Surface expression of CD3 in the absence of T cell receptor (TcR): evidence for sorting of partial TcR/CD3 complexes in a post-endoplasmic reticulum compartment

Murine RAW Macrophages Are a Suitable Model to Study the CD3 Signaling in Myeloid Cells

In recent years, a growing body of evidence has shown the presence of a subpopulation of macrophages that express CD3, especially in the context of mycobacterial infections. Despite these findings, the function of these cells has been poorly understood. Furthermore, the low frequency of CD3+ macrophages in humans limits the study of this subpopulation. This work aimed to evaluate the expression of CD3 in a murine macrophage cell line and its potential for the study of CD3 signaling. The murine macrophage cell line RAW was used to evaluate CD3 expression at the transcriptional and protein levels and the effect of in vitro infection with the Mycobacterium bovis Bacillus Calmette-Guérin (BCG) on these. Our data showed that RAW macrophages express CD3, both the ε and ζ chains, and it is further increased at the transcriptional level after BCG infection. Furthermore, our data suggest that CD3 can be found on the cell surface and intracellularly. However, this molecule is internalized constantly, mainly after activation with anti-CD3 stimulus, but interestingly, it is stably maintained at the transcriptional level. Finally, signaling proteins such as NFAT1, c-Jun, and IKK-α are highly expressed in RAW macrophages. They may play a role in the CD3-controlled signaling pathway to deliver inflammatory cytokines such as TNF and IL-6. Our study provides evidence to support that RAW cells are a suitable model to study the function and signaling of the CD3 complex in myeloid cells.

CAR- and TRuC-redirected regulatory T cells differ in capacity to control adaptive immunity to FVIII

Regulatory T cells (Tregs) control immune responses in autoimmune disease, transplantation, and enable antigen-specific tolerance induction in protein-replacement therapies. Tregs can exert a broad array of suppressive functions through their T cell receptor (TCR) in a tissue-directed and antigen-specific manner. This capacity can now be harnessed for tolerance induction by "redirecting" polyclonal Tregs to overcome low inherent precursor frequencies and simultaneously augment suppressive functions. With the use of hemophilia A as a model, we sought to engineer antigen-specific Tregs to suppress antibody formation against the soluble therapeutic protein factor (F)VIII in a major histocompatibility complex (MHC)-independent fashion. Surprisingly, high-affinity chimeric antigen receptor (CAR)-Treg engagement induced a robust effector phenotype that was distinct from the activation signature observed for endogenous thymic Tregs, which resulted in the loss of suppressive activity. Targeted mutations in the CD3ζ or CD28 signaling motifs or interleukin (IL)-10 overexpression were not sufficient to restore tolerance. In contrast, complexing TCR-based signaling with single-chain variable fragment (scFv) recognition to generate TCR fusion construct (TRuC)-Tregs delivered controlled antigen-specific signaling via engagement of the entire TCR complex, thereby directing functional suppression of the FVIII-specific antibody response. These data suggest that cellular therapies employing engineered receptor Tregs will require regulation of activation thresholds to maintain optimal suppressive function.

Facilitating cells: Novel promoters of stem cell alloengraftment and donor-specific transplantation tolerance in the absence of GVHD

Bone marrow transplantation (BMT) is the treatment of choice for many hematological malignancies and immunopathologies. Unfortunately, success is often impeded by engraftment failure and graft-versus-host disease (GVHD). A rare bone marrow population known as the facilitating cell (FC) has been identified which facilitates stem cell engraftment and circumvents these obstacles in murine experimental models. This review discusses the identification and characterization of this rare population and provides an emerging portrait of FC origin, ontogeny and function. The promotion of durable stem cell engraftment in MHC disparate recipients, GVHD inhibition and tolerance induction by the FC suggests that future therapies in hematopoietic cell transplantation and tolerance induction for solid organ transplants may be significantly improved through the application of FC transplantation.

