Interleukin-4 (IL-4) provides support for humoral immune responses through upregulation of T helper (Th) type 2 cell differentiation, but it is not known whether IL-4 promotes antibodymediated autoimmune diseases such as systemic lupus erythematosus (SLE). Here, we show that the constitutive expression of an IL-4 transgene by B cells completely prevents the development of lethal lupus-like glomerulonephritis in the (NZW × C57BL/6.Yaa)F1 murine model of SLE. This was associated with marked changes in the serum levels of IgG subclasses, rather than in the total levels of anti-DNA antibodies, with a lack of IgG3, a decrease of IgG2a, and an increase in IgG1 subclasses, and by a strong reduction in the serum levels of gp70-antigp70 immune complexes. This effect of the transgene appears to result from a modulation of the Th1 versus Th2 autoimmune response, since the protected mice displayed comparably modified IgG2a and IgG3 antibody response against exogenous T cell–dependent antigen, but not against T cell–independent antigens. Thus, IL-4 prevents the development of this lupuslike autoimmune disease, most likely by downregulating the appearance of Th1-mediated IgG subclasses of autoantibodies such as the IgG3 autoantibodies which have been shown to be especially nephritogenic.