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

The RMA-S mutant T cell line is defective in H-2^b restricted antigen presentation and has markedly reduced surface expression of K^b and D^b. We examined RMA-S for the expression of the medial class I histocompatibility antigens Qa1^b and Mta. While RMA-S targets varied in their susceptibility to lysis by cytotoxic T lymphocytes (CTL) specific for Qa1^b, Mta levels were detectable but consistently low compared to the parent RMA cell line. Addition of synthetic ND1α1 - 26 or ND1α1 - 17 peptides that mimic MTF^α (the ligand of Mta) increased killing of RMA-S by anti-Mta^α 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-2^b or anti-Qa1^b CTL, even at the highest (1 μM) peptide concentrations. RMA-S cells treated with 100 μM of either the ND1α4 - 26 or ND1α1 - 26 peptides showed a small increase in the fluorescent staining for β2-microglobulin but not for H-2K^b or H-2D^b. These results show that Mta and Qa1^b, 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.