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CUTTING EDGE |






* Department of Microbiology and Immunology and
Second Department of Surgery, Nippon Medical School, Tokyo, Japan;
Japan BCG Laboratory, Kiyose, Tokyo, Japan; and
Lymphocyte Biology Section, Division of Rheumatology, Immunology, and Allergy, Brigham and Womens Hospital, Harvard Medical School, Boston, MA 02115
| Abstract |
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, an important cytokine for protection against mycobacterial infection. These results emphasize that lipid-reactive CD8+ T cells may contribute to host defense against mycobacterial infection. | Introduction |
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that they produce in host defense against mycobacterial infection, CD8+ T cells appear to mediate distinct but complementary pathways for controlling intracellular mycobacteria (1, 2). Evidence obtained from analysis of TAP- and CD1d-deficient mice as well as an earlier study of mice lacking the expression of
2-microglobulin unequivocally demonstrated a significant contribution of MHC class I-restricted CD8+ CTLs in clearing mycobacterial infection in mice (3, 4). In humans, the potential role of CD8+ T cells in protection against mycobacterial infection has been appreciated partially by isolating and characterizing mycobacteria-specific, MHC class I-restricted CD8+ T cell lines (5). These studies focused on macrophages, the well-known reservoir for mycobacteria, as CTL target cells, that were either infected with mycobacteria or pulsed with mycobacteria-derived peptide Ags.
Recently, the universe of human CD8+ CTLs has been expanded to include those that recognize mycobacterial cell wall-derived lipid and glycolipid Ags in the context of group 1 CD1 molecules (CD1a, -b, and -c) expressed prominently and almost exclusively on dendritic cells (DCs)3 (6). Given that DCs are the most efficient APCs in the immune system and that, besides macrophages, DCs represent another important reservoir for mycobacteria (7), group 1 CD1-dependent activation of CD8+ T cells may occur during the course of mycobacterial infection (8). In addition, CD1 molecules are expressed intracellularly in endocytic subcompartments where lipid Ags derived from phagocytosed mycobacteria are known to traffic (9, 10), raising the possibility that CD1 molecules may be situated to efficiently monitor infection with live mycobacteria. Indeed, group 1 CD1-restricted CD8+ T cell lines were isolated from healthy subjects as well as patients with mycobacterial infection, and their outstanding ability to recognize mycobacteria-infected cells and kill the intracellular organisms has been noted (11). Thus, despite the previous observations that a majority of human circulating CD8+ T cells may recognize mycobacteria-infected cells in a CD1-independent manner (12, 13), we reasoned that a circulating pool of CD1-restricted CD8+ T cells reactive to live mycobacteria might be detected in those who have established augmented cell-mediated immunity against mycobacterial infection. In the present study, we show that a sizable pool of such CD8+ T cells exists in the peripheral blood of individuals inoculated with live Mycobacterium bovis bacillus Calmette-Guérin (BCG) and that these CD1-restricted T cells produce IFN-
in response to live BCG-infected DCs. Lipid Ag-reactive CD8+ T cells may therefore contribute to host defense against mycobacterial infection.
| Materials and Methods |
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The Tokyo 172 strain of BCG was grown in 7H9 medium and the extraction with chloroform/methanol was performed as described previously (14). TLC of the organic extract was conducted on 200-µm silica-coated glass TLC plates developed in 50% sulfuric acid as described (14). For infection experiments, BCG was harvested at its midlog phase growth, washed, and suspended in RPMI 1640 (Life Technologies, Gaithersburg, MD) supplemented with 10% FCS (HyClone Laboratories, Logan, UT). Without prior sonication, the suspension was passed through a 5-µm pore size filter to obtain single-cell bacteria. The viability of bacteria was constantly >90%. The BCG preparation was divided into equal aliquots; one aliquot was incubated for 30 min at 85°C to kill the bacteria, whereas the other aliquot was left at room temperature. Human peripheral blood monocytes and monocyte-derived DCs were isolated as described (15) and incubated for 4 h with the BCG preparations at the multiplicity of infection of 10. The infected cells were then washed and incubated for an additional 14 h in RPMI 1640 complete medium without antibiotics. At the end of the culture, the cells were harvested, irradiated, and used as APCs in T cell stimulation assays and ELISPOT assays.
T cell transfectants stimulation assays
Reconstitution of the LDN5 TCR in TCR-deficient Jurkat cells (J.RT3/LDN5) was conducted as described (14). The J.RT3/LDN5 cells (5 x 104/well) were cultured for 20 h with either BCG-infected, glucose monomycolate (GMM)-pulsed, or untreated DCs (5 x 104/well), and the amount of IL-2 released into the supernatants was measured as described (14).
