Type-specific and cross-reactive antibodies and T cell responses in norovirus VLP immunized mice are targeted both to conserved and variable domains of capsid VP1 protein.

Norovirus (NoV)-specific antibodies, which block binding of the virus-like particles (VLPs) to the cell receptors are conformation dependent and directed towards the most exposed domain of the NoV capsid VP1 protein, the P2 domain. Limited data are available on the antibodies directed to other domains of the VP1, and even less on the NoV VP1-specific T cell epitopes. In here, BALB/c mice were immunized with six VLPs derived from NoV GII.4-1999, GII.4-2009 (New Orleans), GII.4-2012 (Sydney), GII.12, GI.1, and G1.3. Serum immunoglobulin G binding antibodies, histo-blood group antigen blocking antibodies and T cell responses using type-specific and heterologous NoV VLPs, P-dimers and 76 overlapping synthetic peptides, spanning the entire 539 amino acid sequence of GII.4 VP1, were determined. The results showed that at least half of the total antibody content is directed towards conserved S domain of the VP1. Only a small fraction (<1%) of the VP1 binding antibodies were blocking/neutralizing. With the use of matrix peptide pools and individual peptides, seven CD4+ and CD8+ T cell restricted epitopes were mapped, two located in S domain, four in P2 domain and one in P1 domain of NoV VP1. The epitopes were GII.4 strain-specific but also common GII.4 genotype-specific T cell epitopes were identified. More importantly, the results suggest a 9-amino acids long sequence (318 PAPLGTPDF326 ) in P2 domain of VP1 as a universal NoV genogroup II-specific CD8+ T cell epitope. Distribution of the T cell epitopes alongside the capsid VP1 indicates the need of the complete protein for high immunogenicity.