Role of extracellular calcium in chemokine mediated responses in human and mouse CD4+T lymphocytes

CXCR3, a chemokine receptor preferentially expressed in activated T lymphocytes, plays an important role in the migration of these cells to sites of inflammation in response to specific ligands namely IP10, ITAC, and MIG. In vitro, activated CD4+ T lymphocytes from human peripheral blood or mouse spleen respond to CXCR3 ligands by producing robust calcium flux and chemotaxis. In the present study we evaluated the requirement for extracellular calcium in these responses. CXCR3 ligands-induced calcium flux and chemotaxis was pertussis toxin sensitive which indicates its mediation by the Gi/o family of G proteins. Interestingly, in contrast to Thapsigargin, a SERCA pump inhibitor, or anti-CD3 antibody, CXCR3 ligands did not induce any detectable intracellular calcium mobilization from the internal stores. Instead, extracellular calcium was absolutely required to demonstrate CXCR3 mediated calcium mobilization in T lymphocytes. SDF-la, a CXCR4 ligand, also demonstrated similar requirement for extracellular calcium. The calcium flux was sensitive to 2-aminoethoxydiphenyl borate, which potentiated the calcium flux at low concentrations (3 uM), while abolished it at higher concentrations (100 uM), suggesting an involvement of calcium channels. Extracellular calcium was also required for chemotaxis at low chemokine concentrations. Thus, chemotaxis consisted of calcium-dependent and independent components. The signal transduction pathway involved in calcium flux and chemotaxis is currently being characterized.