Impaired resistance to Mycobacterium tuberculosis infection after selective in vivo depletion of L3T4+ and Lyt-2+ T cells

Mycobacterium tuberculosis and TLR2 agonists inhibit induction of type I IFN and class I MHC antigen cross processing by TLR9

Dendritic cells (DCs) cross process exogenous Ags and present them by class I MHC (MHC-I) molecules to CD8(+) T cells specific for Ags from viruses and bacteria such as Mycobacterium tuberculosis. Unmethylated CpG DNA signals through TLR9 to induce type I IFN (IFN-alpha/beta), which enhances MHC-I Ag cross processing, but lipoproteins that signal through TLR2 do not induce IFN-alpha/beta. In these studies we observed that M. tuberculosis, which expresses agonists of both TLR9 and TLR2, did not induce production of IFN-alpha/beta or cross processing by murine DCs. Furthermore, M. tuberculosis and TLR2 agonists inhibited induction of IFN-alpha/beta and DC cross processing by CpG DNA. Exogenous IFN-alpha/beta effectively enhanced cross processing of M. bovis bacillus Calmette-Guérin expressing OVA, bypassing the inhibition of induction of endogenous IFN-alpha/beta. In addition, inhibition of TLR9-induced cross processing of M. bovis bacillus Calmette-Guérin expressing OVA could be circumvented by pretreating cells with CpG DNA to induce IFN-alpha/beta and MHC-I cross processing before inhibitory mycobacterial TLR2 agonists were present. Inhibition of the response to one TLR by another may affect the ultimate response to pathogens like M. tuberculosis that express agonists of multiple TLRs, including TLR2 and TLR9. This mechanism may contribute to immune evasion and explain why IFN-alpha/beta provides little contribution to host immunity to M. tuberculosis. However, downregulation of certain TLR responses may benefit the host by preventing detrimental excessive inflammation that may occur in the presence of persistent infection.

Towards improved understanding of protective mechanisms induced by the BCG vaccine

EVALUATION OF: Ryan AA, Nambiar JK, Wozniak TM et al. Antigen load governs the differential priming of CD8 T cells in response to the bacille Calmette-Guérin vaccine or Mycobacterium tuberculosis infection. J. Immunol. 182(11), 7172-7177 (2009). Mycobacterium tuberculosis is the causative agent of human TB, which is responsible for 26% of all preventable deaths in the developing world. Mycobacterium bovis bacillus Calmette-Guérin (BCG) has been used as a vaccine against TB since 1921, with protection varying from 0 to 80%. Although the reasons for this variability are unclear, protection is thought to be mediated by Th1-type responses. CD8(+) T cells have been shown to play a role in the response to mycobacteria. However, the nature of the CD8(+) response induced by BCG requires further characterization. In the paper being evaluated, it was shown that the antigenic load is important for the induction of CD8(+) T-cell responses and subsequent protective responses. Identification of the factors affecting induction of CD8(+) responses against mycobacteria will facilitate studies to characterize their nature and provide a platform for the development of systems seeking to improve on protection conferred by BCG.

Factors that deregulate the protective immune response in tuberculosis

Tuberculosis (TB) is a chronic infectious disease which essentially affects the lungs and produces profound abnormalities on the immune system. Although most people infected by the tubercle bacillus (90%) do not develop the disease during their lifetime, when there are alterations in the immune system, such as co-infection with HIV, malnutrition, or diabetes, the risk of developing active disease increases considerably. Interestingly, during the course of active disease, even in the absence of immunosuppressive conditions, there is a profound and prolonged suppression of Mycobacterium tuberculosis-specific protective immune responses. Several immune factors can contribute to downregulate the protective immunity, permitting disease progression. In general, many of these factors are potent anti-inflammatory molecules that are probably overproduced with the intention to protect against tissue damage, but the consequence of this response is a decline in protective immunity facilitating bacilli growth and disease progression. Here the most significant participants in protective immunity are reviewed, in particular the factors that deregulate protective immunity in TB. Their manipulation as novel forms of immunotherapy are also briefly commented.

Mycobacterium tuberculosis cell wall glycolipids directly inhibit CD4+ T-cell activation by interfering with proximal T-cell-receptor signaling

Immune evasion is required for Mycobacterium tuberculosis to survive in the face of robust adaptive CD4(+) T-cell responses. We have previously shown that M. tuberculosis can indirectly inhibit CD4(+) T cells by suppressing the major histocompatibility complex class II antigen-presenting cell function of macrophages. This study was undertaken to determine if M. tuberculosis could directly inhibit CD4(+) T-cell activation. Murine CD4(+) T cells were purified from spleens by negative immunoaffinity selection followed by flow sorting. Purified CD4(+) T cells were activated for 16 to 48 h with CD3 and CD28 monoclonal antibodies in the presence or absence of M. tuberculosis and its subcellular fractions. CD4(+) T-cell activation was measured by interleukin 2 production, proliferation, and expression of activation markers, all of which were decreased in the presence of M. tuberculosis. Fractionation identified that M. tuberculosis cell wall glycolipids, specifically, phosphatidylinositol mannoside and mannose-capped lipoarabinomannan, were potent inhibitors. Glycolipid-mediated inhibition was not dependent on Toll-like receptor signaling and could be bypassed through stimulation with phorbol 12-myristate 13-acetate and ionomycin. ZAP-70 phosphorylation was decreased in the presence of M. tuberculosis glycolipids, indicating that M. tuberculosis glycolipids directly inhibited CD4(+) T-cell activation by interfering with proximal T-cell-receptor signaling.

Specificity of the zebrafish host transcriptome response to acute and chronic mycobacterial infection and the role of innate and adaptive immune components

Pathogenic mycobacteria have the ability to survive within macrophages and persist inside granulomas. The complex host-pathogen interactions that determine the outcome of a mycobacterial infection process result in marked alterations of the host gene expression profile. Here we used the zebrafish model to investigate the specificity of the host response to infections with two mycobacterium strains that give distinct disease outcomes: an acute disease with early lethality or a chronic disease with granuloma formation, caused by Mycobacterium marinum strains Mma20 and E11, respectively. We performed a microarray study of different stages of disease progression in adult zebrafish and found that the acute and the chronic strains evoked partially overlapping host transcriptome signatures, despite that they induce profoundly different disease phenotypes. Both strains affected many signaling cascades, including WNT and TLR pathways. Interestingly, the strongest differences were observed at the initial stage of the disease. The immediate response to the acute strain was characterized by higher expression of genes encoding MHC class I proteins, matrix metalloproteinases, transcription factors, cytokines and other common immune response proteins. In contrast, small GTPase and histone gene groups showed higher expression in response to the chronic strain. We also found that nearly 1000 mycobacterium-responsive genes overlapped between the expression signatures of infected zebrafish adults and embryos at different stages of granuloma formation. Since adult zebrafish possess an adaptive immune system similar to mammals and zebrafish embryos rely solely on innate immunity, this overlap indicates a major contribution of the innate component of the immune system in the response to mycobacterial infection. Taken together, our comparison of the transcriptome responses involved in acute versus chronic infections and in the embryonic versus adult situation provides important new leads for investigating the mechanism of mycobacterial pathogenesis.

Anti-TNF immunotherapy reduces CD8+ T cell-mediated antimicrobial activity against Mycobacterium tuberculosis in humans

The incidence of tuberculosis is increased during treatment of autoimmune diseases with anti-TNF antibodies. This is a significant clinical complication, but also provides a unique model to study immune mechanisms in human tuberculosis. Given the key role for cell-mediated immunity in host defense against Mycobacterium tuberculosis, we hypothesized that anti-TNF treatment impairs T cell-directed antimicrobial activity. Anti-TNF therapy reduced the expression in lymphocytes of perforin and granulysin, 2 components of the T cell-mediated antimicrobial response to intracellular pathogens. Specifically, M. tuberculosis-reactive CD8+CCR7-CD45RA+ effector memory T cells (TEMRA cells) expressed the highest levels of granulysin, lysed M. tuberculosis, and infected macrophages and mediated an antimicrobial activity against intracellular M. tuberculosis. Furthermore, TEMRA cells expressed cell surface TNF and bound the anti-TNF therapeutic infliximab in vitro, making them susceptible to complement-mediated lysis. Immune therapy with anti-TNF was associated with reduced numbers of CD8+ TEMRA cells and decreased antimicrobial activity against M. tuberculosis, which could be rescued by the addition of CD8+ TEMRA cells. These results suggest that anti-TNF therapy triggers a reduction of CD8+ TEMRA cells with antimicrobial activity against M. tuberculosis, providing insight into the mechanism whereby key effector T cell subsets contribute to host defense against tuberculosis.

