Pathogenesis and immunity

Studies on NoVs pathogenicity and immunity have been hampered by the lack of a reproducible culture system or small animal model for the propagation of human norovirus. However, studies using FCV and MNV as models, as well as information from studies using human volunteer have provided valuable information (Karst, 2010). Recently, animals such as pigs and calves as well as non-human primates, infected with HuNoV, were used as experimental animal models for understanding HuNoV biology (Bok et al., 2011; Cheetham

et al., 2006; Souza et al., 2008; Souza et al., 2007). Although viral RNA is detected in sera or

cerebrospinal fluid of infected volunteers, the upper part of intestinal tract (duodenum and upper jejunum) remains the only target for NoVs infection (Ito et al., 2006; Takanashi et al., 2009). Proximal intestinal biopsies have revealed specific histological changes in the intestinal epithelium including broadening and blunting of the intestinal villi, shortening of the microvilli, crypt-cell hyperplasia, and epithelial cell disarray and cytoplasmic vacuolisation. In addition, infiltration of inflammatory cells (polymorphonuclear and mononuclear cells) into the lamina propria is also observed (Troeger et al., 2009). In contrast,

(Levy et al., 1976). Gastric emptying is delayed during infection and this may be responsible for nausea and vomiting (Meeroff et al., 1980). A transient malabsorption of fat, D-xylose and lactose is also reported during infection and associated with the alteration of brush border enzyme activity (alkaline phosphatase, sucrase and trehalase) in infected cells (Agus et al., 1973). The susceptibility or resistance to infections depend primarily on the presence of specific human HBGA receptors in the human gut (Lindesmith et al., 2008). Although more than 80% of adults appear susceptible to HuNoVs infection, some adults display resistance to infection due to a lack of HBGA receptors (Lindesmith et al., 2003). In addition, the expression of HBGAs correlates with strain-specific susceptibility (Lindesmith et al., 2003). HBGAs are a family of carbohydrates expressed in the mucosal surfaces of the respiratory, genitourinary and digestive tracts, or linked to proteins/lipids on the red blood cell surface or present as free oligosaccharides in biological fluids such as milk and saliva (Hutson et al., 2004). They serve as receptors for NoVs and the binding to these receptors varies depending on the strains (Lindesmith et al., 2008). Immunological studies reported that volunteers display short-term immunity against NoV infections, varying from 4 to 16 weeks and potentially as long as 2 years (Atmar et al., 2011; Johnson et al., 1990; Rocha-Pereira et al., 2014). On average infected volunteers were repeatedly susceptible to infection with the same strain or heterologous strains six months after previous exposure (Johnson et al., 1990; Parrino et al., 1977). The viral dose is one of the many concerns with volunteer studies. Although HuNoV has a very low infectious dose (less than 20 viral particles) as well as high rate of shedding in stool (108-1010 RNA copies/gram), most studies challenged the volunteers with doses several thousand-fold times higher than the minimum capable of causing human illness (Teunis et al., 2008). More recently,Simmons et al., 2013 suggested that the duration of immunity to infections lasts from about 4-8 years using six different mathematical models

to estimate immunity duration, depending on factors such as transmission process and natural history of HuNoV (Simmons et al., 2013).

Both types of immunity, innate and acquired immunity have been reported to contribute towards immunity against NoV infections (Parrino et al., 1977). The early volunteers’ studies demonstrated no association between pre-existing serum antibodies and the susceptibility to infection with Norwalk virus in adults (Atmar et al., 2011; Dolin et al., 1971; Graham et al., 1994; Reeck et al., 2010). Volunteers with high levels of pre-existing antibodies were more likely to have symptomatic illness compared to those with low levels of antibodies (Johnson et al., 1990). In contrast, high level of antibodies may reflect short-term immunity and recent exposure to HuNoV in children (Matsui & Greenberg, 2000). Using MNV as a model in mice lacking functional type I interferon (IFN) signalling pathways, studies demonstrated the requirement of type I IFN immune protection against MNV infection (Changotra et al., 2009; Karst et al., 2003; Mumphrey et al., 2007). In addition, (Jung et al., 2012) showed that the oral administration of type 1 IFN to gnotobiotic pigs infected with MNV led to decrease viral shedding. IFN acts via activated STATs causing a cascade of events leading to the induction of antiviral proteins such as RNA-dependent protein kinase (PKR) and RNase L (Samuel, 2001). A study using a Norwalk replicon system demonstrated that norovirus replication is inhibited by type I IFN and type II IFNγ (Chang & George, 2007; Chang et al., 2006). The role of T-cells in this process is not completely understood, but studies have reported that HuNoV infection or vaccination using virus-like particles (VLPs) elicit CD4+ Th1 response leading to an increase of IFNγ and IL-2 cytokine secretion (Lindesmith et al., 2005; Lindesmith et al., 2008). Recently, Troeger et al., 2009 observed an increase in the number of intraepithelial cytotoxic T cells in the duodenum during the first six days of infection (Troeger et al., 2009). Moreover, this study also

as well as Th2 cytokines and interferons was also reported in serum and intestinal contents of gnotobiotic pigs and calves infected with human GII.4 strain of HuNoVs (Souza et al., 2008; Souza et al., 2007).

Despite the complex nature of protective immunity to NoVs infection, due to the antigenic diversity of HuNoVs, and also the lack of an efficient culture model, recent attempts to produce a vaccine against HuNoVs using virus-like particles (VLP) have shown promise (Herbst-Kralovetz et al., 2010; Karst, 2010; Karst et al., 2014a; Rocha-Pereira et al., 2014; Rohayem et al., 2010; Thorne & Goodfellow, 2014; Tome-Amat et al., 2014).