The phenotype of H-2M-deficient mice is dependent on the MHC class II molecules expressed

For a broader view of the role of H-2M as an accessory molecule in antigen presentation, we investigated the degree to which different MHC class II isotypes and alleles depend on H-2M to function in vivo. We generated H-2M-deficient animals expressing E^{k / b} or A^k molecules in addition to the A^b molecules already present in the mutant strain, and compared the ability of the different MHC class II molecules to present antigen at the cell surface for recognition by T cells, and contribute to positive selection of CD4⁺ T cells in the thymus. Biochemical analyses were performed to assess MHC class II maturation, and to determine 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 A^b did, which, unlike A^b -CLIP complexes, were not SDS-stable. Unlike A^b molecules, both E^{k / b} and A^k efficiently presented exogenously added peptides to T cells in the absence of H-2M. In addition, epitopes from some proteins, especially those known to be invariant chain independent, were presented by A^k molecules in the mutant animals. To our surprise, expression of E^{k / b} overcame the positive selection defect observed in H-2M-deficient mice expressing A^b alone. In contrast, A^k expression did not augment positive selection of CD4⁺ T cells in the mutant animals. Some of these findings in vivo contrast significantly with findings from in vitro studies on murine MHC class II molecules in human DM-deficient cell lines.