Inhibition of Pattern-Recognition Receptor Signaling by the Secreted Form of West Nile Virus Non-structural Protein 1

West Nile virus (WNV) is a mosquito-borne flavivirus which can cause severe neurological disease if the proper immune response is not mounted upon infection. In any infection, the initial immune response is not pathogen specific, but rather makes use of pattern-recognition receptors (PRRs) designed to recognize pathogen-associated molecular patterns, or PAMPs. Following recognition of a PAMP a signaling cascade is initiated, resulting in the production of proinflammatory cytokines and interferons. These molecules function to limit the spread of infection by inducing an anti-viral state and also influence the formation of the adaptive immune response.;In order to circumvent these immune responses many pathogens have developed mechanisms to evade and/or inhibit innate immune detection. West Nile virus has developed a number of immune evasion and immunomodulatory mechanisms to counteract the host immune response. Work done previously by this lab identified the WNV non-structural protein 1 (NS1) as a potent immunomodulatory protein, capable of inhibiting signaling through the PRR, Toll-like receptor 3 (TLR3). The work presented in this dissertation characterizes the spectrum of NS1-mediated inhibition of PRR-signaling and describes a role for the secreted form of NS1 in modulating innate immune responses.;A survey of different TLR-overexpressing cell lines identified NS1-mediated inhibition of NFkappaB activation in response to stimulation by their respective ligands. These results were confirmed in the Raw 264.7 macrophage-like cell line where these TLRs are expressed endogenously. Additionally, NS1 expression was able to inhibit innate immune signaling triggered by infection with Sendai virus. These results are presented in Chapter 2.;In Chapter 3 the immunomodulatory activity of the secreted form of the WNV NS1 protein is assessed. Co-culture experiments and experiments using NS1-containing cell supernatants show inhibition of TLR3 signaling by naive cells. These results could be replicated using purified secreted NS1. The inhibitory activity of secreted NS1 was maintained in immune cell types such as bone marrow-derived macrophages and dendritic cells. Importantly, NS1 was able to inhibit the immune response to Poly(IC:LC) and infection by West Nile virus replicon particles in vivo..