Use of a microgravity organ culture dish system to demonstrate the signal dampening effects of modeled microgravity during T cell development

Recently, we have shown that exposure of fetal thymus organ cultures (FTOC) to modeled microgravity (MMG) using a clinostat with a microgravity organ culture dish system (MOCDS) blocks T cell development in a manner independent of steroid stress hormones present in vivo. In this study, we describe the development of the MOCDS system, as well as its use in attempting to understand the mechanism by which T cell development is inhibited in MMG. We show that after MMG exposure FTOC exhibited a significant reduction in CD4+CD8+ double positive (DP) cell production, but those DP cells which remained expressed higher levels of the T cell receptor (TCR) associated molecule, CD3. Interestingly, CD4-CD8- double negative (DN) cells expressed lower levels of CD3 on their surface. DN, as well as immature single positive (ISP) cells, also expressed reduced levels of the IL-7 receptor alpha chain (CD127). These changes in CD3 and CD127 expression were concomitantly associated with an increased production of tumor necrosis factor (TNF)-alpha. We were also able to show that addition of an exogenous signal (anti-CD3epsilon monoclonal antibody) to these cultures effectively mitigated the MMG-induced effects, suggesting that MMG-exposure causes a signal dampening effect on developing thymocytes.

Recent advances in pathogenesis and management of hypereosinophilic syndromes

Idiopathic hypereosinophilic syndrome is a largely heterogeneous disorder defined until now as persistent marked hypereosinophilia of unknown origin generally complicated by end-organ damage. Recent studies clearly indicate that many patients fulfilling the diagnostic criteria of this syndrome can now be classified as presenting one of two major disease variants: the myeloproliferative or the lymphocytic variant. Research in cellular and molecular biology has provided firm evidence for the existence of discrete hematological disorders underlying these variants, questioning the pertinence of continued reference to 'idiopathic' hypereosinophilic syndrome in such patients. Furthermore, identification of these variants has a number of prognostic and therapeutic implications that must be taken into consideration for adequate management of these patients.

Initiation of TCR signaling: regulation within CD3 dimers

The number of possible T cell activation outcomes resulting from T cell receptor (TCR) engagement suggests that the TCR is able to differentially activate a myriad of signaling pathways depending on the nature of the stimulus. The complex structural organization of the TCR itself could underlie this diversity of responses. Assembly and stoichiometric studies have helped us to shed some light on the initiation of TCR signaling. The TCR is composed of TCR and CD3 dimers. Changes in the interaction between CD3 subunits within the CD3 dimers and in the interaction of these dimers with the TCR heterodimer could be the triggering mechanism that initiates the first activation events. One of the hallmarks of these early changes in TCR conformation is the induced recruitment of the adapter protein Nck to a proline-rich sequence of the cytoplasmic tail of CD3epsilon, but there may be others. According to our most recent observations, the TCR is organized in pre-existing clusters within plasma membrane microdomains, exhibiting a complexity above and beyond that of dimer composition complexity. How the presence of TCR in clusters influences TCR avidity and propagation of TCR signals is something that has yet to be investigated.

Characterization of a newly discovered T-cell receptor beta-chain heterodimer expressed on a CD8+ bone marrow subpopulation that promotes allogeneic stem cell engraftment

The facilitating cell is a rare CD8+ bone marrow subpopulation that can enhance allogeneic hematopoietic stem cell engraftment across complete major histocompatibility complex barriers without inducing acute graft-versus-host disease. Here we describe a CD3epsilon-associated complex on the facilitating cell surface that consists of the T-cell receptor beta-chain disulfide-linked to a previously unknown 33-kilodalton glycoprotein. Provisionally called FCp33, this glycoprotein does not represent any of the known protein chains or surrogates associated with CD3-T-cell receptor beta. Expression of this CD3-T-cell receptor beta-FCp33 complex directly correlates with the facilitating cell's functional ability to enhance allogeneic stem cell engraftment in vivo.

