Expression and immunogenicity of novel subunit enterovirus 71 VP1 antigens

Immunogenicity analysis based on VP1 and VP2 proteins of bovine enterovirus

Bovine enterovirus (BEV) infection manifests as a spectrum of clinical signs affecting the respiratory, gastrointestinal, and reproductive systems in cattle. Outbreaks of this disease results in large economic losses to the bovine industry worldwide. Currently there are no efficacious vaccines and medicines to prevent BEV infection. In the present study, reverse transcription-polymerase chain reaction was used to amplify the VP1 and VP2 genes of BEV, enabling the synthesis of a chimeric recombinant protein which contained partial sequences from both proteins. Subsequently, the emulsified purified proteins with Freund's adjuvant were used for triple-fold immunization of 4-week-old Institute of Cancer Research (ICR) mice. The mice were subsequently subjected to a challenge assay which elicited an immune response that was characterized by elevated titers of BEV-specific neutralizing antibodies. Notably, the results showed that the purification of pET32a-VP1 and pET32a-VP2 proteins markedly curtailed virus excretion and mitigated the histopathological damage usually associated with BEV infections. These were observed in the small intestines, kidneys and brain in infected animals. It also alleviated clinical symptoms such as hypothermia and weight loss. In summary, this study offers a theoretical and practical basis for BEV vaccine development.

Recombinant Enterovirus 71 Viral Protein 1 Fused to a Truncated Newcastle Disease Virus NP (NPt) Carrier Protein

Enterovirus 71 (EV71) is the major causative agent in hand, foot, and mouth disease (HFMD), and it mainly infects children worldwide. Despite the risk, there is no effective vaccine available for this disease. Hence, a recombinant protein construct of truncated nucleocapsid protein viral protein 1 (NPt-VP1_198–297), which is capable of inducing neutralizing antibody against EV71, was evaluated in a mouse model. Truncated nucleocapsid protein Newcastle disease virus that was used as immunological carrier fused to VP1 of EV71 as antigen. The recombinant plasmid carrying corresponding genes was constructed by recombinant DNA technology and the corresponding protein was produced in Escherichia coli expression system. The recombinant NPt-VP1_198–297 protein had elicited neutralizing antibodies against EV71 with the titer of 1:16, and this result is higher than the titer that is elicited by VP1 protein alone (1:8). It was shown that NPt containing immunogenic epitope(s) of VP1 was capable of inducing a greater functional immune response when compared to full-length VP1 protein alone. It was capable to carry larger polypeptide compared to full-length NP protein. The current study also proved that NPt-VP1_198–297 protein can be abundantly produced in recombinant protein form by E. coli expression system. The findings from this study support the importance of neutralizing antibodies in EV71 infection and highlight the potential of the recombinant NPt-VP1_198–297 protein as EV71 vaccine.

Identification of three linear B cell epitopes using monoclonal antibodies against bovine enterovirus VP2 protein

Bovine enterovirus (BEV) VP2 protein is a structural protein that plays an important role in inducing protective immunity in the host. The function of VP2 has been characterized, but there is little information on its B cell epitopes. Three monoclonal antibodies (mAbs) directed against BEV VP2 were generated and characterized from mice immunized with the recombinant VP2 protein. Three minimal linear epitopes ¹⁵²FQEAFWLEDG¹⁶¹, ¹⁶⁸LIYPHQ¹⁷³, and ⁴⁶DATSVD⁵¹ reactive to the three mAbs were identified using western blotting analysis. Three-dimensional model of the BEV-E virion and the VP2 monomer showed that epitope ¹⁵²FQEAFWLEDG¹⁶¹ is exposed on surface of the virion and epitopes ⁴⁶DATSVD⁵¹ and ¹⁶⁸LIYPHQ¹⁷³ are located inside the virion. Alignment of the amino acid sequences corresponding to the regions containing the three minimal linear epitopes in the VP2 proteins and their cross-reactivity with the three mAbs showed that epitope ¹⁶⁸LIYPHQ¹⁷³ is completely conserved in all BEV strains. Epitope ⁴⁶DATSVD⁵¹ is highly conserved among BEV-E strains and partly conserved among BEV-F strains. However, epitope ¹⁵²FQEAFWLEDG¹⁶¹ is not conserved among BEV-F strains. Using the mAbs of 3H4 and 1E10, we found that VP2 localized in the cytoplasm during viral replication and could be used to monitor the viral antigen in infected tissues using immunohistochemistry. A preliminary 3H4-epitope-based indirect ELISA allowed us to detect anti-BEV-strain-HY12 antibodies in mice. This study indicates that the three mAbs could be useful tools for investigating the structure and function of the viral VP2 protein and the development of serological diagnostic techniques for BEV infection.

