¹Department of Genetic Medicine, Weill Medical College of Cornell University, New York, NY

Abstract

The aerosol form of the bacterium Yersinia pestis causes the pneumonic plague, a virulent and rapidly fatal disease that develops within days post-infection. In the context of a bioterror threat, an ideal anti-plague vaccine should elicit rapid, robust and long-acting protective immunity after a single dose. At present, no plague vaccines are available for use in the USA. One candidate for developing a subunit vaccine is the Yersinia pestis V antigen, a protein that influences the function of the Yersinia outer membrane proteins (Yops) virulence factors and has local anti-inflammatory effects in the host. Although protective immunity against Y. pestis challenge can be induced with recombinant V antigen, this immunization regimen requires multiple administrations and recent evidence suggests that long-term protection against Y. pestis requires strong cellular immune responses that are not stimulated by protein-based vaccines. Based on the knowledge that adenovirus (Ad)-based vaccine platforms rapidly induce humoral and cellular immune responses against expressed transgenes, we tested the hypothesis that a single administration of a replication-defective Ad gene transfer vector encoding the Y. pestis V antigen (AdsecV) could stimulate long-lasting protective immune responses, including cellular immune responses, without a requirement for repeat vaccine administration. To test the efficacy of the vaccine, mice immunized with a single administration of 10¹¹ pu AdsecV were challenged intranasally 4 wk or 6 months after vaccination with doses of Y. pestis CO92 up to 10⁶ cfu. At four wk post-administration, control mice that received saline (naive) or 10¹¹ pu AdNull, a control vector, died within 3 to 5 days after challenge. In contrast, 100% of AdsecV-immunized mice (10/ 10) survived the intranasal challenge with 10⁴ cfu and 8/10 and 9/10 of mice survived after infection with 10⁵ and 10⁶ cfu Y. pestis CO92, respectively (p<0.0001 for all doses). When mice immunized with a single dose of AdsecV were challenged 6 months post-administration, 100% of animals (10/10) survived intranasal infection with 10⁴ cfu and 9/10 survived intranasal infection with 10⁶ cfu Y. pestis CO92 (p<0.0001 for each dose). None of the control animals (naive or immunized with 10¹¹ pu AdNull) survived the challenge. Protection was correlated with high anti-V antibody titers and, importantly, V- specific T cell responses (measured by ELISPOT). Consistent with the concept that cellular immune responses were relevant to protection, immunization of CD8+ T cell deficient mice (Cd8a^tm1Mak) with AdsecV did not protect any mice (0/10) from an intranasal infection with Y. pestis CO92 4 wk after immunization. These data suggest that AdsecV is a promising candidate as an effective single dose vaccine against the plague and that the cellular immune responses evoked by the vaccine play an important role in protection against respiratory infection with Y. pestis.