MHC genes modify systemic autoimmune disease. The role of the I-E locus

The MHC exerts an important influence on systemic autoimmune disease. In C57BL/6-lpr/lpr (B6/lpr) mice, substitution of the H-2d instead of the H-2b MHC haplotype results in a global reduction in autoantibody levels. Since H-2d expresses both I-A and I-E, while H-2b expresses only I-A, general down-regulation of autoimmunity in the d haplotype might be due to I-E expression. This was tested with I-E alpha d transgenic B6/lpr mice, which expressed a functional surface I-E molecule. Five-month-old transgene-positive B6/lpr mice had much lower total IgG, IgG anti-chromatin, anti-DNA, and IgM rheumatoid factor directed against IgG1 and against IgG2b than transgene-negative littermates (p < or = 0.002), as well as significantly lower spleen and lymph node weights (p < or = 0.002). Decreases in autoantibody levels in the transgenic lpr mice were not due to a nonspecific effect of the I-E alpha d transgene, since transgene-positive B6/lpr.H-2d mice had levels of autoantibodies comparable with transgene-negative B6/lpr.H-2d mice. To determine whether autoantibody was preferentially made by I-E-negative B cells, irradiated (B6/lpr.Igha x B6/lpr.I-E alpha d)F1 mice were reconstituted with equal amounts of B6/lpr.Igha and B6/lpr.I-E alpha d bone marrow. Allotype-specific ELISA showed that most autoantibody was produced by the I-E negative B cells (range 97% to 84%). The results show that a functional I-E molecule in lpr mice leads to generalized reduction in autoantibody levels through a direct effect on the B cell. The molecular mechanism of this effect remains to be determined.