Immunochemical characterization of the MPB70/80 and MPB83 proteins of Mycobacterium bovis

Macrophage-specific responses to human- and animal-adapted tubercle bacilli reveal pathogen and host factors driving multinucleated cell formation

The Mycobacterium tuberculosis complex (MTBC) is a group of related pathogens that cause tuberculosis (TB) in mammals. MTBC species are distinguished by their ability to sustain in distinct host populations. While Mycobacterium bovis (Mbv) sustains transmission cycles in cattle and wild animals and causes zoonotic TB, M. tuberculosis (Mtb) affects human populations and seldom causes disease in cattle. The host and pathogen determinants underlying host tropism between MTBC species are still unknown. Macrophages are the main host cell that encounters mycobacteria upon initial infection, and we hypothesised that early interactions between the macrophage and mycobacteria influence species-specific disease outcome. To identify factors that contribute to host tropism, we analysed blood-derived primary human and bovine macrophages (hMϕ or bMϕ, respectively) infected with Mbv and Mtb. We show that Mbv and Mtb reside in different cellular compartments and differentially replicate in hMϕ whereas both Mbv and Mtb efficiently replicate in bMϕ. Specifically, we show that out of the four infection combinations, only the infection of bMϕ with Mbv promoted the formation of multinucleated giant cells (MNGCs), a hallmark of tuberculous granulomas. Mechanistically, we demonstrate that both MPB70 from Mbv and extracellular vesicles released by Mbv-infected bMϕ promote macrophage multinucleation. Importantly, we extended our in vitro studies to show that granulomas from Mbv-infected but not Mtb-infected cattle contained higher numbers of MNGCs. Our findings implicate MNGC formation in the contrasting pathology between Mtb and Mbv for the bovine host and identify MPB70 from Mbv and extracellular vesicles from bMϕ as mediators of this process.

The identification of host and pathogen factors contributing to host-pathogen interaction is crucial to understand the pathogenesis and dissemination of tuberculosis. This is particularly the case in deciphering the mechanistic basis for host-tropism across the MTBC. Here, we show that in vitro, M. bovis but not M. tuberculosis induces multinucleated cell formation in bovine macrophages. We identified host and pathogen mechanistic drivers of multinucleated cell formation: MPB70 as the M. bovis factor and bovine macrophage extracellular vesicles. Using a cattle experimental infection model, we confirmed differential multinucleated cell formation in vivo. Thus, we have identified host and pathogen factors that contribute to host tropism in human/bovine tuberculosis. Additionally, this work provides an explanation for the long-standing association of multinucleated cells with tuberculosis pathogenesis.

Augmentation of DTH reaction of mycobacterial antigenic cocktail using synthetic mycobacterial 19-kDa lipoprotein as a TLR-stimulant

The current study proposed that previously characterized individual antigenic proteins could represent potential replacement for conventional purified protein derivative (PPD) in tuberculosis skin testing when used in cocktails triggered by suitable TLR-stimulants that would provide the missing pro-inflammatory stimulus. Three different cocktails of previously selected antigens, including C1 (ESAT-6/CPF-10/MPB-83); C2 (ESAT-6/MPB-64/MPB-83); and C3 (CPF-10/MPB-64/MPB-83), were evaluated in vitro using lymphocytic proliferation and IFN-γ production assays, as well as mRNA and protein expression levels of TNF-α, IL-12p40, and IL-2 as pro-inflammatory molecules. C1 showed the highest significant induction of pro-inflammatory molecules as compared to other cocktails, yet still significantly lower than that induced by conventional PPD. Interestingly, inclusion of the synthetic Mycobacterium tuberculosis 19-kDa lipoprotein (Pam3Cys-SSNKSTTGSGETTTA) as a TLR-stimulant resulted in obvious augmentation of C1-induced pro-inflammatory molecules to levels comparable to that of PPD. In addition, skin testing using sensitized guinea pig model revealed comparable significant reaction to that of conventional PPD. ESAT-6/CPF-10/MPB-83 cocktail is suggested as a potential alternative skin-testing reagent when used in combination with the M. tuberculosis 19-kDa lipoprotein as a TLR-stimulant.

Virulent Mycobacterium bovis Beijing Strain Activates the NLRP7 Inflammasome in THP-1 Macrophages

Mycobacterium bovis is the causative agent of tuberculosis in a wide range of mammals, including humans. Macrophages are the first line of host defense. They secrete proinflammatory cytokines, such as interleukin-1 beta (IL-1β), in response to mycobacterial infection, but the underlying mechanisms by which human macrophages are activated and release IL-1β following M. bovis infection are poorly understood. Here we show that the ‘nucleotide binding and oligomerization of domain-like receptor (NLR) family pyrin domain containing 7 protein’ (NLRP7) inflammasome is involved in IL-1β secretion and caspase-1 activation induced by M. bovis infection in THP-1 macrophages. NLRP7 inflammasome activation promotes the induction of pyroptosis as well as the expression of tumor necrosis factor alpha (TNF-α), Chemokine (C-C motif) ligand 3 (CCL3) and IL-1β mRNAs. Thus, the NLRP7 inflammasome contributes to IL-1β secretion and induction of pyroptosis in response to M. bovis infection in THP-1 macrophages.

