Immunogenicity and protective efficacy of DNA vaccines encoding secreted and non-secreted forms of Mycobacterium tuberculosis Ag85A

Enhanced immunogenicity to Mycobacterium tuberculosis by vaccination with an alphavirus plasmid replicon expressing antigen 85A

The immunogenicity of a plasmid DNA vaccine incorporating Sindbis virus RNA replicase functions (pSINCP) and expressing antigen 85A (Ag85A) from Mycobacterium tuberculosis was compared with a conventional plasmid DNA vector encoding Ag85A. pSINCP-85A was highly immunogenic in mice and gave enhanced long-term protection against M. tuberculosis compared with the conventional vector.

Improved tuberculosis DNA vaccines by formulation in cationic lipids

Mice were vaccinated with plasmid DNA (pDNA) encoding antigen 85A (Ag85A), Ag85B, or PstS-3 from Mycobacterium tuberculosis either in saline or formulated for intramuscular injections in VC1052:DPyPE (aminopropyl-dimethyl-myristoleyloxy-propanaminium bromide-diphytanoylphosphatidyl-ethanolamine) (Vaxfectin; Vical, Inc., San Diego, Calif.) or for intranasal instillations in GAP-DLRIE:DOPE (aminopropyl-dimethyl-bis-dodecyloxy-propanaminium bromide-dioleoylphosphatidyl-ethanolamine). These two novel cationic and neutral colipid formulations were previously reported to be effective adjuvants for pDNA-induced antibody responses. The levels of Ag85-specific total immunoglobulin G (IgG) and IgG isotypes were all increased 3- to 10-fold by formulation of pDNA in Vaxfectin. The level of production of splenic T-cell-derived Th1-type cytokines (interleukin-2 and gamma interferon) in response to purified Ag85 and to synthetic peptides spanning the entire Ag85A protein was also significantly higher in animals vaccinated with pDNA formulated in Vaxfectin. Cytolytic T-lymphocyte responses generated by pDNA encoding phosphate-binding protein PstS-3 in Vaxfectin were better sustained over time than were those generated by PstS-3 DNA in saline. Intranasal immunization with Ag85A DNA in saline was completely ineffective, whereas administration in GAP-DLRIE:DOPE induced a positive Th1-type cytokine response; however, the extent of the latter response was clearly lower than that obtained following intramuscular immunization with the same DNA dose. Combined intramuscular and intranasal administrations in cationic lipids resulted in stronger immune responses in the spleen and, more importantly, in the lungs as well. Finally, formulation in Vaxfectin increased the protective efficacy of the Ag85B DNA vaccine, as measured by reduced relative light unit counts and CFU counts in the spleen and lungs from mice challenged with bioluminescent M. tuberculosis H37Rv. These results may be of importance for future clinical use of DNA vaccines in humans.

Role of signal sequence in vaccine-induced protection against experimental coccidioidomycosis

The vaccine efficacy of the gene sequence encoding the signal peptide of the antigen known as antigen 2 or proline-rich antigen (Ag2/PRA), an immunodominant antigen present in the cell wall of the fungal pathogen Coccidioides immitis, was investigated in a murine model of coccidioidomycosis. Expression plasmids for Ag2/PRA(1-18) DNA (signal sequence), Ag2/PRA(19-194) DNA (lacking the signal sequence), and Ag2/PRA(1-194) DNA (full length) were inserted in the pVR1012 vector, and the constructs were used to vaccinate the highly susceptible BALB/c mouse strain. Immunization with the signal gene sequence significantly reduced the fungal burden in the lungs and spleens of mice 12 days after intraperitoneal challenge with a lethal dose of 2,500 C. immitis arthroconidia, to a level comparable to the protection induced in mice immunized with the full-length Ag2/PRA(1-194) DNA. The Ag2/PRA(19-194) gene protected mice but to a significantly lower level than the signal sequence or the full-length Ag2 gene. The immunizing capacity of Ag2/PRA(1-18) was not attributable to a nonspecific immunostimulatory effect of DNA, as evidenced by the fact that mice immunized with a frameshift mutation of Ag2/PRA(1-18) were not protected against challenge. Furthermore, a synthetic peptide corresponding to the translated sequence of Ag2/PRA(1-18) DNA protected mice, albeit at a lower level than the Ag2/PRA(1-18) DNA vaccine. The protection induced with the signal gene vaccine correlated with the production of gamma interferon when splenocytes from Ag2/PRA(1-18)-immunized mice were stimulated with recombinant full-length Ag2 and was not associated with the production of anti-Coccidioides immunoglobulin G antibody. This is the first study to establish that a signal peptide sequence alone, administered as a gene vaccine or synthetic peptide, can induce protective immunity against a microbial pathogen.