Human IgG Fc promotes expression, secretion and immunogenicity of enterovirus 71 VP1 protein

Enterovirus (EV71) can cause severe neurological diseases, but the underlying pathogenesis remains unclear. The capsid protein, viral protein 1 (VP1), plays a critical role in the pathogenicity of EV71. High level expression and secretion of VP1 protein are necessary for structure, function and immunogenicity in its natural conformation. In our previous studies, 5 codon-optimized VP1 DNA vaccines, including wt-VP1, tPA-VP1, VP1-d, VP1-hFc and VP1-mFc, were constructed and analyzed. They expressed VP1 protein, but the levels of secretion and immunogenicity of these VP1 constructs were significantly different (P<0.05). In this study, we further investigated the protein levels of these constructs and determined that all of these constructs expressed VP1 protein. The secretion level was increased by including a tPA leader sequence, which was further increased by fusing human IgG Fc (hFc) to VP1. VP1-hFc demonstrated the most potent immunogenicity in mice. Furthermore, hFc domain could be used to purify VP1-hFc protein for additional studies.

Cell surface nucleolin facilitates enterovirus 71 binding and infection

Because the pathogenesis of enterovirus 71 (EV71) remains mostly ambiguous, identifying the factors that mediate viral binding and entry to host cells is indispensable to ultimately uncover the mechanisms that underlie virus infection and pathogenesis. Despite the identification of several receptors/attachment molecules for EV71, the binding, entry, and infection mechanisms of EV71 remain unclear. Herein, we employed glycoproteomic approaches to identify human nucleolin as a novel binding receptor for EV71. Glycoproteins purified by lectin chromatography from the membrane extraction of human cells were treated with sialidase, followed by immunoprecipitation with EV71 particles. Among the 16 proteins identified by tandem mass spectrometry analysis, cell surface nucleolin attracted our attention. We found that EV71 interacted directly with nucleolin via the VP1 capsid protein and that an antinucleolin antibody reduced the binding of EV71 to human cells. In addition, the knockdown of cell surface nucleolin decreased EV71 binding, infection, and production in human cells. Furthermore, the expression of human nucleolin on the cell surface of a mouse cell line increased EV71 binding and conferred EV71 infection and production in the cells. These results strongly indicate that human nucleolin can mediate EV71 binding to and infection of cells. Our findings also demonstrate that the use of glycoproteomic approaches is a reliable methodology to discover novel receptors for pathogens.

IMPORTANCE

Outbreaks of EV71 have been reported in Asia-Pacific countries and have caused thousands of deaths in young children during the last 2 decades. The discovery of new EV71-interacting molecules to understand the infection mechanism has become an emergent issue. Hence, this study uses glycoproteomic approaches to comprehensively investigate the EV71-interacting glycoproteins. Several EV71-interacting glycoproteins are identified, and the role of cell surface nucleolin in mediating the attachment and entry of EV71 is characterized and validated. Our findings not only indicate a novel target for uncovering the EV71 infection mechanism and anti-EV71 drug discovery but also provide a new strategy for virus receptor identification.

The molecule of DC-SIGN captures enterovirus 71 and confers dendritic cell-mediated viral trans-infection

Background

Enterovirus 71 (EV71) is the main causative agent of hand, foot and mouth disease that occurs in young children. Neither antiviral agents nor vaccines are available for efficiently combating viral infection. Study of EV71–host interplay is important for understanding viral infection and developing strategies for prevention and therapy. Here the interactions of EV71 with human dendritic cells were analyzed.

