Evidence for a rapid rate of molecular evolution at the hypervariable and immunogenic Mycobacterium tuberculosis PPE38 gene region

Large-scale Pan Genomic Analysis of Mycobacterium tuberculosis Reveals Key Insights Into Molecular Evolutionary Rate of Specific Processes and Functions

Mycobacterium tuberculosis (Mtb) is the causative agent of tuberculosis (TB), an infectious disease that is a major killer worldwide. Due to selection pressure caused by the use of antibacterial drugs, Mtb is characterised by mutational events that have given rise to multi drug resistant (MDR) and extensively drug resistant (XDR) phenotypes. The rate at which mutations occur is an important factor in the study of molecular evolution, and it helps understand gene evolution. Within the same species, different protein-coding genes evolve at different rates. To estimate the rates of molecular evolution of protein-coding genes, a commonly used parameter is the ratio dN/dS, where dN is the rate of non-synonymous substitutions and dS is the rate of synonymous substitutions. Here, we determined the estimated rates of molecular evolution of select biological processes and molecular functions across 264 strains of Mtb. We also investigated the molecular evolutionary rates of core genes of Mtb by computing the dN/dS values, and estimated the pan genome of the 264 strains of Mtb. Our results show that the cellular amino acid metabolic process and the kinase activity function evolve at a significantly higher rate, while the carbohydrate metabolic process evolves at a significantly lower rate for M. tuberculosis. These high rates of evolution correlate well with Mtb physiology and pathogenicity. We further propose that the core genome of M. tuberculosis likely experiences varying rates of molecular evolution which may drive an interplay between core genome and accessory genome during M. tuberculosis evolution.

Functional genetic variation in pe/ppe genes contributes to diversity in Mycobacterium tuberculosis lineages and potential interactions with the human host

Introduction

Around 10% of the coding potential of Mycobacterium tuberculosisis constituted by two poorly understood gene families, the pe and ppe loci, thought to be involved in host-pathogen interactions. Their repetitive nature and high GC content have hindered sequence analysis, leading to exclusion from whole-genome studies. Understanding the genetic diversity of pe/ppe families is essential to facilitate their potential translation into tools for tuberculosis prevention and treatment.

Methods

To investigate the genetic diversity of the 169 pe/ppe genes, we performed a sequence analysis across 73 long-read assemblies representing seven different lineages of M. tuberculosis and M. bovis BCG. Individual pe/ppe gene alignments were extracted and diversity and conservation across the different lineages studied.

Results

The pe/ppe genes were classified into three groups based on the level of protein sequence conservation relative to H37Rv, finding that >50% were conserved, with indels in pe_pgrs and ppe_mptr sub-families being major drivers of structural variation. Gene rearrangements, such as duplications and gene fusions, were observed between pe and pe_pgrs genes. Inter-lineage diversity revealed lineage-specific SNPs and indels.

Discussion

The high level of pe/ppe genes conservation, together with the lineage-specific findings, suggest their phylogenetic informativeness. However, structural variants and gene rearrangements differing from the reference were also identified, with potential implications for pathogenicity. Overall, improving our knowledge of these complex gene families may have insights into pathogenicity and inform the development of much-needed tools for tuberculosis control.

The Mycobacterium tuberculosis genome at 25 years: lessons and lingering questions

First achieved in 1998 by Cole et al., the complete genome sequence of Mycobacterium tuberculosis continues to provide an invaluable resource to understand tuberculosis (TB), the leading cause of global infectious disease mortality. At the 25-year anniversary of this accomplishment, we describe how insights gleaned from the M. tuberculosis genome have led to vital tools for TB research, epidemiology, and clinical practice. The increasing accessibility of whole-genome sequencing across research and clinical settings has improved our ability to predict antibacterial susceptibility, to track epidemics at the level of individual outbreaks and wider historical trends, to query the efficacy of the bacille Calmette-Guérin (BCG) vaccine, and to uncover targets for novel antitubercular therapeutics. Likewise, we discuss several recent efforts to extract further discoveries from this powerful resource.

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.

Transcontinental spread and evolution of Mycobacterium tuberculosis W148 European/Russian clade toward extensively drug resistant tuberculosis

Transmission-driven multi-/extensively drug resistant (M/XDR) tuberculosis (TB) is the largest single contributor to human mortality due to antimicrobial resistance. A few major clades of the Mycobacterium tuberculosis complex belonging to lineage 2, responsible for high prevalence of MDR-TB in Eurasia, show outstanding transnational distributions. Here, we determined factors underlying the emergence and epidemic spread of the W148 clade by genome sequencing and Bayesian demogenetic analyses of 720 isolates from 23 countries. We dated a common ancestor around 1963 and identified two successive epidemic expansions in the late 1980s and late 1990s, coinciding with major socio-economic changes in the post-Soviet Era. These population expansions favored accumulation of resistance mutations to up to 11 anti-TB drugs, with MDR evolving toward additional resistances to fluoroquinolones and second-line injectable drugs within 20 years on average. Timescaled haplotypic density analysis revealed that widespread acquisition of compensatory mutations was associated with transmission success of XDR strains. Virtually all W148 strains harbored a hypervirulence-associated ppe38 gene locus, and incipient recurrent emergence of prpR mutation-mediated drug tolerance was detected. The outstanding genetic arsenal of this geographically widespread M/XDR strain clade represents a “perfect storm” that jeopardizes the successful introduction of new anti-M/XDR-TB antibiotic regimens.

An outbreak of the multidrug-resistant Mycobacterium tuberculosis lineage W148 has spread widely across Russia, Central Asia and Europe. Here, the authors use whole genome sequences of ~700 isolates of this lineage collected over ~20 years to analyze its spread, evolution of drug resistance, and impact of compensatory mutations.

In-depth Analysis of IS6110 Genomic Variability in the Mycobacterium tuberculosis Complex

The insertion sequence (IS) 6110 is a repetitive mobile element specific for the Mycobacterium tuberculosis complex (MTBC) used for years to diagnose and genotype this pathogen. It contains the overlapping reading frames orfA and orfB that encode a transposase. Its genetic variability is difficult to study because multiple copies are present in the genome. IS6110 is randomly located, nevertheless some preferential locations have been reported, which could be related to the behaviour of the strains. The aim of this work was to determine the intra- and inter-strain genetic conservation of this element in the MTBC. For this purpose, we analysed 158 sequences of IS6110 copies from 55 strains. Eighty-four copies were from 17 strains for which we knew all the locations in their genome. In addition, we studied 74 IS6110 copies in 38 different MTBC strains in which the location was characteristic of different families including Haarlem, LAM, S, and L6 strains. We observed mutation in 13.3% of the copies studied and we found 10 IS6110 variants in 21 copies belonging to 16 strains. The high copy number strains showed 6.2% of their IS6110 copies mutated, in contrast with the 31.1% in the low-copy-number strains. The apparently more ancient copy localised in the DR region was that with more variant copies, probably because this was the most studied location. Notably, all Haarlem and X family strains studied have an IS6110 in Rv0403c, suggesting a common origin for both families. Nevertheless, we detected a variant specific for the X family that would have occurred in this location after the phylogenetic separation. This variant does not prevent transposition although it may occur at a lower frequency, as X strains remain with low copy number (LCN) of IS6110.

