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 defenc e against gram-negative bacteria. Little is known regarding such antib odies that are specific to outer membrane (OM) antigens as pill and li pooligosaccharides (LOS) in the bactericidal killing of Haemophilus du creyl. Presence of serum antibodies with specificity to a 430 kDa prot ein (polymer of the 24 kDa protein, named fine-tangled pill) and LOS i n serum from chancroid patients and healthy individuals were investiga ted by ELISA. Using a bactericidal assay, we investigated the role of human and rabbit antibodies with the aforementioned specificity. Acces sibility of LOS and of OM antigens, as well as the deposition of compo nents of the complement (C) system on the surface of the bacteria, was further investigated by whole-cell ELISA and immunoelectron microscop y. Immunoglobulin G (IgG) antibodies specific to the 430 kDa polymer a nd to LOS were demonstrated in the majority of sera from chancroid pat ients and healthy individuals. However, sera from chancroid patients d id not significantly enhance the C-mediated killing of H. ducreyl comp ared with normal human serum (NHS). Similar results were demonstrated using rabbit sera to whole bacteria, specific to the 430 kDa protein a nd LOS of H. ducreyl. However, using the same assay noncapsulated H. i nfluenzae was totally killed, as were H. influenzae type b in presence of specific antibodies. This suggests a limited effectiveness of anti bodies specific to surface antigens in C-mediated killing of H. ducrey l. LOS was detectable on the surface of H. ducreyl with a specific mon oclonal antibody in white-cell ELISA. However, a significant enhanceme nt of LOS detection was demonstrated on washed bacteria. OM antigens o f 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 b y specific monoclonal antibodies, indicating a lack of accessibility o f these antigens on the bacterial surface. However, the C6 to C9 compo nents of C were detected on the bacterial surface, suggesting capacity of forming the membrane attack complex. Altogether, these findings im ply 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 dep osition of the membrane attack complex components, but rather to a blo cking of LOS accessibility and OM proteins as potential targets of bac tericidal antibodies and C action. (C) 1998 Academic Press.