A small amount of Factor D activates the complement alternative pathway under conditions of higher levels of C3 and Factor B 4351

Abstract Description FD cleaves C3bB to form AP convertase C3bBb that triggers an amplification loop of complement activation. Relative contributions of C3, FB, and FD to AP activation have not been thoroughly analyzed. While polymorphisms of C3 or FB impacts complement-related human diseases, only homozygous deficiency of human FD has been associated with opportunistic infections. We found that a 50% dilution of C3 or FB significantly impaired AP activity, while a dilution of FD to less than 5% had a similar effect. We aim to further investigate the contribution of C3, FB and FD to AP activation. Breeding Crry+/-FB+/- yielded 2.4% Crry-/-FB+/+, 4.9% Crry-/-FB+/-, and 1.2% Crry-/-FB-/- mice from a total of 243 pups, but breeding Crry+/-FD+/- failed to produce any pups with the Crry-/- genotype among 171 pups, indicating that a half-dose of FD is sufficient to sustain AP mediated-rejection of embryos. Consumption of C3 and FB in Crry-/- mice normalizes in Crry-/-fD-/- mice, indicating a requirement of FD to modulate excessive complement turnover. Disparate dosage of FD (∼ 1/100 of normal) and C3 (∼ 1/3 of normal) were necessary to rescue AP activity. The defect in hemolysis of Crry-/-fD-/-C3-/- TKO serum was rescued by adding back both purified FD and C3; a low dose of FD and a high dose of C3 were required to rescue the hemolytic defect of AP activity in TKO serum. Our results demonstrate that a low amount of FD but relative higher C3 and FB are required to modulate complement homeostasis. Funding Sources 5R35GM136352-05 Topic Categories Innate Immune Responses and Host Defense: Cellular Mechanisms (INC)