Lipoprotein LpqS deficient M. tuberculosis mutant is attenuated for virulence in vivo and shows protective efficacy better than BCG in guinea pigs

Comparative analysis of genomic characteristics and immune response between Mycobacterium tuberculosis strains cultured continuously for 25 years and H37Rv

Tuberculosis (TB) continues to pose a significant global health challenge, emphasizing the critical need for effective preventive measures. Although many studies have tried to develop new attenuated vaccines, there is no effective TB vaccine. In this study, we report a novel attenuated Mycobacterium tuberculosis (M. tb) strain, CHVAC-25, cultured continuously for 25 years in the laboratory. CHVAC-25 exhibited significantly reduced virulence compared to both the virulent H37Rv strain in C57BL/6J and severe combined immunodeficiency disease mice. The comparative genomic analysis identified 93 potential absent genomic segments and 65 single nucleotide polymorphic sites across 47 coding genes. Notably, the deletion mutation of ppsC (Rv2933) involved in phthiocerol dimycocerosate synthesis likely contributes to CHVAC-25 virulence attenuation. Furthermore, the comparative analysis of immune responses between H37Rv- and CHVAC-25-infected macrophages showed that CHVAC-25 triggered a robust upregulation of 173 genes, particularly cytokines crucial for combating M. tb infection. Additionally, the survival of CHVAC-25 was significantly reduced compared to H37Rv in macrophages. These findings reiterate the possibility of obtaining attenuated M. tb strains through prolonged laboratory cultivation, echoing the initial conception of H37Ra nearly a century ago. Additionally, the similarity of CHVAC-25 to genotypes associated with attenuated M. tb vaccine positions it as a promising candidate for TB vaccine development.

Comparative Genomics of Mycobacterium avium Complex Reveals Signatures of Environment-Specific Adaptation and Community Acquisition

Nontuberculous mycobacteria, including those in the Mycobacterium avium complex (MAC), constitute an increasingly urgent threat to global public health. Ubiquitous in soil and water worldwide, MAC members cause a diverse array of infections in humans and animals that are often multidrug resistant, intractable, and deadly. MAC lung disease is of particular concern and is now more prevalent than tuberculosis in many countries, including the United States. Although the clinical importance of these microorganisms continues to expand, our understanding of their genomic diversity is limited, hampering basic and translational studies alike. Here, we leveraged a unique collection of genomes to characterize MAC population structure, gene content, and within-host strain dynamics in unprecedented detail. We found that different MAC species encode distinct suites of biomedically relevant genes, including antibiotic resistance genes and virulence factors, which may influence their distinct clinical manifestations. We observed that M. avium isolates from different sources—human pulmonary infections, human disseminated infections, animals, and natural environments—are readily distinguished by their core and accessory genomes, by their patterns of horizontal gene transfer, and by numerous specific genes, including virulence factors. We identified highly similar MAC strains from distinct patients within and across two geographically distinct clinical cohorts, providing important insights into the reservoirs which seed community acquisition. We also discovered a novel MAC genomospecies in one of these cohorts. Collectively, our results provide key genomic context for these emerging pathogens and will facilitate future exploration of MAC ecology, evolution, and pathogenesis.

IMPORTANCE Members of the Mycobacterium avium complex (MAC), a group of mycobacteria encompassing M. avium and its closest relatives, are omnipresent in natural environments and emerging pathogens of humans and animals. MAC infections are difficult to treat, sometimes fatal, and increasingly common. Here, we used comparative genomics to illuminate key aspects of MAC biology. We found that different MAC species and M. avium isolates from different sources encode distinct suites of clinically relevant genes, including those for virulence and antibiotic resistance. We identified highly similar MAC strains in patients from different states and decades, suggesting community acquisition from dispersed and stable reservoirs, and we discovered a novel MAC species. Our work provides valuable insight into the genomic features underlying these versatile pathogens.

Hydrophobic Mycobacterial Antigens Elicit Polyfunctional T Cells in Mycobacterium bovis Immunized Cattle: Association With Protection Against Challenge?

