Surface expression of a T cell receptor beta (TCR-beta) chain in the absence of TCR-alpha, -delta, and -gamma proteins

The CD3 chains of the T cell antigen receptor associate with the ZAP-70 tyrosine kinase and are tyrosine phosphorylated after receptor stimulation

Recent work indicates that signaling events resulting from stimulation of the T cell antigen receptor (TCR) can be initiated by the CD3 complex (gamma, delta, epsilon) as well as the zeta chains of the receptor. To help characterize the signaling function of CD3 we examined its associated tyrosine kinase activity since induction of tyrosine phosphorylation is one of the earliest signaling events. Our results indicate that at least two kinases, lck and ZAP-70, contribute to the CD3-associated kinase activity. A likely target of this activity is the CD3 complex itself since we observed that TCR stimulation resulted in rapid tyrosine phosphorylation of the CD3 epsilon and delta chains. To examine the function of the CD3 epsilon chain in particular, we constructed a chimera that fused the extracellular and transmembrane domains of CD8 to the cytoplasmic domain of CD3 epsilon. This chimera demonstrated that CD3 epsilon was independently capable of associating with proteins having tyrosine kinase activity, including ZAP-70. Our results show that the kinase activity that associates with the CD3 complex has characteristics that are quite similar to the previously characterized zeta-associated kinase activity. This finding suggests that both these components of the TCR initiate signaling events using a common mechanism. However, differences in their signaling function could result from recognition of distinct substrates.

A novel disulfide-linked heterodimer on pre-T cells consists of the T cell receptor beta chain and a 33 kd glycoprotein

We describe a novel signal-transducing protein complex, which consists of the T cell receptor (TCR) beta chain that is disulfide linked to a 33 kd glycoprotein and noncovalently associated with proteins of the CD3 complex on the surface of the pre-T cell line SCB.29. This 33 kd glycoprotein, provisionally designated gp33, represents neither of the known TCR chains and has escaped previous detection because it labels poorly by surface iodination. This glycoprotein is absent from the surface of mature T cell lines. A TCR beta complex with identical molecular masses before and after reduction can be immunoprecipitated from surface-iodinated large thymocytes of TCR alpha-deficient mice. The novel gp33-TCR beta complex may be entirely or partly responsible for control of early T cell development exerted by the TCR beta protein.

Protein tyrosine kinase p56lck controls allelic exclusion of T-cell receptor beta-chain genes

During T-cell development, site-specific DNA rearrangements mediating assembly of beta- and alpha-chain genes of the T-cell receptor (TCR) are developmentally ordered. In particular, assembly and expression of a complete beta-chain gene blocks further rearrangements at the beta-locus (a process referred to as allelic exclusion) and drives the generation and expansion of CD4+8+ cells. Although the mechanism used by TCR beta chains to deliver such signals is unknown, studies in transgenic animals have suggested that the lymphocyte-specific protein tyrosine kinase p56lck may impinge on a similar signalling pathway. The hypothesis that TCR beta chains deliver intracellular signals via p56lck makes an explicit prediction: that interference with p56lck function will mitigate the effects of a simultaneously expressed TCR beta chain. Here we confirm this prediction through examination of allelic exclusion in mice expressing both a functional TCR beta chain transgene and a catalytically inactive form of p56lck.

The cytoplasmic tail of the T cell receptor zeta chain is dispensable for antigen-mediated T cell activation

