DR-ALPHA-EP HETERODIMERS IN DRA TRANSGENIC MICE HINDER EXPRESSION OF E-ALPHA-EP MOLECULES AND ARE MORE EFFICIENT IN ANTIGEN PRESENTATION

HLA-DRA transgenic (tg) mice on H-2(d) background were constructed to study assembly, expression and function of DR alpha:E beta class II he terodimers when an alternate E alpha chain is available. Cytofluorimet ric analysis and immunoprecipitation studies demonstrate that the majo rity (90%) of E beta(d) molecules on class II-positive splenocytes fro m DRA-tg mice are associated with DR alpha rather than E alpha chains. To characterize the functional role of the interspecies as compared w ith the wild-type I-E molecules, MHC restriction and T cell epitope im munodominance of synthetic peptides spanning the entire sequence of 65 kDa heat shock protein (hsp) from Mycobacterium tuberculosis were det ermined in hsp-primed DRA-tg and DBA/2 mice. A similar pattern of resp onsiveness was observed in both strains, but hsp epitopes recalled a h igher response in DRA-tg as compared with DBA/2 mice. A panel of T cel l hybridomas specific for two hsp peptides or a hen egg white lysozyme peptide presented by both DR alpha:E beta(d) and E alpha(d):E beta(d) was studied in detail. Surprisingly, DR alpha:E beta(d) dimers presen t these peptides more efficiently than E alpha:E beta(d), even when th e TCR was selected in mice expressing only E alpha(d):E beta d molecul es. The higher efficiency of antigen presentation by DR alpha:E beta(d ) dimers does not appear to depend on increased binding affinity for p eptides, as demonstrated by competition for antigen presentation, nor on increased efficiency in the interaction with CD4 molecules. Rather, the higher efficiency of antigen presentation could be explained by a more effective ligand-TCR interaction. This is consistent with molecu lar modeling based on the class II structure, indicating that 16 out o f 17 substitutions between the first domain of E alpha(d) and DR alpha chains lie outside the peptide binding groove and are potentially ava ilable for interaction with the TCR.