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

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Abstract
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 β 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 β chain at a very low level, visible by staining as a “shoulder” on the peak of negative cells. Thymocytes with high-density surface β 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 β 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 β chain occurs in parallel with functional maturation.

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