CD45-mediated control of TCR tuning in naive and memory CD8+ T cells

Continuous contact with self-major histocompatibility complex (MHC) ligands is essential for survival of naive T cells but not memory cells. This surprising finding implies that T cell subsets may vary in their relative T-cell receptor (TCR) sensitivity. Here we show that in CD8(+) T cells TCR sensitivity correlates inversely with levels of CD5, a marker for strong self-MHC reactivity. We also show that TCR sensitivity is lower in memory CD8(+) T cells than naive cells. In both situations, TCR hypo-responsiveness applies only to short-term TCR signalling events and not to proliferation, and correlates directly with increased expression of a phosphatase, CD45 and reciprocal decreased expression of activated LCK. Inhibition by high CD45 on CD8(+) T cells may protect against overt TCR auto-MHC reactivity, while enhanced sensitivity to cytokines ensures strong responses to foreign antigens.