Mycobacterium tuberculosis virulence: insights and impact on vaccine development

Protective RNA nanovaccines against Mycobacterium avium subspecies hominissuis

The induction of an effective immune response is critical for the success of mRNA-based therapeutics. Here, we developed a nanoadjuvant system compromised of Quil-A and DOTAP (dioleoyl 3 trimethylammonium propane), hence named QTAP, for the efficient delivery of mRNA vaccine constructs into cells. Electron microscopy indicated that the complexation of mRNA with QTAP forms nanoparticles with an average size of 75 nm and which have ~90% encapsulation efficiency. The incorporation of pseudouridine-modified mRNA resulted in higher transfection efficiency and protein translation with low cytotoxicity than unmodified mRNA. When QTAP-mRNA or QTAP alone transfected macrophages, pro-inflammatory pathways (e.g., NLRP3, NF-kb, and MyD88) were upregulated, an indication of macrophage activation. In C57Bl/6 mice, QTAP nanovaccines encoding Ag85B and Hsp70 transcripts (QTAP-85B+H70) were able to elicit robust IgG antibody and IFN- ɣ, TNF-α, IL-2, and IL-17 cytokines responses. Following aerosol challenge with a clinical isolate of M. avium ss. hominissuis (M.ah), a significant reduction of mycobacterial counts was observed in lungs and spleens of only immunized animals at both 4- and 8-weeks post-challenge. As expected, reduced levels of M. ah were associated with diminished histological lesions and robust cell-mediated immunity. Interestingly, polyfunctional T-cells expressing IFN- ɣ, IL-2, and TNF- α were detected at 8 but not 4 weeks post-challenge. Overall, our analysis indicated that QTAP is a highly efficient transfection agent and could improve the immunogenicity of mRNA vaccines against pulmonary M. ah, an infection of significant public health importance, especially to the elderly and to those who are immune compromised.

Mycobacterium tuberculosis PE_PGRS1 promotes mycobacteria intracellular survival via reducing the concentration of intracellular free Ca2+ and suppressing endoplasmic reticulum stress

Mycobacterium tuberculosis (M. tuberculosis) is the causative agent of tuberculosis (TB). And the PE_PGRS family members of M. tuberculosis are closely associated with virulence and antigen presentation but with function largely elusive. PE_PGRS1(Rv0109) contained 7 Ca²⁺ binding domains of GGXGXD/NXUX (X is any amino acid), which can reduce intracellular Ca²⁺ surge. In addition, PE_PGRS1 can mitigate the activation of PERK branch in endoplasmic reticulum (ER) stress by down-regulating the expression of CHOP, Bip, p-PERK, p-eIF2α, and ATF4. Interestingly, we found that two splicing variations of Bax/Bcl-2, Baxβ, and Bcl-2α, were differentially expressed after infection with Ms_PE_PGRS1, and may be involved in the regulation of apoptosis. Hence, this study identified that PE_PGRS1 is a novel calcium-associated protein that can decrease intracellular Ca²⁺ levels and the PERK axis. And the weakening of the PERK-eIF2α-ATF4 axis reduces THP-1 macrophages apoptosis, promotes the survival of mycobacteria in macrophages.

PE_PGRS3 ensures provision of the vital phospholipids cardiolipin and phosphatidylinositols by promoting the interaction between M. tuberculosis and host cells

PE_PGRS proteins of Mycobacterium tuberculosis (Mtb) constitute a large family of complex modular proteins whose role is still unclear. Among those, we have previously shown, using the heterologous expression in Mycobacterium smegmatis, that PE_PGRS3 containing a unique arginine-rich C-terminal domain, promotes adhesion to host cells. In this study, we investigate the role of PE_PGRS3 and its C-terminal domain directly in Mtb using functional deletion mutants. The results obtained here show that PE_PGRS3 is localized on the mycobacterial cell wall and its arginine-rich C-terminal region protrudes from the mycobacterial membrane and mediates Mtb entry into epithelial cells. Most importantly, this positively charged helical domain specifically binds phosphorylated phosphatidylinositols and cardiolipin, whereas it is unable to bind other phospholipids. Interestingly, administration of cardiolipin and phosphatidylinositol but no other phospholipids was able to turn-off expression of pe_pgrs3 activated by phosphate starvation conditions. These findings suggest that PE_PGRS3 has the key role to serve as a bridge between mycobacteria and host cells by interacting with specific host phospholipids and extracting them from host cells, for their direct integration or as a source of phosphate, during phases of TB pathogenesis when Mtb is short of phosphate supply.