The CD3ε Subunit of the TCR Contains Endocytosis Signals

Ligand binding to TCR induces its internalization and cell surface down-modulation. These phenomena contribute to the extinction of activation signals. Due to the multicomponent nature of the TCR-CD3 complex, its internalization may be mediated by one or several of its subunits. Although it has been reported that CD3γ and CD3δ contain endocytosis motifs involved in the internalization of the TCR-CD3 complex, other subunits could also be involved in this process. For instance, CD3ε and CDζ display amino acid sequences reminiscent of internalization motifs. To investigate whether CD3ε bears endocytosis signals, we have analyzed the internalization capacity of a panel of deletion and point mutants of CD3ε that were expressed on the cell surface independently of other TCR-CD3 subunits. Here we report that CD3ε displays endocytosis determinants. These data indicate that CD3ε could contribute to the internalization and cell surface down-regulation of TCR-CD3 complexes. Moreover, the existence of endocytosis signals in this polypeptide could serve to retrieve unassembled CD3ε subunits or partial CD3 complexes from the plasma membrane, thus restricting the expression on the cell surface to fully functional TCR-CD3 complexes.

Characterization of the region involved in CD3 pairwise interactions within the T cell receptor complex

Assembly of the six-chain T cell antigen receptor-CD3 complex takes place by pairwise interactions. Thus, CD3-epsilon interacts with either CD3-gamma or CD3-delta, and these dimers then associate with the TCR heterodimer (alpha.beta or gamma.delta) and the CD3-zeta homodimer to constitute a full complex. We have now mapped the site in CD3-epsilon responsible for the interaction with CD3-gamma and CD3-delta by analysis of a series of deletional mutants encompassing the most conserved regions. We found that the highly conserved juxtamembrane domain is mainly responsible for the interaction. Thus, deletion of this 16-amino acid extracellular sequence resulted in the inhibition of up to 95% of the CD3-epsilon/gamma interaction. A highly conserved sequence is also present in both CD3-gamma and CD3-delta, suggesting that the domain in these two chains may reciprocally be involved in the interaction with CD3-epsilon. Indeed, an immobilized synthetic peptide corresponding to the CD3-gamma sequence specifically associated to a bacterially expressed CD3-epsilon protein, suggesting the 16-amino acid domain is sufficient to promote CD3-epsilon/CD3-gamma assembly. The conservation of the motif in the CD3 chains suggest that, in addition to CD3-epsilon/CD3-gamma and CD3-epsilon/CD3-delta interactions, it may also mediate homotypic interactions. Indeed, it is shown that it mediates the formation of disulfide-linked homodimers and that the formation of homo- and heterodimers are mutually excluded. Finally, this domain contains a Cys-X-X-Cys sequence that resembles that of p56(lck), which is responsible for the interaction with the cytoplasmic tails of CD4 and CD8. Since the replacement of the two cysteines (Cys97 and Cys100) in CD3-epsilon by alanines strongly inhibited pair formation, the existence of a Cys-X-X-Cys motif involved in protein-protein interactions is suggested.

The CD3gamma chain is essential for development of both the TCRalphabeta and TCRgammadelta lineages

CD3gamma and CD3delta are the most closely related CD3 components, both of which participate in the TCRalphabeta-CD3 complex expressed on mature T cells. Interestingly, however, CD3delta does not appear to participate functionally in the pre-T-cell receptor (TCR) complex that is expressed on immature T cells: disruption of CD3delta gene expression has no effect on the developmental steps controlled by the pre-TCR. Here we report that in contrast with CD3delta, CD3gamma is an essential component of the pre-TCR. We generated mice selectively lacking expression of CD3gamma, in which expression of CD3delta, CD3epsilon, CD3zeta, pTalpha and TCRbeta remained undisturbed. Thus, all components for composing a pre-TCR are available, with the exception of CD3gamma. Nevertheless, T-cell development is severely inhibited in CD3gamma-deficient mice. The number of cells in the thymus is reduced to <1% of that in normal mice, and the large majority of thymocytes lack CD4 and CD8 and are arrested at the CD44-CD25+ double negative (DN) stage of development. Peripheral lymphoid organs are also practically devoid of T cells, with absolute numbers of peripheral T cells reduced to only 2-5% of those in normal mice. Both TCRalphabeta and TCRgammadelta lineages fail to develop effectively in CD3gamma-deficient mice, although absence of CD3gamma has no effect on gene rearrangements of the TCRbeta, delta and gamma loci. Furthermore, absence of CD3gamma results in a severe reduction in the level of TCR and CD3epsilon expression at the cell surface of thymocytes and peripheral T cells. The defect in the DN to double positive transition in mice lacking CD3gamma can be overcome by anti-CD3epsilon-mediated cross-linking. CD3gamma is thus essential for pre-TCR function.

