Mycobacterium tuberculosis PE_PGRS18 enhances the intracellular survival of M. smegmatis via altering host macrophage cytokine profiling and attenuating the cell apoptosis

Role of PE/PPE proteins of Mycobacterium tuberculosis in triad of host mitochondria, oxidative stress and cell death

The PE and PPE family proteins of Mycobacterium tuberculosis (Mtb) is exclusively found in pathogenic Mycobacterium species, comprising approximately 8-10 % of the Mtb genome. These emerging virulent factors have been observed to play pivotal roles in Mtb pathogenesis and immune evasion through various strategies. These immunogenic proteins are known to modulate the host immune response and cell-death pathways by targeting the powerhouse of the cell, the mitochondria to support Mtb survival. In this article, we are focused on how PE/PPE family proteins target host mitochondria to induce mitochondrial perturbations, modulate the levels of cellular ROS (Reactive oxygen species) and control cell death pathways. We observed that the time of expression of these proteins at different stages of infection is crucial for elucidating their impact on the cell death pathways and eventually on the outcome of infection. This article focuses on understanding the contributions of the PE/PPE proteins by unravelling the triad of host mitochondria, oxidative stress and cell death pathways that facilitate the Mtb persistence. Understanding the role of these proteins in host cellular pathways and the intricate mechanisms paves the way for the development of novel therapeutic strategies to combat TB infections.

Mycobacterium tuberculosis PE_PGRS38 Enhances Intracellular Survival of Mycobacteria by Inhibiting TLR4/NF-κB-Dependent Inflammation and Apoptosis of the Host

Mycobacterium tuberculosis (Mtb) ranks as the most lethal human pathogen, able to fend off repeated attacks by the immune system or medications. PE_PGRS proteins are hallmarks of the pathogenicity of Mtb and contribute to its antigenic diversity, virulence, and persistence during infection. M. smegmatis is a nonpathogenic mycobacterium that naturally lacks PE_PGRS and is used as a model to express Mtb proteins. PE_PGRS has the capability to evade host immune responses and enhance the intracellular survival of M. smegmatis. Despite the intense investigations into PE_PGRS proteins, their role in tuberculosis remains elusive. We engineered the recombinant M. smegmatis strain Ms-PE_PGRS38. The result shows that PE_PGRS38 is expressed in the cell wall of M. smegmatis. PE_PGRS38 contributes to biofilm formation, confers permeability to the cell wall, and shows variable responses to exogenous stresses. PE_PGRS38 downregulated TLR4/NF-κB signaling in RAW264.7 macrophages and lung tissues of infected mice. In addition, PE_PGRS38 decreased NLRP3-dependent IL-1β release and limited pathogen-mediated inflammasome activity during infection. Moreover, PE_PGRS38 inhibited the apoptosis of RAW264.7 cells by downregulating the expression of apoptotic markers including Bax, cytochrome c, caspase-3, and caspase-9. In a nutshell, our findings demonstrate that PE_PGRS38 is a virulence factor for Mtb that enables recombinant M. smegmatis to survive by resisting and evading the host's immune responses during infection.

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.

Mycobacterium tuberculosis-THP-1 like macrophages protein-protein interaction map revealed through dual RNA-seq analysis and a computational approach

Introduction. Infection caused by Mycobacterium tuberculosis (M. tb) is still a leading cause of mortality worldwide with estimated 1.4 million deaths annually.Hypothesis/Gap statement. Despite macrophages' ability to kill bacterium, M. tb can grow inside these innate immune cells and the exploration of the infection has traditionally been characterized by a one-sided relationship, concentrating solely on the host or examining the pathogen in isolation.Aim. Because of only a handful of M. tb-host interactions have been experimentally characterized, our main goal is to predict protein-protein interactions during the early phases of the infection.Methodology. In this work, we performed an integrative computational approach that exploits differentially expressed genes obtained from Dual RNA-seq analysis combined with known domain-domain interactions.Results. A total of 2381 and 7214 genes were identified as differentially expressed in M. tb and in THP-1-like macrophages, respectively, revealing different transcriptional profiles in response to infection. Over 48 h of infection, the host-pathogen network revealed 25 016 PPIs. Analysis of the resulting predicted network based on cellular localization information of M. tb proteins, indicated the implication of interacting nodes including the bacterial PE/PPE/PE_PGRS family. In addition, M. tb proteins interacted with host proteins involved in NF-kB signalling pathway as well as interfering with the host apoptosis ability via the potential interaction of M. tb TB16.3 with human TAB1 and M. tb GroEL2 with host protein kinase C delta, respectively.Conclusion. The prediction of the full range of interactions between M. tb and host will contribute to better understanding of the pathogenesis of this bacterium and may provide advanced approaches to explore new therapeutic targets against tuberculosis.