Vaccination of guinea pigs with DNA encoding the mycobacterial antigen MPB83 influences pulmonary pathology but not hematogenous spread following aerogenic infection with Mycobacterium bovis

Protection of cattle against bovine tuberculosis by vaccination could be an important control strategy in countries where there is persistent Mycobacterium bovis infection in wildlife and in developing countries where it is not economical to implement a tuberculin test and slaughter control program. The main aim of such a vaccination strategy would be to reduce transmission of infection by reducing the lung pathology caused by infection and preventing seeding of the organism to organs from which M. bovis could be excreted. Recent reports of successful DNA vaccination against Mycobacterium tuberculosis in small-animal models have suggested that DNA vaccines act by reducing lung pathology without sensitizing animals to tuberculin testing. We therefore evaluated the ability of vaccines consisting of DNA encoding the mycobacterial antigens MPB83 and 85A to reduce lung pathology and prevent hematogenous spread in guinea pigs challenged with a low dose of aerosolized M. bovis. Vaccination with MPB83 DNA reduced the severity of pulmonary lesions, as assessed by histopathology, and resembled M. bovis BCG vaccination in this respect. However, unlike BCG vaccination, MPB83 DNA vaccination did not protect challenged guinea pigs from hematogenous spread of organisms to the spleen. In contrast, vaccination with antigen 85A DNA, a promising DNA vaccine for human tuberculosis, had no measurable protective effect against infection with M. bovis.

Immunization with a DNA vaccine expressing a truncated form of varicella zoster virus glycoprotein E

The gE glycoprotein of varicella zoster virus (VZV) is involved with cell entry and it is the most abundant glycoprotein produced in VZV-infected cells. It is also the first glycoprotein to be recognized by the immune system and induces neutralizing antibodies and cellular immunity. We have shown previously that immunization with a DNA vaccine encoding full length gE induces high antibody titres in BALB/c mice. In this study, we engineered a truncated form of gE to facilitate secretion of the glycoprotein, which is thought to increase the quantity of antigen available for B cells to mount an immune response. This hypothesis was tested by using inverse PCR mutagenesis (IPCRM) to engineer a mutated form of gE that was secreted from the cell. This construct was then evaluated as a potential DNA vaccine. Following immunization studies, the magnitude of the immune response induced with the mutant form of gE was found to be similar to that induced by membrane bound protein. This finding suggests that, in the case of VZV, a DNA vaccine expressing a secreted protein has no advantage over one expressing a membrane bound protein. However, mice immunized with the truncated form of gE (gED) displayed responses favouring IgG1 (Th2) in comparison with mice immunized with the full length gE construct, which generated an IgG2a (Th1) response. This observation indicates that immunization with a truncated form of a gene may induce immune modulation, a phenomenon that should be taken into account for the design of vaccines.

Tuberculosis DNA vaccines

DNA vaccines have been the subject of intense investigation for the past 10 y, during which time several tuberculosis (TB) DNA vaccines have been shown to confer protective immunity in animal models. So far, proof of principle for priming of immune responses by a naked DNA vaccine (malaria) has been demonstrated in humans, but potency remains a significant limitation. However, new DNA vaccine formulations and delivery systems are being developed with markedly improved potency in animal models. Therefore, there is a clear path to the human clinical testing of TB DNA vaccines.

