Transcripts of functionally rearranged gamma genes in primary T cells of adult immunocompetent mice

Structures of human γδ T cell receptor-CD3 complex

Gamma delta (γδ) T cells, a unique T cell subgroup, are crucial in various immune responses and immunopathology1-3. The γδ T cell receptor (TCR), which is generated by γδ T cells, recognizes a diverse range of antigens independently of the major histocompatibility complex2. The γδ TCR associates with CD3 subunits, initiating T cell activation and holding great potential in immunotherapy4. Here we report the structures of two prototypical human Vγ9Vδ2 and Vγ5Vδ1 TCR-CD3 complexes5,6, revealing two distinct assembly mechanisms that depend on Vγ usage. The Vγ9Vδ2 TCR-CD3 complex is monomeric, with considerable conformational flexibility in the TCRγ-TCRδ extracellular domain and connecting peptides. The length of the connecting peptides regulates the ligand association and T cell activation. A cholesterol-like molecule wedges into the transmembrane region, exerting an inhibitory role in TCR signalling. The Vγ5Vδ1 TCR-CD3 complex displays a dimeric architecture, whereby two protomers nestle back to back through the Vγ5 domains of the TCR extracellular domains. Our biochemical and biophysical assays further corroborate the dimeric structure. Importantly, the dimeric form of the Vγ5Vδ1 TCR is essential for T cell activation. These findings reveal organizing principles of the γδ TCR-CD3 complex, providing insights into the unique properties of γδ TCR and facilitating immunotherapeutic interventions.

gammadelta cells and the regulation of mucosal immune responses

We are only now uncovering the potentially important contributions made to immune responses by gammadelta cells. These contributions are likely to be particularly important at mucosal sites, where gammadelta cells are disproportionately enriched. Indeed, gammadelta cells have proven biological activity in the lung. In addition, gammadelta cells are also enriched in young rather than adult animals. Studies of mutant mice have demonstrated that alphabeta T cells are seemingly essential for high-affinity, cognate immunological memory, whereas gammadelta cells contribute to the early stages of an immune response and to the regulation of alphabeta T cell- and B cell-mediated immunity. To explore further the role of gammadelta cells in immune responses, we have investigated whether their contribution is greater during the early period of life, when the cells are more abundant. In a natural system of coccidial infection of gut epithelial cells, we find that alphabeta T cell responses are less essential for immunoprotection during primary challenge of young mice than is true for adult animals. This "ineffectiveness" creates a "window of importance" for the immunoprotective capacity of gammadelta cells, which seem thereby to be more crucial in young compared with older animals. The relative ineffectiveness of alphabeta T cells in young mice may be attributable to a bias toward Th2 activity. We therefore hypothesize that gammadelta cell activity, elicited by infection early in life, may compensate for defects in Th1 activity and may actually accelerate the bias in alphabeta T cells away from Th2. This has obvious implications for susceptibility to Th2-type allergic responses.

Cutting Edge: The Expression In Vivo of a Second Isoform of pTα: Implications for the Mechanism of pTα Action

A second isoform of pTα, “pTαb,” is derived from the pTα locus by tissue-specific, alternative splicing. pTαb is coexpressed in the thymus with the previously characterized form of pTα (which we term pTαa) and is also expressed in peripheral cells without pTαa. While pTαa acts to retain most TCR β-chains intracellularly, pTαb permits higher levels of cell surface TCRβ expression and facilitates signaling from a CD3-TCRβ complex.

Interleukin 7 induces TCR gene rearrangement in adult marrow-resident murine precursor T cells

Rearrangement of the T cell antigen receptor genes is a complex, highly regulated process. To gain a better understanding of the extracellular factors involved in the regulation of TCR beta and gamma gene rearrangement in adult murine bone marrow-resident precursor T cells, several cytokines were tested for their ability to induce gene recombination. A selected population of C58/J bone marrow cells (Thy 1(low), CD3, CD8, B220) that is enriched for pre-T cell activity was propagated in vitro in medium supplemented with IL-3 and mast cell growth factor (MGF, also referred to as stem cell factor, Steele factor and c-kit ligand). These cytokines were required for the maintenance of pre-T cell activity in culture, but had no effect on TCR gene expression. Several additional cytokines were added to the culture medium. Of all those tested, only IL-7 induced complete rearrangement of the TCR gamma locus. Complete rearrangement of the TCR beta locus was not induced under any of the culture conditions analysed here. The bone marrow cells cultured in IL-3, MGF and IL-7 did not begin to express mature T cell proteins and maintained their in vivo progenitor potential. Furthermore, IL-7 cultured bone marrow cells were capable of differentiation in vivo into all phenotypic subpopulations of T cells, without an apparent bias toward the gammadelta lineage. The data presented here suggest that TCR gamma gene rearrangement in adult pre-T cells is regulated by IL-7, but that the TCR beta locus requires additional or alternative signals for the induction of complete rearrangement.