The T cell antigen receptor consists of an antigen-binding alpha beta heterodimer and a group of invariant polypeptides denoted CD3-gamma, CD3-delta, CD3-epsilon and CD3-zeta. Whether antigen responsiveness is dependent on the expression of functional CD3-zeta subunit remains controversial. For instance, transfection of a zeta-/eta- variant of the 2B4.11. T cell hybridoma with mutated zeta cDNA that encoded a zeta protein truncated at residue 108, restored the surface expression of T cell antigen receptor complexes with, however, impaired antigen responsiveness [Frank, S. J., Niklinska, B. B., Orloff, D. G., Mercep, M., Ashwell, J. D. and Klausner, R. D., Science 1990. 249: 174.]. In marked contrast, BW5147 transfectants that expressed T cell antigen receptors devoid of functional zeta subunits were still able to trigger the production of interleukin-2 in response to antigen [Wegener, A.-M. K., Letourneur, F., Hoeveler, A., Brocker, T., Luton, F. and Malissen, B., Cell 1992. 68: 83.]. To assess if the above discrepancies may have resulted from the use of different recipient T cells, we transfected a zeta/eta-deficient variant of 2B4.11 (MA5.8) with the very same truncated zeta cDNA we previously used in BW5147. Consistent with our initial observations in BW5147, the cytoplasmic tail of the zeta polypeptide was found dispensable for antigenic responsiveness. Furthermore, a difference between the two recipient T cells was detected when cells were challenged via the Thy-1 and Ly-6 molecules. Once expressed in MA5.8, but not in BW5147, T cell antigen receptor complexes devoid of functional zeta subunits were able to sustain activation initiated via Thy-1 and Ly-6 molecules.

Multiple rearrangements in T cell receptor alpha chain genes maximize the production of useful thymocytes

Peripheral T lymphocytes each express surface T cell receptor (TCR) alpha and beta chains of a single specificity. These are produced after random somatic rearrangements in TCR alpha and beta germline genes. Published model systems using mice expressing TCR alpha and/or beta chain transgenes have shown that allelic exclusion occurs conventionally for TCR-beta. TCR alpha chain expression, however, appears to be less strictly regulated, as endogenous TCR alpha chains are often found in association with transgenic TCR beta chains in TCR alpha/beta transgenic mice. This finding, coupled with the unique structure of the TCR alpha locus, has led to the suggestion that unlike TCR beta and immunoglobulin heavy chain genes, TCR alpha genes may make multiple rearrangements on each chromosome. In the current study, we demonstrate that the majority of TCR-, noncycling thymocytes spontaneously acquire surface expression of CD3/TCR. Further, we show that cultured immature thymocytes originally expressing specific TCR alpha and beta chains may lose surface expression of the original TCR alpha, but not beta chains. These data provide evidence that not only must multiple rearrangements occur, but that TCR alpha gene rearrangement continues even after surface expression of a TCR alpha/beta heterodimer, apparently until the recombination process is halted by positive selection, or the cell dies. Sequential rearrangement of TCR alpha chain genes facilitates enhanced production of useful thymocytes, by increasing the frequency of production of both in-frame rearrangements and positively selectable TCR alpha/beta heterodimers.

Transmembrane signalling through the T-cell-receptor-CD3 complex

Recent data support the existence of activation motifs within different subunits of the T-cell-receptor-CD3 complex. This architecture generates a receptor composed of discrete modules, each capable of being coupled to an effector pathway. Although new T-cell specific protein tyrosine kinases have recently been identified, the nature of the proximal non-receptor protein tyrosine kinase linking the T-cell receptor complex to essential signalling effectors remains unknown. Developmentally regulated differences in T-cell-receptor-CD3 assembly or stability may lead to the expression of isoforms displaying different sets of activation motifs. Whether this may be the basis of differential signalling during T-cell development is still a matter of speculation.

T cell receptor beta chain dimers on immature thymocytes from normal mice

During T cell development the T cell receptor (TCR) beta chain is expressed before the TCR alpha chain. Experiments in TCR beta transgenic severe combined immune deficiency (SCID) mice have shown that the TCR beta protein can be expressed on the cell surface of immature thymocytes in the absence of the TCR alpha chain and that the TCR beta protein controls T cell development with regard to cell number, CD4/CD8 expression and allelic exclusion of the TCR beta chain. Subsequent experiments have shown that on the surface of thymocytes from TCR beta transgenic SCID mice the TCR beta protein can be expressed in a monomeric and dimeric form whereas only the dimeric form was found on the surface of a TCR beta-transfected, immature T cell line. The results presented here show that normal thymocytes from 16-day-old fetuses likewise express only the dimeric form and that the monomeric form on the surface of thymocytes from transgenic mice results from glycosyl phosphatidylinositol linkage. Our results show for the first time that under physiological conditions a TCR beta dimer can be expressed on the cell surface without the TCR alpha chain.

