Classical complement pathway activation in membranoproliferative glomerulonephritis.

Levels of components of the classical and alternative complement pathways and the activity of the C3 nephritic factor (C3NeF) were measured in serum specimens from patients with type I (subendothelial deposits) and type II (intramembranous dense deposits) membranoproliferative glomerulonephritis (MPGN) and the results compared with the levels in normal subjects. Although C3 and C5 concentrations were comparably depressed in type I and type II, the levels of Clq and C4 were lower in type I and the correlation coefficients between Clq vs. C4 and C4 vs. C3 were higher than in type II. The profile was highly suggestive of classical pathway activation. In contrast, in type II MPGN, factor B levels were lower and C3NeF activity was more frequently detectable and higher. A feature of interest was that type I, as compared to type II, was characterized by significantly lower serum concentrations of properdin, higher and more significant correlation coefficients between serum properdin concentrations and those of Clq, C4 and C3, and consistent glomerular deposition of properdin. The involvement of properdin in type I may reflect recruitment of a pathway resembling the C1 bypass mechanism, said to involve C1, properdin, factor B and C3-9.