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.

Dengue virus (DV) is a mosquito-borne flavivirus that causes hemorrhagic fever in humans. In the natural infection, DV is introduced into human skin by an infected mosquito vector where it is believed to target immature dendritic cells (DCs) and Langerhans cells (LCs). We found that DV productively infects DCs but not LCs. We show here that the interactions between DV E protein, the sole mannosylated glycoprotein present on DV particles, and the C-type lectin dendritic cell-specific intercellular adhesion molecule 3-grabbing non-integrin (DC-SIGN) are essential for DV infection of DCs. Binding of mannosylated N-glycans on DV E protein to DC-SIGN triggers a rapid and efficient internalization of the viral glycoprotein. However, we observed that endocytosisdefective DC-SIGN molecules allow efficient DV replication, indicating that DC-SIGN endocytosis is dispensable for the internalization step in DV entry. Together, these results argue in favor of a mechanism by which DC-SIGN enhances DV entry and infection in cis. We propose that DC-SIGN concentrates mosquito-derived DV particles at the cell surface to allow efficient interaction with an as yet unidentified entry factor that is ultimately responsible for DV internalization and pHdependent fusion into DCs.

WHIM (warts, hypogammaglobulinemia, infections, and myelokathexis) syndrome is an immune deficiency linked in many cases to heterozygous mutations causing truncations in the cytoplasmic tail of CXC chemokine receptor 4 (CXCR4). Leukocytes expressing truncated CXCR4 display enhanced responses to the receptor ligand CXCL12, including chemotaxis, which likely impair their trafficking and contribute to the immunohematologic clinical manifestations of the syndrome. CXCR4 desensitization and endocytosis are dependent on ␤-arrestin (␤arr) recruitment to the cytoplasmic tail, so that the truncated CXCR4 are refractory to these processes and so have enhanced G protein-dependent signaling. Here, we show that the augmented responsiveness of WHIM leukocytes is also accounted for by enhanced ␤arr2-dependent signaling downstream of the truncated CXCR4 receptor. Indeed, the WHIM-associated receptor CXCR4 1013 maintains association with ␤arr2 and triggers augmented and prolonged ␤arr2-dependent signaling, as revealed by ERK1/2 phosphorylation kinetics. Evidence is also provided that CXCR4 1013 -mediated chemotaxis critically requires ␤arr2, and disrupting the SHSK motif in the third intracellular loop of CXCR4 1013 abrogates ␤arr2-mediated signaling, but not coupling to G proteins, and normalizes chemotaxis. We also demonstrate that CXCR4 1013 spontaneously forms heterodimers with wild-type CXCR4. Accordingly, we propose a model where enhanced functional interactions between ␤arr2 and receptor dimers account for the altered responsiveness of WHIM leukocytes to CXCL12. (Blood. 2008;112:34-44) IntroductionThe G-protein-coupled receptor (GPCR) CXC chemokine receptor 4 (CXCR4) and its ligand, the stromal cell-derived factor-1 (CXCL12/SDF-1), regulate leukocyte hematopoiesis and trafficking. 1 They initiate signal transduction by activating heterotrimeric G␣␤␥-proteins, which then act on effectors to trigger downstream cellular responses. 2 CXCL12 also elicits processes causing receptor desensitization, which results in the uncoupling from G-proteins and involves phosphorylation of the CXCR4 cytoplasmic tail (C-tail) by protein kinase C and GPCR kinases (GRKs) and interaction of the phosphorylated receptor with ␤-arrestins (␤arrs). [3][4][5] ␤arrs then target desensitized CXCR4 to clathrin-coated pits for endocytosis. ␤arrs also link GPCRs to the stimulation of additional signaling molecules, including members of the mitogen-activated protein kinase (MAPK) family. 6 Studies on CXCR4 have demonstrated that ␤-arrestin2 (␤arr2) strengthens activation of the p38 and extracellular signal-regulated kinase (ERK) MAPKs and CXCL12-induced chemotaxis. 5,7,8 WHIM (warts, hypogammaglobulinemia, infections, and myelokathexis) syndrome (WS) is a rare immunodeficiency disease linked to CXCR4 dysfunctions and is characterized by warts, recurrent bacterial infections, hypogammaglobulinemia, lymphopenia, and myelokathexis, a severe neutropenia with abnormal retention of mature neutrophils in the bone marrow (BM). 9,10 WS, most often inherited as an a...

IntroductionCD5 is a lymphoid marker-one of the earliest acquired during T-cell ontogeny. Its expression increases coordinately with that of cell surface CD3. 1 Although expressed on lymphoid-committed progenitors, its expression is lost following natural killer (NK) cell differentiation. 2 In contrast to its being a pan-T-cell marker, CD5 is only expressed on some B cells. Based on its coexpression with CD11b, immunogloublin M (IgMϩ) B cells in mouse and human are classically separated into different subsets, termed B-1a and B-1b, each of which expresses CD11b and B-2. 3 The B-1a subset expresses CD5, and at a functional level these CD5 ϩ B cells frequently produce polyreactive antibodies, mainly IgM, that recognize a variety of self-antigens and foreign antigens. 4 The reason for the expression of CD5 on a particular B-cell subsetsomething Wortis and Berland 5 call "one of many intriguing and seemingly idiosyncratic features of B-1a cells"-has remained obscure. Two hypotheses are proposed to explain the origin of B-1 cells. The lineage hypothesis holds that only certain fetal progenitors are destined to become B-1 cells. 6,7 In contrast, the differentiation hypothesis holds that every B cell is able to acquire B-1 cell characteristics. In brief, the notion of B-cell lineage based on differential CD5 expression is controversial because of the lack of clear identification of a fetal progenitor destined to become a B-1 B cell and the demonstration that CD5 Ϫ cells can acquire CD5 expression in vivo 8 or in vitro 9 after B-cell receptor (BCR) stimulation This up-regulation supports the initial view that CD5 is an activation marker 10 ; however, a definitive function for this antigen remains to be established.It has been shown that CD5 up-regulation in B cells plays a role in tolerance to autoantigens. By setting the threshold level for activation signals, CD5 prevents B cells from activation-induced cell death and maintains tolerance in anergic B cells in vivo. 11 The reason for keeping potentially autoreactive cells alive is that these cells are also necessary for an effective immune response to some pathogens. 12 This supported the role of CD5 as a negative regulator of BCR signaling, which was later demonstrated by the generation of CD5-null mice. 13 In these animals, peritoneal B cells, which are poorly responsive to BCR stimulation, restored their capacity to fully proliferate to anti-IgM. 13 The molecular basis for BCR inhibition by CD5 has been extensively investigated. The physical association of CD5 to the BCR was demonstrated, 14 and our recent work further documented the structural basis of CD5 inhibition of BCR-mediated signals. Using a reconstitution approach, in a murine lymphoma B cell line we showed that Ca 2ϩ response, extracellular signal-related kinase-2 (ERK-2) activation, and the production of interleukin-2 (IL-2) induced by BCR activation were antagonized by CD5. 15 The role of the src autophosphorylationlike motif within the cytoplasmic domain was demonstrated. 16 Of interest, this domain is...