Functions of TCR and pre-TCR subunits: lessons from gene ablation

Recent gene-targeting experiments have highlighted the existence of checkpoints that ensure that alpha beta T cells do not complete intrathymic differentiation if they have not attained certain landmark events. These 'proofreading' mechanisms operate by way of the pre-TCR and TCR complexes, which are sequentially expressed during T-cell development. These complexes are likely to signal via their associated CD3 subunits. By activating intracellular effectors, the CD3 subunits probably modulate gene expression profiles and drive the maturing alpha beta T cells through a precise developmental sequence.

Characterization of immature thymocyte lines derived from T-cell receptor or recombination activating gene 1 and p53 double mutant mice

The T-cell receptor (TCR) beta chain is instrumental in the progression of thymocyte differentiation from the CD4-CD8- to the CD4+CD8+ stage. This differentiation step may involve cell surface expression of novel CD3-TCR complexes. To facilitate biochemical characterization of these complexes, we established cell lines from thymic lymphomas originating from mice carrying a mutation in the p53 gene on the one hand and a mutation in TCR-alpha, TCR-beta, or the recombination activating gene 1 (RAG-1) on the other hand. The cell lines were CD4+CD8+ and appeared to be monoclonal. A cell line derived from a RAG-1 x p53 double mutant thymic lymphoma expressed low levels of CD3-epsilon, -gamma, and -delta on the surface. TCR-alpha x p53 double mutant cell lines were found to express complexes consisting of TCR-beta chains associated with CD3-epsilon, -gamma, and -delta chains and CD3-zeta zeta dimers. These lines will be useful tools to study the molecular structure and signal transducing properties of partial CD3-TCR complexes expressed on the surface of immature thymocytes.

Developmentally regulated expression of CD3 components independent of clonotypic T cell antigen receptor complexes on immature thymocytes

CD3 signal transducing proteins are thought to be expressed on the surface of T cells only as part of clonotypic T cell receptor (TCR) complexes. Contrary to this paradigm, the present study describes surface expression of CD3 proteins independently of clonotypic TCR complexes, but only on immature thymocytes. Such novel clonotype-independent CD3 (CIC) complexes are composed primarily of CD3 gamma epsilon and secondarily of CD3 delta epsilon heterodimers that are independent of one another and are expressed on the cell surface in association with an unknown 90-100 kD protein termed CD3-associated protein (CD3AP). CIC complexes are expressed in normal mice on early thymocytes through the CD4+CD8+ stage of development, but not on mature peripheral T cells. Furthermore, CIC complexes are expressed by both TCR- severe combined immunodeficiency (SCID) thymocytes and thymoma cell lines, in the absence of any clonotypic chains. The isolation and biochemical characterization of surface CIC complexes provides a structural basis for the signaling effects of anti-CD3 epsilon antibody treatment in early thymocyte development.

Probing immune functions in RAG-deficient mice

Inactivation of recombination activating gene (RAG)-1 or RAG-2 in mice results in the inability of developing lymphocytes to initiate V(D)J recombination, leading to the arrest of lymphocyte differentiation at a very early stage. Introduction of functionally assembled antigen-receptor genes or other potentially relevant genes into the RAG-deficient background can bypass the V(D)J recombination block and promote differentiation of the lymphocytes of RAG-deficient mice to various stages. This approach offers new means for analyzing the control of lymphocyte differentiation. In addition, generation of somatic chimeric mice by injecting mutant embryonic stem cells into the RAG-2-deficient blastocysts has also provided a powerful new method for assaying the potential roles of genes or regulatory elements in lymphocyte development or function.