PE_PGRS proteins of Mycobacterium tuberculosis: A specialized molecular task force at the forefront of host-pathogen interaction

To the PE_PGRS protein subfamily belongs a group of surface-exposed mycobacterial antigens that in Mycobacterium tuberculosis (Mtb) H37Rv accounts to more than 65 genes, 51 of which are thought to express a functional protein. PE_PGRS proteins share a conserved structural architecture with three main domains: the N-terminal PE domain; the PGRS domain, that can vary in sequence and size and is characterized by the presence of multiple GGA-GGX amino acid repeats; the highly conserved sequence containing the GRPLI motif that links the PE and PGRS domains; the unique C-terminus end that can vary in size from few to up to ≈ 300 amino acids. pe_pgrs genes emerged in slow-growing mycobacteria and expanded and diversified in MTBC and few other pathogenic mycobacteria. Interestingly, despite sequence homology and apparent redundancy, PE_PGRS proteins seem to have evolved a peculiar function. In this review, we summarize the actual knowledge on this elusive protein family in terms of evolution, structure, and function, focusing on the role of PE_PGRS in TB pathogenesis. We provide an original hypothesis on the role of the PE domain and propose a structural model for the polymorphic PGRS domain that might explain how so similar proteins can have different physiological functions.

PE_PGRS33, an Important Virulence Factor of Mycobacterium tuberculosis and Potential Target of Host Humoral Immune Response

PE_PGRS proteins are surface antigens of Mycobacterium tuberculosis (Mtb) and a few other pathogenic mycobacteria. The PE_PGRS33 protein is among the most studied PE_PGRSs. It is known that the PE domain of PE_PGRS33 is required for the protein translocation through the mycobacterial cell wall, where the PGRS domain remains available for interaction with host receptors. Interaction with Toll like receptor 2 (TLR2) promotes secretion of inflammatory chemokines and cytokines, which are key in the immunopathogenesis of tuberculosis (TB). In this review, we briefly address some key challenges in the development of a TB vaccine and attempt to provide a rationale for the development of new vaccines aimed at fostering a humoral response against Mtb. Using PE_PGRS33 as a model for a surface-exposed antigen, we exploit the availability of current structural data using homology modeling to gather insights on the PGRS domain features. Our study suggests that the PGRS domain of PE_PGRS33 exposes four PGII sandwiches on the outer surface, which, we propose, are directly involved through their loops in the interactions with the host receptors and, as such, are promising targets for a vaccination strategy aimed at inducing a humoral response.

Recent Advances in the Development of Protein- and Peptide-Based Subunit Vaccines against Tuberculosis

The World Health Organization (WHO) herald of the “End TB Strategy” has defined goals and targets for tuberculosis prevention, care, and control to end the global tuberculosis endemic. The emergence of drug resistance and the relative dreadful consequences in treatment outcome has led to increased awareness on immunization against Mycobacterium tuberculosis (Mtb). However, the proven limited efficacy of Bacillus Calmette-Guérin (BCG), the only licensed vaccine against Mtb, has highlighted the need for alternative vaccines. In this review, we seek to give an overview of Mtb infection and failure of BCG to control it. Afterward, we focus on the protein- and peptide-based subunit vaccine subtype, examining the advantages and drawbacks of using this design approach. Finally, we explore the features of subunit vaccine candidates currently in pre-clinical and clinical evaluation, including the antigen repertoire, the exploited adjuvanted delivery systems, as well as the spawned immune response.

Deciphering Molecular Virulence Mechanism of Mycobacterium tuberculosis Dop isopeptidase Based on Its Sequence-Structure-Function Linkage

The pupylation pathway marks proteins for prokaryotic ubiquitin-like protein (Pup)-proteasomal degradation and survival strategy of mycobacteria inside of the host macrophages. Deamidase of Pup (Dop) plays a central role in the pupylation pathway. It is still a matter of investigation to know the function of Dop in virulence of mycobacterial lineage. Hence, the present study was intended to describe the sequence-structure-function-virulence link of Dop for understanding the molecular virulence mechanism of Mycobacterium tuberculosis H37Rv (Mtb). Phylogenetic analysis of this study indicated that Dop has extensively diverged across the proteasome-harboring bacteria. The functional part of Dop was converged across the pathogenic mycobacterial lineage. The genome-wide analysis pointed out that the pupylation gene locus was identical to each other, but its genome neighborhood differed from species to species. Molecular modeling and dynamic studies proved that the predicted structure of Mtb Dop was energetically stable and low conformational freedom. Moreover, evolutionary constraints in Mtb Dop were intensively analyzed for inferring its sequence-structure-function relationships for the full virulence of Mtb. It indicated that evolutionary optimization was extensively required to stabilize its local structural environment at the side chains of mutable residues. The sequence-structure-function-virulence link of Dop might have retained in Mtb by reordering hydrophobic and hydrogen bonding patterns in the local structural environment. Thus, the results of our study provide a quest to understand the molecular virulence and pathogenesis mechanisms of Mtb during the infection process.

