Homing receptor-bearing thymocytes, an immunocompetent cortical subpopulation

Much of the differentiation of murine T cells takes place in the thymus, perhaps influenced by the operation of stringent selection mechanisms whose existence has been inferred from the high rate of thymocyte turnover in the absence of extensive emigration. The origin of those 1% of total thymocytes which leave the thymus and seed the peripheral lymphoid organs is obscure. Recent thymic emigrants are functionally and phenotypically mature, and the purported greater maturity of medullary relative to cortical thymocytes is often cited a evidence for the medullary origin of thymic emigrants, a suggestion not without its critics. To approach this question, we have now isolated a a subpopulation of thymocytes expressing high levels of a receptor that mediates the homing of blood-borne lymphocytes into peripheral lymph nodes. Surprisingly, this population of cells (1-3% of total thymocytes) is both cortical and immunocompetent, containing approximately half of all thymic cytolytic T-lymphocyte precursors. The combination of homing receptor expression and immunocompetence makes this cortical population ideally suited for emigration to peripheral lymphoid organs.