A. Frisk et al., ANTIBODIES SPECIFIC TO SURFACE-ANTIGENS ARE NOT EFFECTIVE IN COMPLEMENT-MEDIATED KILLING OF HAEMOPHILUS-DUCREYI, Microbial pathogenesis, 25(2), 1998, pp. 67-75
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.