THEMES
Current Concepts in Mucosal Immunity
III. Ontogeny and function of gamma delta T cells in the intestine*

Martin F. Kagnoff

Department of Medicine, University of California at San Diego, La Jolla, California 92093

    ABSTRACT
Top
Abstract
Introduction
Summary
References

gamma delta T cells are located in the paracellular space between epithelial cells. In the human colon and small intestine, 5-40% of intraepithelial lymphocytes (IEL) are gamma delta T cells, and in mice an even greater proportion of IEL are gamma delta T cells. The gamma delta T cell receptor repertoire in the human intestine undergoes marked changes in V region gene usage and junctional diversity during development from fetus to newborn to adult, suggesting that gamma delta T cells may mediate qualitatively or quantitatively different functions at various stages of development. gamma delta IEL have been shown to produce cytokines and growth factors and to influence epithelial cell proliferation and differentiation, as well as the mucosal development of immunoglobulin A B cells. gamma delta IEL also manifest cytolytic activity. However, the ligands recognized by intestinal gamma delta T cells and the role they play in intestinal immune responses, in immune defense to enteric pathogens, and in the pathogenesis of intestinal disease are thus far largely unknown.

celiac disease; development; intraepithelial lymphocytes; mucosa; T cell repertoire

    INTRODUCTION
Top
Abstract
Introduction
Summary
References

THERE ARE TWO MAJOR lineages of T cells, those bearing the alpha beta T cell receptor (TCR) and those that express the gamma delta TCR. alpha beta T cells develop in the thymus, where they undergo positive and negative selection with respect to self major histocompatibility complex (MHC) molecules and antigens before populating peripheral lymphoid sites. The restriction elements and many of the functional properties of alpha beta T cells are well characterized. alpha beta T cells that express CD8 or CD4 as coreceptors recognize peptides bound within the peptide-binding groove of MHC class I or class II molecules, respectively. Many of the effector functions of alpha beta T cells, which are important in the regulation of immune and inflammatory responses, are mediated by cytokines secreted by those cells. CD8 alpha beta T cells can also lyse target cells that express foreign proteins (e.g., viral proteins) in the class I peptide-binding groove. The majority of T cells in systemic and mucosal lymphoid tissues are alpha beta T cells.

A second lineage of T cells expresses the gamma delta TCR. This intriguing T cell population is overrepresented in the intestine. gamma delta T cells make up ~50% of intraepithelial lymphocytes (IEL) in the intestinal mucosa of mice, 5-15% of human small intestinal IEL, and as much as 40% of IEL in human colon, although they represent only a minor fraction of T cells in most peripheral lymphoid tissues (<5%) (8, 11). Most intestinal gamma delta T cells are located in the paracellular space between epithelial cells, on the luminal side of the basement membrane, rather than in the underlying lamina propria.

    ONTOGENY OF INTESTINAL gamma delta T CELLS

alpha beta T cell development is tightly regulated within the thymus. In contrast, studies suggest that intestinal gamma delta T cells in mice and humans can develop extrathymically as well as in the thymus. Thus, by 6 wk of gestation, T cells that express the gamma delta -receptor are found in human fetal liver but are not yet detected in human fetal thymus (24). In addition, gamma delta T cells are present in the intestine of congenitally athymic mice (i.e., nude mice) and in thymectomized mice reconstituted with fetal liver (1). Also consistent with extrathymic development, intestinal gamma delta T cells in mice were shown to express 1) recombination-activating gene-1, which is essential for TCR gene rearrangement; 2) a homodimeric alpha alpha form of CD8, rather than the alpha beta heterodimer of CD8 found on most T cells undergoing thymic development; and 3) the Fcepsilon RIgamma chain as a novel component of the TCR-associated CD3 complex (13, 15). The intestinal epithelium may play a role in mucosal gamma delta T cell development analogous to the role the thymic epithelium plays in intrathymic T cell development (13). However, data also suggest the thymus can indirectly contribute to the extrathymic development of gamma delta T cells (23). To the extent that gamma delta IEL undergo extrathymic development, they may recognize a different array of antigens than those recognized by T cells that undergo thymic selection, including self antigens (i.e., the potential for autoimmunity).

