Intrathymic differentiation of Vγ3 T cells

Whereas there is considerable information on the phenotypic and functional maturation of T cell receptor (TCR) α/β thymocytes, comparatively little is known of the maturational processes that affect development of TCR-γ/δ thymocytes. One class of γ/δ T cells, those bearing the Vγ3 gene product, are generated only during the early fetal stages of thymic development, and then migrate to the skin. Here we examine the intrathymic differentiation of these Vγ3⁺ cells. The earliest Vγ3 cells to appear in the thymus expressed low levels of TCR (Vγ3^low) and high levels of heat stable antigen (HSA). Over the next few days, Vγ3⁺ thymocytes appeared which expressed high levels of TCR (Vγ3^high) and very low levels of HSA. The antigens CD5, CD45RB, and MEL14 were also differentially expressed on Vγ3^low versus Vγ3^high thymocytes, but the shift in expression was the opposite as compared with immature and mature TCR-α/β thymocytes. Transfer experiments of sorted Vγ3^low/HSA^high thymocytes to SCID thymic lobes showed that these cells were indeed the precursors of Vγ3^high/HSA^low thymocytes. The phenotype of the Vγ3^high thymocytes was similar to that of the postthymic Vγ3⁺ cells found in the skin of adult mice. The differentiation of Vγ3^low in Vγ3^high thymocytes was also observed in fetal thymic organ culture. Addition of cyclosporin A (CsA) to these cultures had little effect on the appearance of Vγ3^low/HSA^high cells, but blocked the appearance of Vγ3^high/HSA^low cells. These results show that, like α/β T cells, Vγ3⁺ thymocytes differentiate from TCR^low precursors to cells with a mature phenotype and that CsA inhibits this transition.