© 2017, American College of Rheumatology Objective: Both environmental and genetic factors are important in the development of antiphospholipid antibodies (aPL) in patients with antiphospholipid syndrome (APS). Currently, the only available data on predisposing genetic factors have been obtained from epidemiologic studies, without mechanistic evidence. Therefore, we studied the influence of major histocompatibility complex (MHC) class II alleles on the production of aPL in a mouse model of APS. Methods: Three groups of mice, MHC class II–deficient (MHCII−/−) mice, MHCII−/− mice transgenic for human HLA–DQ6 (DQ6), DQ8, or DR4 alleles, and the corresponding wild-type (WT) mouse strains were immunized; half were immunized with human β2-glycoprotein I (β2GPI), and the other half were immunized with control ovalbumin (OVA) protein. Thrombus formation in vivo, tissue factor activity in carotid and peritoneal macrophages, and serum levels of tumor necrosis factor (TNF), IgG anticardiolipin (aCL), antibodies, and anti-OVA antibodies were determined. Results: Immunization with β2GPI induced significant production of aCL and anti-β2GPI in WT mice compared with control mice immunized with OVA (P < 0.001) but diminished aCL (P < 0.001) and anti-β2GPI (P = 0.016) production in MHCII−/− mice. Anti-β2GPI production was fully restored in DQ6 and DQ8 mice, while levels of anti-β2GPI in DR4 mice and aCL in all transgenic lines were only partially restored (P < 0.001 to P < 0.046). Thrombus size in WT mice was twice that in MHCII−/− mice (P < 0.001) but similar to that in all transgenic lines. Carotid and peritoneal macrophage tissue factor levels decreased by >50% in MHCII−/− mice compared with wild-type B6 mice and were restored in DQ8 mice but not DR4 mice or DQ6 mice. TNF levels decreased 4-fold in MHCII−/− mice (P < 0.001) and were not restored in transgenic mice. Conclusion: Our mechanistic study is the first to show that MHC class II alleles influence not only quantitative aPL production but also the pathogenic capacity of induced aPL.