The thymus medulla and its control of alpha beta T cell development

alpha beta T cells are an essential component of effective immune responses. The heterogeneity that lies within them includes subsets that express diverse self-MHC-restricted alpha beta T cell receptors, which can be further subdivided into CD4(+) helper, CD8(+) cytotoxic, and Foxp3(+) regulatory T cells. In addition, alpha beta T cells also include invariant natural killer T cells that are very limited in alpha beta T cell receptor repertoire diversity and recognise non-polymorphic CD1d molecules that present lipid antigens. Importantly, all alpha beta T cell sublineages are dependent upon the thymus as a shared site of their development. Ongoing research has examined how the thymus balances the intrathymic production of multiple alpha beta T cell subsets to ensure correct formation and functioning of the peripheral immune system. Experiments in both wild-type and genetically modified mice have been essential in revealing complex cellular and molecular mechanisms that regulate thymus function. In particular, studies have demonstrated the diverse and critical role that the thymus medulla plays in shaping the peripheral T cell pool. In this review, we summarise current knowledge on functional properties of the thymus medulla that enable the thymus to support the production of diverse alpha beta T cell types.