Isolation and characterization of circulating immune complexes from patients with pneumococcal pneumonia

Circulating immune complexes (CIC) were isolated from serum samples from patients with bacteremic and nonbacteremic pneumococcal pneumonia. Overall, 63% (26 of 41) of patients with pneumococcal pneumonia had elevated levels of immunoglobulin G (IgG)-containing CIC. IgM-containing CIC were identified in samples from only three patients. Serum samples from nonbacteremic patients contained significantly higher levels of IgG-containing CIC (96.6 +/- 111.7 micrograms/ml) than did samples from bacteremic patients (31.7 +/- 26.9 micrograms/ml) during week 1 in hospital (P less than 0.05). Immune complexes levels did not correlate with IgG concentrations in serum or anticapsular antibody levels. Immune complexes from nonbacteremic patients had sedimentation coefficients of greater than 19s by density gradient ultracentrifugation. In contrast, CIC from bacteremic patients had smaller coefficients, of between 9s and 14s. Pneumococcal capsular antigens were identified in concentrated dissociated CIC from both patient groups by counterimmunoelectrophoresis.