In Silico Exploration of Mycobacterium tuberculosis Metabolic Networks Shows Host-Associated Convergent Fluxomic Phenotypes

Mycobacterium tuberculosis, the causative agent of tuberculosis, is composed of several lineages characterized by a genome identity higher than 99%. Although the majority of the lineages are associated with humans, at least four lineages are adapted to other mammals, including different M. tuberculosis ecotypes. Host specificity is associated with higher virulence in its preferred host in ecotypes such as M. bovis. Deciphering what determines the preference of the host can reveal host-specific virulence patterns. However, it is not clear which genomic determinants might be influencing host specificity. In this study, we apply a combination of unsupervised and supervised classification methods on genomic data of ~27,000 M. tuberculosis clinical isolates to decipher host-specific genomic determinants. Host-specific genomic signatures are scarce beyond known lineage-specific mutations. Therefore, we integrated lineage-specific mutations into the iEK1011 2.0 genome-scale metabolic model to obtain lineage-specific versions of it. Flux distributions sampled from the solution spaces of these models can be accurately separated according to host association. This separation correlated with differences in cell wall processes, lipid, amino acid and carbon metabolic subsystems. These differences were observable when more than 95% of the samples had a specific growth rate significantly lower than the maximum achievable by the models. This suggests that these differences might manifest at low growth rate settings, such as the restrictive conditions M. tuberculosis suffers during macrophage infection.

Multiple genetic paths including massive gene amplification allow Mycobacterium tuberculosis to overcome loss of ESX-3 secretion system substrates

The Mycobacterium tuberculosis (Mtb) ESX-3 type VII secretion system plays a critical role in iron acquisition. Infection of mice with highly attenuated Mtb deletion mutants lacking esxG or esxH, genes encoding key ESX-3 substrates, unexpectedly yielded suppressor mutants with restored capacity to grow in vivo and in vitro in the absence of iron supplementation. Whole-genome sequencing identified two mechanisms of suppression, the disruption of a transcriptional repressor that regulates expression of an ESX-3 paralogous region encoding EsxR and EsxS, and a massive 38- to 60-fold gene amplification of this same region. These data are significant because they reveal a previously unrecognized iron acquisition regulon and inform mechanisms of Mtb chromosome evolution.

Mycobacterium tuberculosis (Mtb) possesses five type VII secretion systems (T7SS), virulence determinants that include the secretion apparatus and associated secretion substrates. Mtb strains deleted for the genes encoding substrates of the ESX-3 T7SS, esxG or esxH, require iron supplementation for in vitro growth and are highly attenuated in vivo. In a subset of infected mice, suppressor mutants of esxG or esxH deletions were isolated, which enabled growth to high titers or restored virulence. Suppression was conferred by mechanisms that cause overexpression of an ESX-3 paralogous region that lacks genes for the secretion apparatus but encodes EsxR and EsxS, apparent ESX-3 orphan substrates that functionally compensate for the lack of EsxG or EsxH. The mechanisms include the disruption of a transcriptional repressor and a massive 38- to 60-fold gene amplification. These data identify an iron acquisition regulon, provide insight into T7SS, and reveal a mechanism of Mtb chromosome evolution involving “accordion-type” amplification.

Proline-Glutamate/Proline-Proline-Glutamate (PE/PPE) proteins of Mycobacterium tuberculosis: The multifaceted immune-modulators

The PE/PPE proteins encoded by seven percent (7%) of Mycobacterium tuberculosis (Mtb) genome are the chief constituents to pathogen's virulence reservoir. The fact that these genes have evolved along ESX secretory system in pathogenic Mtb strains make their investigation very intriguing. There is lot of speculation about the prominent role of these proteins at host pathogen interface and in disease pathogenesis. Nevertheless, the exact function of PE/PPE proteins still remains a mystery which calls for further research targeting these proteins. This article is an effort to document all the facts known so far with regard to these unique proteins which involves their origin, evolution, transcriptional control, and most important their role as host immune-modulators. Our understanding strongly points towards the versatile nature of these PE/PPE proteins as Mtb's host immune sensors and as decisive factors in shaping the outcome of infection. Further investigation on these proteins will surely pave way for newer and effective vaccines and therapeutics to control Tuberculosis (TB).

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.

PPE38-Secretion-Dependent Proteins of M. tuberculosis Alter NF-kB Signalling and Inflammatory Responses in Macrophages

It was previously shown that secretion of PE-PGRS and PPE-MPTR proteins is abolished in clinical M. tuberculosis isolates with a deletion in the ppe38-71 operon, which is associated with increased virulence. Here we investigate the proteins dependent on PPE38 for their secretion and their role in the innate immune response using temporal proteomics and protein turnover analysis in a macrophage infection model. A decreased pro-inflammatory response was observed in macrophages infected with PPE38-deficient M. tuberculosis CDC1551 as compared to wild type bacteria. We could show that dampening of the pro-inflammatory response is associated with activation of a RelB/p50 pathway, while the canonical inflammatory pathway is active during infection with wild type M. tuberculosis CDC1551. These results indicate a molecular mechanism by which M. tuberculosis PE/PPE proteins controlled by PPE38 have an effect on modulating macrophage responses through NF-kB signalling.

Insight into the drug resistance whole genome of Mycobacterium tuberculosis isolates from Khyber Pakhtunkhwa, Pakistan

Whole genome sequencing (WGS) is one of the most reliable methods for detection of drug resistance, genetic diversity in other virulence factor and also evolutionary dynamics of Mycobacterium tuberculosis complex (MTBC). First-line anti-tuberculosis drugs are the major weapons against Mycobacterium tuberculosis (MTB). However, the emergence of drug resistance remained a major obstacle towards global tuberculosis (TB) control program 2030, especially in high burden countries including Pakistan. To overcome the resistance and design potent drugs, genomic variations in drugs targets as well as in the virulence and evolutionary factors might be useful for better understanding and designing potential inhibitors. Here we aimed to find genomic variations in the first-line drugs targets, along with other virulence and evolutionary factors among the circulating isolates in Khyber Pakhtunkhwa, Pakistan. Samples were collected and drug susceptibility testing (DST) was performed as per WHO standard. The resistance samples were subjected to WGS. Among the five whole genome sequences, three samples (NCBI BioProject Accession: PRJNA629298, PRJNA629388) harbored 1997, 1162, and 2053 mutations. Some novel mutations have been detected in drugs targets. Similarly, numerous novel variants have also been detected in virulency and evolutionary factors, PE, PPE, and secretory system of MTB isolates. Exploring the genomic variations among the circulating isolates in geographical specific locations might be useful for future drug designing. To the best of our knowledge, this is the first study that provides useful data regarding the insight genomic variations in virulency, evolutionary factors including ESX and PE/PPE as well as drug targets, for better understanding and management of TB in a WHO declared high burden country.

Pathogenomic analyses of Mycobacterium microti, an ESX-1-deleted member of the Mycobacterium tuberculosis complex causing disease in various hosts

Mycobacterium microti is an animal-adapted member of the Mycobacterium tuberculosis complex (MTBC), which was originally isolated from voles, but has more recently also been isolated from other selected mammalian hosts, including occasionally from humans. Here, we have generated and analysed the complete genome sequences of five representative vole and clinical M. microti isolates using PacBio- and Illumina-based technologies, and have tested their virulence and vaccine potential in SCID (severe combined immune deficient) mouse and/or guinea pig infection models. We show that the clinical isolates studied here cluster separately in the phylogenetic tree from vole isolates and other clades from publicly available M. microti genome sequences. These data also confirm that the vole and clinical M. microti isolates were all lacking the specific RD1mic region, which in other tubercle bacilli encodes the ESX-1 type VII secretion system. Biochemical analysis further revealed marked phenotypic differences between isolates in type VII-mediated secretion of selected PE and PPE proteins, which in part were attributed to specific genetic polymorphisms. Infection experiments in the highly susceptible SCID mouse model showed that the clinical isolates were significantly more virulent than the tested vole isolates, but still much less virulent than the M. tuberculosis H37Rv control strain. The strong attenuation of the ATCC 35872 vole isolate in immunocompromised mice, even compared to the attenuated BCG (bacillus Calmette-Guérin) vaccine, and its historic use in human vaccine trials encouraged us to test this strain's vaccine potential in a guinea pig model, where it demonstrated similar protective efficacy as a BCG control, making it a strong candidate for vaccination of immunocompromised individuals in whom BCG vaccination is contra-indicated. Overall, we provide new insights into the genomic and phenotypic variabilities and particularities of members of an understudied clade of the MTBC, which all share a recent common ancestor that is characterized by the deletion of the RD1mic region.