Bovine tuberculosis (bTB), caused by Mycobacterium bovis, is a chronic disease of cattle with a detrimental impact on food quality and production. Research on bTB vaccines has predominantly been focused on proteinaceous antigens. However, mycobacteria have a thick and intricate lipid outer layer and lipids as well as lipopeptides are important for immune-evasion and virulence. In humans, lipid extracts of M. tuberculosis have been shown to elicit immune responses effective against M. tuberculosis in vitro. Chloroform-methanol extraction (CME) was applied to M. bovis BCG to obtain a hydrophobic antigen extract (CMEbcg) containing lipids and lipopeptides. CMEbcg stimulated IFN-γ⁺IL-2⁺ and IL-17A⁺IL-22⁺ polyfunctional T cells and elicited T cell responses with a Th1 and Th17 cytokine release profile in both M. bovis BCG vaccinated and M. bovis challenged calves. Lipopeptides were shown to be the immunodominant antigens in CMEbcg, stimulating CD4 T cells via MHC class II. CMEbcg expanded T cells killed CMEbcg loaded monocytes and the CMEbcg-specific CD3 T cell proliferative response following M. bovis BCG vaccination was the best predictor for reduced pathology following challenge with M. bovis. Although the high predictive value of CMEbcg-specific immune responses does not confirm a causal relationship with protection against M. bovis challenge, when taking into account the in vitro antimycobacterial phenotype of CMEbcg-specific T cells (e.g. Th1/Th17 cytokine profile), it is indicative that CMEbcg-specific immune responses could play a functional role in immunity against M. bovis. Based on these findings we conclude that lipopeptides of M. bovis are potential novel subunit vaccine candidates and that further studies into the functional characterization of lipopeptide-specific immune responses together with their role in protection against bovine tuberculosis are warranted.

Preclinical Progress of Subunit and Live Attenuated Mycobacterium tuberculosis Vaccines: A Review following the First in Human Efficacy Trial

Tuberculosis (TB) is the global leading cause of death from an infectious agent with approximately 10 million new cases of TB and 1.45 million deaths in 2018. Bacille Calmette-Guérin (BCG) remains the only approved vaccine for Mycobacterium tuberculosis (M. tb, causative agent of TB), however clinical studies have shown BCG has variable effectiveness ranging from 0–80% in adults. With 1.7 billion people latently infected, it is becoming clear that vaccine regimens aimed at both post-exposure and pre-exposure to M. tb will be crucial to end the TB epidemic. The two main strategies to improve or replace BCG are subunit and live attenuated vaccines. However, following the failure of the MVA85A phase IIb trial in 2013, more varied and innovative approaches are being developed. These include recombinant BCG strains, genetically attenuated M. tb and naturally attenuated mycobacteria strains, novel methods of immunogenic antigen discovery including for hypervirulent M. tb strains, improved antigen recognition and delivery strategies, and broader selection of viral vectors. This article reviews preclinical vaccine work in the last 5 years with focus on those tested against M. tb challenge in relevant animal models.

Animal Models of Tuberculosis Vaccine Research: An Important Component in the Fight against Tuberculosis

Tuberculosis (TB), an infectious disease caused by Mycobacterium tuberculosis, is one of the top ten infectious diseases worldwide, and is the leading cause of morbidity from a single infectious agent. M. tuberculosis can cause infection in several species of animals in addition to humans as the natural hosts. Although animal models of TB disease cannot completely simulate the occurrence and development of human TB, they play an important role in studying the pathogenesis, immune responses, and pathological changes as well as for vaccine research. This review summarizes the commonly employed animal models, including mouse, guinea pig, rabbit, rat, goat, cattle, and nonhuman primates, and their characteristics as used in TB vaccine research, and provides a basis for selecting appropriate animal models according to specific research needs. Furthermore, some of the newest animal models used for TB vaccine research (such as humanized animal models, zebrafish, Drosophila, and amoeba) are introduced, and their characteristics and research progress are discussed.

Lipoproteins: Structure, Function, Biosynthesis

The Lpp lipoprotein of Escherichia coli is the first identified protein with a covalently linked lipid. It is chemically bound by its C-terminus to murein (peptidoglycan) and inserts by the lipid at the N-terminus into the outer membrane. As the most abundant protein in E. coli (10⁶ molecules per cell) it plays an important role for the integrity of the cell envelope. Lpp represents the type protein of a large variety of lipoproteins found in Gram-negative and Gram-positive bacteria and in archaea that have in common the lipid structure for anchoring the proteins to membranes but otherwise strongly vary in sequence, structure, and function. Predicted lipoproteins in known prokaryotic genomes comprise 2.7% of all proteins. Lipoproteins are modified by a unique phospholipid pathway and transferred from the cytoplasmic membrane into the outer membrane by a special system. They are involved in protein incorporation into the outer membrane, protein secretion across the cytoplasmic membrane, periplasm and outer membrane, signal transduction, conjugation, cell wall metabolism, antibiotic resistance, biofilm formation, and adhesion to host tissues. They are only found in bacteria and function as signal molecules for the innate immune system of vertebrates, where they cause inflammation and elicit innate and adaptive immune response through Toll-like receptors. This review discusses various aspects of Lpp and other lipoproteins of Gram-negative and Gram-positive bacteria and archaea.