Mycobacterium bovis infection in the lion (Panthera leo): Current knowledge, conundrums and research challenges

Mycobacterium bovis has global public-health and socio-economic significance and can infect a wide range of species including the lion (Panthera leo) resulting in tuberculosis. Lions are classified as vulnerable under the IUCN Red List of Threatened Species and have experienced a 30% population decline in the past two decades. However, no attempt has been made to collate and critically evaluate the available knowledge of M. bovis infections in lions and potential effects on population. In this review we set out to redress this. Arguments suggesting that ingestion of infected prey animals are the main route of infection for lions have not been scientifically proven and research is needed into other possible sources and routes of infection. The paucity of knowledge on host susceptibility, transmission directions and therefore host status, manifestation of pathology, and epidemiology of the disease in lions also needs to be addressed. Advances have been made in diagnosing the presence of M. bovis in lions. However, these diagnostic tests are unable to differentiate between exposure, presence of infection, or stage of disease. Furthermore, there are contradictory reports on the effects of M. bovis on lion populations with more data needed on disease dynamics versus the lion population's reproductive dynamics. Knowledge on disease effects on the lion reproduction and how additional stressors such as drought or co-morbidities may interact with tuberculosis is also lacking. Filling these knowledge gaps will contribute to the understanding of mycobacterial infections and disease in captive and wild lions and assist in lion conservation endeavours.

Evaluation of cocktails with recombinant proteins of Mycobacterium bovis for a specific diagnosis of bovine tuberculosis

The Delayed type hypersensitivity skin test (DTH) and interferon-gamma assay are used for the diagnosis of bovine tuberculosis (TBB). The specificity of these diagnoses, however, is compromised because both are based on the response against purified protein derivative of Mycobacterium bovis (PPD-B). In this study, we assessed the potential of two cocktails containing M. bovis recombinant proteins: cocktail 1 (C1): ESAT-6, CFP-10 and MPB83 and cocktail 2 (C2): ESAT-6, CFP-10, MPB83, HspX, TB10.3, and MPB70. C1, C2, and PPD-B showed similar response by DTH in M. bovis-sensitized guinea pigs. Importantly, C1 induced a lower response than PPD-B in M. avium-sensitized guinea pigs. In cattle, C1 displayed better performance than PPD-B and C2; indeed, C1 showed the least detection of animals either vaccinated or Map-infected. To optimize the composition of the cocktails, we obtained protein fractions from PPD-B and tested their immunogenicity in experimentally M. bovis-infected cattle. In one highly reactive fraction, seven proteins were identified. The inclusion of FixB in C1 enhanced the recognition of M. bovis-infected cattle without compromising specificity. Our data provide a promising basis for the future development of a cocktail for TBB detection without interference by the presence of sensitized or infected animals with other mycobacteria.

An Evaluation of MAPIA in Michigan as an Ante-Mortem Supplemental Test for Use in Suspect Tuberculosis Cattle

The objective of this study was to make use of bovine tuberculosis suspect cattle from the state of Michigan to validate a multiantigen print immunoassay for use on sera to serve as an improved supplementary ante-mortem test to increase specificity of current tuberculosis testing methods. Over a 27-month period, 234 sera were collected and tested by MAPIA method, which was evaluated using four different interpretation criteria. These results were subsequently compared to final mycobacterial culture and PCR results obtained by the National Veterinary Services Laboratories, Ames, IA, which served as the true indicator of the cattle's tuberculosis infection status. This study indicates that an interpretation criterion which includes 3 or more positive reactions to the 11 different mycobacteria antigens utilized provided both an acceptable sensitivity (69.39%) and a high specificity (90.27%). This MAPIA technique shows potential for eventual application as a supplementary ante-mortem tuberculosis serologic test following one of the various current or soon-to-be-approved whole herd screening assays as part of a tuberculosis eradication program.

The secreted lipoprotein, MPT83, of Mycobacterium tuberculosis is recognized during human tuberculosis and stimulates protective immunity in mice

