MHC CLASS-I HEAVY CHAIN-DEPENDENT EXPRESSION OF DISCONTINUOUS ANTIGENIC EPITOPES ON BETA(2)-MICROGLOBULIN(B) IS INDUCIBLE WITH PEPTIDE-LIGAND

Previously, we reported that expression of the murine beta 2-microglobulin(b) (beta 2m(b)) antigenic epitopes defined by the mAb S19.8 and 23 (SJL [beta 2m(a)] anti-B10.S [beta 2m(b)]) was dependent upon association of beta 2m with MHC class I heavy chains. We have further explored the antigenic properties of beta 2m under circumstances requiring the induction of MHC class I surface expression with heavy chain-specific peptide-ligand. For the RMA-S cell line, which is class I surface null due to a defect in the TAP-2 peptide transporter, treatment with the H-2K(b)-specific vesicular stomatitis virus-derived N p52-59 peptide resulted in the cell surface expression of the epitopes defined by the anti-H-2K(b) mAb Y-3, as well as equally strong expression of the epitopes defined by the anti-beta 2m(b) mAb S19.8 and 23. Similarly, the FLU-NP p366-374 peptide induced H-2D(b) on the surface of RMA-S cells as determined by cytofluorometry with the mAb MKQ8; however, expression of the epitope defined by S19.8 was only partially recovered and no reactivity was observed for mAb 23. That the H-2D(b) heavy chain was assembled with beta 2m(b) on the cell surface was established from immunoprecipitation experiments with I-125-surface-radiolabeled RMA-S cells treated with FLU-NP p366-374; MKQ8 immunoprecipitated prominent heavy chain and beta 2m bands, whereas S19.8 and 23 isolated a weak beta 2m band (12-15% of that co-immunoprecipitated with MKQ8). These results are consistent with the observation that human beta 2m-deficient cells (designated FO-1) transfected with the B2m(b) allele were induced, in combination with the endogenous HLA class I heavy chains, to express the epitope defined by S19.8, but not mAb 23, whereas both were expressed when cotransfection was performed with the H-2K(b) gene. That the determinants recognized by S19.8 and 23 were formed by a discontinuous cluster of amino acids within beta 2m was established from experiments demonstrating that H-2K(b) heavy chain assembled with a chimeric beta 2m molecule (comprising human beta 2m from 1-69 and mouse beta 2m from amino acid 70-99, including the polymorphic residue Ala 85) did not lead to expression of the S19.8 and 23 epitopes. The results of this study provide evidence that heavy chain polymorphism can affect the antigenic properties of beta 2m and offer insight into the basis by which CTL may react against beta 2m(b) when assembled with the H-2K(b) molecule.