Expression of medial class I histocompatibility antigens on RMA-S mutant cells.

The RMA-S mutant T cell line is defective in H-2b restricted antigen presentation and has markedly reduced surface expression of Kb and Db. We examined RMA-S for the expression of the medial class I histocompatibility antigens Qa1b and Mta. While RMA-S targets varied in their susceptibility to lysis by cytotoxic T lymphocytes (CTL) specific for Qa1b, Mta levels were detectable but consistently low compared to the parent RMA cell line. Addition of synthetic ND1 alpha 1-26 or ND1 alpha 1-17 peptides that mimic MTF alpha (the ligand of Mta) increased killing of RMA-S by anti-Mta alpha CTL to levels comparable to or better than RMA, with 300 nM peptide being fully effective. None of the MTF peptides increased the killing of RMA-S by anti-H-2b or anti-Qa1b CTL, even at the highest (1 microM) peptide concentrations. RMA-S cells treated with 100 microM of either the ND1 alpha 4-26 or ND1 alpha 1-26 peptides showed a small increase in the fluorescent staining for beta 2-microglobulin but not for H-2Kb or H-2Db. These results show that Mta and Qa1b, although affected, are not obliterated by the defect in RMA-S cells; that the association of MTF peptides with HMT is exclusive; and that MTF enters the endoplasmic reticulum in the same fashion as other endogenous peptides.