Several characteristic changes that take place in the intestinal gamma delta T cell repertoire during ontogeny have been described (18). Diversity of gamma - and delta -chains of the gamma delta TCR is determined by combinatorial joining of several gene segments (i.e., V, D, and J segments in TCR delta  genes; V and J segments in TCR gamma -chains), as well as by nucleotide additions and deletions that occur at the junctions of those gene segments. One can categorize the repertoire of gamma delta T cells in the intestinal tract and at other anatomic sites on the basis of the array of V gene segments used by the TCR and by nucleotide/amino acid sequence diversity at the TCR junctional regions (also termed CDR3) (6, 16). Compared with alpha beta T cells, gamma delta T cells use a small number of different V genes. Moreover, the use of different V genes by gamma delta T cells in different anatomic sites is relatively compartmentalized. For example, most gamma delta T cells in the skin, tongue, and female genital tract of mice use a single Vgamma and a single Vdelta gene. Those gamma delta TCR also lack junctional diversity, suggesting they recognize a monomorphic ligand. In contrast, gamma delta T cells in the human and murine intestine use several different Vdelta genes. In the normal adult human intestine, expression of a single Vdelta gene, DV1, is predominant (6). Nonetheless, in some individuals, other Vdelta genes can form a substantial fraction of the repertoire. DV2 expression in particular is prevalent, whereas DV3 is less so (16). Of note, the human TCR delta  locus maps within the TCR alpha  locus, and in some individuals TCR delta -receptors have been shown to use several different Valpha genes (16). In contrast to the human intestinal mucosa, the predominant Vdelta gene used by circulating gamma delta T cells is DV2, and DV1-expressing cells are only a minor component of circulating gamma delta T cells (6).

The gamma delta TCR has the potential for extensive diversity within the junctional region, and it is the junctional region that is thought to be important for TCR ligand binding. Molecular analysis of gamma delta TCR junctional regions has focused mainly on the TCR delta -chain, since TCR delta  genes, unlike TCR gamma  genes, are expressed exclusively in gamma delta T cells. Paralleling their potential for extensive diversity, the TCR delta  genes expressed in the adult human intestine contain highly complex junctional regions (6, 16). Thus it was perhaps surprising when molecular analysis of the expressed TCR delta  genes in the intestine of individual adults revealed that the TCR delta repertoire in the intestine was highly restricted (i.e., oligoclonal), not diverse. In each individual the repertoire consisted of a relatively limited number of different TCR delta  transcripts, some of which were clonally dominant (6, 16). Moreover, oligoclonality was a general feature of the intestinal TCR delta  T cell repertoire in adults, irrespective of V region usage (6, 16). Within individual subjects, identical TCR delta  transcripts were expressed at multiple sites within the small intestine or colon, and no overlap was noted among the TCR delta  transcripts expressed in the intestinal tract of different adults (6, 16). Moreover, the TCR delta  repertoire in the small intestine or colon of each individual was relatively constant when assessed over a 1- to 2-yr period (6, 16). The above findings are consistent with a model in which gamma delta T cells are selected, possibly by ligands in the intestinal tract, and undergo clonal expansion (i.e., either in a mucosal or an extramucosal site) before seeding to multiple different sites in the small intestine and colon. Because the repertoire of TCR delta  transcripts in the intestinal mucosa differs from that in the peripheral circulation, different ligands and/or factors may drive the expansion of the gamma delta T cells in those different locations.

The TCR delta  repertoire in the adult human intestine differs markedly from that in early and midterm fetuses (18). The repertoire in midterm fetuses, in turn, differs from the repertoire in the early postnatal period and during the first few years of life. During fetal intestinal development, expression of the DV2 gene segment, and not DV1, predominates. Moreover, TCR delta  transcripts in the intestinal mucosa of midterm fetuses (i.e., ~20 wk gestation) have relatively simple junctions (i.e., minimal N-region nucleotide additions) and lack the extensive diversity that is characteristic of TCR delta  junctional regions in adults (18). Furthermore, in striking contrast to adults, identical TCR delta  transcripts can be present in the intestinal tract of different fetuses. However, TCR delta  junctional regions in newborns are as complex as those of adults (18). The TCR delta  repertoire is polyclonal in the newborn and during the first few years of life (18). Nonetheless, by the second decade of life the intestinal TCR delta  repertoire is oligoclonal and resembles that of older adults. These marked changes in the TCR delta  repertoire during development from fetus to adult suggest that gamma delta T cells may play different roles at different stages of development. For example, gamma delta T cells that express a diverse repertoire at birth might be important in host mucosal defense early in life, when antigen-specific acquired mucosal immune responses that are mediated by alpha beta T cells and the secretory immunoglobulin A (IgA) system are not fully developed.