Mycobacterial PE12 protein promotes bacterial survival through inhibiting cell apoptosis

Mycobacterial PE_PGRS family proteins play key roles in pathogen-host interaction. However, the function of most PE_PGRS proteins remains unknown. In this study, we characterized the role of PE12 of Mycobacterium bovis (M. bovis) on bacterial growth, bacterial survival, and host cell apoptosis. Transcriptome sequencing of infected THP-1 cells was also performed. Compared to Ms_Vec, we found that M. bovis PE12 did not alter the colony morphology of M. smegmatis. The survival of Ms_PE12 was obviously higher than that of Ms_Vec. Furthermore, PE12 significantly suppressed the apoptosis of THP-1 induced by M. smegmatis infection. Transcriptome analysis results showed that there were 70 downregulated genes in the Ms_PE12 infection group in comparison with the Ms_Vec infection group, and these differentially expressed genes were enriched in 240 downregulated GO terms and 6 KEGG pathways. The downregulated expression genes are involved in cell adhesion, phagocytosis, apoptosis, inflammatory response, glycolysis and transmembrane transporter activity. Taken together, our study reveals that PE12 can suppress apoptosis and inhibit proinflammatory cytokine response. We propose that PE12 is related to macrophage phagocytosis and apoptosis, providing useful information to the pathogenic mechanisms of M. bovis.

Mycobacterium tuberculosis PE_PGRS45 (Rv2615c) Promotes Recombinant Mycobacteria Intracellular Survival via Regulation of Innate Immunity, and Inhibition of Cell Apoptosis

Tuberculosis (TB), a bacterial infectious disease caused by Mycobacterium tuberculosis (M. tuberculosis), is a significant global public health problem. Mycobacterium tuberculosis expresses a unique family of PE_PGRS proteins that have been implicated in pathogenesis. Despite numerous studies, the functions of most PE_PGRS proteins in the pathogenesis of mycobacterium infections remain unclear. PE_PGRS45 (Rv2615c) is only found in pathogenic mycobacteria. In this study, we successfully constructed a recombinant Mycobacterium smegmatis (M. smegmatis) strain which heterologously expresses the PE_PGRS45 protein. We found that overexpression of this cell wall-associated protein enhanced bacterial viability under stress in vitro and cell survival in macrophages. MS_PE_PGRS45 decreased the secretion of pro-inflammatory cytokines such as IL-1β, IL-6, IL-12p40, and TNF-α. We also found that MS_PE_PGRS45 increased the expression of the anti-inflammatory cytokine IL-10 and altered macrophage-mediated immune responses. Furthermore, PE_PGRS45 enhanced the survival rate of M. smegmatis in macrophages by inhibiting cell apoptosis. Collectively, our findings show that PE_PGRS45 is a virulent factor actively involved in the interaction with the host macrophage.

Immunological effects of the PE/PPE family proteins of Mycobacterium tuberculosis and related vaccines

Tuberculosis (TB) is a chronic infectious disease caused by Mycobacterium tuberculosis (Mtb), and its incidence and mortality are increasing. The BCG vaccine was developed in the early 20th century. As the most widely administered vaccine in the world, approximately 100 million newborns are vaccinated with BCG every year, which has saved tens of millions of lives. However, due to differences in region and race, the average protective rate of BCG in preventing tuberculosis in children is still not high in some areas. Moreover, because the immune memory induced by BCG will weaken with the increase of age, it is slightly inferior in preventing adult tuberculosis, and BCG revaccination cannot reduce the incidence of tuberculosis again. Research on the mechanism of Mtb and the development of new vaccines against TB are the main strategies for preventing and treating TB. In recent years, Pro-Glu motif-containing (PE) and Pro-Pro-Glu motif-containing (PPE) family proteins have been found to have an increasingly important role in the pathogenesis and chronic protracted infection observed in TB. The development and clinical trials of vaccines based on Mtb antigens are in progress. Herein, we review the immunological effects of PE/PPE proteins and the development of common PE/PPE vaccines.

