Expression of the T-cell-specific gamma gene is unnecessary in T cells recognizing class II MHC determinants

Subtractive complementary DNA cloning combined with partial protein sequencing has allowed identification of the genes encoding the alpha and beta subunits of T-cell receptors. The subtractive cDNA library prepared from the cytotoxic T lymphocyte (Tc) clone 2C has been found to contain a third type of clone encoding the gamma chain. The gamma gene shares several features with the alpha and beta genes: (1) assembly from gene segments resembling immunoglobulin V, J and C (respectively variable, joining and constant region) DNA segments; (2) rearrangement and expression in T cells and not in B cells; (3) sequences reminiscent of transmembrane and intracytoplasmic regions of integral membrane proteins; (4) a cysteine residue at the position expected for an interchain disulphide bond. The alpha and beta genes are expressed at equivalent levels in both Tc cells and helper T cells (TH). The gamma gene, obtained from 2C, has been found to be expressed in all Tc cells studied. Here we present evidence that strongly suggests that TH cells do not require gamma gene expression.     

The genomic clone G-21 which resembles a beta-adrenergic receptor sequence encodes the 5-HT1A receptor

The recent cloning of the complementary DNAs and/or genes for several receptors linked to guanine nucleotide regulatory proteins including the adrenergic receptors (alpha 1, alpha 2A, alpha 2B, beta 1, beta 2), several subtypes of the muscarinic cholinergic receptors, and the visual 'receptor' rhodopsin has revealed considerable similarity in the primary structure of these proteins. In addition, all of these proteins contain seven putative transmembrane alpha-helices. We have previously described a genomic clone, G-21, isolated by cross-hybridization at reduced stringency with a full length beta 2-adrenergic receptor probe. This clone contains an intronless gene which, because of its striking sequence resemblance to the adrenergic receptors, is presumed to encode a G-protein-coupled receptor. Previous attempts to identify this putative receptor by expression studies have failed. We now report that the protein product of the genomic clone, G21, transiently expressed in monkey kidney cells has all the typical ligand-binding characteristics of the 5-hydroxytryptamine (5-HT1A) receptor.     

Analysis of specificity for antigen, Mls, and allogenic MHC by transfer of T-cell receptor alpha- and beta-chain genes

The majority of peripheral T lymphocytes bear cell-surface antigen receptors comprised of a disulphide-linked alpha beta dimer. In an immune response, this receptor endows T cells with specificities for foreign antigenic protein fragments bound to cell surface glycoproteins encoded in the major histocompatibility complex (MHC). At a high frequency (greater than 1%), the same population of T lymphocytes responds to allogeneic MHC glycoproteins, or to differences at other genetic loci termed Mls, in conjunction with MHC. The alpha beta-antigen receptor has been implicated in alloreactivity and Mls reactivity. In fact, many monoclonal T-cell lines recognize a foreign protein fragment bound to self-MHC molecules and, in addition, recognize allogeneic MHC glycoproteins, an Mls-encoded determinant, or both. For at least one T-cell clone, a monoclonal antibody directed against the alpha beta antigen receptor has been shown to block activation induced by either antigen-bound self-MHC or by allogeneic MHC. However, it remains to be demonstrated directly that a single alpha beta receptor can mediate antigen specificity, alloreactivity and Mls reactivity, a prerequisite to understanding the structural basis of these high-frequency cross-reactivities. To address this issue we have performed transfers of receptor chain genes from a multiple-reactive T-cell clone into an unrelated host T lymphocyte. We now demonstrate definitively that the genes encoding a single alpha beta-receptor chain pair can transfer the recognition of self-MHC molecules complexed with fragments of antigen, allogeneic MHC molecules, and an Mls-encoded determinant (presumably in conjunction with MHC). In this case the transfer of antigen specificity and alloreactivity requires a specific alpha beta-receptor chain combination, whereas Mls reactivity can be transferred with the beta-chain gene alone into a recipient expressing a randomly selected alpha-chain.     

Structural polymorphism of I-E subregion antigens determined by a gene in the H-2K to I-B genetic interval

