67. Protective Immunity Against Yersinia pestis in Mice Immunized with an Ad-Based Vaccine Vector Expressing the Y. pestis F1 and V Antigens

Abstract

The aerosol form of the bacterium Yersinia pestis causes the pneumonic plague, an extremely virulent and rapidly fatal disease that develops within days post-infection. In the context of a bioterror threat, an effective anti-plague vaccine must elicit rapid, robust and long acting protective immunity after a single dose. At present, no plague vaccines are available for use in the USA. Two subunits of Y. pestis, fraction 1 (F1) and V antigen have been identified as vaccine candidates based on: (1) F1 antigen is a temperature-regulated capsular protein of Y. pestis with anti-phagocytic activity in the host; (2) V antigen has local anti-inflammatory effects and influences the function of important virulence factors such as the Yersinia outer membrane proteins (Yops). It is known that protective immunity against Y. pestis challenge can be induced by multiple immunizations with recombinant F1 and V proteins and immediate protection can be achieved by passive transfer of immune sera against these antigens. Based on the knowledge that adenovirus (Ad) gene transfer vectors act as adjuvants in eliciting host immunity against the transgene they carry, replication-defective Ad gene transfer vectors encoding either the Y. pestis V antigen or the F1 protein (AdsecV and AdsecF1, respectively) were constructed. The F1 and V antigen genes were synthesized by an overlap PCR strategy using mammalian preferred codons, and fused to the human Ig signal sequence for extracellular secretion. We tested the hypothesis that AdsecV and AdsecF1 can stimulate strong protective immune responses without a requirement for repeat administration. To test the efficacy of the vaccines, BALB/c mice were immunized with a single intramuscular administration of either vector at a dose of 108 to 1011 particles units (pu). Four wk after immunization, mice were challenged in a BSL3 experimental animal facility (n=10/group) by the intranasal route with 3103 cfu of the fully virulent Yersinia pestis strain, CO92. At challenge time, anti-V antigen IgG reciprocal titers from AdsecV–immunized mice (n=10/group) were 666137 (108 pu), 3045735 (109 pu), 97032193 (1010 pu), and 7611215914 (1011 pu). In contrast, anti-F1 antigen IgG reciprocal titers from AdsecF1-immunized mice (n=10/group) were much lower with 122 (108 pu), 9135 (109 pu), 1154138 (1010 pu), and 939157 (1011 pu). All (10/10) of the animals survived the Y. pestis challenge in the group vaccinated with 1011 pu AdsecV, and 40% (4/10) survived in the 1010 pu AdsecV dose, while in all the other experimental groups immunized with AdsecV and in the control groups the mice died within 2 to 5 days after challenge. No mice vaccinated with AdsecF1 survived the Y. pestis challenge. These observations suggest that an Ad-based vaccine encoding for the V antigen of Y. pestis (AdsecV) is an excellent candidate as a effective single dose vaccine against the plague.