Potentiation of antigen-specific, Th1 immune responses by multiple DNA vaccination with an ovalbumin/interferon-gamma hybrid construct

Potentiating the Cross-Reactive IFN-γ T Cell and Polyfunctional T Cell Responses by Heterologous GX-19N DNA Booster in Mice Primed with Either a COVID-19 mRNA Vaccine or Inactivated Vaccine

Waning vaccine-induced immunity, coupled with the emergence of SARS-CoV-2 variants, has inspired the widespread implementation of COVID-19 booster vaccinations. Here, we evaluated the potential of the GX-19N DNA vaccine as a heterologous booster to enhance the protective immune response to SARS-CoV-2 in mice primed with either an inactivated virus particle (VP) or an mRNA vaccine. We found that in the VP-primed condition, GX-19N enhanced the response of both vaccine-specific antibodies and cross-reactive T Cells to the SARS-CoV-2 variant of concern (VOC), compared to the homologous VP vaccine prime-boost. Under the mRNA-primed condition, GX-19N induced higher vaccine-induced T Cell responses but lower antibody responses than the homologous mRNA vaccine prime-boost. Furthermore, the heterologous GX-19N boost induced higher S-specific polyfunctional CD4⁺ and CD8⁺ T cell responses than the homologous VP or mRNA prime-boost vaccinations. Our results provide new insights into booster vaccination strategies for the management of novel COVID-19 variants.

Immunization with a chimera consisting of the B subunit of Shiga toxin type 2 and brucella lumazine synthase confers total protection against Shiga toxins in mice

The striking feature of enterohemorrhagic Escherichia coli (EHEC) infections is the production of Shiga toxins (Stx) implicated in the development of the life-threatening hemolytic uremic syndrome. Despite the magnitude of the social impact of EHEC infections, no licensed vaccine or effective therapy is available for human use. One of the biggest challenges is to develop an effective and safe immunogen to ensure nontoxicity, as well as a strong input to the immune system to induce long-lasting, high-affinity Abs with anti-Stx-neutralizing capacity. The enzyme lumazine synthase from Brucella spp. (BLS) is a highly stable dimer of pentamers and a scaffold with enormous plasticity on which to display foreign Ags. Taking into account the advantages of BLS and the potential capacity of the B subunit of Stx2 to induce Abs that prevent Stx2 toxicity by blocking its entrance into the host cells, we engineered a new immunogen by inserting the B subunit of Stx2 at the amino termini of BLS. The resulting chimera demonstrated a strong capacity to induce a long-lasting humoral immune response in mice. The chimera induced Abs with high neutralizing capacity for Stx2 and its variants. Moreover, immunized mice were completely protected against i.v. Stx2 challenge, and weaned mice receiving an oral challenge with EHEC were completely protected by the transference of immune sera. We conclude that this novel immunogen represents a promising candidate for vaccine or Ab development with preventive or therapeutic ends, for use in hemolytic uremic syndrome-endemic areas or during future outbreaks caused by pathogenic strains of Stx-producing E. coli.

Keratinocytes function as accessory cells for presentation of endogenous antigen expressed in the epidermis

