Complement (C) fixing properties of glomerular immune deposits were studied in vitro in two groups of rabbits with acute
serum sickness (ASS). Deposits were studied early in decomplemented (
cobra venom factor-treated) rabbits (Group I) and during the evolution of the
nephritis in non-decomplemented rabbits (Group II). Following perfusion of kidneys of animals from Group I (which contained no autologous C3) with rabbit serum deficient in
factor B (classical pathway) or serum pre-treated with Mg++
EGTA (alternative pathway), fixation of C3 was observed in both systems. No C3 deposition was found when
EDTA-rabbit serum was used. When kidney sections obtained from serial biopsies and autopsy material from Group II (which contained rabbit C3-C3rb) were incubated with purified factors B and D, followed by either purified human C3 (C3hu) or
EDTA-guinea-pig serum (C3gp), fixation of C3hu or C3gp to early C3 deposits was observed. When sections were pre-incubated with heat-inactivated rabbit serum (source of
C3b-inactivator), C3 fixation was substantially reduced. Little or no C3 fixation was found after incubation of sections with human or guinea-pig serum depleted of
factor D (classical pathway). Thus, in ASS, glomerular
complement deposition appears to be predominantly dependent on the activation of the alternative pathway (C3b-feedback) which is progressively lost during the evolution of the
nephritis.