A Norovirus Vaccine on the Horizon?

Human noroviruses are the leading cause of epidemic gastroenteritis and an important cause of sporadic disease in humans of all ages in industrialized countries. Noroviruses are highly contagious, have a low infectious dose, and are persistent in the environment. These characteristics allow them to spread easily among individuals by direct contact, contaminated fomites (ie, environmental surfaces), and aerosol droplets. The ease of close contact transmission makes closed communities such as long-term care facilities, hospitals, military barracks, schools, and cruise ships especially vulnerable to outbreaks. Recent data also demonstrate that noroviruses are a leading cause of foodborne disease outbreaks in the United States [1, 2] and Europe [3]. Although norovirus infection is usually self-limiting and most people recover within a few days, norovirus gastroenteritis can be severe and sometimes fatal, especially among the very young, the elderly, the chronically ill, and immunocompromised individuals; norovirus gastroenteritis is also a frequent cause of hospitalization. Long-term effects of norovirus gastroenteritis, such as necrotizing enterocolitis, chronic diarrhea, and post-infectious irritable bowel syndrome, have been reported, but more data are needed to confirm a causal link with these conditions. Noroviruses are highly heterogeneous and can be genetically divided into 5 different genogroups (GI-GV), with human strains classified in genogroups GI, GII, and GIV, and at least 25 genotypes [4]. The prototype norovirus strain, Norwalk virus, is designated GI.1. The majority of the norovirus outbreaks are caused by GII viruses; in particular, GII.4 viruses have been associated with pandemics since the mid-1990s. Novel epidemic GII.4 strains emerge with 2–4-year intervals, and they demonstrate a different epidemiological profile compared with other genotypes. These findings suggest an evolution pattern similar to influenza viruses; it has been hypothesized that herd immunity may contribute to a proposed epochal evolution model for GII.4 noroviruses [5], although there are other possible explanations that could explain these observations (eg, natural fluctuation). Because human noroviruses cannot be cultivated in vitro [6], norovirus recombinant virus-like particles (VLPs) produced by the expression and spontaneous self-assembly of the major capsid protein VP1 in recombinant systems (eg, insect, plant, and eukaryotic cells) have played a major role in increasing fundamental knowledge of this group of viruses. These VLPs are antigenically and morphologically similar to native viruses. Both genetic factors and acquired immunity are involved in resistance to norovirus infection. On the basis of data from experimentally infected human volunteers, immunity to norovirus is not long-lasting and is likely strain specific [7, 8]. Genetic resistance to Norwalk virus was suggested 33 years ago when experimental infections in human volunteers demonstrated that subjects were repeatedly susceptible or resistant to symptomatic infection [7]. Subsequent studies of Norwalk VLPs demonstrated that its binding patterns to histo-blood group antigens (HBGAs) correlate with susceptibility to infection and illness, and HBGAs are regarded as putative receptors and host-susceptibility factors of infection [9, 10]. Several enzymes are important in the synthesis of HBGAs, including fucosyl transferase 2 (FUT-2); mutations in the FUT-2 gene can render them nonfunctional and, in the case of Norwalk virus, make a person resistant to infection. However, the differential binding patterns of noroviruses to HBGAs are genotype dependent and sometimes even strain dependent; thus, it is likely that each person is genetically susceptible to multiple norovirus strains. HBGA is the putative receptor for noroviruses; this is analogous to other pathogens that use similar carbohydrates as receptors, including the well-established link between resistance to human immunodeficiency virus (HIV) and polymorphisms in the CCR5 coreceptor of HIV [11, 12]. Currently, several Food and Drug Administration (FDA)-approved VLP-based vaccines are available, including hepatitis B virus (HBV) and human papillomavirus (HPV) vaccines [13]. Norwalk VLPs induce both systemic and mucosal immune responses in mice and humans when delivered intranasally or orally [14], but although these VLPs are highly immunogenic, it is unknown whether the elicited immune responses are strong enough to prevent norovirus illness. Mucosal vaccine adjuvants may increase norovirus antigenicity, because they have the ability to skew adaptive immune responses toward a CD4+ T helper type 1 phenotype associated with effective protection against viruses [15]. A modified cholera toxin as a potential mucosal adjuvant has shown to enhance systemic immunoglobulin G (IgG) immune response in mice when codelivered with Norwalk VLPs administered intranasally or orally; however, because cholera toxin may have potential toxic adverse effects, it is not ideal for use in a human norovirus vaccine [16, 17]. Monophosphoryl lipid A (MPL), which is a Toll-like receptor (TLR) agonist, has been shown to induce higher antibody titers than those of commonly used adjuvants. Several additional mucosal adjuvants, including TLR7 agonists, which have a high safety profile in humans, are currently being developed as adjuvants. In this issue of the Journal , El-Kamary et al [18] report clinical data from two phase I clinical trials evaluating the immune responses to a monovalent adjuvanted Norwalk VLP vaccine administered intranasally. The mucosal and systemic immune responses are very promising, compared with results from previous phase I studies with VLPs administered orally without adjuvant, which were only modestly immunogenic as measured by serum antibody and specific antibody-secreting cells (ASCs) [19, 20]. In the first clinical trial, 18 healthy H type 1 secretor adults received 2 intranasal doses of MPL-adjuvanted VLP vaccine separated by 21 days, and 7 (70%) of the 10 subjects who received 50 mg of vaccine developed rises in specific immunoglobulin A (IgA) ASCs, whereas in trial 2, all 10 subjects who received 50 or 100 μg of vaccine developed IgA ASCs at days 7 and 28. These data demonstrate that priming of the nasal mucosa with adjuvanted Nor-walk VLPs is highly effective in generating ASCs in peripheral blood. The IgA and IgG seroconversion rates showed a dose-dependent response, with the highest titers at the highest vaccine dose tested; these antibodies were also able to inhibit hemagglutination of type O red blood cells. The fact that after nasal vaccination ASCs expressed homing receptors with a diverse homing profile, including the gut mucosa and peripheral lymphoid tissues, is very encouraging and demonstrates that the nasal route is able to induce systemic and mucosal immune responses. Nasal vaccination has several advantages, including ease of administration and stimulation of mucosal dendritic cells. However, a special delivery device is needed because of rapid clearance of antigens from the nose. El-Kamary et al [18] used a Norwalk virus vaccine that included both MPL and chitosan, a bioadhesive material that is able to decrease the clearance of formulations from the nasal cavity and also has the effect of transiently opening the tight junctions in mucosal membranes [21]. This is the first study to our knowledge that demonstrates that Norwalk VLP vaccine administered intranasally is safe and immunogenic, an important step in developing a vaccine. Next steps should include larger clinical trials to assess whether this vaccine protects against a live virus challenge. As a norovirus vaccine is developed, several challenges to determine its use remain. First, we still have an incomplete understanding of the immune correlates of protection, in part because of the lack of a small animal model and a simple cell culture system to measure neutralizing antibodies. Lack of correlates of protection make it more difficult to assess likely efficacy and disease risk, which is especially important for one of the largest risk groups for norovirus, the elderly. Interestingly, a recent study demonstrated that the ability of serum antibodies to block VLP-binding to HBGA correlates with decreased risk of norovirus gastroenteritis [22]. Concerns about waning immunity and impact of preexisting antibodies need to be considered in both the design and use of vaccines. In addition, the frequency of norovirus infections in the community suggests that natural infection does not provide long-term immunity, which raises concerns that vaccines will also not provide long-term immunity. Second, there appears to be little cross-protection among strains from different genogroups; therefore, multivalent vaccines will likely be needed. The lack of cross-protection among strains is compounded by the apparent rapid evolution of noroviruses. Norovirus vaccines may encounter challenges similar to those for influenza, in which comprehensive strain surveillance is needed to identify and evaluate the most prevalent strains that need to be included in a vaccine. Electronic norovirus surveillance systems, such as CaliciNet and NoroNet, may help detect and, perhaps in the future, predict the most prevalent strains prior to each norovirus season.