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(+) t o single positive T cells. It has been proposed that this process occu rs by an instructive or a stochastic mechanism. In this report, we sho w that in recombination-deficient mice (RAG-1(-1-)) constitutive expre ssion 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 leve ls of T cell receptor expression as CD8end(+) cells, respond to cognat e antigen and, upon stimulation, they exhibit a phenotype characterist ic of CD4(+) helper T cells. These data are consistent with a model of differentiation that predicts that thymocytes become functionally com mitted to a helper or cytotoxic lineage before the final step of posit ive selection and independently of MHC specificity of their T cell rec eptor.