Antibodies specific to surface antigens are not effective in complement-mediated killing of Haemophilus ducreyi

The bactericidal activity of serum is an important primary host defence against gram-negative bacteria. Little is known regarding such antibodies that are specific to outer membrane (OM) antigens as pili and lipooligosaccharides (LOS) in the bactericidal killing of Haemophilus ducreyi. Presence of serum antibodies with specificity to a 430 kDa protein (polymer of the 24 kDa protein, named fine-tangled pili) and LOS in serum from chancroid patients and healthy individuals were investigated by ELISA. Using a bactericidal assay, we investigated the role of human and rabbit antibodies with the aforementioned specificity. Accessibility of LOS and of OM antigens, as well as the deposition of components of the complement (C) system on the surface of the bacteria, was further investigated by whole-cell ELISA and immunoelectron microscopy. Immunoglobulin G (IgG) antibodies specific to the 430 kDa polymer and to LOS were demonstrated in the majority of sera from chancroid patients and healthy individuals. However, sera from chancroid patients did not significantly enhance the C-mediated killing of H. ducreyi compared with normal human serum (NHS). Similar results were demonstrated using rabbit sera to whole bacteria, specific to the 430 kDa protein and LOS of H. ducreyi. However, using the same assay noncapsulated H. influenzae was totally killed, as were H. influenzae type b in presence of specific antibodies. This suggests a limited effectiveness of antibodies specific to surface antigens in C-mediated killing of H. ducreyi. LOS was detectable on the surface of H. ducreyi with a specific monoclonal antibody in white-cell ELISA. However, a significant enhancement of LOS detection was demonstrated on washed bacteria. OM antigens of 26, 40, 45 kDa and the major outer membrane protein (MOMP) of 43 kDa were not detectable on the surface of nonwashed and washed bacteria by specific monoclonal antibodies, indicating a lack of accessibility of these antigens on the bacterial surface. However, the C6 to C9 components of C were detected on the bacterial surface, suggesting capacity of forming the membrane attack complex. Altogether, these findings imply that antibodies specific to surface antigens, such as the 430 kDa protein and LOS, are not capable of enhancing killing of bacteria. The demonstrated relative resistance is probably due not to a lack of deposition of the membrane attack complex components, but rather to a blocking of LOS accessibility and OM proteins as potential targets of bactericidal antibodies and C action.