Transfection of the CD8 gene enhances T-cell recognition

Deletion of autospecific T cells in T cell receptor (TCR) transgenic mice spares cells with normal TCR levels and low levels of CD8 molecules

Transgenic mice that carry on a large fraction of their T cells an alpha/beta T cell receptor that recognizes the male antigen in the context of H-2Db molecules were constructed. An mAb specific for the transgenic receptor was developed and used to analyze T cell subsets in male transgenic H-2b mice. The vast majority of immature CD4+8+ T cells that express the transgenic TCR were deleted in the male transgenic mouse. Nevertheless, the majority of T cells spared by this deletion process expressed a high level of the transgenic TCR. These T cells, however, had an abnormal CD4/CD8 phenotype in that they expressed either no CD8 molecules or only low levels.

The thymus selects the useful, neglects the useless and destroys the harmful

Although efficient at reacting to foreign antigen in the context of MHC, mature T cells do not normally react to self antigens presented by self MHC. In this review, Harald von Boehmer and colleagues describe the investigation of self MHC restriction and self-tolerance using TCR transgenic mice expressing a receptor for the male-specific minor histocompatibility antigen, H-Y, in the context of class I H-2Db MHC antigens, on many of their T cells. CD4-8+ T cells expressing the transgenic receptor were positively selected by the restricting H-2Db MHC antigens in female transgenic mice. In the male TCR transgenic mice, CD4+8+ thymocytes were deleted, and transgene-expressing T cells with high surface-density of CD8 were-absent from the periphery. The remaining T cells could not be activated by male H-Y stimulator cells, as they lacked or expressed only low levels of CD8 molecules.

Substitution at residue 227 of H-2 class I molecules abrogates recognition by CD8-dependent, but not CD8-independent, cytotoxic T lymphocytes

The CD8 (Lyt 2) molecule is a phenotypic marker for T lymphocytes that recognize and react with major histocompatibility complex (MHC) class I molecules. Antibody blocking experiments and gene transfection studies indicate that CD8 binds to a determinant on MHC class I molecules on the target cells, facilitating interaction between effector T lymphocytes and the target cell. The CD8 molecule may also be involved in transmembrane signalling during T-cell activation. The existence of CD8- cytotoxic T lymphocytes (CTL) and class I-reactive CTL that are not inhibited by antibody to CD8 suggests that at least some CTL do not require the CD8 molecule to interact with and lyse target cells. We have recently demonstrated that cells transfected with an H-2Dd gene that carries a mutation at residue 227 are not killed by primary CTL8. Here we show that although this mutation abrogates recognition by primary CTL, it does not affect recognition by CD8-independent CTL, suggesting that residue 227 of class I molecules might contribute to a determinant that is the ligand of the CD8 molecule.

Direct evidence for binding of CD8 to HLA class I antigens

Adhesion of T lymphocytes is an essential step for antigen recognition and lymphocyte activation. mAbs to T cell surface proteins have been used to define the receptor-ligand proteins that appear to be involved in adhesion. Since most assays measure the effects of mAbs on T lymphocyte function, it is not known whether mAb-mediated blocking is due to a disruption of receptor-ligand interactions or results in inhibition of some aspect of receptor-mediated triggering. It has been suggested that the CD8 molecule augments T cell avidity for the target cells by binding to determinants on target cell MHC class I molecules. In the present report, we demonstrated that purified CD8 molecules incorporated into large lipid vesicles (artificial target cells) mediate the adhesion of these vesicles to cells expressing HLA proteins, while vesicles expressing purified HLA class I antigens bind to CD8+ T cells. Furthermore, vesicles bearing CD8 will form conjugates with vesicles expressing HLA class I proteins. These conjugates were found to be specifically inhibited by mAbs to CD8 or HLA class I molecules. We also demonstrate that CD2-reconstituted vesicles can form conjugates with vesicles bearing LFA-3. These experiments provide direct evidence for an interaction of the CD8 molecule with class I MHC proteins as well as between CD2 proteins and LFA-3 proteins, thus supporting the hypothesis that these molecules can mediate cell-cell adhesion.

