Aerosol-, but not intradermal-immunization with the live vaccine strain of Francisella tularensis protects mice against subsequent aerosol challenge with a highly virulent type A strain of the pathogen by an αβ T cell- and interferon gamma- dependent mechanism

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Francisella tularensis is an extremely virulent facultative intracellular bacterial pathogen of many mammalian species including mice and humans in which it causes a spectrum of disease collectively called tularemia. In humans, intradermal or inhaled inocula of 10 cfu or less of the most virulent strains of the pathogen are sufficient to cause severe infection and possible death; in mice similar inocula are routinely lethal. An attenuated live vaccine strain, F. tularensis LVS, was developed almost 50 years ago, and remains the sole prophylactic against virulent strains of the pathogen. Using F. tularensis LVS as a model vaccine, we recently showed that it was possible to systemically immunize various mouse strains and protect them against subsequent massive (2000 cfu) intradermal (i.d.) challenge, but not against low dose (∼10 cfu) aerosol challenge, with virulent strains of the pathogen. This is troubling because the latter route is considered an important means of deliberately disseminating F. tularensis in a bioterrorist attack. Others have previously shown that administering LVS to humans, guinea pigs and monkeys as an aerosol enhanced protection against subsequent aerosol challenge with virulent F. tularensis. In the present study, we show the same phenomenon in BALB/c and C3H/HeN mice. In this model, interferon gamma (IFNγ) and CD4 + and CD8 + T cells are essential for the expression of anti- Francisella immunity in the lungs. Combined this immune response operates by limiting dissemination of the pathogen to susceptible internal organs. Further, understanding of how inhaled LVS elicits local cell-mediated protective immunity will be critical for devising improved vaccines against pulmonary tularemia.