Immune responses following neonatal DNA vaccination are long-lived, abundant, and qualitatively similar to those induced by conventional immunization

HER2 peptide-specific CD8(+) T cells are proportionally detectable long after multiple DNA vaccinations

We prepared a plasmid encoding 147 amino acid residues from the N terminus of c-erbB-2/HER2/neu (HER2), which included both a cytotoxic T lymphocyte (CTL) epitope (HER2p63) and a helper epitope (HER2p1), using the mammalian expression vector pCAGGS-New (pCAGGS147HER2). In a parallel analysis with a Tetramer assay and CTL assay, good specificity and sensitivity of a quantitative enzyme-linked immunospot (ELISPOT) assay to detect functional HER2p63-specific CD8(+) T cells were demonstrated after intramuscular immunization of pCAGGS147HER2. In an ELISPOT assay for HER2p63, spots of IFN gamma-producing cells were first detected 10 days after the first immunization, and additional immunizations increased the number of spots. HER2p63-specific CD8(+) T cells were detected over a period of more than 10 months after the last immunization. In hosts receiving more than three immunizations, surprisingly high numbers of specific CD8(+) T cells were persistently detectable. HER2 protein-specific antibodies of IgG class with dominance of IgG2a remain detectable 6 months after single or multiple immunizations. The antibodies however, were not reactive with cell surface HER2 antigens. Total suppression of tumor growth was observed when syngeneic HER2(+) tumor cells (2 x 10(6)) were injected subcutaneously 14 days after a single immunization with pCAGGS147HER2. Furthermore, the number of pulmonary metastases decreased significantly when DNA vaccination was initiated on the day of, or 3 days after, intravenous injection (1 x 10(6) cells).

Development of new vaccines against dengue fever and Japanese encephalitis

Mosquito-borne dengue (DEN) and Japanese encephalitis (JE) viruses are the leading causes of arthropod-transmitted viral disease in humans. A licensed tetravalent vaccine that provides effective, long-term immunity against all four serotypes of DEN virus is needed, but is currently unavailable. Improvements to currently available JE vaccines are also needed. Past and recent strategies for the development of new DEN and JE vaccines include inactivated and live attenuated viruses, engineered viruses and chimeric viruses derived from infectious cDNA clones of DEN or JE virus, recombinant poxviruses, recombinant baculoviruses, protein expression in Escherichia coli, and naked DNA vaccines. This report summarizes some of the recent developments in DEN and JE vaccinology, particularly vaccine strategies that involve live attenuated viruses, engineered viruses derived from infectious cDNA clones, and naked DNA vaccines.

Development of a vaccine against murine cytomegalovirus (MCMV), consisting of plasmid DNA and formalin-inactivated MCMV, that provides long-term, complete protection against viral replication

We previously demonstrated that immunization of mice with plasmid DNAs (pDNAs) expressing the murine cytomegalovirus (MCMV) genes IE1-pp89 and M84 provided synergistic protection against sublethal viral challenge, while immunization with plasmids expressing putative virion proteins provided no or inconsistent protection. In this report, we sought to augment protection by increasing the breadth of the immune response. We identified another MCMV gene (m04 encoding gp34) that provided strong and consistent protection against viral replication in the spleen. We also found that immunization with a DNA pool containing 10 MCMV genes that individually were nonprotective elicited reproducible protection against low to intermediate doses of challenge virus. Moreover, inclusion of these plasmids into a mixture with gp34, pp89, and M84 DNAs provided even greater protection than did coimmunization with pp89 and M84. The highest level of protection was achieved by immunization of mice with the pool of 13 pDNAs, followed by formalin-inactivated MCMV (FI-MCMV). Immunization with FI-MCMV elicited neutralizing antibodies against salivary gland-derived MCMV, and of greatest importance, mice immunized with both the combined pDNA pool and FI-MCMV had undetectable levels of virus in the spleen and salivary glands after challenge. Intracellular cytokine staining of splenocytes from pDNA- and FI-MCMV-immunized mice showed that pDNA immunization elicited high levels of pp89- and M83-specific CD8(+) T cells, whereas both pDNA and FI-MCMV immunizations generated strong CD8(+)-T-cell responses against virion-associated antigens. Taken together, these results show that immunization with pDNA and inactivated virus provides strong antibody and cell-mediated immunity against CMV infection.

Genetic immunization of neonates

The vaccination of neonates is generally difficult due to immaturity of the immune system, higher susceptibility to tolerance and potential negative interference of maternal antibodies. Studies carried out in rodents and non-human primates showed that plasmid vaccines expressing microbial antigens, rather than inducing tolerance, triggered significant humoral and cellular immunity with a Th1 component. The ability of bacterial CpG motifs to activate immature antigen-presenting cells is critical for the neonatal immunogenicity of DNA vaccines. In addition, the endogenous production of antigen subsequent to transfection of antigen-presenting cells may explain the lack of inhibition by maternal antibodies of cellular responses. Together, these features make the plasmid vaccines an appealing strategy to prime immune responses against foreign pathogens, during early life. In combination with subsequent boosting using conventional vaccines, DNA vaccine-based regimens may provide a qualitatively superior immunity against microbes. Thorough understanding of immunomodulatory properties of plasmid-vectors may extend their use for early prophylaxis of inflammatory disorders.

