IgG autoantibodies to C1q do not detectably influence complement activation in vivo and in vitro in systemic lupus erythematosus

The influence of IgG antibodies to C1q (C1qAb) on activation of the classical pathway of the complement system was investigated in patients with systemic lupus erythematosus (SLE). In in vivo experiments, a prototype for immune complexes was administered intravenously to 14 patients and 9 healthy controls. Eight SLE patients had increased C1qAb titers. The increase of C3a levels, which was measured as a parameter of C1 activation, was significantly lower in SLE patients than in the healthy controls (p = 0.01). No correlation was found between C3a increases and C1qAb titers. In in vitro experiments the influence on C1 activation of monomeric IgG isolated from serum of 11 SLE patients, 7 of whom had increased C1qAb titers, was measured in a C4 consumption assay. The presence of C1qAb did not influence C4 consumption. The results demonstrate that C1qAb do not influence C1 activation by immune complexes in SLE patients.