From the AGA Journals

Characterization of norovirus immunity in nonsecretor adults might provide vaccine model for children

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Study showed broad immune activation against natural norovirus infection

Noroviruses belonging to genogroup II.4 are the leading cause of acute gastroenteritis, but our understanding of norovirus immunity remains incomplete. Most studies have focused on humoral responses and have shown that antibodies may be short lived, strain specific, and not always protective against rechallenge. On the other hand, human innate and T-cell immunity have received little attention despite evidence from the mouse norovirus model that they are critical for limiting viral spread and clearing antigen.

In this study, Lindesmith et al. conducted broad phenotypic and functional analysis of innate and adaptive immune responses following infection with a GII.2 strain of norovirus. Their cohort consists of “nonsecretors,” subjects who express a limited repertoire of histoblood group antigens and are therefore naturally resistant to GII.4 infection. Since nonsecretors have no pre-existing immunity against GII.4 viruses, this system enables the authors to test cross-reactivity of GII.2-specific T cells against GII.4 virus-like particles (VLPs).

The authors showed broad immune activation against natural norovirus infection. Following GII.2 infection, T-cell responses persist for at least a month and, importantly, are cross-reactive against GII.4 VLPs. These findings suggest that T cells may target conserved viral epitopes and play an important role in long-term protection against reinfection.

Developing an effective norovirus vaccine will require a detailed understanding of immune correlates of protection, and this study is a step in the right direction. In future work, tracking epitope-specific T cells must further define the phenotype, functionality, and localization of the norovirus T-cell repertoire.

Vesselin Tomov, MD, PhD, is assistant professor of medicine at the Hospital of the University of Pennsylvania, Philadelphia. He has no conflicts of interest.



Among nonsecretors – individuals who express a less diverse array of fucosylated histoblood group antigen carbohydrates (HBGAs) and consequently are less susceptible to some norovirus strains – natural infection with norovirus strain GII.2 induced cellular and antibody immunity that lasted for at least 30 days for T cells, monocytes, and dendritic cells and for at least 180 days for blocking antibodies, researchers reported.

“Multiple cellular lineages expressing interferon-gamma and tumor necrosis factor [TNF]–alpha dominated the response. Both T-cell and B-cell responses were cross-reactive with other GII strains, but not GI strains,” Lisa C. Lindesmith of the University of North Carolina, Chapel Hill, and her associates wrote in Cellular and Molecular Gastroenterology and Hepatology. The researchers also found that bile salts enable GII.2 to bind HBGAs produced by nonsecretors. “[I]n addition to HBGAs, one or more specific components of bile also is likely to be an essential co-factor for human norovirus attachment and infection,” the researchers wrote.

Susceptibility to norovirus depends on whether individuals express secretor enzyme, which is encoded by the FUT2 gene. Nonsecretors (who are FUT2–/–) express less varied HBGA, are susceptible to fewer norovirus strains, and are resistant to the predominant norovirus strain, GII.4. “Because future human norovirus vaccines will comprise GII.4 antigen, and because secretor phenotype impacts GII.4 infection and immunity, nonsecretors may mimic young children immunologically in response to GII.4 vaccination,” the researchers explained. But until now, most vaccines have focused on adult secretors, they said.

Their study focused on a familial norovirus outbreak in Chapel Hill that was the first to be characterized among nonsecretors who were naturally infected with norovirus GII.2. Four adults provided blood samples, and one provided a stool sample from which the researchers isolated and cloned the G11.2 capsid gene sequence. They used neutralization assays to study serologic immunity and flow cytometry to assess cellular activation and cytokine production in blood samples from the four cases and from seven healthy donors.

Norovirus GII.2 infection activated both innate and adaptive immunity and typical production of antiviral helper T cell (Th)1 and Th2 cytokines. The cellular immune response lasted at least 30 days, “long after symptom resolution,” the investigators wrote.

Compared with healthy donors, blood specimens from infected nonsecretors showed increases in non-class-switched memory, transitional B cells, and plasmablast B cells, and both naive and memory B cells also were positive for activation markers for at least 30 days after infection. Activated interferon-gamma+ T cells, natural killer cells, TNF-alpha+ monocytes, IL-10+, TNF-alpha+ myeloid dendritic cells, and TNF plasmacytoid dendritic cells also persisted for at least 30 days. Cross-reactive GII immunity was evident for at least 180 days. “GII.2 infection boosted cross-reactive blocking antibodies to GII.3, GII.14, and GII.17, as well as T-cell responses to GII.4, despite the lack of clear serologic evidence of previous GII.4 exposure,” the investigators wrote.

Based on prior reports that bile enhances norovirus growth or ligand binding, they inoculated specimens with chenodeoxycholic acid (CDCA) and glycochenodeoxycholic acid (GCDCA), pig bile, ox bile, or human bile. “Strikingly, the addition of bile enabled GII.2 Chapel Hill outbreak virus-like particle to bind to saliva from the four nonsecretor donors,” the researchers wrote. Bile acids “may override the genetic advantage of less-diverse HBGA expression in nonsecretors by improving the avidity of GII.2 binding to nonsecretor HBGAs, potentially paving the way for infection.” However, bile salts did not enable the GII.2 strain to replicate in human intestinal enteroid cells, which suggests that additional factors play into how norovirus enters human cells, according to the researchers.

The findings, they wrote, “support development of within-genogroup, cross-reactive antibody and T-cell immunity, key outcomes that may provide the foundation for eliciting broad immune responses after GII.4 vaccination in individuals with limited GII.4 immunity, including young children.”

The National Institutes of Health, the Wellcome Trust, the Centers for Disease Control and Prevention, and a Cancer Center Core support provided funding. Ms. Lindesmith and her associates reported having no relevant conflicts of interest.

SOURCE: Lindesmith LC et al. Cell Molec Gastroenterol Hepatol. 2020;10:245-67.

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