T cell receptor gamma/delta expression on lymphocyte populations of breast cancer patients

The quantitative distribution and phenotype of gamma/delta lymphocytes in the peripheral blood (PBL), tumour draining lymph node (LNL) and tumour infiltrating lymphocytes (TIL) from breast carcinoma patients were determined by one- and two-colour flow cytometry. The TCR-gamma/delta + cells generally expressed the T cell lineage antigen CD3. The proportions of such cells were variable but generally small from all the three sources. Phenotypic analysis revealed that the CD8 marker was consistently and predominantly observed on gamma/delta + CD3+ cells in the tumour infiltrate, whereas CD4 expression, while generally low, was noted on a significant percentage (median 10%) of LNL gamma/delta + lymphocytes. In both PBL and LNL the predominant gamma/delta cell population was CD4-8-.

Novel excision products of T cell receptor gamma gene rearrangements and developmental stage specificity implied by the frequency of nucleotide insertions at signal joints

We have cloned circular DNA excised by T cell receptor (TcR) gamma 1, gamma 2 and gamma 3 gene rearrangements in fetal and adult mouse thymocytes. Circular DNA contained a signal joint reciprocal to the genomic V-J coding joint. Although signal joints without nucleotide insertions are common in immunoglobulin (Ig) and TcR gene rearrangements, the signal joint of gamma found in adult thymocytes contained non-germ-line element (N) insertions at high frequency, while no insertions were found in fetal thymocytes. Thus developmental stage specificity of TcR gamma gene rearrangements is faithfully reflected on the signal joint of excision products. In addition, examination of gamma gene excision products revealed circular DNA products of TcR gamma-alpha transrearrangements, but no evidence of V gamma gene replacement in a rearranged segment.

Rearrangement patterns of T-cell receptor genes in the spleen of athymic (nu/nu) young mice

Although the athymic nude mouse is grossly deficient in peripheral T cells, the number of lymphocytes bearing T-cell markers (L3T4, LyT2) and the alpha beta or gamma delta T-cell receptor (Tcr) increases steadily with age. The anatomical site(s) where these cells arise are unknown. Splenocytes from 3-5-week-old C57BL/6 (nu/nu) mice contain 2%-5% Pro-T cell progenitors identified with the Joro 37-5 and Joro 75 antibodies, but not mature T cells. To study Tcr gene rearrangement outside the thymus, we fused splenocytes from 3-5-week-old C57BL/6 nude mice with the T-cell lymphoma BW 100.129. Of 22 hybrids that grew stably in culture, four had Tcrd-VD1-D2-J1, two had Tcrd-VD2-J1, and seven had Tcrd-D1-D2 types of rearrangement. Eight hybrids had rearranged the Tcrg-2 gene cluster, but none had rearranged Tcrg-1, -3, or -4. None of the hybrids had rearranged the Tcrb gene cluster and 13 contained DJ rearrangements at the Igh locus. We conclude that the spleen is one of the extrathymic sites where T-cell progenitors can rearranged Tcrd and Tcrg genes. However, there was no evidence for Tcrb gene rearrangements in this organ. Furthermore, the analysis of this limited number of hybrids suggests that extrathymic Tcr gene rearrangements seem to be distinct and much less diverse than those found in the developing thymocytes.

