In this study, however, we were not able to detect binding of the human anti-V mAbs on the surface of Y. pestis cells by flow cytometry or fluorescent microscopy, suggesting that the Vantigen was not on the bacterial cell surface under the conditions used in our studies or the expression level of V-antigen was below the level of detection. As has been discussed previously, however, the presence of V-antigen on the surface of the cell may be dependent on contact with the eukaryotic host cell. The highly specific region of the neutralizing epitope (s) on the V-antigen suggests a possible ligand-receptor interaction between the Vantigen and a cellular factor. This indicates that perhaps the Vantigen exerts its biological effect through mechanisms other than mediating the TTSS pathway.