T cell adolescence: Maturation events beyond positive selection

Single-positive thymocytes that successfully complete positive and negative selection must still undergo one final step, generally termed T cell maturation, before they gain functional competency and enter the longlived T cell pool. Maturation initiates after positive selection in single-positive thymocytes and continues in the periphery in recent thymic emigrants, before these newly produced T cells gain functional competency and are ready to participate in the immune response as peripheral naive T cells. Recent work using genetically altered mice demonstrates that T cell maturation is not a single process, but a series of steps that occur independently and sequentially after positive selection. This review focuses on the changes that occur during T cell maturation, as well as the molecules and pathways that are critical at each step.