The long-term control of tuberculosis (TB) will require the development of more effective anti-TB vaccines, as the only licensed vaccine, Mycobacterium bovis bacille Calmette-Guérin (BCG), has limited protective efficacy against infectious pulmonary TB. Subunit vaccines have an improved safety profile over live, attenuated vaccines, such as BCG, and may be used in immuno-compromised individuals. MPT83 (Rv2873) is a secreted mycobacterial lipoprotein expressed on the surface of Mycobacterium tuberculosis. In this study, we examined whether recombinant MPT83 is recognized during human and murine M. tuberculosis infection. We assessed the immunogenicity and protective efficacy of MPT83 as a protein vaccine, with monophosphyl lipid A (MPLA) in dimethyl-dioctadecyl ammonium bromide (DDA) as adjuvant, or as a DNA vaccine in C57BL/6 mice and mapped the T cell epitopes with peptide scanning. We demonstrated that rMPT83 was recognised by strong proliferative and Interferon (IFN)-γ-secreting T cell responses in peripheral blood mononuclear cells (PBMC) from patients with active TB, but not from healthy, tuberculin skin test-negative control subjects. MPT83 also stimulated strong IFN-γ T cell responses during experimental murine M. tuberculosis infection. Immunization with either rMPT83 in MPLA/DDA or DNA-MPT83 stimulated antigen-specific T cell responses, and we identified MPT83_127–135 (PTNAAFDKL) as the dominant H-2^b-restricted CD8⁺ T cell epitope within MPT83. Further, immunization of C57BL/6 mice with rMPT83/MPLA/DDA or DNA-MPT83 stimulated significant levels of protection in the lungs and spleens against aerosol challenge with M. tuberculosis. Interestingly, immunization with rMPT83 in MPLA/DDA primed for stronger IFN-γ T cell responses to the whole protein following challenge, while DNA-MPT83 primed for stronger CD8⁺ T cell responses to MPT83_127–135. Therefore MPT83 is a protective T cell antigen commonly recognized during human M. tuberculosis infection and should be considered for inclusion in future TB subunit vaccines.

Production and evaluation of antibodies and phage display-derived peptide ligands for immunomagnetic separation of Mycobacterium bovis

This study describes the development and optimization of an immunomagnetic separation (IMS) method to isolate Mycobacterium bovis cells from lymph node tissues. Gamma-irradiated whole M. bovis AF2122/97 cells and ethanol-extracted surface antigens of such cells were used to produce M. bovis-specific polyclonal and monoclonal antibodies in rabbits and mice. They were also used to generate M. bovis-specific peptide ligands by phage display biopanning. The various antibodies and peptide ligands obtained were used to coat MyOne tosyl-activated Dynabeads (Life Technologies), singly or in combination, and evaluated for IMS. Initially, M. bovis capture from Middlebrook 7H9 broth suspensions (concentration range, 10 to 10(5) CFU/ml) was evaluated by IMS combined with an M. bovis-specific touchdown PCR. IMS-PCR results and, subsequently, IMS-culture results indicated that the beads with greatest immunocapture capability for M. bovis in broth were those coated simultaneously with a monoclonal antibody and a biotinylated 12-mer peptide. These dually coated beads exhibited minimal capture (mean of 0.36% recovery) of 12 other Mycobacterium spp. occasionally encountered in veterinary tuberculosis (TB) diagnostic laboratories. When the optimized IMS method was applied to various M. bovis-spiked lymph node matrices, it demonstrated excellent detection sensitivities (50% limits of detection of 3.16 and 57.7 CFU/ml of lymph node tissue homogenate for IMS-PCR and IMS-culture, respectively). The optimized IMS method therefore has the potential to improve isolation of M. bovis from lymph nodes and hence the diagnosis of bovine tuberculosis.

Potential application of new diagnostic methods for controlling bovine tuberculosis in Brazil

Bovine tuberculosis, a chronic infection in cattle caused by Mycobacterium bovis, remains an economic and public health problem for several countries. Due to its economic impact on international trade, contagious nature, and implications for human health, global programs to eradicate the disease were implemented worldwide. Those programs are based on slaughtering PPD-reactive animals. Despite the National Programs in Brazil, complete eradication has not been achieved, and the disease remains, albeit at a lower prevalence. The central purpose of this review is to address diagnostic tests for tuberculosis. Considering the course of the infection in cattle, at least two tests, ideally complementary to one another, may be necessary for an adequate diagnosis: the first based on the cellular response, and the second capable of identifying anergic animals by detection of specific anti-M.bovis antibodies.

Dynamic antibody responses to the Mycobacterium tuberculosis proteome

Considerable effort has been directed toward controlling tuberculosis, which kills almost two million people yearly. High on the research agenda is the discovery of biomarkers of active tuberculosis (TB) for diagnosis and for monitoring treatment outcome. Rational biomarker discovery requires understanding host-pathogen interactions leading to biomarker expression. Here we report a systems immunology approach integrating clinical data and bacterial metabolic and regulatory information with high-throughput detection in human serum of antibodies to the entire Mycobacterium tuberculosis proteome. Sera from worldwide TB suspects recognized approximately 10% of the bacterial proteome. This result defines the M. tuberculosis immunoproteome, which is rich in membrane-associated and extracellular proteins. Additional analyses revealed that during active tuberculosis (i) antibody responses focused on an approximately 0.5% of the proteome enriched for extracellular proteins, (ii) relative target preference varied among patients, and (iii) responses correlated with bacillary burden. These results indicate that the B cell response tracks the evolution of infection and the pathogen burden and replicative state and suggest functions associated with B cell-rich foci seen in tuberculous lung granulomas. Our integrated proteome-scale approach is applicable to other chronic infections characterized by diverse antibody target recognition.

