Characterization of kinetics and target proteins for binding of human complement component C3 to the surface-exposed outer membrane of Chlamydia trachomatis serovar L2

Abstract

In order to characterize the interaction of human complement with Chlamydia trachomatis, flow cytometry was used to quantitate binding of complement component C3 to elementary bodies of C. trachomatis serovar L2 preincubated in fresh serum in the presence or absence of human polyclonal chlamydial antibody. Isolation of each of the complement activation pathways revealed that C3 was activated most effectively by the alternative pathway. The degree of binding by the classical pathway was proportional to the concentration of antibody, but dual-pathway-mediated binding was not greater than antibody-independent alternative pathway binding. Electrophoresis and immunoblotting of detergent-extracted outer membrane protein-C3b complexes indicated that the chlamydial major outer membrane protein was the primary cell surface moiety binding C3b in both the presence and absence of specific antibody. Hydroxylamine cleavage of outer membrane protein-C3b complexes provided evidence that the majority of C3b is bound to the major outer membrane protein by hydroxyl ester bonds. This result was also unchanged by the presence of specific antibody. An unexpected finding was the apparent binding of anti-C3 antibody to a 40-kDa protein of the chlamydial outer membrane complex, perhaps indicating C3 mimicry on the part of the chlamydial major outer membrane protein.Chlamydia trachomatis is an obligate intracellular bacterium causing conjunctivitis, urethritis, and pelvic inflammatory disease. It is characterized by a dimorphic life cycle, consisting of the intracellular, metabolically active reticulate body and, after lysis of the host cell, the inert, infectious elementary body (EB), which is released into the extracellular environment (18). The interaction of EBs with host resistance factors in the extracellular milieu is an important area of chlamydial research in which there remain many unanswered questions.The cell surface of C. trachomatis EBs has been studied extensively with monoclonal antibodies (MAb) to map and catalog the antigenic profiles of the outer membrane proteins (OMPs). Studies of these membrane antigens have revealed that the major OMP (MOMP) binds specific antibody and plays an important role both structurally and immunologically (20, 21). It is surface exposed at its four immunochemically variable domains (1), which bear the species and subspecies determinants for C. trachomatis serovars. Su and Caldwell (25) showed that Fab fragments of an MAb directed against the MOMP exerted a neutralizing effect by preventing chlamydial attachment to the host cell, indicating a possible role for the MOMP as an adhesin molecule in infectivity. Neutralization of chlamydial infectivity probably occurs both before and after entry into the host cell (4, 20) through a variety of mechanisms, some of which are complement dependent.Several studies have shown that chlamydiae can activate the complement cascade in fresh serum in vitro (16) and that complement greatly enhances the neutralization of infectiv-* Corresponding author. ity by a...

Problems with Non-Gonococcal Urethritis

“…vaccines may have a role in the damage caused by re-exposure to chlamydia'". The serum antibody level also appears to correlate with the damafe in fallopian tubes in women 88 , infertility in men 1 and scarring in the eye 63 • There is evidence that chlamydia activates the alternative complement pathway and the recently reported mimicry between the 40 kDa MOMP protein and C3b 89 suggests that complement may also be involved in the tissue damage.…”

Immunobiology of monocytes and macrophages during Chlamydia trachomatis infection

“…trachomatis is opsonized by the complement C3 fragment iC3b which is recognized by CR3 expressed on monocytes and macrophages [58,59]. We recently demonstrated that complement C3 facilitates rapid uptake of C. trachomatis in human monocytes supporting the role for CR3 in chlamydial uptake, [59].…”

Complement C3 opsonization of Chlamydia trachomatis facilitates uptake in human monocytes

“…C. trachomatis is able to activate the complement system and it has been demonstrated that C. trachomatis induced complement activation leads to binding of C3 to the bacterium [15].…”

“…Our observations suggest that complement opsonization of C. trachomatis D and L2 facilitates uptake into monocytes. Monocytes express different receptors recognizing the C3 opsonins, C3b and iC3b, and it was previously demonstrated that these complement proteins bind to C. trachomatis L2 [15]. We therefore used a polyclonal antibody against C3c to IgG as secondary antibody [22].…”