A critical role for CD8 T cells in a nonhuman primate model of tuberculosis

The role of CD8 T cells in anti-tuberculosis immunity in humans remains unknown, and studies of CD8 T cell–mediated protection against tuberculosis in mice have yielded controversial results. Unlike mice, humans and nonhuman primates share a number of important features of the immune system that relate directly to the specificity and functions of CD8 T cells, such as the expression of group 1 CD1 proteins that are capable of presenting Mycobacterium tuberculosis lipids antigens and the cytotoxic/bactericidal protein granulysin. Employing a more relevant nonhuman primate model of human tuberculosis, we examined the contribution of BCG- or M. tuberculosis-elicited CD8 T cells to vaccine-induced immunity against tuberculosis. CD8 depletion compromised BCG vaccine-induced immune control of M. tuberculosis replication in the vaccinated rhesus macaques. Depletion of CD8 T cells in BCG-vaccinated rhesus macaques led to a significant decrease in the vaccine-induced immunity against tuberculosis. Consistently, depletion of CD8 T cells in rhesus macaques that had been previously infected with M. tuberculosis and cured by antibiotic therapy also resulted in a loss of anti-tuberculosis immunity upon M. tuberculosis re-infection. The current study demonstrates a major role for CD8 T cells in anti-tuberculosis immunity, and supports the view that CD8 T cells should be included in strategies for development of new tuberculosis vaccines and immunotherapeutics.

Tuberculosis, HIV/AIDS and malaria remain top killers worldwide. Cell-mediated immune responses play a crucial role in immunity against tuberculosis. While CD4 T cells are well described for their protection against tuberculosis, little is known about the role of human CD8 T cells in anti-tuberculosis immunity. Studies done to date in mice have yielded conflicting results regarding the role of mouse CD8 T cells in tuberculosis. Since there are considerable differences in CD8 T cell biology between mice and primates including humans and macaques, studies in humans or macaques are crucial for clarifying human CD8 T cell–mediated immunity against tuberculosis. Thus, we used a macaque tuberculosis model to examine the contribution of CD8 T cells to vaccine-induced immunity against tuberculosis. We found that CD8 T cells play a role in BCG vaccine-induced immune control of Mycobacterium tuberculosis replication and in the vaccine-induced immunity against tuberculosis. Consistently, memory CD8 T cells also play a crucial role in anti-tuberculosis immunity upon M. tuberculosis re-infection. The findings in the current study provide evidence that human CD8 T cells are of importance for anti-tuberculosis immunity, and support the view that CD8 T cells should be targeted for development of new tuberculosis vaccines and immunotherapeutics.

CD4 and CD8 T cell responses to the M. tuberculosis Ag85B-TB10.4 promoted by adjuvanted subunit, adenovector or heterologous prime boost vaccination

BACKGROUND

Although CD4 T cells are crucial for defense against M.tb, it is still not clear whether the optimal response against M.tb in fact involves both CD4 and CD8 T cells. To test this, we used a new vaccine strategy that generated a strong balanced T cell response consisting of both CD4 and CD8 T cells.

METHODS AND FINDINGS

To compare CD4 and CD8 responses against Ag85B-TB10.4 (H4), H4 was delivered as a subunit vaccine in cationic liposomes (CAF01), expressed in Ad5 (Ad-H4) or as a heterologous prime boost vaccination. H4/CAF01 induced primarily CD4 T cells and Ad-H4 gave predominantly a CD8 T cell response. In contrast, the heterologous prime boost combination resulted in augmentation of both the CD4 and CD8 response. The majority (>40%) of the CD4 T cells induced by the heterologous prime boost protocol were polyfunctional, and expressed IFN-gamma(+), IL-2(+), and TNF-alpha(+), whereas most of the CD8 T cells expressed IFN-gamma(+) and TNF-alpha(+) and possessed strong cytotoxic potential. The heterologous prime boost protocol also gave an increase in protective efficacy against M.tb challenge compared to H4/CAF01 and Ad-H4. Both the H4 specific CD4 and CD8 T cells were recruited to the site of infection, at the onset of infection. However, compared to CD8 T cells, CD4 T cells showed more extensive recruitment and were the main T cell subset proliferating at the site of infection.

CONCLUSIONS/SIGNIFICANCE

Heterologous prime boost based on H4, produced an additive effect on the priming of CD4 and CD8 cells and in terms of the protective capacity of the vaccine, and therefore represent an interesting new vaccine strategy against M.tb. However, CD4 and CD8 T cells respond very differently to live M.tb challenge, in a manner which supports the consensus that CD4 T cells do play the major role during the early stages of an M.tb infection.

TB vaccines: current status and future perspectives

Vaccines against intracellular pathogens such as Mycobacterium tuberculosis need to induce strong cellular immune responses. Antigen discovery programs have exploited this and used proteome studies and T-cell recognition in PPD-positive individuals to select proteins and after testing for protective efficacy in animals the most promising proteins have been put together in fusion molecules. Three such fusion proteins are currently in clinical trials, the two most advanced have already passed phase I trials and are entering phase II.

T-cell epitope mapping in Mycobacterium tuberculosis using pepmixes created by micro-scale SPOT- synthesis

Mycobacterium tuberculosis (Mtb) remains a major threat to human health worldwide. Although treatment of infection is an important part of tuberculosis control, an improved vaccine is essential for the elimination of this disease. Control of infection with Mtb is dependent on the cellular immune system, which in turn requires an understanding of those antigens that are capable of stimulating CD4+ and CD8+ T-cell responses. Peptide libraries provide a high-throughput system for identifying novel T-cell epitopes. They can also be used to assess the hierarchy of immunodominance of these novel antigens and epitopes that are associated with infection with Mtb. This T-cell-driven means of antigen discovery is well adapted to vaccine development as well as developing the tools necessary to understand the natural history of this important human pathogen.

T cell responses to mycobacterial catalase-peroxidase profile a pathogenic antigen in systemic sarcoidosis

Sarcoidosis is a systemic granulomatous disease associated with local epithelioid granulomas, CD4(+) T cells, and Th1 cytokines. The tissue Ags that drive this granulomatous inflammation are uncertain. In this study, we used IFN-gamma-ELISPOT assays and flow cytometry to assess lung and blood T cell responses to the candidate pathogenic Ag, Mycobacterium tuberculosis catalase-peroxidase (mKatG) in patients with sarcoidosis from two centers. Despite differences in patient phenotypic, genetic, and prognostic characteristics, we report that T cell responses to mKatG were remarkably similar in these cohorts, with higher frequencies of mKatG-reactive, IFN-gamma-expressing T cells in the blood of sarcoidosis patients compared with nontuberculosis sensitized healthy controls, and (in a subset) in greater numbers than T cells reactive to purified protein derivative. In sarcoidosis, mKatG-reactive CD4(+) Th1 cells preferentially accumulated in the lung, indicating a compartmentalized response. Patients with or without Löfgren syndrome had similar frequencies of mKatG specific IFN-gamma-expressing blood T cells. Circulating mKatG-reactive T cells were found in chronic active sarcoidosis but not in patients with inactive disease. Together, these results demonstrate that T cell responses to mKatG in sarcoidosis fit a profile expected for a pathogenic Ag, supporting an immunotherapeutic approach to this disease.

Identification of Mycobacterium tuberculosis vaccine candidates using human CD4+ T-cells expression cloning

To identify Mycobacterium tuberculosis (Mtb) antigens as candidates for a subunit vaccine against tuberculosis (TB), we have employed a CD4+ T-cell expression screening method. Mtb-specific CD4+ T-cell lines from nine healthy PPD positive donors were stimulated with different antigenic substrates including autologous dendritic cells (DC) infected with Mtb, or cultured with culture filtrate proteins (CFP), and purified protein derivative of Mtb (PPD). These lines were used to screen a genomic Mtb library expressed in Escherichia coli and processed and presented by autologous DC. This screening led to the recovery of numerous T-cell antigens, including both novel and previously described antigens. One of these novel antigens, referred to as Mtb9.8 (Rv0287), was recognized by multiple T-cell lines, stimulated with either Mtb-infected DC or CFP. Using the mouse and guinea pig models of TB, high levels of IFN-gamma were produced, and solid protection from Mtb challenge was observed following immunization with Mtb9.8 formulated in either AS02A or AS01B Adjuvant Systems. These results demonstrate that T-cell screening of the Mtb genome can be used to identify CD4+ T-cell antigens that are candidates for vaccine development.

