Pattern recognition receptors, including members of the NBD and LLR-containing (NLR) family, are key sensors of infection and injury. Early sensing of pathogen invasion and subsequent activation of pro-inflammatory signaling cascades is essential for controlling infection. However, signaling pathways activated upon pathogen recognition can also contribute to inflammation-mediated tissue damage. The studies detailed in Chapters 3 and 4 are primarily concerned with the roles of two NLR family members, Nlrp12 and Nlrc4, during influenza A virus (IAV) infection. While IAV itself is cytopathic, the immune response is responsible for a great deal of the tissue damage during infection in some contexts. NLR family members are involved in both pathogen sensing and modulation of pro-inflammatory signaling, thus they are perfectly situated to shift the balance between pathogen clearance and immunopathology.
Nlrp12 has been implicated in regulation of pro-inflammatory signaling through NFκB family members. In Chapter 3 we report that during IAV infection, we find no differences in those pathways, and instead we describe a novel role for Nlrp12 in regulating transcript stability. Previous work has shown that one of the key differences between lethal and sublethal IAV infections is the early and exaggerated recruitment of neutrophils. Previous studies in our laboratory had established a role for Nlrp12 in CXCL1-mediated neutrophil recruitment during respiratory bacterial infections. We therefore hypothesized that Nlrp12-/- mice would be protected from pathogenic neutrophil recruitment during lethal IAV infection due to decreased CXCL1 production. In Chapter 3 we show that indeed, Nlrp12-/- mice have improved survival, decreased pulmonary microvascular permeability, and decreased necrosis and hemorrhage in their airways compared to WT mice. Nlrp12-/- mice also have fewer neutrophils in their lungs, due to decreased production of CXCL1 by neutrophils, DCs and macrophages. Our data showing decreased Cxcl1 transcript stability in R848-treated Nlrp12-/- BMDCs strongly suggest that the reduction in CXCL1 production by DCs in the Nlrp12-/- lungs is a result of decreased Cxcl1 transcript stability.
Nlrc4 is a best known as a member of the Nlrc4 inflammasome, which is activated upon sensing of Gram-negative bacterial pathogens. However, a recent study from our laboratory showed an inflammasome-independent role for Nlrc4 in supporting critical anti-tumor T cell responses. Given that T cells are also critical for successful resolution of IAV infection, we hypothesized that during IAV infection, Nlrc4-/- mice would have compromised IAV-specific T cell responses and therefore poorer survival. Indeed, our studies in Chapter 4 show that in IAV-infected Nlrc4-/- mice, the pulmonary IAV-specific CD4 T cell response is significantly diminished and mortality is significantly increased compared to WT mice. During IAV infection, the blunted CD4 T cell response is a result of increased death of the CD4 T cells, perhaps due to increased expression of FasL on CD11c+ cells in the Nlrc4-/- lung environment.