ELISPOT assays
PBLs were obtained from 11 healthy Japanese subjects (donors 111) who had positive purified protein derivative (PPD) test conversion due to BCG vaccine inoculation at their infancy. The subjects included 8 men (donor 1, 30 years old; donor 3, 32 years old; donor 5, 30 years old; donor 6, 32 years old; donor 7, 34 years old; donor 8, 33 years old; donor 9, 30 years old; donor 10, 44 years old) and 3 women (donor 2, 32 years old; donor 4, 33 years old; donor 11, 33 years old). After incubation of the PBLs at 4°C in the presence of anti-CD19 and anti-CD56 Abs (both from BD PharMingen, San Diego, CA) together with Abs against either CD4 (OKT4) or CD8
(OKT8) (15), the labeled cell populations were removed by two cycles of separation with magnetic beads coated with goat anti-mouse IgG Abs. The efficiency in depletion of the corresponding T cell populations was determined by flow cytometry as described (14). The CD8 T cell-enriched and the CD4 T cell-enriched populations (1 x 105/well) were separately incubated for 24 h with APCs (1 x 104/well), and ELISPOT assays were performed as described (5). In some experiments, the culture was performed in the presence of saturating amounts of either W6/32 (anti-MHC class I) (16, 17), L243 (anti-HLA-DR) (18), 10H3.9.3 (anti-CD1a) (14), BCD1b3.1 (anti-CD1b) (14), F10/21A3.1 (anti-CD1c) (14), or RPC5.4 (control; obtained from the American Tissue Culture Collection, Manassas, VA). Each experiment was repeated two to four times to confirm reproducibility of the results.
| Results |
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To evaluate the ability of CD1 to monitor mycobacterial infection, we first used a CD1-restricted T cell line, LDN5 (19), that was specific for GMM produced by several species of mycobacteria. Indeed, BCG grown for 7 days in 7H9 medium supplemented with 3% D-glucose produced a significant amount of GMM in culture (Fig. 1A, arrowhead). To examine GMM recognition in BCG-infected CD1+ cells, monocyte-derived DCs were infected with either live or heat-killed BCG and used as APCs. For detection of GMM presentation by these DCs, TCR-deficient Jurkat cells (J.RT3) reconstituted by transfection with the LDN5 TCR
- and
-chains (J.RT3/LDN5) (14) were used as responder cells that were capable of recognizing GMM in the context of CD1b molecules. As expected, GMM-pulsed DCs, but not unpulsed cells, were able to stimulate the J.RT3/LDN5 cells to produce IL-2 (Fig. 1B, DC + GMM and untreated DC, respectively). DCs infected with live BCG were efficiently recognized by J.RT3/LDN5 cells, indicating that GMM was presented by live BCG-infected DCs (Fig. 1B, DC + live BCG). Despite the fact that phagocytosis of live and dead BCG occurred in similar efficiency and the bacilli contained a significant amount of GMM in their cell wall (Fig. 1A), DCs treated with dead BCG were incapable of or markedly inefficient in stimulating J.RT3/LDN5 cells. The level of IL-2 produced by J.RT3/LDN5 in response to dead BCG-infected DCs was similar to that produced in the presence of untreated DCs (Fig. 1B, DC + dead BCG and untreated DC, respectively). Purified GMM that was heat treated was presented to J.RT3/LDN5 cells as efficiently as non-heat-treated GMM (data not shown), ruling out the possibility that heat-killed BCG might contain denatured GMM. These observations suggested that CD1b molecules functioned efficiently in monitoring live mycobacterial infection but that they might not present all Ags equally well from dead bacteria.
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The importance of CD8+ CTLs in efficient host defense against mycobacterial infection has been well noted in mice (3, 4). Yet, it has proved difficult to identify the CD8 T cell response in humans. Thus, we sought to compare CD1-restricted and MHC-restricted responses of T cells in human peripheral blood. We used BCG-immunized subjects and examined the ability of the peripheral blood T cells to recognize live BCG-infected APCs. Given that group 1 CD1-restricted T cells have often been detected in CD4-negative T cell populations and are capable of producing IFN-
on antigenic stimulation (11, 20), we prepared CD8+ T cell-enriched and CD4+ T cell-enriched populations by negative selection with appropriate Abs and magnetic beads (Fig. 2A, left and right, respectively) before performing IFN-
ELISPOT assays. As shown in Fig. 2B, these two T cell populations were separately analyzed in triplicate in ELISPOT assays in the presence of autologous DCs that were uninfected or infected with either dead or live BCG (DC + dead BCG and DC + live BCG, respectively), and spots representing IFN-
-producing cells were visualized (Fig. 2B, top) and counted (Fig. 2B, bottom). Whereas only a small number of CD8+ T cells showed reactivity to DCs infected with dead BCG, a much larger pool was detected for live BCG-reactive CD8+T cells (Fig. 2B, left). For comparison, the CD4+ T cell population from the same subject was also analyzed for its reactivity to dead and live BCG (Fig. 2B, right). In contrast to the CD8+T cell population that preferentially recognized live BCG, the CD4+ T cell population responded better to dead BCG than to live BCG.