Activation of human T lymphocytes: differential effects of CD3- and CD8-mediated signals

T cells are activated physiologically by triggering the T-cell receptor-CD3 complex. There is evidence that invariant accessory molecules on the T-cell membrane (CD8 and CD4) are involved in the major histocompatibility complex-restricted recognition process. Moreover, binding and crosslinking of these accessory molecules to the T-cell receptor-CD3 complex exerts a positive synergistic signal, as has been shown by stimulation with crosslinked antibodies. Here we demonstrate that stimulation mediated by immobilized anti-CD3/CD8 antibodies differs from stimulation mediated solely by anti-CD3. Whereas interleukin 2 receptor expression and interferon gamma production are seen to a similar extent in both cases, a second signal provided by the additional involvement of CD8 seems to be essential for interleukin 2 production and full interleukin 2 responsiveness in CD8+ T cells. This second signal is much more sensitive to inhibition by 1-(5-isoquinolinylsulfonyl)-2-methylpiperazine, an inhibitor of protein kinase C and cGMP/cAMP-dependent kinases. Our results also show that substantial modulation of the T-cell receptor complex and most likely CD3 phosphorylation are not essential for initiating the activation of resting T cells. Instead, we found a 22- to 24-kDa phosphoprotein whose strong phosphorylation correlated reliably with T-cell activation.

Class I alloantigen is sufficient for cytolytic T lymphocyte binding and transmembrane signaling

Based largely on antibody blocking studies, a number of surface "accessory" molecules on effector cytolytic T lymphocytes (CTL) have been implicated as having a role in mediating CTL binding and lysis of target cells, possibly via binding to ligands on the target cell surface. Despite this, cloned allogeneic CTL were able to specifically bind cell-size, artificial membranes (pseudocytes) bearing only class I alloantigen. This binding triggered CTL degranulation, as measured by serine esterase release. Thus, class I alloantigen alone is both a necessary and sufficient ligand for specific binding and effective transmembrane signaling to occur.

A second chain of human CD8 is expressed on peripheral blood lymphocytes

Human CD8 has been thought to consist of disulfide-linked homodimers and homomultimers of a single polypeptide chain homologous to mouse and rat CD8 alpha. In contrast, mouse and rat CD8 are composed of disulfide-linked heterodimers of alpha and beta chains. We have now isolated and sequenced cDNA clones encoding a human homologue of mouse and rat CD8 beta. One such clone was inserted into an expression vector and its encoded product was shown to be expressed on the cell surface after cotransfection into L cells with the human CD8 alpha gene. A second form of human CD8 beta cDNA encoding a protein with an altered cytoplasmic tail was similarly transfected, but its product could not be demonstrated on the cell surface. CD8 beta was further shown to be expressed on the surface of almost all CD8+ human peripheral blood T cells. These data provide the first evidence that human CD8 is a heterodimeric protein.

Regulatory role of the CD8 antigen in both CD3 and CD2 monoclonal antibody-induced nonspecific cytotoxicity of class I- and class II-allospecific cytotoxic T cell clones

We investigated the function of the CD8 moiety in antigen-specific and alternative activation of HLA class I- and HLA class II-allospecific CD8+ cytotoxic T lymphocyte (CTL) clones. Monoclonal antibodies (mAb) directed against the CD8 structure were only found to inhibit antigen-specific cytotoxicity of class I-allospecific CD8+ CTL clones and not of a class II-allospecific CD8+ CTL clone. However, cytotoxicity induced by CD3 mAb (used at suboptimal concentrations) or CD2 mAb in both types of CTL clone was blocked by CD8 mAb. The class II-allospecific CD8+ CTL clone was uniformly more difficult to inhibit than the class I-allospecific CD8+ CTL clones and, moreover, also easier to induce to exert nonspecific cytotoxicity by CD2 mAb and CD3 mAb. The absence of CD8 mAb blocking of antigen-specific cytotoxicity of the class II-specific CD8+ CTL clone is, therefore, assumed to result from too strong a triggering signal to be overcome by the down-regulatory signal of the CD8 antigen. These combined findings suggest a down-regulatory function of CD8 not only in T cell receptor (TcR)/CD3 activation, but also in TcR/CD3-controlled alternative activation routes such as the CD2 activation pathway.

