Combined phylogenetic and chromosomal location studies suggest that the orphan receptor RDC1 is related to CXC chemokine receptors. RDC1 provides a co-receptor function for a restricted number of human immunodeficiency virus (HIV) isolates, in particular for the CXCR4-using HIV-2 ROD strain. Here we show that CXCL12, the only known natural ligand for CXCR4, binds to and signals through RDC1. We demonstrate that RDC1 is expressed in T lymphocytes and that CXCL12-promoted chemotaxis is inhibited by an anti-RDC1 monoclonal antibody. Concomitant blockade of RDC1 and CXCR4 produced additive inhibitory effects in CXCL12-induced T cell migration. Furthermore, we provide evidence that interaction of CXCL12 with RDC1 is specific, saturable, and of high affinity (apparent K_D ≈ 0.4 nm). In CXCR4-negative cells expressing RDC1, CXCL12 promotes internalization of the receptor and chemotactic signals through RDC1. Collectively, our data indicate that RDC1, which we propose to rename as CXCR7, is a receptor for CXCL12.

The stromal cell-derived factor-1/CXCL12 chemokine engages the CXCR4 and CXCR7 receptors and regulates homeostatic and pathologic processes, including organogenesis, leukocyte homeostasis, and tumorigenesis. Both receptors are widely expressed in mammalian cells, but how they cooperate to respond to CXCL12 is not well understood. Here, we show that CXCR7 per se does not trigger Gαi protein–dependent signaling, although energy transfer assays indicate that it constitutively interacts with Gαi proteins and undergoes CXCL12-mediated conformational changes. Moreover, when CXCR4 and CXCR7 are coexpressed, we show that receptor heterodimers form as efficiently as receptor homodimers, thus opening the possibility that CXCR4/CXCR7 heterodimer formation has consequences on CXCL12-mediated signals. Indeed, expression of CXCR7 induces conformational rearrangements within preassembled CXCR4/Gαi protein complexes and impairs CXCR4-promoted Gαi-protein activation and calcium responses. Varying CXCR7 expression levels and blocking CXCL12/CXCR7 interactions in primary T cells suggest that CXCR4/CXCR7 heterodimers form in primary lymphocytes and regulate CXCL12-promoted chemotaxis. Taken together, these results identify CXCR4/CXCR7 heterodimers as distinct functional units with novel properties, which can contribute to the functional plasticity of CXCL12.

Inflammatory mechanisms appear to play a significant role in some types of pulmonary hypertension (PH), including monocrotaline-induced PH in rats and pulmonary arterial hypertension of various origins in humans, such as connective tissue diseases (scleroderma, systemic lupus erythematosus, mixed connective disease), human immunodeficiency virus infection, or plasma cell dyscrasia with polyneuropathy, organomegaly, endocrinopathy, monoclonal (M) protein and skin changes (POEMS) syndrome.Interestingly, some patients with severe pulmonary arterial hypertension associated with systemic lupus erythematosus have experienced significant improvements with immunosuppressive therapy, emphasising the relevance of inflammation in a subset of patients presenting with PH. Patients with primary PH (PPH) also have some immunological disturbances, suggesting a possible role for inflammation in the pathophysiology of this disease. A subset of PPH patients have been shown to have circulating autoantibodies, including antinuclear antibodies, as well as elevated circulating levels of the pro-infammatory cytokines, interleukins ‐1 and ‐6. Lung histology has also revealed inflammatory infiltrates in the range of plexiform lesions in patients displaying severe PPH, as well as an increased expression of the chemokines regulated upon activation, normal T‐cell expressed and secreted (RANTES) and fractalkine.Further analysis of the role of inflammatory mechanisms is necessary to understand whether this component of the disease is relevant to its pathophysiology.

