ESX-5-targeted export of ESAT-6 in BCG combines enhanced immunogenicity & efficacy against murine tuberculosis with low virulence and reduced persistence

Antigen identification strategies and preclinical evaluation models for advancing tuberculosis vaccine development

In its myriad devastating forms, Tuberculosis (TB) has existed for centuries, and humanity is still affected by it. Mycobacterium tuberculosis (M. tuberculosis), the causative agent of TB, was the foremost killer among infectious agents until the COVID-19 pandemic. One of the key healthcare strategies available to reduce the risk of TB is immunization with bacilli Calmette-Guerin (BCG). Although BCG has been widely used to protect against TB, reports show that BCG confers highly variable efficacy (0-80%) against adult pulmonary TB. Unwavering efforts have been made over the past 20 years to develop and evaluate new TB vaccine candidates. The failure of conventional preclinical animal models to fully recapitulate human response to TB, as also seen for the failure of MVA85A in clinical trials, signifies the need to develop better preclinical models for TB vaccine evaluation. In the present review article, we outline various approaches used to identify protective mycobacterial antigens and recent advancements in preclinical models for assessing the efficacy of candidate TB vaccines.

Impaired ESX-3 Induces Bedaquiline Persistence in Mycobacterium abscessus Growing Under Iron-Limited Conditions

ESX-3 is a secretion pathway which is essential for mycobactin-mediated iron acquisition under iron-limited conditions. Although present in all Mycobacterium sp., ESX-3 remains to be elucidated in Mycobacterium abscessus. In the study reported here, impaired ESX-3 seriously restricts the growth of M. abscesses under iron-limited conditions; growth is salvaged by functional ESX-3 or iron supplementation. Notably, impaired ESX-3 does not kill M. abscesses when environmental iron is insufficient but induces persistence to bedaquiline, a diarylquinoline class antibiotic used to treat multidrug-resistant mycobacteria. One potential mechanism contributing to persistence is the iron deficiency due to impaired ESX-3 suppressing succinate dehydrogenase activity, which dysregulates the tricarboxylic acid cycle and inactivates bedaquiline. Experiments conducted here also demonstrate that the regulator, MtrA, can bind ESX-3 and promote the survival of M. abscessus. As such, this study suggests that a novel pathway involving MtrA, ESX-3, iron metabolism, and the TCA cycle contributes to bedaquiline persistence in M. abscesses growing under iron-limited conditions.

The Immunogenicity and Safety of Mycobacterium tuberculosis-mosR-Based Double Deletion Strain in Mice

Mycobacterium tuberculosis (M. tuberculosis) remains a significant global health threat, accounting for ~1.7 million deaths annually. The efficacy of the current vaccine, M. bovis BCG, ranges from 0 to 80% in children and does not prevent adulthood tuberculosis. We explored the immune profile and safety of a live-attenuated M. tuberculosis construct with double deletions of the mosR and echA7 genes, where previously, single mutations were protective against an M. tuberculosis aerosol challenge. Over 32 weeks post-vaccination (WPV), immunized mice with M. tuberculosisΔmosRΔechA7 (double mutant) were sacrificed to evaluate the vaccine persistence, histopathology, and immune responses. Interestingly, despite similar tissue colonization between the vaccine double mutant and wild-type M. tuberculosis, the vaccine construct showed a greater reaction to the ESAT-6, TB.10, and Ag85B antigens with peptide stimulation. Additionally, there was a greater number of antigen-specific CD4 T cells in the vaccine group, accompanied by significant polyfunctional T-cell responses not observed in the other groups. Histologically, mild but widely distributed inflammatory responses were recorded in the livers and lungs of the immunized animals at early timepoints, which turned into organized inflammatory foci via 32WPV, a pathology not observed in BCG-immunized mice. A lower double-mutant dose resulted in significantly less tissue colonization and less tissue inflammation. Overall, the double-mutant vaccine elicited robust immune responses dominated by antigen-specific CD4 T cells, but also triggered tissue damage and vaccine persistence. The findings highlight key features associated with the immunogenicity and safety of the examined vaccine construct that can benefit the future evaluation of other live vaccines.

Repurposing mucosal delivery devices for live attenuated tuberculosis vaccines

Tuberculosis (TB) remains one of the most lethal infectious diseases globally. The only TB vaccine approved by the World Health Organization, Bacille Calmette-Guérin (BCG), protects children against severe and disseminated TB but provides limited protection against pulmonary TB in adults. Although several vaccine candidates have been developed to prevent TB and are undergoing preclinical and clinical testing, BCG remains the gold standard. Currently, BCG is administered as an intradermal injection, particularly in TB endemic countries. However, mounting evidence from experimental animal and human studies indicates that delivering BCG directly into the lungs provides enhanced immune responses and greater protection against TB. Inhalation therapy using handheld delivery devices is used for some diseases and allows the delivery of drugs or vaccines directly into the human respiratory tract. Whether this mode of delivery could also be applicable for live attenuated bacterial vaccines such as BCG or other TB vaccine candidates remains unknown. Here we discuss how two existing inhalation devices, the mucosal atomization device (MAD) syringe, used for influenza vaccines, and the Respimat® Soft Mist™ inhaler, used for chronic obstructive pulmonary disease (COPD) therapy, could be repurposed for mucosal delivery of live attenuated TB vaccines. We also outline the challenges and outstanding research questions that will require further investigations to ensure usefulness of respiratory delivery devices that are cost-effective and accessible to lower- and middle-income TB endemic countries.

