Antibody-dependent alternative pathway killing of Haemophilus influenzae type b.

The bactericidal activities of human complement and human antibody directed against specific Haemophilus influenzae type b cell surface determinants were investigated. Strain Eagan, a laboratory isolate, and strain Kn, a clinical isolate, were used as the test organisms and gave qualitatively similar results. In the absence of antibody, both isolates were resistant to killing by 60% agammaglobulinemic serum (AGS) containing normal complement levels. The addition of affinity-purified immunoglobulin G anticapsular antibody was bactericidal with 15% AGS as the complement source. Bactericidal activity was also demonstrated with this antibody when the complement source was AGS-Mg-EGTA [ethylene glycol-bis(beta-aminoethyl ether)-N,N-tetraacetic acid], C2-deficient human serum (alternative complement pathway), or AGS in which factor D and properdin had been selectively inactivated (classical pathway). Immunoglobulin G fractions from a human serum pool or from serum from an adult who had recovered from H. influenzae type b (Kn) sepsis were absorbed to remove anticapsular antibody. The absorbed fractions containing noncapsular antibodies also activated complement-dependent bactericidal activity. But, in contrast to the results with anticapsular antibody, noncapsular antibodies did not elicit alternative pathway bactericidal activity. Incubation of cells of H. influenzae type b in C2-deficient serum or AGS-Mg-EGTA did not cause complement consumption (total hemolytic complement and C3). The addition of immunoglobulin G anticapsular antibody (but not noncapsular antibody) increased consumption of total complement and C3, paralleling the results of the bactericidal assays. These studies demonstrated an absolute requirement for anticapsular antibody in alternative pathway activation and killing of H. influenzae type b.(ABSTRACT TRUNCATED AT 250 WORDS)