Changes in C3 metabolism during protozoan infection (Babesia rodhaini) in rats.

Metabolism of the third component of complement (C3) and IgG was measured in rats before and during infection with the hemosporidium agent Babesia rodhaina. In the course of infection, hypocomplementemia and immune complex nephritis developed. During babesial infection, in most animals the half-life of C3 fell sharply, as did serum levels of C3; the catabolic rate for C3 sharply increased, whereas the synthetic rate sharply decreased. In contrast, the catabolic rate for IgG remained unchanged. The alteration in the metabolism of C3 in the face of nonparallel changes in IgG metabolism suggests that abnormal glomerular filtration and increased vasopermeability cannot explain the findings. Babesial infection in the rat provides a useful model for the study of acquired C3 metabolic defects that have been observed in humans with immune complex diseases.