Highly variable antigenic site located at the apex of GII.4 norovirus capsid protein induces cross-reactive blocking antibodies in a variant-specific manner.

GII.4 norovirus is responsible for most cases of viral gastroenteritis worldwide. The complex epidemiology and extreme GII.4 antigenic diversity could challenge the development of a vaccine. Most antibody responses to GII.4 norovirus target five antigenic sites mapping to the major viral capsid, whose variability has been associated with the emergence of antigenically distinct variants. Bioinformatics analyses of amino acid sequences from these antigenic sites demonstrated that antigenic site A is the most variable and relevant site for antigenic diversification. Despite this complex diversity, cross-blocking was demonstrated in the profiling of site A-mapping mouse monoclonal antibodies generated against two antigenically distinct GII.4 variants. Using a novel immunoassay, we determined the contribution of specific epitopes to blockade function in polyclonal sera and observed extensive cross-blocking in which 36% was attributed to antibodies mapping to antigenic site A. However, cross-reactivity was virus-dependent, with the greatest breadth exhibited by contemporary variants that emerged after 2006. Notably, cross-blocking titers associated with antibodies mapping to antigenic site A are differentially affected by an increasing number of mutations on the capsid. Since the elicitation of blocking antibodies to conserved regions of the GII.4 norovirus capsid is infrequent, a better understanding of how to elicit cross-protective responses to immunodominant variable antigenic sites could guide norovirus vaccine development.IMPORTANCEGII.4 noroviruses exhibit an accumulation of mutations on their capsid protein, leading to the continuous emergence and turnover of new variants that can escape herd immunity. Despite the fact that most antibodies mapping to the variable antigenic sites of GII.4 norovirus show exquisite specificity, cross-neutralizing antibodies mapping to these variable sites have also been described. In this study, we systematically evaluate the antigenicity of a panel of different GII.4 antigens to demonstrate that cross-reactive responses are elicited in a virus-dependent manner in naïve mice. Notably, one wild-type virus demonstrated multiple instances of potent cross-blocking responses, providing new hopes for the development of cross-protective vaccines against human norovirus.