Differential determinants of virulence in two Mycobacterium tuberculosis Colombian clinical isolates of the LAM09 family

The heterogeneity of the clinical outcome of Mycobacterium tuberculosis (Mtb) infection may be due in part to different strategies used by circulating strains to cause disease. This heterogeneity is one of the main limitations to eradicate tuberculosis disease. In this study, we have compared the transcriptional response of two closely related Colombian clinical isolates (UT127 and UT205) of the LAM family under two axenic media conditions. These clinical isolates are phenotypically different at the level of cell death, cytokine production, growth kinetics upon in vitro infection of human tissue macrophages, and membrane vesicle secretion upon culture in synthetic medium. Using RNA-seq, we have identified different pathways that account for two different strategies to cope with the stressful condition of a carbon-poor media such as Sauton’s. We showed that the clinical isolate UT205 focus mainly in the activation of virulence systems such as the ESX-1, synthesis of diacyl-trehalose, polyacyl-trehalose, and sulfolipids, while UT127 concentrates its efforts mainly in the survival mode by the activation of the DNA replication, cell division, and lipid biosynthesis. This is an example of two Mtb isolates that belong to the same family and lineage, and even though they have a very similar genome, its transcriptional regulation showed important differences. This results in summary highlight the necessity to reach a better understanding of the heterogeneity in the behavior of these circulating Mtb strains which may help us to design better treatments and vaccines and to identify new targets for drugs.

In vitro activity of collinin isolated from the leaves of Zanthoxylum schinifolium against multidrug- and extensively drug-resistant Mycobacterium tuberculosis

BACKGROUND

Tuberculosis is a very serious infectious disease that threatens humanity, and the emergence of multidrug-resistant (MDR), extensively drug-resistant (XDR) strains resistant to drugs suggests that new drug development is urgent. In order to develop new tuberculosis drug, we have conducted in vitro anti-tubercular tests on thousands of plant-derived substances and finally found collinin extracted from the leaves of Zanthoxylum schinifolium, which has an excellent anti-tuberculosis effect.

PURPOSE

To isolate an anti-tubercular bioactive compound from the leaves of Z. schinifolium and evaluate whether this agent demonstrates any potential in vitro characteristics suitable for the development of future anti-tubercular drugs to treat MDR and XDR Mycobacterium tuberculosis.

METHODS

The methanolic extracts of the leaves of Z. schinifolium were subjected to bioassay-guided fractionation against M. tuberculosis using a microbial cell viability assay. In addition, following cell cytotoxicity assay, an intracellular anti-mycobacterial activity of the most active anti-tubercular compound was investigated after it was purified.

RESULTS

The active compound with anti-tubercular activity isolated from leaves of Z. schinifolium was identified as a collinin. The extracted collinin showed anti-tubercular activity against both drug-susceptible and -resistant strains of M. tuberculosis at 50% minimum inhibitory concentrations (MIC₅₀s) of 3.13-6.25 µg/ml in culture broth and MIC₅₀s of 6.25-12.50 µg/ml inside Raw264.7 and A549 cells. Collinin had no cytotoxicity against human lung pneumocytes up to a concentration of 100 µg/ml (selectivity index > 16-32).

CONCLUSIONS

Collinin extracted from the leaves of Z. schinifolium significantly inhibits the growth of MDR and XDR M. tuberculosis in the culture broth. In addition, it also inhibits the growth of intracellular drug-susceptible and drug-resistant tuberculosis in Raw264.7 and A549 cells. To our knowledge, this is the first report on the in vitro anti-tubercular activity of collinin, and our data suggest collinin as a potential drug to treat drug-resistant tuberculosis. Further studies are warranted to assess the in vivo efficacy and therapeutic potential of collinin.