THE generation of immune responses in mice is influenced by Ir genes located in the I region of the major histocompatibility complex (MHC)(1). In some instances maximum responses require complementation by two genes, one in the I-A or I-B and the other in the I-E or I-C subregion(2,3). The effects of these genes are thought to be mediated by Ia alloantigens, which are cell surface molecules whose expression is controlled by the I region(4). This is based on the observations that anti-Ia sera inhibit in vitro immune responses(5,6), and soluble factors that enhance in vitro immune responses express Ia alloantigenic determinants(7,9). Jones et al.(10), using two-dimensional gel electrophoresis, observed that the expression of I-E subregion antigens is controlled by two genes, one in the I-A subregion, the other in the I-E subregion, and that the polymorphism of these antigens is influenced by an I-A subregion gene. As an explanation, the authors proposed that only one of the two polypeptide chains present in I-E immunoprecipitates is an I-E subregion product, the second being a product of the I-A subregion. Antisera obtained by cross-immunisation of I-E subregion-disparate strains of mice immunoprecipitates a molecular complex consisting of two chains, designated alpha and beta, with molecular weights of 32,000 and 29,000 respectively(11-14). Previous studies suggested that I-E antigens isolated from B10.A(5R) and B10.D2 mice had identical alpha-chains but different (beta)-chains(15). However, as these mice differed at multiple genetic regions, it was not possible to show which I subregion(s) determined the polymorphism of the E(beta) chain. Therefore, we investigated the effects of the I-A subregion on the polymorphism of I-E subregion antigens. We have now shown by peptide mapping that the I-E subregion polymorphism which Jones et al. found to be controlled by the I-A subregion probably reflects structural polymorphism of beta-chains controlled by an I-A subregion gene.     

T gamma protein is expressed on murine fetal thymocytes as a disulphide-linked heterodimer

During the search for genes coding for the mouse alpha and beta subunits of the antigen-specific receptor of mouse T cells we encountered a third gene, subsequently designated gamma. This gene has many properties in common with the alpha and beta genes, somatic assembly from gene segments that resemble the gene segments for immunoglobulin variable (V), joining (J) and constant (C) regions; rearrangement and expression in T cells and not in B cells; low but distinct sequence homology to immunoglobulin V, J and C regions; other sequences that are reminiscent of the transmembrane and intracytoplasmic regions of integral membrane proteins; and a cysteine residue at the position expected for a disulphide bond linking two subunits of a dimeric membrane protein. Despite these similarities the gamma gene also shows some interesting unique features. These include a relatively limited repertoire of the germ-line gene segments, more pronounced expression at the RNA level in immature T cells such as fetal thymocytes and an apparent absence of in-frame RNA in some functional, alpha beta heterodimer-bearing T cells or cultured T clones and hybridomas. To understand the function of the putative gamma protein it is essential to define the cell population that expresses this protein. To this end we produced a fusion protein composed of Escherichia coli beta-galactosidase and the gamma-chain (hereafter referred to a beta-gal-gamma) using the phage expression vector lambda gt11 and raised rabbit antisera against the gamma determinants. Using the purified anti-gamma antibody we detected a polypeptide chain of relative molecular mass 35,000 (Mr 35K) on the surface of 16-day old fetal thymocytes. The gamma-chain is linked by a disulphide bridge to another component of 45K. No such heterodimer was detected on the surface of a cytotoxic T lymphocyte (CTL) clone 2C from which an in-phase gamma cDNA clone was originally isolated.     

Precursor and effector phenotypes of activated human T lymphocytes

In mice, thymus-derived lymphocytes are differentiated into functional subclasses by their cell surface antigens. The Ly 1 determinants are present on T cells with a helper function, whereas Ly 2 and Ly 3 antigens are expressed on the surface of lymphocytes with suppressor or cytotoxic functions. In man also, T-cell subsets have been identified using allo- and heteroimmune sera and, more recently, using monoclonal antibodies, which seem to identify helper and suppressor or cytotoxic subpopulations. The major histocompatibility system (MHS)-encoded Ia antigens belong to several polymorphic families of membrane associated glycoproteins originally found on B lymphocytes; however, they have also been shown to be markers for suppressor T cells in mice. Recent studies have shown that in both mouse and man, T cells activated by a mixed lymphocyte reaction or by mitogens become Ia+. Furthermore, some human T lymphoid cells, either freshly isolated from peripheral blood or after in vitro activation by lectins or alloantigens, possess suppressor properties. We report here the phenotype of a T suppressor-cell subpopulation which was induced in long-term culture of lymphoid cells after activation with phytohaemagglutinin (PHA). Our results suggest that a subset of T cells was progressively expanded over a period of 8 days in culture and that, with the expression on the surface of these cells of 'Ia-like' antigens, they acquired the capacity to suppress the proliferative response of syngeneic or allogeneic lymphocytes to alloantigens or mitogens.     

