Circulating immune complexes associated with decreased complement-mediated inhibition of immune precipitation in sera from patients with bacterial endocarditis.

The sera of patients with bacterial endocarditis frequently contain high levels of circulating immune complexes. In in vitro assays these sera have been shown to be deficient in the complement mediated inhibition of immune precipitation (immune complex solubilization) although C3 and C4 levels were often normal. The deficiency is due to the presence of a factor which also inhibits the ability of normal serum to solubilize immune complexes. This inhibitor is possibly rheumatoid factor which is frequently detected in endocarditis. Serial studies on 16 patients showed the levels of immune complexes, the ability to prevent immune precipitation and rheumatoid factor to correlate with disease activity. The similarity of the findings to those in rheumatoid arthritis are discussed.