Mucosal and systemic neutralizing antibodies to norovirus induced in infant mice orally inoculated with recombinant rotaviruses

Rotaviruses (RVs) preferentially replicate in the small intestine and frequently cause severe diarrheal disease, and the following enteric infection generally induces variable levels of protective systemic and mucosal immune responses in humans and other animals. Rhesus rotavirus (RRV) is a simian RV that was previously used as a human RV vaccine and has been extensively studied in mice. Although RRV replicates poorly in the suckling mouse intestine, infection induces a robust and protective antibody response. The recent availability of plasmid only-based RV reverse genetics systems has enabled the generation of recombinant RVs expressing foreign proteins. However, recombinant RVs have not yet been experimentally tested as potential vaccine vectors to immunize against other gastrointestinal pathogens in vivo. This is a newly available opportunity because several live-attenuated RV vaccines are already widely administered to infants and young children worldwide. To explore the feasibility of using RV as a dual vaccine vector, we rescued replication-competent recombinant RRVs harboring bicistronic gene segment 7 that encodes the native RV nonstructural protein 3 (NSP3) protein and a human norovirus (HuNoV) VP1 protein or P domain from the predominant genotype GII.4. The rescued viruses expressed HuNoV VP1 or P protein in infected cells in vitro and elicited systemic and local antibody responses to HuNoV and RRV following oral infection of suckling mice. Serum IgG and fecal IgA from infected suckling mice bound to and neutralized both RRV and HuNoV. These findings have encouraging practical implications for the design of RV-based next-generation multivalent enteric vaccines to target HuNoV and other human enteric pathogens.

Mucosal immunity plays a critical role in protecting against many pathogens in the respiratory and intestinal tracts. Live virus infections generally trigger more robust and effective mucosal immune response than oral administration of inactivated viruses or target protein antigens because they are self-amplifying and can more effectively elicit cellular as well as humoral immunity (1–4). Several studies have attempted to utilize recombinant viruses as vaccine vectors to induce an immune response against enteric pathogens (5–8); however, the most advanced of such enteric vaccine vectors are still in early stages of clinical development.Rotaviruses (RVs), the leading cause of acute gastroenteritis in infants, are a promising candidate for enteric vaccine vectors for several reasons. A) RV preferentially replicates in the small intestine, distinguishing it from several other enteric viruses that can also infect systemically or the colon. B) RV infection is acute, and the virus does not integrate into the host genome. C) RV is highly immunogenic and induces both systemic and mucosal immune responses in infected animals and humans (9, 10). D) Several live-attenuated human RV vaccines have been shown to be both safe and effective to use in very young children [e.g., RotaTeq (Merck) and Rotarix (GlaxoSmithKline)]. Other effective live-attenuated RV vaccines [Rotasiil, Rotavac, Lanzhou lamb rotavirus vaccine (LLR), and Rotavin-M1] are also licensed for use globally or primarily in their country of origin (11). E) Following substantial public health efforts, RV vaccines are now widely available in many low- and middle-income countries, as well as the more developed countries, and hence the administration of RV-based vaccines that included other heterologous antigens could potentially be piggybacked onto current RV immunization programs used globally. F) The RV double-stranded RNA (dsRNA) genome is segmented in nature, permitting easy genetic manipulation. G) With the insertion of heterologous antigens, RV replication can become attenuated in vitro (12, 13).Since a plasmid-based reverse genetics system was established in 2017, several studies have reported the generation of recombinant RVs that express fluorescent and bioluminescent reporter proteins (GFP, RFP, luciferase, etc.) and exogenous nucleotide sequences [e.g., endoribonuclease Csy4 target sequence and sequences encoding the receptor binding domain of the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) spike protein] in vitro (12–22). To facilitate the assessment and development of RVs as potential enteric vaccine vectors, the capacity of recombinant RVs to induce an enteric immune response against other gastrointestinal (GI) pathogens needs to be evaluated in well-characterized preclinical small animal models. Rhesus rotavirus (RRV) is a prototype laboratory strain of simian RV that efficiently replicates in vitro (23, 24). Although RRV does not replicate well in a murine model (25–27), it does induce both systemic and mucosal immune responses in infected mice (28). In addition, RRV itself and RRV-based RV vaccine candidates have previously been shown to be a highly immunogenic and protective in several human vaccine trials and were, for a time, licensed for use in children in the United States (29, 30).Human norovirus (HuNoV) is a major cause of acute gastroenteritis in both young children and adults. Although B cells and human intestinal organoids support HuNoV replication (31, 32), there is not yet a widely available robust cell culture system for efficient HuNoV cultivation, which has impeded both the assessment of HuNoV immunity and vaccine development. The HuNoV virion consists of major capsid protein VP1 and minor capsid protein VP2 surrounding a positive-sense RNA genome (33–35). Exogenously expressed VP1 can form virus-like particles (VLPs) that are structurally and antigenically similar to HuNoV virions (36–38), and the parenteral administration of such VLPs provides some level of protective immunity to HuNoV in adults (39–41). Moreover, expression of the protruding or P domain of VP1 that bears the major antigenic sites of HuNoV can yield subunit “P particles” that can also induce immune responses (42, 43). Here, we demonstrate the induction of both systemic and mucosal antibody responses against HuNoV in suckling mice using recombinant RRVs expressing HuNoV VP1 or P domain. Our data suggest that recombinant RVs represent a potentially effective small-intestine–targeted vaccination platform to express exogenous genes in the human intestine and to protect people from other enteric pathogens such as HuNoV as well as RV.