The majority of H2-M3 is retained intracellularly in a peptide-receptive state and traffics to the cell surface in the presence of N-formylated peptides

We used a new monoclonal antibody (mAb 130) to analyze the intracellular tr afficking and surface expression of H2-M3, the major histocompatibility com plex class Ib molecule that presents AT-formylated peptides to cytotoxic T cells. M3 surface expression is undetectable in most cell types due to the paucity of endogenous antigen. M3 is induced on the cell surface by additio n of high-affinity N-formylated peptides from mitochondria and listeria. Pe ptide-induced M3 expression is most efficient on antigen presenting cells. Basal and inducible expression of M3 is transporter associated with antigen processing (TAP)-dependent, distinguishing M3 from the class Ib molecules TL and CD1. Unlike the expression of class Ia molecules and a previously de scribed M3/L-d chimera, surface expression of M3 cannot be rescued by lower ed temperature, suggesting that the alpha 3 domain and transmembrane region of M3 may control trafficking. Pulse-chase analysis and use of trafficking inhibitors revealed a pool of empty M3 in the endoplasmic reticulum or ear ly Golgi apparatus. Addition of exogenous peptide allows maturation with ki netics matching those of D-d. The lack of endogenous N-formylated peptide a llows discovery of novel pathogen-derived peptides in normal antigen presen ting cells. The nonpolymorphic nature of M3 and its ability to present bact erial antigens rapidly and dominantly make it an attractive target for pept ide vaccination strategies.