DNA vaccines for bacteria and viruses

Cysteine proteases as potential antigens in antiparasitic DNA vaccines

Cysteine proteases in parasites are potent inducers of vertebrate host immune responses and may under certain circumstances take part in the pathogen's immune evasion strategies. These capacities place these parasite molecules as interesting candidate antigens in antiparasitic vaccines for use in vertebrates. Parasite cysteine proteases are able to skew the Th1/Th2 profile in mammals towards a response which allows sustainable parasite burdens in the host. DNA vaccines are also able to skew the Th1/Th2 profile by different administration techniques and the use of cysteine proteases in these genetic immunizations open perspectives for manipulation of the host immune response towards higher protection.

Graphical representation and mathematical characterization of protein sequences and applications to viral proteins

Graphical representation and numerical characterization (GRANCH) of nucleotide and protein sequences is a new field that is showing a lot of promise in analysis of such sequences. While formulation and applications of GRANCH techniques for DNA/RNA sequences started just over a decade ago, analyses of protein sequences by these techniques are of more recent origin. The emphasis is still on developing the underlying technique, but significant results have been achieved in using these methods for protein phylogeny, mass spectral data of proteins and protein serum profiles in parasites, toxicoproteomics, determination of different indices for use in QSAR studies, among others. We briefly mention these in this chapter, with some details on protein phylogeny and viral diseases. In particular, we cover a systematic method developed in GRANCH to determine conserved surface exposed peptide segments in selected viral proteins that can be used for drug and vaccine targeting. The new GRANCH techniques and applications for DNAs and proteins are covered briefly to provide an overview to this nascent field.

MG7 mimotope-based DNA vaccination for gastric cancer

Gastric cancer is still one of the leading causes of cancer-related death worldwide. Prevention and treatment of gastric cancer through vaccination has been difficult owing to lack of a specific target and poor immunity. A number of vaccination strategies have been used to augment immune responses against gastric cancer and some progress has been made. In a series of studies, the authors have focused on gastric cancer vaccination approaches based on MG7 mimotopes, which are mimicry epitopes selected from phage-displayed oligopeptide libraries with a gastric cancer cell-specific monoclonal antibody, MG7-Ab. Strategies employed in these studies include viral or plasmid vectors in combination with carrier sequence or unmethylated CpG with synthetic peptides in nanoemulsion. The results demonstrated that MG7 mimotopes could effectively and specifically induce both cellular and humoral immune reactions and in vivo antitumor responses. In particular, a four-MG7 mimotope DNA vaccine was found to elicit much stronger antitumor immune responses in mice compared with its single-mimotope counterpart. These encouraging findings might pave the way for the development of novel MG7 antigen-based vaccination approaches for human gastric cancer. The review also discusses other immune-enhancing vaccination strategies for gastric cancer.

A retroviral DNA vaccine vector

A versatile DNA vaccine (pdIV3) was constructed by replacing the integrase, vif, vpx, and vpr genes of a pathogenic simian immunodeficiency virus (SIV) molecular clone with a linker containing unique cloning sites. The 5' long terminal repeat (LTR) is truncated and transcription is controlled by a cytomegalovirus (CMV) promoter. The construct expresses Gag and Env in vitro and noninfectious virus particles are produced from transfected cells. The ability of pdIV3 to promote cellular and humoral immune responses, along with the flexibility of the linker design to allow insertion of immunostimulatory genes in future constructs, makes this a useful base vector for immunization against primate lentiviruses. We present the construction of a retroviral plasmid designed to serve as a template for the development of safe and effective vaccines against primate immunodeficiency retroviruses. This vaccine component should facilitate the simultaneous induction of cellular and humoral immune responses that protect primates against infection with SIV and human immunodeficiency virus (HIV) and the development of acquired immune deficiency syndrome (AIDS). This plasmid could induce the appropriate immune response required to attack both cell-free and cell-associated viruses. The lack of infectivity, the inability to integrate, and the SIV origin make this construct a safe alternative to attenuated vaccines based on HIV. In addition, we intend to develop this construct as an immunotherapeutic approach to lower the viremia in AIDS patients.

Simian immunodeficiency virus (SIV) gag DNA-vaccinated rhesus monkeys develop secondary cytotoxic T-lymphocyte responses and control viral replication after pathogenic SIV infection

The potential contribution of a plasmid DNA construct to vaccine-elicited protective immunity was explored in the simian immunodeficiency virus (SIV)/macaque model of AIDS. Making use of soluble major histocompatibility class I/peptide tetramers and peptide-specific killing assays to monitor CD8(+) T-lymphocyte responses to a dominant SIV Gag epitope in genetically selected rhesus monkeys, a codon-optimized SIV gag DNA vaccine construct was shown to elicit a high-frequency SIV-specific cytotoxic T-lymphocyte (CTL) response. This CTL response was demonstrable in both peripheral blood and lymph node lymphocytes. Following an intravenous challenge with the highly pathogenic viral isolate SIVsm E660, these vaccinated monkeys developed a secondary CTL response that arose with more rapid kinetics and reached a higher frequency than did the postchallenge CTL response in control plasmid-vaccinated monkeys. While peak plasma SIV RNA levels were comparable in the experimentally and control-vaccinated monkeys during the period of primary infection, the gag plasmid DNA-vaccinated monkeys demonstrated better containment of viral replication by 50 days following SIV challenge. These findings indicate that a plasmid DNA vaccine can elicit SIV-specific CTL responses in rhesus monkeys, and this vaccine-elicited immunity can facilitate the generation of secondary CTL responses and control of viral replication following a pathogenic SIV challenge. These observations suggest that plasmid DNA may prove a useful component of a human immunodeficiency virus type 1 vaccine.

