Role of C3 in humoral immunity. Defective antibody production in C3-deficient dogs

Previous studies have shown that animals pharmacologically depleted of C3 have impaired antibody responses. However, such C depletion is neither complete nor sustained, and the C3 cleavage products generated by C3 depletion can both enhance and inhibit the immune response. To clarify the role of C3 in humoral immunity, the antibody response of dogs with genetically determined total deficiency of C3 (C3D) was examined. Serum IgG levels of the C3D animals were within the normal range, but were significantly lower than levels seen in normal controls or C3D heterozygotes. Specific antibody production was defective: the antibody titers of C3D dogs in response to primary intravenous immunization with two different T cell-dependent Ag (sheep E and bacteriophage phi X-174) were markedly reduced when compared to either normal controls or C3D heterozygotes. After secondary immunization with T-dependent Ag, the total antibody titers were normal, but the C3D dogs made proportionately more IgM and less IgG antibody than did either control group. After i.v. immunization with a T cell-independent Ag (DNP-Ficoll), the C3D dogs had reduced levels of IgM and IgG antibody after primary and secondary immunization. Neither i.m. immunization nor the use of a 20-fold increase in Ag dose i.v. could correct the defect seen in the antibody response of C3D dogs. The results herein demonstrate that C3 plays a critical role in the generation of a normal humoral immune response.