The precise contribution(s) of skin dendritic cells (DCs) to immune responses in the skin has not been well delineated. We developed an intradermal (i.d.) injection model in which CD8+ T (OT-I) cells that express ovalbumin (OVA) peptide-specific TCRs (Valpha2/Vbeta5) are delivered directly to the dermis of transgenic (Tg) mice expressing OVA in the epidermis. After i.d. injection, these mice reliably develop skin graft-versus-host disease (GVHD) by day 7. To determine the relative contribution of Langerhans cells (LCs) to the ensuing GVHD-like reaction, we generated K14-OVA x Langerin-diphtheria-toxin-receptor (Langerin-DTR) Tg mice to allow conditional ablation of LCs in the epidermis. To delineate the role of dermal DCs (dDCs) in the reaction, we also generated K14-OVA Tg chimeras using beta(2)-microglobulin-deficient (beta(2)m) congenic donor bone marrow cells. Dermal DCs in these mice cannot present OVA to autoreactive T cells (OT-I cells), whereas the LCs are antigen presentation-competent. Unexpectedly, OT-I cell injection into diphtheria toxin (DT)-treated beta(2)m --> K14-OVA x Langerin-DTR Tg mice resulted in skin GVHD. Thus, in vivo, both LC and dDC appear to be dispensable for the induction of keratinocyte-directed, CD8-mediated effector immune responses. Furthermore and surprisingly, OVA-expressing epidermal cells depleted of LCs that could not initiate allogeneic epidermal lymphocyte reactions activated naive OT-I cells in vitro. These results indicate that keratinocytes may function as accessory cells competent to prime naive skin-reactive T cells.JID JOURNAL CLUB ARTICLE: For questions, answers, and open discussion about this article, please go to http://network.nature.com/group/jidclub.

Induction of GVHD-Like Skin Disease by Passively Transferred CD8+ T-Cell Receptor Transgenic T Cells into Keratin 14-Ovalbumin Transgenic Mice

To understand the mechanisms involved in immunological tolerance to skin-associated antigens, we have developed transgenic (Tg) mice that express a model self-antigen, membrane-bound chicken ovalbumin (OVA), under the control of a keratin 14 (K14) promoter. K14-mOVA Tg mice express OVA mRNA in the epidermis, and appear normal. K14-mOVA Tg mice failed to mount T cell and delayed type hypersensitivity reactions to OVA, suggesting that the Tg mice were tolerant to OVA. Skin dendritic cells, including Langerhans cells, may contribute to the tolerance induction because migratory skin DC derived from K14-mOVA efficiently activated CD8(+) T cells from OVA-specific T-cell receptor (Va2/Vb5) Tg (OT-I) mice. OT-I cells expanded and accumulated in skin-draining lymph nodes after intravenous injected into K14-mOVA mice and exhibited activation markers. Graft-versus-host disease-like skin lesions appeared in K14-mOVA mice by day 7 after injection of OT-I cells. These studies demonstrate that K14-mOVA Tg mice are susceptible to an autoimmunelike skin disease induced by passively transferred naïve CD8(+) OVA T-cell receptor Tg T cells, and serve as a good model for understanding self-tolerance and for the investigation of the pathogenesis, treatment and potential prevention of cell-mediated autoimmune reactions in skin.

Development of DNA vaccines against hemolytic-uremic syndrome in a murine model

Shiga toxin type 2 (Stx2) produced by Escherichia coli O:157H7 can cause hemolytic-uremic syndrome in children, a disease for which there is neither a vaccine nor an effective treatment. This toxin consists of an enzymatically active A subunit and a pentameric B subunit responsible for the toxin binding to host cells, and also found to be immunogenic in rabbits. In this study we developed eukaryotic plasmids expressing the B subunit gene of Stx2 (pStx2B) and the B subunit plus the gene coding for the A subunit with an active-site deletion (pStx2 Delta A). Transfection of eukaryotic cells with these plasmids produced proteins of the expected molecular weight which reacted with specific monoclonal antibodies. Newborn and adult BALB/c mice immunized with two intramuscular injections of each plasmid, either alone or together with the same vector expressing the granulocyte and monocyte colony-stimulating factor (pGM-CSF), elicited a specific Th1-biased humoral response. The effect of pGM-CSF as an adjuvant plasmid was particularly notable in newborn mice and in pStx2B-vaccinated adult mice. Stx2-neutralizing activity, evaluated in vitro on VERO cell monolayers, correlated with in vivo protection. This is the first report using plasmids to induce a neutralizing humoral immune response against the Stx2.