Early T-cell responses in tuberculosis immunity

SUMMARY

Tuberculosis (TB) has plagued mankind for millennia yet is classified as an emerging infectious disease, because its prevalence in the human population continues to increase. Immunity to TB depends critically on the generation of effective CD4(+) T-cell responses. Sterile immunity has not been achieved through vaccination, although early T-cell responses are effective in controlling steady-state infection in the lungs. Although such early T-cell responses are clearly protective, the initiation of the Mycobacterium tuberculosis (Mtb) T-cell response occurs much later than is the case following other aerogenic infections. This fact suggests that there is a critical period, before the activation of the T-cell response, in which Mtb is able to establish infection. An understanding of the factors that regulate early T-cell activation should, therefore, lead to better control of the disease. This review discusses recent work that has investigated the early development of T-cell immunity following Mtb infection in the mouse.

Bovine TB and the development of new vaccines

Bovine tuberculosis (bTB) is caused by Mycobacterium bovis. The incidence of bTB is increasing in cattle herds of developed countries that have a wild life reservoir of M. bovis, such as the UK, New Zealand and the USA. The increase in the incidence of bTB is thought to be due, at least in part, to a wildlife reservoir of M. bovis. M. bovis is also capable of infecting humans and on a worldwide basis, M. bovis is thought to account for up to 10% of cases of human TB [Cosivi O, Grange JM, Daborn CJ et al. Zoonotic tuberculosis due to Mycobacterium bovis in developing countries. Emerg Infect Dis 1998;4(1):59-70]. Thus, the increased incidence of bTB, besides being a major economic problem, poses an increased risk to human health. In the UK, the incidence of bTB continues to rise despite the use of the tuberculin test and slaughter control policy, highlighting the need for improved control strategies. Vaccination of cattle, in combination with more specific and sensitive diagnostic tests, is suggested as the most effective strategy for bovine TB control. The only vaccine currently available for human and bovine TB is the live attenuated Bacille Calmette Guerin (BCG). BCG is thought to confer protection through the induction of Th1 responses against mycobacteria. However, protection against TB conferred by BCG is variable and to this date the reasons for the successes and failures of BCG are not clear. Therefore, there is a need to develop vaccines that confer greater and more consistent protection against bTB than that afforded by BCG. Given that BCG is currently the only licensed vaccine against human TB, it is likely that any new vaccine or vaccination strategy will be based around BCG. In this review we discuss immune responses elicited by mycobacteria in cattle and the novel approaches emerging for the control of bovine TB based on our increasing knowledge of protective immune responses.

An analysis of the epitope knowledge related to Mycobacteria

BACKGROUND

Tuberculosis, caused by the bacterium Mycobacterium tuberculosis, remains a leading cause of infectious disease morbidity and mortality, and is responsible for more than 2 million deaths a year. Reports about extremely drug resistant (XDR) strains have further heightened the sense of urgency for the development of novel strategies to prevent and treat TB. Detailed knowledge of the epitopes recognized by immune responses can aid in vaccine and diagnostics development, and provides important tools for basic research. The analysis of epitope data corresponding to M. tuberculosis can also identify gaps in our knowledge, and suggest potential areas for further research and discovery. The Immune Epitope Database (IEDB) is compiled mainly from literature sources, and describes a broad array of source organisms, including M. tuberculosis and other Mycobacterial species.

DESCRIPTION

A comprehensive analysis of IEDB data regarding the genus Mycobacteria was performed. The distribution of antibody/B cell and T cell epitopes was analyzed in terms of their associated recognition cell type effector function and chemical properties. The various species, strains and proteins which the epitope were derived, were also examined. Additional variables considered were the host in which the epitopes were defined, the specific TB disease state associated with epitope recognition, and the HLA associated with disease susceptibility and endemic regions were also scrutinized. Finally, based on these results, standardized reference datasets of mycobacterial epitopes were generated.

CONCLUSION

All current TB-related epitope data was cataloged for the first time from the published literature. The resulting inventory of more than a thousand different epitopes should prove a useful tool for the broad scientific community. Knowledge gaps specific to TB epitope data were also identified. In summary, few non-peptidic or post-translationally modified epitopes have been defined. Most importantly epitopes have apparently been defined from only 7% of all ORFs, and the top 30 most frequently studied protein antigens contain 65% of the epitopes, leaving the majority of M. tuberculosis genome unexplored. A lack of information related to the specific strains from which epitopes are derived is also evident. Finally, the generation of reference lists of mycobacterial epitopes should also facilitate future vaccine and diagnostic research.

Mycobacterial glycolipid trehalose 6,6'-dimycolate-induced hypersensitive granulomas: contribution of CD4+ lymphocytes

The granulomatous response is a characteristic histological feature of Mycobacterium tuberculosis infection responsible for organism containment. The development of cell-mediated immunity is essential for protection against disease, as well as being required for maintenance of the sequestering granulomatous response. Trehalose 6,6'-dimycolate (TDM; cord factor), a glycolipid associated with the cell wall of mycobacteria, is implicated as a key immunogenic component in M. tuberculosis infection. Models of TDM-induced hypersensitive granulomatous response have similar pathologies to that of active tuberculosis infection. Prior immunization (sensitization) of mice with TDM results in exacerbated histological damage, inflammation and lymphocytic infiltration upon subsequent TDM challenge. Adoptive transfer experiments were performed to ascertain the cell phenotype governing this response; CD4(+) cells were identified as critical for development of related pathology. Mice receiving CD4(+) cells from donor TDM-immunized mice demonstrated significantly increased production of Th1-type cytokines IFN-gamma and IL-12 within the lung upon subsequent TDM challenge. Control groups receiving naïve CD4(+) cells, or CD8(+) or CD19(+) cells isolated from TDM-immunized donors, did not exhibit an exacerbated response. The identified CD4(+) cells isolated from TDM-immunized mice produced significant amounts of IFN-gamma and IL-2 when exposed to TDM-pulsed macrophages in vitro. These experiments provide further evidence for involvement of a cell-mediated response in TDM-induced granuloma formation, which mimics pathological damage elicited during M. tuberculosis infection.

Induction of CD8 T cells against a novel epitope in TB10.4: correlation with mycobacterial virulence and the presence of a functional region of difference-1

Although infection with Mycobacterium tuberculosis (M.tb) induces a robust CD8 T cell response, the role of CD8 T cells in the defense against M.tb, and the mechanisms behind the induction of CD8 T cells, is still not clear. TB10.4 is a recently described Ag that is expressed by both bacillus Calmette-Guérin (BCG) and M.tb. In the present study, we describe a novel CD8 T cell epitope in TB10.4, TB10.4(3-11). We show that TB10.4(3-11)-specific CD8 T cells are induced at the onset of infection and are present throughout the infection in high numbers. TB10.4(3-11) CD8 T cells were recruited to the site of infection and expressed CD44, TNF-alpha, and IFN-gamma. In addition, TB10.4(3-11) CD8 T cells showed an up-regulation of FasL and LAMP-1/2 (CD107A/B), which correlated with a strong in vivo cytolytic activity. The induction of TB10.4(3-11)-specific CD8 T cells was less pronounced following infection with BCG compared to infection with M.tb. By using a rBCG expressing the genetic region of difference-1 (RD1), we show that the presence of a functional RD1 region increases the induction of TB10.4(3-11)-specific CD8 T cells as well as the bacterial virulence. Finally, as an M.tb variant lacking the genetic region RD1 also induced a significant amount of TB10.4(3-11)-specific CD8 T cells, and exhibited increased virulence compared with BCG, our data suggest that virulence in itself is also involved in generating a robust CD8 T cell response against mycobacterial epitopes, such as TB10.4(3-11).

Immunodominant tuberculosis CD8 antigens preferentially restricted by HLA-B

CD8(+) T cells are essential for host defense to intracellular bacterial pathogens such as Mycobacterium tuberculosis (Mtb), Salmonella species, and Listeria monocytogenes, yet the repertoire and dominance pattern of human CD8 antigens for these pathogens remains poorly characterized. Tuberculosis (TB), the disease caused by Mtb infection, remains one of the leading causes of infectious morbidity and mortality worldwide and is the most frequent opportunistic infection in individuals with HIV/AIDS. Therefore, we undertook this study to define immunodominant CD8 Mtb antigens. First, using IFN-gamma ELISPOT and synthetic peptide arrays as a source of antigen, we measured ex vivo frequencies of CD8(+) T cells recognizing known immunodominant CD4(+) T cell antigens in persons with latent tuberculosis infection. In addition, limiting dilution was used to generate panels of Mtb-specific T cell clones. Using the peptide arrays, we identified the antigenic specificity of the majority of T cell clones, defining several new epitopes. In all cases, peptide representing the minimal epitope bound to the major histocompatibility complex (MHC)-restricting allele with high affinity, and in all but one case the restricting allele was an HLA-B allele. Furthermore, individuals from whom the T cell clone was isolated harbored high ex vivo frequency CD8(+) T cell responses specific for the epitope, and in individuals tested, the epitope represented the single immunodominant response within the CD8 antigen. We conclude that Mtb-specific CD8(+) T cells are found in high frequency in infected individuals and are restricted predominantly by HLA-B alleles, and that synthetic peptide arrays can be used to define epitope specificities without prior bias as to MHC binding affinity. These findings provide an improved understanding of immunodominance in humans and may contribute to a development of an effective TB vaccine and improved immunodiagnostics.

