Source: Dissertation Abstracts International, Volume: 74-11(E), Section: B.

Adviser: Akiko Iwasaki.

Host recognition of influenza A virus (IAV) infection relies on receptors of the innate immune system that can directly detect pathogen-associated molecular patterns (PAMPs) through pattern recognition receptors (PRRs), as well as those that sense perturbation of cellular integrity as a result of virus-inflicted damage. Activation of the innate immune system triggers rapid innate defense and transducer signals necessary for initiating and maintaining the adaptive immune responses. Microbial sensing by innate pattern recognition receptors is not restricted to pathogens. Rather, proper development, function, and maintenance of innate and adaptive immunity rely on continuous recognition of products derived from the microorganisms indigenous to the internal and external surfaces of mammalian host. However, the role of commensal bacteria in immune responses to mucosal virus infections, and the contribution of various innate viral recognition pathways to the establishment of protective adaptive immunity during live virus infections have remained unclear. The goal of this thesis is to investigate the potential link between viruses, commensal microbiota and the initiation of host immune responses using a respiratory model of IAV infection.

First, to understand the role of commensal microbiota in regulating respiratory immune responses to IAV, we examined the antiviral responses in antibiotic-treated mice depleted of commensal bacteria following IAV infections. We showed that commensal bacteria composition critically controlled the generation of adaptive T cell responses and B cell-mediated antibody responses to IAV. Intact microbiota provided signals leading to constitutive expressions of inflammasome-priming factors including pro-IL-1beta, pro-IL-18 and Nlrp3 in the lung at steady state and optimal inflammasome-mediated IL-1 secretion after IAV infections. Our data reveal the importance of commensal-derived signals in potentiating respiratory antiviral defense by proper activation of the inflammasomes essential for the generation of adaptive immunity to IAV.

Second, we examined the mechanism by which inflammasome-mediated IL-1 release and IL-1 receptor (IL-1R) signaling elicits adaptive antiviral immunity following host recognition of IAV infection. We observed that signaling through IL-1R, but not PRRs, is required for the activation and expansion of virus-specific CD8 T cell responses following IAV infections. Our data further reveal that activation of dendritic cells by a bystander cytokine, IL-1 in trans is both required and sufficient to promote CD8 T cell priming during a live virus infection.

Finally, we examined the functional interaction of two major innate viral recognition pathways, namely TLR7 and RIG-I in modulating host defense against IAV infections in vivo. We found that deficiency in both the TLR7 and RIG-I mediated viral recognition pathway significantly lowered the production of infectious virions in the airway of mice following a low dose of respiratory IAV infection. Reduction in viral replication efficiency in the absence of TLR7 and RIG-I mediated signaling was associated with severely reduced proinflammatory cytokine production and leukocyte infiltration into the airway, which could be restored by transfer of inflammatory mediators from WT to Tlr7/Mavs -/-mice. Our study provides evidence that IAV might utilize the host inflammatory response to promote its replication efficiency in vivo.

In all, these studies contribute to our knowledge of how the microbiome potentiates antiviral immunity and how host viral recognition influences the generation of protective immunity to respiratory IAV infections.