Mycobacterium smegmatis, a Promising Vaccine Vector for Preventing TB and Other Diseases: Vaccinomics Insights and Applications

Mycobacterium smegmatis (M.sm) is frequently used as an alternative model organism in Mycobacterium tuberculosis (M.tb) studies. While containing high sequence homology with M.tb, it is considered non-pathogenic in humans. As such it has been used to study M.tb and other infections in vivo and more recently been explored for potential therapeutic applications. A body of previous research has highlighted the potential of using genetically modified M.sm displaying rapid growth and unique immunostimulatory characteristics as an effective vaccine vector. Novel systems biology techniques can further serve to optimize these delivery constructs. In this article, we review recent advancements in vaccinomics tools that support the efficacy of a M.sm-based vaccine vector. Moreover, the integration of systems biology and molecular omics techniques in these pioneering studies heralds a potential accelerated pipeline for the development of next-generation recombinant vaccines against rapidly developing diseases.

Mycobacterium tuberculosis-macrophage interaction: Molecular updates

Mycobacterium tuberculosis (Mtb), the causative agent of Tuberculosis (TB), remains a pathogen of great interest on a global scale. This airborne pathogen affects the lungs, where it interacts with macrophages. Acidic pH, oxidative and nitrosative stressors, and food restrictions make the macrophage's internal milieu unfriendly to foreign bodies. Mtb subverts the host immune system and causes infection due to its genetic arsenal and secreted effector proteins. In vivo and in vitro research have examined Mtb-host macrophage interaction. This interaction is a crucial stage in Mtb infection because lung macrophages are the first immune cells Mtb encounters in the host. This review summarizes Mtb effectors that interact with macrophages. It also examines how macrophages control and eliminate Mtb and how Mtb manipulates macrophage defense mechanisms for its own survival. Understanding these mechanisms is crucial for TB prevention, diagnosis, and treatment.

Mycobacterium tuberculosis PE_PGRS19 Induces Pyroptosis through a Non-Classical Caspase-11/GSDMD Pathway in Macrophages

The PE/PPE protein family commonly exists in pathogenic species, such as Mycobacterium tuberculosis, suggesting a role in virulence and its maintenance. However, the exact role of most PE/PPE proteins in host-pathogen interactions remains unknown. Here, we constructed a recombinant Mycobacterium smegmatis expressing M. tuberculosis PE_PGRS19 (Ms_PE_PGRS19) and found that PE_PGRS19 overexpression resulted in accelerated bacterial growth in vitro, increased bacterial survival in macrophages, and enhanced cell damage capacity. Ms_PE_PGRS19 also induced the expression of pro-inflammatory cytokines, such as IL-6, TNF-α, IL-1β, and IL-18. Furthermore, we demonstrated that Ms_PE_PGRS19 induced cell pyroptosis by cleaving caspase-11 via a non-classical pathway rather than caspase-1 activation and further inducing the cleavage of gasdermin D, which led to the release of IL-1β and IL-18. To the best of our current knowledge, this is the first report of a PE/PPE family protein activating cell pyroptosis via a non-classical pathway, which expands the knowledge on PE/PPE protein functions, and these pathogenic factors involved in bacterial survival and spread could be potential drug targets for anti-tuberculosis therapy.

Different modalities of host cell death and their impact on Mycobacterium tuberculosis infection

