The chemokine SDF-1/CXCL12 binds to and signals through the orphan receptor RDC1 in T lymphocytes

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.