Identification of novel Mycobacterium bovis antigens by dissection of crude protein fractions

Culture filtrate and cell extracts from Mycobacterium bovis cultures contain molecules which could promote protective immunity to tuberculosis in animals. Different protein fractions of M. bovis cultures were obtained by elution electrophoresis and were tested in experimentally infected cattle. The fractions that elicited gamma interferon (IFN-gamma) responses were resolved by two-dimensional gel electrophoresis, and individual proteins were identified by matrix-assisted laser desorption ionization-time of flight mass spectrometry. The open reading frames were cloned, expressed as their recombinant forms, and retested with naturally and experimentally infected animals. Eleven protein fractions were highly reactive, from which the Rv1636, HspX, Rv0138, Rv2524, EsxI, and Rv3740 recombinant proteins were obtained. EsxI and HspX were the antigens most recognized by the IFN-gamma release assay. In summary, a proteomic approach allowed the identification of novel antigens useful for the diagnosis of bovine tuberculosis.

Bacterial species exhibit diversity in their mechanisms and capacity for protein disulfide bond formation

Protein disulfide bond formation contributes to the folding and activity of many exported proteins in bacteria. However, information about disulfide bond formation is limited to only a few bacterial species. We used a multifaceted bioinformatic approach to assess the capacity for disulfide bond formation across this biologically diverse group of organisms. We combined data from a cysteine counting method, in which a significant bias for even numbers of cysteine in proteins is taken as an indicator of disulfide bond formation, with data on the presence of homologs of known disulfide bond formation enzymes. These combined data enabled us to make predictions about disulfide bond formation in the cell envelope across bacterial species. Our bioinformatic and experimental results suggest that many bacteria may not generally oxidatively fold proteins, and implicate the bacterial homolog of the enzyme vitamin K epoxide reductase, a protein required for blood clotting in humans, as part of a disulfide bond formation pathway present in several major bacterial phyla.

Environmental monitoring of Mycobacterium bovis in badger feces and badger sett soil by real-time PCR, as confirmed by immunofluorescence, immunocapture, and cultivation

Real-time PCR was used to detect and quantify Mycobacterium bovis cells in naturally infected soil and badger feces. Immunomagnetic capture, immunofluorescence, and selective culture confirmed species identification and cell viability. These techniques will prove useful for monitoring M. bovis in the environment and for elucidating transmission routes between wildlife and cattle.

A novel fusion protein-based indirect enzyme-linked immunosorbent assay for the detection of bovine tuberculosis

Enzyme-linked immunosorbent assay (ELISA) for diagnosis of bovine tuberculosis has been widely explored over the years. Three Mycobacterium bovis-specific antigen genes, namely, mpb70, mpb83, and esat-6 were recombined in tandem by spliced overlap extension technology and expressed in Escherichia coli to obtain the fusion protein (rM70-83-E6). Western blot analysis showed that rM70-83-E6 can specifically react with bovine tuberculosis-positive sera but not those from cattle infected with other bovine diseases such as bovine paratuberculosis. An indirect ELISA (iELISA) method was established with rM70-83-E6 as the diagnostic antigen. The diagnostic criteria were determined using 150 serum samples from healthy cattle. Analyses of 85 serum samples from cattle with bovine tuberculosis and 100 serum samples from healthy cattle demonstrated that the sensitivity of the iELISA was 69.4% (59/85) and the specificity was 96.0% (96/100). Moreover, 46 out of 67 purified protein derivative (PPD) skin test-positive samples were also positive by iELISA, giving a positive coincidence of 68.7%, while all 50 PPD skin test-negative samples were negative by iELISA, giving a negative coincidence of 100%. The total coincidence between iELISA and the PPD skin test was 82.1%. This study demonstrated that iELISA using rM70-83-E6 antigen is simple, sensitive and easy to perform and can be used to analysis of a large number of samples for serodiagnosis of bovine tubercuiosis.

Reduced expression of antigenic proteins MPB70 and MPB83 in Mycobacterium bovis BCG strains due to a start codon mutation in sigK

Mycobacterium bovis Bacille Calmette-Guerin (BCG) strains are genetically and phenotypically heterogeneous. Expression of the antigenic proteins MPB70 and MPB83 is known to vary considerably across BCG strains; however, the reason for this phenotypic difference has remained unknown. By immunoblot, we separated BCG into high- and low-producing strains. By quantitative reverse transcription polymerase chain reaction (RT-PCR), we determined that transcription of the antigen-encoding genes, mpb70 and mpb83, follows the same strain pattern with mRNA levels reduced over 50-fold in low-producing strains. Transcriptome comparison of the same BCG strains by DNA microarray revealed two gene regions consistently downregulated in low-producing strains compared with high-producing strains, one including mpb70 (Rv2875) and mpb83 (Rv2873) and a second that includes the predicted sigma factor, sigK. DNA sequence analysis revealed a point mutation in the start codon of sigK in all low-producing BCG strains. Complementation of a low-producing strain, BCG Pasteur, with wild-type sigK fully restored MPB70 and MPB83 production. Microarray-based analysis and confirmatory RT-PCR of the complemented strains revealed an upregulation in gene transcription limited to the sigK and the mpb83/mpb70 gene regions. These data demonstrate that a mutation of sigK is responsible for decreased expression of MPB70 and MPB83 in low-producing BCG strains and provide clues into the role of Mycobacterium tuberculosis SigK.