Methods

EV71 capture, endocytosis, infection, and degradation in monocyte-derived dendritic cells (MDDCs) were detected by Flow cytometry or real-time (RT-) PCR, and MDDCs-mediated EV71 trans-infection of RD cells was determined via coculture system. Cell morphology or viability was monitored with microscopy or flow cytometry. SiRNA interference was used to knock down gene expression.

Results

MDDCs can bind EV71, but these loaded-EV71 particles in MDDCs underwent a rapid degradation in the absence of efficient replication; once the captured EV71 encountered susceptible cells, MDDCs efficiently transferred surface-bound viruses to target cells. The molecule of DC-SIGN (DC-specific intercellular adhesion molecule-3 grabbing nonintegrin) mediated viral binding and transfer, because interference of DC-SIGN expression with specific siRNAs reduced EV71 binding and impaired MDDC-mediated viral trans-infection, and exogenous expression of DC-SIGN molecule on Raji cell initiated viral binding and subsequent transmission.

Conclusion

MDDCs could bind efficiently EV71 viruses through viral binding to DC-SIGN molecule, and these captured-viruses could be transferred to susceptible cells for robust infection. The novel finding of DC-mediated EV71 dissemination might facilitate elucidation of EV71 primary infection and benefit searching for new clues for preventing viruses from initial infection.

VP2 dominated CD4+ T cell responses against enterovirus 71 and cross-reactivity against coxsackievirus A16 and polioviruses in a healthy population

Enterovirus 71 (EV71)-associated hand-foot-mouth disease has become a major threat to public health in the Asia-Pacific region. Although T cell immunity is closely correlated with clinical outcomes of EV71 infection, little is known about T cell immunity baseline against EV71 and T cell immunogenecity of EV71 Ags in the population, which has restricted our understanding of immunoprotection mechanisms. In this study, we investigated the cellular immune responses against the four structural Ags of EV71 and determined the immunohierarchy of these Ags in healthy adults. A low frequency of EV71-responsive T cells was detected circulating in peripheral blood, and broad T cell immune responses could be identified in most of the subjects after in vitro expansion. We demonstrated that the VP2 Ag with broad distribution of immunogenic peptides dominates T cell responses against EV71 compared with VP1, VP3, and VP4. Furthermore, the responses were illuminated to be mainly single IFN-γ-secreting CD4(+) T cell dependent, indicating the previous natural acute viral infection of the adult population. Conservancy analysis of the immunogenic peptides revealed that moderately variant peptides were in the majority in coxsackievirus A16 (CV-A16) whereas most of the peptides were highly variant in polioviruses. Less efficient cross-reactivity against CV-A16 might broadly exist among individuals, whereas influences derived from poliovirus vaccination would be limited. Our findings suggest that the significance of VP2 Ag should be addressed in the future EV71-responsive immunological investigations. And the findings concerning the less efficient cross-reactivity against CV-A16 and limited influences from poliovirus vaccination in EV71-contacted population would contribute to a better understanding of immunoprotection mechanisms against enteroviruses.

Construction of a prokaryotic expression vector containing the EV71 VP1-VP4 fusion gene and detection of its expressions

AIM: To construct a prokaryotic vector expressing human enterovirus 71 (EV71) VP1-VP4 fusion antigen.

METHODS: A prokaryotic expression plasmid carrying the VP1-VP4 fusion gene was constructed and transformed into E. coli DH5α. VP1-VP4 fusion protein was induced to express with IPTG. SDS-PAGE and Western blot were performed to detect VP1-VP4 fusion protein. Purified VP1-VP4 fusion protein was coated onto ELISA plates to detect 41 serum samples for screening EV71 positive serum samples.

RESULTS: The sequence of recombinant VP1-VP4 fragment was the same as the expected sequence, indicating that the recombinant vector was successfully constructed. SDS-PAGE showed that the fusion protein had a molecular weight of 42.8 kDa. Western blot showed that fusion protein can be specifically recognized by VP1 antibody and VP4 antibody. Fusion protein coated onto ELISA plates could accurately detect 16 EV71 positive serum samples from 41 serum samples without cross-reactivity with coxsackievirus16 (CA16).

CONCLUSION: The VP1-VP4 fusion protein has good antigenicity and can be used as a diagnostic antigen to detect EV71 infection. Our results provide a experimental basis for development of EV71 diagnostic kits.