Nucleotide sequence analysis of the C.AKR Lyt-2a gene: structural polymorphism in alleles encoding the Lyt-2.1 T-cell surface alloantigen

The Lyt-2a allele of the C.AKR strain of mice (genotype Lyt-2a, Lyt-3a) was cloned, and its complete nucleotide sequence as well as that of 2 kb of 5' flanking DNA was determined. The sequence was compared with the partial sequence of the Lyt-2a allele of DBA/2 (genotype Lyt-2a, Lyt-3b) and the nearly complete sequence of the B10.CAS2 Lyt-2b allele reported by Liaw and coworkers (1986). The coding regions of the two Lyt-2a alleles differ from each other by two nucleotide substitutions in the three exons over which they could be compared, resulting in two amino acid substitutions in the leader and transmembrane segments. The coding region of the C.AKR Lyt-2a allele differs from the Lyt-2b allele by two nucleotide substitutions in the extracellular V-like domain, one of which is silent and the second of which leads to substitution of valine for methionine at amino acid position 78 giving rise to the Lyt-2.1 allotypic specificity. The coding region of the DBA/2 Lyt-2a allele shares with C.AKR the allotypic substitution at position 78 and differs from Lyt-2b by three additional nucleotide substitutions in the coding regions, two of which lead to amino acid substitutions in the leader and transmembrane segments. It would therefore appear that the Lyt-2 alleles of the three strains analyzed are distinct, and the nomenclature Lyt-2a1 and Lyt-2a2 is suggested to distinguish the alleles of C.AKR and DBA/2, respectively. These alleles share a common difference from the Lyt-2b gene product at position 78, and since the amino acid substitutions which distinguish them from each other are in the leader and transmembrane segments, their mature Lyt-2 gene products appear antigenically identical.

Structure and expression of the Lyt-3a gene of C.AKR mice

The mouse Lyt-3a gene, which encodes the Lyt-3.1 T-cell surface alloantigen of the C.AKR strain, has been cloned, and the nucleotide sequence of its exons and more than 2 kb of 5' flanking sequence have been determined. The gene extends over approximately 16 kb of DNA and consists of six exons encoding leader, leader plus V-like domain, membrane-proximal, transmembrane, and cytoplasmic domains. The only difference between the coding region of the Lyt-3a gene and the cDNA sequences reported for Lyt-3b (Nakauchi et al. 1987. Panaccio et al. 1987) is at position 77 of the mature protein where Lyt-3a encodes serine and Lyt-3b encodes arginine. This substitution must therefore be the basis for the serological distinction between the Lyt-3.1 and Lyt-3.2 alloantigens. Potential TATA and CAAT sequences, two Sp1 protein binding sites, two extended repeats of the dinucleotide, CA, a number of short inverted repeats, and an inverted segment of the mouse B1 repetitive sequence are found 5' to the Lyt-3a gene. Two consensus poly-A addition signals and a complete copy of the mouse B1 sequence are found 3' to the gene. Both B1-related regions are flanked by short direct repeats suggesting that they arose by an insertional mechanism. Cotransfection of the Lyt-3a gene together with a cloned Lyt-2a gene resulted in expression of both Lyt-2 and Lyt-3.1 on the surface of Ltk- and BW5147 cells.(ABSTRACT TRUNCATED AT 250 WORDS)

Electroporation of mammalian cells with a firefly luciferase expression plasmid: kinetics of transient expression differ markedly among cell types

Electroporation in combination with the luciferase reporter system provides a rapid and sensitive means of determining transient expression from plasmid constructs in various mammalian cells. Conditions are described allowing efficient electroporation of several cell types with a commercial electroporation device. Measurements on extracts prepared at various times after electroporation showed a rapid rise in luciferase activity for up to 12 hr, which was followed, in certain cell types, by a rapid decline. With such cells, determinations made after 48-72 hr, as is conventional, would greatly underestimate expression of the luciferase reporter gene. Therefore, to observe optimum activity, the kinetics of expression must be determined for each cell type.

Functional consequences of anti-sense RNA-mediated inhibition of CD8 surface expression in a human T cell clone

An experimental approach for defining the function of CD8 has been developed by linking anti-sense RNA mutagenesis and T cell cloning technologies. We have transfected an anti-sense CD8 episomal expression vector into a CD8+ nontransformed human T cell clone that is specific for the human class I alloantigen HLA-B35. Expression of CD8 on this T cell clone, JH.ARL.1, was selectively and efficiently inhibited. Stimulation of this CD8- variant with specific alloantigen resulted in a marked loss of a number of functional responses, including cytotoxicity, proliferation, IL-2 secretion, and IL-2-R expression. However, these same functional responses could be elicited with stimuli that do not require antigen recognition to activate the T cell (anti-CD3 mAbs, PHA). The results of our study support the hypothesis that CD8 is required for recognition of class I MHC alloantigens that results in activation of T cell functional responses.

