A species-specific monoclonal IgM antibody (mAb) 9BF8 directed against the major outer membrane protein (MOMP) of neutralized several chlamydial serovars in a complement-independent manner. The presence of Mg ions negated the neutralization in serovars F, L1 and L2, but not in serovars A, B, E, D and K. The ability of monovalent Fab-fragments of this mAb to neutralize chlamydial infectivity in a Mg-independent manner suggested that conformational alterations on the chlamydial surface induced by the cation hindered the IgM but allowed the smaller Fab fragment access to its epitope. In order to determine the chlamydial component that binds Mg, elementary bodies (EB) of serovars E and L1 were treated with EDTA at pHs 8 and 9. The infectivity of the treated EB and the amount of released LPS were determined. Only after EDTA treatment at pH 9, as the LPS release increased, did the binding of the mAb on the chlamydial surface become Mg-independent. The infectivity of the EB was almost completely lost after such a treatment. These results suggest that the chlamydial LPS has the potential to modulate the exposure of antigenic sites on the MOMP, when it is cross-linked by Mg. They further imply that serovars protected by Mg and those that are not differ in the surface topology of one particular MOMP epitope, but are antigenically very similar. This difference might be of considerable importance .


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