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The causative agent of white tail disease (WTD) in the giant freshwater prawn is Macrobrachium rosenbergii nodavirus (MrNV). The recombinant capsid protein (CP) of MrNV was previously expressed in Escherichia coli, and it self-assembled into icosahedral virus-like particles (VLPs) with a diameter of approximately 30 nm. Extensive studies on the MrNV CP VLPs have attracted widespread attention in their potential applications as biological nano-containers for targeted drug delivery and antigen display scaffolds for vaccine developments. Despite their advantageous features, the recombinant MrNV CP VLPs produced in E. coli are seriously affected by protease degradations, which significantly affect the yield and stability of the VLPs. Therefore, the aim of this study is to enhance the stability of MrNV CP by modulating the protease degradation activity. Edman degradation amino acid sequencing revealed that the proteolytic cleavage occurred at arginine 26 of the MrNV CP. The potential proteases responsible for the degradation were predicted in silico using the Peptidecutter, Expasy. To circumvent proteolysis, specific protease inhibitors (PMSF, AEBSF and E-64) were tested to reduce the degradation rates. Modulation of proteolytic activity demonstrated that a cysteine protease was responsible for the MrNV CP degradation. The addition of E-64, a cysteine protease inhibitor, remarkably improved the yield of MrNV CP by 2.3-fold compared to the control. This innovative approach generates an economical method to improve the scalability of MrNV CP VLPs using individual protease inhibitors, enabling the protein to retain their structural integrity and stability for prominent downstream applications including drug delivery and vaccine development. [ABSTRACT FROM AUTHOR]

Hepatitis B is a major global health challenge. In the absence of an effective treatment for the disease, hepatitis B vaccines provide protection against the viral infection. However, some individuals do not have positive immune responses after being vaccinated with the hepatitis B vaccines available in the market. Thus, it is important to develop a more protective vaccine. Previously, we showed that hepatitis B virus (HBV) 'a' determinant (aD) displayed on the prawn nodavirus capsid (Nc) and expressed in Spodoptera frugiperda (Sf9) cells (namely, Nc-aD-Sf9) self-assembled into virus-like particles (VLPs). Immunisation of BALB/c mice with the Nc-aD-Sf9 VLPs showed significant induction of humoral, cellular and memory B-cell immunity. In the present study, the biophysical properties of the Nc-aD-Sf9 VLPs were studied using dynamic light scattering (DLS) and circular dichroism (CD) spectroscopy. Enzyme-linked immunosorbent assay (ELISA) was used to determine the antigenicity of the Nc-aD-Sf9 VLPs, and multiplex ELISA was employed to quantify the cytokine response induced by the VLPs administered intramuscularly into BALB/c mice (n = 8). CD spectroscopy of Nc-aD-Sf9 VLPs showed that the secondary structure of the VLPs predominantly consisted of beta (β)-sheets (44.8%), and they were thermally stable up to ~52 °C. ELISA revealed that the aD epitope of the VLPs was significantly antigenic to anti-HBV surface antigen (HBsAg) antibodies. In addition, multiplex ELISA of serum samples from the vaccinated mice showed a significant induction (p < 0.001) of IFN-γ, IL-4, IL-5, IL-6, IL-10, and IL-12p70. This cytokine profile is indicative of natural killer cell, macrophage, dendritic cell and cytotoxic T-lymphocyte activities, which suggests a prophylactic innate and adaptive cellular immune response mediated by Nc-aD-Sf9 VLPs. Interestingly, Nc-aD-Sf9 induced a more robust release of the aforementioned cytokines than that of Nc-aD VLPs produced in Escherichia coli and a commercially used hepatitis B vaccine. Overall, Nc-aD-Sf9 VLPs are thermally stable and significantly antigenic, demonstrating their potential as an HBV vaccine candidate. [ABSTRACT FROM AUTHOR]

Macrobrachium rosenbergii nodavirus (Mr Nv) causes white tail disease (WTD) in the giant freshwater prawn, which poses a major issue in the aquaculture sector. The capsid protein (CP) of Mr Nv expressed in Escherichia coli self-assembles into virus-like particles (VLPs) with a triangulation number, T = 3. However, the region in Mr Nv CP that involved in the viral capsid formation has yet to be identified. In the present study, a series of N- and C-terminal Mr Nv CP deletion mutants were constructed based on the viral subatomic structure. These mutants were then expressed in E. coli and purified with an immobilized metal affinity chromatography (IMAC). The formation of VLPs was determined using transmission electron microscopy (TEM). The results showed that, the first 29 amino acid residues at the N-terminal end of CP are dispensable for VLP formation, but deletion of these residues produced a smaller VLP. Disruption of the CP N-terminal arm (NTA) that contains a β-annulus motif resulted in no protein expression in E. coli. The β-annulus peptide was chemically synthesized, and analysed with TEM, dynamic light scattering (DLS), size-exclusion chromatography (SEC) and native agarose gel electrophoresis, but it did not assemble into VLPs. Truncation of the entire protruding (P) domain did not disrupt the VLP formation. However, deletion of the amino acid residues involved in dimeric interaction between subunits in the shell (S) domain impaired VLP assembly. In conclusion, the minimum contiguous sequence of the CP required for VLP formation was delineated in residues 30 and 252. This study provides mechanistic insights into the assembly of Mr Nv VLP, which will further broaden its applications in vaccine development and nucleic acid delivery. • M. rosenbergii nodavirus (Mr Nv) capsid protein (CP) forms virus-like particle (VLP). • The assembly domain of Mr Nv CP was mapped with deletion mutagenesis. • RNA binding and protruding domains of Mr Nv CP are dispensable for VLP assembly. • β-annulus peptide in Mr Nv CP does not self-assemble into VLP. • Amino acids 30 to 252 of Mr Nv are involved in VLP formation. [ABSTRACT FROM AUTHOR]