Influence of pathological progression on the balance between cellular and humoral immune responses in bovine tuberculosis

Studies of tuberculosis have suggested a shift in dominance from a T helper type 1 (Th1) towards a Th2 immune response that is associated with suppressed cell-mediated immune (CMI) responses and increased humoral responses as the disease progresses. In this study a natural host disease model was used to investigate the balance of the evolving immune response towards Mycobacterium bovis infection in cattle with respect to pathogenesis. Cytokine analysis of CD4 T-cell clones derived from M. bovis-infected animals gave some indication that there was a possible relationship between enhanced pathogenesis and an increased ratio of Th0 [interleukin-4-positive/interferon-gamma-positive (IL-4(+)/IFN-gamma(+))] clones to Th1 (IFN-gamma(+)) clones. All animals developed strong antimycobacterial CMI responses, but depressed cellular responses were evident as the disease progressed, with the IFN-gamma test failing to give consistently positive results in the latter stages. Furthermore, a stronger Th0 immune bias, depressed in vitro CMI responses, elevated levels of IL-10 expression and enhanced humoral responses were also associated with increased pathology. In minimal disease, however, a strong Th1 immune bias was maintained and an anti-M. bovis humoral response failed to develop. It was also seen that the level of the anti-M. bovis immunoglobulin G1 (IgG1) isotype antibody responses correlated with the pathology scores, whereas CMI responses did not have as strong a relationship with the development of pathology. Therefore, the development and maintenance of a Th1 IFN-gamma response is associated with a greater control of M. bovis infection. Animals progressing from a Th1-biased to a Th0-biased immune response developed more extensive pathology and performed less well in CMI-based diagnostic tests but developed strong IgG1 humoral responses.

Lipoproteins ofMycobacterium tuberculosis: an abundant and functionally diverse class of cell envelope components

Mycobacterium tuberculosis remains the predominant bacterial scourge of mankind. Understanding of its biology and pathogenicity has been greatly advanced by the determination of whole genome sequences for this organism. Bacterial lipoproteins are a functionally diverse class of membrane-anchored proteins. The signal peptides of these proteins direct their export and post-translational lipid modification. These signal peptides are amenable to bioinformatic analysis, allowing the lipoproteins encoded in whole genomes to be catalogued. This review applies bioinformatic methods to the identification and functional characterisation of the lipoproteins encoded in the M. tuberculosis genomes. Ninety nine putative lipoproteins were identified and so this family of proteins represents ca. 2.5% of the M. tuberculosis predicted proteome. Thus, lipoproteins represent an important class of cell envelope proteins that may contribute to the virulence of this major pathogen.

Computer-assisted prediction of HLA-DR binding and experimental analysis for human promiscuous Th1-cell peptides in the 24 kDa secreted lipoprotein (LppX) of Mycobacterium tuberculosis

The secreted 24 kDa lipoprotein (LppX) is an antigen that is specific for Mycobacterium tuberculosis complex and M. leprae. The present study was carried out to identify the promiscuous T helper 1 (Th1)-cell epitopes of the M. tuberculosis LppX (MT24, Rv2945c) antigen by using 15 overlapping synthetic peptides (25 mers overlapping by 10 residues) covering the sequence of the complete protein. The analysis of Rv2945c sequence for binding to 51 alleles of nine serologically defined HLA-DR molecules, by using a virtual matrix-based prediction program (propred), showed that eight of the 15 peptides of Rv2945c were predicted to bind promiscuously to >/=10 alleles from more than or equal to three serologically defined HLA-DR molecules. The Th1-cell reactivity of all the peptides was assessed in antigen-induced proliferation and interferon-gamma (IFN-gamma)-secretion assays with peripheral blood mononuclear cells (PBMCs) from 37 bacille Calmette-Guérin (BCG)-vaccinated healthy subjects. The results showed that 17 of the 37 donors, which represented an HLA-DR-heterogeneous group, responded to one or more peptides of Rv2945c in the Th1-cell assays. Although each peptide stimulated PBMCs from one or more donors in the above assays, the best positive responses (12/17 (71%) responders) were observed with the peptide p14 (aa 196-220). This suggested a highly promiscuous presentation of p14 to Th1 cells. In addition, the sequence of p14 is completely identical among the LppX of M. tuberculosis, M. bovis and M. leprae, which further supports the usefulness of Rv2945c and p14 in the subunit vaccine design against both tuberculosis and leprosy.

Advances in antibody-mediated immunity againstMycobacterium tuberculosis: implications for a novel vaccine strategy

Cell-mediated immunity is considered to be the major component of the host response against Mycobacterium tuberculosis, whereas antibody-mediated immunity historically has been considered inconsequential. In recent years, studies from several groups have challenged the traditional dogma and demonstrated that monoclonal antibodies can modify various aspects of mycobacterial infections. This review describes the experimental evidence supporting a role for antibodies in defense against mycobacterial infections and outlines future challenges to the field of antibody-mediated immunity against M. tuberculosis, with particular emphasis on the implications of these findings for a novel vaccine strategy.

