Norovirus Cell Tropism Is Determined by Combinatorial Action of a Viral Non-structural Protein and Host Cytokine

Abstract

Summary Cellular tropism during persistent viral infection is commonly conferred by the interaction of a viral surface protein with a host receptor complex. Norovirus, the leading global cause of gastroenteritis, can be persistently shed during infection, but its in vivo cellular tropism and tropism determinants remain unidentified. Using murine norovirus (MNoV), we determine that a small number of intestinal epithelial cells (IECs) serve as the reservoir for fecal shedding and persistence. The viral non-structural protein NS1, rather than a viral surface protein, determines IEC tropism. Expression of NS1 from a persistent MNoV strain is sufficient for an acute MNoV strain to target IECs and persist. Additionally, interferon-lambda (IFN-λ) is a key host determinant blocking MNoV infection in IECs. The inability of acute MNoV to shed and persist is rescued in Ifnlr1−/− mice, suggesting that NS1 evades IFN-λ-mediated antiviral immunity. Thus, NS1 and IFN-λ interactions govern IEC tropism and persistence of MNoV.

The Role of Interferon in Persistent Viral Infection: Insights from Murine Norovirus

“…NS1 CR6 promotes early growth in the cecum and proximal colon where MNV ultimately persists most abundantly [53,54,57]. Moreover, NS1 CR6 is necessary and sufficient for intestinal epithelial cell (IEC) tropism in vivo , indicating that IECs of the cecum and colon are the cellular reservoir of persistence [68]. NS1 is an essential part of the viral replication complex, interacts with the membrane protein VAPA, and localizes with the replication complex on cytoplasmic membranes [69–71].…”

“…Strikingly, IFN-λ is uniquely responsible for control of early and persistent CR6 replication in the IECs of the colon [68,72]. Additionally, treatment with recombinant IFN-λ [72] or stimulation of IFN-λ production by intra-epithelial T cells [76] clears persistent MNV from the intestine.…”

“…The compartmentalized roles of IFN-λ and IFN-α/β are underpinned by compartmentalized expression of IFN receptors; IFNLR is highly expressed in IECs but not on most immune cells whereas IFNAR is broadly expressed on immune cell types but is expressed poorly by IECs of adult mice [79–82]. Indeed, IFNLR must be expressed specifically on IECs for IFN-λ to clear reovirus or MNV from the intestine [68,83]. From these studies, the specific role for IFN-λ in IECs is clear.…”

“…IECs do not commonly express the MNV receptor, CD300lf, whereas myeloid cells express it abundantly [73,94,95]. Additionally, work from our lab and others have not found MNV to grow in IECs in vitro unless they ectopically express CD300lf [68]. A receptor for human noroviruses is not known, but Estes and colleagues recently developed an in vitro method for human norovirus infection of primary human epithelial cells using bile acids to promote infection [96].…”

“…Therefore, genetic differences between CW3 and CR6 other than the capsid may provide resistance to the anti-viral effects of endogenous IFN-λ (Figure 2). This may be part of NS1’s role, either alone or in co-operation with VF1 [68,75]. Identifying viral mechanisms of endogenous IFN-λ suppression and IFN-λ resistance will be important for understanding the immunological basis of intestinal persistence.…”