Protection against plague following immunisation with microencapsulated V antigen is reduced by co-encapsulation with IFN-gamma or IL-4, but not IL-6

We have investigated intranasal delivery of novel vaccines for plague, based on poly-L-lactide (PLLA) microencapsulated recombinant V antigen (rV) of Yersinia pestis. Microspheres containing rV alone or co-encapsulated with the cytokines IFN-gamma, IL-4 or IL-6 were administered in a two-dose regimen and antibody responses and protective efficacy were monitored. All treatment groups stimulated high rV-specific antibody titres in serum, predominantly of the IgG1 isotype, which were maintained over several months. There was evidence of both IgG and IgA responses in lung samples from all groups. Formulations based on rV antigen alone or rV co-encapsulated with IL-6 provided complete protection against systemic challenge with Y. pestis strain GB; however protective efficacy was impaired by co-encapsulating either IFN-gamma or IL-4 with rV.

Efficient priming of CD4+ and CD8+ T cells by DNA vaccination depends on appropriate targeting of sufficient levels of immunologically relevant antigen to appropriate processing pathways

The initial cellular events and interactions that occur following DNA immunization are likely to be key to determining the character and magnitude of the resulting immune response, and as such, a better understanding of these events could ultimately lead to the design of more effective pathogen-appropriate DNA vaccines. Therefore, we have used a variety of sensitive cell-based techniques to study the induction of adaptive immunity in vivo. We examined the efficacy of induction of Ag-specific CD4(+) and CD8(+) T cell responses in vivo by the adoptive transfer of fluorescently labeled Ag-specific TCR transgenic T cells and have demonstrated how such approaches can be used to study the effect of simple DNA construct manipulations on immunological priming. OVA-specific CD8(+) and CD4(+) T cells were activated and divided in vivo following immunization with DNA constructs that targeted OVA expression to different subcellular locations; however, the kinetics and degree of cell proliferation were dependent on the cellular location of the expressed protein. DNA vectors encoding cell-associated OVA resulted in greater CD8(+) T cell division compared with other forms of OVA. In contrast, soluble secreted OVA targeted to the classical secretory pathway enhanced division of CD4(+) T cells. Furthermore, the inclusion of mammalian introns to enhance protein expression increased the ability of poorly immunogenic forms of Ag to activate naive T cells, indicating that not only the location, but also the amount of Ag expression, is important for efficient T cell priming following DNA injection.

Dendritic cells transduced with protein antigens induce cytotoxic lymphocytes and elicit antitumor immunity

Dendritic cell (DC)-based vaccines are being developed for treatment of patients with cancer, in part because DC are potent inducers of CD8(+) CTL. DC MHC class I:antigenic peptide complexes that are required for CTL elicitation are most often generated by incubating DC with peptides or by transfecting (or transducing) DC with cDNAs (or viral vectors) that encode protein Ags. The former approach is feasible when MHC class I Ags and relevant peptides are known. The latter approach may be hampered by inefficient DC transfection (transduction) and/or difficulties associated with preparation and use of viral vectors. Herein we demonstrate that a bacterial recombinant model tumor-associated Ag (OVA) that contains the HIV TAT protein transduction domain (PTD) was readily engineered and purified, efficiently transduced murine lymphocytes and DC, and was processed by proteasomes for MHC class I-restricted presentation to CTL. In addition, PTD-containing rOVA was processed and presented by DC to CD4 T cells as efficiently as native OVA or rOVA lacking the PTD. PTD-OVA-transduced DC induced CTL in vivo in a Th cell-independent fashion and vaccinated against OVA-expressing tumors. In contrast, rOVA lacking the PTD did not enter the DC MHC class I presentation pathway and DC treated with this protein did not prime OVA-specific CTL in vivo. Treatment of mice harboring clinically apparent OVA-expressing tumors with PTD-OVA-transduced DC resulted in tumor regression in some animals. This straightforward vaccination strategy may translate into DC-based treatments for patients with cancer and other serious diseases.