Developmentally regulated fetal thymic and extrathymic T-cell receptor gamma delta gene expression

The gamma delta T-cell receptor (TCR) is the first TCR to be expressed in ontogeny in all vertebrates in which it has been examined thoroughly. Murine gamma delta cell-surface protein is detected by the fourteenth day of gestation. In this work, the activation of gamma delta RNA has been studied. Data indicate that the first TCR protein to appear in the thymus is encoded by gamma genes that are activated after cells colonize the thymus. However, the sequential appearance of different gamma delta TCR proteins during thymic ontogeny cannot be readily explained by differential temporal activation of V gamma genes in the thymus. There are distinct patterns of gamma and delta gene expression during fetal liver development and in the fetal gut (or tissue associated with it). Cells apparent in the liver of mice at birth express gamma delta cell-surface protein, but they disappear from the liver very soon afterward. One V gamma gene is rearranged and expressed prethymically. In addition, gamma gene expression is detectable in the livers of newborn athymic mice. Together, these observations indicate a thymic-independent pathway of activation of TCR genes.

Structure and specificity of a class II MHC alloreactive gamma delta T cell receptor heterodimer

Two distinct CD3-associated T cell receptors (TCR alpha beta and TCR gamma delta) are expressed in a mutually exclusive fashion on separate subsets of T lymphocytes. While the specificity of the TCR alpha beta repertoire for major histocompatibility complex (MHC) antigens is well established, the diversity of expressed gamma delta receptors and the ligands they recognize are less well understood. An alloreactive CD3+CD4-CD8- T cell line specific for murine class II MHC (Ia) antigens encoded in the I-E subregion of the H-2 gene complex was identified, and the primary structure of its gamma delta receptor heterodimer was characterized. In contrast to a TCR alpha beta-expressing alloreactive T cell line selected for similar specificity, the TCR gamma delta line displayed broad cross-reactivity for multiple distinct I-E-encoded allogeneic Ia molecules.

Analysis of T cell receptor gamma chains from adult CD4- CD8- thymocytes

The role of T cell receptor (TcR) gamma genes in T cell development has not been determined. To extend our understanding of the repertoire of TcR gamma expression, we prepared a cDNA library from CD4-CD8- adult BALB/c thymocytes and cloned and sequenced 15 TcR gamma genes from this cDNA library. We found that two clones were transcripts of the unrearranged C gamma 2 gene and that three clones terminated in the J gamma 2 region. Nine of the remaining clones were V gamma 1.2 J gamma 2 C gamma 2 genes and five of these were in frame. Only one clone corresponded to C gamma 1 and resulted from an in-frame V gamma 2 J gamma 1 C gamma 1 join. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis analysis of the gamma chain proteins from the surface of both BALB/c and C57BL/6 adult CD4-CD8- thymocytes did not detect the 32-kDa V gamma 1.2 C gamma 2 protein, but did detect the 35-kDa V gamma C gamma 1 protein. These results suggest that despite the abundance of full-length functional V gamma 1.2 C gamma 2 transcripts in the thymocyte subset, the protein product is not expressed on the cell surface as the predicted 32-kDa gamma protein. Finally, our analysis of the V-J joining of the gamma genes reveals both flexibility at the V-J junction and extensive N-region nucleotide addition that lead to diversity of the predicted protein sequence.

Diversity in T-cell receptor gamma gene usage in intestinal epithelium

The intraepithelial cells of the murine small intestine include a significant number of CD3+ T cells that use T-cell receptor gamma genes rather than T-cell receptor beta genes. As with other sites of T-cell receptor gamma expression, combinatorial diversity is limited, but there is junctional diversity, and this, together with the specific variable region gamma gene segments used, distinguishes gamma gene expression in the gut epithelium from that in cells derived from the dermal epithelium. The restriction of productive gamma gene expression largely to one V-J-C (V, variable; J, joining; C, constant) gene combination may result from nonproductive joining of other V-J combinations and from productively rearranged genes rendered nonfunctional by incorrect splicing.

Developmental biology of T cell receptors

T cell receptors are the antigen-recognizing elements found on the effector cells of the immune system. Two isotypes have been discovered, TCR-gamma delta and TCR-alpha beta, which appear in that order during ontogeny. The maturation of prothymocytes that colonize the thymic rudiment at defined gestational stages occurs principally within the thymus, although some evidence for extrathymic maturation also exists. The maturation process includes the rearrangement and expression of the T cell receptor genes. Determination of these mechanisms, the lineages of the cells, and the subsequent thymic selection that results in self-tolerance is the central problem in developmental immunology and is important for the understanding of autoimmune diseases.