XCL1 (lymphotactin) chemokine produced by activated CD8 T cells during the chronic stage of infection withMycobacterium tuberculosisnegatively affects production of IFN-γ by CD4 T cells and participates in granuloma stability

CD8 T cell immune responses are known not to be essential during the initial stages of infection with Mycobacterium tuberculosis (Mtb), but their presence becomes important as the chronic infection ensues. The basis of this is still not clear. In previous studies, we showed that CD8 T cells have a distinctive positioning in the architecture of the granuloma lesion, with further changes throughout the course of the chronic infection. We have also hypothesized that further movement of lymphocytes once they are within the lung lesions could be associated with the levels of expression of the chemokine XCL1 (lymphotactin). XCL1 is produced mainly by activated CD8 T cells, and its chemotactic activity seems primarily controlling movement of CD4 and CD8 T cells. In this study, using a murine low-dose aerosol infection model coupled with antibody depletion of T cell subsets, we investigated the role of CD8 T cells in the control of the bacterial growth and in the pathogenesis of the disease in mice at early, mid, or late stages of the chronic disease state. Additionally, we also describe for the first time that during Mtb infection, activated CD8 T cells in the lungs produce XCL1 and that this chemokine is capable of controlling IFN-gamma production by CD4 T cells.

Impaired expression of perforin and granulysin in CD8+ T cells at the site of infection in human chronic pulmonary tuberculosis

Protective immunity in tuberculosis is dependent on the coordinated release of cytolytic effector molecules from effector T cells and the subsequent granule-associated killing of infected target cells. In this study, we investigated the expression of cytolytic (perforin and granzyme A) and antimicrobial (granulysin) molecules at the single-cell level in cryopreserved lung tissue from patients with chronic, progressive tuberculosis disease. Quantification of protein-expressing cells was performed by in situ imaging, while mRNA levels in the infected tissue were analyzed by real-time PCR. Persistent inflammation, including excessive expression of inducible nitric oxide synthase in CD68+ macrophages and significant infiltration of CD3+, CD8+ and CD4+ T cells, was evident in tuberculosis lesions in all patients. However, despite the accumulation of CD3+ T cells, perforin- and granulysin-expressing CD3+ T cells were detected at two- to threefold-lower ratios in the tuberculosis lesions than in distal lung parenchyma and uninfected control lungs, respectively. This was evident at both the protein and mRNA levels. Moreover, perforin- and granulysin-expressing CD8+ T cells were scarce in individual granulomas within the tuberculosis lesions. In contrast, significant up-regulation of granzyme A-expressing CD3+ T cells was evident in the lesions from all patients. Confocal microscopy revealed coexpression of perforin and granulysin, primarily in CD8+ T cells; however, this expression was lower in the tuberculosis lesions. These findings suggest that symptomatic, chronic tuberculosis disease is associated with insufficient up-regulation of perforin and granulysin coexpression in CD8+ T cells at the local site of infection.

Next generation: tuberculosis vaccines that elicit protective CD8+ T cells

Tuberculosis continues to cause considerable human morbidity and mortality worldwide, particularly in people coinfected with HIV. The emergence of multidrug resistance makes the medical treatment of tuberculosis even more difficult. Thus, the development of a tuberculosis vaccine is a global health priority. Here we review the data concerning the role of CD8+ T cells in immunity to tuberculosis and consider how CD8+ T cells can be elicited by vaccination. Many immunization strategies have the potential to elicit CD8+ T cells and we critically review the data supporting a role for vaccine-induced CD8+ T cells in protective immunity. The synergy between CD4+ and CD8+ T cells suggests that a vaccine that elicits both T-cell subsets has the best chance at preventing tuberculosis.

Translational mini-review series on vaccines: Development and evaluation of improved vaccines against tuberculosis

Tuberculosis (TB) continues to be a major global health disaster, despite the widespread use of BCG and effective drug therapies. The development of an efficacious new TB vaccine would be an important component of disease control in the future. Many approaches are being utilised to enhance understanding of the requirements of a successful vaccine. Numerous vaccines are being designed and assessed in a series of animal models, with a few progressing to clinical trials. Here, the steps involved in the development and evaluation of TB vaccines will be discussed, including description of the most frequently used animal models and the processes involved in advancing vaccines to phase III trials.

Mycobacterium tuberculosis-specific CD8+ T cells and their role in immunity

There are more cases of tuberculosis in the world today than at any other time in history. The global epidemic has generated intense interest into the immunological mechanisms that control infection. Although CD4+ T cells play a critical role in host immunity to Mycobacterium tuberculosis, there is considerable interest in understanding the role of other T cell subsets in preventing disease development following infection. CD8+ T cells are required for optimum host defense following M. tuberculosis infection, which has led to investigation into how this protective effect is mediated. A critical review of recent literature regarding the role of CD8+ T cells in protective immunity to M. tuberculosis infection is now required to address the strengths and weaknesses of these studies. In this article, we evaluate the evidence that CD8+ T cells are critical in immunity to M. tuberculosis infection. We discuss the specific mycobacterial proteins that are recognized by CD8+ T cells elicited during infection. Finally, we examine the effector mechanisms of CD8+ T cells generated during infection and synthesize recent studies to consider the protective roles that these T cells serve in vivo.

High frequency of CD4+ T cells specific for the TB10.4 protein correlates with protection against Mycobacterium tuberculosis infection

TB10.4 is a newly identified antigen of Mycobacterium tuberculosis recognized by human and murine T cells upon mycobacterial infection. Here, we show that immunization with Mycobacterium bovis BCG induces a strong, genetically controlled, Th1 immune response against TB10.4 in mice. BALB/c and C57BL/6 strains behave as high and low responders to TB10.4 protein, respectively. The TB10.4:74-88 peptide was identified as an immunodominant CD4+ T-cell epitope for H-2d mice. Since recent results, as well as the present study, have raised interest in TB10.4 as a subunit vaccine, we analyzed immune responses induced by this antigen delivered by a new vector, the adenylate cyclase (CyaA) of Bordetella pertussis. CyaA is able to target dendritic cells and to deliver CD4+ or CD8+ T-cell epitopes to the major histocompatibility complex class II/I molecule presentation pathways, triggering specific Th1 or cytotoxic T-lymphocyte (CTL) responses. Several CyaA harboring either the entire TB10.4 protein or various subfragments containing the TB10.4:20-28 CTL epitope were shown to induce TB10.4-specific Th1 CD4+ and CD8+ T-cell responses. However, none of the recombinant CyaA, injected in the absence of adjuvant, was able to induce protection against M. tuberculosis infection. In contrast, TB10.4 protein administered with a cocktail of strong adjuvants that triggered a strong Th1 CD4+ T-cell response induced significant protection against M. tuberculosis challenge. These results confirm the potential value of the TB10.4 protein as a candidate vaccine and show that the presence of high frequencies of CD4+ T cells specific to this strong immunogen correlates with protection against M. tuberculosis infection.

Antigen-specific proliferation and activation of peripheral blood mononuclear cells from Mycobacterium bovis-infected reindeer

To evaluate antigen-specific proliferative and activation-associated responses from Mycobacterium bovis-infected reindeer, blood mononuclear cells from M. bovis- (n = 10) and non-infected reindeer (n = 4) were stimulated with a recombinant early secretory antigenic target-6 and culture filtrate protein-10 fusion protein (rESAT6:CFP10), M. bovis purified protein derivative, pokeweed mitogen, or medium alone and evaluated by flow cytometry using dye tracker analysis and cell surface marker staining. gammadelta TCR+ and CD8+ cells, but not CD4+ cells, from M. bovis-infected reindeer proliferated in response to specific antigen stimulation. Expression (i.e., mean fluorescence intensity) of CD44 was increased and CD62L decreased on proliferative as compared to non-proliferative fractions in antigen- and mitogen-stimulated cultures. In response rESAT6:CFP10 stimulation, MHC II fluorescence intensity was increased on CD4+, gammadelta TCR+, CD172a+, and IgM+ cells from infected reindeer as compared to that of non-stimulated cells from the same reindeer. Recombinant ESAT6:CFP10 stimulation also induced expansion of a CD172a+, MHC II+ population within mononuclear cell cultures from M. bovis-infected reindeer. Despite a moderate challenge dose and extended duration of incubation, experimental infection of reindeer was generally limited to lymph nodes draining the inoculation site, suggestive of host resistance to progressive disease. Present in vitro findings, therefore, may be predictive of host responses by reindeer that limit progression to disseminated disease.

