Hypocomplementaemia caused by C3 nephritic factors (C3 NeF): clinical findings and the coincidence of C3 NeF type II with anti-C1q autoantibodies.

OBJECTIVES

The main purposes were to document manifestations associated with prolonged or clinically unexplained C3 deficiency and to approximate how often hypocomplementaemia of this kind is caused by C3 nephritic factors (C3 NeF), i.e. autoantibodies to alternative pathway C3 convertases. We also wished to distinguish between C3 NeF types I and II and to assess coincident autoantibody responses to the collagen-like region of C1q (C1qCLR).

SETTING

The investigation was based on serum samples referred to a specialized laboratory for complement analysis in the course of several years.

SUBJECTS: Twenty-five persons with C3 concentrations lower than 0.43 g L-1, a third of the normal, were included in the study.

RESULTS

Analysis using three methods provided evidence of C3 NeF in 20 persons with equal frequencies of C3 NeF types I and II. We also gave evidence of antibody specificity differences for the two types of C3 NeF. Six patients with C3 NeF type II showed antibodies to C1qCLR. Membranoproliferative glomerulonephritis was the predominant diagnosis and two patients had partial lipodystrophy reflecting the well-known association between these diseases and C3 NeF. Anaphylactoid purpura, systemic lupus erythematosus, and severe infection, mainly meningococcal disease, were also observed.

CONCLUSIONS

The study group was probably fairly representative of C3 deficiency syndromes as encountered in clinical practice. The findings emphasize the heterogeneity of C3 NeF, and that acquired C3 deficiency syndromes caused by C3 NeF should perhaps be considered more often in diagnostic work.