Main features of DNA-based immunization vectors

Next-Generation Vaccines: Nanovaccines in the Fight against SARS-CoV-2 Virus and beyond SARS-CoV-2

The virus responsible for the coronavirus viral pandemic is the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). Emerging SARS-CoV-2 variants caused by distinctive mutations within the viral spike glycoprotein of SARS-CoV-2 are considered the cause for the rapid spread of the disease and make it challenging to treat SARS-CoV-2. The manufacturing of appropriate efficient vaccines and therapeutics is the only option to combat this pandemic. Nanomedicine has enabled the delivery of nucleic acids and protein-based vaccines to antigen-presenting cells to produce protective immunity against the coronavirus. Nucleic acid-based vaccines, particularly mRNA nanotechnology vaccines, are the best prevention option against the SARS-CoV-2 pandemic worldwide, and they are effective against the novel coronavirus and its multiple variants. This review will report on progress made thus far with SARS-CoV-2 vaccines and beyond employing nanotechnology-based nucleic acid vaccine approaches.

Immunizing Animals

The traditional method for generating polyclonal and monoclonal antibodies requires the immunization of an animal. Selecting the best species of animal and getting that animal's immune system to respond to a target antigen with an antibody response are essential to obtaining good-quality antibodies and hybridomas. There are only a limited number of opportunities for a researcher to intervene to manipulate and tailor the response to a particular antigen. Here we present advice and methods for designing the way in which the antigen is presented to the immune system (i.e., the immunization protocol), including the choice of animal, the antigen dose, the use of adjuvants, the route and number of injections, and the period between injections.

Improvement of different vaccine delivery systems for cancer therapy

Cancer vaccines are the promising tools in the hands of the clinical oncologist. Many tumor-associated antigens are excellent targets for immune therapy and vaccine design. Optimally designed cancer vaccines should combine the best tumor antigens with the most effective immunotherapy agents and/or delivery strategies to achieve positive clinical results. Various vaccine delivery systems such as different routes of immunization and physical/chemical delivery methods have been used in cancer therapy with the goal to induce immunity against tumor-associated antigens. Two basic delivery approaches including physical delivery to achieve higher levels of antigen production and formulation with microparticles to target antigen-presenting cells (APCs) have demonstrated to be effective in animal models. New developments in vaccine delivery systems will improve the efficiency of clinical trials in the near future. Among them, nanoparticles (NPs) such as dendrimers, polymeric NPs, metallic NPs, magnetic NPs and quantum dots have emerged as effective vaccine adjuvants for infectious diseases and cancer therapy. Furthermore, cell-penetrating peptides (CPP) have been known as attractive carrier having applications in drug delivery, gene transfer and DNA vaccination. This review will focus on the utilization of different vaccine delivery systems for prevention or treatment of cancer. We will discuss their clinical applications and the future prospects for cancer vaccine development.

Immunological analysis of a Lactococcus lactis-based DNA vaccine expressing HIV gp120

For reasons of efficiency Escherichia coli is used today as the microbial factory for production of plasmid DNA vaccines. To avoid hazardous antibiotic resistance genes and endotoxins from plasmid systems used nowadays, we have developed a system based on the food-grade Lactococcus lactis and a plasmid without antibiotic resistance genes. We compared the L. lactis system to a traditional one in E. coli using identical vaccine constructs encoding the gp120 of HIV-1. Transfection studies showed comparable gp120 expression levels using both vector systems. Intramuscular immunization of mice with L. lactis vectors developed comparable gp120 antibody titers as mice receiving E. coli vectors. In contrast, the induction of the cytolytic response was lower using the L. lactis vector. Inclusion of CpG motifs in the plasmids increased T-cell activation more when the E. coli rather than the L. lactis vector was used. This could be due to the different DNA content of the vector backbones. Interestingly, stimulation of splenocytes showed higher adjuvant effect of the L. lactis plasmid. The study suggests the developed L. lactis plasmid system as new alternative DNA vaccine system with improved safety features. The different immune inducing properties using similar gene expression units, but different vector backbones and production hosts give information of the adjuvant role of the silent plasmid backbone. The results also show that correlation between the in vitro adjuvanticity of plasmid DNA and its capacity to induce cellular and humoral immune responses in mice is not straight forward.

Ensuring safety of DNA vaccines

In 1990 a new approach for vaccination was invented involving injection of plasmid DNA in vivo, which elicits an immune response to the encoded protein. DNA vaccination can overcome most disadvantages of conventional vaccine strategies and has potential for vaccines of the future. However, today 15 years on, a commercial product still has not reached the market. One possible explanation could be the technique's failure to induce an efficient immune response in humans, but safety may also be a fundamental issue. This review focuses on the safety of the genetic elements of DNA vaccines and on the safety of the microbial host for the production of plasmid DNA. We also propose candidates for the vaccine's genetic elements and for its microbial production host that can heighten the vaccine's safety and facilitate its entry to the market.

