DNA-Launched Alphavirus Replicons Encoding a Fusion of Mycobacterial Antigens Acr and Ag85B Are Immunogenic and Protective in a Murine Model of TB Infection

Application of DNA Replicons in Gene Therapy and Vaccine Development

DNA-based gene therapy and vaccine development has received plenty of attention lately. DNA replicons based on self-replicating RNA viruses such as alphaviruses and flaviviruses have been of particular interest due to the amplification of RNA transcripts leading to enhanced transgene expression in transfected host cells. Moreover, significantly reduced doses of DNA replicons compared to conventional DNA plasmids can elicit equivalent immune responses. DNA replicons have been evaluated in preclinical animal models for cancer immunotherapy and for vaccines against infectious diseases and various cancers. Strong immune responses and tumor regression have been obtained in rodent tumor models. Immunization with DNA replicons has provided robust immune responses and protection against challenges with pathogens and tumor cells. DNA replicon-based COVID-19 vaccines have shown positive results in preclinical animal models.

A peptide-based vaccine ACP derived from antigens of Mycobacterium tuberculosis induced Th1 response but failed to enhance the protective efficacy of BCG in mice

BACKGROUND

Tuberculosis (TB) is a global infectious disease, but there is no ideal vaccine against TB except the Bacille Calmette-Guérin (BCG) vaccine.

METHODS

Herein, 25 candidate peptides were predicted from four antigens of Mycobacterium tuberculosis based on their high-affinity binding capacity for the human leukocyte antigen (HLA) DRB1∗0101. Three T-helper 1 (Th1) immunodominant peptides (Ag85B_12-26, CFP21_12-26, and PPE18_149-163) were identified by ELISPOT assays in the humanized C57BL/6 mice. They resulted in a novel Th1 peptide-based vaccine ACP named by the first letter of the three peptides. In addition, the protective efficacy was evaluated in humanized or wild-type C57BL/6 mice and the humoral and cellular immune responses were confirmed in vitro.

RESULTS

Compared with the PBS group, the ACP vaccinated mice showed slight decreases in colony-forming units (CFUs) and pathological lesions. However, when using it as a booster, the ACP vaccine did not significantly enhance the protective efficacy of BCG in humanized or wild-type mice. Interestingly, we found that ACP vaccination significantly increased the number of interferon-γ positive (IFN-γ⁺) T lymphocytes and the levels of IFN-γ cytokines as well as antibodies. Furthermore, the IL-2 level was significantly higher in humanized mice prime-boosted with BCG and ACP.

CONCLUSIONS

Our results suggested that ACP vaccination could stimulate higher levels of cytokines and antibodies but failed to improve the protective efficacy of BCG in mice, indicating that the secretion level of IFN-γ may not be positively correlated with the protection efficiency of the vaccine. These findings provided important information on the feasibility of a peptide vaccine as a booster for enhancing the protective efficacy of BCG.

Intranasal boosting with MVA encoding secreted mycobacterial proteins Ag85A and ESAT-6 generates strong pulmonary immune responses and protection against M. tuberculosis in mice given BCG as neonates

Bacille-Calmette-Guerin (BCG) has variable efficacy as an adult tuberculosis (TB) vaccine but can reduce the incidence and severity of TB infection in humans. We have engineered modified vaccinia Ankara (MVA) strain vaccine constructs to express the secreted mycobacterial proteins Ag85A and ESAT-6 (MVA-AE) and evaluated their immunogenicity and protective efficacy as mucosal booster vaccines for BCG given subcutaneously in early life. Intranasal delivery of MVA-AE to young adult mice induced CD4+ and CD8+ T cell responses to both Ag85A and ESAT-6 in lung mucosae. These responses were markedly enhanced in mice that had been primed neonatally with BCG prior to intranasal MVA-AE immunization (BCG/MVA-AE), as evidenced by numbers of pulmonary Ag85A-, ESAT-6-, and PPD-specific CD4+ and CD8+ T cells and by their capacity to secrete multiple antimicrobial factors, including IFNγ, IL-2 and IL-17. Moreover, MVA-AE boosting generated multifunctional lung CD4+ T cells responding to ESAT-6, which were not, as expected, detected in control mice given BCG, and elevated Ag85A-specific circulating antibody responses. After aerosol challenge with M. tuberculosis H37Rv (Mtb), the BCG/MVA-AE group had significantly reduced mycobacterial burden in the lungs, compared with either BCG primed mice boosted with control MVA or mice given only BCG. These data indicate that intranasal delivery of MVA-AE can boost BCG-induced Th1 and Th17-based immunity locally in the lungs and improve the protective efficacy of neonatally-administered BCG against M. tuberculosis infection.

