Vaccination with an Attenuated Strain of Francisella novicida Prevents T-Cell Depletion and Protects Mice Infected with the Wild-Type Strain from Severe Sepsis

Abstract

Francisella tularensis is the causative agent of zoonotic tularemia, a severe pneumonia in humans, and Francisella novicida causes a similarly severe tularemia in mice upon inhalation. The correlates of protective immunity, as well as the virulence mechanisms of this deadly pathogen, are not well understood. In the present study, we compared the host immune responses of lethally infected and vaccinated mice to highlight the host determinants of protection from this disease. Intranasal infection with an attenuated mutant (Mut) of F. novicida lacking a 58-kDa hypothetical protein protected C57BL/6 mice from a subsequent challenge with the fully virulent wild-type strain U112 via the same route. The protection conferred by Mut vaccination was associated with reduced bacterial burdens in systemic organs, as well as the absence of bacteremia. Also, there was reduced lung pathology and associated cell death in the lungs of vaccinated mice. Both vaccinated and nonvaccinated mice displayed an initial 2-day delay in upregulation of signature inflammatory mediators after challenge. Whereas the nonvaccinated mice developed severe sepsis characterized by hypercytokinemia and T-cell depletion, the vaccinated mice displayed moderated cytokine induction and contained increased numbers of ␣␤ T cells. The recall response in vaccinated mice consisted of a characteristic Th1-type response in terms of cytokines, as well as antibody isotypes. Our results show that a regulated Th1 type of cell-mediated and humoral immunity in the absence of severe sepsis is associated with protection from respiratory tularemia, whereas a deregulated host response leading to severe sepsis contributes to mortality.

Features of sepsis caused by pulmonary infection with Francisella tularensis Type A strain

“…The exaggerated inflammatory response culminates in extensive cell death which results in tissue pathology and depletion of immune cells necessary for fighting the infection [29]. Our previous studies with F. novicida have shown extensive cell death as well as apoptosis induced loss of immune cells during respiratory infection [14, 17], and our results with SchuS4 infected mice parallel these observations. We have also observed a depletion of T cells in the lungs of SchuS4 infected mice (data not shown).…”

“…Indeed, recent studies from our laboratory showed that respiratory infection with the murine model strain F. novicida results in inflammatory cell infiltration and profound cytokine and chemokine expression consistent with severe sepsis [13-15]. In addition, bacteremia and dissemination of bacteria to systemic organs followed by extensive cell death in these F. novicida infected mice correlated with upregulation and release of alarmins [14-16]. Furthermore, a depletion of CD4+ T cells by apoptosis in lungs of F. novicida infected mice was observed consistent with previous studies showing extensive lymphocyte apoptosis during sepsis [17, 18].…”

Glycosylation of a Capsule-Like Complex (CLC) by Francisella novicida Is Required for Virulence and Partial Protective Immunity in Mice

“…The pathology of tularemia is largely due to tissue damage following severe inflammation and hypercytokinemia that occurs after Francisella has replicated extensively (Mares et al, 2008; Sharma et al, 2009a,b, 2011). In this study, control mice infected with F. novicida did not show clinical signs until ~48 h after infection and became moribund between 72 and 96 h post-infection.…”

Members of the Francisella tularensis Phagosomal Transporter Subfamily of Major Facilitator Superfamily Transporters Are Critical for Pathogenesis

“…In contrast, the Fpt mutant strains induced significantly less proinflammatory gene expression, and this reduction correlated with lower bacterial burdens and reduced pathology. It was shown previously that a cytokine storm resulting in severe tissue damage is a factor contributing to F. tularensis virulence in mice (3,21,22,38). The reduced induction of gene expression of the proinflammatory cytokines TNF-␣, IFN-␥, and iNOS in the livers of mice infected with Fpt mutant strains corresponded to decreased liver pathology compared with that of mice infected with LVS, where more inflammatory foci, apoptosis, and necrosis were observed.…”