CD3 components at the surface of pro-T cells can mediate pre-T cell development in vivo

Developmentally arrested pro-T cells (CD4-8-, IL-2R+, HSA++) of RAG-1-deficient mice appear to express low levels of CD3 molecules in the absence of T cell receptor (TcR) chains at their surface, while developmentally arrested pre-T cells of TcR alpha-deficient mice express low levels of a disulfide-linked TcR beta chain in association with CD3 molecules. Cross-linking of the CD3 modules on pro-T cells of RAG-1-/- mice in vivo, with either of two different CD3 epsilon-specific monoclonal antibodies, induces differentiation of these pro-T cells into pre-T cells (CD4+8+, IL-2R-, HSA+), concomitant with a rapid expansion of the thymic T cell compartment, up to 175-fold within 12 days. The same effects can be produced by introduction of a mutant TcR beta transgene lacking most of the variable domain (delta V-TcR beta) into the RAG-1-/- background. These experiments suggest that cross-linking of the CD3 modules on pro-T cells mimics the signaling function expected of the pre-TcR complex, which is found at the surface of pre-T cells prior to functional TcR alpha gene rearrangement. The variable domain of the TcR beta chain is apparently not essential for inducing these aspects of T cell development.

Restoration of early thymocyte differentiation in T-cell receptor beta-chain-deficient mutant mice by transmembrane signaling through CD3 epsilon

Thymic repertoire selection requires the expression of the alpha beta CD3 T-cell receptor (TCR) together with the coreceptors CD4 and CD8. The appearance of CD4 and CD8 on thymocytes is the hallmark of a complex maturation step, accompanied by downregulation of the interleukin 2 receptor (IL-2R) alpha chain, arrest of rearrangement (i.e., allelic exclusion) of the TCR beta-chain locus, a burst of cell divisions, and reduction in cell size. This maturation step is inhibited in TCR beta-chain-deficient mouse strains and may depend on surface expression of an immature TCR complex containing CD3 and TCR beta chains but no TCR alpha chain. Here we show that the CD4+8+ double-positive (DP) stage can be induced by treatment of fetal thymic organ cultures with anti-CD3 epsilon monoclonal antibodies in several TCR beta-chain-deficient mouse strains: severe combined immunodeficient (scid) mice, mice carrying a mutation in the recombination activating gene 1 (Rag-1), or mice carrying a deletion in the TCR beta-chain locus itself. These findings suggest that CD3 epsilon is expressed on the thymocyte surface independent of and prior to the TCR beta chain. The data are consistent with the notion that in wild-type mice the DP stage is induced by transmembrane signaling through an immature CD3-TCR beta-chain complex, which can be bypassed by crosslinking of CD3 epsilon alone.

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.

Surface expression of alternative forms of the TCR/CD3 complex

T-cell antigen receptor (TCR) heterodimers of both the alpha beta and gamma delta types are expressed at the surface of T cells only in association with a complex of invariant chains called CD3. The requirement for individual CD3 components to achieve TCR surface expression was examined by cotransfection of a non-T-cell line with TCR alpha and beta, as well as CD3 delta, epsilon, gamma, and zeta, cDNAs. Both transient and stable transfectants expressing TCR and CD3 epitopes at the cell surface were generated. By transfection of TCR and CD3 components in different combinations, the TCR chains, as well as the CD3 epsilon and zeta chains, were each shown to be essential for reconstituting surface expression. On the other hand, CD3 delta and gamma chains could be used alternatively, providing evidence for two different types of TCR/CD3 complexes.

The T cell receptor/CD3 complex and CD2 stimulate the tyrosine phosphorylation of indistinguishable patterns of polypeptides in the human T leukemic cell line Jurkat

Stimulation of the T cell receptor (TcR)/CD3 complex of Jurkat T cells with a monoclonal antibody to the CD3 epsilon chain induced the tyrosine phosphorylation of multiple polypeptides, ranging in size from 21 to 155 kDa. The protein tyrosine phosphorylation was characterized by its rapidity and its transient nature, returning to baseline levels by 60 min. Protein tyrosine kinase activity was also induced when the Jurkat T cells were stimulated with a mitogenic pair of antibodies directed against CD2. Comparison of the polypeptides which were phosphorylated on tyrosine in response to stimulation of the two receptors, by either one- or two-dimensional analysis, failed to reveal any differences. These data suggest that the TcR/CD3 complex and CD2 activated the same tyrosine kinase or kinases. A model is proposed in which CD2 functions as a signal amplifier in physiological responses to antigen/major histocompatibility complex without changing the qualitative nature of the signal generated via the TcR/CD3 complex.