Specific delayed-type hypersensitivity responses to ESAT-6 identify tuberculosis-infected cattle

Human and bovine tuberculosis have long been detected by skin testing with purified protein derivative (PPD), a complex mix of partly denatured mycobacterial antigens with suboptimal specificity. In the present study, skin tests based on ESAT-6, a recombinantly produced antigen highly specific for tuberculosis infection, were investigated. Although ESAT-6 was strongly recognized in vitro and induced high levels of gamma interferon, initial investigations demonstrated that higher doses of ESAT-6 than of PPD were needed to induce substantial delayed-type hypersensitivity reactions. Also, the kinetics of the skin test response differed for the two reagents; PPD showed maximal response at 72 h, but the response to ESAT-6 often peaked later at 96 h. Tests based on an optimized strategy (400 micro g of ESAT-6 measured between 72 and 96 h), in cattle infected with Mycobacterium bovis (n = 22) and animals sensitized by exposure to environmental mycobacteria showed ESAT-6 to have a promising diagnostic potential (sensitivity, 82%; specificity, 100%; optimal cutoff, 3 mm), compared with PPD (sensitivity, 86%; specificity, 90%; optimal cutoff, 4 mm). Larger investigations are required to refine cutoff points for any new diagnostic test, but the present results indicate great potential for skin tests based on specific antigens for accurate in vivo diagnosis of tuberculosis.

Synthetic peptides identify promiscuous human Th1 cell epitopes of the secreted mycobacterial antigen MPB70

MPB70 is a secreted protein of Mycobacterium bovis and Mycobacterium tuberculosis which stimulates both cellular and humoral immune responses during infection with bovine and human tubercle bacilli. In addition, vaccination with MPB70 has been shown to induce Th1 cell responses and protection in animal models of tuberculosis. The present study was carried out to map the dominant human Th1 cell epitopes of MPB70 in relation to major histocompatibility complex (MHC) class II restriction in healthy subjects showing strong T-cell responses to complex mycobacterial antigens. Peripheral blood mononuclear cells (PBMC) from HLA-DR-typed donors were tested with complex mycobacterial antigens (whole-cell M. tuberculosis and M. tuberculosis culture filtrates), with MPB70 purified from the culture filtrate of M. bovis BCG Tokyo, and with 13 synthetic peptides (25-mers overlapping by 10 residues) covering the sequence of MPB70. The donors that responded to the complex antigens and MPB70 also responded to the cocktail of synthetic MPB70 peptides. Testing of PBMC with individual peptides showed that peptides p5 (amino acids [aa] 61 to 85), p6 (aa 76 to 100), p8 (aa 106 to 130), p12 (aa 166 to 190), and p13 (aa 181 to 193) were most frequently recognized in proliferation and gamma interferon (IFN-gamma) assays. Testing of antigen-specific CD4(+) T-cell lines with the individual peptides of MPB70 confirmed that peptides p8, p12, and p13 contain immunodominant Th1 cell epitopes of MPB70. MHC restriction analysis with HLA-typed donors showed that MPB70 and its immunodominant peptides were presented to T cells promiscuously. The T-cell lines responding to MPB70 and peptides p8, p12, and p13 in IFN-gamma assays mediated antigen-peptide-specific cytotoxic activity against monocytes/macrophages pulsed with the whole-protein antigen or the peptides. In conclusion, the promiscuous recognition of MPB70 and its immunodominant peptide defined epitopes (aa 106 to 130 and 166 to 193) by IFN-gamma-producing Th1 cells supports possible application of this secreted antigen to subunit vaccine design.

T- and B-cell epitopes in the secreted Mycobacterium bovis antigen MPB70 in mice

MPB70 is a soluble secreted protein highly expressed in Mycobacterium bovis and strains of bacille Calmette-Guérin (BCG); as such, it is a candidate for subunit and DNA vaccines against tuberculosis. MPB70 was screened for T-cell epitopes in four different inbred mouse strains. Major histocompatibility complex (MHC) H-2b-expressing mice (C57BL/6) secreted interferon-gamma (IFN-gamma) after stimulation with peptides from the regions 1-20, 41-50, 81-110, 121-150 and 161-193 of the MPB70 sequence. H-2db mouse (B6D2) splenocytes secreted IFN-gamma after stimulation with some of the same peptides, whereas H-2d mice (BALB/c and DBA/2) did not secrete IFN-gamma upon stimulation with the peptides. Sera from H-2db mice immunized with native MPB70 in incomplete Freund's adjuvant (IFA), mpb70 DNA or live BCG Moreau were found to contain antibodies against the native MPB70 antigen. H-2db mice immunized with native MPB70 in IFA exhibited high titres of peptide-reactive immunoglobulin G1 (IgG1) antibodies, whereas DNA-immunized mice reacted with IgG2a antibodies against some of the same peptides. As some of the epitopes recognized by mouse T and B cells have previously been found to stimulate immune responses in humans, cattle and rabbits, we conclude that these epitopes may be good general epitopes for the stimulation of T- and B-cell responses and candidates for a DNA vaccine with a broad applicability.