Evaluation of PE_PGRS33 as a potential surface target for humoral responses against Mycobacterium tuberculosis

Mycobacterium tuberculosis (Mtb) PE_PGRS33 is a surface-exposed protein that was shown to interact with Toll-like receptor 2 on host macrophages to induce inflammatory signals and promote entry in macrophages. In this study, we investigated PE_PGRS33 as a potential target of a humoral response aimed at hampering key processes in tuberculosis pathogenesis. PE_PGRS33 protein was successfully expressed and purified under native condition in Escherichia coli. The purified protein retained its native functional and biological properties, showing the ability to elicit proinflammatory signals in murine and human macrophages. Interestingly, a polyclonal antiserum raised against native PE_PGRS33 showed no cross-reactions with other mycobacterial proteins. The anti-PE_PGRS33 serum was also able to inhibit Mtb entry into macrophages, but it did not reduce entry of the MtbΔpe_pgrs33 strain. Addition of native recombinant PE_PGRS33 to the MtbΔpe_pgrs33 strain during infection restored the Mtb wild-type entry phenotype in macrophage. Moreover, the anti-PE_PGRS33 serum was able to neutralize the proinflammatory activity of PE_PGRS33 in vitro. Furthermore, mice immunized with native recombinant PE_PGRS33, but not with a DNA vaccine expressing PE_PGRS33, were able to restrict M. smegmatis in vivo. These results highlight the potential use of PE_PGRS33 as a target of a neutralizing humoral response against tuberculosis.

The patterns of in vitro cell-death and inflammatory cytokines induced by distinct BCG vaccine strains are differentially induced in human mononuclear cells

Tuberculosis (TB) remains among the world's leading cause of mortality. For its control, studies of TB vaccines are needed. Since live-attenuated Mycobacterium bovis bacillus Calmette-Guérin (BCG) is the only TB vaccine currently in use, studies on the protective role of BCG are required. In this study, we analyzed host cells purified directly from whole blood of human immunodeficiency virus (HIV)-negative volunteers, comprising adult healthy donors (HD) and neonates (umbilical cord bloods, UCB), with the aim to directly compare in vitro immune responses with distinct BCG strains in human mononuclear cells. The Moreau, Pasteur, and Danish BCG strains were used to infect mononuclear cells in vitro for 48 h; bacilli viability and cell-death were subsequently detected by flow cytometry. In addition, cell culture supernatants were used in cytokine detection assays. Overall, the Moreau BCG strain induced higher levels of apoptosis than the Pasteur and Danish BCG strains in both the HD and UCB groups (p-value < 0.05), and a human monocytic cell-line mirrored those cell-death patterns after BCG infection. The Moreau BCG strain, exclusively, induced Th1 cytokines at the highest levels in cells from adults (p-value < 0.05) when compared with both Pasteur and Danish BCG strains, whereas TGF-β1 levels were reduced significantly (p-value < 0.01) in the HD group when cells were infected with the Moreau BCG vaccine. As expected, eight out of 22 pro-inflammatory cytokines were secreted at significant levels (p-value < 0.05) above the baseline rates in all BCG-infected cell cultures, in the HD group only. When analyzing these results, we excluded confounding factors related to storage and viability of the BCG strains used. These findings suggest that Moreau BCG is a more potent immunostimulating agent than the Pasteur and Danish BCG strains. Clinical trials will be needed to confirm these findings.

The Alternative Sigma Factors SigE and SigB Are Involved in Tolerance and Persistence to Antitubercular Drugs

The emergence and spread of drug-resistant Mycobacterium tuberculosis strains possibly threaten our ability to treat this disease in the future. Even though two new antitubercular drugs have recently been introduced, there is still the need to design new molecules whose mechanisms of action could reduce the length of treatment. We show that two alternative sigma factors of M. tuberculosis (SigE and SigB) have a major role in determining the level of basal resistance to several drugs and the amount of persisters surviving long-duration drug treatment. We also demonstrate that ethambutol, a bacteriostatic drug, is highly bactericidal for M. tuberculosis mutants missing either SigE or SigB. We suggest that molecules able to interfere with the activity of SigE or SigB not only could reduce M. tuberculosis virulence in vivo but also could boost the effect of other drugs by increasing the sensitivity of the organism and reducing the number of persisters able to escape killing.

