New vaccine strategies against enterotoxigenic Escherichia coli. I: DNA vaccines against the CFA/I fimbrial adhesin

Dengue Virus and Vaccines: How Can DNA Immunization Contribute to This Challenge?

Dengue infections still have a tremendous impact on public health systems in most countries in tropical and subtropical regions. The disease is systemic and dynamic with broad range of manifestations, varying from mild symptoms to severe dengue (Dengue Hemorrhagic Fever and Dengue Shock Syndrome). The only licensed tetravalent dengue vaccine, Dengvaxia, is a chimeric yellow fever virus with prM and E genes from the different dengue serotypes. However, recent results indicated that seronegative individuals became more susceptible to develop severe dengue when infected after vaccination, and now WHO recommends vaccination only to dengue seropositive people. One possibility to explain these data is the lack of robust T-cell responses and antibody-dependent enhancement of virus replication in vaccinated people. On the other hand, DNA vaccines are excellent inducers of T-cell responses in experimental animals and it can also elicit antibody production. Clinical trials with DNA vaccines have improved and shown promising results regarding the use of this approach for human vaccination. Therefore, in this paper we review preclinical and clinical tests with DNA vaccines against the dengue virus. Most of the studies are based on the E protein since this antigen is the main target for neutralizing antibody production. Yet, there are other reports with DNA vaccines based on non-structural dengue proteins with protective results, as well. Combining structural and non-structural genes may be a solution for inducing immune responses aging in different infection moments. Furthermore, DNA immunizations are also a very good approach in combining strategies for vaccines against dengue, in heterologous prime/boost regimen or even administering different vaccines at the same time, in order to induce efficient humoral and cellular immune responses.

Cooperation between CD4+ T Cells and Humoral Immunity Is Critical for Protection against Dengue Using a DNA Vaccine Based on the NS1 Antigen

Dengue virus (DENV) is spread through most tropical and subtropical areas of the world and represents a serious public health problem. At present, the control of dengue disease is mainly hampered by the absence of antivirals or a vaccine, which results in an estimated half worldwide population at risk of infection. The immune response against DENV is not yet fully understood and a better knowledge of it is now recognized as one of the main challenge for vaccine development. In previous studies, we reported that a DNA vaccine containing the signal peptide sequence from the human tissue plasminogen activator (t-PA) fused to the DENV2 NS1 gene (pcTPANS1) induced protection against dengue in mice. In the present work, we aimed to elucidate the contribution of cellular and humoral responses elicited by this vaccine candidate for protective immunity. We observed that pcTPANS1 exerts a robust protection against dengue, inducing considerable levels of anti-NS1 antibodies and T cell responses. Passive immunization with anti-NS1 antibodies conferred partial protection in mice infected with low virus load (4 LD₅₀), which was abrogated with the increase of viral dose (40 LD₅₀). The pcTPANS1 also induced activation of CD4⁺ and CD8⁺ T cells. We detected production of IFN-γ and a cytotoxic activity by CD8⁺ T lymphocytes induced by this vaccine, although its contribution in the protection was not so evident when compared to CD4⁺ cells. Depletion of CD4⁺ cells in immunized mice completely abolished protection. Furthermore, transfer experiments revealed that animals receiving CD4⁺ T cells combined with anti-NS1 antiserum, both obtained from vaccinated mice, survived virus infection with survival rates not significantly different from pcTPANS1-immunized animals. Taken together, results showed that the protective immune response induced by the expression of NS1 antigen mediated by the pcTPANS1 requires a cooperation between CD4⁺ T cells and the humoral immunity.

Dengue is an emerging mosquito-borne disease present in an extensive area of the globe with an estimated risk exposure of half of the world’s population. Unfortunately, no specific treatment or vaccine is available to control this disease, which leads to approximately 20,000 casualties annually. The protective immune response against this pathogen consists of an important goal for the development of anti-dengue strategies. For years, the presence of neutralizing antibodies was believed to represent the major response for protection against dengue. However, a recent clinical trial showed that despite the induction of a balanced antibody response against all serotypes, vaccination had only a partial efficacy. In the present work, we aimed to elucidate the contribution of the cellular and humoral responses elicited by a DNA vaccine candidate encoding the non-structural 1 protein (NS1) from dengue virus. We observed that antibody as well as T cell responses are important for protection against dengue in a cooperative way. Our results demonstrated that an effective defense against virus was not achieved with antibodies or T cells alone, but rather with the combination of both responses. Therefore, we suggest that an ideal vaccine against dengue should induce both arms of the immune system.

