Expression of Fc gamma RIII on HeLa 229 cells: possible effect on in vitro neutralization of Chlamydia trachomatis.

The neutralizing activities of a murine immunoglobulin G3 (IgG3) monoclonal antibody specific for the major outer membrane protein of Chlamydia trachomatis and its monovalent Fab fragments were studied by using Syrian hamster kidney (HaK) cells and human epithelial (HeLa 229) cells. The intact IgG3 antibody was neutralizing for HaK cells but was nonneutralizing for HeLa cells. In contrast, monovalent Fab antibody fragments neutralized chlamydial infectivity for both HaK and HeLa cells. Immunofluorescence analysis of HeLa 229 cells with a panel of monoclonal antibodies specific to human Fc gamma receptors revealed the expression of cell surface Fc gamma RIII. We propose that Fc gamma RIII may obscure the chlamydia-neutralizing activity of certain IgG isotypes by facilitating the Fc gamma R-mediated entry of chlamydiae into HeLa 229 cells. These findings may help explain the inconsistencies that are commonly observed in results when HeLa 229 cells are used in chlamydia neutralization assays.