Hypocomplementaemia of poststreptococcal acute glomerulonephritis is associated with C3 nephritic factor (C3NeF) IgG autoantibody activity

We have observed that decreased plasma levels of C3 in the serumof three children with poststreptococcal acute glomerulonephritis(PSAGN) at the time of presentation were associated with thepresence of C3NeF activity in purified serum IgG from the patients.C3NeF activity was determined using a sensitive assay measuringthe ability of patients’ IgG to stabilize a preformedcell-bound alternative pathway convertase complex. C3NeF activityof patients’ IgG decreased within weeks as plasma levelsof C3 progress ively returned to normal values. C3NeF activitybecame undetectable within 1–4 months following normalizationof plasma C3 levels. Our observations suggest that early alternativepathway-dependent hypo complementaemia, a cardinal feature ofPSAGN, is mediated by the transient expression of C3NeF autoantibody activity by patients’ IgG.