Multi-epitope recombinant vaccine induces immunoprotection against mixed infection of Eimeria spp

Immunity to Eimeria is species-specific, and chickens with immunity to one species of Eimeria remain susceptible to other Eimeria species. This presents a major challenge in the development of effective vaccines against multiple Eimeria species. In this study, we cloned the antigenic epitope of a tachyzoite surface protein gene of Eimeria tenella, a tachyzoite surface protein gene of Eimeria acervulina and the gametocyte protein gene of Eimeria maxima, and constructed prokaryotic and eukaryotic plasmids carrying the multi-epitope antigenic gene. Immunization of chickens with the multivalent DNA and protein conferred partial protection against infection by the three Eimeria species, as shown by increased CD4+ T lymphocytes in the intestinal mucosa, decreased oocyst excretion and intestinal lesions, and increased body weight gain compared with non-immunized controls. The DNA prime-protein boost immunization schedule induced greater cellular immunity and protection from Eimeria infection than immunization with DNA or protein alone. Our findings demonstrated that DNA prime-protein boost immunization with a multivalent vaccine could stimulate protective immunity against challenge infection of multiple Eimeria species. This work provides a promising step towards DNA-protein vaccination against multiple species of pathogens.

Induction of a protective response in mice by the dengue virus NS3 protein using DNA vaccines

The dengue non-structural 3 (NS3) is a multifunctional protein, containing a serino-protease domain, located at the N-terminal portion, and helicase, NTPase and RTPase domains present in the C-terminal region. This protein is considered the main target for CD4+ and CD8+ T cell responses during dengue infection, which may be involved in protection. However, few studies have been undertaken evaluating the use of this protein as a protective antigen against dengue, as well as other flavivirus. In the present work, we investigate the protective efficacy of DNA vaccines based on the NS3 protein from DENV2. Different recombinant plasmids were constructed, encoding either the full-length NS3 protein or only its functional domains (protease and helicase), fused or not to a signal peptide (t-PA). The recombinant proteins were successfully expressed in transfected BHK-21 cells, and only plasmids encoding the t-PA signal sequence mediated protein secretion. Balb/c mice were immunized with the different DNA vaccines and challenged with a lethal dose of DENV2. Most animals immunized with plasmids encoding the full-length NS3 or the helicase domain survived challenge, regardless of the presence of the t-PA. However, some mice presented clinical signs of infection with high morbidity (hind leg paralysis and hunched posture), mainly in animal groups immunized with the DNA vaccines based on the helicase domain. On the other hand, inoculation with plasmids encoding the protease domain did not induce any protection, since mortality and morbidity rates in these mouse groups were similar to those detected in the control animals. The cellular immune response was analyzed by ELISPOT with a specific-CD8+ T cell NS3 peptide. Results revealed that the DNA vaccines based on the full-length protein induced the production of INF-γ, thus suggesting the involvement of this branch of the immune system in the protection.

Boosting systemic and secreted antibody responses in mice orally immunized with recombinant Bacillus subtilis strains following parenteral priming with a DNA vaccine encoding the enterotoxigenic Escherichia coli (ETEC) CFA/I fimbriae B subunit

Recombinant Bacillus subtilis strains, either spores or vegetative cells, may be employed as safe and low cost orally delivered live vaccine vehicles. In this study, we report the use of an orally delivered B. subtilis vaccine strain to boost systemic and secreted antibody responses in mice i.m. primed with a DNA vaccine encoding the structural subunit (CfaB) of the CFA/I fimbriae encoded by enterotoxigenic Escherichia coli (ETEC), an important etiological agent of diarrhea among travelers and children living in endemic regions. DBA/2 female mice submitted to the prime-boost immunization regimen developed synergic serum (IgG) and mucosal (IgA) antibody responses to the target CfaB antigen. Moreover, in contrast to mice immunized only with one vaccine formulation, sera harvested from prime-boosted vaccinated individuals inhibited adhesion of ETEC cells to human red blood cells. Additionally, vaccinated dams conferred full passive protection to suckling newborn mice challenged with a virulent ETEC strain. Taken together the present results further demonstrate the potential use of recombinant B. subtilis strains as an alternative live vaccine vehicle.