Association of ESAT-6/CFP-10-induced IFN-γ, TNF-α and IL-10 with clinical tuberculosis: evidence from cohorts of pulmonary tuberculosis patients, household contacts and community controls in an endemic setting

Mycobacterium tuberculosis (Mtb) early secreted protein antigen 6 (ESAT-6) and culture filtrate protein 10 (CFP-10) are among candidate vaccines against tuberculosis (TB). Results of experimental animal models show that these antigens are associated with induction of strong T cell immunity [interferon (IFN)-γ production], while others report that these proteins as virulent factors involved in pathogenicity of Mtb infection. However, the role of ESAT-6/CFP-10 during natural Mtb infections in humans has not been established. In this paper we present results of a longitudinal study from an Mtb-infected human population from an endemic setting. Whole blood assay was used to determine levels of IFN-γ, tumour necrosis factor (TNF)-α and interleukin (IL)-10 against rESAT-6/CFP-10 in TB patients, household contacts and community controls. The levels of IFN-γ, TNF-α and IL-10 against rESAT-6/CFP-10 at baseline were significantly higher in patients and community controls than in household contacts. In patients, no significant difference was observed in the level of these cytokines before and after chemotherapy whereas, in contacts, the level of these cytokines increased significantly and progressively over time. The study shows that the levels of IFN-γ, TNF-α and IL-10 against rESAT-6/CFP-10 are depressed during Mtb infection or exposure but are elevated during clinical TB. Our findings from a study of naturally infected human population suggest that IFN-γ, TNF-α and IL-10 against rESAT-6/CFP-10 are markers for clinical TB but not for protective immunity.

Mycobacterium tuberculosis sigE mutant ST28 used as a vaccine induces protective immunity in the guinea pig model

With more than 9 million new infections and 1.5 million deaths claimed every year, tuberculosis remains one of the major scourges of humankind. The only vaccine available against this disease, the attenuated strain Mycobacterium bovis, BCG is effective against severe forms of the disease in infants, but scarcely effective in protecting adults from the pulmonary form of the disease, thus not stopping transmission. Consequently, the development of an effective anti-tuberculosis vaccine is a major goal for improving global health. The most common concept is that a more effective vaccine should include a first immunization with a live vaccine followed by the administration of an acellular boosting vaccine. In this approach, the live vaccine might be either BCG or a different, more efficient attenuated strain. Recently, we showed that a Mycobacterium tuberculosis mutant missing the gene encoding for the extracellular function sigma factor SigE, is strongly attenuated and is able to induce a more effective protection from M. tuberculosis infection compared to BCG in mice. We now further characterize the protective potential of this novel strain in the guinea pig model of tuberculosis. In the guinea pig, it had limited growth but induced a Th1 immune response and was able to significantly reduce the number of colony forming units as well as prolong survival. Taken together these data provide evidence for the use of the M. tuberculosis sigE mutant as the basis for further development as a vaccine against infection.

Bacterial Virulence Factors: Secreted for Survival

Virulence is described as an ability of an organism to infect the host and cause a disease. Virulence factors are the molecules that assist the bacterium colonize the host at the cellular level. These factors are either secretory, membrane associated or cytosolic in nature. The cytosolic factors facilitate the bacterium to undergo quick adaptive-metabolic, physiological and morphological shifts. The membrane associated virulence factors aid the bacterium in adhesion and evasion of the host cell. The secretory factors are important components of bacterial armoury which help the bacterium wade through the innate and adaptive immune response mounted within the host. In extracellular pathogens, the secretory virulence factors act synergistically to kill the host cells. In this review, we revisit the role of some of the secreted virulence factors of two human pathogens: Mycobacterium tuberculosis-an intracellular pathogen and Bacillus anthracis-an extracellular pathogen. The advances in research on the role of secretory factors of these pathogens during infection are discussed.

EBP50 induces apoptosis in macrophages by upregulating nitric oxide production to eliminate intracellular Mycobacterium tuberculosis

Mycobacterium bovis BCG is known to have the capacity to inhibit the positioning of iNOS on BCG-containing phagosomes by interfering with EBP50, a scaffolding protein that controls the recruitment of inducible nitric oxide synthase (iNOS) at the vicinity of phagosomes in macrophages. However, knockdown of the expression of EBP50 still facilitates the intracellular survival of BCG, which suggested that EBP50 may have some other unknown antimycobacterial properties. In this study we show that overexpression of EBP50 by a recombinant lentivirus had no effect on the iNOS recruitment to M.tuberculosis-containing phagosomes, but significantly promoted the elimination of intracellular M.tuberculosis. We revealed in the present study that the enhancement of intracellular killing to M. tuberculosis upon EBP50 overexpression was due to the increased level of apoptosis in macrophages. We showed that EBP50 overexpression significantly increased the expression of iNOS and generation of nitric oxide (NO), and EBP50-induced apoptosis was NO-dependent and mediated by Bax and caspase-3. We found that M. tuberculosis decreases while Mycobacterium smegmatis increases the expression of EBP50 in RAW264.7 cells, which suggested that virulent mycobacteria are capable of modulating the antimycobacterial properties of macrophages by inhibiting the expression and interfering with the function of EBP50.