Enhanced Macrophage M1 Polarization and Resistance to Apoptosis Enable Resistance to Plague.

Background: Susceptibility to infection is in part genetically driven, and C57BL/6 mice resist various pathogens through the proinflammatory response of their M1 macrophages (MPs). However, they are susceptible to plague. It has been reported elsewhere that Mus spretus SEG mice resist plague and develop an immune response characterized by a strong recruitment of MPs. Methods: The responses of C57BL/6 and SEG MPs exposed to Yersinia pestis in vitro were examined. Results: SEG MPs exhibit a stronger bactericidal activity with higher nitric oxide production, a more proinflammatory polarized cytokine response, and a higher resistance to Y. pestis-induced apoptosis. This response was not specific to Y. pestis and involved a reduced sensitivity to M2 polarization/signal transducer and activator of transcription 6 activation and inhibition of caspase 8. The enhanced M1 profile was inducible in C57BL/6 MPs in vitro, and when transferred to susceptible C57BL/6 mice, these MPs significantly increased survival of bubonic plague. Conclusions: MPs can develop an enhanced functional profile beyond the prototypic M1, characterized by an even more potent proinflammatory response coordinated with resistance to killing. This programming plays a key role in the plague-resistance phenotype and may be similarly significant in other highly lethal infections, suggesting that orienting the MP response may represent a new therapeutic approach.

T cell expression of IL-18R and DR3 is essential for non-cognate stimulation of Th1 cells and optimal clearance of intracellular bacteria.

Th1 cells can be activated by TCR-independent stimuli, but the importance of this pathway in vivo and the precise mechanisms involved require further investigation. Here, we used a simple model of non-cognate Th1 cell stimulation in Salmonella-infected mice to examine these issues. CD4 Th1 cell expression of both IL-18R and DR3 was required for optimal IFN-γ induction in response to non-cognate stimulation, while IL-15R expression was dispensable. Interestingly, effector Th1 cells generated by immunization rather than live infection had lower non-cognate activity despite comparable IL-18R and DR3 expression. Mice lacking T cell intrinsic expression of MyD88, an important adapter molecule in non-cognate T cell stimulation, exhibited higher bacterial burdens upon infection with Salmonella, Chlamydia or Brucella, suggesting that non-cognate Th1 stimulation is a critical element of efficient bacterial clearance. Thus, IL-18R and DR3 are critical players in non-cognate stimulation of Th1 cells and this response plays an important role in protection against intracellular bacteria.

B7-H1 (programmed cell death ligand 1) is required for the development of multifunctional Th1 cells and immunity to primary, but not secondary, Salmonella infection.

Robust Ab and CD4 T cell responses are required for the resolution of Salmonella infection in susceptible mice. In this study, we examined the role of B7-H1 (programmed cell death ligand 1) in resistance to primary Salmonella infection. Infected B7-H1-deficient mice had significantly higher bacterial burdens at day 21 and day 35 postinfection compared with wild-type mice, demonstrating that B7-H1 plays an important role in immunity to Salmonella. B7-H1-deficient and wild-type mice both generated Salmonella-specific IgM and IgG2c Ab responses to infection, and clonal expansion of endogenous and adoptively transferred Salmonella-specific CD4 T cells was similar in both groups. However, although Salmonella-specific IFN-gamma-producing Th1 CD4 T cells were generated in Salmonella-infected B7-H1-deficient mice, these cells did not expand to the level observed in wild-type mice. Furthermore, fewer multifunctional Th1 cells that simultaneously secreted IFN-gamma, TNF-alpha, and IL-2 were detected in Salmonella-infected B7-H1-deficient mice. Together, these data demonstrate that B7-H1 is required for the generation of multifunctional Th1 responses and optimal protective immunity to primary Salmonella infection.