Complement-mediated solubilization of immune complexes. Solubilization inhibition and complement factor levels in SLE patients.

Thirty-two of 36 serum samples from 19 SLE patients showed reduced capacity to mediate complement-dependent solubilization of immune complexes (IC). SLE patients with nephritis exerted the lowest complement-mediated solubilization capacity (CMSC) whereas sera from patients with inactive disease gave the highest CMSC values, with three out of four samples within the normal reference range. Thirty-five of the 36 serum samples showed inhibition of CMSC in a newly developed CMSC inhibition assay. The strongest CMSC inhibition was exerted by sera from newly discovered cases of SLE who received no medical treatment and the lowest inhibition by sera from patients with inactive disease. There was a significant negative correlation between CMSC and CMSC inhibition (r = -0.67, P less than 0.001). Sera with low concentrations of C1q, C3, factor B or high C3d levels showed markedly reduced CMSC values. Pronounced CMSC inhibition was observed only in samples with normal or high factor H values. No significant correlation was found between CMSC or CMSC inhibition and circulating IC levels, but pronounced CMSC inhibition was registered only in strongly IC positive sera.