The role of CD4 in antigen-independent activation of isolated single T lymphocytes

The membrane molecule CD4 (L3T4) is thought to facilitate activation of Class II H-2-restricted T cells by binding to Ia determinants on antigen-presenting cells. Recent reports suggest that CD4 can also contribute to antigen-independent activation by anti-T cell receptor (TCR) antibodies. An assay which measures the secretion of two lymphokines, granulocyte-macrophage colony-stimulating factor and interleukin 3 (IL-3), by single T cells activated with an anti-TCR antibody, F23.1, was used to analyze the effects of anti-CD4 antibodies on antigen-independent T cell activation. Single cells of a CD4+F23.1+ clone were micromanipulated into wells to which F23.1 had been immobilized, and their lymphokine secretion was measured 24 hr later. The frequency of lymphokine-secreting cells was consistently reduced up to 10-fold in the presence of soluble anti-CD4 antibody (GK1.5) but only up to 2.5-fold by an antibody to the cell adhesion molecule, LFA-1. In both bulk and single-cell cultures, responses to suboptimal concentrations of F23.1 were more susceptible to inhibition by GK1.5 than responses to optimal F23.1. The failure of GK1.5 to inhibit IL-2-stimulated lymphokine synthesis in bulk cultures suggested that CD4 ligation did not deliver a negative signal to the clone. By contrast, when either anti-CD4 or anti-LFA-1 was immobilized on the same surface as F23.1, the frequency of lymphokine-secreting cells could be increased up to 10-fold. It is concluded that anti-CD4 antibodies can act directly on the responding T cell to affect TCR-dependent activation, in the absence of interaction with antigen-presenting cells or any other cell type.

Thymic major histocompatibility complex antigens and the alpha beta T-cell receptor determine the CD4/CD8 phenotype of T cells

T-cell receptors and T-cell subsets were analysed in T-cell receptor transgenic mice expressing alpha and beta T-cell receptor genes isolated from a male-specific, H-2Db-restricted CD4-8+ T-cell clone. The results indicate that the specific interaction of the T-cell receptor on immature thymocytes with thymic major histocompatibility complex antigens determines the differentiation of CD4+8+ thymocytes into either CD4+8- or CD4-8+ mature T cells.

Evidence for specific association between class I major histocompatibility antigens and the CD8 molecules of human suppressor/cytotoxic cells

Human T lymphocytes, metabolically labeled with 35S-cysteine and 35S-methionine, were reacted with the homobifunctional cross-linking reagent, dithiobis (succinimidyl propionate) (DSP). When detergent lysates from these cells were immunoprecipitated with a monoclonal antibody reactive with the CD8 antigen, a radiolabeled protein of approximately 44 kd was coprecipitated with the CD8 molecule. Immunoprecipitates from detergent lysates prepared without prior chemical cross-linking contained only the 33 kd CD8 molecule. Similar results were obtained when T lymphocytes or a cytotoxic T cell clone (T4T8Cl) were radiolabeled with 32P-orthophosphoric acid. The 44 kd CD8-associated protein was identified as the heavy chain of the class I major histocompatibility antigen by depletion in preclearing experiments with anti-class I MHC antibody and by peptide mapping. Further analyses indicated that the CD8-class I MHC association is due, in part at least, to disulfide bonding, which may be susceptible to cleavage during processing of cell lysates.

The Lyt-2 molecule recognizes residues in the class I alpha 3 domain in allogeneic cytotoxic T cell responses