T cell depletion in transgenic mice carrying a mutant gene for TCR-beta

Classical T lymphocytes recognize foreign antigens in the context of self major histocompatibility complex (MHC) molecules by means of the T-cell receptor (TCR)alpha beta heterodimer. The genes for TCR beta-chains, like immunoglobulin genes, are subject to allelic exclusion. The introduction of a functional TCR-beta gene into the germline of mice prevents rearrangement of endogenous TCR-beta genes. Here we report that the introduction of a non-functional TCR-beta genes. Here we report that the introduction of a non-functional TCR-beta gene with a deletion of the major part of the variable region (delta V-TCR-beta), also inhibits endogenous TCR-beta gene rearrangement. This inhibition is mediated via the encoded protein because impairment of endogenous TCR-beta gene rearrangement is not found if a frameshift mutation is introduced into the DJ region of the delta V-TCR-beta transgene. The delta V-TCR-beta transgene can lead to two phenotypes, in which lymphoid development is perturbed. Phenotype A is characterized by a severe impairment of both T and B cell development as reflected by the complete absence of certain lymphoid organs. In phenotype B, lymphoid organs are macroscopically normal, but T cell differentiation is impeded. Virtually all thymocytes lack membrane expression of TCR-alpha beta, but nevertheless carry the CD4 and CD8 antigens (CD4+CD8+ phenotype); they do not, however, mature further. The defect in mice of phenotype B but not of phenotype A can be corrected by the introduction of a functional TCR-beta gene.     

The gene encoding the Ia antigen-associated invariant chain (Ii) is not linked to the H-2 complex

The invariant (Ii) chain of murine Ia antigens is associated with the intracellular but not the cell-surface forms of the A alpha:A beta and E alpha:E beta Ia complexes. Due to its unique subcellular localization, Ii has been postulated to play a part in the assembly or intracellular transport of the Ia alpha:beta complexes, which function in immune recognition. A more general role for Ii in the transport of other cell proteins has also been suggested. Because of the unusual subunit composition of Ia antigens and because the synthesis of alpha, beta and Ii chains is coordinately regulated, it was of interest to determine whether, like the alpha and beta chains, Ii is encoded by a gene in the I region of the H-2 histocompatibility complex. We report here the use of an Ii chain polymorphism present in Mus spretus to demonstrate that the gene for Ii is not linked to the H-2 complex. Thus, intracellular Ia antigens consist of the products of two linked genes and one unlinked gene.     

Developmental regulation of T-cell receptor gene expression

In contrast to B cells or their antibody products, T lymphocytes have a dual specificity, for both the eliciting foreign antigen and for polymorphic determinants on cell surface glycoproteins encoded in the major histocompatibility complex (MHC restriction). The recent identification of T-cell receptor glycoproteins as well as the genes encoding T-cell receptor subunits will help to elucidate whether MHC proteins and foreign antigens are recognized by two T-cell receptors or by a single receptor. An important feature of MHC restriction is that it appears to be largely acquired by a differentiating T-cell population under the influence of MHC antigens expressed in the thymus, suggesting that precursor T cells are selected on the basis of their reactivity with MHC determinants expressed in the host thymus. To understand this process of 'thymus education', knowledge of the developmental regulation of T-cell receptor gene expression is necessary. Here we report that whereas messenger RNAs encoding the beta-and gamma-subunits are relatively abundant in immature thymocytes, alpha mRNA levels are very low. Interestingly, whereas alpha mRNA levels increase during further development and beta mRNA levels stay roughly constant, gamma mRNA falls to very low levels in mature T cells, suggesting a role for the gamma gene in T-cell differentiation.     

Transfer of specificity by murine alpha and beta T-cell receptor genes

T-cell receptor alpha- and beta-chain genes were isolated from a class I major histocompatibility complex-restricted cytotoxic T-cell clone and transferred by protoplast fusion into another cytolytic T-cell clone of different specificity. Expression of the transfected alpha and beta genes endowed the recipient cell with the specificity of the donor cell.     

A minimum of four human class II alpha-chain genes are encoded in the HLA region of chromosome 6

The major histocompatibility complex (MHC) in man, also called the HLA region, is located on the short arm of chromosome 6 and encodes antigens involved in immunological processes. The class II HLA antigens consist of two noncovalently associated polypeptide chains, one of molecular weight 34,000 (alpha) and the other of molecular weight 29,000 (beta). The extensive polymorphism of the beta chain(s) has allowed the genetic mapping of the corresponding beta gene(s) to the HLA-DR region. cDNA clones for the HLA-DR alpha chain have been used to map the non-polymorphic DR alpha-chain gene to chromosome 6 using mouse-human somatic cell hybrids. Similarly, the DR alpha-chain gene has been mapped to the short arm of chromosome 6 centromeric to the HLA-A, -B and -C loci by in situ hybridization experiments. We isolated a cDNA clone that is related to the DR alpha chain and encodes the class II antigen DC alpha chain. We describe here how this DC alpha clone was used to find two or three additional alpha-chain genes by cross-hybridization and how HLA-antigen loss mutants of a human lymphoblastoid cell line (LCL) were used to ascertain that these additional class II antigen alpha-chain genes are also located in the HLA region.     