T-cell receptor delta-chain diversity in peripheral lymphocytes

A small percentage (approximately 5%) of the cells in the adult thymus expresses a heterodimeric receptor, gamma delta, that exhibits extensive clonal diversity. The specificity and function of these cells are unclear. Furthermore, it is not known if their role in the immune system is primarily one that operates within the thymus during the selection of the T-cell repertoire or if they function primarily in an antigen-recognition capacity in the peripheral lymphoid system. To examine if gamma delta+ T cells in the periphery are as diverse as those in the thymus, we used the polymerase chain reaction to amplify delta-chain transcripts from polyclonal populations of thymic and splenic lymphocytes (the latter were derived from allogeneic mixed lymphocyte cultures). The nucleotide sequences of delta chains from the spleen, like those from the thymus, were all different. Most of the diversity was present in the region between the variable (V) and joining (J) gene segments and was generated through the use of the two known diversity (D) elements, D delta 1 and D delta 2, and by the addition or deletion of bases at the V delta D delta 1, D delta 1D delta 2, and D delta 2J delta junctions. The extensive gamma delta repertoire among peripheral cells suggests that they have the potential to recognize an array of ligands that could be as diverse as those recognized by alpha beta+ cells. The amplification strategy described here can be used to analyze rapidly the diversity exhibited by any of the members of the immunoglobulin-like gene families that undergo rearrangement.

Age-associated increase in the expression of T cell antigen receptor gamma chain genes in mice

T cell function generally declines with age. To determine the underlying cause of the age-related decline, we compared the expression levels of T cell antigen receptor genes encoding gamma, beta and alpha chains in the lymphoid tissues of young (8 wk old) vs. aged (40 wk old) mice. An age-associated increase in gamma chain gene transcripts was evident in the thymocytes, spleen cells and mesenteric lymph node cells. Aged mice had a relatively high proportion of CD3+CD4-CD8- cells but a reduced level of CD3+CD4-CD8+ cells in the lymphoid tissues, as compared with young counterparts. The allo-reactivity in the lymphoid cells, as assessed by mixed lymphocyte reaction (MLR) responses, decreased with advancing age. On the contrary, increased levels of syngeneic MLR and spontaneous cytolytic activity were noted in the lymphoid cells of aged mice, as compared with findings in their young counterparts. A remarkable increase in the number of CD3+CD4-CD8- cells and level of the gamma chain gene messages was also detected in the responder cells of day 4 syngeneic MLR culture from aged mice. An increase in the number of T cells bearing gamma chain may be related to the alterations in immunological functions in aged mice.

T-cell receptor gene rearrangement in primary tumors: effect of genetic background and inducing agent

The status of T-cell receptor beta and gamma genes has been assessed in a series of primary tumors induced by a chemical carcinogen or by gamma-irradiation using two inbred strains of mice. It appears that these well-characterized regimens of carcinogenesis yield T-cell tumors showing gene rearrangements consistent with a clonal origin of the tumors. Individual rearranged bands seem to represent orthodox, intralocus recombination events. A variety of rearrangement phenotypes are observed, most strikingly for the gamma genes, and differences in the degree of T-cell receptor gene rearrangements observed can be categorized according to the inducing agent and to the genetic background of the mice, with the implication that premalignant thymocytes have been captured in different stages of T-cell development. Additionally, primary tumors were shown to express significant levels of mature beta gene mRNA.

Diversity of the mouse T cell receptor C gamma 1 gene: structural analysis in C57BL/Ka

We have isolated an unusual T cell receptor gamma chain cDNA clone (gamma 7.1) from a library made from RNA derived from adult thymus of C57BL/Ka mice. This cDNA clone corresponds to the appropriately processed C gamma 1 constant region exons preceded by 1.5 kb of J-C gamma 1 intron. The gamma 7.1 coding region is extremely homologous to the C gamma 1 gene of BALB/c mice, differing at the protein level by a single deletion (alanine 139) and a single substitution. This latter change eliminates the sole N-linked sugar attachment site, providing a basis for strain-specific glycosylation patterns. The J-C gamma 1 intronic region contains two DNA segments (termed psi J gamma 1 and psi J gamma 2) that are highly reminiscent of joining (J) segments; both have potentially functional recombination and donor splice sequences flanking an open reading frame. Northern analysis suggests that gamma 7.1 may be derived from a large, variable region-containing precursor.