Internalin-expressing Lactococcus lactis is able to invade small intestine of guinea pigs and deliver DNA into mammalian epithelial cells

The use of the food-grade bacterium Lactococcus lactis as antigen delivery vehicle at the mucosal level is an attractive vaccination strategy intensively explored during the last decade. In this study, we developed L. lactis strains which could be used as a DNA delivery vector to combine both advantages of mucosal delivery and of DNA vaccination. To render lactococci capable of invading epithelial cells, the Listeria monocytogenes inlA gene was cloned and expressed in L. lactis under transcriptional control of the native promoter. Western blot and immunofluorescence assays revealed that recombinant lactococci efficiently displayed the cell wall anchored form of InlA. We demonstrated that this expression promotes internalization of L. lactis inlA+ into the human epithelial cell line Caco-2. Gentamicin assay showed that invasiveness of L. lactis in these cells is approximately 100-fold higher for L. lactis inlA+ than for wild type (wt) L. lactis strains. Moreover, we showed that L. lactis inlA+ is able to enter intestinal cells in vivo, after oral inoculation of guinea pigs. After internalization, L. lactis inlA+ was able to deliver a functional eukaryotic gfp gene into epithelial Caco-2 cells; GFP was detected in 1% of internalized cells. The L. lactis inlA+ strain will be a useful bacterial vector for the development of new live oral DNA vaccines. It also constitutes an interesting new model to study the role of internalin in bacterial localization in the animal host.

Nonviral DNA vectors for immunization and therapy: design and methods for their obtention

The use of plasmid DNA for vaccination and therapy is a relatively novel technology, with advantages and limitations as with other gene transfer techniques. The technology is based on DNA vectors designed for administering genes coding for relevant proteins into a given organism, fulfilling requirements of the regulatory agencies that once properly formulated and delivered the desired vaccine/therapeutic effect can be achieved. Starting from conventional plasmid DNA vectors currently tested in clinical trials, improvement resulted in bacterial element-less vectors, increasing the complexity of the developmental process. The present review focuses on systems described for generating these nonviral DNA vectors for immunization and therapy from bacterial hosts (conventional and conditionally replicating plasmids, nonreplicating minicircles, and linear dumbbell-shaped expression cassettes) in vivo or in vitro. Additionally, nontherapeutic genetic sequences with a negative or positive effect according to the specific application are described, bringing a better comprehension of the technology's state of the art.

Enhanced cell-mediated IFN-gamma-secreting activity against the HIV-1IIIB V3 peptide of the TAB9 multiepitope after DNA vaccine backbone engineering

The SV40t polyadenylation and splicing signals of the pAEC plasmid vectors were replaced by synthetic intron and synthetic rabbit beta globin-based termination/polyadenylation sequences, and 5, 10, and 20 copies of the 5'-AACGTT-3' CpG motif were inserted. Balb/c mice were immunized by intramuscular injection of 200 microg of each plasmid, coding for the HIV-1 multiepitope TAB9, under the control of the human cytomegalovirus promoter. After three doses of DNA, a fourth boost with plasmid DNA or a TAB9-expressing recombinant fowlpox virus rFPTAB9LZ was administered. ELISA and ELISPOT assays were conducted for antibody and IFN-gamma-secreting cell-mediated responses' evaluation against the whole TAB9 and the TAB9's IIIB V3 peptide, respectively. Serum IgG antibodies were not detected. Effector IFN-gamma-secreting responses were only detected on the animals receiving the new set of DNA constructs, alone or in combination with a recombinant virus boost, with or without in vitro re-stimulation. The response was dependent on the new transcriptional unit and influenced by the number of CpG motifs. We showed that plasmid backbone optimization based on these two factors could enhance the response against a multiepitope-based DNA vaccine. A new family of plasmid vectors is also available for evaluation with desired antigens.

Modulating gene expression using DNA vaccines with different 3'-UTRs influences antibody titer, seroconversion and cytokine profiles

To determine if modulating the amount of foreign antigen produced by a DNA vaccine can influence the overall intensity and cytokine polarization of the ensuing immune response, three different plasmids, each encoding the hepatitis B (HB) surface antigen, were constructed. In each construct, HBs gene expression was driven by the cytomegalovirus immediate early promoter, but differed in the 3'-untranslated regions (3'-UTR) containing the polyadenylation sequence. These 3'-UTR sequences were derived from either the hepatitis B virus (HBVpA), bovine growth hormone (BGHpA), or rabbit beta-globin (betapA). BALB/c mice were immunized intramuscularly with equimolar amounts of each plasmid and blood was collected bi-weekly. Following immunization, total IgG titers correlated with in vitro antigen production levels (from transfected CHO cells), as evidenced by the following response pattern: HBVpA>BGHpA>>betapA. All groups demonstrated a heavy bias toward a Th1 immune response, as evidenced by high serum IgG2a/IgG1 ratios and the predominance of IFN-gamma over IL-4 secretion from cultured splenocytes. In addition, the HBVpA construct resulted in a seroconversion rate of 100%, in comparison to 40-50% in the BGHpA, and 0% in the betapA group. Surprisingly, splenocytes isolated from mice immunized with the betapA construct secreted the highest levels of IFN-gamma. Taken together, these findings suggest that altering the level of gene expression not only affects the overall titer and seroconversion rates of vaccinated animals, but also may play a role in modulating cytokine profiles.