Protective efficacy of a DNA vaccine encoding antigen 85A from Mycobacterium bovis BCG against Buruli ulcer

Buruli ulcer, caused by Mycobacterium ulcerans, is characterized by deep and necrotizing skin lesions, mostly on the arms and legs. Together with tuberculosis and leprosy, this mycobacterial disease has become a major health problem in tropical and subtropical regions, particularly in central and western Africa. No specific vaccine is available for Buruli ulcer. There is, however, evidence in the literature that suggests a cross-reactive protective role of the tuberculosis vaccine M. bovis BCG. To identify potential mechanisms for this cross-protection, we identified and characterized the M. ulcerans homologue of the important protective mycobacterial antigen 85 (Ag85A) from BCG. The homologue is well conserved in M. ulcerans, showing 84.1% amino acid sequence identity and 91% conserved residues compared to the sequence from BCG. This antigen was sufficiently conserved to allow cross-reactive protection, as demonstrated by the ability of M. ulcerans- infected mice to exhibit strong cellular immune responses to both BCG and its purified Ag85 complex. To further address the mechanism of cross-reactive protection, we demonstrate here that prior vaccination with either BCG or plasmid DNA encoding BCG Ag85A is capable of significantly reducing the bacterial load in the footpads of M. ulcerans- infected mice, as determined by Ziehl-Neelsen staining and by actual counting of CFU on 7H11 Middlebrook agar. Together, the results reported here support the potential of a cross-protective Ag85-based future vaccine against tuberculosis, Buruli ulcer, and leprosy.

Relative potency of cellular and humoral immune responses induced by DNA vaccination

DNA vaccines can prime broad-based immune responses in small animal models. In the present study, we sought to evaluate the relative ability of DNA vaccines to induce humoral and cellular immune responses. Using a DNA vaccine encoding HIV gag in mice, we observed that CD8+ T cell responses were primed more readily than were antibody responses, particularly at low doses of DNA. These CD8+ T cell responses were detected in spleen cells, as well as at local sites such as the lung and draining lymph nodes. The potency of the HIV gag DNA vaccine used was sufficient to prime strong CTL responses in macaques, but only low to undetectable antibody responses. Therefore, DNA vaccines appear able to prime strong, broad CTL but only modest antibody responses. These results may have implications on the development of vaccines against infectious diseases where both CTL and antibody responses are desired, such as HIV.

Improved immunogenicity and protective efficacy of a tuberculosis DNA vaccine encoding Ag85 by protein boosting

C57BL/6 mice were vaccinated with plasmid DNA encoding Ag85 from Mycobacterium tuberculosis, with Ag85 protein in adjuvant, or with a combined DNA prime-protein boost regimen. While DNA immunization, as previously described, induced robust Th1-type cytokine responses, protein-in-adjuvant vaccination elicited very poor cytokine responses, which were 10-fold lower than those observed with DNA immunization alone. Injection of Ag85 DNA-primed mice with 30 to 100 microg of purified Ag85 protein in adjuvant increased the interleukin-2 and gamma interferon (IFN-gamma) response in spleen two- to fourfold. Further, intracellular cytokine analysis by flow cytometry also showed an increase in IFN-gamma-producing CD4(+) T cells in DNA-primed-protein-boosted animals, compared to those that received only the DNA vaccination. Moreover, these responses appeared to be better sustained over time. Antibodies were readily produced by all three methods of immunization but were exclusively of the immunoglobulin G1 (IgG1) isotype following protein immunization in adjuvant and preferentially of the IgG2a isotype following DNA and DNA prime-protein boost vaccination. Finally, protein boosting increased the protective efficacy of the DNA vaccine against an intravenous M. tuberculosis H37Rv challenge infection, as measured by CFU or relative light unit counts in lungs 1 and 2 months after infection. The capacity of exogenously given protein to boost the DNA-primed vaccination effect underlines the dominant role of Th1-type CD4(+) helper T cells in mediating protection.