    INTESTINAL gamma delta T CELLS: WHAT LIGANDS DO THEY RECOGNIZE AND WHAT FUNCTIONS DO THEY MEDIATE?

There are, on average, 20-100 epithelial cells for every gamma delta IEL in the adult human intestine. This relatively sparse distribution of gamma delta IEL and the oligoclonal nature of the adult gamma delta repertoire must be taken into account when considering mucosal gamma delta T cell function.

Consistent with their location in the paracellular space between intestinal epithelial cells (IEC), gamma delta T cells initially were envisioned to be a first line of defense against mucosal pathogens. However, there are few data to support that notion, and the intestinal gamma delta T cell population appears to be similar in germ-free and conventional mice, whereas alpha beta IEL are markedly influenced by the microbial flora (2). Furthermore, in the adult human colon, the TCR gamma delta repertoire is oligoclonal and relatively stable over time, despite a highly diverse enteric microbial flora. Nonetheless, the possibility exists that gamma delta T cells in adult humans recognize a limited array of antigens that are highly conserved among different bacterial strains.

Alternatively, direct microbial infection of epithelial cells or other causes of epithelial cell stress or damage might result in the expression of molecules on the epithelial cell membrane that can activate adjacent gamma delta IEL. In the periphery, gamma delta T cells in mice are known to play a role in host defense, primarily after infection with a spectrum of intracellular microbial pathogens (e.g., Listeria, Mycobacteria, Plasmodia, Toxoplasma), but few data are available regarding the role of intestinal gamma delta IEL in host defense against intraepithelial pathogens. Moreover, given the scattered distribution of gamma delta T cells in the epithelium, it is possible that gamma delta T cells that are not directly adjacent to stressed, damaged, or infected epithelial cells are activated at a distance from those cells, either by mediators released from epithelial cells or by signals conducted across the intercellular junctions between IEC. In this regard, epithelial cells are known to engage in crosstalk with gamma delta IEL, since they produce stem cell factor (SCF) and interleukin-7 (IL-7), which signal IEL via c-kit ligand (i.e., the receptor for SCF) and IL-7R, respectively (9, 25, 28).

The nature of the ligands recognized by gamma delta T cells is unknown thus far. On the basis of both the greater length and marked variability in length of the junctional regions of TCR delta  chains relative to other TCR chains, it has been proposed that gamma delta T cells might recognize cell surface ligands in a manner analogous to that of immunoglobulin molecules (5a). Whether gamma delta T cells recognize MHC class I-like molecules such as CD1d in humans (see Ref. 3) or MHC class Ib molecules such as MICA (i.e., a class Ib molecule expressed almost exclusively on IEC and whose promoter contains heat shock elements similar to HSP70 genes) is not yet known (7, 12).

The fact that gamma delta IEL reside within a sea of epithelial cells suggests that an individual gamma delta T cell might monitor a group of epithelial cells. Moreover, when activated, gamma delta IEL may signal multiple target cells in the adjacent and underlying mucosa. In this regard, gamma delta IEL produce an array of mediators, including cytokines, that are usually associated with antigen-specific immune responses (e.g., interferon-gamma ) as well as proinflammatory chemokines that are essential components of host innate immunity (4, 29). These findings suggest gamma delta T cells can provide important signals to adjacent epithelial cells and/or immune and inflammatory cells. Furthermore, gamma delta T cells in mice produce growth factors, which may promote epithelial cell growth or play a role in healing a damaged epithelium (5). In addition, gamma delta IEL can mediate cytotoxic functions. Thus, gamma delta T cells can be activated to express Fas ligand and are known to have cytolytic activity for target cells in in vitro lysis assays, suggesting they may play a role in deleting damaged IEC (19, 27). Other studies in mice have reported the induction of natural killer cell markers on gamma delta T cells from the gut and natural killer activity of those cells in vitro (14).

