The virus-induced signaling adaptor molecule enhances DNA-raised immune protection against H5N1 influenza virus infection in mice

Conserved Functions of Orthohepadnavirus X Proteins to Inhibit Type-I Interferon Signaling

Orthohepadnavirus causes chronic hepatitis in a broad range of mammals, including primates, cats, woodchucks, and bats. Hepatitis B virus (HBV) X protein inhibits type-I interferon (IFN) signaling, thereby promoting HBV escape from the human innate immune system and establishing persistent infection. However, whether X proteins of Orthohepadnavirus viruses in other species display a similar inhibitory activity remains unknown. Here, we investigated the anti-IFN activity of 17 Orthohepadnavirus X proteins derived from various hosts. We observed conserved activity of Orthohepadnavirus X proteins in inhibiting TIR-domain-containing adaptor protein inducing IFN-β (TRIF)-mediated IFN-β signaling pathway through TRIF degradation. X proteins from domestic cat hepadnavirus (DCH), a novel member of Orthohepadnavirus, inhibited mitochondrial antiviral signaling protein (MAVS)-mediated IFNβ signaling pathway comparable with HBV X. These results indicate that inhibition of IFN signaling is conserved in Orthohepadnavirus X proteins.

MDA5 with Complete CARD2 Region Inhibits the Early Replication of H9N2 AIV and Enhances the Immune Response during Vaccination

Chicken melanoma differentiation-associated gene 5 (MDA5) is a member of the RLRs family that recognizes the viral RNAs invading cells and activates downstream interferon regulatory pathways, thereby inhibiting viral replication. The caspase activation and recruitment domain (CARD) is the most important region in MDA5 protein. However, the antiviral and immune enhancement of MDA5 with the CARD region remains unclear. In this study, two truncated MDA5 genes with different CARD regions, namely MDA5-1 with CARD1 plus partial CARD2 domain and MDA5-2 with CARD1 plus complete CARD2 domain, were cloned via reverse transcription PCR and ligated into plasmid Flag-N vector to be Flag-MDA5-1 and Flag-MDA5-2 plasmids. DF-1 cells were transfected with two plasmids for 24 h and then inoculated with H9N2 virus (0.1 MOI) for 6 h to detect the levels of IFN-β, PKR, MAVS, and viral HA, NA, and NS proteins expression. The results showed that MDA5-1 and MDA5-2 increased the expression of IFN-β and PKR, activated the downstream molecule MAVS production, and inhibited the expression of HA, NA, and NS proteins. The knockdown of MDA5 genes confirmed that MDA5-2 had a stronger antiviral effect than that of MDA5-1. Furthermore, the recombinant proteins MDA5-1 and MDA5-2 were combined with H9N2 inactivated vaccine to immunize SPF chickens subcutaneously injected in the neck three times. The immune response of the immunized chicken was investigated. It was observed that the antibody titers and expressions of immune-related molecules from the chicken immunized with MDA5-1 and MDA5-2 group were increased, in which the inducing function of MDA5-2 groups was the highest among all immunization groups. These results suggested that the truncated MDA5 recombinant proteins with complete CARD2 region could play vital roles in antiviral and immune enhancement. This study provides important material for the further study of the immunoregulatory function and clinical applications of MDA5 protein.

A Genetically Engineered Bivalent Vaccine Coexpressing a Molecular Adjuvant against Classical Swine Fever and Porcine Epidemic Diarrhea

