Inactivated Francisella tularensis LVS Protects Against Respiratory Tularemia By Intranasal Vaccination in an IgA-Dependent Fashion

Center for Immunology & Microbial Disease, Albany Medical College, Albany, NY 12208

^* To whom correspondence should be addressed. Email: metzged{at}mail.amc.edu.

	Abstract

Francisella tularensis is a gram-negative intracellular bacterium that is considered to be a potential category A biological weapon due to its extreme virulence. Although vaccination with the attenuated Live Vaccine Strain (LVS) of F. tularensis can protect against lethal challenge, use of inactivated or subunit forms as vaccine candidates for induction of protective antibody responses has not been fully evaluated. In the present study, we tested whether immune protection in the lung could be stimulated by intranasal administration of inactivated LVS together with IL-12 as an adjuvant. LVS was inactivated by heat, paraformaldehyde treatment or exposure to UV, and inactivation of the preparations was confirmed by assessing bacterial growth and survival of mice after direct inoculation. It was found that mucosal vaccination with inactivated LVS provided 90-100% protection to mice after lethal intranasal challenge with 10⁴ CFU of LVS, and this protection was dependent upon inclusion of exogenous IL-12 during vaccine administration. Survival of vaccinated mice after live bacterial challenge was correlated with reduced bacterial burden, decreased pulmonary inflammation, increased serum antibody titers and lower levels of IFN-gamma, TNF-alpha, and IL-6 in lungs, livers and spleens. Whereas NK cells were primarily responsible for the production of IFN-gamma in unvaccinated, challenged animals, vaccinated mice showed increased levels of lung IFN-gamma⁺ CD4⁺ T cells after challenge. Significantly, mice genetically deficient in IgA expression were unable to survive lethal challenge after vaccination. These results are the first to demonstrate IgA-mediated protection against lethal respiratory tularemia by mucosal vaccination with inactivated F. tularensis LVS.