Studies in mice lacking gamma delta T cells (i.e., gamma delta knockout mice) have provided additional insights regarding gamma delta T cell function. Such studies suggest gamma delta T cells are important for the development of mucosal IgA B cells and that gamma delta T cells in the intestinal mucosa can play a role in epithelial cell differentiation (10, 22). Although gamma delta knockout mice manifest greater intestinal immunopathology after Eimeria infection than controls (26), it is not known whether the latter reflects 1) a direct effect of gamma delta T cells on the parasite or parasite-infected cells, 2) the absence of an immunoregulatory role of gamma delta T cells on the remaining alpha beta T cells, or 3) a secondary effect, in that gamma delta knockout mice are also IgA deficient. Other studies in mice depleted of gamma delta T cells suggest those cells may play a role in regulating oral tolerance (21).

    gamma delta T CELLS AND CELIAC DISEASE

Celiac disease is characterized by small intestinal mucosal damage and nutrient malabsorption after the dietary ingestion of prolamins in wheat, rye, and barley in genetically susceptible individuals (20). This disease is unique among small intestinal disorders in that these patients manifest a striking increase in the proportion of gamma delta relative to alpha beta T cells in the intraepithelial region of the small intestinal mucosa; this is recognized as one of the hallmarks of celiac disease (20). Moreover, the proportion of gamma delta IEL relative to alpha beta IEL remains increased even in celiac disease patients in remission, which suggests a fundamental role for these cells in the pathogenesis of this disease. Monozygotic twins concordant for celiac disease were shown to have nonoverlapping intestinal TCR delta  repertoires, indicating that the genetic factors that determine celiac disease susceptibility do not appear to select for specific TCR delta  sequences or junctional region amino acid motifs (17). Nonetheless, the role gamma delta T cells play in the pathogenesis of celiac disease, whether protective of the mucosa or damaging to it, is not known.

    SUMMARY
Top
Abstract
Introduction
Summary
References

gamma delta T cells represent an intriguing, but as yet somewhat mysterious, component of the intestinal mucosal immune system. They are located predominately in the paracellular space between epithelial cells and are therefore ideally situated for crosstalk with IEC and other immune and nonimmune populations of cells in the adjacent lamina propria. During the first few decades of life, mucosal gamma delta T cells appear to be selected by ligands and undergo clonal expansion and recirculation before lodging throughout the small intestine and colon. In adults the repertoire of receptors used by this cell population is markedly restricted and unique in each individual, despite the potential for extensive diversity seen in the newborn. This suggests gamma delta T cells in the intestinal mucosa may play different roles in the fetus and newborn compared with their roles later in life. The unique structural and developmental characteristics of the gamma delta TCR, their largely extrathymic differentiation, and the production of an array of mediators, including those characteristic of host innate immune responses, further suggest that these fascinating cells may bridge innate and acquired mucosal immunity. Currently the function of these cells, the nature of the ligands they recognize, and their role in normal intestinal immunophysiology and disease represent key unsolved issues in mucosal T cell biology.

    FOOTNOTES

* Third in a series of invited articles on Current Concepts in Mucosal Immunity.

Address reprint requests to Univ. of California, San Diego, Dept. of Medicine, 0623D, 9500 Gilman Dr., La Jolla, CA 92093-0623.

    REFERENCES
Top
Abstract
Introduction
Summary
References

1.   Bandeira, A., S. Itohara, M. Bonneville, O. Burlen-Defranoux, T. Mota-Santos, A. Coutinho, and S. Tonegawa. Extrathymic origin of intestinal intraepithelial lymphocytes bearing T-cell antigen receptor gamma delta . Proc. Natl. Acad. Sci. USA 88: 43-47, 1991[Abstract].

2.   Bandeira, A., T. Mota-Santos, S. Itohara, S. Degermann, C. Heusser, S. Tonegawa, and A. Coutinho. Localization of gamma /delta T cells to the intestinal epithelium is independent of normal microbial colonization. J. Exp. Med. 172: 239-244, 1990[Abstract].

3.   Blumberg, R. S. Current concepts in mucosal immunity. II. One size fits all: nonclassical MHC molecules fulfill multiple roles in epithelial cell function. Am. J. Physiol. 274 (Gastrointest. Liver Physiol. 37): G227-G231, 1998[Abstract/Free Full Text].