Human CD8+ CTL specific for the mycobacterial major secreted antigen 85A

The role of CD8(+) CTL in protection against tuberculosis in human disease is unclear. In this study, we stimulated the peripheral blood mononuclear cells of bacillus Calmette-Guérin (BCG)-vaccinated individuals with live Mycobacterium bovis BCG bacilli to establish short-term cell lines and then purified the CD8(+) T cells. A highly sensitive enzyme-linked immunospot (ELISPOT) assay for single cell IFN-gamma release was used to screen CD8(+) T cells with overlapping peptides spanning the mycobacterial major secreted protein, Ag85A. Three peptides consistently induced a high frequency of IFN-gamma responsive CD8(+) T cells, and two HLA-A*0201 binding motifs, P(48-56) and P(242-250), were revealed within the core sequences. CD8(+) T cells responding to the 9-mer epitopes were visualized within fresh blood by ELISPOT using free peptide or by binding of HLA-A*0201 tetrameric complexes. The class I-restricted CD8(+) T cells were potent CTL effector cells that efficiently lysed an HLA-A2-matched monocyte cell line pulsed with peptide as well as autologous macrophages infected with Mycobacterium tuberculosis or recombinant vaccinia virus expressing the whole Ag85A protein. Tetramer assays revealed a 6-fold higher frequency of peptide-specific T cells than IFN-gamma ELISPOT assays, indicating functional heterogeneity within the CD8(+) T cell population. These results demonstrate a previously unrecognized, MHC class I-restricted, CD8(+) CTL response to a major secreted Ag of mycobacteria and supports the use of Ag85A as a candidate vaccine against tuberculosis.

Vaccinia expression of Mycobacterium tuberculosis-secreted proteins: tissue plasminogen activator signal sequence enhances expression and immunogenicity of M. tuberculosis Ag85

There is increasing evidence to implicate a role for CD8(+) T cells in protective immunity against tuberculosis. Recombinant vaccinia (rVV) expressing Mycobacterium tuberculosis (MTB) proteins can be used both as tools to dissect CD8(+) T-cell responses and, in attenuated form, as candidate vaccines capable of inducing a balanced CD4(+)/CD8(+) T-cell response. A panel of rVV was constructed to express four immunodominant secreted proteins of MTB: 85A, 85B and 85C and ESAT-6. A parallel group of rVV was constructed to include the heterologous eukaryotic tissue plasminogen activator (tPA) signal sequence to assess if this would enhance expression and immunogenicity. Clear expression was obtained for 85A, 85B and ESAT-6 and the addition of tPA resulted in N-glycosylation and a 4-10-fold increase in expression. Female C57BL/6 mice were immunised using the rVV-Ag85 constructs, and interleukin-2 and gamma-interferon were assayed using a co-culture of immune splenocytes and recall antigen. There was a marked increase in cytokine production in mice immunised with the tPA-containing constructs. We report the first data demonstrating enhanced immunogenicity of rVV using a tPA signal sequence, which has significant implications for future vaccine design.

Tuberculosis DNA vaccine encoding Ag85A is immunogenic and protective when administered by intramuscular needle injection but not by epidermal gene gun bombardment

Immunogenicity and protective efficacy of a DNA vaccine encoding Ag85A from Mycobacterium tuberculosis were compared in BALB/c and C57BL (B6 and B10) mice immunized by intramuscular (i.m.) needle injection or epidermal gene gun (gg) bombardment. In BALB/c mice, gg immunization could induce elevated antibody and cytotoxic T lymphocyte responses with plasmid doses 50-fold lower than those required for i.m. immunization. Interleukin-2 (IL-2) and gamma interferon (IFN-gamma) secretion, however, was much lower in gg-immunized than in i.m.-immunized BALB/c mice. On the other hand, C57BL mice reacted only very weakly to gg immunization, whereas elevated Ag85A-specific antibody, IL-2, and IFN-gamma responses (significantly higher than in BALB/c mice) were detected following vaccination by the i.m. route. Antibody isotypes were indicative of Th2 activation following gg injection of BALB/c and of Th1 activation following i.m. injection of C57BL mice. Finally, C57BL but not BALB/c mice were protected by i.m. Ag85A DNA immunization against intravenous M. tuberculosis challenge, as measured by reduced numbers of CFU in spleen and lungs, compared to animals vaccinated with control DNA. Gene gun immunization was not effective in either BALB/c or C57BL mice. These results indicate that i.m. DNA vaccination is the method of choice for the induction of protective Th1 type immune responses with the Ag85A tuberculosis DNA vaccine.