Lectin-induced expression of DR antigen on human cultured follicular thyroid cells

HLA-DR antigens, the human equivalent of mouse I region-associated or Ia products, are polymorphic cell surface sialoglycoproteins involved in initiation of the immune response. Their expression is normally restricted to B lymphocytes, macrophages, dendritic and other antigen-presenting cells and vascular endothelium and possibly some cells of the mucosa lining body cavities. HLA-DR expression can be modified during cell differentiation; B lymphocytes become negative on maturing to plasma cells and human T lymphocytes acquire these antigens when activated in vitro or in vivo. We report here that human thyroid follicular cells which are normally negative for HLA-DR molecules, can be induced to express these antigens when cultured with phytohaemagglutinin (PHA), concanavalin A (Con A) or pokeweed mitogen (PWM). These lectins exert their action directly on the thyroid cells with no concomitant mitogenic effect.     

Death of mature T cells by separate ligation of CD4 and the T-cell receptor for antigen

Effector T cells are restricted to recognizing antigens associated with major histocompatibility complex (MHC) molecules. Specific recognition is mediated by the alpha beta heterodimer of the T-cell receptor (TCR)/CD3 complex, although other membrane components are involved in T-cell antigen recognition and functions. There has been much controversy in this regard over the part played by the CD4 glycoprotein. It is known that expression of CD4 correlates closely with the cell's ability to recognize antigens bound to class II MHC molecules and that CD4 can bind to class II molecules. Also monoclonal antibodies to CD4 can modify signals generated through the TCR/CD3 complex. It has therefore been proposed that CD4 binds to class II molecules, coaggregates with the TCR-CD3 complex and aids the activation of T cells. But given that TCR can itself impart restriction on the cell, it remains unclear whether the contribution of CD4-derived signals to those generated through the TCR alpha beta-CD3 complex is central to this activation. Here we report that when preceded by ligation of CD4, signalling through TCR alpha beta results in T cell unresponsiveness due to the induction of activation dependent cell death by apoptosis. These results imply that CD4 is critically involved in determining the outcome of signals generated through TCR, and could explain why the induction of effector T cells needs to be MHC-restricted.     

Phenotypic differences between alpha beta versus beta T-cell receptor transgenic mice undergoing negative selection

T-cell differentiation in the thymus is thought to involve a progression from the CD4-CD8- phenotype through CD4+CD8+ intermediates to mature CD4+ or CD8+ cells. There is evidence that during this process T cells bearing receptors potentially reactive to 'self' are deleted by a process termed 'negative selection' One example of this process occurs in mice carrying polymorphic Mls antigens, against which a detectable proportion of T cells are autoreactive. These mice show clonal deletion of thymic and peripheral T-cell subsets that express the autoreactive V beta 3 segment of the T-cell antigen receptor, but at most a two-fold depletion of thymic cells at the CD4+CD8+ stage. By contrast, transgenic mice bearing both alpha and beta chain genes encoding autoreactive receptors recognizing other ligands, show severe depletion of CD4+CD8+ thymocytes as well, suggesting that negative selection occurs much earlier. We report here the Mls 2a/3a mediated elimination of T cells expressing a transgene encoded V beta 3-segment, in T-cell receptor alpha/beta and beta-transgenic mice. Severe depletion of CD4+CD8+ thymocytes is seen only in the alpha/beta chain transgenic mice, whereas both strains delete mature V beta 3 bearing CD4+ and CD8+ T cells efficiently. We conclude that severe CD4+CD8+ thymocyte deletion in alpha/beta transgenic mice results from the premature expression of both receptor chains, and does not reflect a difference in the timing or mechanism of negative selection for Mls antigens as against the allo- and MHC class 1-restricted antigens used in the other studies.     

Structural relationship of the SB beta-chain gene to HLA-D-region genes and murine I-region genes

The major histocompatibility complex (MHC) regulates several aspects of the immune response. Class II antigens of the MHC control cellular interactions between lymphocytes. In man, at least three class II antigens (DR, DC and SB), consisting of distinct alpha- and beta-chains, are encoded in the HLA complex. Sequence analysis has established that the DR and DC antigens are the respective structural counterparts of the murine I-E and I-A antigens. Molecular cloning of the SB beta-chain gene has now enabled us to define its relationship to other class II genes. The DR, DC and SB beta genes have diverged from each other to the same extent. In murine DNA and in cloned genes from the I region, the best hybridization of SB beta DNA is with the E beta 2 sequence. E beta 2 may belong to a complete gene (E' beta) because first domain sequences were found adjacent to it.