DNA vaccines against dengue virus based on the ns1 gene: The influence of different signal sequences on the protein expression and its correlation to the immune response elicited in mice

We analyzed four DNA vaccines based on DENV-2 NS1: pcENS1, encoding the C-terminal from E protein plus the NS1 region; pcENS1ANC, similar to pcENS1 plus the N-terminal sequence from NS2a (ANC); pcTPANS1, coding the t-PA signal sequence fused to NS1; and pcTPANS1ANC, similar to pcTPANS1 plus the ANC sequence. The NS1 was detected in lysates and culture supernatants from pcTPANS1-, pcENS1- and pcENS1ANC-transfected cells and not in cells with pcTPANS1ANC. Only the pcENS1ANC leads the expression of NS1 in plasma membrane, confirming the importance of ANC sequence for targeting NS1 to cell surface. High levels of antibodies recognizing conformational epitopes of NS1 were induced in mice immunized with pcTPANS1 and pcENS1, while only few pcENS1ANC-inoculated animals presented detectable anti-NS1 IgG. Protection against DENV-2 was verified in pcTPANS1- and pcENS1-immunized mice, although the plasmid pcTPANS1 induced slight higher protective immunity. These plasmids seem to activate distinct patterns of the immune system.

Analysis of strategies to successfully vaccinate infants in developing countries against enterotoxigenic E. coli (ETEC) disease

Enterotoxigenic Escherichia coli (ETEC) is the most common bacterial cause of diarrhoea in the world, annually affecting up to 400,000,000 children under 5 years of age living in developing countries (DCs). Although ETEC possesses numerous antigens, the relatively conserved colonization factor (CF) antigens and the heat labile enterotoxin (LT) have been associated with protection and most vaccine candidates have exploited these antigens. A safe and effective vaccine against ETEC is a feasible goal as supported by the acquisition of protective immunity. The success of an ETEC vaccine targeting infants and children in DCs will depend on a combination of maximally antigenic vaccine preparations and regimens for their delivery which will produce optimal immune responses to these antigens. Vaccine candidates having a high priority for accelerated development and clinical testing for eventual use in infants would include inactivated ETEC or Shigella hybrids expressing ETEC antigens as well as attenuated ETEC strains which express the major CF antigens and LT toxin B-subunit, as well as attenuated Shigella, Vibrio cholerae and Salmonella typhi hybrids engineered to deliver antigens of ETEC. Candidates for an ETEC vaccine would have to meet the minimal requirement of providing at least 50% protection against severe disease in DCs during the first 2 years of life. The critical roadblock to achieving this goal has not been the science as much as the lack of a sufficiently funded and focused effort to bring it to realization. However, a Product Development Partnership to overcome this hurdle could accelerate the time lines towards when control of ETEC disease in DCs is substantially closer.

DNA vaccines against enteric infections

The first DNA vaccines for prevention of infectious diseases were described in 1993 and have since been shown to generate protective humoral and cellular immune responses to numerous infectious agents. For enteric infections, protective immunity has been obtained with DNA vaccines against several enteric viral, bacterial, and parasitic agents. Inoculation of DNA vaccines has generally been by intramuscular injection or by gene gun delivery of vaccine DNA-coated gold microparticles into the skin. Administration of DNA vaccines by the oral route would target the vaccines to enteric mucosal tissues, as well as providing a convenient means for vaccine delivery. Orally administered plasmid DNAs encapsulated in polymeric microparticles or inserted in live bacterial vectors have been effective in animal models for rotavirus DNA vaccines and Listeria monocytogenes DNA vaccines, respectively. Human trials of enteric DNA vaccines have not been initiated, but trials of veterinary vaccines have shown promise.

A plasmid DNA encoding chicken interleukin-6 and Escherichia coli K88 fimbrial protein FaeG stimulates the production of anti-K88 fimbrial antibodies in chickens

Immunization using a plasmid to deliver an encoded protein for expression in situ as the antigen is a promising technology. A plasmid encoding the enterotoxigenic Escherichia coli K88 fimbrial protein FaeG when injected into chickens stimulates the production of antibodies against the fimbrial protein, similar to what has been observed in mice. The efficacy of a genetic adjuvant on fimbrial antibody production was tested by introducing the gene for chicken interleukin-6 in tandem with the faeG gene. Expression of both the fimbrial FaeG protein and chicken interleukin-6 protein was confirmed in COS-M6 cells. Slightly higher antiFaeG antibody titer in chickens was obtained compared with immunization with the plasmid encoding FaeG alone, especially at 10 (19%, P < 0.05) and 12 (27%, P < 0.05) wk, respectively, after the secondary immunization. Elevated antiFaeG antibody titer induced by chicken interleukin-6 and FaeG proteins expressed jointly persisted longer than when induced by FaeG protein alone. This is the first report of an avian cytokine enhancing an immune response, and confirms that coexpression of the antigen and adjuvant from a plasmid delivered by DNA immunization is an effective protocol.

