The selection of M3-restricted T cells is dependent on M3 expression and presentation of N-formylated peptides in the thymus

The major histocompatibility complex (MHC) class Ib molecule H2-M3 binds N- formylated peptides from mitochondria and bacteria. To explore the role of MS expression and peptide supply in positive and negative selection, we gen erated transgenic mice expressing an M3-restricted TCR-alpha/beta from a CD 8(+) T cell hybridoma (D7) specific for a listerial peptide (LemA). Develop ment of M3-restricted transgenic T cells is impaired in both beta 2-microgl obulin-deficient and transporter associated with antigen processing (TAP)-d eficient mice, but is not diminished by changes in the H-2 haplotype. Matur ation of M3/LemA-specific CD8(+) single positive cells in fetal thymic orga n culture was sensitive to M3 expression levels as determined by antibody b locking and use of the castaneus mutant allele of M3. positive selection wa s rescued in TAP(-/-) lobes by nonagonist mitochondrial and bacterial pepti des, whereas LemA and a partial agonist variant caused negative selection. Thus, MS-restricted CD8(+) T cells are positively and negatively selected b y M3, with no contribution from the more abundant class Ia molecules. These results demonstrate that class Ib molecules can function in thymic educati on like class Ia molecules, despite limited ligand diversity and low levels of expression.