The Phosphoenolpyruvate Carboxykinase of Mycobacterium Tuberculosis Induces Strong Cell-Mediated Immune Responses in Mice

Phosphoenolpyruvate carboxykinase (PEPCK) catalyzes guanosine or adenosine mononucleotide-dependent reversible conversion of oxaloacetate (OAA) and phosphoenolpyruvate (PEP). Mycobacterium (M) tuberculosis possesses a putative GTP-dependent PEPCK. To analyze the immune responses caused by PEPCK, the effects of PEPCK on the induction of CD4(+) T cells and cytokines such as IFN-gamma, IL-12 and TNF-alpha were evaluated in mice. It was found that the number of CD4(+) T cells was increased in the PEPCK immunized mice although the change of the number of CD8(+) T cells was not significant. The cytokines IFN-gamma, IL-12 and TNF-alpha were increased significantly in the mice immunized with PEPCK than those of incomplete adjuvant. These characteristics were further demonstrated in the mice infected by pckA mutated BCG strain. The results indicate that PEPCK can effectively induce cell-mediated immune response by increasing activity of cytokines and PEPCK may be a promising new subunit vaccine candidate for tuberculosis.

CD4+Cells Play a Limited Role in Murine Lung Infection withMycobacterium kansasii

Mycobacterium kansasii has emerged as an important nontuberculous mycobacterium that can cause severe infection in the immunocompromised host, especially in human immunodeficiency virus-infected patients. However, little is known about the pathogenesis of this infection. Because patients suffering from M. kansasii infection are severely compromised in their cellular immune response, we studied the course of infection in CD4+ cell knockout (KO) mice. Wild-type (WT) mice and CD4+ KO mice were infected with 10(5) cfu of M. kansasii. Although previously shown to be susceptible to Mycobacterium tuberculosis infection, CD4+ KO mice demonstrated no impairment in clearing infection with M. kansasii when compared with WT animals, despite reduced pulmonary inflammation (reduced granuloma formation and lymphocyte infiltration in the lungs). Pulmonary IFN-gamma levels and M. kansasii-induced IFN-gamma production by splenocytes from infected animals were reduced in CD4+ KO mice, confirming that these mice were defective in the M. kansasii-specific T helper cell type 1 immune response. Furthermore, mice deficient for IFN-gamma, IL-12p35, IL-12p40, or IL-18 also displayed a normal host defense against pulmonary infection with M. kansasii. These data suggest that CD4+ cells, IFN-gamma, and an intact T helper cell type 1 response play a limited role in protective immunity against pulmonary M. kansasii infection.

Animal models of tuberculosis

It was Robert Koch who recognized the spectrum of pathology of tuberculosis (TB) in different animal species. The examination of clinical specimens from infected humans and animals confirmed the variable patterns of pathological reactions in different species. Guinea pigs are innately susceptible while humans, mice and rabbits show different level of resistance depending upon their genotype. The studies of TB in laboratory animals such as mice, rabbits and guinea pigs have significantly increased our understanding of the aetiology, virulence and pathogenesis of the disease. The introduction of less than five virulent organisms into guinea pigs by the respiratory route can produce lung lesions, bacteraemia and fatal diseases, which helped the extrapolation of results of such experiments to humans. The similarities in the course of clinical infection between guinea pigs and humans allow us to model different forms of TB and to evaluate the protective efficacy of candidate vaccines in such systems. The only limitation of this model, however, is a dearth of immunological reagents that are required for the qualitative and quantitative evaluation of the immune responses, with special reference to cytokines and cell phenotypes. Another limitation is the higher cost of guinea pigs compared with mice. The rabbit is relatively resistant to Mycobacterium tuberculosis, however following infection with virulent Mycobacterium bovis, the rabbit produces pulmonary cavities like humans. The rabbit model, however, is also limited by the lack of the immunological reagents. Mice are the animal of choice for studying the immunology of mycobacterial infections and have contributed much to our current understanding of the roles of various immunological mechanisms of resistance. The resistance of mice to the development of classic TB disease, however, represents a significant disadvantage of the mouse model. Although non-human primates are closely related to humans, owing to high cost and handing difficulties they have not been exploited to a large extent. As all existing animal models fail to mimic the human disease perfectly, efforts should be focused on the development of the non-human primate(s) as the alternative animal model for TB.

Tracking antigen-specific CD8 T lymphocytes in the lungs of mice vaccinated with the Mtb72F polyprotein

This study used a major histocompatibility complex class I tetramer reagent to track antigen-specific CD8 T cells in the lungs of mice immunized with the tuberculosis vaccine candidate Mtb72F. The results show that CD8 T cells recognizing an immunodominant Mtb32-specific epitope could be detected in significant numbers over the course of infection in mice exposed to low-dose aerosol challenge with Mycobacterium tuberculosis and that prior vaccination substantially increased the numbers of these cells early in the lungs. The effector phenotype of the cells was shown by the demonstration that many secreted gamma interferon, but very few contained granzyme B. As the course of the infection progressed, many activated CD8 T cells down-regulated expression of CD45RB and upregulated expression of the interleukin-7 receptor alpha chain, indicating a transition of these cells to a state of memory. These data support the hypothesis that M. tuberculosis-specific CD8 T cells can be targeted by vaccination with the Mtb72F polyprotein.

Protection of mice and guinea pigs against tuberculosis induced by immunization with a single Mycobacterium tuberculosis recombinant antigen, MTB41

MTB41 is a Mycobacterium antigen that is recognized by CD4+ T cells early after experimental infection of mice with Mycobacterium tuberculosis and by PBMC from healthy PPD positive individuals. Immunization of mice with plasmid DNA encoding the MTB41 gene sequence results in the development of antigen-specific CD4+ and CD8+ T cells, and protection against challenge with virulent M. tuberculosis. In the present studies, in contrast to DNA immunization, we show, that a strong MTB41-specific CD4+ T cell response, but no MHC class I restricted cytotoxic T lymphocyte (CTL) activity is detected in the spleen cells of infected mice. Therefore, this data suggests that the induction of CD8+ T cell response to MTB41 epitopes by DNA immunization may not be relevant to protection because these epitopes are not recognized during the infectious process. We also compared the repertoire of rMTB41 epitope recognition by CD4+ T cells of M. tuberculosis-infected mice with the recognition repertoire of mice immunized with the recombinant rMTB41 protein. Both regimens of sensitization lead to the recognition of the same molecular epitope. Coincidentally, immunization with the soluble recombinant protein plus adjuvant, a regimen known to generate primarily CD4+ T cells, resulted in induction of protection comparable to BCG in two well-established animal models of tuberculosis (mice and guinea pigs).

Role of CD8+ T lymphocytes in control of Mycobacterium tuberculosis infection

Tuberculosis remains a global health concern. Control of infection is dependent on cell-mediated immune responses, with CD4+ T lymphocytes playing a central role. In this article, data supporting the importance of CD8+ T lymphocytes is reviewed, with an emphasis on the unique functional roles that this lymphocyte subset may play.

Bystander activation of CD8+ T lymphocytes during experimental mycobacterial infection

Infection of C57BL/6 mice with Mycobacterium avium leads to the activation of both CD4+ and CD8+ gamma interferon (IFN-gamma)-producing T cells, although the CD8+ cells play no role in protection against infection. Using transfer of different lines of transgenic T cells with T-cell receptors (TCRs) which recognize irrelevant antigens, we show here that transferred CD8+ T cells from two of the three lines were activated to the same degree as the host cells, suggesting that the majority of the IFN-gamma-producing CD8+ T cells of the host represented bystander activation. The third line, specific for the male HY antigen, showed no activation. Activation required the participation of the CD28 coreceptor on T cells and was unaffected by the removal of CD44(hi) (memory phenotype) T cells. The transferred CD8+ T cells proliferated in vivo, although this was not essential for IFN-gamma production. Taken together, these data are highly reminiscent of homeostatic proliferation of TCR transgenic T cells upon transfer to lymphopenic hosts, and suggest low-affinity stimulation through the TCR, possibly by self peptides. The findings are discussed in relation to homeostatic proliferation and their significance in the possible induction of autoimmune disease.

