The surface-exposed loop region of norovirus GII.3 VP1 plays an essential role in binding histo-blood group antigens

An enterovirus A71 virus-like particle with replaced loops confers partial cross-protection in mice

Enterovirus A71 (EV-A71), coxsackievirus A16 (CV-A16), and CV-A10 belong to the main prevailing types causing hand-foot-and-mouth disease. Since EV-A71 monovalent vaccine does not confer cross-protection, developing a multivalent vaccine is essential. In this study, a trivalent chimeric virus-like particle of EV-A71 (EV-A71-VLPCHI3) was constructed based on EV-A71-VLP backbone by replacing the corresponding surface loops with CV-A16 VP1 G-H, CV-A10 VP1 B-C and E-F loops, which are critical for immunogenic neutralization. The baculovirus-insect cell expression system was employed for EV-A71-VLPCHI3 production. EV-A71-VLPCHI3 was purified by sucrose density gradient and observed by transmission electron microscopy. The immunogenicity and protective efficacy of EV-A71-VLPCHI3 were evaluated in mice. Our results revealed that EV-A71-VLPCHI3 had a similar morphology to inactivated EV-A71 particles and could induce specific IgG antibodies against EV-A71, CV-A16 and CV-A10 in mice. More importantly, EV-A71-VLPCHI3 enhanced cross-reactive protection against CV-A16 and CV-A10, by 20 % and 40 %, compared to inactivated EV-A71 counterparts, respectively. In conclusion, the successful construction of EV-A71-VLPCHI3 suggested that loop-dependent heterologous protection could be transferred by loops replacement on the surface of viral capsid. This strategy may also supplement the development of multivalent vaccines against other infectious viral diseases.

Expression of chimeric proteins based on a backbone of the GII.4 norovirus VP1 and their application in the study of a GII.6 norovirus-specific blockade epitope

The surface-exposed loop regions of the protruding domain of the norovirus (NoV) major capsid protein VP1 can tolerate the insertion of foreign antigens without affecting its assembly into subviral particles. In this study, we investigated the tolerance of the surface-exposed loop region of the GII.4 NoV VP1 by replacing it with homologous or heterologous sequences. We designed a panel of constructs in which the amino acid sequence from position 298-305 of the GII.4 NoV VP1 was replaced by sequences derived from the same region of GI.3, GII.3, GII.6, and GII.17 NoVs as well as neutralizing epitopes of enterovirus type 71 and varicella-zoster virus. The constructs were synthesized and expressed using a recombinant baculovirus expression system. The expression of target proteins was measured by indirect enzyme-linked immunosorbent assay (ELISA), and the assembly of virus-like particles (VLPs) was confirmed by electron microscopy. Our results showed that all of the constructs expressed high levels of target chimeric proteins, and all of the chimeric proteins successfully assembled into VLPs or subviral particles. An in vitro VLP-histo-blood group antigen (HBGA) binding assay revealed that chimeric-protein-containing VLPs did not bind or showed reduced binding to salivary HBGAs, a ligand for NoV particles. The results of an in vitro VLP-HBGA binding blockade assay indicated that the predicted surface-exposed loop region of the GII.6 NoV VP1 may comprise a blockade epitope. In summary, the surface-exposed loop region of the GII.4 NoV VP1 can be replaced by foreign sequences of a certain length. Using this strategy, we found that the predicted surface-exposed loop region of GII.6 NoV VP1 might contain a blockade epitope.

Production and characterization of monoclonal antibodies against GII.6 norovirus virus-like particles

In this study we generated and characterized a series of monoclonal antibodies (mAbs) against GII.6 norovirus (NoV) virus like particles (VLPs). Mice were immunized with purified GII.6 NoV VLPs and peptide-bovine serum albumin (BSA) conjugates with the peptide sequence (31 aa) derived from the trypsin cleavage region. An indirect enzyme-linked immunosorbent assay was used to identify positive cell clones during cloning and subcloning, and an in vitro VLP-histo-blood group antigens (HBGAs) binding blockade assay was used to identify mAbs with blocking ability. A total of seven mAbs comprising five (1F7, 1F11, 2B6, 2C4, and 2E10) reactive with major capsid proteins (VP1) and two (1E5 and 2B2) reactive with both VP1 proteins and the peptide were identified. mAb 1F7, 1F11, and 2B6 were identified as blocking antibodies. Sandwich ELISA indicated that all these mAbs recognized soluble GII.6 NoV VLPs. Cross-reactivities with GI.7, GII.3, and GII.4 NoV VLPs were observed in indirect and sandwich ELISA. Western blot analysis indicated that all non-blocking mAbs recognized denatured GII.6 VP1 proteins and blocking mAbs only recognized non-denatured proteins. The in vitro VLP-HBGA binding blockade assay indicated that the three blocking antibodies exhibited blocking effects against GII.6 NoV VLPs, but not GI.7, GII.3, and GII.4 NoV VLPs. Epitope mapping and HBGA blocking assay indicated that mAbs targeting the predicted surface-exposed loop region did not have blocking effects, suggesting a possible important role of this region in regulating NoV-HBGA interactions. This is the first report regarding the characterization of mAbs with blocking ability against GII.6 NoV VLPs. These mAbs might be useful in facilitating our understanding of this group of viruses.