Role of peptide ligand in the positive selection of T lymphocytes

T lymphocytes mature in the thymus where they encounter peptides derived from self proteins of the body presented by MHC class I and class II molecules. Recognition of both peptides and MHC leads to negative and positive selection of T cells bearing appropriate antigen receptors. The role of self-peptides bound to MHC molecules in negative selection is well established. Such a role in positive selection has only recently been scrutinized and many issues are still controversial. These topics are discussed in this review. A new transgenic mouse has been recently generated that displays only one peptide covalently bound to all class II MHC molecules in the body. The process of positive selection in that mouse is presented here in more detail.