Activation of CD8 T cells by mycobacterial vaccination protects against pulmonary tuberculosis in the absence of CD4 T cells

We have investigated whether both primary CD8 T cell activation and CD8 T cell-mediated protection from Mycobacterium tuberculosis challenge could occur in mycobacterial-vaccinated CD4 T cell-deficient (CD4KO) mice. Different from wild-type C57BL/6 mice, s.c. vaccination with bacillus Calmette-Guérin (BCG) in CD4KO mice failed to provide protection from secondary M. tuberculosis challenge at 3 wk postvaccination. However, similar to C57BL/6 mice, CD4KO mice were well protected from M. tuberculosis at weeks 6 and 12 postvaccination. This protection was mediated by CD8 T cells. The maintenance of protective effector/memory CD8 T cells in CD4KO mice did not require the continuous presence of live BCG vaccine. As in C57BL/6 mice, similar levels of primary activation of CD8 T cells in CD4KO mice occurred in the draining lymph nodes at 3 wk after BCG vaccination, but different from C57BL/6 mice, the distribution of these cells to the spleen and lungs of CD4KO mice was delayed, which coincided with delayed acquisition of protection in CD4KO mice. Our results suggest that both the primary and secondary activation of CD8 T cells is CD4 T cell independent and that the maintenance of these CD8 T cells is also independent of CD4 T cells and no longer requires the presence of live mycobacteria. However, the lack of CD4 T cells may result in delayed distribution of activated CD8 T cells from draining lymph nodes to distant organs and consequently a delayed acquisition of immune protection. Our findings hold implications in rational design of tuberculosis vaccination strategies for humans with impaired CD4 T cell function.

Identification of the new T-cell-stimulating antigens from Mycobacterium tuberculosis culture filtrate

The proteins secreted by Mycobacterium tuberculosis are an important target for vaccine development. To identify the antigens from M. tuberculosis culture filtrate (CF) that strongly stimulate T-cells, the CF was fractionated by ion-exchange chromatography and then non-reducing sodium dodecyl sulfate-polyacrylamide gel electrophoresis with mini-whole gel elution. Each fraction was screened for its ability to induce interferon-gamma (IFN-gamma) production in peripheral blood mononuclear cells isolated from healthy tuberculin reactors. The protein bands that strongly induced IFN-gamma production were subjected to N-terminal sequencing. Two new proteins, a 17-kDa protein (Rv0164, MTSP17) and an 11-kDa (Rv3204, MTSP11) protein, were identified. The recombinant MTSP17 (rMTSP17) and rMTSP11 induced significant production of IFN-gamma and interleukin (IL)-12p40 in peripheral blood mononuclear cells from healthy tuberculin reactors. Interestingly, IL-12p40 production in response to rMTSP11 was significantly higher than that in response to rMTSP17 or the three components of the antigen 85 complex. These results suggest that MTSP11 antigen should be further evaluated as a component of a subunit vaccine.

Immunology of tuberculosis and implications in vaccine development

Mycobacterium tuberculosis is a very successful pathogen that can survive and persist in the human host in the face of a robust immune response. This immune response is sufficient to prevent disease in the majority of infected persons, providing compelling evidence that immunity to tuberculosis is possible. However, it is more striking that the strong immune response is not generally effective at eliminating the organisms, during either initial infection or the persistent or latent phase of infection. Studies in animal models and in humans have demonstrated the wide range of immune components involved in the effective response against M. tuberculosis. These components include T cells (both CD4+ and CD8+), cytokines, including IFN-gamma, IL-12, TNF-alpha, and IL-6, and macrophages. The precise roles and functions of these cells and molecules (and others) are still being defined and may differ in acute and chronic infection. These immune responses are directed towards containing or eliminating the tubercle bacillus within the tissues of the host. The estimated eight million new cases of tuberculosis each year clearly demonstrate that these responses are not always effective. M. tuberculosis has obviously evolved a variety of mechanisms to evade destruction by the immune response. Studying both the host and the pathogen will elucidate potential vaccine candidates. In this review, the known functions of immune components in the response to M. tuberculosis and implications for vaccine development will be discussed.

Differential Immune Responses and Protective Efficacy Induced by Components of a Tuberculosis Polyprotein Vaccine, Mtb72F, Delivered as Naked DNA or Recombinant Protein

Key Ags of Mycobacterium tuberculosis initially identified in the context of host responses in healthy purified protein derivative-positive donors and infected C57BL/6 mice were prioritized for the development of a subunit vaccine against tuberculosis. Our lead construct, Mtb72F, codes for a 72-kDa polyprotein genetically linked in tandem in the linear order Mtb32(C)-Mtb39-Mtb32(N). Immunization of C57BL/6 mice with Mtb72F DNA resulted in the generation of IFN-gamma responses directed against the first two components of the polyprotein and a strong CD8(+) T cell response directed exclusively against Mtb32(C). In contrast, immunization of mice with Mtb72F protein formulated in the adjuvant AS02A resulted in the elicitation of a moderate IFN-gamma response and a weak CD8(+) T cell response to Mtb32c. However, immunization with a formulation of Mtb72F protein in AS01B adjuvant generated a comprehensive and robust immune response, resulting in the elicitation of strong IFN-gamma and Ab responses encompassing all three components of the polyprotein vaccine and a strong CD8(+) response directed against the same Mtb32(C) epitope identified by DNA immunization. All three forms of Mtb72F immunization resulted in the protection of C57BL/6 mice against aerosol challenge with a virulent strain of M. tuberculosis. Most importantly, immunization of guinea pigs with Mtb72F, delivered either as DNA or as a rAg-based vaccine, resulted in prolonged survival (>1 year) after aerosol challenge with virulent M. tuberculosis comparable to bacillus Calmette-Guérin immunization. Mtb72F in AS02A formulation is currently in phase I clinical trial, making it the first recombinant tuberculosis vaccine to be tested in humans.

Alcohol exacerbates murine pulmonary tuberculosis

Alcohol consumption has been described as a risk factor for infection with Mycobacterium tuberculosis, but its contribution to tuberculosis has been difficult to isolate from other adverse socioeconomic factors. Our objective was to evaluate the impact of alcohol consumption on pulmonary infection with M. tuberculosis in a murine model. BALB/c mice were maintained on the Lieber-DeCarli liquid ethanol diet or a liquid control diet and infected intratracheally with low-dose M. tuberculosis H37Rv. Lung organism burdens, lung and lung-associated lymph node CD4(+)- and CD8(+)- lymphocyte numbers and rates of proliferation, and CD4(+)-lymphocyte cytokine production levels were compared between the groups. The alcohol-consuming mice had significantly higher lung organism burdens than the control mice, and the CD4(+)- and CD8(+)-lymphocyte responses to pulmonary infection with M. tuberculosis were blunted in the alcohol group. Lymphocyte proliferation and production of gamma interferon were decreased in the CD4(+) lymphocytes from the alcohol-consuming mice. Additionally, lung granulomas were significantly smaller in the alcohol-consuming mice. In conclusion, murine alcohol consumption is associated with decreased control of pulmonary infection with M. tuberculosis, which is accompanied by alterations in the region-specific CD4(+)- and CD8(+)-lymphocyte responses and defective lung granuloma formation.

Mycobacterium tuberculosis-specific CD8+ T cells preferentially recognize heavily infected cells

Both CD4+ and CD8+ T cells are important for successful immunity to tuberculosis and have redundant effector functions, such as cytolysis and release of potent antimycobacterial cytokines such as interferon-gamma and tumor necrosis factor-alpha. We hypothesized that CD8+ T cells play a unique role in host defense to Mycobacterium tuberculosis infection as well. Possibilities include preferential and/or enhanced release of granular constituents and/or preferential recognition of heavily infected cells. Utilizing human, Mycobacterium tuberculosis-specific, CD4+ and CD8+ T cell clones, we demonstrate that, after recognition of antigen-presenting cells displaying peptide antigen, CD4+ T cells preferentially release interferon-gamma, whereas CD8+ T cells preferentially lyse antigen-presenting cells. Furthermore, utilizing dendritic cells infected with Mycobacterium tuberculosis expressing green fluorescent protein, we show that CD8+ T cells preferentially recognize heavily infected cells that constitute the minority of infected cells. These data support the hypothesis that the central role of CD8+ T cells in the control of infection with Mycobacterium tuberculosis may be that of surveillance; in essence, recognition of cells in which the containment of Mycobacterium tuberculosis is no longer effective.