Mycobacterium tuberculosis (Mtb) is the pathogen that causes tuberculosis (TB), a leading infectious disease of humans worldwide. One of the main histopathological hallmarks of TB is the formation of granulomas comprised of elaborately organized aggregates of immune cells containing the pathogen. Dissemination of Mtb from infected cells in the granulomas due to host and mycobacterial factors induces multiple cell death modalities in infected cells. Based on molecular mechanism, morphological characteristics, and signal dependency, there are two main categories of cell death: programmed and nonprogrammed. Programmed cell death (PCD), such as apoptosis and autophagy, is associated with a protective response to Mtb by keeping the bacteria encased within dead macrophages that can be readily phagocytosed by arriving in uninfected or neighboring cells. In contrast, non-PCD necrotic cell death favors the pathogen, resulting in bacterial release into the extracellular environment. Multiple types of cell death in the PCD category, including pyroptosis, necroptosis, ferroptosis, ETosis, parthanatos, and PANoptosis, may be involved in Mtb infection. Since PCD pathways are essential for host immunity to Mtb, therapeutic compounds targeting cell death signaling pathways have been experimentally tested for TB treatment. This review summarizes different modalities of Mtb-mediated host cell deaths, the molecular mechanisms underpinning host cell death during Mtb infection, and its potential implications for host immunity. In addition, targeting host cell death pathways as potential therapeutic and preventive approaches against Mtb infection is also discussed.

PE_PGRS38 Interaction With HAUSP Downregulates Antimycobacterial Host Defense via TRAF6

Mycobacterium tuberculosis (Mtb) is the causative pathogen of tuberculosis (TB), which manipulates the host immunity to ensure survival and colonization in the host. Mtb possess a unique family of proteins, named PE_PGRS, associated with Mtb pathogenesis. Thus, elucidation of the functions of PE_PGRS proteins is necessary to understand TB pathogenesis. Here, we investigated the role of PE_PGRS38 binding to herpesvirus-associated ubiquitin-specific protease (HAUSP, USP7) in regulating the activity of various substrate proteins by modulating their state of ubiquitination. We constructed the recombinant PE_PGRS38 expressed in M. smegmatis (Ms_PE_PGRS38) to investigate the role of PE_PGRS38. We found that Ms_PE_PGRS38 regulated the cytokine levels in murine bone marrow-derived macrophages by inhibiting the deubiquitination of tumor necrosis factor receptor-associated factor (TRAF) 6 by HAUSP. Furthermore, the PE domain in PE_PGRS38 was identified as essential for mediating TRAF6 deubiquitination. Ms_PE_PGRS38 increased the intracellular burden of bacteria by manipulating cytokine levels in vitro and in vivo. Overall, we revealed that the interplay between HAUSP and PE_PGRS38 regulated the inflammatory response to increase the survival of mycobacteria.

LysX2 is a Mycobacterium tuberculosis membrane protein with an extracytoplasmic MprF-like domain

Background

Aminoacyl-phosphatidylglycerol (aaPG) synthases are bacterial enzymes that usually catalyze transfer of aminoacyl residues to the plasma membrane phospholipid phosphatidylglycerol (PG). The result is introduction of positive charges onto the cytoplasmic membrane, yielding reduced affinity towards cationic antimicrobial peptides, and increased resistance to acidic environments. Therefore, these enzymes represent an important defense mechanism for many pathogens, including Staphylococcus aureus and Mycobacterium tuberculosis (Mtb), which are known to encode for lysyl-(Lys)-PG synthase MprF and LysX, respectively. Here, we used a combination of bioinformatic, genetic and bacteriological methods to characterize a protein encoded by the Mtb genome, Rv1619, carrying a domain with high similarity to MprF-like domains, suggesting that this protein could be a new aaPG synthase family member. However, unlike homologous domains of MprF and LysX that are positioned in the cytoplasm, we predicted that the MprF-like domain in LysX2 is in the extracytoplasmic region.

Results

Using genetic fusions to the Escherichia coli proteins PhoA and LacZ of LysX2, we confirmed this unique membrane topology, as well as LysX and MprF as benchmarks. Expression of lysX2 in Mycobacterium smegmatis increased cell resistance to human β-defensin 2 and sodium nitrite, enhanced cell viability and delayed biofilm formation in acidic pH environment. Remarkably, MtLysX2 significantly reduced the negative charge on the bacterial surface upon exposure to an acidic environment. Additionally, we found LysX2 orthologues in major human pathogens and in rapid-growing mycobacteria frequently associated with human infections, but not in environmental and non-pathogenic mycobacteria.

Conclusions

Overall, our data suggest that LysX2 is a prototype of a new class within the MprF-like protein family that likely enhances survival of the pathogenic species through its catalytic domain which is exposed to the extracytoplasmic side of the cell membrane and is required to decrease the negative charge on the bacterial surface through a yet uncharacterized mechanism.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12866-022-02493-2.