Identification of gene fusion events in Mycobacterium tuberculosis that encode chimeric proteins

Mycobacterium tuberculosis is a facultative intracellular pathogen responsible for causing tuberculosis. The harsh environment in which M. tuberculosis survives requires this pathogen to continuously adapt in order to maintain an evolutionary advantage. However, the apparent absence of horizontal gene transfer in M. tuberculosis imposes restrictions in the ways by which evolution can occur. Large-scale changes in the genome can be introduced through genome reduction, recombination events and structural variation. Here, we identify a functional chimeric protein in the ppe38-71 locus, the absence of which is known to have an impact on protein secretion and virulence. To examine whether this approach was used more often by this pathogen, we further develop software that detects potential gene fusion events from multigene deletions using whole genome sequencing data. With this software we could identify a number of other putative gene fusion events within the genomes of M. tuberculosis isolates. We were able to demonstrate the expression of one of these gene fusions at the protein level using mass spectrometry. Therefore, gene fusions may provide an additional means of evolution for M. tuberculosis in its natural environment whereby novel chimeric proteins and functions can arise.

Live attenuated TB vaccines representing the three modern Mycobacterium tuberculosis lineages reveal that the Euro-American genetic background confers optimal vaccine potential

Background

Human tuberculosis (TB) is caused by a plethora of Mycobacterium tuberculosis complex (MTBC) strains belonging to seven phylogenetic branches. Lineages 2, 3 and 4 are considered “modern” branches of the MTBC responsible for the majority of worldwide TB. Since the current BCG vaccine confers variable protection against pulmonary TB, new candidates are investigated. MTBVAC is the unique live attenuated vaccine based on M. tuberculosis in human clinical trials.

Methods

MTBVAC was originally constructed by unmarked phoP and fadD26 deletions in a clinical isolate belonging to L4. Here we construct new vaccines based on isogenic gene deletions in clinical isolates of the L2 and L3 modern lineages. These three vaccine candidates were characterized at molecular level and also in animal experiments of protection and safety.

Findings

Safety studies in immunocompromised mice showed that MTBVAC-L2 was less attenuated than BCG Pasteur, while the original MTBVAC was found even more attenuated than BCG and MTBVAC-L3 showed an intermediate phenotype. The three MTBVAC candidates showed similar or superior protection compared to BCG in immunocompetent mice vaccinated with each MTBVAC candidate and challenged with three representative strains of the modern lineages.

Interpretation

MTBVAC vaccines, based on double phoP and fadD26 deletions, protect against TB independently of the phylogenetic linage used as template strain for their construction. Nevertheless, lineage L4 confers the best safety profile.

Funding

European Commission (TBVAC2020, H2020-PHC-643381), Spanish Ministry of Science (RTI2018-097625-B-I00), Instituto de Salud Carlos III (PI18/0336), Gobierno de Aragón/Fondo Social Europeo and the French National Research Council (ANR-10-LABX-62-IBEID, ANR-16-CE35-0009, ANR-16-CE15-0003).

New insights into the mycobacterial PE and PPE proteins provide a framework for future research

The PE and PPE proteins of Mycobacterium tuberculosis have been studied with great interest since their discovery. Named after the conserved proline (P) and glutamic acid (E) residues in their N-terminal domains, these proteins are postulated to perform wide-ranging roles in virulence and immune modulation. However, technical challenges in studying these proteins and their encoding genes have hampered the elucidation of molecular mechanisms and leave many open questions regarding the biological functions mediated by these proteins. Here, I review the shared and unique characteristics of PE and PPE proteins from a molecular perspective linking this information to their functions in mycobacterial virulence. I discuss how the different subgroups (PE_PGRS, PPE-PPW, PPE-SVP and PPE-MPTR) are defined and why this classification of paramount importance to understand the PE and PPE proteins as individuals and or groups. The goal of this MicroReview is to summarize and structure the existing information on this gene family into a simplified framework of thinking about PE and PPE proteins and genes. Thereby, I hope to provide helpful starting points in studying these genes and proteins for researchers with different backgrounds. This has particular implications for the design and monitoring of novel vaccine candidates and in understanding the evolution of the M. tuberculosis complex.

PPE39 of the Mycobacterium tuberculosis strain Beijing/K induces Th1-cell polarization through dendritic cell maturation

In a previous study, we have identified MTBK_24820, the complete protein form of PPE39 in the hypervirulent Mycobacterium tuberculosis (Mtb) strain Beijing/K by using comparative genomic analysis. PPE39 exhibited vaccine potential against Mtb challenge in a murine model. Thus, in this present study, we characterize PPE39-induced immunological features by investigating the interaction of PPE39 with dendritic cells (DCs). PPE39-treated DCs display reduced dextran uptake and enhanced MHC-I, MHC-II, CD80 and CD86 expression, indicating that this PPE protein induces phenotypic DC maturation. In addition, PPE39-treated DCs produce TNF-α, IL-6 and IL-12p70 to a similar and/or greater extent than lipopolysaccharide-treated DCs in a dose-dependent manner. The activating effect of PPE39 on DCs was mediated by TLR4 through downstream MAPK and NF-κB signaling pathways. Moreover, PPE39-treated DCs promoted naïve CD4⁺ T-cell proliferation accompanied by remarkable increases of IFN-γ and IL-2 secretion levels, and an increase in the Th1-related transcription factor T-bet but not in Th2-associated expression of GATA-3, suggesting that PPE39 induces Th1-type T-cell responses through DC activation. Collectively, the results indicate that the complete form of PPE39 is a so-far-unknown TLR4 agonist that induces Th1-cell biased immune responses by interacting with DCs.This article has an associated First Person interview with the first author of the paper.

Unexpected Genomic and Phenotypic Diversity of Mycobacterium africanum Lineage 5 Affects Drug Resistance, Protein Secretion, and Immunogenicity

Mycobacterium africanum consists of Lineages L5 and L6 of the Mycobacterium tuberculosis complex (MTBC) and causes human tuberculosis in specific regions of Western Africa, but is generally not transmitted in other parts of the world. Since M. africanum is evolutionarily closely placed between the globally dispersed Mycobacterium tuberculosis and animal-adapted MTBC-members, these lineages provide valuable insight into M. tuberculosis evolution. Here, we have collected 15 M. africanum L5 strains isolated in France over 4 decades. Illumina sequencing and phylogenomic analysis revealed a previously underappreciated diversity within L5, which consists of distinct sublineages. L5 strains caused relatively high levels of extrapulmonary tuberculosis and included multi- and extensively drug-resistant isolates, especially in the newly defined sublineage L5.2. The specific L5 sublineages also exhibit distinct phenotypic characteristics related to in vitro growth, protein secretion and in vivo immunogenicity. In particular, we identified a PE_PGRS and PPE-MPTR secretion defect specific for sublineage L5.2, which was independent of PPE38. Furthermore, L5 isolates were able to efficiently secrete and induce immune responses against ESX-1 substrates contrary to previous predictions. These phenotypes of Type VII protein secretion and immunogenicity provide valuable information to better link genome sequences to phenotypic traits and thereby understand the evolution of the MTBC.

