Pathogenesis, immunity, and prevention of human norovirus infection in gnotobiotic pigs
Human noroviruses (HuNoVs) are the leading cause of viral epidemic acute gastroenteritis and responsible for the deaths of over 200,000 children each year worldwide. HuNoV research has been hampered by the long absence of a readily reproducible cell culture system and a suitable small animal model, while gnotobiotic (Gn) pigs have been a unique animal model for understanding HuNoV pathogenesis and immunity, as well as evaluating vaccine and therapeutics. Recent reports of HuNoVs infection and replication in B cells supplemented with commensal bacteria Enterobacter cloacae and in Blab/c mice deficient in RAG/IL2RG have gained extensive attention, and my studies utilized the well-established Gn pig model to investigate the effects of these two interventions on HuNoV infection. Surprisingly, the colonization of E. cloacae inhibited HuNoV infectivity in Gn pigs, evidenced by the significantly reduced HuNoV shedding in feces and HuNoV titers in intestinal tissues and blood compared to control pigs. Moreover, HuNoV infection of enterocytes but not B cells was observed with or without E. cloacae colonization, indicating B cells were not a target cell type for HuNoV in Gn pigs. On the other hand, using RAG2/IL2RG deficient pigs generated by CRISPR/Cas9 system, with confirmed severe combined immunodeficiency, I evaluated the effects of host immune responses on HuNoV infection. Compared to wild-type Gn pigs, longer HuNoV shedding was observed in RAG2/IL2RG deficient pigs (16 versus 27 days), and higher HuNoV titers were detected in intestinal tissues and contents and in blood, indicating increased and prolonged HuNoV infection in RAG2/IL2RG deficient pigs. In addition, I evaluated dietary interventions including probiotics and rice bran using Gn pig model of HuNoV infection and diarrhea. While the colonization of probiotic bacteria Lactobacillus rhamnosus GG (LGG) and Escherichia coli Nissle 1917 (EcN) in Gn pigs completely inhibited HuNoV fecal shedding, the two cocktail regimens, in which rice bran feeding started either 7 days prior to or 1 day after viral inoculation in the LGG+EcN colonized Gn pigs, exhibited dramatic anti-HuNoV effects, including reduced incidence and shorter duration of diarrhea, as well as shorter duration of virus fecal shedding. The anti-HuNoV effects of the cocktail regimens were associated with the enhanced IFN-𝛾⁺ T cell responses, increased production of intestinal IgA and IgG, and longer villus length. Taken together, my dissertation work improves our understanding of HuNoV infection and immunity, and further supports for Gn pigs as a valuable model for future studies of human enteric virus infection, host immunity, and interventions.