Advances in norovirus biology

Abstract

Human noroviruses are a major cause of epidemic and sporadic gastroenteritis worldwide and can chronically infect immunocompromised patients. Efforts to develop effective vaccines and antivirals have been hindered by the uncultivable nature and extreme genetic diversity of human noroviruses. Although they remain a particularly challenging pathogen to study, recent advances in norovirus animal models and in vitro cultivation systems have led to an increased understanding of norovirus molecular biology and replication, pathogenesis, cell tropism, and innate and adaptive immunity. Furthermore, clinical trials of vaccines consisting of nonreplicating virus-like particles have shown promise. In this review, we summarize these recent advances and discuss controversies in the field, which is rapidly progressing toward generation of antiviral agents and increasingly effective vaccines.

Figure 1. Norovirus virulence determinants

The NoV genome is depicted. The 5′ proximal ORF1 (shown in orange) encodes a nonstructural polyprotein which is cleaved into six mature products by the virally encoded protease (NS6, or Pro). Other nonstructural proteins include NS1/2 (also referred to as p48), NS3 (NTPase), NS4 (p22), NS5 (VPg), and NS7 (the RNA-dependent RNA polymerase; RdRp). ORF2 encodes the capsid protein referred to as VP1; this protein can be divided into shell (S; shown in yellow) and protruding (P) domains, and the P domain further subdivided into the P1 stalk domain (shown in blue) and the hypervariable P2 domain comprising the tips of the arches (shown in red). ORF3 encodes the minor structural protein VP2, and ORF4 of MuNoV genomes encodes a newly defined protein called virulence factor 1, or VF1. NoV genomes are covalently linked to VPg at their 5′ ends and polyadenylated at their 3′ ends. Studies in the MuNoV model system have identified a number of NoV virulence determinants which are indicated along the viral genome.

Figure 2. Human norovirus mouse model

The schematic overview summarizes the findings of Taube et al. (Taube et al., 2013). (Top) Balb/c mice deficient in the recombination activating gene (RAG) and common gamma chain (γc) were “humanized” with human CD34+ positive stem cells following irradiation. Mice were infected with filtered HuNoV-containing stool by combined peroral (p.o.) and intraperitoneal (i.p.) routes. Infection was detected by measuring increased genome titers over input by qRT-PCR and viral protein expression by immunohistochemistry (IHC). The image shows two VP1-positive Kupffer cells. (Middle) Balb/c RAG/γc-deficient mice were infected with HuNoV-containing stool filtrate by the oral and/or intraperitoneal routes and infection measured as above. The image shows two NS6-positive cells in the spleen. No infection was seen following oral infection. (Bottom) B6/B10 RAG/γc-deficient mice or wild-type (wt) Balb/c were infected by the p.o. or i.p. routes but no increases in viral genome titers over input were detected, demonstrating both the immune status and genetic background are important susceptibility factors in this model.