T cell receptor-beta mRNA splicing: regulation of unusual splicing intermediates

The expression of functional T cell receptor-beta (TCR-beta) transcripts requires the activation of programmed DNA rearrangement events. It is not clear whether other mechanisms dictate TCR-beta mRNA levels during thymic ontogeny. We examined the potential role of RNA splicing as a regulatory mechanism. As a model system, we used an immature T cell clone, SL12.4, that transcribes a fully rearranged TCR-beta gene but essentially lacks mature 1.3-kb TCR-beta transcripts in the cytoplasm. Abundant TCR-beta splicing intermediates accumulate in the nucleus of this cell clone. These splicing intermediates result from inefficient or inhibited excision of four of the five TCR-beta introns; the only intron that is efficiently spliced is the most 5' intron, IVSL. The focal point for the regulation appears to be IVS1C beta 1 and IVS2C beta 1, since unusual splicing intermediates that have cleaved the 5' splice site but not the 3' splice site of these two introns accumulate in vivo. The block in 3' splice site cleavage is of interest since sequence analysis reveals that these two introns possess canonical splice sites. A repressional mechanism involving a labile repressor protein may be responsible for the inhibition of RNA splicing since treatment of SL12.4 cells with the protein synthesis inhibitor cycloheximide reversibly induces a rapid and dramatic accumulation of fully spliced TCR-beta transcripts in the cytoplasm, concomitant with a decline in TCR-beta pre-mRNAs in the nucleus. This inducible system may be useful for future studies analyzing the underlying molecular mechanisms that regulate RNA splicing.

Development of alpha beta T cells

Developing T cells in the thymus are subject to a screening process, through interactions with thymic stromal cells, from which T cells with appropriate T-cell receptors are selected. The recent generation of T-cell receptor transgenic mice and mice homozygous for disrupted T-cell receptor genes have now supplied tools that improve the prospect for a better understanding of the molecular mechanisms of this thymic selection process. In addition these model systems appear to indicate a role for a, not yet fully characterized, pre-T cell receptor complex in survival and further differentiation of pre-T cells.

Regulation of thymocyte development through CD3. I. Timepoint of ligation of CD3 epsilon determines clonal deletion or induction of developmental program

Several recent observations suggest that successful rearrangement of the T cell receptor (TCR) beta locus induces several important events in thymocyte maturation. Allelic exclusion is achieved by interruption of further rearrangement of the beta locus, and CD4-8- interleukin (IL)-2R+ cells enter the CD4+8+IL-2R- stage. The actual molecular events regulating this important control point are unknown, but may be related to the expression of the TCR-beta locus in immature CD4-8- thymocytes. It is not clear whether maturation is induced by intracellular appearance of TCR-beta chain or by signal transduction through an immature TCR complex on the thymocyte membrane, possibly involving TCR-beta chain homodimers and CD3. Here we show that early addition of anti-CD3 mAb to fetal thymic organ cultures induces all known events associated with the acquisition of the CD4+8+ stage. Expression of CD4 and CD8 is accelerated, IL-2R alpha is downregulated, and the cells fail to produce TCR-beta, possibly based on premature cessation of beta gene rearrangement. Upon stimulation with anti-CD3 antibodies, we see calcium mobilization in 15% of all CD4-8- thymocytes with no detectable surface TCR expression. These results suggest that functional CD3 is expressed on immature thymocytes at very low concentrations before the appearance of a complete TCR-beta chain. Ligation of CD3 at this stage may mimic the maturation signal normally generated by the immature TCR-beta homodimer-CD3 complex. The results are consistent with the notion that acquisition of the CD4+8+ stage involves signal transduction through an immature TCR complex. Later in thymocyte development, ligation of CD3 results in deletion of CD4+8+ cells. Thus, signal transduction through CD3 may result in entirely different cellular responses, depending on the stage of thymocyte differentiation. These results suggest an involvement of CD3 as a link in signal transduction for at least two different decision points in the development of a thymocyte.

