Endogenous peptides associated with MHC class II and selection of CD4 T cells.

CD4 T cells undergo positive and negative selection during thymic development mediated by the interaction of T-cell receptors with MHC class II molecules expressed by thymic epithelial cells and bone marrow-derived antigen presenting cells in the thymus. The majority of MHC molecules are occupied with peptides derived from self proteins. Although the role of self peptides in the negative selection of CD4 cells is well established, the influence of peptides on positive selection of CD4 T cells remained elusive. However, some recent results suggest a role of peptides in the positive selection of CD4 T cells.