Impact of a Plasmid DNA-Based Alphavirus Vaccine on Immunization Efficiency

Alphavirus vectors have been engineered for high-level gene expression relying originally on replication-deficient recombinant particles, more recently designed for plasmid DNA-based administration. As alphavirus-based DNA vectors encode the alphavirus RNA replicon genes, enhanced transgene expression in comparison to conventional DNA plasmids is achieved. Immunization studies with alphavirus-based DNA plasmids have elicited specific antibody production, have generated tumor regression and protection against challenges with infectious agents and tumor cells in various animal models. A limited number of clinical trials have been conducted with alphavirus DNA vectors. Compared to conventional plasmid DNA-based immunization, alphavirus DNA vectors required 1000-fold less DNA to elicit similar immune responses in rodents.

Plasmid DNA-based Alphavirus Vaccines

Alphaviruses have been engineered as vectors for high-level transgene expression. Originally, alphavirus-based vectors were applied as recombinant replication-deficient particles, subjected to expression studies in mammalian and non-mammalian cell lines, primary cell cultures, and in vivo. However, vector engineering has expanded the application range to plasmid DNA-based delivery and expression. Immunization studies with DNA-based alphavirus vectors have demonstrated tumor regression and protection against challenges with infectious agents and tumor cells in animal tumor models. The presence of the RNA replicon genes responsible for extensive RNA replication in the RNA/DNA layered alphavirus vectors provides superior transgene expression in comparison to conventional plasmid DNA-based expression. Immunization with alphavirus DNA vectors revealed that 1000-fold less DNA was required to elicit similar immune responses compared to conventional plasmid DNA. In addition to DNA-based delivery, immunization with recombinant alphavirus particles and RNA replicons has demonstrated efficacy in providing protection against lethal challenges by infectious agents and tumor cells.

Expression of nuclear factor, erythroid 2-like 2-mediated genes differentiates tuberculosis

During infection and host defense, nuclear factor, erythroid 2-like 2 (Nrf2) dependent signaling is an efficient antioxidant defensive mechanism used by host cells to control the destructive effects of reactive oxygen species. This allows for effective defense responses against microbes while minimizing oxidative injury to the host cell itself. As a central regulator of antioxidant genes, Nrf2 has gained great attention in its pivotal role in infection, especially in tuberculosis (TB), the top infectious disease killer worldwide. To elucidate the genes potentially regulated by Nrf2 in TB, we conducted a meta-analysis on published gene expression datasets. Firstly, we compared the global gene expression profiles between control and Nrf2-deficient human cells. The differentially expressed genes were deemed as "Nrf2-mediated genes". Next, the whole blood gene expression pattern of TB patients was compared with that of healthy controls, pneumonia patients, and lung cancer patients. We found that the genes deregulated in TB significantly overlap with the Nrf2-mediated genes. Based on the intersection of Nrf2-mediated and TB-regulated genes, we identified an Nrf2-mediated 17-gene signature, which reflects a cluster of gene ontology terms highly related to TB physiology. We demonstrated that the 17-gene signature can be used to distinguish TB patients from healthy controls and patients with latent TB infection, pneumonia, or lung cancer. Also, the Nrf2-mediated gene signature can be used as an indicator of the anti-TB therapeutic response. More importantly, we confirmed that the predictive power of the Nrf2-mediated 17-gene signature is significantly better than the random gene sets selected from the human transcriptome. Also, the 17-gene signature performs even better than the random gene signatures selected from TB-associated genes. Our study confirms the central role of Nrf2 in TB pathogenesis and provides a novel and useful diagnostic method to differentiate TB patients from other human subjects.