Classical swine fever (CSF) and porcine epidemic diarrhea (PED) are highly contagious viral diseases that pose a significant threat to piglets and cause substantial economic losses in the global swine industry. Therefore, the development of a bivalent vaccine capable of targeting both CSF and PED simultaneously is crucial. In this study, we genetically engineered a recombinant classical swine fever virus (rCSFV) expressing the antigenic domains of the porcine epidemic diarrhea virus (PEDV) based on the modified infectious cDNA clone of the vaccine strain C-strain. The S1N and COE domains of PEDV were inserted into C-strain cDNA clone harboring the mutated 136th residue of Npro and substituted 3'UTR to generate the recombinant chimeric virus vC/SM3'UTRN-S1NCOE. To improve the efficacy of the vaccine, we introduced the tissue plasminogen activator signal (tPAs) and CARD domain of the signaling molecule VISA into vC/SM3'UTRN-S1NCOE to obtain vC/SM3'UTRN-tPAsS1NCOE and vC/SM3'UTRN-CARD/tPAsS1NCOE, respectively. We characterized three vaccine candidates in vitro and investigated their immune responses in rabbits and pigs. The NproD136N mutant exhibited normal autoprotease activity and mitigated the inhibition of IFN-β induction. The introduction of tPAs and the CARD domain led to the secretory expression of the S1NCOE protein and upregulated IFN-β induction in infected cells. Immunization with recombinant CSFVs expressing secretory S1NCOE resulted in a significantly increased in PEDV-specific antibody production, and coexpression of the CARD domain of VISA upregulated the PEDV-specific IFN-γ level in the serum of vaccinated animals. Notably, vaccination with vC/SM3'UTRN-CARD/tPAsS1NCOE conferred protection against virulent CSFV and PEDV challenge in pigs. Collectively, these findings demonstrate that the engineered vC/SM3'UTRN-CARD/tPAsS1NCOE is a promising bivalent vaccine candidate against both CSFV and PEDV infections.

The impact of immuno-aging on SARS-CoV-2 vaccine development

The SARS-CoV-2 pandemic has almost 56 million confirmed cases resulting in over 1.3 million deaths as of November 2020. This infection has proved more deadly to older adults (those >65 years of age) and those with immunocompromising conditions. The worldwide population aged 65 years and older is increasing, and the total number of aged individuals will outnumber those younger than 65 years by the year 2050. Aging is associated with a decline in immune function and chronic activation of inflammation that contributes to enhanced viral susceptibility and reduced responses to vaccination. Here we briefly review the pathogenicity of the virus, epidemiology and clinical response, and the underlying mechanisms of human aging in improving vaccination. We review current methods to improve vaccination in the older adults using novel vaccine platforms and adjuvant systems. We conclude by summarizing the existing clinical trials for a SARS-CoV-2 vaccine and discussing how to address the unique challenges for vaccine development presented with an aging immune system.

A systematic review of SARS-CoV-2 vaccine candidates

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is an emerging virus that is highly pathogenic and has caused the recent worldwide pandemic officially named coronavirus disease (COVID-19). Currently, considerable efforts have been put into developing effective and safe drugs and vaccines against SARS-CoV-2. Vaccines, such as inactivated vaccines, nucleic acid-based vaccines, and vector vaccines, have already entered clinical trials. In this review, we provide an overview of the experimental and clinical data obtained from recent SARS-CoV-2 vaccines trials, and highlight certain potential safety issues that require consideration when developing vaccines. Furthermore, we summarize several strategies utilized in the development of vaccines against other infectious viruses, such as severe acute respiratory syndrome coronavirus (SARS-CoV) and Middle East respiratory syndrome coronavirus (MERS-CoV), with the aim of aiding in the design of effective therapeutic approaches against SARS-CoV-2.

Innate signalling molecules as genetic adjuvants do not alter the efficacy of a DNA-based influenza A vaccine

In respect to the heterogeneity among influenza A virus strains and the shortcomings of current vaccination programs, there is a huge interest in the development of alternative vaccines that provide a broader and more long-lasting protection. Gene-based approaches are considered as promising candidates for such flu vaccines. In our study, innate signalling molecules from the RIG-I and the NALP3 pathways were evaluated as genetic adjuvants in intramuscular DNA immunizations. Plasmids encoding a constitutive active form of RIG-I (cRIG-I), IPS-1, IL-1β, or IL-18 were co-administered with plasmids encoding the hemagglutinin and nucleoprotein derived from H1N1/Puerto Rico/8/1934 via electroporation in BALB/c mice. Immunogenicity was analysed in detail and efficacy was demonstrated in homologous and heterologous influenza challenge experiments. Although the biological activities of the adjuvants have been confirmed by in vitro reporter assays, their single or combined inclusion in the vaccine did not result in superior vaccine efficacy. With the exception of significantly increased levels of antigen-specific IgG1 after the co-administration of IL-1β, there were only minor alterations concerning the immunogenicity. Since DNA electroporation alone induced substantial inflammation at the injection site, as demonstrated in this study using Mx2-Luc reporter mice, it might override the adjuvants´ contribution to the inflammatory microenvironment and thereby minimizes the influence on the immunogenicity. Taken together, the DNA immunization was protective against subsequent challenge infections but could not be further improved by the genetic adjuvants analysed in this study.

