Regulation of Norovirus Virulence by the VP1 Protruding Domain Correlates with B Cell Infection Efficiency

Abstract

Human noroviruses are a leading cause of gastroenteritis across the globe, but the pathogenic mechanisms responsible for disease are not well established. The availability of a murine norovirus model system provides the opportunity to elucidate viral and host determinants of virulence in a natural host. For example, previous studies have revealed that the protruding domain of the murine norovirus capsid protein VP1, specifically residue 296 of VP1, regulates virulent infection. We identified a panel of nonsynonymous mutations in the open reading frame 2 (ORF2) gene encoding VP1 that arose in persistently infected mice and tested whether these mutations conferred phenotypic changes to viral replication and virulence. Consistent with previous studies, we demonstrate that a glutamic acid at position 296 results in attenuation. For the first time, we also demonstrate that a lysine at this position is sufficient to confer virulence on an otherwise attenuated murine norovirus strain. Moreover, our studies reveal a direct correlation between the efficiency of viral replication in B cells and virulence. These data are especially striking because mutations causing reduced B cell replication and attenuation had minimal effects on the ability of the virus to replicate in macrophages. Thus, norovirus infection of B cells may directly contribute to disease outcome. IMPORTANCE Human noroviruses are a major global cause of disease, yet we know very little about their pathogenic mechanisms. The availability of a murine norovirus model system facilitates investigation of noroviruses in a natural host organism and the identification of viral and host determinants of pathogenesis. We have identified a panel of mutations arising in the viral capsid protein VP1 during persistent infection of mice. Our data reveal that the protruding domain of VP1 regulates the ability of the virus to replicate in B cells, and this directly correlates with virulence. Importantly, mutations impairing B cell infection had minimal effects on macrophage infection, revealing a potentially critical role for B cell infection in norovirus pathogenesis.H uman noroviruses (HuNoVs) are the major cause of nonbacterial gastroenteritis worldwide, causing rapid bouts of nausea, severe vomiting, and watery diarrhea in all age groups. They are the leading cause of foodborne disease globally and of severe childhood diarrhea in parts of the world where rotavirus vaccination is being implemented (1-3). Although HuNoV infection is typically self-limited in immunocompetent adults, more severe consequences can occur in risk groups, including the elderly, immunocompromised and transplant patients, and infants (4-6). Considering the huge disease burden attributable to HuNoV infections (7) and the tremendous economic loss associated with these infections (8), development of effective therapeutics and vaccines is highly warranted. However, HuNoV studies are greatly hampered by the lack of robust culturing systems and small-animal models. Although there have been exciti...

The intestinal regionalization of acute norovirus infection is regulated by the microbiota via bile acid-mediated priming of type III interferon

“…Importantly, type I IFN has been demonstrated to be dispensable for the effect of commensal bacteria on MNV-CR6 infection (7). As expected based on our previous work with Ifnar −/− mice (19), PBS-treated control mice rapidly lost weight ( Fig. 1a) and succumbed to lethal infection by 9 dpi (Fig.…”

IFN-I and IL-22 mediate protective effects of intestinal viral infection

“…Although MNV CR6 and CW3 share ~95% amino acid sequence identity, they display significant differences in their virulence. The VP1 major capsid protein of MNV CW3 accounts for the ability of this strain to cause lethality when introduced into immunocompromised Stat1 −/− mice 13–15 . The less virulent CR6 strain evades recognition by lymphocytes to establish persistent infection 16 , which is attributed to sequence variation in NS1/2 17 .…”

Recent advances in understanding norovirus pathogenesis

“…In addition, as is the case for several other important pathogens, dendritic cells have been implicated as critical mediators of murine norovirus dissemination to secondary lymphoid tissues [Elftman et al, 2013]. Finally, a recent in vivo murine norovirus study reveals a correlation between the efficiency of B cell infection and the extent of virulence [Zhu et al, 2015]. These collective in vitro and in vivo data prove conclusively that noroviruses can infect both innate (macrophages and dendritic cells) and adaptive (B cells) immune cells along the intestinal tract, and suggest that infection of these cell types is a crucial determinant of viral pathogenesis.…”