Expression and function of a variant T cell receptor complex lacking CD3-gamma

A T cell line termed DIL2 has been derived from an infant with a polyclonal T cell receptor (TCR)/CD3 cell surface expression defect. Indirect immunofluorescence showed that the expression of certain TCR/CD3 epitopes (like those detected by WT31 and BMA031 monoclonals) was strongly reduced (around five-fold) on DIL2, whereas other epitopes (like those detected by SP34 and Leu4) were only around two-fold lower than in normal T cell lines. Specific immunoprecipitates of surface-radioiodinated DIL2 cells contained TCR-alpha, TCR-beta, CD3-delta, CD3-epsilon and TCR-zeta chains, but lacked CD3-gamma. This structural TCR/CD3 variant was, however, capable of transducing certain activation signals, since normal proliferation and a low but significant calcium flux was observed in DIL2 cells after engagement with specific antibodies. Our data suggest that a functional TCR/CD3 complex can be expressed on the surface of T cells in the absence of CD3-gamma.

The CD3-gamma and CD3-delta subunits of the T cell antigen receptor can be expressed within distinct functional TCR/CD3 complexes

The T cell receptor for antigen (TCR) consists of two glycoproteins containing variable regions (TCR-alpha/beta or TCR-gamma/delta) which are expressed on the cell surface in association with at least four invariant proteins (CD3-gamma, -delta, -epsilon and -zeta). CD3-gamma and CD3-delta chains are highly homologous, especially in the cytoplasmic domain. The similarity observed in their genomic organization and their proximity in the chromosome indicate that both genes arose from duplication of a single gene. Here, we provide several lines of evidence which indicate that in human and murine T cells which expressed both the CD3-gamma and CD3-delta chains on their surface, the TCR/CD3 complex consisted of a mixture of alpha beta gamma epsilon zeta and alpha beta delta epsilon zeta complexes rather than a single alpha beta gamma delta epsilon zeta complex. First, a CD3-gamma specific antibody failed to co-immunoprecipitate CD3-delta and conversely, several CD3-delta specific antibodies did not coprecipitate CD3-gamma. Secondly, analysis of a panel of human and murine T cell lines demonstrated that CD3-gamma and CD3-delta were expressed at highly variable ratios on their surface. This suggested that these chains were not expressed as a single complex. Thirdly, CD3-gamma and CD3-delta competed for binding to CD3-epsilon in transfected COS cells, suggesting that CD3-gamma and CD3-delta formed mutually exclusive complexes. The existence of these two forms of TCR/CD3 complexes could have important implications in the understanding of T cell receptor function and its role in T cell development.

Characterization of two human lymphoid cell lines producing human T-lymphotropic virus type I (HTLV-I) isolated from patients with HTLV-I-associated myelopathy or encephalopathy

Two cell lines, CNS-5 and CNS-6, were established by cocultivation of sedimented cells in cerebrospinal fluid (CSF) from two anti-human T-lymphotropic virus type I (HTLV-I) antibody-positive male patients with encephalopathy and HTLV-I-associated myelopathy, respectively, with peripheral blood mononuclear cells from a healthy seronegative female. These cell lines, possessing a normal female karyotype, revealed similar characteristics as follows; they expressed HTLV-I-related antigens, they produced C-type retrovirus particles, HTLV-I provirus genomes were integrated into their DNAs, and they had CD4+ activated T-cell markers. In addition, immunocytochemical and immunoelectron microscopic studies showed peculiar immunoreactivity of these cell lines with anti-alpha/beta T-cell antigen receptor (TCR) antibodies; beta Fl, defining beta chain epitope, was only positive in the perinuclear spaces and rough endoplasmic reticulum in some cells, and WT31, recognizing alpha/beta framework, was mostly negative, while CD3 was expressed in the majority of the cells. These facts indicate that HTLV-I-infected cells were present in CSF of these two patients, and suggest that neurological disorders associated with HTLV-I may not be restricted to myelopathy and may include brain abnormalities.