The involvement of the different domains of the MHC class I molecule in CTL recognition was investigated. mAbs specific for the alpha 1/alpha 2 domains of H-2Ld interfered with both the primary and secondary generation and effector function of in vitro Ld-specific CTL. mAbs specific for the alpha 3 domain of H-2Ld interfered with the generation and function of primary in vitro Ld-specific CTL; however, there was no effect on the in vitro generation of secondary CTL and only partial inhibition of their function. In vivo treatment with graft-specific antibodies to both the alpha 3 domain and the alpha 1/alpha 2 domains together resulted in a dramatic enhancement of Ld- or Dd-disparate skin grafts, whereas the individual mAbs showed minimal effects. This suggested that the class I alpha 3 domain is recognized by alloreactive CTL. Several approaches were undertaken to examine whether recognition of the alpha 3 domain determinants is mediated by the Lyt-2 molecule. When mAbs specific for the alpha 3 domain of either H-2Ld or H-2Dd were used in vivo and in vitro, the resulting CTL population was not inhibited by antibody to the alpha 3 domain and was only partially inhibited by antibody to Lyt-2. We therefore observed a correlation between the effects of antibody to the class I alpha 3 domain of the target molecule and antibody to the Lyt-2 molecule on the CTL. To further test the relationship between CTL recognition of the alpha 3 domain and the involvement of Lyt-2, we used a cell expressing a mutation in the alpha 3 domain of the Dd molecule. The mutation resulted in a single amino acid substitution of glu to lys at residue 227 of the alpha 3 domain. Consistent with an earlier report, cells expressing the mutant Dd lys molecule were not lysed by CTL from a primary stimulation against the wild-type Dd glu molecule. However, this same cell line was killed by the Lyt-2-independent secondary Dd-specific CTL generated in the presence of antibody to the alpha 3 domain in vivo and in vitro. Furthermore, cells expressing the mutant Dd lys molecule failed to stimulate a primary response. In conclusion, several independent lines of evidence indicate that residues in the alpha 3 domain of the class I molecule are involved in recognition by the Lyt-2 molecule, and that Lyt-2-mediated recognition can be specifically blocked using mAb to determinants in the alpha 3 domain.

Role of CD8 (Lyt-2) in cytotoxic T-cell function. Analysis of variants of a cytotoxic T-cell clone with reduced Lyt-2 expression

The role of CD8 (Lyt-2) in the function of a long-term cytotoxic T-cell (CTL) clone (Cl96) was analysed. Previous studies had shown that C196 cells utilize the alpha beta T-cell receptor and depend on Lyt-2 to recognize and lyse P815 mastocytoma target cells. Recognition is H-2Kd-restricted, presumably involving a P815 specific antigenic structure. Here we analyse a number of variants selected from Cl96 that have reduced or virtually abolished expression of Lyt-2, alone or in combination with other deficiencies. We studied the ability of these variant cells to lyse either P815, a process requiring both specific antigen recognition and triggering of cytolytic function, or to lyse the anti-CD3 hybridoma 145-2C11, a process that requires the triggering of cytolytic function only. The results shows that in the case or a permanently activated CTL effector cell such as Cl96, Lyt-2 is required for antigen recognition but is not essential for the triggering of cytotoxicity.

Tolerance in T-cell-receptor transgenic mice involves deletion of nonmature CD4+8+ thymocytes

The mechanism of self-tolerance is studied in T-cell-receptor transgenic mice expressing a receptor in many of their T cells for the male (H-Y) antigen in the context of class I H-2Db MHC antigens. Autospecific T cells are deleted in male mice. The deletion affects only transgene-expressing cells with a relatively high surface-density of CD8 molecules, including nonmature CD4+ CD8+ thymocytes, and is not caused by anti-idiotype cells.

Use of an Epstein-Barr virus episomal replicon for anti-sense RNA-mediated gene inhibition in a human cytotoxic T-cell clone

A methodology was developed for stable gene transfer into cloned nontransformed human T lymphocytes. Stable high-level gene expression was achieved in cloned human T cells by using a self-replicating Epstein-Barr virus (EBV) episomal replicon. A comparison of five eukaryotic promoters established that the Rous sarcoma virus 3' long terminal repeat (RSV 3' LTR) and the lymphopapilloma virus (LPV) 5' LTR are optimal for episome-based expression in T cells. Effective (greater than 95%), selective, and reversible anti-sense RNA-mediated gene inhibition of a model T-cell-associated molecule (CD8) was achieved in a cytotoxic human T-cell clone by using an EBV episome-based, RSV 3' LTR-driven expression system. The linking of anti-sense RNA mutagenesis and T-cell cloning technologies should contribute significantly to studies of human T-cell function.

Physical association between the CD8 and HLA class I molecules on the surface of activated human T lymphocytes

Immune recognition by cytotoxic effector T cells requires participation of the CD8 and major histocompatibility complex class I antigens. We found that the CD8 molecule is noncovalently associated with the HLA class I heavy chain on the surface of human T cells activated by Con A. Accordingly, anti-CD8 monoclonal antibodies precipitated a heterodimer containing polypeptides of 32 and 43 kDa from the lysates of activated T cells. The 43-kDa chain of this heterodimer can be adsorbed from cell lysates with anti-HLA-A, -B, and -C antibodies. Endoglycosidase F treatment and chymotryptic peptide mapping identified a structural similarity between this 43-kDa molecule and the HLA class I heavy chain precipitated by the anti-HLA-A, -B, and -C antibody W6/32. Analysis of anti-CD8 precipitates under nonreducing and reducing conditions indicated a lack of interchain disulfide bonding between the CD8 and HLA heavy chain molecules. The CD8-HLA heavy chain complex was also detected in mixed lymphocyte cultures and a cloned cytotoxic T-lymphocyte line but not in purified natural killer cells. The present study indicates that CD8 is complexed with HLA heavy chain on the same cells, and the complex may have functional relevance in the T-cell recognition process.

