THE PHENOTYPE OF H-2M-DEFICIENT MICE IS DEPENDENT ON THE MHC CLASS-IIMOLECULES EXPRESSED

For a broader view of the role of H-2M as an accessory molecule in ant igen presentation, we investigated the degree to which different MHC c lass II isotypes and alleles depend on H-2M to function in vivo. We ge nerated H-2M-deficient animals expressing E-k/b Or A(k) molecules in a ddition to the Ab molecules already present in the mutant strain, and compared the ability of the different MHC class II molecules to presen t antigen at the cell surface for recognition by T cells, and contribu te to positive selection of CD4(+) T cells in the thymus. Biochemical analyses were performed to assess MHC class II maturation, and to dete rmine the peptide content of the molecules. In the absence of H-2M, E- k/b molecules containd a more heterogeneous set of class Ii-associated invariant chain peptides (CLIP) than Ab did, which, unlike Ab-CLIP co mplexes, were not SDS-stable. Unlike Ab molecules, both E-k/b and A(k) efficiently presented exogenously added peptides to T cells in the ab sence of H-2M. In addition, epitopes from some proteins, especially th ose known to be invariant chain independent, were presented by A(k) mo lecules in the mutant animals. To our surprise, expression of Ek/b ove rcame the positive selection defect observed in H-2M-deficient mice ex pressing Ab alone. In contrast, PT expression did not augment positive selection of CD4(+) T cells in the mutant animals. Some of these find ings in vivo contrast significantly with findings from in vitro studie s on murine MHC class II molecules in human DM-deficient cell lines.