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To clearly determine restriction elements that were involved in live BCG recognition by these two T cell populations, ELISPOT assays with live BCG-infected DCs were performed in the presence of blocking Abs against MHC and CD1 molecules, and the percent inhibition for each Ab was calculated. Whereas the CD4+T cell response to live BCG was blocked solely by anti-MHC class II Ab (Fig. 3A, right), the CD8+ T cell response to live BCG was blocked most significantly by anti-CD1b Ab and moderately by anti-CD1a and anti-CD1c Abs, but not by Abs to MHC molecules (Fig. 3A, left) and anti-CD1d Abs (data not shown). Significant blockade of the CD8+ T cell response to live BCG with anti-CD1 Abs (p < 0.05 in F test), but not by anti-MHC Abs, was observed in all 11 individuals thus far examined (Fig. 3B). Thus, these studies detect a circulating pool of CD1-restricted CD8+ T cells that potentially monitor live mycobacterial infection.
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| Discussion |
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In contrast, the intracellular delivery of phagocytosed dead mycobacteria is different from that for live mycobacteria, resulting in distinct pathways for Ag processing and presentation. Phagocytosed dead mycobacteria reside in phagosomes only transiently and are quickly delivered to the acidic, proteolytically active lysosomal compartments, where MHC class II molecules readily bind processed peptide Ags. The cell wall structure of dead mycobacteria that contains CD1-restricted lipid Ags also reaches the lysosome where CD1b and CD1c molecules are expressed; yet the rigid cell wall packed densely with extremely hydrophobic lipid molecules that are either covalently or noncovalently attached to each other and to the underlying arabinogalactan and peptidoglycan multilayers is hard to digest to release individual lipid components in a form that is readily sampled by CD1 molecules. Thus, the intracellular availability and distinct pathways for lipid and protein Ag processing and trafficking are likely to account for differential recognition of live and dead mycobacteria by CD1 and MHC class II molecules.
In contrast to the results presented here, previous studies have failed to detect mycobacteria-specific, CD1-restricted CD8+ T cells in the peripheral blood of PPD+ individuals (12, 13). These individuals may have had positive PPD test conversion due to natural infection with certain mycobacteria species, whereas the individuals analyzed in the present study had positive skin test conversion due to percutaneous BCG vaccine inoculation. Because alveolar mucosa and skin contain distinct subsets of macrophages and DCs, differences in the route of infection may significantly affect the quality and magnitude of T cell responses induced.
Most mycobacterial lipid-specific, CD1-restricted T cell lines thus far established are CD4-, and their potential ability to control mycobacterial infection has been demonstrated in vitro, especially for CD8+ T cells that produce granulysin, a bactericidal protein that directly kills mycobacteria (23). These CD1-restricted T cells notably show broad reactivity to a variety of mycobacteria species (19), contrasting MHC-restricted T cells that are highly specific and often lose reactivity when single amino acid mutation is introduced into the specific peptide Ag. Further, recent evidence has suggested that CD1-dependent lipid Ag presentation may play a crucial role in the early stages of host defense by interacting with immature DCs, even before peptide Ag-specific T cells are activated and fully expand (15, 24). Thus, the ability of CD1 to monitor infection with a broad spectrum of live mycobacteria may provide the immune system with an efficient way for the early defense against invading mycobacteria. The detection of a significant circulating pool of live mycobacteria-reactive, CD1-restricted T cells in previously infected donors emphasizes the possibility that lipid reactive T cells may contribute to host defense against mycobacterial infection.
| Acknowledgments |
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| Footnotes |
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2 Address correspondence and reprint requests to Dr. Masahiko Sugita, Department of Microbiology and Immunology, Nippon Medical School, 1-1-5 Sendagi, Bunkyo-ku, Tokyo 113-8602, Japan. E-mail address: msugita{at}nms.ac.jp ![]()
3 Abbreviations used in this paper: DC, dendritic cell; BCG, bacillus Calmette-Guérin; GMM, glucose monomycolate; PPD, purified protein derivative. ![]()
Received for publication December 5, 2002. Accepted for publication March 27, 2003.
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