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CD8 Cytotoxic T Cells and the Development of New Tuberculosis Vaccines

Nuffield Department of Clinical Medicine University of Oxford John Radcliffe Hospital Oxford, United Kingdom

The majority of immunocompetent people exposed to Mycobacterium tuberculosis (MTB) acquire lifelong protective immunity against active tuberculosis (TB). An understanding of the components of this naturally acquired protective immune response and its antigenic targets is a prerequisite for the rational design of an improved TB vaccine. MTB infection induces a potent human leukocyte antigen (HLA) class II–restricted Th1-type CD4 T cell response, and the human immunodeficiency virus epidemic has taught us that this limb of the cellular immune system is an essential part of the protective immune response against TB. The role of HLA class I–restricted CD8 cytotoxic T cells (CTLs) in host defense against MTB is less clear. In murine models, CD8 CTLs are essential for protection against MTB (1) and specifically seem to play a role in long-term immune control of the bacillus (2), but it was not until 1998 that the first HLA class I–restricted antigen-specific CD8 CTLs were identified in MTB-infected humans (3). Since then, our understanding of this important T cell population in the host response to MTB infection in humans has expanded apace. A total of 10 secreted and somatic MTB protein antigens have now been identified as targets of CD8 CTL in humans. In general, these T cells secrete interferon-, have cytolytic activity, recognize MTB-infected macrophages in vitro (4), and in some reports, actually suppress the growth of intracellular bacilli (5). These are attributes that are consistent with a protective role for these cells in vivo.

Recently, novel vaccination strategies using naked DNA and recombinant viral vectors have made it possible to induce HLA class I–restricted CD8 CTLs, as well as HLA class II–restricted CD4 T cells in humans. Hence, the characterization of new MTB antigens as targets of both CD4 and CD8 T cells is an important part of the global effort to develop an effective vaccine against TB. If we could generate an array of defined peptide epitopes covering a broad enough range of common HLA types, HLA restriction of immune responsiveness would no longer be a barrier to the use of a multisubunit vaccine (based on multiple epitopes or the antigens that contain them) in genetically heterogeneous human populations (6). Delineating immunodominant epitopes will of course also help to monitor the immunogenicity of novel vaccines in clinical trials (6).

It is in this context that the report by Lewinsohn and colleagues (7) in this issue of AJRCCM (pp. 843–848) represents another step forward on the long path toward an effective TB vaccine. This article demonstrates that Mtb39, already known to be a potent CD4 T cell antigen (8), is also a target of CD8 CTL in MTB-infected people. In an elegant series of experiments, Lewinsohn and coworkers (7) have exploited the highly sensitive interferon- enzyme-linked immunospot (ELISPOT) assay (9) to identify circulating Mtb39-specific CD8 T cells that secrete interferon-, display cytolytic activity, and recognize MTB-infected target cells in vitro. These CD8 T cells were identified directly ex vivo in the ELISPOT assay, using autologous dendritic cells infected with an adenovirus recombinant for the Mtb39 gene (adenoMtb39). Each spot in the ex vivo ELISPOT assay represents the "footprint" of an interferon-–secreting CD8 T cell specific for an HLA class I–restricted epitope expressed at the surface of the adenoMtb39-infected dendritic cell. Lewinsohn and coworkers (7) went on to delineate the minimal peptide epitope in Mtb39 by testing cloned CTL against a panel of overlapping peptides representative of the entire Mtb39 protein. The minimal epitope is 10 amino acids long and is restricted through the HLA class I molecule HLA-B44. Interestingly, this immunodominant epitope might well have been overlooked if computerized peptide motif algorithms (for predicting which peptides are likely to bind HLA class I molecules) had been used to select candidate epitopes for screening. This study thus adds to the growing body of evidence that highlights overlapping peptides, with or without recombinant viral vectors (10), as the most efficient strategy for new epitope discovery.

Another advantage of the ex vivo ELISPOT assay is that it gives the actual frequency of circulating epitope-specific interferon-–secreting CD8 T cells in peripheral blood (9). In the three donors in this report, this frequency was quite high, ranging from 1:3,000 to 1:15,000 of all circulating CD8 T cells, almost as high as the frequencies of CD8 CTLs specific for two other MTB antigens, ESAT-6 (3, 11) and CFP10 (12), noted previously. These CTL frequencies are of a similar level to those directed against viral epitopes in people with latent EBV or CMV infection, in whom CD8 CTL are believed to mediate long-term immune control of the virus. Of note, the donors in the article by Lewinsohn and coworkers (7), as well as the previously described donors with a potent CD8 CTL response to ESAT-6 (11) and CFP10 (12), had latent TB infection. We know little about the biology of the pivotal host–pathogen relationship that underlies latent TB infection, but the bacillary burden (and hence antigen load) in this state is considered to be very low, and the vast majority of people with latent TB infection have mounted a successful, long-lived, and protective immune response. Since it is antigen load that drives antigen-specific T cell frequencies in vivo, the high frequency of MTB antigen-specific CD8 CTLs in healthy people with latent TB infection is all the more remarkable. Moreover, in some donors, longitudinal follow-up over 2 years has shown that this potent, highly focused CTL response can also be very durable (11).