The WHIM syndrome is a rare immunodeficiency disorder characterized by warts, hypogammaglobulinemia, infections, and myelokathexis. Dominant heterozygous mutations of the gene encoding CXCR4, a G-protein–coupled receptor with a unique ligand, CXCL12, have been associated with this pathology. We studied patients belonging to 3 different pedigrees. Two siblings inherited a CXCR4 mutation encoding a novel C-terminally truncated receptor. Two unrelated patients were found to bear a wild-type CXCR4 open reading frame. Circulating lymphocytes and neutrophils from all patients displayed similar functional alterations of CXCR4-mediated responses featured by a marked enhancement of G-protein–dependent responses. This phenomenon relies on the refractoriness of CXCR4 to be both desensitized and internalized in response to CXCL12. Therefore, the aberrant dysfunction of the CXCR4-mediated signaling constitutes a common biologic trait of WHIM syndromes with different causative genetic anomalies. Responses to other chemokines, namely CCL4, CCL5, and CCL21, were preserved, suggesting that, in clinical forms associated with a wild-type CXCR4 open reading frame, the genetic anomaly might target an effector with some degree of selectivity for the CXCL12/CXCR4 axis. We propose that the sustained CXCR4 activity in patient cells accounts for the immune-hematologic clinical manifestations and the profusion of warts characteristic of the WHIM syndrome.

Perivascular infiltrates composed of macrophages and lymphocytes have been described in lung biopsies of patients displaying pulmonary arterial hypertension (PAH), suggesting that circulating inflammatory cells can be recruited in affected vessels. CX₃C chemokine fractalkine is produced by endothelial cells and promotes leukocyte recruitment, but unlike other chemokines, it can capture leukocytes rapidly and firmly in an integrin-independent manner under high blood flow. We therefore hypothesized that fractalkine may contribute to pulmonary inflammatory cell recruitment in PAH. Expression and function of the fractalkine receptor (CX₃CR1) were studied by use of triple-color flow cytometry on circulating T-lymphocyte subpopulations in freshly isolated peripheral blood mononuclear cells from control subjects and patients with PAH. Plasma-soluble fractalkine concentrations were measured by enzyme-linked immunosorbent assay. Finally, fractalkine mRNA and protein expression were analyzed in lung samples by reverse transcriptase-polymerase chain reaction or in situ hybridization and immunohistochemistry, respectively. In patients with PAH, CX₃CR1 expression and function are upregulated in circulating T-lymphocytes, mostly of the CD4+ subset, and plasma soluble fractalkine concentrations are elevated, as compared with control subjects. Fractalkine mRNA and protein product are expressed in pulmonary artery endothelial cells. We conclude that inflammatory mechanisms involving chemokine fractalkine and its receptor CX₃CR1 may have a role in the natural history of PAH.

The recent discovery that sporadic and familial primary pulmonary hypertension can be associated with germline mutations of genes encoding receptor members of the transforming growth factor-beta family has focused much attention on cytokines and growth factors in pulmonary vascular disorders. Production of several cytokines has been demonstrated in severe pulmonary arterial hypertension, emphasizing the possible influence of inflammatory mechanisms in this condition. Moreover, perivascular inflammatory cell infiltrates composed of macrophages and lymphocytes have been detected in plexiform lesions of primary pulmonary hypertension. Chemokine RANTES is an important chemoattractant for monocytes and T cells. We therefore hypothesize that chemokine RANTES promotes cell recruitment in the lungs of patients displaying severe pulmonary arterial hypertension. Reverse transcriptase polymerase chain reaction demonstrated elevated RANTES mRNA expression in 10 lung samples from patients with severe pulmonary arterial hypertension, as compared with seven control subjects. In situ hybridization and immunohistochemistry confirmed that endothelial cells were the major source of RANTES within the pulmonary artery wall of the patients. Serial sections analysis showed that RANTES expression was associated with CD45 + inflammatory cell infiltrates. These results support the concept that inflammatory mechanisms play a role in the natural history of pulmonary arterial hypertension.