Evaluating peptide repertoires within the context of thymocyte development

The process of antigen presentation by MHC molecules allows T cells to samp le the proteins expressed within a particular cell. This sampling is in the form of short peptides bound within the grooves of MHC molecules displayed on the surface of cells. In the context of immune surveillance, this prese ntation allows the identification of infected cells by displaying peptides originating from foreign proteins within the cell. However, MHC-bound pepti des play additional roles beyond serving as antigenic stimuli during an imm une response. In fact, it has become clear that MHC-bound peptides derived from self proteins are critically involved in the development of T cells du ring selective events in the thymus. In this review we will discuss the nat ure of the population of MHC-bound peptides as it relates to thymocyte deve lopment, with particular emphasis on the recent finding that peptide- MHC c omplexes present at low levels can drive the positive selection of thymocyt es.