Strong CD8 T-cell responses following coimmunization with plasmids expressing the dominant pp89 and subdominant M84 antigens of murine cytomegalovirus correlate with long-term protection against subsequent viral challenge

We previously showed that intradermal immunization with plasmids expressing the murine cytomegalovirus (MCMV) protein IE1-pp89 or M84 protects against viral challenge and that coimmunization has a synergistic protective effect (C. S. Morello, L. D. Cranmer, and D. H. Spector, J. Virol. 74:3696-3708, 2000). Using an intracellular gamma interferon cytokine staining assay, we have now characterized the CD8+ T-cell response after DNA immunization with pp89, M84, or pp89 plus M84. The pp89- and M84-specific CD8+ T-cell responses peaked rapidly after three immunizations. DNA immunization and MCMV infection generated similar levels of pp89-specific CD8+ T cells. In contrast, a significantly higher level of M84-specific CD8+ T cells was elicited by DNA immunization than by MCMV infection. Fusion of ubiquitin to pp89 enhanced the CD8+ T-cell response only under conditions where vaccination was suboptimal. Three immunizations with either pp89, M84, or pp89 plus M84 DNA also provided significant protection against MCMV infection for at least 6 months, with the best protection produced by coimmunization. A substantial percentage of antigen-specific CD8+ T cells remained detectable, and they responded rapidly to the MCMV challenge. These results underscore the importance of considering antigens that do not appear to be highly immunogenic during infection as DNA vaccine candidates.

Oral DNA vaccination in utero induces mucosal immunity and immune memory in the neonate

Infectious diseases are responsible for a significant number of deaths during the first weeks of life. Some of the salient pathogens include HSV, HIV, hepatitis B virus, group B streptococcus, Haemophilus sp., and Chlamydia sp. The vertical transmission of many of these pathogens significantly increases the risk of neonatal infection. We recently reported that oral DNA immunization in utero induced high serum Ab titers and cell-mediated immunity in fetal lambs. In this study, we demonstrate immune memory and mucosal immunity in newborn lambs following oral DNA immunization of the fetus. A single oral exposure in utero to plasmid DNA encoding a truncated form of glycoprotein D of bovine herpesvirus-1 induced detectable immune responses in 80% (12 of 15) of newborn lambs. There was no evidence for the induction of immune tolerance in nonresponding lambs. Responding lambs displayed both systemic and mucosal immune responses and reduced virus shedding following intranasal challenge. Furthermore, strong anamnestic responses were evident for at least 3 mo after birth. The efficacy of in utero oral DNA immunization was further demonstrated with the hepatitis B surface Ag, and protective serum Ab titers occurred in 75% of immunized lambs. Thus, the present investigation confirms that oral DNA immunization in utero can induce both mucosal and systemic immune responses in the neonate and that this immunity has the potential to prevent vertical disease transmission.

Protection induced by intramuscular immunization with DNA vaccines of pseudorabies in mice, rabbits and piglets

Glycoprotein gene gB, gC and gD of pseudorabies virus (PrV) strain Ea, which was isolated locally in Wuhan, were cloned from the viral genome DNA and expressed in vitro controlled by the major immediately-early promotor/enhancer of HCMV. In the presented paper, Balb/c mice, rabbits and piglets were vaccinated intramuscularly two times at 2-week interval with those eukaryotic expression plasmid pcDB, pcDC and pcDD, respectively. The animals injected with pcDB, pcDC, pcDD or mix DNA developed anti-PrV antibodies. Neutralizing antibody titers obtained 2-5log(2), 2 weeks after the second vaccination. Cellular immune responses were also detected by lymphoproliferation assay and cytotoxic T lymphocyte (CTL) activity assay in all groups vaccinated with DNA. Immune responses elicited by DNA vaccines provided protections with different degrees against lethal dose PrV challenge. In mice, protections induced by pcDC, pcDD or mix DNA were 100%, similar to that by inactivated vaccine. Protections were more than 50% induced by pcDC, pcDD or mix DNA in rabbits. Protections induced by pcDB were the lowest among DNA immunization in mice or rabbits. However, pcDB could elicit the higher cellular responses in rabbits or piglets. In piglets, body temperatures of animals injected with pcDB, pcDC, pcDD or mix DNA did not change significantly after challenge with 2x10(5) pfu of PrV strain Ea, and the means daily growth post-challenge of those animals were higher than those injected with inactivated vaccine or parental plasmid. Neither DNA vaccines nor inactivated vaccine could prevent or delay virus excretion after challenge. Our experiments in experimental animals and natural hosts suggested the efficiency and potential application of DNA vaccines for pseudorabies in pigs.

