Abstract
A woman with systemic lupus erythematosus (SLE) was found to have undetectable serum total complement (C) activity with marked reductions in levels of several C components. Recognition of rising C4, C3, and C5 levels coupled with sustained absence of hemolytic activity led to studies of individual C components that identified a persisting selective and complete absence of C7 hemolytic activity. Review of the literature indicated that the frequency of SLE is increased in patients with complete deficiency of a lateβacting C component (7% as compared to 0.05% in the general population); however, the disease frequency is less than that observed in patients with complete deficiency of an earlyβacting C component (41% incidence in patients lacking C1, C4, or C2). In order to explore one possible basis for this association, the capacity of serum from the patient described herein and from patients with other C component deficiencies to mediate in vitro solubilization of immune complexes was studied by using a recently developed assay. The patient's serum had a markedly decreased capacity to induce solubilization of immune complexes, as did sera of other patients with active SLE and sera of individuals with inborn deficiencies of C1, C2, or C3 without SLE. By contrast, the sera of patients with a selective terminal C component deficiency (C5, C6, C7, or C8) but without SLE readily supported immune complex solubilization. These results suggest that patients lacking a terminal C component are predisposed to the development of SLE by a process which is independent of Cβmediated immune complex solubilization. Whether the compromised capacity for Cβmediated immune complex solubilization contributes to the even greater frequency of SLE in patients lacking an earlyβacting C component remains to be established.