The Mycobacterium tuberculosis PE_PGRS Protein Family Acts as an Immunological Decoy to Subvert Host Immune Response

Mycobacterium tuberculosis (M.tb) is a successful pathogen that can reside within the alveolar macrophages of the host and can survive in a latent stage. The pathogen has evolved and developed multiple strategies to resist the host immune responses. M.tb escapes from host macrophage through evasion or subversion of immune effector functions. M.tb genome codes for PE/PPE/PE_PGRS proteins, which are intrinsically disordered, redundant and antigenic in nature. These proteins perform multiple functions that intensify the virulence competence of M.tb majorly by modulating immune responses, thereby affecting immune mediated clearance of the pathogen. The highly repetitive, redundant and antigenic nature of PE/PPE/PE_PGRS proteins provide a critical edge over other M.tb proteins in terms of imparting a higher level of virulence and also as a decoy molecule that masks the effect of effector molecules, thereby modulating immuno-surveillance. An understanding of how these proteins subvert the host immunological machinery may add to the current knowledge about M.tb virulence and pathogenesis. This can help in redirecting our strategies for tackling M.tb infections.

Codon Optimization, Soluble Expression and Purification of PE_PGRS45 Gene from Mycobacterium tuberculosis and Preparation of Its Polyclonal Antibody Protein

Studies have demonstrated that PE_PGRS45 is constitutively expressed under various environmental conditions (such as nutrient depletion, hypoxia, and low pH) of the in vitro growth conditions examined, indicating that PE_PGRS45 protein is critical to the basic functions of Mycobacterium tuberculosis. However, there are few reports about the biochemical function and pathogenic mechanism of PE_PGRS45 protein. The fact that this M. tuberculosis gene is not easily expressed in E. coli may be mainly due to the high content of G+C and the use of unique codons. Fusion tags are indispensable tools used to improve the soluble expression of recombinant proteins and accelerate the characterization of protein structure and function. In the present study, His6, Trx, and His6-MBP were used as fusion tags, but only MBP-PE_PGRS45 was expressed solubly. The purification using His6-MBP tag-specific binding to the Ni column was easy to separate after the tag cleavage. We used the purified PE_PGRS45 to immunize New Zealand rabbits and obtained anti- PE_PGRS45 serum. We found that the titer of polyclonal antibodies against PE_PGR45 was higher than 1:256000. The result shows that purified PE_PGRS45 can induce New Zealand rabbits to produce high-titer antibodies. In conclusion, the recombinant protein PE_PGRS45 was successfully expressed in E. coli and specific antiserum was prepared, which will be followed by further evaluation of these specific antigens to develop highly sensitive and specific diagnostic tests for tuberculosis.

PGRS Domain of Rv0297 of Mycobacterium tuberculosis Functions in A Calcium Dependent Manner

Mycobacterium tuberculosis (M.tb), the pathogen causing tuberculosis, is a major threat to human health worldwide. Nearly 10% of M.tb genome encodes for a unique family of PE/PPE/PGRS proteins present exclusively in the genus Mycobacterium. The functions of most of these proteins are yet unexplored. The PGRS domains of these proteins have been hypothesized to consist of Ca²⁺ binding motifs that help these intrinsically disordered proteins to modulate the host cellular responses. Ca²⁺ is an important secondary messenger that is involved in the pathogenesis of tuberculosis in diverse ways. This study presents the calcium-dependent function of the PGRS domain of Rv0297 (PE_PGRS5) in M.tb virulence and pathogenesis. Tandem repeat search revealed the presence of repetitive Ca²⁺ binding motifs in the PGRS domain of the Rv0297 protein (Rv0297PGRS). Molecular Dynamics simulations and fluorescence spectroscopy revealed Ca²⁺ dependent stabilization of the Rv0297PGRS protein. Calcium stabilized Rv0297PGRS enhances the interaction of Rv0297PGRS with surface localized Toll like receptor 4 (TLR4) of macrophages. The Ca²⁺ stabilized binding of Rv0297PGRS with the surface receptor of macrophages enhances its downstream consequences in terms of Nitric Oxide (NO) production and cytokine release. Thus, this study points to hitherto unidentified roles of calcium-modulated PE_PGRS proteins in the virulence of M.tb. Understanding the pathogenic potential of Ca²⁺ dependent PE_PGRS proteins can aid in targeting these proteins for therapeutic interventions.