Recent Developments in Mycobacteria-Based Live Attenuated Vaccine Candidates for Tuberculosis

Vaccination is an excellent approach to stimulating the host immune response and reducing human morbidity and mortality against microbial infections, such as tuberculosis (TB). Bacillus Calmette-Guerin (BCG) is the most widely administered vaccine in the world and the only vaccine approved by the World Health Organization (WHO) to protect against TB. Although BCG confers "protective" immunity in children against the progression of Mycobacterium tuberculosis (Mtb) infection into active TB, this vaccine is ineffective in protecting adults with active TB manifestations, such as multiple-, extensive-, and total-drug-resistant (MDR/XDR/TDR) cases and the co-existence of TB with immune-compromising health conditions, such as HIV infection or diabetes. Moreover, BCG can cause disease in individuals with HIV infection or other immune compromises. Due to these limitations of BCG, novel strategies are urgently needed to improve global TB control measures. Since live vaccines elicit a broader immune response and do not require an adjuvant, developing recombinant BCG (rBCG) vaccine candidates have received significant attention as a potential replacement for the currently approved BCG vaccine for TB prevention. In this report, we aim to present the latest findings and outstanding questions that we consider worth investigating regarding novel mycobacteria-based live attenuated TB vaccine candidates. We also specifically discuss the important features of two key animal models, mice and rabbits, that are relevant to TB vaccine testing. Our review emphasizes that the development of vaccines that block the reactivation of latent Mtb infection (LTBI) into active TB would have a significant impact in reducing the spread and transmission of Mtb. The results and ideas discussed here are only based on reports from the last five years to keep the focus on recent developments.

BCGΔBCG1419c increased memory CD8+ T cell-associated immunogenicity and mitigated pulmonary inflammation compared with BCG in a model of chronic tuberculosis

Previously, we reported that a hygromycin resistant version of the BCGΔBCG1419c vaccine candidate reduced tuberculosis (TB) disease in BALB/c, C57BL/6, and B6D2F1 mice infected with Mycobacterium tuberculosis (Mtb) H37Rv. Here, the second-generation version of BCGΔBCG1419c (based on BCG Pasteur ATCC 35734, without antibiotic resistance markers, and a complete deletion of BCG1419c) was compared to its parental BCG for immunogenicity and protective efficacy against the Mtb clinical isolate M2 in C57BL/6 mice. Both BCG and BCGΔBCG1419c induced production of IFN-γ, TNF-α, and/or IL-2 by effector memory (CD44⁺CD62L^-), PPD-specific, CD4⁺ T cells, and only BCGΔBCG1419c increased effector memory, PPD-specific CD8⁺ T cell responses in the lungs and spleens compared with unvaccinated mice before challenge. BCGΔBCG1419c increased levels of central memory (CD62L⁺CD44⁺) T CD4⁺ and CD8⁺ cells compared to those of BCG-vaccinated mice. Both BCG strains elicited Th1-biased antigen-specific polyfunctional effector memory CD4⁺/CD8⁺ T cell responses at 10 weeks post-infection, and both vaccines controlled Mtb M2 growth in the lung and spleen. Only BCGΔBCG1419c significantly ameliorated pulmonary inflammation and decreased neutrophil infiltration into the lung compared to BCG-vaccinated and unvaccinated mice. Both BCG strains reduced pulmonary TNF-α, IFN-γ, and IL-10 levels. Taken together, BCGΔBCG1419c increased memory CD8+T cell-associated immunogenicity and mitigated pulmonary inflammation compared with BCG.

After 100 Years of BCG Immunization against Tuberculosis, What Is New and Still Outstanding for This Vaccine?

In 2021, most of the world was reasonably still concerned about the COVID-19 pandemic, how cases were up and down in different countries, how the vaccination campaigns were ongoing, and most people were familiar with the speed with which vaccines against SARS-Co-V2 were developed, analyzed, and started to be applied in an attempt to curb the pandemic. Because of this, it may have somehow passed relatively inadvertently for people outside of the field that the vaccine used to control tuberculosis (TB), Mycobacterium bovis Bacille Calmette-Guérin (BCG), was first applied to humans a century ago. Over these years, BCG has been the vaccine applied to most human beings in the world, despite its known lack of efficacy to fully prevent respiratory TB. Several strategies have been employed in the last 20 years to produce a novel vaccine that would replace, or boost, immunity and protection elicited by BCG. In this work, to avoid potential redundancies with recently published reviews, I only aim to present my current thoughts about some of the latest findings and outstanding questions that I consider worth investigating to help develop a replacement or modified BCG in order to successfully fight TB, based on BCG itself.