Transcriptional analyses of peripheral blood lymphocytes obtained shortly after primary immunization discriminate vaccines against <i>Francisella tularensis</i>

ABSTRACT The high virulence and pathogenesis of Francisella tularensis ( Ft ), responsible for tularemia, made it a target for bioweapon development during the “Cold War.” This prompted investigation of vaccines, starting with an older vaccine candidate denoted Ft live vaccine strain (LVS). LVS has limited efficacy against aerosol challenge with Ft subsp. Tularensis (Type A), the most virulent subtype. However, no vaccines, including LVS, have been licensed in the U.S. Animal studies indicate that Ft vaccines induce protective T-cell-mediated immune responses. In previous studies, analyses of leukocytes from vaccinated animals that were restimulated in vitro facilitated screening and selection of vaccines for more extensive in vivo evaluation. These studies demonstrated that a novel live attenuated vaccine strain derived from Type A Ft , denoted ΔclpB , was protective against aerosol challenge in animal models. In this study, we focused on evaluating immune responses immediately after vaccination of Fischer 344 rats with LVS and ΔclpB using multiple analytical approaches. While humoral immune responses were comparable between the two vaccines, we identified multiple differences in immune responses by analyses of peripheral blood leukocytes by flow cytometry and by relative gene expression. In particular, vaccination with LVS or ΔclpB resulted in differential expression of multiple genes at selected time points. Gene expression, in turn, predicted activation or inhibition of multiple biological pathways. These results suggest that immune responses shortly after primary vaccination may discriminate vaccines with different degrees of in vivo protection that could potentially be used as correlates of protection and extrapolate efficacy from animals to people.