The role of IL-4, a key Th2 cytokine, in promoting or inhibiting active Heymann nephritis (HN) was examined. HN is induced by immunization with Fx1A in CFA, and proteinuria in HN is associated with subepithelial IgG and C3 deposition and infiltration of CD8(+) T-cytotoxic 1 (Tc1) cells and macrophages into glomeruli, as well as induction of Abs to Crry. Treatment with rIL-4 from the time of Fx1A/CFA immunization stimulated an earlier IgG1 response to Fx1A, induced anti-Crry Abs, and up-regulated IL-4 mRNA in lymphoid tissue, but did not alter proteinuria. Treatment with MRCOx-81, an IL-4-blocking mAb, resulted in greater proteinuria, which suggests endogenous IL-4 regulated the autoimmune response. Delay of rIL-4 treatment until 4 wk post-Fx1A/CFA immunization and just before the onset of proteinuria prevented the development of proteinuria and reduced Tc1 cell infiltrate in glomeruli. Delayed treatment with IL-4 had no effect on titer or isotype of Abs to Fx1A or on Ig, C3, and C9 accumulation in glomeruli. Treatment with rIL-13, a cytokine that alters macrophage function such as rIL-4, but has no direct effect on T or B cell function, reduced glomerular macrophage infiltrate, but did not prevent proteinuria or CD8+ T cell infiltrate. Anti-Crry Abs were paradoxically only induced with rIL-4 therapy, not in HN controls with proteinuria. It was concluded that the rIL-4 effect was probably by inhibition of Tc1 cells, which normally mediate the glomerular injury that results in proteinuria.