Thymocyte Cell Fate Alteration by High Levels of CD8α Is Independent of CD8-lck Association

CD8 expression on the surface of developing thymocytes is essential for the positive selection and maturation of CD8 single positive T cells. CD8 is physically associated with the cytoplasmic tyrosine kinase, p56^lck and, in addition to increasing the avidity between the TCR and MHC/peptide complex, may transduce intracellular signals. Here we address the extent to which the requirement for CD8 in thymocyte selection depends on its capacity to signal vs its contribution to avidity. We have previously reported that elevated levels of CD8α can alter the fate of developing 2C TCR⁺ thymocytes from positive to negative selection in H-2^b mice. We show here that the fate of 2C TCR thymocytes overexpressing a mutant form of CD8α that does not bind to p56^lck is the same as that of thymocytes overexpressing the wild-type CD8α-chain. This finding demonstrates that, for a TCR-MHC system in which the level of CD8α is the deciding factor in cell fate, negative selection does not require increased CD8-dependent signaling through p56^lck. We have also found that highly elevated levels of CD8α expression (10-fold) block thymocyte maturation in both CD8α single and CD8α/HY TCR double transgenic mice. Although these may be additional examples of CD8α overexpression altering cell fate from positive to negative selection, we present arguments in favor of the interpretation that high levels of CD8α actually may block positive selection.