Thus, careful immunologic studies of MTB-exposed healthy donors with latent TB infection, of which the article by Lewinsohn and coworkers (7) is the latest example, suggest that potent, long-lived CD8 CTL responses to certain MTB antigens may play a role in long-term control of MTB infection. In the next few years, it will be important to track these antigen-specific T cell populations in latently infected TB contacts over time and correlate changes in their frequency or phenotype with clinical outcomes. This is a long and arduous task; however, it should provide a definitive answer as to the role of CD8 CTL in immunity to TB, and it will help to accelerate the rational development of an effective TB vaccine.

REFERENCES

1. Flynn JL, Goldstein MM, Triebold KJ, Koller B, Bloom BR. Major histocompatibility complex class I-restricted T cells are required for resistance to Mycobacterium tuberculosis infection. Proc Natl Acad Sci USA 1992;89:12013–12017.[Abstract/Free Full Text]
2. van Pinxteren LA, Cassidy JP, Smedegaard BH, Agger EM, Andersen P. Control of latent Mycobacterium tuberculosis infection is dependent on CD8 T cells. Eur J Immunol 2000;30:3689–3698.[CrossRef][Medline]
3. Lalvani A, Brookes R, Wilkinson RJ, Malin AS, Pathan AA, Andersen P, Dockrell H, Pasvol G, Hill AV. Human cytolytic and interferon gamma-secreting CD8+ T lymphocytes specific for Mycobacterium tuberculosis. Proc Natl Acad Sci USA 1998;95:270–275.[Abstract/Free Full Text]
4. Mohagheghpour N, Gammon D, Kawamura LM, van Vollenhoven A, Benike CJ, Engleman EG. CTL response to Mycobacterium tuberculosis: identification of an immunogenic epitope in the 19-kDa lipoprotein. J Immunol 1998;161:2400–2406.[Abstract/Free Full Text]
5. Cho S, Mehra V, Thoma-Uszynski S, Stenger S, Serbina N, Mazzaccaro RJ, Flynn JL, Barnes PF, Southwood S, Celis E, et al. Antimicrobial activity of MHC class I-restricted CD8+ T cells in human tuberculosis. Proc Natl Acad Sci USA 2000;97:12210–12215.[Abstract/Free Full Text]
6. Lalvani A, Hill A. Cytotoxic T-lymphocytes against malaria and tuberculosis: from natural immunity to vaccine design. Clin Sci (Lond) 1998;95:531–538.[Medline]
7. Lewinsohn DA, Lines RA, Lewinsohn DM. Human dendritic cells presenting adenovirally expressed antigen elicit Mycobacterium tuberculosis–specific CD8⁺ T cells. Am J Respir Crit Care Med 2002;166:843–848.[Abstract/Free Full Text]
8. Dillon DC, Alderson MR, Day CH, Lewinsohn DM, Coler R, Bement T, Campos-Neto A, Skeiky YA, Orme IM, Roberts A, et al. Molecular characterization and human T-cell responses to a member of a novel Mycobacterium tuberculosis mtb39 gene family. Infect Immun 1999;67:2941–2950.[Abstract/Free Full Text]
9. Lalvani A, Brookes R, Hambleton S, Britton WJ, Hill AV, McMichael AJ. Rapid effector function in CD8+ memory T cells. J Exp Med 1997;186:859–865.[Abstract/Free Full Text]
10. Wilkinson RJ, Zhu X, Wilkinson KA, Lalvani A, Ivanyi J, Pasvol G, Vordermeier HM. 38000 MW antigen-specific major histocompatibility complex class I restricted interferon-gamma-secreting CD8+ T cells in healthy contacts of tuberculosis. Immunology 1998;95:585–590.[CrossRef][Medline]
11. Pathan AA, Wilkinson KA, Wilkinson RJ, Latif M, McShane H, Pasvol G, Hill AV, Lalvani A. High frequencies of circulating IFN-gamma-secreting CD8 cytotoxic T cells specific for a novel MHC class I-restricted Mycobacterium tuberculosis epitope in M. tuberculosis-infected subjects without disease. Eur J Immunol 2000;30:2713–2721.[CrossRef][Medline]
12. Lewinsohn DM, Zhu L, Madison VJ, Dillon DC, Fling SP, Reed SG, Grabstein KH, Alderson MR. Classically restricted human CD8+ T lymphocytes derived from Mycobacterium tuberculosis-infected cells: definition of antigenic specificity. J Immunol 2001;166:439–446.[Abstract/Free Full Text]

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