Using thymus anatomy to dissect T cell repertoire selection

The thymic development of CD4(+) T cells incorporates the opposing processes of positive and negative selection to produce mature lymphocytes which respond to foreign pep tides in the context of self-major histocompatibility complex class II molecules. Here, we present a model in which these events occur in two temporally and anatomically distinct steps. Mie propose that an initial positive selection step is mediated exclusively by thymic cortical epithelium. Subsequently, all those thymocytes which have been positively selected will interact with medullary epithelium and bone marrow-derived cells. Those thymocytes reacting with excess affinity or avidity to these antigen presenting cells will be negatively selected. Although acknowledging the importance of differential sig-nailing to the developing thymocyte, we will emphasize the centrality of the phenotype of the tissues which comprise the thymus.