Functional commitment to helper T cell lineage precedes positive selection and is independent of T cell receptor MHC specificity.

Thymocyte differentiation proceeds from double positive CD4+CD8+ to single positive T cells. It has been proposed that this process occurs by an instructive or a stochastic mechanism. In this report, we show that in recombination-deficient mice (RAG-1-I-) constitutive expression of a CD8 transgene allows maturation of CD4+(CD8tg+) cells, which express mature levels of a transgenic class I-restricted T cell receptor, F5. Rescued F5+CD4+(CD8tg+) cells have equivalent levels of T cell receptor expression as CD8end+ cells, respond to cognate antigen and, upon stimulation, they exhibit a phenotype characteristic of CD4+ helper T cells. These data are consistent with a model of differentiation that predicts that thymocytes become functionally committed to a helper or cytotoxic lineage before the final step of positive selection and independently of MHC specificity of their T cell receptor.