Mycobacterium tuberculosis PE17 (Rv1646) promotes host cell apoptosis via host chromatin remodeling mediated by reduced H3K9me3 occupancy

Tuberculosis caused by Mycobacterium tuberculosis remains a serious global public health threat. M. tuberculosis PE and PPE proteins are closely involved in pathogen-host interaction. To explore the predicted function of the M. tuberculosis PE17 (Rv1646), we heterologously expressed PE17 in a non-pathogenic Mycobacterium smegmatis strain (Ms_PE17). PE17 can reduce the survival of M. smegmatis within macrophages associated with altering the transcription levels of inflammatory cytokines IL1β, IL6, TNFα, and IL10 in Ms_PE17 infected macrophages through JNK signaling. Furthermore, macrophages apoptosis was increased upon Ms_PE17 infection in a caspases-dependent manner, accompanied by the activation of the Endoplasmic Reticulum stress IRE1α/ASK1/JNK signaling pathway. This can be largely interpreted by the epigenetic changes through reduced H3K9me3 chromatin occupancy post Ms_PE17 infection. To our knowledge, this is the first report that PE17 altered the macrophages apoptosis via H3K9me3 mediated chromatin remodeling.

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.

Mycobacterium PPE31 Contributes to Host Cell Death

Genome scale mutagenesis identifies many genes required for mycobacterial infectivity and survival, but their contributions and mechanisms of action within the host are poorly understood. Using CRISPR interference, we created a knockdown of ppe31^Mm gene in Mycobacterium marinum (M. marinum), which reduced the resistance to acid medium. To further explore the function of PPE31, the ppe31 mutant strain was generated in M. marinum and Mycobacterium tuberculosis (M. tuberculosis), respectively. Macrophages infected with the ppe31^Mm mutant strain caused a reduced inflammatory mediator expressions. In addition, macrophages infected with M. marinum Δppe31^Mm had decreased host cell death dependent on JNK signaling. Consistent with these results, deletion of ppe31^Mtb from M. tuberculosis increased the sensitivity to acid medium and reduced cell death in macrophages. Furthermore, we demonstrate that both ppe31 mutants from M. marinum and M. tuberculosis resulted in reduced survival in macrophages, and the survivability of M. marinum was deceased in zebrafish due to loss of ppe31^Mm. Our findings confirm that PPE31 as a virulence associated factor that modulates innate immune responses to mycobacterial infection.

PE_PGRS: Vital proteins in promoting mycobacterial survival and modulating host immunity and metabolism

Tuberculosis (TB), caused by Mycobacterium tuberculosis (M. tb), is the leading infectious cause of mortality worldwide. One of the key reasons for M. tb pathogenesis is the capability of M. tb to evade immune elimination and survive in macrophage, eventually causing chronic infection. However the pathogenicity mechanism of M. tb is not unclear yet, and thus diagnosis and therapy for TB remains a challenge. The genome of M. tb, encodes a unique protein family known as the PGRS family, with largely unexplored functions. Recently, an increasing number of reports have shown that the PE_PGRS proteins play critical roles in bacterial pathogenesis and immune evasion. The PE_PGRS protein family, characterized by a special N-terminal PE (Pro (P)-Glu (E) motif) domain and a C-terminal PGRS (Polymorphic GC-rich Repetitive Sequences) domain, is restricted mainly to pathogenic mycobacteria. Here we summarize current literature on the PE_PGRS as vital proteins in promoting bacterial survival and modulating host immunity, cell death and metabolism. We also highlight the potential of PE_PGRS as novel targets of anti-mycobacterial interventions for TB control.

Role of the PE/PPE Family in Host-Pathogen Interactions and Prospects for Anti-Tuberculosis Vaccine and Diagnostic Tool Design

The pe/ppe genes are found in pathogenic, slow-growing Mycobacterium tuberculosis and other M. tuberculosis complex (MTBC) species. These genes are considered key factors in host-pathogen interactions. Although the function of most PE/PPE family proteins remains unclear, accumulating evidence suggests that this family is involved in M. tuberculosis infection. Here, we review the role of PE/PPE proteins, which are believed to be linked to the ESX system function. Further, we highlight the reported functions of PE/PPE proteins, including their roles in host cell interaction, immune response regulation, and cell fate determination during complex host-pathogen processes. Finally, we propose future directions for PE/PPE protein research and consider how the current knowledge might be applied to design more specific diagnostics and effective vaccines for global tuberculosis control.

