In view of the controversy concerning the expression of chlamydial lipopolysaccharide (LPS) in the eukaryotic host cell, the presence of this molecule was examined in three cell types which were experimentally infected with serotype E. LPS was detected in the McCoy cell line, human endometrial epithelium and human amniotic epithelium with two monoclonal antibodies. The appearance and distribution of LPS at the host cell surface during the chlamydial developmental cycle and its transmission to neighbouring cells were examined by immunofluorescence microscopy after air drying of the host cells. LPS distribution was not uniform; it was first observed on regions of the cell surface in close proximity to the chlamydial endosome (inclusion). Soon after, the antigen was also detected at points of contact with neighbouring uninfected cells. Immunofluorescent plaques of host cells contaminated with LPS were thus formed in the vicinity of infected cells. These plaques increased in size over 2 d before becoming smaller as host cell lysis occurred. The major outer-membrane protein (MOMP) was not visualized on the host cell surface after air drying. No cell-surface LPS antigen was observed in live cells or those fixed in formaldehyde without air drying. Conventional methanol fixation and immunolocalization of LPS and MOMP in parallel infected cultures stained these antigens within inclusions in the expected fashion. Radio-immunoassays were used to quantify LPS in confluent McCoy cell monolayers during the chlamydial developmental cycle. Cell-surface-associated and inclusion-associated LPS, measured by direct binding of I-labelled anti-LPS monoclonal antibodies to air-dried or methanol-fixed monolayers respectively, increased for up to 3 d then declined. Cell-surface-associated LPS is not directly accessible to antibodies in the hydrated cell. This apparent masking of the antigen may have a significant advantage for persistence of the parasite , since such host-cell-associated antigen is unlikely to be a target for immune attack.


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