Safety and immunogenicity in neonatal mice of a hyperattenuated Listeria vaccine directed against human immunodeficiency virus

CD8(+) T cells are a major component of the adaptive response of a host to infections by viruses and other intracellular pathogenic agents. However, because of the intrinsic immaturity of the immune system of neonatal animals, neonates are highly sensitive to a variety of pathogens and may be unable to respond in a protective manner. Here we explore whether a hyperattenuated strain of Listeria monocytogenes that can be used as a live vaccine vector in adults is safe and able to induce an effective response in neonates. We answer both questions affirmatively.

Priming by DNA immunization augments T-cell responses induced by modified live bovine herpesvirus vaccine

DNA vaccines have several advantages over conventional vaccines. One of the most important characteristics is the presentation of antigen via both MHC class I and class II receptors. Although this generally results in strong T-cell responses, antibody production and protection achieved by DNA immunization are unfortunately not always adequate. In contrast, modified live virus (MLV) vaccines usually induce adequate antibody and moderate cellular responses, whereas killed vaccines tend to elicit weak immune responses in general. A DNA prime-MLV boost regimen should result in enhanced cellular immunity and possibly improved antibody production. To test this hypothesis, plasmids encoding bovine herpesvirus-1 (BHV-1) glycoproteins B and D were delivered by gene gun to the genital mucosa of cattle prior to immunization with modified live BHV-1 vaccine. The immune responses induced were compared to those of an MLV-vaccinated group and a negative control group. Although significantly enhanced T-cell responses were induced by priming with the DNA vaccine, there was no increase in antibody titres. Similar levels of protection were induced by the MLV vaccine alone and the DNA prime and MLV boost regimen, which suggests that there is no correlation between the induction of T-cell responses and protection from BHV-1 challenge.

Immunity to viral haemorrhagic septicaemia (VHS) following DNA vaccination of rainbow trout at an early life-stage

Rainbow trout fry of average weight 0.5 g were vaccinated against viral haemorrhagic septicaemia (VHS) by intramuscular injection of 1 microg of plasmid DNA encoding the VHS virus glycoprotein gene. Challenge with a lethal dose of virus at two different time points, 9 and 71 days post-vaccination respectively, revealed that a highly protective and lasting immunity was established shortly after vaccination, in accordance with earlier experiments with larger fish. The defence mechanisms activated by the DNA vaccine are thus functional at an early life-stage in rainbow trout.

Neonatal and early life vaccinology

Preclinical and human vaccine studies indicate that, although neonatal immunisation does not generally lead to rapid and strong antibody responses, it may result in an efficient immunological priming, which can serve as an excellent basis for future responses. The apparent impairment of CD4 and CD8 T-cell function in early life seems to result from suboptimal antigen-presenting cells-T cell interactions, which can be overcome by use of specific adjuvants or delivery systems. Although persistence of maternal antibodies may limit infant antibody responses, induction of T-cell responses largely remain unaffected by these passively transferred antibodies. Thus, neonatal priming and early boosting with vaccine formulations optimised for sufficient early life immunogenicity and maximal safety profiles, could allow better control of the huge infectious disease burden in early life.

The regulation of DNA vaccines

The framework for regulating DNA vaccines has been in place since the first clinical trial was initiated in the mid-1990s. American and European regulatory guidance has evolved on the basis of insights provided by ongoing preclinical and clinical studies. These include analyses of the safety of DNA vaccines in normal volunteers, and recent data concerning the tissue distribution, persistence, and integration potential of DNA plasmids.

Antiviral CD8+ T cell responses in neonatal mice: susceptibility to polyoma virus-induced tumors is associated with lack of cytotoxic function by viral antigen-specific T cells

Polyoma virus is a potent oncogenic pathogen when inoculated into newborn mice of particular H-2(k) strains. Using D(k) tetramers containing the dominant antipolyoma CD8(+) T cell epitope, middle T protein (MT)389-397, and intracellular interferon gamma staining, we enumerated MT389-specific CD8(+) T cells in infected neonates having opposite susceptibilities to polyoma virus-induced tumors. In resistant mice, MT389-specific CD8(+) T cells dramatically expanded during acute infection in neonates to a frequency rivaling that in adults; furthermore, in both neonatal and adult mice, this antipolyoma CD8(+) T cell response exhibited nearly identical T cell receptor (TCR) functional avidities and TCR functional fingerprints. Susceptible mice mounted an MT389-specific CD8(+) T cell response of only fourfold lower magnitude than resistant mice; but, in clear contrast to resistant mice, these CD8(+) T cells lacked ex vivo MT389-specific cytotoxic activity. However, MT389-specific CD8(+) T cells in resistant and susceptible mice expressed similar TCR avidities, perforin levels, and surface type O-glycan levels indicative of mature CD8(+) T cell effectors. Upon in vitro restimulation with infected antigen-presenting cells, CD8(+) T cells from acutely infected susceptible neonates acquired strong MT389-specific cytotoxicity. These findings indicate that polyoma-specific CD8(+) T cells are armed with, but restrained from deploying, their cytotoxic effector function in mice susceptible to polyoma virus tumorigenesis.

