SERUM BACTERICIDAL ACTIVITY AND PHAGOCYTOSIS IN HOST-DEFENSE AGAINST HAEMOPHILUS-DUCREYI

Serum bactericidal activity and phagocytic killing are two important m echanisms involved in the host defence against bacteria. Using some in vitro methods, serum bactericidal assay, phagocytic killing by polymo rphonuclear leukocytes (PMN) and chemiluminescence, we have evaluated the significance of these mechanisms in the killing of Haemophilus duc reyi bacteria. Furthermore, induction of C3 conversion and deposition of immunoglobulins, C1q and C3, on the surface of bacteria was studied by crossed immunoelectrophoresis and ELISA, respectively. Transmissio n electron microscopy was employed to study internalization of bacteri a by PMN. H. ducreyi and lipooligosaccharide preparations from these b acteria were able to induce conversion of complement factor C3 in norm al human serum (NHS). Exposure of bacteria to NHS resulted in depositi on of IgG, IgM and complement factors C1q and C3 on the surface of bac teria. H. ducreyi bacteria lost their viability when incubated with fr esh but not inactivated normal serum at high concentrations, indicatin g that the bacteria are sensitive to the complement-dependent bacteric idal activity of serum. There were some variations between different s trains regarding their susceptibility to the bactericidal activity of NHS, but for eight strains tested, all of the bacteria exposed were no t killed in medium containing up to 70% of fresh serum. Complement-med iated opsonophagocytic killing of H. ducreyi by PMN was more effective than complement-dependent bactericidal activity of fresh normal sera. Bacteria treated with heat inactivated immune sera, on the other hand , were as sensitive to the bactericidal effect of PMN as those treated with non-inactivated immune sera, indicating the role of antibodies i n opsonophagocytosis. H. ducreyi bacteria were also killed by PMN in t he absence of serum antibodies and complement. Using the chemiluminesc ence assay, H. ducreyi was shown to activate PMN in the absence of ser um as well as after opsonization with complement and antibodies. Our r esults therefore indicate that both opsonic- and non-opsonic mechanism s are involved in the phagocytosis and the subsequent killing of H. du creyi bacteria. Although both complement and antibodies enhance the ab ility of phagocytes to kill H. ducreyi, neither component is sufficien t for effective killing of H. ducreyi.