The molecular basis of MHC-restricted antigen recognition by T cells

In order to determine the contribution of the clonotypic T cell receptor (Ti) alpha beta heterodimer to the antigen/MHC specificity of mature T cells, we have transfected cloned Ti alpha and/or beta genes into either human or mouse T cells, and analyzed the transfectants for Ti-T3 expression and responses to antigen and Ia molecules. Our analysis establishes that a single receptor structure (the Ti alpha beta heterodimer) is necessary and sufficient to define the dual specificity of T cell antigen recognition and suggests that in at least certain instances Ti beta chains play a predominant role in MHC restriction specificity, raising the possibility of a "one receptor, two sites" model of T cell recognition.

Growth-dependent variation of major histocompatibility complex (MHC) restriction and expression of Ly-2 and CD3/alpha/beta T cell receptor in cloned cytotoxic T cells

The specificity of major histocompatibility complex (MHC)-restricted antigen recognition by cytotoxic T cells (CTL) has been clearly correlated to the alpha/beta T cell receptor (TcR) complex on the T cell surface. Occasional changes in the specificity of in vitro cultivated CTL clones, therefore, have been suspected to result from alterations of the genes coding for the TcR alpha and/or beta chain. Here we demonstrate that pronounced variations in the stringency of MHC restriction, previously reported to occur during long-term culture of 2,4,6-trinitrophenyl (TNP)-specific CTL clones, may occur rapidly in a growth-dependent, reversible manner, i.e. without structural TcR variation. Several H-2b TNP-specific CTL clones were shown to possess strong cross-reactivity for H-2k TNP target cells when seeded at low cell numbers, but exhibit reduced or undetectable cross-reaction to H-2k TNP in high-density cultures. Another clone revealed "heteroclitic" properties with significantly stronger cytotoxic activity towards allogeneic (H-2k) than syngeneic (H-2b) TNP-modified target cells. In this case dilute cultures appeared as exclusively allo-MHC restricted, whereas dense cultures were allo/self cross-restricted. In all instances these phenomena were accompanied by cell density-dependent quantitative changes in the expression of Ly-2 and T cell antigen receptor. CTL from dilute cultures had at least 2-fold higher surface concentrations of Ly-2 and CD3 antigens than cells from dense cultures while other surface markers such as Thy-1 or LFA-1 were completely identical. No such effects were observed for CTL clones exhibiting cell density-independent specificity patterns. We conclude from these findings that (a) the stringency of MHC restriction specificity may be significantly affected by the amount of expressed TcR and/or Ly-2 molecules, (b) CTL possess mechanisms to regulate Ly-2 and TcR expression and, hence, their MHC-restricted antigen recognition, and (c) the ability to regulate Ly-2 and TcR expression may be altered during prolonged culture of a CTL clone.

The generation and selection of the T cell repertoire: insights from studies of the molecular basis of T cell recognition