4.   Boismenu, R., L. Feng, Y. Y. Xia, J. C. Chang, and W. L. Havran. Chemokine expression by intraepithelial gamma delta T cells. Implications for the recruitment of inflammatory cells to damaged epithelia. J. Immunol. 157: 985-992, 1996[Abstract].

5.   Boismenu, R., and W. L. Havran. Modulation of epithelial cell growth by intraepithelial gamma delta T cells. Science 266: 1253-1255, 1994[Medline].

5a.   Chien, Y. H., R. Jores, and M. P. Crowley. Recognition by gamma /delta T cells. Annu. Rev. Immunol. 14: 511-532, 1996[Medline].

6.   Chowers, Y., W. Holtmeier, J. Harwood, E. Morzycka-Wroblewska, and M. F. Kagnoff. The V delta 1 T cell receptor repertoire in human small intestine and colon. J. Exp. Med. 180: 183-190, 1994[Abstract].

7.   Crowley, M. P., Z. Reich, N. Mavaddat, J. D. Altman, and Y. Chien. The recognition of the nonclassical major histocompatibility complex (MHC) class I molecule, T10, by the gamma delta T cell, G8. J. Exp. Med. 185: 1223-1230, 1997[Abstract/Free Full Text].

8.   Deusch, K., F. Luling, K. Reich, M. Classen, H. Wagner, and K. Pfeffer. A major fraction of human intraepithelial lymphocytes simultaneously expresses the gamma /delta T cell receptor, the CD8 accessory molecule and preferentially uses the Vdelta 1 gene segment. Eur. J. Immunol. 21: 1053-1059, 1991[Medline].

9.   Fujihashi, K., S. Kawabata, T. Hiroi, M. Yamamoto, J. R. McGhee, S. Nishikawa, and H. Kiyono. Interleukin 2 (IL-2) and interleukin 7 (IL-7) reciprocally induce IL-7 and IL-2 receptors on gamma delta T-cell receptor-positive intraepithelial lymphocytes. Proc. Natl. Acad. Sci. USA 93: 3613-3618, 1996[Abstract/Free Full Text].

10.   Fujihashi, K., J. R. McGhee, M. N. Kweon, M. D. Cooper, S. Tonegawa, I. Takahashi, T. Hiroi, J. Mestecky, and H. Kiyono. gamma /delta T cell-deficient mice have impaired mucosal immunoglobulin A responses. J. Exp. Med. 183: 1929-1935, 1996[Abstract].

11.   Goodman, T., and L. Lefrancois. Expression of the gamma -delta T-cell receptor on intestinal CD8+ intraepithelial lymphocytes. Nature 333: 855-858, 1988[Medline].

12.   Groh, V., S. Bahram, S. Bauer, A. Herman, M. Beauchamp, and T. Spies. Cell stress-regulated human major histocompatibility complex class I gene expressed in gastrointestinal epithelium. Proc. Natl. Acad. Sci. USA 93: 12445-12450, 1996[Abstract/Free Full Text].

13.   Guy-Grand, D., N. Cerf-Bensussan, B. Malissen, M. Malassis-Seris, C. Briottet, and P. Vassalli. Two gut intraepithelial CD8+ lymphocyte populations with different T cell receptors: a role for the gut epithelium in T cell differentiation. J. Exp. Med. 173: 471-481, 1991[Abstract].

14.   Guy-Grand, D., B. Cuenod-Jabri, M. Malassis-Seris, F. Selz, and P. Vassalli. Complexity of the mouse gut T cell immune system: identification of two distinct natural killer T cell intraepithelial lineages. Eur. J. Immunol. 26: 2248-2256, 1996[Medline].

15.   Guy-Grand, D., B. Rocha, P. Mintz, M. Malassis-Seris, F. Selz, B. Malissen, and P. Vassalli. Different use of T cell receptor transducing modules in two populations of gut intraepithelial lymphocytes are related to distinct pathways of T cell differentiation. J. Exp. Med. 180: 673-679, 1994[Abstract].

16.   Holtmeier, W., Y. Chowers, A. Lumeng, E. Morzycka-Wroblewska, and M. F. Kagnoff. The delta  T cell receptor repertoire in human colon and peripheral blood is oligoclonal irrespective of V region usage. J. Clin. Invest. 96: 1108-1117, 1995[Medline].

