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

Chimeric virus-like particles of human norovirus constructed by structure-guided epitope grafting elicit cross-reactive immunity against both GI.1 and GII.4 genotypes

IMPORTANCE

Human norovirus (HuNoV) is highly infectious and can result in severe illnesses in the elderly and children. So far, there is no effective antiviral drug to treat HuNoV infection, and thus, the development of HuNoV vaccines is urgent. However, NoV evolves rapidly, and currently, at least 10 genogroups with numerous genotypes have been found. The genetic diversity of NoV and the lack of cross-protection between different genotypes pose challenges to the development of broadly protective vaccines. In this study, guided by structural alignment between GI.1 and GII.4 HuNoV VP1 proteins, several chimeric-type virus-like particles (VLPs) were designed through surface-exposed loop grafting. Mouse immunization studies show that two of the designed chimeric VLPs induced cross-immunity against both GI.1 and GII.4 HuNoVs. To our knowledge, this is the first designed chimeric VLPs that can induce cross-immune activities across different genogroups of HuNoV, which provides valuable strategies for the development of cross-reactive HuNoV vaccines.

Linear epitopes on the capsid protein of norovirus commonly elicit high antibody response among past-infected individuals

BACKGROUND

Human norovirus (HuNoV) is the leading cause of acute nonbacterial gastroenteritis globally, and its infection is usually self-limited, so most people become past Norovirus (NoV)-infected individuals. It is known that some antibody responses may play a critical role in preventing viral infection and alleviating disease; however, the characteristics and functions of particular antibody responses in persons with previous infections are not fully understood. Capsid proteins, including VP1 and VP2, are crucial antigenic components of NoV and may regulate antibody immune responses, while epitope-specific antibody responses to capsid proteins have not been comprehensively characterized.

METHODS

We prepared purified VP1 and VP2 proteins by ion exchange chromatography and measured serum antigen-specific IgG levels in 398 individuals by ELISA. Overlapping 18-mer peptides covering the full length of VP1 and VP2 were synthesized, and then we identified linear antigenic epitopes from 20 subjects with strong IgG positivity. Subsequently, specific antibody responses to these epitopes were validated in 185 past infected individuals, and the conservation of epitopes was analyzed. Finally, we obtained epitope-specific antiserum by immunizing mice and expressed virus-like particles (VLPs) in an insect expression system for a blockade antibody assay to evaluate the receptor-blocking ability of epitope-specific antibodies.

RESULTS

The IgG responses of VP1 were significantly stronger than those of VP2, both of which had high positive rates of over 80%. The overall positive rate of VP1-IgG and/or VP2-IgG was approximately 94%, which may be past NoV-infected individuals. Four linear antigenic B-cell epitopes of capsid proteins were identified, namely, VP1_199-216, VP1_469-492, VP2_97-120, and VP2_241-264, all of which were conserved. The IgG response rates of the above epitopes in past NoV-infected individuals were 38.92%, 22.16%, 8.11% and 28.11%, respectively. In addition, VP1_199-216- and VP1_469-492-specific antibodies can partially block the binding of VLPs to the receptor histo-blood group antigen (HBGA).

CONCLUSION

This is the first study to describe specific antibody responses of VP2 and to identify its B-cell epitopes. Our findings offer data for a more thorough understanding of norovirus capsid protein-specific IgG responses and could provide useful information for designing and developing vaccines.

The Magic Staff: A Comprehensive Overview of Baculovirus-Based Technologies Applied to Human and Animal Health

Baculoviruses are enveloped, insect-specific viruses with large double-stranded DNA genomes. Among all the baculovirus species, Autographa californica multiple nucleopolyhedrovirus (AcMNPV) is the most studied. Due to its characteristics regarding biosafety, narrow host range and the availability of different platforms for modifying its genome, AcMNPV has become a powerful biotechnological tool. In this review, we will address the most widespread technological applications of baculoviruses. We will begin by summarizing their natural cycle both in larvae and in cell culture and how it can be exploited. Secondly, we will explore the different baculovirus-based protein expression systems (BEVS) and their multiple applications in the pharmaceutical and biotechnological industry. We will focus particularly on the production of vaccines, many of which are either currently commercialized or in advanced stages of development (e.g., Novavax, COVID-19 vaccine). In addition, recombinant baculoviruses can be used as efficient gene transduction and protein expression vectors in vertebrate cells (e.g., BacMam). Finally, we will extensively describe various gene therapy strategies based on baculoviruses applied to the treatment of different diseases. The main objective of this work is to provide an extensive up-to-date summary of the different biotechnological applications of baculoviruses, emphasizing the genetic modification strategies used in each field.

Sequence addition to the N- or C-terminus of the major capsid protein VP1 of norovirus affects its cleavage and assembly into virus-like particles

Norovirus (NoV) infection is a leading cause of non-bacterial gastroenteritis worldwide and there are currently no effective therapeutics available to target the virus. The norovirus major capsid protein VP1 is a potential candidate for the development of vaccines due to the similar morphology and immunogenicity of its assembled virus-like particles (VLPs) compared to native virions. In this study, we explored the effects of N- and C-terminal sequence additions to the VP1 of a GII.4 NoV during its assembly into VLPs. A series of sequences of different lengths derived from the minor capsid protein VP2 of the GII.4 NoV were added to the N- and C-terminus of VP1. The fusion proteins were expressed using a recombinant baculovirus expression system and the assembly of the expressed fusion proteins was subsequently observed under electron microscopy (EM). Our results indicated that all constructed fusion proteins were successfully expressed with different degrees of enzyme cleavage at the N-terminus. Electron microscopy revealed the successful assembly of VLPs of different sizes for all fusion proteins. An in vitro binding assay for VLP-histo-blood group antigens (HBGAs) indicated that all fusion proteins exhibited similar binding patterns compared with their wild-type VP1. Our results demonstrate that (Xi et al., 1990) [1] NoV VP1 can tolerate foreign sequences at its N- or C-terminus without affecting its ability to assemble into VLPs, and (Jiang et al., 1992) [2] that the cleavage pattern and effects of foreign sequences on the sizes of assembled VLPs observed in this study might represent important experimental data that can be used to elucidate VP1 self-assembly.