Distinct immune responses of recombinant plasmid DNA replicon vaccines expressing two types of antigens with or without signal sequences

Enhanced effects of DNA vaccine against botulinum neurotoxin serotype A by targeting antigen to dendritic cells

As dendritic cells (DCs) play a critical role in priming antigen-specific immune responses, the efficacy of DNA vaccines may be enhanced by targeting the encoded antigen proteins to DCs. In this study, we constructed a DC-targeted DNA vaccine encoding the Hc domain of botulinum neurotoxin serotype A (AHc) fused with scDEC, a single-chain Fv antibody (scFv) specific for the DC-restricted antigen-uptake receptor DEC205. Intramuscular injections of mice with the DC-targeted DNA vaccine (pVAX1-scDEC-AHc) stimulated more DCs to mature than the non-targeted DNA vaccine (pVAX1-SAHc) in the splenocytes. The DC-targeted DNA vaccine could induce more DCs maturation at the site of inoculation. The DC-targeted DNA vaccine induced stronger AHc-specific humoral immune responses, lymphocyte proliferative responses and protective potency against BoNT/A in mice than did pVAX1-SAHc. Moreover, the DC-targeting DNA vaccine provided effective protection after only two inoculations. In summary, these results showed that the DC-targeted fusion DNA vaccine could generate strong immunity, indicating that maturation of DCs induced by pVAX1-scDEC-AHc may be helpful for priming and boosting immune responses. Thus, we propose that the strategy of targeting antigen to DCs in vivo via DEC205 can enhance effectively the potency of DNA vaccines against BoNTs or other pathogens in an animal model.

Enhanced Immune Response to DNA Vaccine Encoding Bacillus anthracis PA-D4 Protects Mice against Anthrax Spore Challenge

Anthrax has long been considered the most probable bioweapon-induced disease. The protective antigen (PA) of Bacillus anthracis plays a crucial role in the pathogenesis of anthrax. In the current study, we evaluated the efficiency of a genetic vaccination with the fourth domain (D4) of PA, which is responsible for initial binding of the anthrax toxin to the cellular receptor. The eukaryotic expression vector was designed with the immunoglobulin M (IgM) signal sequence encoding for PA-D4, which contains codon-optimized genes. The expression and secretion of recombinant protein was confirmed in vitro in 293T cells transfected with plasmid and detected by western blotting, confocal microscopy, and enzyme-linked immunosorbent assay (ELISA). The results revealed that PA-D4 protein can be efficiently expressed and secreted at high levels into the culture medium. When plasmid DNA was given intramuscularly to mice, a significant PA-D4-specific antibody response was induced. Importantly, high titers of antibodies were maintained for nearly 1 year. Furthermore, incorporation of the SV40 enhancer in the plasmid DNA resulted in approximately a 15-fold increase in serum antibody levels in comparison with the plasmid without enhancer. The antibodies produced were predominantly the immunoglobulin G2 (IgG2) type, indicating the predominance of the Th1 response. In addition, splenocytes collected from immunized mice produced PA-D4-specific interferon gamma (IFN-γ). The biodistribution study showed that plasmid DNA was detected in most organs and it rapidly cleared from the injection site. Finally, DNA vaccination with electroporation induced a significant increase in immunogenicity and successfully protected the mice against anthrax spore challenge. Our approach to enhancing the immune response contributes to the development of DNA vaccines against anthrax and other biothreats.

Pentavalent replicon vaccines against botulinum neurotoxins and tetanus toxin using DNA-based Semliki Forest virus replicon vectors

The clostridial neurotoxin (CNT) family includes botulinum neurotoxin (BoNT), serotypes A, B, E, and F of which can cause human botulism, and tetanus neurotoxin (TeNT), which is the causative agent of tetanus. This suggests that the greatest need is for a multivalent or multiagent vaccine that provides protection against all 5 agents. In this study, we investigated the feasibility of generating several pentavalent replicon vaccines that protected mice against BoNTs and TeNT. First, we evaluated the potency of individual replicon DNA or particle vaccine against TeNT, which induced strong antibody and protective responses in BALB/c mice following 2 or 3 immunizations. Then, the individual replicon TeNT vaccines were combined with tetravalent BoNTs vaccines to prepare 4 types of pentavalent replicon vaccines. These replicon DNA or particle pentavalent vaccines could simultaneously and effectively induce antibody responses and protect effects against the 5 agents. Finally, a solid-phase assay showed that the sera of pentavalent replicon formulations-immunized mice inhibited the binding of THc to the ganglioside GT1b as the sera of individual replicon DNA or particle-immunized mice. These results indicated these pentavalent replicon vaccines could protect against the 4 BoNT serotypes and effectively neutralize and protect the TeNT. Therefore, our studies demonstrate the utility of combining replicon DNA or particle vaccines into multi-agent formulations as potent pentavalent vaccines for eliciting protective responses against BoNTs and TeNT.

In vitro and in vivo activities of recombinant anthrax protective antigen co-expressed with thioredoxin in Escherichia coli

Because of the central role it plays in the formation of lethal toxin and edema toxin, protective antigen (PA) is the principal target for the development of vaccines against anthrax. In the present study, we explored and compared the in vitro and in vivo activities of recombinant anthrax protective antigen (rPA) and receptor binding domain of protective antigen (PA4). As a result, the fully soluble rPA and PA4 proteins were successfully expressed in Escherichia coli by co-expression with thioredoxin (Trx), and the rPA was active in forming cytotoxic lethal toxins, indicating that the rPA protein retains a functionally biological activity. Furthermore, immunization with rPA protein induced stronger PA-specific immune responses in mice than PA4 protein. The protection elicited by immunization with PA4 suggests the presence of common neutralizing epitopes between rPA and PA4, but the immunization with rPA protein induced stronger neutralizing antibodies and protective levels against challenge with the B. anthracis strain A16R than the PA4 protein. The sera neutralizing antibodies titers correlated well with anti-PA group ELISA antibodies titers and the in vivo protective potency. Based on the results of cell cytotoxicity assays and the observed immune responses and protective potency, we concluded that the soluble rPA protein retains the in vitro and in vivo functionally biological activity and can be developed into a highly effective human subunit vaccine candidate against anthrax.

Three types of human CpG motifs differentially modulate and augment immunogenicity of nonviral and viral replicon DNA vaccines as built-in adjuvants

NakedDNA vaccines given by intramuscular injection are efficient in mouse models, but they require improvement for human use. As the immunogenicity of DNA vaccines depends, to a large extent, on the presence of CpG motifs as built-in adjuvants, we addressed this issue by inserting three types of human CpG motifs (A-type, B-type, and C-type) into the backbone of nonviral DNA and viral DNA replicon vectors with distinct immunostimulatory activities on human PBMCs. The adjuvant effects of CpG modifications in DNA vaccines expressing three types of antigens (β-Gal, AHc, or PA4) were then characterized in mice and found to significantly enhance antigen-specific humoral and cell-mediated immune responses. The three types of CpG motifs also differentially affected and modulated immune responses and protective potency against botulinum neurotoxin serotype A and Bacillus anthracis A16R challenge. Taken together, these results demonstrate that insertion of human CpG motifs can differentially modulate the immunogenicity of nonviral DNA vaccines as well as viral DNA replicon vaccines. Our study provides not only a better understanding of the in vivo activities of CpG motif adjuvants but implications for the rational design of such motifs as built-in adjuvants for DNA vectors targeting specific antigens.