The development of helper T cell precursors in mouse thymus

We have examined the appearance in mouse ontogeny of thymocyte precursors for Ag-specific, MHC-restricted Th. These cells are first detectable at day 18 of fetal life, about 1 day after alpha/beta, TCR-positive cells begin to appear. These early Th precursors are not dependent on the thymus for priming with Ag and MHC, and are L3T4+, Lyt-2-. Thus, these cells already have the phenotype of mature Th. In neonatal F1 animals expressing both IAk and IAb, the appearance of Th precursors restricted by either IAk or IAb is specifically inhibited by treatment of the mice with anti-IAk or anti-IAb antibodies, respectively. These results indicate that cells of mature T cell phenotype and function can arise fairly rapidly from immature, receptor-bearing precursors, once these appear. Moreover the results are in line with those previously obtained in chimeric animal experiments which suggested that specific interaction of TCR on thymocytes with class II alleles in the thymus is required for the subsequent appearance of T cells restricted by those class II alleles.