Among all of the genetic factors associated with multiple sclerosis (MS) susceptibility, MHC class II molecules have the strongest association. Although a direct role of DR alleles in MS have been confirmed, it has been difficult to understand the role of DQ alleles in disease pathogenesis due to strong linkage disequilibrium with certain DR alleles. Population studies have indicated that DQ alleles may play a modulatory role in progression of MS. Using HLA class II transgenic (Tg) mice, we investigated gene complementation between DR and DQ genes in the disease process. Previously, using single Tg mice (expressing HLA-DR or DQ gene), we showed that PLP 91–110 peptide induced experimental autoimmune encephalomyelitis (EAE) only in DR3. Aβmice, suggesting that DR3 (DRB1* 0301) is a disease susceptibility gene in the context of PLP. We also showed that DQ6 protects development of EAE in DQ6/DR3 double Tg mice by production of anti-inflammatory IFN-γ. In this study, we investigated the ability of DQ8 to modulate disease in DR3/DQ8 double Tg mice. Introduction of DQ8 onto DR3 Tg mice led to higher disease incidence and increased disease severity on immunization with PLP 91–110, indicating that DQ8 had an exacerbating effect on the development of EAE. Increased susceptibility in DR3/DQ8 Tg mice was due to increased production of proinflammatory cytokine IL-17 by DQ8-restricted T cells. HLA-DR3/DQ8 mice with EAE also demonstrated increased inflammation and demyelination in CNS as compared with single DR3 Tg mice. Thus double Tg mouse provides a novel model to study epistatic interactions between HLA class II molecules in inflammatory and demyelinating disease.