PROJECT SUMMARY Human norovirus (HuNoV) is the leading cause of acute gastroenteritis in the United States, resulting in $4.2 billion in direct health system costs annually. Due to underlying immunosuppression and the lack of effective antiviral therapeutics, transplant patients may develop serious sequelae from HuNoV infections. Though bacterial gut microbiota has been shown to enhance replication and pathogenesis of enteric viruses in preclinical models, its role in HuNoV infection remains largely unknown. In this project, we aim to gain greater mechanistic insights into how gut microbiota modulate HuNoV infection in transplant patients. Our central hypothesis is that transplant patients with symptomatic HuNoV infection will have a gut microbiome signature showing an enrichment of specific gut microbiota (Enterobacteriaceae) that facilitate infection of HIEs, and those who develop chronic symptoms from HuNoV infection will have a concomitant depletion of specific gut microbiota that modulate host innate immune responses (type 1 interferons). In Aim 1, we will further define both gut microbiome and host factor differences in transplant patients ±HuNoV infections. First, we will establish a larger cohort of adult and pediatric transplant patients and collect longitudinal stool specimens. Then, we will perform comprehensive gut microbiome profiling (metagenomic shotgun sequencing and bacterial qPCR) to confirm our preliminary results. We predict that HuNoV-infected transplant patients will have significantly different gut microbiota signatures compared to uninfected counterparts, and that higher intensity of immunosuppression and high antibiotic load will correspond with chronic diarrhea in HuNoV-infected transplant patients. In Aim 2, we will determine if Enterobacteriaceae facilitate HuNoV infection using an in vitro HIE model. First, we will infect jejunal HIEs with HuNoV and co-incubate HuNoV with various Enterobacteriaceae spp. Then, we will perform RNA extractions and quantitative HuNoV RT-qPCR to evaluate the effect of co-incubating various Enterobacteriaceae spp. on viral replication. We predict that Enterobacteriaceae promotes HuNoV infection of HIEs. If this is the case, we will investigate if this is a phenomenon observed only in HBGA-expressing members of Enterobacteriaceae. In Aim 3, we will determine if transplant patients with chronic symptomatic HuNoV infections have different systemic cytokine signatures compared to uninfected counterparts. We will perform bulk cytokine analysis on serum samples utilizing Isoplexis, a novel functional proteomic profiling platform. We predict that transplant patients with chronic diarrhea from HuNoV infection will exhibit a paucity of genus Bacteroides, resulting in depletion of type 1 interferons, which then leads to a decreased Th1 immune response, increased Th2 immune response and increased expression of Th2-predominant cytokines (IL-4, IL-5, IL-10). The proposed experiments, training and didact...