DNA vaccines: future strategies and relevance to intracellular pathogens

Increasing awareness of microbial threat has rekindled interest in the great potential of vaccines for controlling infectious diseases. The fact that diseases caused by intracellular pathogens cannot be overcome by chemotherapy alone has increased our interest in the generation of highly efficacious novel vaccines. Vaccines have proven their efficacy, as the immunoprotection they induce appears to be mediated by long-lived humoral immune responses. However, there are no consistently effective vaccines available against diseases such as tuberculosis and HIV, and other infections caused by intracellular pathogens, which are predominantly controlled by T lymphocytes. This review describes the T-cell populations and the type of immunity that should be activated by successful DNA vaccines against intracellular pathogens. It further discusses the parameters that need to be fulfilled by protective T-cell Ag. We then discuss future approaches for DNA vaccination against diseases in which cell-mediated immune responses are essential for providing protection.

Genetic adjuvants for DNA vaccines

The relatively low efficacy of DNA vaccines in inducing immune responses, especially in large animal species and humans, has impaired their practical use. Despite considerable effort expended on improving DNA vaccine delivery, only minute amounts of Ag are available for immune induction following DNA vaccination. Two complementary strategies have been used to improve and modulate the immune response induced by DNA vaccines: (i) supplementing DNA vaccines with plasmids encoding cytokines and (ii) targeting the Ag encoded by DNA vaccine through genetically fusing the Ag to molecules binding cell surface receptors. This paper reviews recent progress in these two areas and possible mechanisms responsible for the observed effects.

DNA encoding the attachment (G) or fusion (F) protein of respiratory syncytial virus induces protection in the absence of pulmonary inflammation

Significant protection against respiratory syncytial virus (RSV) infection was induced in mice vaccinated intramuscularly (i.m.) with DNA encoding the F or G protein of RSV. The amounts of IgG1 of IgG2a antibodies in mice immunized with DNA-G alone were similar. However, the antibody response in mice co-immunized with DNA-G and DNA encoding IL-4 (DNA-IL-4) was strongly biased towards IgG1. In contrast, the antibody response in mice co-immunized with DNA-G and DNA-IL-2, -IL-12 or-IFN-gamma was biased towards IgG2a. Mice vaccinated with DNA-F either alone or in combination with DNA encoding cytokines developed a predominant RSV-specific IgG2a response, which was most pronounced in mice co-immunized with DNA-F and DNA-IL-12 or -IFN-gamma. Vaccinated mice developed only a slightly enhanced pulmonary inflammatory response following RSV challenge. More significantly, and in contrast to mice scarified with recombinant vaccinia virus expressing the G protein, mice vaccinated i.m. with DNA-G did not develop pulmonary eosinophilia, even when the immune response was biased towards a Th2 response by co-administration of DNA-IL-4.

DNA vaccines: immunology, application, and optimization*

The development and widespread use of vaccines against infectious agents have been a great triumph of medical science. One reason for the success of currently available vaccines is that they are capable of inducing long-lived antibody responses, which are the principal agents of immune protection against most viruses and bacteria. Despite these successes, vaccination against intracellular organisms that require cell-mediated immunity, such as the agents of tuberculosis, malaria, leishmaniasis, and human immunodeficiency virus infection, are either not available or not uniformly effective. Owing to the substantial morbidity and mortality associated with these diseases worldwide, an understanding of the mechanisms involved in generating long-lived cellular immune responses has tremendous practical importance. For these reasons, a new form of vaccination, using DNA that contains the gene for the antigen of interest, is under intensive investigation, because it can engender both humoral and cellular immune responses. This review focuses on the mechanisms by which DNA vaccines elicit immune responses. In addition, a list of potential applications in a variety of preclinical models is provided.