RD5-mediated lack of PE_PGRS and PPE-MPTR export in BCG vaccine strains results in strong reduction of antigenic repertoire but little impact on protection

Tuberculosis is the deadliest infectious disease worldwide. Although the BCG vaccine is widely used, it does not efficiently protect against pulmonary tuberculosis and an improved tuberculosis vaccine is therefore urgently needed. Mycobacterium tuberculosis uses different ESX/Type VII secretion (T7S) systems to transport proteins important for virulence and host immune responses. We recently reported that secretion of T7S substrates belonging to the mycobacteria-specific Pro-Glu (PE) and Pro-Pro-Glu (PPE) proteins of the PGRS (polymorphic GC-rich sequences) and MPTR (major polymorphic tandem repeat) subfamilies required both a functional ESX-5 system and a functional PPE38/71 protein for secretion. Inactivation of ppe38/71 and the resulting loss of PE_PGRS/PPE-MPTR secretion were linked to increased virulence of M. tuberculosis strains. Here, we show that a predicted total of 89 PE_PGRS/PPE-MPTR surface proteins are not exported by certain animal-adapted strains of the M. tuberculosis complex including M. bovis. This Δppe38/71-associated secretion defect therefore also occurs in the M. bovis-derived tuberculosis vaccine BCG and could be partially restored by introduction of the M. tuberculosis ppe38-locus. Epitope mapping of the PPE-MPTR protein PPE10, further allowed us to monitor T-cell responses in splenocytes from BCG/M. tuberculosis immunized mice, confirming the dependence of PPE10-specific immune-induction on ESX-5/PPE38-mediated secretion. Restoration of PE_PGRS/PPE-MPTR secretion in recombinant BCG neither altered global antigenic presentation or activation of innate immune cells, nor protective efficacy in two different mouse vaccination-infection models. This unexpected finding stimulates a reassessment of the immunomodulatory properties of PE_PGRS/PPE-MPTR proteins, some of which are contained in vaccine formulations currently in clinical evaluation.

One of the major findings of the pioneering Mycobacterium tuberculosis H37Rv genome sequencing project was the identification of the highly abundant PE and PPE proteins, named after their N-terminal motifs Pro–Glu (PE) or Pro–Pro–Glu (PPE). Within the 20 years of research since then, many claims were made that PE/PPE proteins, including the two large subgroups encoded by repetitive sequences with very high GC content (PE_PGRS and PPE-MPTR families), are exported to the bacterial surface or beyond, and show broad immunomodulatory impact on host-pathogen interaction. We thus screened strains from different branches of the M. tuberculosis complex, including the attenuated Mycobacterium bovis BCG vaccine strains, for their capacity to export PE_PGRS/PPE-MPTR proteins. Strikingly, we found that BCG strains were unable to export the plethora of PE_PGRS/PPE-MPTR proteins due to the absence of the region of difference RD5, which in M. tuberculosis encodes PPE38, required for PE_PGRS/PPE-MPTR export. Surprisingly, the restoration of PE_PGRS/PPE-MPTR export by genetic complementation in recombinant BCG did not result in immunomodulatory changes or altered protection in mouse models. Our results thus put into perspective the numerous reports on virulence-associated immunomodulatory impact of individual PE_PGRS and PPE-MPTR proteins and open novel questions on their biological function(s).

Protective Vaccine Efficacy of the Complete Form of PPE39 Protein from Mycobacterium tuberculosis Beijing/K Strain in Mice

The aim of this study was to evaluate the protective efficacy of MTBK_24820, a complete form of PPE39 protein derived from a predominant Beijing/K strain of Mycobacterium tuberculosis in South Korea. Mice were immunized with MTKB_24820, M. bovis Bacilli Calmette-Guérin (BCG), or adjuvant prior to a high-dosed Beijing/K strain aerosol infection. After 4 and 9 weeks, bacterial loads were determined and histopathologic and immunologic features in the lungs and spleens of the M. tuberculosis-infected mice were analyzed. Putative immunogenic T-cell epitopes were examined using synthetic overlapping peptides. Successful immunization of MTBK_24820 in mice was confirmed by increased IgG responses (P < 0.05) and recalled gamma interferon (IFN-γ), interleukin-2 (IL-2), IL-6, and IL-17 responses (P < 0.05 or P < 0.01) to MTBK_24820. After challenge with the Beijing/K strain, an approximately 0.5 to 1.0 log₁₀ reduction in CFU in lungs and fewer lung inflammation lesions were observed in MTBK_24820-immunized mice compared to those for control mice. Moreover, MTBK_24820 immunization elicited significantly higher numbers of CD4⁺ T cells producing protective cytokines, such as IFN-γ and IL-17, in lungs and spleens (P < 0.01) and CD4⁺ multifunctional T cells producing IFN-γ, tumor necrosis factor alpha (TNF-α), and/or IL-17 (P < 0.01) than in control mice, suggesting protection comparable to that of BCG against the hypervirulent Beijing/K strain. The dominant immunogenic T-cell epitopes that induced IFN-γ production were at the N terminus (amino acids 85 to 102 and 217 to 234). Its vaccine potential, along with protective immune responses in vivo, may be informative for vaccine development, particularly in regions where the M. tuberculosis Beijing/K-strain is frequently isolated from TB patients.

The Enigmatic PE/PPE Multigene Family of Mycobacteria and Tuberculosis Vaccination

The genome of Mycobacterium tuberculosis, the bacterium responsible for the disease tuberculosis, contains an unusual family of abundant antigens (PE/PPEs). To date, certain members of this multigene family occur only in mycobacteria that cause disease. It is possible that the numerous proteins encoded by these mycobacterial genes dictate the immune pathogenesis of this bacterial pathogen. There is also evidence that some of these antigens are present at the cell surface and that they affect the pathology and immunology of the organism in many ways. Also, they elicit both antibodies and T cells, they may be involved in antigenic variation, and they may be good candidates for vaccines and drugs. However, since they are plentiful and extremely homologous, these PE/PPEs are very challenging to study, and it is difficult to be certain what role(s) they have in the pathogenesis of tuberculosis. Consequently, how to develop treatments like vaccines using these antigens as candidates is complex.

SMRT genome assembly corrects reference errors, resolving the genetic basis of virulence in Mycobacterium tuberculosis

BACKGROUND

The genetic basis of virulence in Mycobacterium tuberculosis has been investigated through genome comparisons of virulent (H37Rv) and attenuated (H37Ra) sister strains. Such analysis, however, relies heavily on the accuracy of the sequences. While the H37Rv reference genome has had several corrections to date, that of H37Ra is unmodified since its original publication.

RESULTS

Here, we report the assembly and finishing of the H37Ra genome from single-molecule, real-time (SMRT) sequencing. Our assembly reveals that the number of H37Ra-specific variants is less than half of what the Sanger-based H37Ra reference sequence indicates, undermining and, in some cases, invalidating the conclusions of several studies. PE_PPE family genes, which are intractable to commonly-used sequencing platforms because of their repetitive and GC-rich nature, are overrepresented in the set of genes in which all reported H37Ra-specific variants are contradicted. Further, one of the sequencing errors in H37Ra masks a true variant in common with the clinical strain CDC1551 which, when considered in the context of previous work, corresponds to a sequencing error in the H37Rv reference genome.

CONCLUSIONS

Our results constrain the set of genomic differences possibly affecting virulence by more than half, which focuses laboratory investigation on pertinent targets and demonstrates the power of SMRT sequencing for producing high-quality reference genomes.

Patterns and processes of Mycobacterium bovis evolution revealed by phylogenomic analyses

Mycobacterium bovis is an important animal pathogen worldwide that parasitizes wild and domesticated vertebrate livestock as well as humans. A comparison of the five M. bovis complete genomes from the United Kingdom, South Korea, Brazil, and the United States revealed four novel large-scale structural variations of at least 2,000 bp. A comparative phylogenomic study including 2,483 core genes of 38 taxa from eight countries showed conflicting phylogenetic signal among sites. By minimizing this effect, we obtained a tree that better agrees with sampling locality. Results supported a relatively basal position of African strains (all isolated from Homo sapiens ), confirming that Africa was an important region for early diversification and that humans were one of the earliest hosts. Selection analyses revealed that functional categories such as “Lipid transport and metabolism,” “Cell cycle control, cell division, chromosome partitioning” and “Cell motility” were significant for the evolution of the group, besides other categories previously described, showing importance of genes associated with virulence and cholesterol metabolism in the evolution of M. bovis . PE/PPE genes, many of which are known to be associated with virulence, were major targets for large-scale polymorphisms, homologous recombination, and positive selection, evincing for the first time a plethora of evolutionary forces possibly contributing to differential adaptability in M. bovis . By assuming different priors, US strains originated and started to diversify around 150–5,210 ya. By further analyzing the largest set of US genomes to date (76 in total), obtained from 14 host species, we detected that hosts were not clustered in clades (except for a few cases), with some faster-evolving strains being detected, suggesting fast and ongoing reinfections across host species, and therefore, the possibility of new bovine tuberculosis outbreaks.

