Development of In Vitro Correlate Assays of Immunity to Infection with Yersinia Pestis
ARMY MEDICAL RESEARCH INST OF INFECTIOUS DISEASES FORT DETRICK MD BACTERIOLOGY DIV
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Pneumonic plague is a severe, rapidly progressing disease for which there is no effective vaccine. As the efficacy of new vaccines cannot be tested in humans, it is essential to develop in vitro surrogate assays that are valid predictors of immunity. The F1 capsule antigen stimulates a protective immune response to most strains of Y. pestis. However, strains of Y. pestis that are F1- but still virulent have been isolated, and an in vitro assay, the results which can predict protection against both F1 and F1- strains, is needed. The virulence antigen V is an essential virulence factor of Y. pestis and stimulates protective antibodies. We investigated potential correlates of plague immunity that are based on anti-V Ab-mediated neutralization of Yersinia-induced macrophage cytotoxicity. The neutralizing activity of sera from mice vaccinated with an F1-V fusion candidate vaccine was determined. The decrease in the level of the apoptosis-specific enzyme caspase-3 significantly predicted survival in one- and two-dose vaccination experiments. Sera from F1-V vaccinated nonhuman primates were evaluated with MPhis assays based on caspase-3 and on other markers manifested at the different stages in cell death. Using murine- and human-derived MPhis in microscopic and fluorescence activated cell sorting-based livedead staining assays of terminal necrosis, we demonstrated a strong association between in vitro neutralization of macrophage cytotoxicity induced by sera-treated Yersinia and in vivo protection against lethal infection. These results provide a strong base for the development of reliable in vitro correlate bioassays which are predictive of protective immunity to plague.
- Medicine and Medical Research