Rearrangement and expression of T cell receptor genes in cloned murine natural suppressor cell lines

Naturally occurring suppressor cells of the in vitro mixed leukocyte culture reaction and of in vivo graft-vs.-host disease have been identified in the spleens of neonatal mice (1) and of adult mice recovering from total lymphoid irradiation (2), whole-body irradiation (3), and syngeneic marrow transplantation (4), or cyclophosphamide therapy (5). Using both positive and negative selection procedures, the suppressors were reported to be null lymphocytes that did not express mature macrophage surface markers, nor differentiate into mature macrophages in vitro, nor demonstrate natural killer (NK) activity (1). Subsequently, cloned lines of these natural suppressor (NS) cells were derived from either adult mice given total lymphoid irradiation (TLI) (2) or from neonates (6). The cloned NS cell lines expressed a surface phenotype (2, 6) similar to that reported previously for cloned NK cells (Thy-1(+), asialo-GM1(+), Ig(-), Lyt-1(-), Lyt-2(-), Ia(-), MAC-1(-)) (7-9). However, the NS cells did not show NK activity in the standard assay with YAC-1 target cells. The cloned NS lines suppressed the proliferation of responder cells and the generation of cytolytic cells in the mixed leukocyte reaction (MLR), and suppressed lethal graft-vs.-host disease in vivo (10, 11). In view of the unusual function and surface phenotype of the cells, the lineage of these cells remained unclear. To determine the lineage of the cloned NS cells, we searched for expression and rearrangement of the alpha and beta chain genes of the T cell antigen receptor, as well as that of the gamma chain gene. Studies of the phenotypically similar NK cell yielded conflicting results. Thus, cloned lines of murine NK cells were reported to have rearrangements of the beta chain genes, and to express mRNA for all three chains (12). In contrast, freshly purified rat or human large granular lymphocytes (LGL) were shown to express only the 1.0 kb mRNA species of the beta chain gene (13), indicative of D-J joining (14). Thus, some but not all cells with NK function express the T cell receptor and are members of the T cell lineage. The current report shows that the NS lines express full-length mRNA transcripts for the a and beta chain of the T cell receptor, as well as the gamma chain gene.

Expression of the T-cell receptor gamma-chain gene products on the surface of peripheral T cells and T-cell blasts generated by allogeneic mixed lymphocyte reaction

The gamma-chain genes of the T-cell receptors form a family of related genes that are specifically expressed and somatically rearranged in T cells. Using poly- and monoclonal anti-gamma antibodies, we studied the cell-surface expression of the gamma-chain gene products in mouse peripheral T cells as well as in the T-cell blasts generated by allogeneic mixed lymphocyte reactions. The gamma chains are expressed in the Lyt2-,L3T4- subsets of these T-cell populations as disulfide-linked heterodimers. Whereas the electrophoretic mobility and the N-glycosylation of the spleen and lymph-node gamma chains are indistinguishable from those of the reported thymocyte gamma chain, a minor fraction of the T blasts generated by allogeneic stimulation of B10 lymph-node T cells with B10.BR spleen cells seems to express gamma chains with distinct properties. This suggests that the mixed lymphocyte culture conditions exert a selective effect on the expression of gamma chains among peripheral T-cell populations.

Mammalian T-lymphocyte antigen receptor genes: genetic and nongenetic potential to generate variability

T lymphocytes of higher vertebrates are able to specifically recognize a seemingly unlimited number of foreign antigens via their receptors, the T cell antigen receptors (TCRs). T lymphocytes mature by passing through the thymus and acquire antigen specificity by expressing the TCR molecules on their cell surface. Genetic and somatic diversification mechanisms give rise to the enormous degree of TCR variability observed in mature T cells: germline and combinatorial diversity as well as junctional and the so-called N-region diversity. In contrast to the situation in immunoglobulin genes somatic hypermutation does not seem to play a significant role in TCR diversification. It is argued here that the enzyme terminal nucleotidyl-transferase is potentially a major factor in generating the immense diversity. We propose furthermore that this enzyme ensures the flexibility of T cell responses to novel antigens by random insertion of so-called N-region nucleotides. Apart from the physiological functions of TCR genes any involvement in the etiology of T cell neoplasia remains to be proven.