Host-Pathogen Dialogues in Autophagy, Apoptosis, and Necrosis during Mycobacterial Infection

Mycobacterium tuberculosis (Mtb) is an etiologic pathogen of human tuberculosis (TB), a serious infectious disease with high morbidity and mortality. In addition, the threat of drug resistance in anti-TB therapy is of global concern. Despite this, it remains urgent to research for understanding the molecular nature of dynamic interactions between host and pathogens during TB infection. While Mtb evasion from phagolysosomal acidification is a well-known virulence mechanism, the molecular events to promote intracellular parasitism remains elusive. To combat intracellular Mtb infection, several defensive processes, including autophagy and apoptosis, are activated. In addition, Mtb-ingested phagocytes trigger inflammation, and undergo necrotic cell death, potentially harmful responses in case of uncontrolled pathological condition. In this review, we focus on Mtb evasion from phagosomal acidification, and Mtb interaction with host autophagy, apoptosis, and necrosis. Elucidation of the molecular dialogue will shed light on Mtb pathogenesis, host defense, and development of new paradigms of therapeutics.

PE_PGRS31-S100A9 Interaction Promotes Mycobacterial Survival in Macrophages Through the Regulation of NF-κB-TNF-α Signaling and Arachidonic Acid Metabolism

Mycobacterium tuberculosis (M. tb) evades the surveillance of immune responses for survival in macrophages. However, the precise mechanism and toxins/proteins encoded by M. tb involved in the bacterial escape remain elusive. The function of Rv1768 protein (also referred to as PE_PGRS31, belonging to the PE_PGRS family) encoded by the region of deletion 14 (RD-14) in the virulent M. tb H37Rv strain has not, to the best of our knowledge, been reported previously. Here, we found that Rv1768 remarkably promotes bacterial survival in macrophages. Compared to wild type (WT) H37Rv, the Rv1768 deficient strain (H37RvΔ1768) showed significantly decreased colony-forming units in the lungs, spleen, and liver of the murine M. tb infection model. The bacterial burdens of WT H37Rv in WT macrophages and C57BL/6 mice were significantly higher than those in S100A9 deficiency cells and mice, but there were no significant differences for H37RvΔRv1768. Rv1768 binds S100A9 with the proline-glutamic acid domain (PE domain) and blocks the interaction between S100A9 and Toll-like receptor 4 (TLR4), and suppresses TLR4-myeloid differentiation factor 88-nuclear factor-kappa B (NF-κB)-tumor necrosis factor α (TNF-α) signaling in macrophages. Interestingly, Rv1768 binding to S100A9 also disturbs the metabolism of arachidonic acid by activating 5-lipoxygenase, increasing lipotoxin A4, and down-regulating cyclooxygenase-2 and prostaglandin E2 expression, thus, promoting mycobacterial survival. Our results revealed that M. tb Rv1768 promotes mycobacterial survival in macrophages by regulating NF-κB-TNF-α signaling and arachidonic acid metabolism via S100A9. Disturbing the interaction between Rv1768 and S100A9 may be a potential therapeutic target for tuberculosis.

Intelligent Mechanisms of Macrophage Apoptosis Subversion by Mycobacterium

Macrophages are one of the first innate defense barriers and play an indispensable role in communication between innate and adaptive immune responses, leading to restricted Mycobacterium tuberculosis (Mtb) infection. The macrophages can undergo programmed cell death (apoptosis), which is a crucial step to limit the intracellular growth of bacilli by liberating them into extracellular milieu in the form of apoptotic bodies. These bodies can be taken up by the macrophages for the further degradation of bacilli or by the dendritic cells, thereby leading to the activation of T lymphocytes. However, Mtb has the ability to interplay with complex signaling networks to subvert macrophage apoptosis. Here, we describe the intelligent strategies of Mtb inhibition of macrophages apoptosis. This review provides a platform for the future study of unrevealed Mtb anti-apoptotic mechanisms and the design of therapeutic interventions.