Isolation of a 19-kDa mycobacterium, bovis-specific antigen, different from MPB70/80, by chromatofocusing

Two antigens, 19-kDa each, were purified from Mycobacterium bovis culture filtrate protein extract by chromatofocusing. Antigen I had a 4.5 pI, and its amino terminal (DPVDAVINTTCNYGQVVAALNATDP) showed a 100% homology with the hypothetical protein Rv1174c. Antigen II had a pI of 6.0 pI and its amino terminal (GDLVGPGCAEYAAANPTGPASVQGM) showed a 100% homology with M. bovis MPB70/80. Antigen I is a hetero-dimer formed by a glycosylated, 10.5-kDa, monomer and a non-glycosylated 8-kDa monomer with identical amino terminal sequences. Both antigens were recognized by the sera of PPD+ animals, but antigen I did not crossreact with sera of human PPD+ individuals. Antigen I was a weak inducer of lymphocyte proliferation and IFN-gamma production. Our results show that M. bovis expresses a 19 kDa glycoprotein, homologue to the product of M. tuberculosis gen Rv1174c, which may prove useful for bovine TB diagnostic assays.

Purification, biochemical characterization and immunogenicity of SA5K, a secretion antigen of Mycobacterium tuberculosis

Mycobacterium tuberculosis (MTB) secretory proteins are generally considered important antigens for immune protection against tuberculosis (TB). An 8.3-kDa secretory antigen of MTB and Mycobacterium bovis bacillus Calmette-Guérin (BCG), called SA5K, was recently identified and cloned in our laboratory. In this report, recombinant SA5K containing a histidine hexamer was expressed in Escherichia coli and purified to investigate its biochemical structure and to establish whether it was immunogenic for healthy sensitized and nonsensitized human donors and for patients infected with MTB. The protein nucleotide sequence was shown to be identical in BCG and in MTB. SA5K revealed an abnormal electrophoretic mobility in SDS-PAGE that made it look lighter than it is in Western blotting. While recombinant SA5K was poorly recognized by T lymphocytes from patients with pulmonary TB, it elicited proliferation of CD4+ T lymphocytes in the vast majority of healthy individuals sensitized to mycobacterial antigens by BCG vaccination. At a serum dilution of 1 : 80, antibodies reacting against recombinant SA5K were found in 67% of sera from TB patients and in 73% of sera from healthy subjects. The percentage of positive subjects dropped at higher serum dilutions, but no significant difference in the recognition rate was observed between TB patients and healthy donors and between healthy vaccinated and nonvaccinated subjects. Owing to the high percentage of sera from healthy subjects who recognized SA5K in Western blotting, the antigen seems to exhibit, at least in the present form, a poor specificity for an employment for a serodiagnosis of TB.

Generation of antibodies to the signal peptide of the MPT83 lipoprotein of Mycobacterium tuberculosis

The mpt83 gene (Rv2873) encodes the exported MPT83 lipoprotein of Mycobacterium tuberculosis. The corresponding identical mpb83 gene of Mycobacterium bovis is expressed to varying extents in different substrains of M. bovis Bacille Calmette Guerin (BCG), BCG Tokyo and BCG Moreau being high producers and BCG Danish 1331, a low producer of the MPB83 protein. Immunization with the 13-mer N-terminal part of the signal peptide of MPT83, MINVQAKPAAAASC, coupled to keyhole limpet haemocyanin (KLH) through the added C-terminal cysteine resulted in rapid antibody formation monitored by enzyme-linked immunosorbent assay (ELISA) with free immunizing peptide on the solid phase. In ELISA, with four 20-mer overlapping peptides covering the N-terminal part of the MPT83 sequence, three polyclonal rabbit antisera reacted only with the N-terminal peptide. Antigenic signal peptide could not be detected in sonicates of BCG Tokyo and BCG Moreau. After SDS-PAGE and blotting, the antibodies reacted with sonicates of recombinant Escherichia coli containing the entire mpt83 gene including the signal sequence, but not with the 22 kDa form of native MPB83 purified from BCG culture filtrate. In partition chromatography the recMPT83 partitioned in the water phase while 26 kDa MPB83 in BCG culture filtrate partitioned in the lipid phase confirming that lipidation at the N-terminal cysteine residue occurs after the splitting of the polypeptide chain by signal peptidase II.

Characterization of the Mycobacterium tuberculosis region containing the mpt83 and mpt70 genes

Like the products of the genes mpt83 and mpt70, the putative protein encoded by the gene located between these genes was undetectable in Mycobacterium tuberculosis with an antiserum raised against the recombinant protein. The protein showed 100% homology with M. tuberculosis Rv2874 and similarities with CcdA and DipZ proteins involved in cytochrome-c biogenesis in bacteria. Expression analysis by RT-PCR and transcriptional fusions of Rv2874 and their neighbor genes Rv2871, Rv2872, mpt83 and mpt70 with lacZ suggest that these genes are part of an operon and their transcription is driven by promoter regions located 5' upstream of mpt83 and of Rv2874 genes.

