Detection of C-reactive protein, streptolysin O, and anti-streptolysin O antibodies in immune complexes isolated from the sera of patients with acute rheumatic fever

Circulating immune complexes (IC) of 42 patients with acute rheumatic fever from Santiago, Chile, were studied. The complexes were isolated by polyethylene glycol precipitation and were analyzed for antibodies, antigens, and C-reactive protein. We found the complexes to be enriched in antibody to streptolysin O, particularly in the group of patients with elevated levels of IC. IgM was the predominant class of Ig present in the complexes. Western blots from 12 patients to detect antigens in the complexes showed proteins of m.w. 50,000, 60,000, and 69,000, consistent with the polypeptides of streptolysin O. Such antigens were absent in the complexes from patients with post-streptococcal glomerulonephritis and pharyngitis. Eluted antibodies from these protein bands on the nitrocellulose sheets reacted with the streptolysin O in Western blots and neutralized the hemolytic activity of streptolysin O in a microhemolysin assay. In addition, isolated complexes from several sera showed the presence of C-reactive protein bound to complexes. In vitro experiments demonstrated that 125I]C-reactive protein was not precipitated by polyethylene glycol either alone or when added to monomeric IgG, whereas it precipitated significantly when added to aggregated IgG. The detectable C-reactive protein in isolated complexes and sera samples increased after treatment with sodium dodecyl sulfate. These data suggest that circulating immune complexes in acute rheumatic fever contain streptolysin O and its antibody and raise interesting questions regarding the pathogenetic significance of C-reactive protein in the complexes.