PE_PGRS62 promotes the survival of Mycobacterium smegmatis within macrophages via disrupting ER stress-mediated apoptosis

Mycobacterium tuberculosis, the leading causative agent of tuberculosis, remains one of the most deadly infectious pathogens. PE_PGRS proteins become a new focus as their species specificity in mycobacteria, especially in pathogenic mycobacteria. Despite intensive research, PE_PGRS proteins are still a mysterious aspect of mycobacterial pathogenesis with unknown mechanism. Herein, we focused on a PE_PGRS member from M. tuberculosis, PE_PGRS62, characterized by a surface-exposed protein function in disrupting phagolysosome maturation. Expression of PE_PGRS62 in Mycobacterium smegmatis, a nonpathogenic species naturally deficient in PE_PGRS genes, resulted in enhanced resistance to various in vitro stresses and cellular survival in macrophage. As a consequence, the cytokine profiles of macrophage were disturbed and cell apoptosis were inhibited via decreasing endoplasmic reticulum stress response.

PPE11 of Mycobacterium tuberculosis can alter host inflammatory response and trigger cell death

Tuberculosis (TB), which is caused by Mycobacterium tuberculosis (Mtb), remains a serious global health problem. The PE/PPE family, featuring unique sequences, structures and expression in Mtb, is reported to interfere with the macrophage response to the pathogen and facilitate its infection. PPE11 (Rv0453) existed in pathogenic mycobacteria and was persistently expressed in the infected guinea pig lungs. However, the role it played in the pathogenesis remains unclear. Here, to investigate the interaction and potential mechanism of PPE11 between pathogens and hosts, we heterologously expressed PPE11 in non-pathogenic, rapidly growing Mycobacterium smegmatis strains. We found that the overexpression of the cell wall-associated protein, PPE11, can improve the viability of bacteria in the presence of lysozyme, hydrogen peroxide and acid stress. Expression of PPE11 enhanced the early survival of M. smegmatis in macrophages and sustained a higher bacterial load in mouse tissues that showed exacerbated organ pathology. Macrophages infected with recombinant M. smegmatis produced significantly greater amounts of interleukin (IL)-1β, IL-6, tumour necrosis factor (TNF)-α and an early decrease in IL-10 along with higher levels of host cell death. Similar cytokines changes were observed in the sera of infected mice. Accordingly, PPE11 protein causes histopathological changes by disrupting the dynamic balance of the inflammatory factors and promoting host-cell death, indicating a potential role in the virulence of Mtb.

Cell death at the cross roads of host-pathogen interaction in Mycobacterium tuberculosis infection

Tuberculosis (TB) continues to be the leading cause of death by any single infectious agent, accounting for around 1.7 million annual deaths globally, despite several interventions and support programs by national and international agencies. With the development of drug resistance in Mycobacterium tuberculosis (M. tb), there has been a paradigm shift in TB research towards host-directed therapy. The potential targets include the interactions between host and bacterial proteins that are crucial for pathogenesis. Hence, collective efforts are being made to understand the molecular details of host-pathogen interaction for possible translation into host-directed therapy. The present review focuses on 'host cell death modalities' of host-pathogen interaction, which play a crucial role in determining the outcome of TB disease progression. Several cell death modalities that occur in response to mycobacterial infection have been identified in human macrophages either as host defences for bacterial clearance or as pathogen strategies for multiplication and dissemination. These cell death modalities include apoptosis, necrosis, pyroptosis, necroptosis, pyronecrosis, NETosis, and autophagy. These processes are highly overlapping with several mycobacterial proteins participating in more than one cell death pathway. Until now, reviews in M. tb and host cell death have discussed either focusing on host evasion strategies, apoptosis, autophagy, and necrosis or describing all these forms with limited discussions of their role in host-pathogen interactions. Here, we present a comprehensive review of various mycobacterial factors modulating host cell death pathways and the cross-talk between them. Besides this, we have discussed the networking of host cell death pathways including the interference of host miRNA during M. tb infection with their respective targets. Through this review, we present the host targets that overlap across several cell death modalities and the technical limitations of methodology in cell death research. Given the compelling need to discover alternative drug target(s), this review identifies these overlapping cell death factors as potential targets for host-directed therapy.