- Gene-wide-association and candidate gene studies indicate that the greatest effect on multiple sclerosis (MS) risk is driven by the HLA-DRB1*15:01 allele within the HLA-DR15-haplotype (HLA-DRB1*15:01-DQA1*01:02-DQB1*0602-DRB5*01:01). Nevertheless, linkage-disequilibrium makes it difficult to define, without functional studies, whether the functionally relevant effect derives from DRB1*15:01 only, its neighboring DQA1*01:02-DQB1*06:02 or DRB5*01:01 genes of HLA-DR15-haplotype, or from their combinations or epistatic interactions. Here, we analyzed the impact of the different HLA-DR15-haplotype alleles on disease-susceptibility in a new humanized model of MS induced in HLA-transgenic (Tg) mice by human oligodendrocyte-specific protein (OSP)/claudin-11 (hOSP), one of the bone fide potential primary target antigens in MS. We show that the hOSP-associated MS-like disease is dominated by DRB1*15:01 allele not only as the DRA1*01:01;DRB1*15:01 isotypic-heterodimer, but also, unexpectedly, as a functional DQA1*01:02;DRB1*15:01 mixed-isotype heterodimer. The contribution of HLA-DQA1/DRB1 mixed-isotype heterodimer to OSP-pathogenesis was revealed in (DRB1*1501xDQB1*0602)F1-double-Tg mice immunized with hOSP142-161 peptide, where the encephalitogenic potential of prevalent DRB1*1501/hOSP142-161-reactive Th1/Th17-cells is hindered due to a single amino acid difference in the OSP142-161 region between human and mouse; this impedes binding of DRB1*1501 to mouse-OSP142-161 epitope in mouse CNS while exposing functional binding of mouse-OSP142-161 to DQA1*01:02;DRB1*15:01-mixed-isotype heterodimer. This study, which shows for the first time a functional HLA-DQA1/DRB1 mixed-isotype-heterodimer and its potential association with disease-susceptibility, provides a rationale for potential effect on MS-risk from DQA1*01:02 through functional DQA1*01:02;DRB1*15:01-antigen presentation. Furthermore, it highlights a potential contribution to MS-risk also from interisotypic combination between products of neighboring HLA-DR15-haplotype alleles, in this case the DQA1/DRB1 combination. Copyright © 2015, The American Society for Biochemistry and Molecular Biology.