The pH 6 antigen is an antiphagocytic factor produced by Yersinia pestis independent of Yersinia outer proteins and capsule antigen.

The pH 6 antigen (pH 6 Ag; PsaA) of Yersinia pestis has been shown to be a virulence factor. In this study, we set out to investigate the possible function of Y. pestis PsaA in a host cell line, RAW264.7 mouse macrophages, in order to better understand the role it might play in virulence. Y. pestis KIM5 derivatives with and without the pCD1 plasmid and their psaA isogenic counterparts and Escherichia coli HB101 and DEta5alpha carrying a psaA clone or a vector control were used for macrophage infections. Macrophage-related bacteria and gentamicin-resistant intracellular bacteria generated from plate counting and direct microscopic examinations were used to evaluate these RAW264.7 macrophage infections. Y. pestis psaA isogenic strains did not show any significant difference in their abilities to associate with or bind to mouse macrophage cells. However, expression of psaA appeared to significantly reduce phagocytosis of both Y. pestis and E. coli by mouse macrophages (P < 0.05). Furthermore, we found that complementation of psaA mutant Y. pestis strains could completely restore the ability of the bacteria to resist phagocytosis. Fluorescence microscopy following differential labeling of intracellular and extracellular Y. pestis revealed that significantly lower numbers of psaA-expressing bacteria were located inside the macrophages. Enhanced phagocytosis resistance was specific for bacteria expressing psaA and did not influence the ability of the macrophages to engulf other bacteria. Our data demonstrate that Y. pestis pH 6 Ag does not enhance adhesion to mouse macrophages but rather promotes resistance to phagocytosis.