The molecular basis of T cell corecognition of antigen and products of the major histocompatibility complex (MHC) was examined using the technique of DNA-mediated gene transfer. Introduction of the genes encoding the alpha and beta chains of the clonotypic molecule (Ti) of the pigeon cytochrome c-specific, I-Ek-restricted murine T cell hybridoma 2B4 into the human leukemic T cell Jurkat or its T3- variants led to the surface expression of various dimeric combinations of human and mouse Ti chains. All alpha beta dimers were associated in a 1:1 ratio with the chains of the T3 complex and all Ti-T3 complexes could transmit effective transmembrane signals for IL2 gene activation following binding and cross-linking by anti-T3 or anti-clonotypic antibodies. However, only the reconstituted mouse 2B4 alpha beta dimer mediated functional responses to antigen-presenting cells bearing the appropriate combination of antigen and Ia molecules. The fine specificity of the antigen-MHC molecule responses precisely matched that of the 2B4 T cell gene donor, establishing that the alpha beta clonotypic molecule is both necessary and sufficient to fully define the dual specificity of a T cell. This result, together with similar data from other laboratories, provides direct evidence in favor of the "single receptor" model of T cell recognition. Analysis of past experiments looking for independent recognition of antigen vs. MHC molecules led us to conclude that limitations on either formation of mixed receptor dimers or of appropriate antigen peptide-Ia molecule pairs may have limited the utility of these studies in providing an answer to this question. Therefore, additional gene transfer experiments were carried out to investigate the role of the two chains of the alpha beta heterodimer in controlling antigen vs. MHC molecule specificity. Our results showed that under conditions in which it was assured that antigen-Ia complexes formed, one could produce a receptor molecule composed of the alpha chain of one T cell hybridoma (2H10) and the beta chain of a second hybridoma (2B4) with a mixed antigen and MHC molecule specificity that could be predicted based on the specificities of the parent T cells. We discuss at length the implication of this result for the issue of single- vs. dual-site recognition models of T cell receptor function. In addition to these results looking directly at specificity of receptor molecules, we have also analyzed expression of various combinations of Ti chains. For alpha and beta, clear asymmetries in the efficiency of surface receptor expression were observed for certain pairs of chains.(ABSTRACT TRUNCATED AT 400 WORDS)

L3T4 and the immunoglobulin gene superfamily: new relationships between the immune system and the nervous system

L3T4 is a mouse cell surface protein expressed on most thymocytes and on the subset of mature T cells that recognizes class II MHC molecules. Its primary function on T cells is most likely that of increasing the avidity of the interaction between T cells and antigen-presenting or target cells. It may accomplish this by binding to a nonpolymorphic region on class II MHC molecules. The cDNA and gene encoding L3T4 have been isolated and sequenced. Analysis of the amino acid sequence predicted by the nucleotide sequence indicates that L3T4 is a member of the Ig gene superfamily. It is most closely related to Ig and Tcr V regions. Although the amino-terminal domain of L3T4 is the portion of the molecule that is most similar to V-regions, L3T4 is one of the polydomain members of the Ig gene superfamily. Studies of the expression of L3T4 mRNA in various tissues led to the surprising finding that this gene is transcribed not only in T lymphoid cells, but also in brain. The predominant form of L3T4 mRNA expressed in brain is foreshortened as compared to that in T lineage cells, and it is most likely the product of a distinct transcriptional start site. If translated, the protein encoded by this brain transcript would be 217 amino acids in length and would lack the signal peptide and the amino-terminal 214 amino acids of the mature protein. It is not known whether a stable protein product is synthesized from this mRNA or what its function might be. However, these findings place L3T4 in an intriguing class of Ig gene superfamily members characterized by coexpression in the immune system and the nervous system.

Coaggregation of the T-cell receptor with CD4 and other T-cell surface molecules enhances T-cell activation

The CD4 molecule, expressed by T cells restricted by class II major histocompatibility complex (MHC) molecules, is believed to play a role in T-cell activation. We have previously suggested that CD4 interacts with the T-cell receptor for antigen (TCR) and with class II MHC and that this dual interaction stabilizes the bond between the TCR and antigen in association with MHC. To investigate the contribution of CD4-TCR interaction, we have used the murine monoclonal anti-TCR V beta 8 antibody F23.1 to activate cloned T cells. Weak activation by soluble biotinylated F23.1 was markedly enhanced by crosslinking with either avidin or with anti-immunoglobulin (anti-Ig). The monoclonal anti-L3T4 antibody GK1.5, which normally inhibits the activation induced by F23.1, did not inhibit when GK1.5 and F23.1 were coaggregated on T cells by anti-Ig, and in many experiments activation was enhanced. Coaggregation of anti-Thy-1.2, anti-H-2Kk, or anti-LFA-1 with F23.1 also enhanced T-cell activation, although, unlike GK1.5, these antibodies in soluble form had no effect on the response to F23.1. These results are consistent with a model for T-cell activation that proposes a primary interaction between L3T4 and the TCR to stabilize TCR complexes and so to enhance T-cell activation. A related but less specific accessory role for other T-cell surface molecules is also suggested. We propose that the cellular interaction that leads to physiological T-cell activation not only achieves TCR ligation but also promotes through their ligation or redistribution the interaction of other T-cell surface molecules, all of which contribute to the overall strength of the activation signal.