PE and PPE Genes: A Tale of Conservation and Diversity

PE and PPE are two large families of proteins typical of mycobacteria whose structural genes in the Mycobacterium tuberculosis complex (MTBC) occupy about 7% of the total genome. The most ancestral PE and PPE proteins are expressed by genes that belong to the same operon and in most cases are found inserted in the esx clusters, encoding a type VII secretion system. Duplication and expansion of pe and ppe genes, coupled with intragenomic and intergenomic recombination events, led to the emergence of the polymorphic pe_pgrs and ppe_mptr genes in the MTBC genome. The role and function of these proteins, and particularly of the polymorphic subfamilies, remains elusive, although it is widely accepted that PE and PPE proteins may represent a specialized collection used by MTBC to interact with the complex host immune system of mammals. In this chapter, we summarize what has been discovered since the identification of these genes in 1998, focusing on M. tuberculosis genetic variability, host-pathogen interaction and TB pathogenesis.

Identification of a Non-Pentapeptide Region Associated with Rapid Mycobacterial Evolution

A large portion of the coding capacity of Mycobacterium tuberculosis is devoted to the production of proteins containing several copies of the pentapeptide-2 repeat, namely the PE/PPE_MPTR proteins. Protein domain repeats have a variety of binding properties and are involved in protein-protein interactions as well as binding to other ligands such as DNA and RNA. They are not as common in prokaryotes, compared to eukaryotes, but the enrichment of pentapeptide-2 repeats in Mycobacteria constitutes an exception to that rule. The genes encoding the PE/PPE_MPTR proteins have undergone many rearrangements and here we have identified the expansion patterns across the Mycobacteria. We have performed a reclassification of the PE/PPE_MPTR proteins using cohesive regions rather than sparse domain architectures. It is clear that these proteins have undergone large insertions of several pentapeptide-2 domains appearing adjacent to one another in a repetitive pattern. Further, we have identified a non-pentapeptide motif associated with rapid mycobacterial evolution. The sequence composition of this region suggests a different structure compared to pentapeptide-2 repeats. By studying the evolution of the PE/PPE_MPTR proteins, we have distinguished features pertaining to tuberculosis-inducing species. Further studies of the non-pentapeptide region associated with repeat expansions promises to shed light on the pathogenicity of Mycobacterium tuberculosis.

Double-Strand DNA Break Repair in Mycobacteria

Discontinuity of both strands of the chromosome is a lethal event in all living organisms because it compromises chromosome replication. As such, a diversity of DNA repair systems has evolved to repair double-strand DNA breaks (DSBs). In part, this diversity of DSB repair systems has evolved to repair breaks that arise in diverse physiologic circumstances or sequence contexts, including cellular states of nonreplication or breaks that arise between repeats. Mycobacteria elaborate a set of three genetically distinct DNA repair pathways: homologous recombination, nonhomologous end joining, and single-strand annealing. As such, mycobacterial DSB repair diverges substantially from the standard model of prokaryotic DSB repair and represents an attractive new model system. In addition, the presence in mycobacteria of a DSB repair system that can repair DSBs in nonreplicating cells (nonhomologous end joining) or when DSBs arise between repeats (single-strand annealing) has clear potential relevance to Mycobacterium tuberculosis pathogenesis, although the exact role of these systems in M. tuberculosis pathogenesis is still being elucidated. In this article we will review the genetics of mycobacterial DSB repair systems, focusing on recent insights.

Mycobacterium marinum mmar_2318 and mmar_2319 are Responsible for Lipooligosaccharide Biosynthesis and Virulence Toward Dictyostelium

Resistance to phagocyte killing is an important virulence factor in mycobacteria. Dictyostelium has been used to study the interaction between phagocytes and bacteria, given its similarity to the mammalian macrophage. Here, we investigated the genes responsible for virulence to Dictyostelium by screening 1728 transposon mutants of the Mycobacterium marinum NTUH-M6094 strain. A total of 30 mutants that permissive for Dictyostelium growth were identified. These mutants revealed interruptions in 20 distinct loci. Of the 20 loci, six genes (losA, mmar_2318, mmar_2319, wecE, mmar_2323 and mmar_2353) were located in the lipooligosaccharide (LOS) synthesis cluster. LOS are antigenic glycolipids and the core LOS structure from LOS-I to LOS-IV have been reported to exist in M. marinum. Two-dimensional thin-layer chromatography (2D-TLC) glycolipid profiles revealed that deletion of mmar_2318 or mmar_2319 resulted in the accumulation of LOS-III and deficiency of LOS-IV. Deletion and complementation of mmar_2318 or mmar_2319 confirmed that these genes both contributed to virulence toward Dictyostelium but not entry and replication inside Dictyostelium. Co-incubation with a murine macrophage cell line J774a.1 or PMA-induced human monocytic cell line THP-1 demonstrated that mmar_2318 or mmar_2319 deletion mutant could grow in macrophages, and their initial entry rate was not affected in J774a.1 but significantly increased in THP-1. In conclusion, although mmar_2319 has been reported to involve LOS biosynthesis in a previous study, we identified a new gene, mmar_2318 that is also involved in the biosynthesis of LOS. Deletion of mmar_2318 or mmar_2319 both exhibits reduction of virulence toward Dictyostelium and increased entry into THP-1 cells.

Mycobacterium tuberculosis PE27 activates dendritic cells and contributes to Th1-polarized memory immune responses during in vivo infection

A gradual understanding of the proline-glutamate (PE) and proline-proline-glutamate (PPE) families, which compromise 10% of the coding regions in the Mycobacterium tuberculosis (Mtb) genome, has uncovered unique roles in host-pathogen interactions. However, the immunological function of PE27 (Rv2769c), the largest PE member, remains unclear. Here, we explored the functional roles and related signaling mechanisms of PE27 in the interaction with dendritic cells (DCs) to shape the T cell response. PE27 phenotypically and functionally induces DC maturation by up-regulating CD80, CD86, MHC class I and MHC class II expression on the DC surface to promote the production of TNF-α, IL-1β, IL-6, and IL-12p70 but not IL-10. Additionally, we found that PE27-mediated DC activation requires the participation of mitogen-activated protein kinases (MAPKs) and nuclear factor κB (NF-κB) signaling pathways. Interestingly, PE27-treated DCs directed naïve CD4(+) T cells to secrete IFN-γ and activate T-bet but not GATA-3. PE27 also induced IFN-γ-producing memory T cell responses in Mtb-infected mice, indicating that PE27 contributes to Th1-polarization. Taken together, these findings suggest that PE27 possesses Th1-polarizing potential through DC maturation and could be useful in the design of TB vaccines.