Antigen-specific cytotoxicity and cell number of adoptively transferred T cells are efficiently maintained in vivo by re-stimulation with an antigen/interleukin-2 fusion protein

In order to maintain in vivo anti-tumor efficacy of antigen (Ag)-specific T cells in adoptive immunotherapy for a prolonged period, we constructed a fusion protein (OVA/IL-2) containing ovalbumin (OVA) as a model tumor Ag, co-valently linked to murine interleukin-2 (IL-2). The OVA/IL-2 protein produced in a baculovirus expression system displayed potent IL-2 bio-activity. Immunization with the OVA/IL-2 protein after adoptive transfer of OVA-specific T cells maintained the OVA-specific cytotoxicity and cell number of adoptively transferred T cells long term in vivo, while a simple mixture of recombinant OVA (rOVA) and rIL-2 did not. The response was dependent on the injection doses and times of the OVA/IL-2 protein. Furthermore, weekly re-stimulation of adoptively transferred OVA-specific T cells with the OVA/IL-2 protein cured 70% of tumor-bearing mice. In contrast, re-stimulation with a mixture of rOVA and rIL-2 could not significantly prolong the survival period of tumor-bearing mice. These studies suggest that the co-valent linkage between IL-2 and antigen confines the effect of IL-2 to antigen-specific T cells, leading to efficient maintenance of the anti-tumor activity of adoptively transferred T cells.

An in vivo comparison of bacillus Calmette-Guérin (BCG) and cytokine-secreting BCG vaccines

A recombinant bacillus Calmette-Guérin (BCG) vaccine has been developed, which constitutively secretes interleukin (IL)-2. Groups of deer were immunized with either normal BCG (Pasteur 1173 P2 strain) or recombinant BCG (rBCG/IL-2) and their immune responses were monitored over 3 months. Animals gained weight over this period and showed no signs of adverse reactions to either vaccine. Lymphocyte transformation responses did not differ significantly between the two groups. No antibody that was specific for BCG was detected in any animal. Intradermal skin-test responses to BCG antigens showed that the rBCG/IL-2 induced a smaller delayed-type hypersensitivity response than the normal BCG. Cytokine transcription was determined by reverse transcription-polymerase chain reaction (RT-PCR). While IL-2 and interferon-gamma (IFN-gamma) levels did not differ significantly between the two groups, the level of IL-4 was found to be lower in the group given rBCG/IL-2. This resulted in a strong interferon-gamma:IL-4 ratio, suggesting a skewing of the immune response towards a Type 1 response. The rate at which the vaccine was eliminated from the host was the same regardless of whether BCG or rBCG was used. At autopsy (3 months after vaccination) 99.99% of the organisms had been eliminated. The small number of organisms isolated from the draining lymph node of animals given rBCG/IL-2 were grown in antibiotic-containing media. They were shown to still contain the shuttle plasmid and to secrete biologically active IL-2, indicating that the plasmid was stably maintained despite the host's immune response and in the absence of antibiotic selection.

Vaccination with an ovalbumin/interleukin-4 fusion DNA efficiently induces Th2 cell-mediated immune responses in an ovalbumin-specific manner

To more effectively drive immune responses toward antigen-specific T helper type 2 (Th2) cell-mediated responses, we constructed a mammalian expression vector (pOVA/IL4) carrying a fused gene in which the ovalbumin (OVA) cDNA was covalently linked to murine interleukin-4 (IL-4) cDNA. A biologically active OVA/IL4 protein was expressed by the transfected COS cells with the pOVA/IL4 DNA, as demonstrated by Western blotting and cytokine bioassay. Intramuscular injection of BALB/c mice with the pOVA/IL4 DNA increased both the production of OVA-specific IL-4 by CD4+ T cells and the ratio of anti-OVA IgG1 to anti-OVA IgG2a isotypes, while the injection with the pOVA DNA alone, or with the mixture of the pOVA and pIL4 DNA did no or little increase. Furthermore, the OVA-specific, Th2 cell-mediated immune responses were significantly enhanced by multiple injections with the pOVA/IL4 DNA. These studies indicate that the direct linkage of an OVA gene to an IL-4 gene in the expression plasmid confines the effects of IL-4 to the OVA-specific cells, efficiently driving the immune response toward OVA-specific, Th2 cell-mediated responses.