Experimental Mycobacterium tuberculosis infection of cynomolgus macaques closely resembles the various manifestations of human M. tuberculosis infection

Nonhuman primates were used to develop an animal model that closely mimics human Mycobacterium tuberculosis infection. Cynomolgus macaques were infected with low doses of virulent M. tuberculosis via bronchoscopic instillation into the lung. All monkeys were successfully infected, based on tuberculin skin test conversion and peripheral immune responses to M. tuberculosis antigens. Progression of infection in the 17 monkeys studied was variable. Active-chronic infection, observed in 50 to 60% of monkeys, was characterized by clear signs of infection or disease on serial thoracic radiographs and in other tests and was typified by eventual progression to advanced disease. Approximately 40% of monkeys did not progress to disease in the 15 to 20 months of study, although they were clearly infected initially. These monkeys had clinical characteristics of latent tuberculosis in humans. Low-dose infection of cynomolgus macaques appears to represent the full spectrum of human M. tuberculosis infection and will be an excellent model for the study of pathogenesis and immunology of this infection. In addition, this model will provide an opportunity to study the latent M. tuberculosis infection observed in approximately 90% of all infected humans.

CD8 T cell responses to infectious pathogens

CD8 T cells respond to viral infections but also participate in defense against bacterial and protozoal infections. In the last few years, as new methods to accurately quantify and characterize pathogen-specific CD8 T cells have become available, our understanding of in vivo T cell responses has increased dramatically. Pathogen-specific T cells, once thought to be quite rare following infection, are now known to be present at very high frequencies, particularly in peripheral, nonlymphoid tissues. With the ability to visualize in vivo CD8 T cell responses has come the recognition that T cell expansion is programmed and, to a great extent, independent of antigen concentrations. Comparison of CD8 T cell responses to different pathogens also highlights the intricate relationship between microbially induced innate inflammatory responses and the kinetics, magnitude, and character of long-term T cell responses. This review describes recent progress in some of the major murine models of CD8 T cell-mediated immunity to viral, bacterial, and protozoal infection.

Human immunity to M. tuberculosis: T cell subsets and antigen processing

A hallmark of M. tuberculosis infection is the ability of most (90-95%) healthy adults to control infection through acquired immunity, in which antigen specific T cells and macrophages arrest growth of M. tuberculosis bacilli and maintain control over persistent bacilli. In addition to CD4+ T cells, other T cell subsets such as, gammadelta, CD8+ and CD1-restricted T cells have roles in the immune response to M. tuberculosis. A diverse T cell response allows the host to recognize a wider range of mycobacterial antigens presented by different families of antigen-presenting molecules, and thus greater ability to detect the pathogen. Macrophages are key antigen presenting cells for T cells, and M. tuberculosis survives and persists in this central immune cell. This is likely an important factor in generating this T cell diversity. Furthermore, the slow growth and chronic nature of M. tuberculosis infection results in prolonged exposure to antigens, and hence further T cell sensitization. The effector mechanisms used by T cells to control M. tuberculosis are poorly understood. To survive in macrophages, M. tuberculosis has evolved mechanisms to block immune responses. These include modulation of phagosomes, neutralization of macrophage effector molecules, stimulating the secretion of inhibitory cytokines, and interfering with processing of antigens for T cells. The relative importance of these blocking mechanisms likely depends on the stage of M. tuberculosis infection: primary infection, persistence, reactivation or active tuberculosis. The balance of the host-pathogen interaction in M. tuberculosis infection is determined by the interaction of T cells and infected macrophages. The outcome of this interaction results either in control of M. tuberculosis infection or active disease. A better understanding of this interaction will result in improved approaches to treatment and prevention of tuberculosis.

Alternate class I MHC antigen processing is inhibited by Toll-like receptor signaling pathogen-associated molecular patterns: Mycobacterium tuberculosis 19-kDa lipoprotein, CpG DNA, and lipopolysaccharide

Pathogen-associated molecular patterns (PAMPs) signal through Toll-like receptors (TLRs) to activate immune responses, but prolonged exposure to PAMPs from Mycobacterium tuberculosis (MTB) and other pathogens inhibits class II MHC (MHC-II) expression and Ag processing, which may allow MTB to evade CD4(+) T cell immunity. Alternate class I MHC (MHC-I) processing allows macrophages to present Ags from MTB and other bacteria to CD8(+) T cells, but the effect of PAMPs on this processing pathway is unknown. In our studies, MTB and TLR-signaling PAMPs, MTB 19-kDa lipoprotein, CpG DNA, and LPS, inhibited alternate MHC-I processing of latex-conjugated Ag by IFN-gamma-activated macrophages. Inhibition was dependent on TLR-2 for MTB 19-kDa lipoprotein (but not whole MTB or the other PAMPs); inhibition was dependent on myeloid differentiation factor 88 for MTB and all of the individual PAMPs. Inhibition of MHC-II and alternate MHC-I processing was delayed, appearing after 16 h of PAMP exposure, as would occur in chronically infected macrophages. Despite inhibition of alternate MHC-I Ag processing, there was no inhibition of MHC-I expression, MHC-I-restricted presentation of exogenous peptide or conventional MHC-I processing of cytosolic Ag. MTB 19-kDa lipoprotein and other PAMPs inhibited phagosome maturation and phagosome Ag degradation in a myeloid differentiation factor 88-dependent manner; this may limit availability of peptides to bind MHC-I. By inhibiting both MHC-II and alternate MHC-I Ag processing, pathogens that establish prolonged infection of macrophages (>16 h), e.g., MTB, may immunologically silence macrophages and evade surveillance by both CD4(+) and CD8(+) T cells, promoting chronic infection.

Investigation of the role of CD8+ T cells in bovine tuberculosis in vivo

Mycobacterium bovis is the causative agent of bovine tuberculosis (TB), and it has the potential to induce disease in humans. CD8(+) T cells (CD8 cells) have been shown to respond to mycobacterial antigens in humans, cattle, and mice. In mice, CD8 cells have been shown to play a role in protection against mycobacterial infection. To determine the role of CD8 cells in bovine TB in vivo, two groups of calves were infected with the virulent M. bovis strain AF2122/97. After infection, one group was injected with a CD8 cell-depleting monoclonal antibody (MAb), and the other group was injected with an isotype control MAb. Immune responses to mycobacterial antigens were measured weekly in vitro. After 8 weeks, the animals were killed, and postmortem examinations were carried out. In vitro proliferation responses were similar in both calf groups, but in vitro gamma interferon (IFN-gamma) production in 24-h whole-blood cultures was significantly higher in control cattle than in CD8 cell-depleted calves. Postmortem examination showed that calves in both groups had developed comparable TB lesions in the lower respiratory tract and associated lymph nodes. Head lymph node lesion scores, on the other hand, were higher in control calves than in CD8 cell-depleted calves. Furthermore, there was significant correlation between the level of IFN-gamma and the head lymph node lesion score. These experiments indicate that CD8 cells play a role in the immune response to M. bovis in cattle by contributing to the IFN-gamma response. However, CD8 cells may also play a deleterious role by contributing to the immunopathology of bovine TB.

The Mycobacterium tuberculosis 19-kilodalton lipoprotein inhibits gamma interferon-regulated HLA-DR and Fc gamma R1 on human macrophages through Toll-like receptor 2

Mycobacterium tuberculosis survives in macrophages in the face of acquired CD4(+) T-cell immunity, which controls but does not eliminate the organism. Gamma interferon (IFN-gamma) has a central role in host defenses against M. tuberculosis by activating macrophages and regulating major histocompatibility complex class II (MHC-II) antigen (Ag) processing. M. tuberculosis interferes with IFN-gamma receptor (IFN-gamma R) signaling in macrophages, but the molecules responsible for this inhibition are poorly defined. This study determined that the 19-kDa lipoprotein from M. tuberculosis inhibits IFN-gamma-regulated HLA-DR protein and mRNA expression in human macrophages. Inhibition of HLA-DR expression was associated with decreased processing and presentation of soluble protein Ags and M. tuberculosis bacilli to MHC-II-restricted T cells. Inhibition of HLA-DR required prolonged exposure to 19-kDa lipoprotein and was blocked with a monoclonal antibody specific for Toll-like receptor 2 (TLR-2). The 19-kDa lipoprotein also inhibited IFN-gamma-induced expression of Fc gamma RI. Thus, M. tuberculosis, through 19-kDa lipoprotein activation of TLR-2, inhibits IFN-gamma R signaling in human macrophages, resulting in decreased MHC-II Ag processing and recognition by MHC-II-restricted CD4 T cells. These findings provide a mechanism for M. tuberculosis persistence in macrophages.