Novel monoclonal antibodies against major antigens of Mycobacterium bovis

MPB70 and MPB83 are among the most characteristically exported proteins defining a strongly expressed phenotype of Mycobacterium bovis. These proteins are known to be homologous to osteoblast-specific factor 2. By in vitro culture of mycobacteria they appear to have a limited species distribution and to be relatively specific for M. bovis. Virtually identical genes are however, present in Mycobacterium tuberculosis. In order to facilitate further research into the immunobiology of these proteins and their potential application for differential diagnosis of tuberculosis as a result of M. bovis, we describe the reactivities of 20 monoclonal antibodies (MoAbs) to these proteins. Immunizing with bovine PPD generated 10 MoAbs. These antibodies reacted preferentially with the soluble MPB70 antigen using reducing conditions in SDS-PAGE with western blotting. Ten MoAbs were generated by immunizing mice with fractions derived from a whole cell sonic extract of M. bovis. These antibodies reacted preferentially with the surface exposed MPB83 lipoglycoprotein.

Characterization of the early antibody response in bovine tuberculosis: MPB83 is an early target with diagnostic potential

A 26-kDa antigen has been shown to be a dominant antibody target in Mycobacterium bovis-infected cattle. In this study, that antigen was used as an immunogen to raise a panel of mouse monoclonal antibodies. The majority of those bound to native protein with a molecular mass of 26 kDa and to recombinant MPB83, strongly suggesting that MPB83 is an important B-cell antigenic target in bovine tuberculosis. In order to provide assessment of the potential of measuring antibody responses to the native protein, one monoclonal antibody, 1F11, was incorporated into an enzyme-linked immunosorbant assay format to trap antigen from a crude bacterial extract. Despite some disadvantages of this format, serum samples from cattle which had been infected experimentally with M. bovis, and from tuberculin skin-test-negative and -positive field cattle were tested for the presence of antibodies. Data from the skin-test-negative cattle allowed an arbitrary cut-off value to be established and, under these conditions, test sensitivity and specificity were estimated at 37.5 and 89%, respectively. These results indicate potential for MPB83 in the development of assays for serological diagnosis of bovine tuberculosis.

IgG isotype antibody responses to epitopes of the Mycobacterium bovis protein MPB70 in immunised and in tuberculin skin test-reactor cattle

Serological assays may have merit in identifying animals in advanced stages of bovine tuberculosis, but most tests have had sub-optimal sensitivities and specificities. The Mycobacterium bovis protein MPB70 has been identified as a B-cell target with diagnostic potential in measurement of pre- and post-skin-test antibody responses. One observation, which has potential practical application, has been that skin testing with tuberculin boosts IgG(1) anti-MPB70 antibody responses in cattle with tuberculous lesions. However, serological cross-reactivities with bacteria, such as Nocardia asteroides, have been described for this protein. With the aim of identifying candidate reagents for improved diagnostic tests, this study investigated IgG isotype antibody responses to MPB70 at the epitope level and, because of the previous findings, focused on IgG(1) responses following skin testing. Screening of a panel of overlapping synthetic peptides using sera from cattle immunised with MPB70 and cattle infected with M. bovis showed that two regions of the protein (residues 21-70 and 101-120) contain dominant B-cell epitopes. No individual epitope appeared to be selectively recognised by one isotype of IgG antibody. Investigation of IgG(1) responses showed that recognition of the epitope within residues 51-70 was boosted strongly by tuberculin injections in skin-test positive cattle and that this memory response was generally a feature of cattle which were found to have macroscopic, tuberculous lesions.

Negligible genetic diversity of mycobacterium tuberculosis host immune system protein targets: evidence of limited selective pressure

A common theme in medical microbiology is that the amount of amino acid sequence variation in proteins that are targets of the host immune system greatly exceeds that found in metabolic enzymes or other housekeeping proteins. Twenty-four Mycobacterium tuberculosis genes coding for targets of the host immune system were sequenced in 16 strains representing the breadth of genomic diversity in the species. Of the 24 genes, 19 were invariant and only six polymorphic nucleotide sites were identified in the 5 genes that did have variation. The results document the highly unusual circumstance that prominent M. tuberculosis antigenic proteins have negligible structural variation worldwide. The data are best explained by a combination of three factors: (i) evolutionarily recent global dissemination in humans, (ii) lengthy intracellular quiescence, and (iii) active replication in relatively few fully immunocompetent hosts. The very low level of amino acid diversity in antigenic proteins may be cause for optimism in the difficult fight to control global tuberculosis.

Antigen specificity in experimental bovine tuberculosis

This report describes the kinetics of T-cell responses to a panel of mycobacterial antigens (PPD-M, PPD-A, ESAT-6, Ag85, 38kD, MPB64, MPB70, MPB83, hsp16.1, hsp65, and hsp70) following experimental infection of cattle with Mycobacterium bovis. Increased antigen-specific lymphocyte proliferation, gamma interferon, and interleukin-2 responses were observed in all calves following infection. Positive lymphocyte proliferation and cytokine responses to PPD-M and ESAT-6 were observed throughout the infection period studied. In contrast, responses to all other antigens were more variable and were not constantly present, suggesting that antigen cocktails rather than individual antigens should be used for immunodiagnosis. The detection of cytokine responses in the absence of lymphocyte proliferation, particularly during the early stages of infection, suggests a role for antigen-specific cytokine readout systems in the early identification of M. bovis infection in cattle.