Strength of signaling by CD4 and CD8 coreceptor tails determines the number but not the lineage direction of positively selected thymocytes

The present study has assessed the impact of the intracellular domains of C D4 and CD8 on positive selection and lineage direction of MHC class I-restr icted thymocytes. Contrary to current presumption, we found that the CD4 ta il promotes the generation of both CD4' and CD8(+) T cells without preferen ce for the CD4' T cell lineage. We also found that the identity of the core ceptor tail and hence the strength of coreceptor signaling determine the nu mber of thymocytes undergoing positive selection but not their ultimate CD4 / CD8 phenotype. These findings demonstrate that the strength of coreceptor signaling has a significant quantitative but not qualitative impact on pos itive selection and provide a simple explanation for the greater numbers of CD4(+) than CD8(+) T cells selected in the normal thymus.