T cell receptor expression and receptor-mediated induction of clonal growth in the developing mouse thymus. High surface beta-chain density is a requirement for functional maturity.

The ontogeny of T cell antigen receptor expression and function in the mouse thymus has been studied using a monoclonal antibody, F23.1, which recognizes a determinant on the beta chain of the receptor, and stains 25% of mature T cells and around 7-15% of adult thymocytes from most mouse strains. The same monoclonal antibody selectively activates Lyt-2+ peripheral T cells. Receptors are detectable by staining and fluorescence-activated cell sorter analysis from fetal day 17, and thereafter the overall frequency increases steadily towards adult levels. However, late fetal thymocytes express all of their antigen receptor beta chain at a very low level, visible by staining as a "shoulder" on the peak of negative cells. Thymocytes with high-density surface beta chain, visible by staining as a distinct peak, appear only after birth and are a prominent feature at neonatal day 4. In the late fetus, expression of beta chain can be detected on thymocytes with the "mature" L3T4-, Lyt-2+ phenotype. Despite this, F23.1-responsive precursors are not found in the fetal thymus, and appear in two waves, the first during day 1 of postnatal life and the second between days 3 and 4. These data suggest that high-density surface expression of T cell receptor beta chain occurs in parallel with functional maturation.