Characterization of thermostable Newcastle disease virus recombinants expressing the hemagglutinin of H5N1 avian influenza virus as bivalent vaccine candidates

Newcastle disease virus (NDV) has been used as a vector in the development of vaccines and gene delivery. In the present study, we generated the thermostable recombinant NDV (rNDV) expressing the different forms of hemagglutinin (HA) of highly pathogenic avian influenza virus (HPAIV) H5N1 based on the full-length cDNA clone of thermostable TS09-C strain. The recombinant thermostable Newcastle disease viruses, rTS-HA, rTS-HA1 and rTS-tPAs/HA1, expressed the HA, HA1 or modified HA1 protein with the tissue plasminogen activator signal sequence (tPAs), respectively. The rNDVs displayed similar thermostability, growth kinetics and pathogenicity compared with the parental TS09-C virus. The tPAs facilitated the expression and secretion of HA1 protein in cells infected with rNDV. Animal studies demonstrated that immunization with rNDVs elicited effective H5N1- and NDV-specific antibody responses and conferred immune protection against lethal H5N1 and NDV challenges in chickens and mice. Importantly, vaccination of rTS-tPAs/HA1 resulted in enhanced protective immunity in chickens and mice. Our study thus provides a novel thermostable NDV-vectored vaccine candidate expressing a soluble form of a heterologous viral protein, which will greatly aid the poultry industry in developing countries.

DNA Vaccines-How Far From Clinical Use?

Two decades ago successful transfection of antigen presenting cells (APC) in vivo was demonstrated which resulted in the induction of primary adaptive immune responses. Due to the good biocompatibility of plasmid DNA, their cost-efficient production and long shelf life, many researchers aimed to develop DNA vaccine-based immunotherapeutic strategies for treatment of infections and cancer, but also autoimmune diseases and allergies. This review aims to summarize our current knowledge on the course of action of DNA vaccines, and which factors are responsible for the poor immunogenicity in human so far. Important optimization steps that improve DNA transfection efficiency comprise the introduction of DNA-complexing nano-carriers aimed to prevent extracellular DNA degradation, enabling APC targeting, and enhanced endo/lysosomal escape of DNA. Attachment of virus-derived nuclear localization sequences facilitates nuclear entry of DNA. Improvements in DNA vaccine design include the use of APC-specific promotors for transcriptional targeting, the arrangement of multiple antigen sequences, the co-delivery of molecular adjuvants to prevent tolerance induction, and strategies to circumvent potential inhibitory effects of the vector backbone. Successful clinical use of DNA vaccines may require combined employment of all of these parameters, and combination treatment with additional drugs.

Baculovirus vectors expressing F proteins in combination with virus-induced signaling adaptor (VISA) molecules confer protection against respiratory syncytial virus infection

Baculovirus has been exploited for use as a novel vaccine vector. To investigate the feasibility and efficacy of recombinant baculoviruses (rBVs) expressing respiratory syncytial virus (RSV) fusion (F) proteins, four constructs (Bac-tF/64, Bac-CF, Bac-CF/tF64 and Bac-CF/tF64-VISA) were generated. Bac-tF64 displays the F ectodomain (tF) on the envelope of rBVs, whereas Bac-CF expresses full-length F protein in transduced mammalian cells. Bac-CF/tF64 not only displays tF on the envelope but also expresses F in cells. Bac-CF/tF64-VISA comprises Bac-CF/tF64 harboring the virus-induced signaling adaptor (VISA) gene. After administration to BALB/c mice, all four vectors elicited RSV neutralizing antibody (Ab), systemic Ab (IgG, IgG1, and IgG2a), and cytokine responses. Compared with Bac-tF64, mice inoculated with Bac-CF and Bac-CF/tF64 exhibited an increased mixed Th1/Th2 cytokine response, increased ratios of IgG2a/IgG1 antibody responses, and reduced immunopathology upon RSV challenge. Intriguingly, co-expression of VISA reduced Th2 cytokine (IL-4, IL-5, and IL-10) production induced by Bac-CF/tF64, thus relieving lung pathology upon a subsequent RSV challenge. Our results indicated that the Bac-CF/tF64 vector incorporated with the VISA molecule may provide an effective vaccine strategy for protection against RSV.

