis that revealed a new form of severe immunodeficiency syndrome. Both clinical observations initially appeared to be very similar to WHIM syndrome (Warts, Hypogammaglobulinemia, Infection, Myelokathexis), a rare immunodeficiency disease correlated with CXC chemokine receptor 4 (CXCR4) mutation leading to an impaired internalization of the receptor upon its ligand CXCL12. We investigated the CXCR4 status of the lymphocytes in both patients and found a severe defect in CXCL12-promoted internalization but no mutation of its gene. Moreover, myelokathexis was not noted in bone marrow biopsies and therefore a diagnosis of WHIM syndrome could not be assessed. This immunodeficiency syndrome associated with CXCR4 dysfunction was responsible for severe MAC infection in our patients, with a fatal outcome in one case. It may be possible that these patients would have benefited from early antimycobacterial infection or azythromycin prophylaxis.

CXCL12/CXCR4-axis dysfunctions: Markers of the rare immunodeficiency disorder WHIM syndrome

The WHIM syndrome features susceptibility to human Papillomavirus infection-induced warts and carcinomas, hypogammaglobulinemia, recurrent bacterial infections, B and T-cell lymphopenia, and neutropenia associated with retention of senescent neutrophils in the bone marrow (i.e. myelokathexis). This rare disorder is mostly linked to inherited heterozygous autosomal dominant mutations in the gene encoding CXCR4, a G protein coupled receptor with a unique ligand, the chemokine CXCL12/SDF-1. Some individuals who have full clinical forms of the syndrome carry a wild type CXCR4 gene. In spite of this genetic heterogeneity, leukocytes from WHIM patients share in common dysfunctions of the CXCR4-mediated signaling pathway upon exposure to CXCL12. Dysfunctions are characterized by impaired desensitization and receptor internalization, which are associated with enhanced responses to the chemokine. Our increasing understanding of the mechanisms that account for the aberrant CXCL12/CXCR4-mediated responses is beginning to provide insight into the pathogenesis of the disorder. As a result we can expect to identify markers of the WHIM syndrome, as well as other disorders with WHIM-like features that are associated with dysfunctions of the CXCL12/CXCR4 axis.

Leukocyte analysis from WHIM syndrome patients reveals a pivotal role for GRK3 in CXCR4 signaling

Leukocytes from individuals with warts, hypogammaglobulinemia, infections, and myelokathexis (WHIM) syndrome, a rare immunodeficiency, and bearing a wild-type CXCR4 ORF (WHIM(WT)) display impaired CXCR4 internalization and desensitization upon exposure to CXCL12. The resulting enhanced CXCR4-dependent responses, including chemotaxis, probably impair leukocyte trafficking and account for the immunohematologic clinical manifestations of WHIM syndrome. We provided here evidence that GPCR kinase-3 (GRK3) specifically regulates CXCL12-promoted internalization and desensitization of CXCR4. GRK3-silenced control cells displayed altered CXCR4 attenuation and enhanced chemotaxis, as did WHIM(WT) cells. These findings identified GRK3 as a negative regulator of CXCL12-induced chemotaxis and as a candidate responsible for CXCR4 dysfunction in WHIM(WT) leukocytes. Consistent with this, we showed that GRK3 overexpression in both leukocytes and skin fibroblasts from 2 unrelated WHIM(WT) patients restored CXCL12-induced internalization and desensitization of CXCR4 and normalized chemotaxis. Moreover, we found in cells derived from one patient a profound and selective decrease in GRK3 products that probably resulted from defective mRNA synthesis. Taken together, these results have revealed a pivotal role for GRK3 in regulating CXCR4 attenuation and have provided a mechanistic link between the GRK3 pathway and the CXCR4-related WHIM(WT) disorder.

CD4 interacts constitutively with multiple CCR5 at the plasma membrane of living cells. A fluorescence recovery after photobleaching at variable radii approach

The entry of human immunodeficiency virus into target cells requires successive interactions of the viral envelope glycoprotein gp120 with CD4 and the chemokine receptors CCR5 or CXCR4. We previously demonstrated, by Förster resonance energy transfer experiments, the constitutive association of CD4 and CCR5 at the surface of living cells. We therefore speculated that this interaction may correlate with compartmentalization of CD4 and CCR5 within the plasma membrane. Here, we characterize the lateral distribution, the dynamics, and the stoichiometry of these receptors in living cells stably expressing CD4 and/or CCR5 by means of fluorescence recovery after photobleaching at variable radii experiments. We found that (i) these receptors expressed alone are confined into 1-microm-sized domains, (ii) CD4-CCR5 associations occur outside and inside smaller domains, and (iii) these interactions involve multiple CCR5 molecules per CD4.

Direct Assessment of CXCR4 Mutant Conformations Reveals Complex Link between Receptor Structure and Gαi Activation

Ligand binding to G protein-coupled receptors (GPCRs) is thought to induce changes in receptor conformation that translate into activation of downstream effectors. The link between receptor conformation and activity is still insufficiently understood, as current models of GPCR activation fail to take an increasing amount of experimental data into account. To elucidate structure-function relationships in GPCR activation, we used bioluminescence resonance energy transfer to directly assess the conformation of mutants of the chemokine receptor CXCR4. We analyzed substitutions in the arginine cage DRY motif and in the conserved asparagine N(3.35)119, which are pivotal molecular switches for receptor conformation and activation. G(alpha)(i) activation of the mutants was either similar to wild-type CXCR4 (D133N, Y135A, and N119D) or resulted in loss of activity (R134A and N119K). Mutant N119S was constitutively active but further activated by agonist. Bioluminescence resonance energy transfer analysis suggested no simple correlation between conformational changes in response to ligand binding and activation of G(alpha)(i) by the mutants. Different conformations of active receptors were detected (for wild-type CXCR4, D133N, and N119S), suggesting that different receptor conformations are able to trigger G(alpha)(i) activity. Several conformations were also found for inactive mutants. These data provide biophysical evidence for different receptor conformations being active with respect to a single readout. They support models of GPCR structure-activity relationships that take this conformational flexibility of active receptors into account.

CD4 and CCR5 constitutively interact at the plasma membrane of living cells: a confocal fluorescence resonance energy transfer-based approach

Human immunodeficiency virus entry into target cells requires sequential interactions of the viral glycoprotein envelope gp120 with CD4 and chemokine receptors CCR5 or CXCR4. CD4 interaction with the chemokine receptor is suggested to play a critical role in this process but to what extent such a mechanism takes place at the surface of target cells remains elusive. To address this issue, we used a confocal microspectrofluorimetric approach to monitor fluorescence resonance energy transfer at the cell plasma membrane between enhanced blue and green fluorescent proteins fused to CD4 and CCR5 receptors. We developed an efficient fluorescence resonance energy transfer analysis from experiments carried out on individual cells, revealing that receptors constitutively interact at the plasma membrane. Binding of R5-tropic HIV gp120 stabilizes these associations thus highlighting that ternary complexes between CD4, gp120, and CCR5 occur before the fusion process starts. Furthermore, the ability of CD4 truncated mutants and CCR5 ligands to prevent association of CD4 with CCR5 reveals that this interaction notably engages extracellular parts of receptors. Finally, we provide evidence that this interaction takes place outside raft domains of the plasma membrane.