T cell receptor-beta mRNA splicing: regulation of unusual splicing intermediates

The expression of functional T cell receptor-beta (TCR-beta) transcripts requires the activation of programmed DNA rearrangement events. It is not clear whether other mechanisms dictate TCR-beta mRNA levels during thymic ontogeny. We examined the potential role of RNA splicing as a regulatory mechanism. As a model system, we used an immature T cell clone, SL12.4, that transcribes a fully rearranged TCR-beta gene but essentially lacks mature 1.3-kb TCR-beta transcripts in the cytoplasm. Abundant TCR-beta splicing intermediates accumulate in the nucleus of this cell clone. These splicing intermediates result from inefficient or inhibited excision of four of the five TCR-beta introns; the only intron that is efficiently spliced is the most 5' intron, IVSL. The focal point for the regulation appears to be IVS1C beta 1 and IVS2C beta 1, since unusual splicing intermediates that have cleaved the 5' splice site but not the 3' splice site of these two introns accumulate in vivo. The block in 3' splice site cleavage is of interest since sequence analysis reveals that these two introns possess canonical splice sites. A repressional mechanism involving a labile repressor protein may be responsible for the inhibition of RNA splicing since treatment of SL12.4 cells with the protein synthesis inhibitor cycloheximide reversibly induces a rapid and dramatic accumulation of fully spliced TCR-beta transcripts in the cytoplasm, concomitant with a decline in TCR-beta pre-mRNAs in the nucleus. This inducible system may be useful for future studies analyzing the underlying molecular mechanisms that regulate RNA splicing.

Mutations in T-cell antigen receptor genes alpha and beta block thymocyte development at different stages

Analysis of mice carrying mutant T-cell antigen receptor (TCR) genes indicates that TCR-beta gene rearrangement or expression is critical for the differentiation of CD4-CD8- thymocytes to CD4+CD8+ thymocytes, as well as for the expansion of the pool of CD4+CD8+ cells. TCR-alpha is irrelevant in these developmental processes. The development of gamma delta T cells does not depend on either TCR-alpha or TCR-beta.

Thymic selection: a matter of life and death

Mice transgenic for the T-cell receptor (TCR) were instrumental to the understanding that developing alpha beta T cells undergo programmed cell death unless the TCR beta gene properly rearranges and produces receptors of appropriate specificity. The delineation of developmental pathways for 'conventional' alpha beta T cells has resulted in the recognition of a different lineage of alpha beta T cells that develop in the thymus, the function of which remains to be elucidated.

Superantigen staphylococcal enterotoxin B-induced T-helper cell activation is independent of CD4 molecules and phosphatidylinositol hydrolysis

The role of the CD4 molecule in activation of T-helper cells was examined by investigating the effect of an anti-CD4 monoclonal antibody (Leu3a) in conventional peptide antigen-specific cloned T-helper cells that are also reactive to staphylococcal enterotoxin B (SEB). These T-helper cell clones are CD4+/CD45RO+/T-cell antigen receptor beta-chain variable region 12-positive and can respond to nominal peptide antigens and SEB by proliferation in the presence of class II major histocompatibility complex-expressing accessory cells. Although antigen and SEB were comparable in their ability to induce proliferative responses, interleukin 2 (IL-2) production, and IL-2 receptor alpha-chain expression, stimulation with SEB failed to trigger phosphatidylinositol hydrolysis or a rise in the intracellular free calcium ion concentration. Leu3a treatment inhibited antigen-induced proliferative responses of T cells with concomitant suppression of IL-2 production and IL-2 receptor expression. In contrast, SEB-induced responses were unaffected by Leu3a. These findings indicate that the functional consequences of binding (ligation) of conventional antigen and of superantigen with the T-cell receptor are distinct in the context of both signal transduction pathways and participation of CD4 molecules.

