Chimeric VLPs with GII.3 P2 domain in a backbone of GII.4 VP1 confers novel HBGA binding ability

Identification of a GII.6 norovirus blockade antibody epitope

Noroviruses (NoVs) are the leading agent that causes acute viral gastroenteritis worldwide. Sporadic cases of GII.6 NoV have been reported primarily in addition to occasional outbreaks. Using the major capsid protein VP1 of GII.6 NoV derived from three distinct clusters, we demonstrated three blockade monoclonal antibodies (mAbs, 1F7, 1F11, and 2B6) generated previously exhibited cluster-specific binding effects. Combining sequence alignment and blocking immune epitopes, we sequentially designed a total of 18 mutant proteins containing one, two, or three mutations, or swapped regions. Indirect enzyme-linked immunosorbent assay (ELISA) demonstrated that the three blocking mAbs lost or showed significantly reduced binding for H383Y, D387N, V390D, and T391D mutant proteins. Combining data from mutant proteins with swapping regions and point mutations, the binding region of the three mAbs was mapped to residues 380-395. Sequence alignment of this region showed within-cluster conservation and between-cluster variations, further strengthening the idea of blockade epitope-mediated evolution of NoV.

Immunogenicity and protective potency of Norovirus GII.17 virus-like particle-based vaccine

OBJECTIVES

Noroviruses (NoVs) are major cause of acute viral gastroenteritis in worldwide, and the lack of a cell culture system that must be considered the virus like particles (VLPs) are used as an effective vaccine development.

MATERIALS AND METHODS

In the present study, we investigated the expression of the major capsid protein (VP1) of the Genogroup II, genotype 17 (GII.17) NoV, using recombinant baculovirus system in insect cells, as well as a saliva binding blockade assay to detect their protective potency.

RESULTS

Our results showed that GII.17 VLPs could be successfully generated in sf9 insect cells, and electron microscopic revealed that GII.17 VLPs appeared as spherical particles with a - 35 nm diameter. Immunized mice with purified VLPs produced GII.17 specific sera and could efficiently block GII.17 VLPs binding to the saliva histo-blood group antigens (HBGAs).

CONCLUSIONS

Together, these results suggested that GII.17 VLPs represent a promising vaccine candidate against NoV GII.17 infection and strongly support further preclinical and clinical studies.

Linear epitope binding antibodies against GII.3 Norovirus exhibit no histo-blood group antigens (HBGAs) blocking effects

Noroviruses are leading cause of acute gastroenteritis worldwide. In our previous study, we established an in vitro histo-blood group antigens (HBGAs) binding blockade assay against GII.3 Norovirus virus like particles (VLPs) with trypsin digestion. In this study, we characterized the blocking antibody binding site and epitope type (linear or conformational) by using hyperimmune sera produced against different antigens. VP1 from Jingzhou402 (GII.3, JZ402) strain was expressed by using pGEX-6p-1 expression vector and the insoluble proteins were purified for immunization in rabbit. Previously characterized chimeric VP1-assembled VLPs (GII.4-VP1/GII.3-P2) were used to immunize guinea pig. Peptides reactive with hyperimmune serum against VLPs derived from the VP1 of JZ402 strain were conjugated with BSA and used to immunize rabbits. Hyperimmune sera against above antigens and JZ402 and JZ403 strain-derived VLPs were used to compare their HBGAs blocking effects. Rabbit anti-GST-VP1 and BSA-peptide conjugated hyperimmune sera demonstrated no blocking effects against the binding of GII.3 and GII.4 NoV VLPs to salivary HBGAs. Guinea pig anti-GII.4-VP1/GII.3-P2 hyperimmune serum blocked the binding of trypsin cleaved GII.3 VLPs to salivary HBGAs with no or very weak blocking effects against the binding of GII.4 VLPs to salivary HBGAs. Our data indicated that HBGAs blocking antibodies primarily bound the P2 domain of GII.3 NoV VP1 and their binding epitopes were most probably conformation-dependent.

Enzymatic cleavage promotes disassembly of GII.3 norovirus virus like particles and its binding to salivary histo-blood group antigens

In this study, we found that addition of fecal extract significantly promoted the binding of GII.3 NoV VLPs to salivary HBGAs. SDS-PAGE analysis indicated that major capsid proteins (VP1) were cleaved into two major bands with molecular weights of 26 and 31kD, respectively. Pretreatment of fecal extract by boiling or addition of protease inhibitor cocktail or type specific protease inhibitor (leupeptin and aprotinin) during incubation all decreased VP1 cleavage and its binding to salivary HBGAs. Trypsin digestion led to cleavage of VP1 and promoted its binding to HBGAs, suggesting that the active enzyme(s) might be trypsin or trypsin-like enzymes. Trypsin and fecal extract pretreatment all led to loss of morphological intact VLPs, indicating enhanced signal was possible due to increased binding of fragmented subunits. N-terminal sequencing was performed to characterize the cleavage sites with indecisive results. In vitro VLP-salivary HBGAs binding blockade assay using VLPs derived from VP1 of different GII.3 strains and rabbit anti-genotype specific hyperimmune serum indicated that GII.3 NoVs might have conservative HBGA binding sites. In summary, our results provide evidence about the widespread presence of active enzyme in fecal samples that can cleave GII.3 NoV VLPs and demonstrate that GII.3 NoVs have conservative HBGA binding sites which might have implications in the design of multivalent NoV vaccines.