The current status, challenges, and future developments of new tuberculosis vaccines

Mycobacterium tuberculosis complex causes tuberculosis (TB), one of the top 10 causes of death worldwide. TB results in more fatalities than multi-drug resistant (MDR) HIV strain related coinfection. Vaccines play a key role in the prevention and control of infectious diseases. Unfortunately, the only licensed preventive vaccine against TB, bacilli Calmette-Guérin (BCG), is ineffective for prevention of pulmonary TB in adults. Therefore, it is very important to develop novel vaccines for TB prevention and control. This literature review provides an overview of the innate and adaptive immune response during M. tuberculosis infection, and presents current developments and challenges to novel TB vaccines. A comprehensive understanding of vaccines in preclinical and clinical studies provides extensive insight for the development of safer and more efficient vaccines, and may inspire new ideas for TB prevention and treatment.

An attenuated quadruple gene mutant of Mycobacterium tuberculosis imparts protection against tuberculosis in guinea pigs

Previously we had developed a triple gene mutant of Mycobacterium tuberculosis (MtbΔmms) harboring disruption in three genes, namely mptpA, mptpB and sapM. Though vaccination with MtbΔmms strain induced protection in the lungs of guinea pigs, the mutant strain failed to control the hematogenous spread of the challenge strain to the spleen. Additionally, inoculation with MtbΔmms resulted in some pathological damage to the spleens in the early phase of infection. In order to generate a strain that overcomes the pathology caused by MtbΔmms in spleen of guinea pigs and controls dissemination of the challenge strain, MtbΔmms was genetically modified by disrupting bioA gene to generate MtbΔmmsb strain. Further, in vivo attenuation of MtbΔmmsb was evaluated and its protective efficacy was assessed against virulent M. tuberculosis challenge in guinea pigs. MtbΔmmsb mutant strain was highly attenuated for growth and virulence in guinea pigs. Vaccination with MtbΔmmsb mutant generated significant protection in comparison to sham-immunized animals at 4 and 12 weeks post-infection in lungs and spleen of infected animals. However, the protection imparted by MtbΔmmsb was significantly less in comparison to BCG immunized animals. This study indicates the importance of attenuated multiple gene deletion mutants of M. tuberculosis for generating protection against tuberculosis.

Summary: In this study, a mutant of M. tuberculosis with the deletion of four important genes has been evaluated in guinea pigs for its attenuation and protective efficacy against tuberculosis.

Mycobacterium tuberculosis lipoproteins in virulence and immunity - fighting with a double-edged sword

Bacterial lipoproteins are secreted membrane-anchored proteins characterized by a lipobox motif. This lipobox motif directs post-translational modifications at the conserved cysteine through the consecutive action of three enzymes: Lgt, LspA and Lnt, which results in di- or triacylated forms. Lipoproteins are abundant in all bacteria including Mycobacterium tuberculosis and often involved in virulence and immunoregulatory processes. On the one hand, disruption of the biosynthesis pathway of lipoproteins leads to attenuation of M. tuberculosis in vivo, and mycobacteria deficient for certain lipoproteins have been assessed as attenuated live vaccine candidates. On the other hand, several mycobacterial lipoproteins form immunodominant antigens which promote an immune response. Some of these have been explored in DNA or subunit vaccination approaches against tuberculosis. The immune recognition of specific lipoproteins, however, might also benefit long-term survival of M. tuberculosis through immune modulation, while others induce protective responses. Exploiting lipoproteins as vaccines is thus a complex matter which requires deliberative investigation. The dual role of lipoproteins in the immunity to and pathogenicity of mycobacteria is discussed here.

Current perspective in tuberculosis vaccine development for high TB endemic regions

Tuberculosis (TB) continues to be a global epidemic, despite of the availability of Bacillus Calmette Guerin (BCG) vaccine for more than six decades. In an effort to eradicate TB, vaccinologist around the world have made considerable efforts to develop improved vaccine candidates, based on the understanding of BCG failure in developing world and immune response thought to be protective against TB. The present review represents a current perspective on TB vaccination research, including additional research strategies needed for increasing the efficacy of BCG, and for the development of new effective vaccines for high TB endemic regions.