Development of bactericidal antibody during Branhamella catarrhalis infection

The recent observation that Branhamella catarrhalis may cause a variety of infections in humans has stimulated interest in human host defenses against this organism. We encountered 21 patients with B. catarrhalis infection: seven with pneumonia, 13 with a purulent exacerbation of chronic bronchitis, and one with purulent sinusitis. Normal human serum (NHS) demonstrated no bactericidal activity against 20 of the 21 isolates. In contrast, 7 of 19 acute and 18 of 20 convalescent sera demonstrated significant bactericidal effects against the corresponding B. catarrhalis isolate. Heating convalescent sera to 56 C for 30 min abolished bactericidal activity. This activity was restored by NHS but not by complement-rich guinea pig serum. Selective blockage of the classic complement pathway eliminated bactericidal activity, whereas selective blockage of the alternative pathway did not. IgG isolated from convalescent serum plus NHS was bactericidal for the corresponding B. catarrhalis isolate. These results suggest that most patients with pulmonary infections due to B. catarrhalis develop a convalescent IgG antibody response that mediates serum bactericidal activity by the classic complement pathway.