Heterologous desensitization of T cell functions by CCR5 and CXCR4 ligands: Inhibition of cellular signaling, adhesion and chemotaxis

T cells migrate into inflamed sites through the extracellular matrix (ECM) in response to chemotactic areas and are then simultaneously or sequentially exposed to multiple chemotactic ligands. We examined the responses of human peripheral blood T cells, present in an ECM-like context, to combinatorial signaling transduced by SDF-1α (CXCL12), and two CCR5 ligands, RANTES (CCL5) and MIP-1β (CCL4). Separately, these chemokines, at G protein-coupled receptor (GPCR)-stimulating concentrations, induced T cell adhesion to fibronectin (FN) and T cell chemotaxis. However, the pro-adhesive and pro-migratory capacities of SDF-1α and RANTES or MIP-1β were mutually suppressed by the simultaneous or sequential exposure of the cells to these CCR5 or CXCR4 ligands. This cross-talk did not involve the internalization of the SDF-1α receptor, CXCR4, but rather, a decrease in phosphorylation of ERK and Pyk-2, as well as inhibition of Ca²⁺ mobilization. Strikingly, early CXCR4 signaling of phosphatidylinositol-3-kinase, detected by SDF-1α-induced AKT phosphorylation, was insensitive to RANTES-CCR5 signals. Accordingly, early chemotaxis to SDF-1α was not susceptible to CCR5 occupancy, whereas late stages of T cell chemotaxis were markedly down-regulated. This is an example of a specialized functional desensitization of heterologous chemokine receptors that induces GPCR interference with T cell adhesion to ECM ligands and chemotaxis within chemokine-rich extravascular contexts.