Expression and function of CD8 in a murine T cell hybridoma

In general, the human CD8 molecule is expressed on T cells specific for HLA class I molecules. Studies designed to delineate the function and to define the ligand of the CD8 molecule have been complicated by the fact that the presumptive ligand for CD8 is on the HLA class I molecule, the same molecule encoding the ligand for the antigen-specific T cell receptor. The ability to express genes in cells other than their natural host has produced a new technology with which to approach CD8 functional studies. The insertion of a cDNA clone for CD8 in a defective retroviral vector has allowed the transfer of CD8 by infection with the resulting defective retrovirus. CD8 was then expressed in an HLA class II-specific T cell, thus separating the ligand requirements of the TCR and CD8. By this approach, the human CD8 molecule was expressed in a murine T cell hybridoma specific for human class II antigens. The resulting CD8+ hybridomas demonstrated a 10-fold increase in IL-2 production over the parent cell line when stimulated with JY, a human B lymphoblastoid cell line expressing both class I and II HLA antigens, demonstrating that expression of CD8 increases T cell activation. mAbs directed against the CD8 molecule inhibited the response of CD8+ hybridomas to JY, supporting the conclusion that the CD8 molecule was fractional. The role of CD8 as a receptor for class I MHC antigens was addressed by stimulation with a cell line expressing HLA-DR antigens, but lacking the expression of HLA class I antigens (Daudi). Stimulation of the CD8+ hybridomas by Daudi did not result in increased IL-2 production. The response to Daudi was unaltered by the addition of anti-CD8 mAb, in contrast to the ability of anti-CD8 mAb to block JY stimulation. Furthermore, mAbs directed against the class I antigens present on JY cells were able to block the enhanced response of the CD8+ hybridomas to JY. These data support the hypothesis that HLA class I molecules are the ligands involved in the CD8-dependent enhancement of T cell activation.

A single amino acid substitution in the alpha 3 domain of an H-2 class I molecule abrogates reactivity with CTL

We previously described a somatic cell expressing a variant H-2Dd molecule that did not serve as a target for alloreactive anti-Dd CTL. The mutant cell line had been isolated by its failure to express a serological epitope present on the H-2Dd alpha 3 domain. In the present study the alpha 3 domain of the Dd molecule of this somatic cell variant was sequenced and a single nucleotide change resulting in a glutamic acid to lysine substitution at residue 227 was identified. This change was reproduced in the cloned H-2Dd gene by oligonucleotide-directed mutagenesis. Cells transfected with this mutant gene were not killed by anti-H-2Dd CTL. Because previous studies using hybrid H-2 class I molecules had established that the alpha 3 domain does not express allele-specific determinants recognized by CTL, our results raise the possibility that residues in the alpha 3 domain of H-2 class I molecules are critical for CTL recognition and constitute a conserved (or monomorphic) determinant recognized by CTL.

Reconstitution of MHC class I specificity by transfer of the T cell receptor and Lyt-2 genes

The T cell receptor alpha and beta chain genes donated by an H-2 class I-specific, CD8-dependent cytotoxic T cell clone were transferred, alone or in combination with the Lyt-2 gene, into a class II-restricted, CD4+ T cell hybridoma. Two important points emerged. First, the alpha and beta T cell receptor genes endowed the recipient with the H-2 class I specificity of the donor only if the same cell had also been transfected with the Lyt-2 gene. Second, the functional Lyt-2 molecule was expressed on the transfected cells in the absence of the Lyt-3 polypeptide. These results demonstrate that, besides the T cell receptor, the Lyt-2 polypeptide is the only subset-specific molecule required to retarget a class II-reactive, CD4+ T cell line toward H-2 class I molecules.

Quantitative considerations of T-cell activation and self tolerance

We have reviewed evidence for the hypothesis that T-cell activation by antigen is an all or none process which is triggered when signal strength exceeds a certain threshold. Signal is generated by multivalent interaction between T-cell antigen receptors (TCR) and antigenic epitopes on the surface of antigen-presenting cells (APC) or target cells. Therefore total signal strength (Stot) will depend upon the concentration of TCR (and other accessory molecules that bind to cell surface ligands, e.g. CD4 and CD8) on T cells, the concentration of antigen on APC or targets and the affinity of interaction between receptors (a broad term incorporating TCR plus CD4 and CD8) and antigen. This hypothesis means that T-cell self tolerance is quantitatively determined by self-antigen concentrations on cell surfaces. Implications for a variety of immunological phenomenona are reviewed.