Effect of Haemophilus influenzae type b lipopolysaccahride on complement activation and polymorphonuclear leukocyte function

Purified lipopolysaccharide (LPS) from Haemophilus influenzae type (Hib) was examined for its capacity to interact with human hemolytic complement, generate conversion products of C3, C4, and factor B, stimulate C5a activity, and affect human neutrophil chemiluminescence and phagocytosis. Salmonella typhimurium LPS and Salmonella minnesota Rb LPS (R345 mutant) were examined for comparison. Incubation of Hib LPS with human serum deficient in γ-globulin or with normal human serum containing 10 mM EGTA and 7 mM MgCl₂ resulted in some depletion of hemolytic complement and conversion of C3 to degradation products (determined by inhibition of passive hemolysis and electrophoresis/immunofixation, respectively), indicating that complement activity occurred by the alternative pathway. Complement activation by Hib LPS and S. minnesota Rb LPS was similar, but significantly less effective than by S. typhimurium LPS (p < 0..01). Solubilized Hib lipid A, but not LPS, induced conversion products of C5 in hypogammaglobulinemic serum, indicating activation of the classical pathway. Similar levels of C5a activity were generated by incubation of Hib LPS and S. tryphimurium LPS in hypogammaglobulinemic serum, as determined by neutrophil shape change and neutrophil aggregation. Hib LPS directly stimulated neutrophil chemiluminexcence, whereas S. typhimurium LPS had little effect. Phagocytosis of radiolabeled, opsonized Hib by neutrophils was diminished by S. minnesota Rb LPS, Hib LPS or solubilized Hib lipid A (p < 0.001), but was slightly increased by S. typhimurium LPS. Neither the oligosaccharide of Hib LPS or Hib capsular polysaccharide was capable of interacting with complement or altering neutrophil chemiluminescence or phagocytosis. These results indicate that in comparison to S. typhimurium LPS, Hib LPS was less effective at activation complement, but more effective at impairing polymorphonuclear leukocyte function.