17.   Holtmeier, W., D. L. Rowell, A. Nyberg, and M. F. Kagnoff. Distinct delta  T cell receptor repertoires in monozygotic twins concordant for coeliac disease. Clin. Exp. Immunol. 107: 148-157, 1997[Medline].

18.   Holtmeier, W., T. Witthoft, A. Hennemann, H. S. Winter, and M. F. Kagnoff. The TCR-delta repertoire in human intestine undergoes characteristic changes during fetal to adult development. J. Immunol. 158: 5632-5641, 1997[Abstract].

19.   Ishikawa, H., Y. Li, A. Abeliovich, S. Yamamoto, S. H. Kaufmann, and S. Tonegawa. Cytotoxic and interferon-gamma -producing activities of gamma delta T cells in the mouse intestinal epithelium are strain dependent. Proc. Natl. Acad. Sci. USA 90: 8204-8208, 1993[Abstract/Free Full Text].

20.   Kagnoff, M. F. Celiac disease. In: Textbook of Gastroenterology, edited by T. Yamada, D. H. Alpers, C. Owyang, D. W. Powell, and F. Silverstein. New York: J. B. Lippincott, 1995, p. 1643-1661.

21.   Ke, Y., K. Pearce, J. P. Lake, H. K. Ziegler, and J. A. Kapp. gamma delta T lymphocytes regulate the induction and maintenance of oral tolerance. J. Immunol. 158: 3610-3618, 1997[Abstract].

22.   Komano, H., Y. Fujiura, M. Kawaguchi, S. Matsumoto, Y. Hashimoto, S. Obana, P. Mombaerts, S. Tonegawa, H. Yamamoto, and S. Itohara. Homeostatic regulation of intestinal epithelia by intraepithelial gamma delta T cells. Proc. Natl. Acad. Sci. USA 92: 6147-6151, 1995[Abstract/Free Full Text].

23.   Lefrancois, L., and L. Puddington. The role of the thymus in intestinal intraepithelial T-cell development. Ann. NY Acad. Sci. 778: 36-46, 1996[Medline].

24.   McVay, L. D., and S. R. Carding. Extrathymic origin of human gamma delta T cells during fetal development. J. Immunol. 157: 2873-2882, 1996[Abstract].

25.   Puddington, L., S. Olson, and L. Lefrancois. Interactions between stem cell factor and c-Kit are required for intestinal immune system homeostasis. Immunity 1: 733-739, 1994[Medline].

26.   Roberts, S. J., A. L. Smith, A. B. West, L. Wen, R. C. Findly, M. J. Owen, and A. C. Hayday. T-cell alpha beta + and gamma delta + deficient mice display abnormal but distinct phenotypes toward a natural, widespread infection of the intestinal epithelium. Proc. Natl. Acad. Sci. USA 93: 11774-11779, 1996[Abstract/Free Full Text].

27.   Sakai, T., Y. Kimura, K. Inagaki-Ohara, K. Kusugami, D. H. Lynch, and Y. Yoshikai. Fas-mediated cytotoxicity by intestinal intraepithelial lymphocytes during acute graft-versus-host disease in mice. Gastroenterology 113: 168-174, 1997[Medline].

28.   Watanabe, M., Y. Ueno, T. Yajima, Y. Iwao, M. Tsuchiya, H. Ishikawa, S. Aiso, T. Hibi, and H. Ishii. Interleukin 7 is produced by human intestinal epithelial cells and regulates the proliferation of intestinal mucosal lymphocytes. J. Clin. Invest. 95: 2945-2953, 1995[Medline].

29.   Yamamoto, M., K. Fujihashi, M. Amano, J. R. McGhee, K. W. Beagley, and H. Kiyono. Cytokine synthesis and apoptosis by intestinal intraepithelial lymphocytes: signaling of high density alpha beta T cell receptor+ and gamma delta T cell receptor+ T cells via T cell receptor-CD3 complex results in interferon-gamma and interleukin-5 production, while low density T cells undergo DNA fragmentation. Eur. J. Immunol. 24: 1301-1306, 1994[Medline].


AJP Gastroint Liver Physiol 274(3):G455-G458
0193-1857/98 $5.00 Copyright © 1998 the American Physiological Society