Immune complex glomerulonephritis in C4- and C3-deficient mice. In this study, we examined the roles of C4 and C3 in immune complex glomerulonephritis by actively immunizing C4-deficient (C4 –/–), C3 deficient (C3 –/–) and wild-type mice with apoferritin. Wild-type animals with an intact complement system produced anti-apoferritin IgG and IgM antibodies, and developed mesangial proliferative glomerulonephritis characterized by hypercellularity, matrix expansion, deposition of IgG, IgM, IgA and C3, and the presence of electron dense deposits. In the majority of animals, the peripheral capillaries also contained IgG, C3 and subendothelial and subepithelial electron dense deposits. In contrast to wild-type animals, all apoferritin-immunized C4 –/– and C3 –/– mice had serum cryoprecipitates containing polyclonal IgM and the variable presence of polyclonal IgG. These animals also developed immune complex glomerulonephritis, but their disease manifestations were distinctly different from that of their wild-type littermates. In apoferritin-immunized C4 –/– and C3 –/– mice, IgG was either absent or present in reduced quantities in glomeruli, yet IgM and IgA were present in greater intensity in glomeruli. Capillary wall IgG deposits were absent in all C4 –/– and C3 –/– animals. C4 –/– animals also had significant glomerular C3 deposition, hypercellularity and neutrophil infiltration, which were not present in C3 –/– animals. These results illustrate the complex interplay between the effects of complement to process immune complexes and to lead to inflammation and tissue injury.