CD4+8- thymocytes bearing major histocompatibility complex class I-restricted T cell receptors: evidence for homeostatic control of early stages of CD4/CD8 lineage development

During thymus development CD4+ CD8+ precursor cells differentiate into mature CD4+ and CD8+ T cells expressing T cell receptors (TCR) that recognize foreign antigens in association with major histocompatibility complex (MHC) class II or I molecules, respectively. Studies with TCR transgenic mice have shown that the accumulation of mature CD4+ and CD8+ thymocytes is strongly skewed by the MHC restriction specificity of the TCR, thus suggesting that commitment of CD4+ CD8+ precursors to the CD4 or CD8 lineage is a direct consequence of TCR/MHC interactions. However, we show here that CD4+ cells expressing an inappropriate (MHC class I-specific) TCR appear transiently in the neonatal thymus of TCR transgenic mice and can also be found in the periphery of adult TCR transgenic recombination-deficient SCID mice. These data argue that the early stages of CD4 and CD8 lineage development in the thymus are (at least in part) controlled by homeostatic mechanisms independent of appropriate TCR/MHC interactions.

Regulation of TCR alpha and beta gene allelic exclusion during T-cell development

Early in their development, most T cells become committed to the expression of one, and only one, TCR alpha beta combination. How do T cells achieve this TCR allelic exclusion? This article discusses the configuration and expression of TCR alpha and beta genes in mature T-cell lines and TCR alpha beta transgenic mice, and proposes three nonexclusive models to account for the significant occurrence of T cells with two productive alpha gene rearrangements.

A naturally occurring peptide recognized by alloreactive CD8+ cytotoxic T lymphocytes in association with a class I MHC protein

The antigenic structures that initiate T cell responses to foreign (allogeneic) cells have long attracted considerable interest. We have purified and sequenced a peptide from mouse spleen that is recognized in association with the class I MHC protein H-2Ld by 2C, an alloreactive CD8+ T cell clone. The peptide (LSP-FPFDL) greatly enhances the susceptibility of Ld+ cells to lysis by 2C, and this activity is completely blocked by a clonotypic antibody against the 2C T cell receptor. Thus, this study characterizes the naturally occurring peptide moiety of an MHC-I/peptide complex recognized by alloreactive CD8+ T cells. The peptide, which occurs in the thymus of MHC-disparate mice, can be used to study T cell development in mice expressing transgenes for the 2C T cell receptor.

Generation of the alpha beta T-cell repertoire

The sequence of events leading to a diverse and competent alpha beta T-cell repertoire has been known in outline form for some time. Details continue to be sketched in, however, and some of these suggest previously unsuspected influences on repertoire content.

The development of functionally responsive T cells

The work reviewed in this article separates T cell development into four phases. First is an expansion phase prior to TCR rearrangement, which appears to be correlated with programming of at least some response genes for inducibility. This phase can occur to some extent outside of the thymus. However, the profound T cell deficit of nude mice indicates that the thymus is by far the most potent site for inducing the expansion per se, even if other sites can induce some response acquisition. Second is a controlled phase of TCR gene rearrangement. The details of the regulatory mechanism that selects particular loci for rearrangement are still not known. It seems that the rearrangement of the TCR gamma loci in the gamma delta lineage may not always take place at a developmental stage strictly equivalent to the rearrangement of TCR beta in the alpha beta lineage, and it is not clear just how early the two lineages diverge. In the TCR alpha beta lineage, however, the final gene rearrangement events are accompanied by rapid proliferation and an interruption in cellular response gene inducibility. The loss of conventional responsiveness is probably caused by alterations at the level of signaling, and may be a manifestation of the physiological state that is a precondition for selection. Third is the complex process of selection. Whereas peripheral T cells can undergo forms of positive selection (by antigen-driven clonal expansion) and negative selection (by abortive stimulation leading to anergy or death), neither is exactly the same phenomenon that occurs in the thymic cortex. Negative selection in the cortex appears to be a suicidal inversion of antigen responsiveness: instead of turning on IL-2 expression, the activated cell destroys its own chromatin. The genes that need to be induced for this response are not yet identified, but it is unquestionably a form of activation. It is interesting that in humans and rats, cortical thymocytes undergoing negative selection can still induce IL-2R alpha expression and even be rescued in vitro, if exogenous IL-2 is provided. Perhaps murine thymocytes are denied this form of rescue because they shut off IL-2R beta chain expression at an earlier stage or because they may be uncommonly Bcl-2 deficient (cf. Sentman et al., 1991; Strasser et al., 1991). Even so, medullary thymocytes remain at least partially susceptible to negative selection even as they continue to mature.(ABSTRACT TRUNCATED AT 400 WORDS)