DNA immunization and central nervous system viral infection

This chapter discusses the virus infections of the central nervous system (CNS) and DNA vaccines. Mild central nervous system (CNS) symptoms, such as headache and drowsiness, can result from systemically elevated cytokine levels and therefore are common in many virus infections, even in the absence of the infection of the CNS. CNS infection is quite unusual and is initiated either as a result of the viremia or, more rarely, as a result of neural spread. The poliovirus infects the anterior horn motor neurons of the spinal cord, causing poliomyelitis, the disease for which the virus is named. DNA vaccination is a relatively new entrant in the vaccine sweepstakes, but is viewed with optimism, for a number of reasons. DNA vaccines encoding the nucleoprotein from lymphocytic choriomeningitis virus can confer protection against the normally lethal intracranial challenge. In rabies, in a mouse model, immunization with plasmids encoding the rabies glycoprotein conferred complete protection against subsequent viral challenge. Several virus-induced CNS diseases may be explained by their triggering of autoimmunity. Experimental autoimmune encephalomyelitis is a well-characterized CNS disease induced by the administration of certain CNS proteins.

Vaccination with empty plasmid DNA or CpG oligonucleotide inhibits diabetes in nonobese diabetic mice: modulation of spontaneous 60-kDa heat shock protein autoimmunity

Nonobese diabetic (NOD) mice develop insulitis and diabetes through a process involving autoimmunity to the 60-kDa heat shock protein (HSP60). Treatment of NOD mice with HSP60 or with peptides derived from HSP60 inhibits this diabetogenic process. We now report that NOD diabetes can be inhibited by vaccination with a DNA construct encoding human HSP60, with the pcDNA3 empty vector, or with an oligonucleotide containing the CpG motif. Prevention of diabetes was associated with a decrease in the degree of insulitis and with down-regulation of spontaneous proliferative T cell responses to HSP60 and its peptide p277. Moreover, both the pcDNA3 vector and the CpG oligonucleotide induced specific Abs, primarily of the IgG2b isotype, to HSP60 and p277, and not to other islet Ags (glutamic acid decarboxylase or insulin) or to an unrelated recombinant Ag expressed in bacteria (GST). The IgG2b isotype of the specific Abs together with the decrease in T cell proliferative responses indicate a shift of the autoimmune process to a Th2 type in treated mice. These results suggest that immunostimulation by bacterial DNA motifs can modulate spontaneous HSP60 autoimmunity and inhibit NOD diabetes.

Direct ex vivo kinetic and phenotypic analyses of CD8(+) T-cell responses induced by DNA immunization

CD8(+) T-cell responses can be induced by DNA immunization, but little is known about the kinetics of these responses in vivo in the absence of restimulation or how soon protective immunity is conferred by a DNA vaccine. It is also unclear if CD8(+) T cells primed by DNA vaccines express the vigorous effector functions characteristic of cells primed by natural infection or by immunization with a recombinant live virus vaccine. To address these issues, we have used the sensitive technique of intracellular cytokine staining to carry out direct ex vivo kinetic and phenotypic analyses of antigen-specific CD8(+) T cells present in the spleens of mice at various times after (i) a single intramuscular administration of a plasmid expressing the nucleoprotein (NP) gene from lymphocytic choriomeningitis virus (LCMV), (ii) infection by a recombinant vaccinia virus carrying the same protein (vvNP), or (iii) LCMV infection. In addition, we have evaluated the rapidity with which protective immunity against both lethal and sublethal LCMV infections is achieved following DNA vaccination. The CD8(+) T-cell response in DNA-vaccinated mice was slightly delayed compared to LCMV or vvNP vaccinees, peaking at 15 days postimmunization. Interestingly, the percentage of antigen-specific CD8(+) T cells present in the spleen at day 15 and later time points was similar to that observed following vvNP infection. T cells primed by DNA vaccination or by infection exhibited similar cytokine expression profiles and had similar avidities for an immunodominant cytotoxic T lymphocyte epitope peptide, implying that the responses induced by DNA vaccination differ quantitatively but not qualitatively from those induced by live virus infection. Surprisingly, protection from both lethal and sublethal LCMV infections was conferred within 1 week of DNA vaccination, well before the peak of the CD8(+) T-cell response.