Both immature and mature T cells mobilize Ca2+ in response to antigen receptor crosslinking

T cells develop from prothymocytes which express no detectable antigen receptors to immature thymocytes with few receptors, eventually becoming mature thymocytes and peripheral T cells with 20,000-40,000 receptors per cell. Recent studies suggest that immature thymocytes are immunologically unresponsive. We have suggested that an early step in signal transduction following engagement of the T cell receptor might differ in immature and mature T cells. Here we examine anti-receptor antibody mediated induction of calcium mobilization in immature and mature T cells. Results indicate that antigen receptors on both immature and mature receptor-positive T cells transduce signals via calcium mobilization. Significant differences were observed, however, between these populations in the magnitude of influx of extracellular Ca2+ following binding of antireceptor antibody. Specifically immature cells show a much reduced Ca2+ influx response compared to mature cells which could result from a low Ca2+ channel frequency in the plasma membranes of immature T cells, or from less efficient activation of existing channels.