Antibody-inducing properties of a prototype bivalent herpes simplex virus/enterotoxigenic Escherichia coli DNA vaccine

The antibody-inducing properties of a bacterial/viral bivalent DNA vaccine (pRECFA), expressing a peptide composed of N- and C-terminal amino acid sequences of the herpes simplex virus type 1 (HSV-1) glycoprotein D (gD) fused with an inner segment encoding the major structural subunit of enterotoxigenic Escherichia coli (ETEC) CFA/I fimbriae (CFA/I), was evaluated in BALB/c mice following intramuscular immunization. The bivalent pRECFA vaccine elicited serum antibody responses, belonging mainly to the IgG2a subclass, against both CFA/I and HSV gD proteins. pRECFA-elicited antibody responses cross-reacted with homologous and heterologous ETEC fimbrial antigens as well as with type 1 and type 2 HSV gD proteins, which could bind and inactivate intact HSV-2 particles. On the other hand, CFA/I-specific antibodies could bind but did not neutralize the adhesive functions of the bacterial CFA/I fimbriae. In spite of the functional restriction of the antibodies targeting the bacterial antigen, the present evidence suggests that fusion of heterologous peptides to the HSV gD protein represents an alternative for the design of bivalent DNA vaccines able to elicit serum antibody responses.

DNA immunisation against the CFA/I fimbriae of enterotoxigenic Escherichia coli (ETEC)

The CFA/I fimbria promotes the attachment of enterotoxigenic Escherichia coli (ETEC) to the surface of human enterocytes. The generation of a protective immune response requires the induction of antibodies able to block the CFA/I-mediated binding of ETEC to receptors located on the small intestine epithelium or on the surface of human red blood cells, in hemagglutination tests. An eukaryotic expression plasmid, pBLCFA, encoding the CFA/I gene under the control of the human cytomegalovirus major immediate-early promoter was constructed as a prototype DNA vaccine against ETEC. pBLCFA-tranfected BHK-21 cells secreted a peptide cross-reacting with a monoclonal antibody raised against CFA/I subunits. BALB/c mice immunized intramuscularly with one or two doses of purified pBLCFA developed CFA/I-specific serum antibodies for at least 52 weeks, composed predominantly of the IgG1 subclass. pBLCFA-induced antibodies bind mainly to epitopes exposed on the surface of intact CFA/I fimbriae and do not react with immune recessive epitopes found in other ETEC fimbra sharing amino acid homologies with CFA/I. Furthermore, pBLCFA-induced antibodies were able to block the adhesive properties of the CFA/I fimbriae, as evaluated by the ability to inhibit the hemagglutination promoted by CFA/I-expressing ETEC cells. These results suggest that secretion of CFA/I encoded by pBLCFA preserves important conformational epitopes required for the generation of protective antibodies against the adhesive properties of the CFA/I fimbriae and open new perspectives for the development of DNA vaccines against enteric bacterial pathogens.

New vaccine strategies against enterotoxigenic Escherichia coli. II: Enhanced systemic and secreted antibody responses against the CFA/I fimbriae by priming with DNA and boosting with a live recombinant Salmonella vaccine

The induction of systemic (IgG) and mucosal (IgA) antibody responses against the colonization factor I antigen (CFA/I) of enterotoxigenic Escherichia coli (ETEC) was evaluated in mice primed with an intramuscularly delivered CFA/I-encoding DNA vaccine followed by two oral immunizations with a live recombinant Salmonella typhimurium vaccine strain expressing the ETEC antigen. The booster effect induced by the oral immunization was detected two weeks and one year after the administration of the DNA vaccine. The DNA-primed/Salmonella-boosted vaccination regime showed a synergistic effect on the induced CFA/I-specific systemic and secreted antibody levels which could not be attained by either immunization strategy alone. These results suggest that the combined use of DNA vaccines and recombinant Salmonella vaccine strains can be a useful immunization strategy against enteric pathogens.