Genetic reconstitution of the T cell receptor (TcR) alpha/beta heterodimer restores the association of CD3 zeta 2 with the TcR/CD3 complex

The cell surface expression of the T cell receptor (TcR)/CD3 complex and, consequently, the functional competence of the cell is partly dependent on CD3 zeta. In its absence, a pentameric complex (TcR alpha/beta/CD3 gamma delta epsilon) is formed which is inefficiently transported to the cell surface. Reconstitution of CD3 zeta by transfection, in turn, restores the cell surface expression and function of the complex. Through the use of transfection experiments, we here provide direct evidence that the association of CD3 zeta 2 with the TcR/CD3 complex is dependent on the presence of both the TcR alpha and beta polypeptide chains. Despite wild-type levels of the CD3 zeta protein in a TcR alpha-negative mutant human T cell line, a complex was formed intracellularly which lacked CD3 zeta 2 and consisted of beta gamma delta epsilon and beta 2 gamma delta epsilon. Upon transfection of the mutant with a TcR alpha cDNA, a TcR/CD3 complex which contained CD3 zeta 2 was observed intracellularly. In contrast to the partial subcomplex on the cell surface of the untransfected cell line, the TcR/CD3 complex on the transfectant was functional as demonstrated by its ability to mobilize intracellular calcium after stimulation with a mitogenic CD3 epsilon-specific monoclonal antibody. Transient transfection studies performed in COS cell fibroblasts indicated that CD3 zeta 2 was not interacting with the TcR alpha protein alone, implying that a conformation provided by either the TcR alpha/beta heterodimer or the TcR alpha/beta/CD3 gamma delta epsilon complex was necessary for the association of CD3 zeta 2. Transfection studies performed in a TcR alpha/beta-negative murine T-T hybridoma confirmed the requirement of both the TcR alpha and beta proteins in CD3 zeta 2 binding. We conclude that the TcR alpha and beta chains harbor polypeptide sequences essential for the association of CD3 zeta 2 with the TcR/CD3 complex.

Intradecidual T lymphocytes lack immunohistochemically detectable T-cell receptors

The decidua apparently plays a major role in immune tolerance to the semiallogeneic embryo. In the present study we applied the immunohistochemical ABC method to investigate the expression of CD3, TCR alpha/beta and TCR gamma/delta molecules on intradecidual T cells with a panel of mAbs. Whereas intradecidual T cells expressed readily detectable amounts of CD3, they lacked immunohistochemically detectable amounts of either alpha/beta or gamma/delta TCR heterodimers on their cell surface. Peripheral blood smears of the same patients showed normal expression of alpha/beta and gamma/delta TCR on CD3+ T cells. The specific absence of (alpha/beta or gamma/delta) TCR molecules on the surface of intradecidual T cells could be one of the decisive factors in tolerance of the maternal immune system towards the fetus.

Structure of the T-cell antigen receptor: evidence for two CD3 epsilon subunits in the T-cell receptor-CD3 complex

The T-cell antigen receptor (TCR) consists of heterodimeric glycoproteins (TCR alpha beta or gamma delta) that demonstrate homology with immunoglobulins. Noncovalently associated with the alpha beta (or gamma delta) heterodimer are at least five nonvariant proteins (CD3-gamma, -delta, -epsilon, -zeta, and -eta), which together comprise the TCR-CD3 complex. The stoichiometry of the antigen receptor has been assumed to be either alpha beta gamma delta epsilon zeta zeta or alpha beta gamma delta epsilon zeta eta. In this paper we provide several lines of evidence that support the notion that the mature TCR-CD3 complex on the cell surface contains two CD3-epsilon polypeptide chains. Transfection of two murine T cell-T cell hybridomas with the human DNA encoding CD3-epsilon protein demonstrated that both murine and human CD3-epsilon chains were present within the same TCR-CD3 complex. Analysis of thymocytes isolated from transgenic mice that expressed high copy numbers of the human CD3-epsilon gene showed that the heterologous human CD3-epsilon subunits were coexpressed with murine CD3-epsilon in the same TCR-CD3 complex. Since CD3-epsilon was shown to form disulfide-linked homodimers both in human and murine T cells, the two CD3-epsilon subunits present in the TCR-CD3 complex were in direct contact with one another. The presence of two CD3-epsilon polypeptide chains in close proximity to one another in the TCR-CD3 complex may have important implications for its assembly and its signal transduction mechanisms.