Identification of Novel Potential Vaccine Candidates against Tuberculosis Based on Reverse Vaccinology

Tuberculosis (TB) is a chronic infectious disease, considered as the second leading cause of death worldwide, caused by Mycobacterium tuberculosis. The limited efficacy of the bacillus Calmette-Guérin (BCG) vaccine against pulmonary TB and the emergence of multidrug-resistant TB warrants the need for more efficacious vaccines. Reverse vaccinology uses the entire proteome of a pathogen to select the best vaccine antigens by in silico approaches. M. tuberculosis H37Rv proteome was analyzed with NERVE (New Enhanced Reverse Vaccinology Environment) prediction software to identify potential vaccine targets; these 331 proteins were further analyzed with VaxiJen for the determination of their antigenicity value. Only candidates with values ≥0.5 of antigenicity and 50% of adhesin probability and without homology with human proteins or transmembrane regions were selected, resulting in 73 antigens. These proteins were grouped by families in seven groups and analyzed by amino acid sequence alignments, selecting 16 representative proteins. For each candidate, a search of the literature and protein analysis with different bioinformatics tools, as well as a simulation of the immune response, was conducted. Finally, we selected six novel vaccine candidates, EsxL, PE26, PPE65, PE_PGRS49, PBP1, and Erp, from M. tuberculosis that can be used to improve or design new TB vaccines.

Phylogeny to function: PE/PPE protein evolution and impact on Mycobacterium tuberculosis pathogenicity

The pe/ppe genes represent one of the most intriguing aspects of the Mycobacterium tuberculosis genome. These genes are especially abundant in pathogenic mycobacteria, with more than 160 members in M. tuberculosis. Despite being discovered over 15 years ago, their function remains unclear, although various lines of evidence implicate selected family members in mycobacterial virulence. In this review, we use PE/PPE phylogeny as a framework within which we examine the diversity and putative functions of these proteins. We report on the evolution and diversity of the respective gene families, as well as the implications thereof for function and host immune recognition. We summarize recent findings on pe/ppe gene regulation, also placing this in the context of PE/PPE phylogeny. We collate data from several large proteomics datasets, providing an overview of PE/PPE localization, and discuss the implications this may have for host responses. Assessment of the current knowledge of PE/PPE diversity suggests that these proteins are not variable antigens as has been so widely speculated; however, they do clearly play important roles in virulence. Viewing the growing body of pe/ppe literature through the lens of phylogeny reveals trends in features and function that may be associated with the evolution of mycobacterial pathogenicity.

Consequences of genomic diversity in Mycobacterium tuberculosis

The causative agent of human tuberculosis, Mycobacterium tuberculosis complex (MTBC), comprises seven phylogenetically distinct lineages associated with different geographical regions. Here we review the latest findings on the nature and amount of genomic diversity within and between MTBC lineages. We then review recent evidence for the effect of this genomic diversity on mycobacterial phenotypes measured experimentally and in clinical settings. We conclude that overall, the most geographically widespread Lineage 2 (includes Beijing) and Lineage 4 (also known as Euro-American) are more virulent than other lineages that are more geographically restricted. This increased virulence is associated with delayed or reduced pro-inflammatory host immune responses, greater severity of disease, and enhanced transmission. Future work should focus on the interaction between MTBC and human genetic diversity, as well as on the environmental factors that modulate these interactions.

Origins of a 350-kilobase genomic duplication in Mycobacterium tuberculosis and its impact on virulence

In the present study, we have investigated the evolution and impact on virulence of a 350-kb genomic duplication present in the most recently evolved members of the Mycobacterium tuberculosis East Asian lineage. In a mouse model of infection, comparing HN878 subclones HN878-27 (no duplication) and HN878-45 (with the 350-kb duplication) revealed that the latter is impaired for in vivo growth during the initial 3 weeks of infection. Furthermore, the median survival time of mice infected with isolate HN878-45 is significantly longer (77 days) than that of mice infected with HN878-27. Whole-genome sequencing of both isolates failed to reveal any mutational events other than the duplication that could account for such a substantial difference in virulence. Although we and others had previously speculated that the 350-kb duplication arose in response to some form of host-applied selective pressure (P. Domenech, G. S. Kolly, L. Leon-Solis, A. Fallow, M. B. Reed, J. Bacteriol. 192: 4562-4570, 2010, and B. Weiner, J. Gomez, T. C. Victor, R. M. Warren, A. Sloutsky, B. B. Plikaytis, J. E. Posey, P. D. van Helden, N. C. Gey van Pittius, M. Koehrsen, P. Sisk, C. Stolte, J. White, S. Gagneux, B. Birren, D. Hung, M. Murray, J. Galagan, PLoS One 7: e26038, 2012), here we show that this large chromosomal amplification event is very rapidly selected within standard in vitro broth cultures in a range of isolates. Indeed, subclones harboring the duplication were detectable after just five rounds of in vitro passage. In contrast, the duplication appears to be highly unstable in vivo and is negatively selected during the later stages of infection in mice. We believe that the rapid in vitro evolution of M. tuberculosis is an underappreciated aspect of its biology that is often ignored, despite the fact that it has the potential to confound the data and conclusions arising from comparative studies of isolates at both the genotypic and phenotypic levels.

On the age of leprosy

Leprosy is a chronic infection of the skin and nerves caused by Mycobacterium leprae and the newly discovered Mycobacterium lepromatosis. Human leprosy has been documented for millennia in ancient cultures. Recent genomic studies of worldwide M. leprae strains have further traced it along global human dispersals during the past ∼ 100,000 years. Because leprosy bacilli are strictly intracellular, we wonder how long humans have been affected by this disease-causing parasite. Based on recently published data on M. leprae genomes, M. lepromatosis discovery, leprosy bacilli evolution, and human evolution, it is most likely that the leprosy bacilli started parasitic evolution in humans or early hominids millions of years ago. This makes leprosy the oldest human-specific infection. The unique adaptive evolution has likely molded the indolent growth and evasion from human immune defense that may explain leprosy pathogenesis. Accordingly, leprosy can be viewed as a natural consequence of a long parasitism. The burden of leprosy may have affected minor selection on human genetic polymorphisms.

Mapping IS6110 in high-copy number Mycobacterium tuberculosis strains shows specific insertion points in the Beijing genotype

BACKGROUND

Mycobacterium tuberculosis Beijing strains are characterized by a large number of IS6110 copies, suggesting the potential implication of this element in the virulence and capacity for rapid dissemination characteristic of this family. This work studies the insetion points of IS6110 in high-copy clinical isolates specifically focusing on the Beijing genotype.

RESULTS

In the present work we mapped the insertion points of IS6110 in all the Beijing strains available in the literature and in the DNA sequence databases. We generated a representative primer collection of the IS6110 locations, which was used to analyse 61 high-copy clinical isolates. A total of 440 points of insertion were identified and analysis of their flanking regions determined the exact location, the direct repeats (DRs), the orientation and the distance to neighboring genes of each copy of IS6110. We identified specific points of insertion in Beijing strains that enabled us to obtain a dendrogram that groups the Beijing genotype.

CONCLUSIONS

This work presents a detailed analysis of locations of IS6110 in high-copy clinical isolates, showing points of insertion present with high frequency in the Beijing family and absent in other strains.

Evolution of smooth tubercle Bacilli PE and PE_PGRS genes: evidence for a prominent role of recombination and imprint of positive selection

BACKGROUND

PE and PE_PGRS are two mycobateria-restricted multigene families encoding membrane associated and secreted proteins that have expanded mainly in the pathogenic species, notably the Mycobacterium tuberculosis complex (MTBC). Several lines of evidence attribute to PE and PE_PGRS genes critical roles in mycobacterial pathogenicity. To get more insight into the nature of these genes, we sought to address their evolutionary trajectories in the group of smooth tubercle bacilli (STB), the putative ancestor of the clonal MTBC.