Genetic immunization with lung-targeting macroaggregated polyethyleneimine-albumin conjugates elicits combined systemic and mucosal immune responses

Genetic immunization is a novel form of vaccination in which transgenes are delivered into the host to produce the foreign protein within host cells. Although systemic immune responses have been relatively easy to induce by genetic immunization, the induction of regional and mucosal immunity has often been more challenging. To address the problem of eliciting mucosal immunity in the lung, we utilized macroaggregated albumin to target plasmid DNA to the lung. Macroaggregated albumin is trapped in the lung after i. v. injection, and it is routinely used in radiolabeled form as an imaging modality to evaluate pulmonary blood flow. To couple DNA to this targeting agent, polyethyleneimine (a polycation that binds DNA and enhances transfection) was conjugated to serum albumin, and the conjugate was aggregated by heating to produce particles of 25-100 microm. The resulting particles bound plasmid DNA avidly, and when injected i.v. in mice, the particles distributed in the peripheral lung tissue in the alveolar interstitium. Particle-bound luciferase plasmid transfected a variety of cell lines in vitro, and after i.v. injection, gene expression was detected exclusively in the lung. Using human growth hormone as the encoded foreign Ag for immunization, i.v. injection of the particle-bound plasmid elicited both pulmonary mucosal and systemic immune responses, whereas naked DNA injected either i.v. or i.m. elicited only systemic responses. Thus, particle-bound plasmid DNA may have utility for genetic immunization by intravascular delivery to the lung and potentially to other organs and tissues.

Protective CD4+ and CD8+ T cells against influenza virus induced by vaccination with nucleoprotein DNA

DNA vaccination is an effective means of eliciting both humoral and cellular immunity, including cytotoxic T lymphocytes (CTL). Using an influenza virus model, we previously demonstrated that injection of DNA encoding influenza virus nucleoprotein (NP) induced major histocompatibility complex class I-restricted CTL and cross-strain protection from lethal virus challenge in mice (J. B. Ulmer et al., Science 259:1745-1749, 1993). In the present study, we have characterized in more detail the cellular immune responses induced by NP DNA, which included robust lymphoproliferation and Th1-type cytokine secretion (high levels of gamma interferon and interleukin-2 [IL-2], with little IL-4 or IL-10) in response to antigen-specific restimulation of splenocytes in vitro. These responses were mediated by CD4+ T cells, as shown by in vitro depletion of T-cell subsets. Taken together, these results indicate that immunization with NP DNA primes both cytolytic CD8+ T cells and cytokine-secreting CD4+ T cells. Further, we demonstrate by adoptive transfer and in vivo depletion of T-cell subsets that both of these types of T cells act as effectors in protective immunity against influenza virus challenge conferred by NP DNA.

DNA vaccines expressing either the GP or NP genes of Ebola virus protect mice from lethal challenge

DNA vaccines expressing the envelope glycoprotein (GP) or nucleocapsid protein (NP) genes of Ebola virus were evaluated in adult, immunocompetent mice. The vaccines were delivered into the skin by particle bombardment of DNA-coated gold beads with the Powderject-XR gene gun. Both vaccines elicited antibody responses as measured by ELISA and elicited cytotoxic T cell responses as measured by chromium release assays. From one to four vaccinations with 0.5 microgram of the GP DNA vaccine resulted in a dose-dependent protection from Ebola virus challenge. Maximal protection (78% survival) was achieved after four vaccinations. Mice were completely protected with a priming dose of 0.5 microgram of GP DNA followed by three or four subsequent vaccinations with 1.5 micrograms of DNA. Partial protection could be observed for at least 9 months after three immunizations with 0.5 microgram of the GP DNA vaccine. Comparing the GP and NP vaccines indicated that approximately the same level of protection could be achieved with either vaccine.

Naked DNA vaccines expressing the prM and E genes of Russian spring summer encephalitis virus and Central European encephalitis virus protect mice from homologous and heterologous challenge

Naked DNA vaccines expressing the prM and E genes of two tick-borne flaviviruses, Russian spring summer encephalitis (RSSE) virus and Central European encephalitis (CEE) virus were evaluated in mice. The vaccines were administered by particle bombardment of DNA-coated gold beads by Accell gene gun inoculation. Two immunizations of 0.5 to 1 microg of RSSE or CEE constructs/dose, delivered at 4-week intervals, elicited cross-reactive antibodies detectable by enzyme-linked immunosorbent assay and high-titer neutralizing antibodies to CEE virus. Cross-challenge experiments demonstrated that either vaccine induced protective immunity to homologous or heterologous RSSE or CEE virus challenge. The absence of antibody titer increases after challenge and the presence of antibodies to E and prM, but not NS1, both before and after challenge suggest that the vaccines prevented productive replication of the challenge virus. One vaccination with 0.5 microg of CEE virus DNA provided protective immunity for at least 2 months, and two vaccinations protected mice from challenge with CEE virus for at least 6 months.

DNA vaccines

Observations in the early 1990s that plasmid DNA could directly transfect animal cells in vivo sparked exploration of the use of DNA plasmids to induce immune responses by direct injection into animals of DNA encoding antigenic proteins. This method, termed DNA immunization, now has been used to elicit protective antibody and cell-mediated immune responses in a wide variety of preclinical animal models for viral, bacterial, and parasitic diseases. DNA vaccination is particularly useful for the induction of cytotoxic T cells. This review summarizes current knowledge on the vectors, immune responses, immunological mechanisms, safety considerations, and potential for further application of this novel method of immunization.