Immune biology of macaque lymphocyte populations during mycobacterial infection

Immune responses of lymphocyte populations during early phases of mycobacterial infection and reinfection have not been well characterized in humans. A non-human primate model of Mycobacterium bovis bacille Calmette-Guerin (BCG) infection was employed to characterize optimally the immune responses of mycobacteria-specific T cells. Primary BCG infection induced biphasic immune responses, characterized by initial lymphocytopenia and subsequent expansion of CD4+, CD8+ and gammadelta T cell populations in the blood, lymph nodes and the pulmonary compartment. The potency of detectable T cell immune responses appears to be influenced by the timing and route of infection as well as challenge doses of BCG organisms. Systemic BCG infection introduced by intravenous challenge induced a dose-dependent expansion of circulating CD4+, CD8+ and gammadelta T cells whereas, in the pulmonary compartment, the systemic infection resulted in a predominant increase in numbers of gammadelta T cells. In contrast, pulmonary exposure to BCG through the bronchial route induced detectable expansions of CD4+, CD8+ and gammadelta T cell populations in only the lung but not in the blood. A rapid recall expansion of these T cell populations was seen in the macaques reinfected intravenously and bronchially with BCG. The expanded alphabeta and gammadelta T cell populations exhibited their antigen specificity for mycobacterial peptides and non-peptide phospholigands, respectively. Finally, the major expansion of T cells was associated with a resolution of active BCG infection and reinfection. The patterns and kinetics of CD4+, CD8+ and gammadelta T cell immune responses during BCG infection might contribute to characterizing immune protection against tuberculosis and testing new tuberculosis vaccines in primates.

The Apa protein of Mycobacterium tuberculosis stimulates gamma interferon-secreting CD4+ and CD8+ T cells from purified protein derivative-positive individuals and affords protection in a guinea pig model

The search to identify Mycobacterium tuberculosis antigens capable of conferring protective immunity against tuberculosis has received a boost owing to the resurgence of tuberculosis over the past two decades. It has long been recognized that lymphoid cells are required for protection against M. tuberculosis. While traditionally the CD4(+) populations of T cells were believed to predominantly serve this protective function, a pivotal role for CD8(+) T cells in this task has been increasingly appreciated. We show that the 50- to 55-kDa Apa protein, specified by the Rv1860 gene of M. tuberculosis, can elicit both lymphoproliferative response and gamma interferon (IFN-gamma) production from peripheral blood mononuclear cells (PBMC) of purified protein derivative (PPD)-positive individuals, with significant differences recorded in the levels of responsiveness between PPD-positive healthy controls and pulmonary tuberculosis patients. Flow cytometric analysis of whole blood stimulated with the recombinant Apa protein revealed a sizeable proportion of CD8(+) T cells in addition to CD4(+) T cells contributing to IFN-gamma secretion. PBMC responding to the Apa protein produced no interleukin-4, revealing a Th1 phenotype. A DNA vaccine and a poxvirus recombinant expressing the Apa protein were constructed and tested for their ability to protect immunized guinea pigs against a challenge dose of virulent M. tuberculosis. Although the DNA vaccine afforded little protection, the poxvirus recombinant boost after DNA vaccine priming conferred a significant level of protective immunity, bringing about a considerable reduction in mycobacterial counts from the challenge bacilli in spleens of immunized guinea pigs, a result comparable to that achieved by BCG vaccination.

A DNA prime-live vaccine boost strategy in mice can augment IFN-gamma responses to mycobacterial antigens but does not increase the protective efficacy of two attenuated strains of Mycobacterium bovis against bovine tuberculosis

The Mycobacterium bovis bacille Calmette-Guérin (BCG) vaccine has variable efficacy for both human and bovine tuberculosis. There is a need for improved vaccines or vaccine strategies for control of these diseases. A recently developed prime-boost strategy was investigated for vaccination against M. bovis infection in mice. BALB/c and C57BL/6 mice were primed with a DNA vaccine, expressing two mycobacterial antigens, ESAT-6 and antigen 85 A and boosted with attenuated M. bovis strains, BCG or WAg520, a newly attenuated strain, prior to aerosol challenge. Before challenge, the antigen-specific production of interferon-gamma (IFN-gamma) was evaluated by ELISPOT and antibody responses were measured. The prime-boost stimulated an increase in the numbers of IFN-gamma producing cells compared with DNA or live vaccination alone, but this varied according to the attenuated vaccine strain, time of challenge and the strain of mouse used. Animals vaccinated with DNA alone generated the strongest antibody response to mycobacterial antigens, which was predominantly IgG1. BCG and WAg520 alone generally gave a 1-2 log10 reduction in bacterial load in lungs or spleen, compared to non-vaccinated or plasmid DNA only control groups. The prime-boost regimen was not more effective than BCG or WAg520 alone. These observations demonstrate the comparable efficacy of BCG and WAg520 in a mouse model of bovine tuberculosis. However, priming with the DNA vaccine and boosting with an attenuated M. bovis vaccine enhanced IFN-gamma immune responses compared to vaccinating with an attenuated M. bovis vaccine alone, but did not increase protection against a virulent M. bovis infection.

Persistence and turnover of antigen-specific CD4 T cells during chronic tuberculosis infection in the mouse

CD4 T cells are critical for resistance to Mycobacterium tuberculosis infection, but how effective T cell responses are maintained during chronic infection is not well understood. To address this question we examined the CD4 T cell response to a peptide from ESAT-6 during tuberculosis infection in the mouse. The ESAT-6(1-20)/IA(b)-specific CD4 T cell response in the lungs, mediastinal lymph nodes, and spleen reached maxima 3-4 wk postinfection, when the bacteria came under the control of the immune response. Once chronic infection was established, the relative frequencies of Ag-specific CD4 T cells were maintained at nearly constant levels for at least 160 days. ESAT-6(1-20)/IA(b)-specific CD4 T cells that responded in vitro expressed activation markers characteristic of chronically activated effector cells and used a limited Vbeta repertoire that was clonally stable in vivo for at least 12 wk. 5-Bromo-2-deoxyuridine incorporation studies indicated a relatively high rate of cell division among both total CD4 and ESAT-6(1-20)/IA(b)-specific CD4 T cells during acute infection, but the degree of 5-bromo-2-deoxyuridine incorporation by both the CD4 T cells and the Ag-specific cells declined at least 3-fold during chronic infection. The data indicate that the peripheral ESAT-6(1-20)/IA(b)-specific CD4 T cell response to M. tuberculosis is characterized during the acute phase of infection by a period of extensive proliferation, but once bacterial control is achieved, this is followed during chronic infection by an extended containment phase that is associated with a persistent response of activated, yet more slowly proliferating, T cells.

Expression of L-Selectin (CD62L), CD44, and CD25 on activated bovine T cells

Mycobacterium bovis infection of cattle represents a natural host-pathogen interaction and, in addition to its economic and zoonotic impact, represents a model for human tuberculosis. Extravasation and trafficking of activated lymphocytes to inflammatory sites is modulated by differential expression of multiple surface adhesion molecules. However, effects of M. bovis infection on adhesion molecule expression have not been characterized. To determine these changes, peripheral blood mononuclear cells from M. bovis-infected cattle were stimulated with M. bovis purified protein derivative (PPD) or pokeweed mitogen (PWM) and evaluated concurrently for proliferation and activation marker expression. Stimulation with PPD or PWM increased CD25 and CD44 mean fluorescence intensity (MFI) and decreased CD62L MFI on CD4(+) cells from infected animals. CD62L MFI on PPD- and PWM-stimulated gammadelta T-cell receptor-positive (TCR(+)) and CD8(+) cells was also reduced compared to that of nonstimulated gammadelta TCR(+) and CD8(+) cells. Using a flow cytometry-based proliferation assay, it was determined that proliferating cells, regardless of lymphocyte subset, exhibited increased expression of CD25 and CD44 and decreased expression of CD62L compared to cells that had not proliferated. In contrast to proliferation, activation-induced apoptosis of CD4(+) cells resulted in a significant down regulation of CD44 expression. Lymphocytes obtained from lungs of M. bovis-infected cattle also had reduced expression of CD44 compared to lymphocytes from lungs of noninfected cattle. These alterations in surface molecule expression upon activation likely impact trafficking to sites of inflammation and the functional capacity of these cells within tuberculous granulomas.