METHODOLOGY/PRINCIPAL FINDINGS

By focussing on six polymorphic STB PE/PE_PGRS genes, we demonstrate significant incongruence among single gene genealogies and detect strong signals of recombination using various approaches. Coalescent-based estimation of population recombination and mutation rates (ρ and θ, respectively) indicates that the two mechanisms are of roughly equal importance in generating diversity (ρ/θ = 1.457), a finding in a marked contrast to house keeping genes (HKG) whose evolution is chiefly brought about by mutation (ρ/θ = 0.012). In comparison to HKG, we found 15 times higher mean rate of nonsynonymous substitutions, with strong evidence of positive selection acting on PE_PGRS62 (dN/dS = 1.42), a gene that has previously been shown to be essential for mycobacterial survival in macrophages and granulomas. Imprint of positive selection operating on specific amino acid residues or along branches of PE_PGRS62 phylogenetic tree was further demonstrated using maximum likelihood- and covarion-based approaches, respectively. Strikingly, PE_PGR62 proved highly conserved in present-day MTBC strains.

CONCLUSIONS/SIGNIFICANCE

Overall the data indicate that, in STB, PE/PE_PGRS genes have undergone a strong diversification process that is speeded up by recombination, with evidence of positive selection. The finding that positive selection involved an essential PE_PGRS gene whose sequence appears to be driven to fixation in present-day MTBC strains lends further support to the critical role of PE/PE_PGRS genes in the evolution of mycobacterial pathogenicity.

PPE38 of Mycobacterium marinum triggers the cross-talk of multiple pathways involved in the host response, as revealed by subcellular quantitative proteomics

The PE/PPE family of proteins which are in high abundance in pathogenic species such as Mycobacterium tuberculosis and M. marinum, play the critical role in generating antigenic variation and evasion of host immune responses. However, little is known about their functional roles in mycobacterial pathogenesis. Previously, we found that PPE38 is associated with the virulence of mycobacteria, presumably by modulating the host immune response. To clarify the link between PPE38 and host response, we employed a subcellular, amino acid-coded mass tagging (AACT)/SILAC-based quantitative proteomic approach to determine the proteome changes during host response to M. marinum PPE38. As a result, 291 or 290 proteins were found respectively to be up- or down-regulated in the nucleus. Meanwhile, 576 upregulated and 272 downregulated proteins were respectively detected in the cytosol. The data of quantitative proteomic changes and concurrent biological validations revealed that M. marinum PPE38 could trigger extensive inflammatory responses in macrophages, probably through interacting with toll-like receptor 2 (TLR2). We also found that PPE38 may arrest MHC-1 processing and presentation in infected macrophages. Using bioinformatics tools to analyze global changes in the host proteome, we obtained a PPE38-respondor network involved in various transcriptional factors (TFs) and TF-associated proteins. The results of our systems investigation now indicate that there is cross-talk involving a broad range of diverse biological pathways/processes that coordinate the host response to M. marinum PPE38.

Role of PPE18 protein in intracellular survival and pathogenicity of Mycobacterium tuberculosis in mice

Background

Ever since its discovery the mycobacterial proline-proline-glutamic acid (PPE) family of proteins has generated a huge amount of interest. Understanding the role of these proteins in the pathogenesis of Mycobacterium tuberculosis (Mtb) is important. We have demonstrated earlier that the PPE18 protein of Mtb induces IL-10 production in macrophages with subsequent downregulation of pro-inflammatory cytokines like IL-12 and TNF-α and favors a T-helper (Th) 2-type of immune response.

Methodology/Principal Findings

Using a ppe18 genetic knock-out Mtb strain, we have now carried out infection studies in mice to understand the role of PPE18 in Mtb virulence. The studies reveal that lack of PPE18 leads to attenuation of Mtb in vivo. Mice infected with the ppe18 deleted strain have reduced infection burden in lung, liver and spleen and have better survival rates compared to mice infected with the wild-type Mtb strain.

Conclusions/Significance

Taken together our data suggest that PPE18 could be a crucial virulence factor for intracellular survival of Mtb.

Comparative analysis of Mycobacterium tuberculosis pe and ppe genes reveals high sequence variation and an apparent absence of selective constraints

Mycobacterium tuberculosis complex (MTBC) genomes contain 2 large gene families termed pe and ppe. The function of pe/ppe proteins remains enigmatic but studies suggest that they are secreted or cell surface associated and are involved in bacterial virulence. Previous studies have also shown that some pe/ppe genes are polymorphic, a finding that suggests involvement in antigenic variation. Using comparative sequence analysis of 18 publicly available MTBC whole genome sequences, we have performed alignments of 33 pe (excluding pe_pgrs) and 66 ppe genes in order to detect the frequency and nature of genetic variation. This work has been supplemented by whole gene sequencing of 14 pe/ppe (including 5 pe_pgrs) genes in a cohort of 40 diverse and well defined clinical isolates covering all the main lineages of the M. tuberculosis phylogenetic tree. We show that nsSNP's in pe (excluding pgrs) and ppe genes are 3.0 and 3.3 times higher than in non-pe/ppe genes respectively and that numerous other mutation types are also present at a high frequency. It has previously been shown that non-pe/ppe M. tuberculosis genes display a remarkably low level of purifying selection. Here, we also show that compared to these genes those of the pe/ppe families show a further reduction of selection pressure that suggests neutral evolution. This is inconsistent with the positive selection pressure of "classical" antigenic variation. Finally, by analyzing such a large number of genes we were able to detect large differences in mutation type and frequency between both individual genes and gene sub-families. The high variation rates and absence of selective constraints provides valuable insights into potential pe/ppe function. Since pe/ppe proteins are highly antigenic and have been studied as potential vaccine components these results should also prove informative for aspects of M. tuberculosis vaccine design.

The Mycobacterium avium ESX-5 PPE protein, PPE25-MAV, interacts with an ESAT-6 family Protein, MAV_2921, and localizes to the bacterial surface

Previous research has demonstrated that inactivation of the Mycobacterium avium gene, PPE25-MAV (MAV_2928), leads to a significant attenuation of virulence in both in vitro and in vivo models. PPE25-MAV encodes for a PPE family protein, a family from which many members have been implicated in both bacterial virulence and host immune recognition. Recent research has shown that many PPE family proteins are exported by a specialized Type VII secretion system in mycobacteria. In this context, the mechanisms of PPE25-MAV in M. avium pathogenesis were investigated. A mycobacterial 2-hybrid system was used to perform a directed search for M. avium proteins that interact directly with PPE25-MAV. An interaction was observed between PPE25-MAV and the ESAT-6 family protein, MAV_2921, and was further defined by 2-hybrid analysis of truncated PPE25-MAV, and confirmed by co-immunoprecipitation. Localization of the PPE25-MAV protein was analyzed in Mycobacterium smegmatis expressing the recombinant protein and a significant percentage of PPE25-MAV was shown to be exposed at the bacterial surface by surface biotinylation and trypsin protection assays. Finally, transcriptional analysis of PPE25-MAV and its associated operon suggested that nutrient limitation, a condition which occurs in the phagosome, plays a role in regulating expression of the PPE25-MAV gene.

Combination of single nucleotide polymorphism and variable-number tandem repeats for genotyping a homogenous population of Mycobacterium tuberculosis Beijing strains in China

The standard 15- and 24-locus variable-number tandem repeat (VNTR) genotyping methods have demonstrated adequate discriminatory power and a small homoplasy effect for tracing tuberculosis (TB) transmission and predicting Mycobacterium tuberculosis lineages in European and North American countries. However, its validity for the definition of transmission in homogenous M. tuberculosis populations in settings with high TB burdens has been questioned. Here, we genotyped a population-based collection of 191 Beijing strains based on standard 15-locus VNTR (VNTR-15) and 8 single nucleotide polymorphisms (SNPs) in Shanghai, China. Limited discriminatory power and high rates of VNTR homoplasy were observed in the homogenous population of evolutionarily "modern" Beijing strains. Additional typing of three hypervariable loci (VNTR3820, VNTR4120, and VNTR3232) was performed for VNTR-15-based clusters. High variations of hypervariable alleles were observed in clusters with inconsistent SNP sublineages. We concluded that SNPs and hypervariable VNTR loci are helpful to enhance the discriminatory power and decrease the VNTR homoplasy effect for defining clusters. We recommend the combination of standard VNTR-15 and SNPs as first-line typing methods and the hypervariable loci for second-line typing of clustered strains for molecular epidemiology studies of homogenous M. tuberculosis populations.