Oral Delivery of a Novel Attenuated Salmonella Vaccine Expressing Influenza A Virus Proteins Protects Mice against H5N1 and H1N1 Viral Infection

Attenuated strains of invasive enteric bacteria, such as Salmonella, represent promising gene delivery agents for nucleic acid-based vaccines as they can be administrated orally. In this study, we constructed a novel attenuated strain of Salmonella for the delivery and expression of the hemagglutinin (HA) and neuraminidase (NA) of a highly pathogenic H5N1 influenza virus. We showed that the constructed Salmonella strain exhibited efficient gene transfer activity for HA and NA expression and little cytotoxicity and pathogenicity in mice. Using BALB/c mice as the model, we evaluated the immune responses and protection induced by the constructed Salmonella-based vaccine. Our study showed that the Salmonella-based vaccine induced significant production of anti-HA serum IgG and mucosal IgA, and of anti-HA interferon-γ producing T cells in orally vaccinated mice. Furthermore, mice orally vaccinated with the Salmonella vaccine expressing viral HA and NA proteins were completely protected from lethal challenge of highly pathogenic H5N1 as well as H1N1 influenza viruses while none of the animals treated with the Salmonella vaccine carrying the empty expression vector with no viral antigen expression was protected. These results suggest that the Salmonella-based vaccine elicits strong antigen-specific humoral and cellular immune responses and provides effective immune protection against multiple strains of influenza viruses. Furthermore, our study demonstrates the feasibility of developing novel attenuated Salmonella strains as new oral vaccine vectors against influenza viruses.

Improvement of DNA vaccination by adjuvants and sophisticated delivery devices: vaccine-platforms for the battle against infectious diseases

Advantages of DNA vaccination against infectious diseases over more classical immunization methods include the possibilities for rapid manufacture, fast adaptation to newly emerging pathogens and high stability at ambient temperatures. In addition, upon DNA immunization the antigen is produced by the cells of the vaccinated individual, which leads to activation of both cellular and humoral immune responses due to antigen presentation via MHC I and MHC II molecules. However, so far DNA vaccines have shown most efficient immunogenicity mainly in small rodent models, whereas in larger animals including humans there is still the need to improve effectiveness. This is mostly due to inefficient delivery of the DNA plasmid into cells and nuclei. Here, we discuss technologies used to overcome this problem, including physical means such as in vivo electroporation and co-administration of adjuvants. Several of these methods have already entered clinical testing in humans.

MDA5 can be exploited as efficacious genetic adjuvant for DNA vaccination against lethal H5N1 influenza virus infection in chickens

Chickens lack the retinoic acid-inducible gene I (RIG-I) and sense avian influenza virus (AIV) infections by means of the melanoma differentiation-associated gene 5 product (chMDA5). Plasmid-driven expression of the N-terminal half of chMDA5 containing the caspase activation and recruitment domains [chMDA5(1-483)] triggers interferon-β responses in chicken cells. We hypothesized that mimicking virus infection by chMDA5(1-483) expression may enhance vaccine-induced adaptive immunity. In order to test this, the potential genetic adjuvant properties of chMDA5(1-483) were evaluated in vivo in combination with a suboptimal quantity of a plasmid DNA vaccine expressing haemagglutinin (HA) of H5N1 AIV. Co-administration of the HA plasmid with plasmid DNA for chMDA5(1-483) expression resulted in approximately 10-fold higher HA-specific antibody responses than injection of the HA plasmid mixed with empty vector DNA as control. Accordingly, compared with HA DNA vaccination alone, the chMDA5(1-483)-adjuvanted HA DNA vaccine mediated enhanced protection against a lethal H5N1 challenge infection in chickens, with reduced clinical signs and cloacal virus shedding. These data demonstrate that innate immune activation by expression of signaling domains of RIG-I-like receptors can be exploited to enhance vaccine efficacy.