PPE38 modulates the innate immune response and is required for Mycobacterium marinum virulence

The proline-glutamic acid (PE) and proline-proline-glutamic acid (PPE) family proteins are prevalent in pathogenic mycobacteria and play a diverse role in mycobacterial pathogenesis. While some members have been studied, the function of most PE/PPE proteins remains unknown. In this study, we isolated a transposon-inactivated PPE38 mutant of Mycobacterium marinum and characterized its phenotype. We found that the PPE38 protein is associated with the cell wall and exposed on the cell surface. The inactivation of PPE38 altered the bacterial cell surface properties and led to deficiencies in cord formation, sliding motility, and biofilm formation. The PPE38 mutant was defective in phagocytosis by macrophages and exhibited reduced virulence in adult zebrafish. We also found that PPE38 is involved in the induction of proinflammatory cytokines in infected macrophages. Together, our results indicate that PPE38, a previously uncharacterized protein, plays a role in mycobacterial virulence, presumably by modulating the host innate immune response.

Overview of errors in the reference sequence and annotation of Mycobacterium tuberculosis H37Rv, and variation amongst its isolates

Since its publication in 1998, the genome sequence of the Mycobacterium tuberculosis H37Rv laboratory strain has acted as the cornerstone for the study of tuberculosis. In this review we address some of the practical aspects that have come to light relating to the use of H37Rv throughout the past decade which are of relevance for the ongoing genomic and laboratory studies of this pathogen. These include errors in the genome reference sequence and its annotation, as well as the recently detected variation amongst isolates of H37Rv from different laboratories.

Mutations in the regulatory network underlie the recent clonal expansion of a dominant subclone of the Mycobacterium tuberculosis Beijing genotype

The Beijing genotype family is an epidemiologically important sub-group of Mycobacterium tuberculosis. It has been suggested that the high frequency of the Beijing isolates in some areas could be explained by selective advantages. Some evidence suggests that the emerging and most frequently isolated "Typical Beijing" lineage has the ability to circumvent BCG-induced immunity. To investigate the phylogeny of the Beijing genotype of M. tuberculosis, the genome of six Beijing strains from three different countries was sequenced with next-generation sequencing. The phylogeny of these strains was established using single nucleotide polymorphisms (SNPs). The three Typical Beijing strains clustered very tightly in the Beijing phylogeny suggesting that Typical Beijing strains represent a monophyletic lineage and resulted from recent diversification. Typing of 150 M. tuberculosis strains with a subset of the SNPs and comparison of the IS6110 restriction-fragment length polymorphism (RFLP) patterns of these strains to a database of 1522 Beijing RFLP patterns revealed that about 80% of all Beijing strains belong to the Typical Beijing subclone, which indicates clonal expansion. To identify the genomic changes that are characteristic for all Typical Beijing strains and to reconstruct their most recent common ancestor, the presence of SNPs was assayed in other Beijing strains. We identified 51 SNPs that define the minimal set of polymorphisms for all Typical Beijing strains. Nonsynonymous polymorphisms in genes coding for the regulatory network were over-represented in this set of mutations. We suggest that alterations in the response to environmental signals may have enabled Typical Beijing strains to develop the emerging phenotype.

Mycobacterial PE/PPE proteins at the host-pathogen interface

The mycobacterial PE/PPE proteins have attracted much interest since their formal identification just over a decade ago. It has been widely speculated that these proteins may play a role in evasion of host immune responses, possibly via antigenic variation. Although a cohesive understanding of their function(s) has yet to be established, emerging data increasingly supports a role for the PE/PPE proteins at multiple levels of the infectious process. This paper will delineate salient features of the families revealed by comparative genomics, bioinformatic analyses and genome-wide screening approaches and will summarise existing knowledge of subcellular localization, secretion pathways, and protein structure. These characteristics will be considered in light of findings on innate and adaptive host responses to PE/PPE proteins, and we will review the increasing body of data on B and T cell recognition of these proteins. Finally, we will consider how current knowledge and future explorations may contribute to a more comprehensive understanding of these intriguing proteins and their involvement in host pathogen interactions. Ultimately this information could underpin future intervention strategies, for example, in the area of new and improved diagnostic tools and vaccine candidates.

Horizontal gene transfers with or without cell fusions in all categories of the living matter

This article reviews the history of widespread exchanges of genetic segments initiated over 3 billion years ago, to be part of their life style, by sphero-protoplastic cells, the ancestors of archaea, prokaryota, and eukaryota. These primordial cells shared a hostile anaerobic and overheated environment and competed for survival. "Coexist with, or subdue and conquer, expropriate its most useful possessions, or symbiose with it, your competitor" remain cellular life's basic rules. This author emphasizes the role of viruses, both in mediating cell fusions, such as the formation of the first eukaryotic cell(s) from a united crenarchaeon and prokaryota, and the transfer of host cell genes integrated into viral (phages) genomes. After rising above the Darwinian threshold, rigid rules of speciation and vertical inheritance in the three domains of life were established, but horizontal gene transfers with or without cell fusions were never abolished. The author proves with extensive, yet highly selective documentation, that not only unicellular microorganisms, but the most complex multicellular entities of the highest ranks resort to, and practice, cell fusions, and donate and accept horizontally (laterally) transferred genes. Cell fusions and horizontally exchanged genetic materials remain the fundamental attributes and inherent characteristics of the living matter, whether occurring accidentally or sought after intentionally. These events occur to cells stagnating for some 3 milliard years at a lower yet amazingly sophisticated level of evolution, and to cells achieving the highest degree of differentiation, and thus functioning in dependence on the support of a most advanced multicellular host, like those of the human brain. No living cell is completely exempt from gene drains or gene insertions.

Immunogenicity and reactivity of novel Mycobacterium avium subsp. paratuberculosis PPE MAP1152 and conserved MAP1156 proteins with sera from experimentally and naturally infected animals

Mycobacterium avium subsp. paratuberculosis causes Johne's disease (JD) in ruminants. Development of genetic tools and completion of the M. avium subsp. paratuberculosis genome sequencing project have expanded the opportunities for antigen discovery. In this study, we determined the seroreactivities of two proteins encoded at the 5' and 3' regions of the MAP1152-MAP1156 gene cluster. MAP1152 encodes a PPE protein, and MAP1156 encodes a diacylglycerol acyltransferase involved in triglyceride metabolism and classified in the uncharacterized protein family UPF0089. Recombinant MAP proteins were overproduced and purified from Escherichia coli as maltose-binding protein (MBP) fusions. Immunoblotting analysis indicated that both MAP1152 and MAP1156 displayed reactivity against sera of mice and rabbits immunized with live M. avium subsp. paratuberculosis cells and against samples from naturally infected cattle. In immunoblot assays, MAP1156 yielded a stronger positive signal than MAP1152 against sera from cattle with JD. An enzyme-linked immunosorbent assay for the recombinant proteins was developed and used to test preclassified positive and negative serum samples from naturally infected and noninfected cattle. Samples, with one exception, displayed no seroreactivity against the MBP-LacZ fusion protein (P > 0.05), the negative-control antigen. MAP1152 displayed seroreactivity against all positive sera but no seroreactivity to the negative sera (P < 0.01). MAP1156 displayed stronger and more variable reactivity than MAP1152, but significant differences were observed between noninfected and infected cattle (P < 0.05). Otherwise, degrees of reactivity followed the same trend as the positive reference antigen. In conclusion, both proteins are immunogenic in mice and rabbits, and M. avium subsp. paratuberculosis-infected cattle mount a humoral response to both MAP1152 and MAP1156 cross-reactive epitopes. These findings have potential applications to diagnostics, vaccine production, and elucidation of the immunopathogenesis of JD.