Activated CD8 binding to class I protein mediated by the T-cell receptor results in signalling

The CD8 glycoprotein of T cells bind nonpolymorphic regions of class I major histocompatibility complex proteins on target cells and these interactions promote antigen recognition and signalling by the T-cell receptor. Studies using artificial membranes indicated that effective CD8/class I interaction is critical for response by alloantigen-specific cytotoxic T lymphocytes when class I protein is the only ligand on the antigen-bearing surface. But significant CD8-mediated binding of cytotoxic T lymphocytes to non-antigenic class I protein could not be detected in the absence of the alloantigen. These apparently contradictory findings indicate that CD8 binding to class I protein might be activated through the T-cell receptor and the results reported here demonstrate that this is the case. Treatment of cytotoxic T lymphocytes with soluble anti-T-cell receptor antibody activates adhesion of the cytotoxic T lymphocytes to class I, but not class II proteins. The specificity of this binding implies that it is mediated by CD8 and blocking by anti-CD8 antibodies confirmed this. Furthermore, binding of CD8 to class I protein resulted in generation of an additional signal(s) necessary to initiate response at low T-cell receptor occupancy levels.     

Association of tyrosine kinase p56lck with CD4 inhibits the induction of growth through the alpha beta T-cell receptor.

The membrane glycoprotein CD4 enhances antigen-mediated activation of T cells restricted by class II molecules of the major histocompatibility complex (MHC). This positive function has been attributed to the protein tyrosine kinase p56lck (ref. 4), which is noncovalently associated with the cytoplasmic portion of CD4, and is activated on CD4 aggregation. Antigen presentation by MHC class II molecules coaggregates CD4 and the T-cell antigen receptor (TCR alpha beta-CD3). Thus, the mutual specificity of CD4 and TCR alpha beta for the MHC-antigen complex results in the juxtaposition of p56lck and TCR alpha beta-CD3. In contrast, anti-CD4 antibodies can abrogate antigen-induced, as well as anti-TCR-induced T-cell activation, indicating that CD4 might also transduce negative signals. The molecular basis for this opposing function remains unclear. Here we show that the CD4-p56lck complex prohibits the induction of activation signals through the TCR-CD3 complex when not specifically included in the signalling process. This negative effect does not require anti-CD4 treatment, indicating that the induction of distinct negative signals is probably not involved. Rather, the results demonstrate that the CD4-p56lck complex provides prerequisite signals for antigen-receptor-induced T-cell growth and thus characterize a molecular mechanism for functional constraints imposed on T-cell activation by the MHC.     

Intrathymic signalling in immature CD4+CD8+ thymocytes results in tyrosine phosphorylation of the T-cell receptor zeta chain

Thymic selection of the developing T-cell repertoire occurs in immature CD4+CD8+ double-positive thymocytes and is thought to be mediated by signals transduced by T-cell antigen receptor (TCR) molecules and possibly by CD4 and CD8 accessory molecules as well. It is not known, however, which signal-transduction mechanisms function in immature CD4+CD8+ thymocytes on engagement of TCR, CD4 or CD8 molecules. In mature T cells, CD4 and CD8 molecules are each associated with the src-like protein tyrosine kinase p56 lck and signals transduced by TCR and CD4 activate tyrosine kinases that phosphorylate TCR-zeta chains and other intracellular substrates. Consequently, we examined whether tyrosine kinases could be similarly activated in immature CD4+CD8+ thymocytes. Unexpectedly, we found that TCR-zeta chains from CD4+CD8+ thymocytes were already phosphorylated in vivo, and that dephosphorylation of this TCR subunit occurred on removal of CD4+CD8+ cells from their intrathymic environment. Rephosphorylation of TCR-zeta in cultured CD4+CD8+ thymocytes occurred rapidly in vitro, either in response to cross-linking of TCR, CD4 or CD8 by specific monoclonal antibodies, or on cell-cell contact. These observations indicate that tyrosine kinases are activated in vivo in immature CD4+CD8+ thymocytes undergoing thymic differentiation and selection. They also indicate that TCR, CD4 and CD8 molecules can function in CD4+CD8+ thymocytes as signalling molecules to activate tyrosine kinases and that phosphorylated TCR-zeta serves as a marker of these signalling events.     

Major histocompatibility complex-linked specificity of gamma delta receptor-bearing T lymphocytes

Several recent studies have identified a distinct subset of CD3(T3)+CD4-CD8-T lymphocytes that express a CD3-associated heterodimer made up of the protein encoded by the T-cell receptor (TCR) gamma-gene and a second glycoprotein termed TCR delta (refs 1-4). TCR gamma delta is expressed on CD3+ thymocytes during fetal ontogeny before the appearance of TCR alpha-beta (alpha beta) (refs 5-7), on CD3+CD4-CD8- adult thymocytes, and on a subset (1-10%) of CD3+ cells in adult peripheral lymphoid organs and the peripheral blood. TCR gamma delta-expressing T cells probably represent a distinct mature T-cell lineage with the capacity to proliferate in response to receptor-mediated signals, and to display non-major histocompatibility complex (MHC)-restricted cytolysis. Critical to understanding the function of this T-cell subset is the identification of the ligand(s) recognized by TCR gamma delta. Here we describe an alloreactive CD3+CD4-CD8-TCR gamma delta-expressing, TCR alpha beta-negative, T-cell line that manifests MHC-linked recognition specificity for both proliferation and cytotoxicity. Our results suggest that T cells expressing TCR gamma delta are capable of self-non-self MHC discrimination and that they can undergo MHC-influenced selection during differentiation like TCR alpha beta-expressing T cells.     

Structural basis for co-stimulation by the human CTLA-4/B7-2 complex

Regulation of T-cell activity is dependent on antigen-independent co-stimulatory signals provided by the disulphide-linked homodimeric T-cell surface receptors, CD28 and CTLA-4 (ref. 1). Engagement of CD28 with B7-1 and B7-2 ligands on antigen-presenting cells (APCs) provides a stimulatory signal for T-cell activation, whereas subsequent engagement of CTLA-4 with these same ligands results in attenuation of the response. Given their central function in immune modulation, CTLA-4- and CD28-associated signalling pathways are primary therapeutic targets for preventing autoimmune disease, graft versus host disease, graft rejection and promoting tumour immunity. However, little is known about the cell-surface organization of these receptor/ligand complexes and the structural basis for signal transduction. Here we report the 3.2-A resolution structure of the complex between the disulphide-linked homodimer of human CTLA-4 and the receptor-binding domain of human B7-2. The unusual dimerization properties of both CTLA-4 and B7-2 place their respective ligand-binding sites distal to the dimer interface in each molecule and promote the formation of an alternating arrangement of bivalent CTLA-4 and B7-2 dimers that extends throughout the crystal. Direct observation of this CTLA-4/B7-2 network provides a model for the periodic organization of these molecules within the immunological synapse and suggests a distinct mechanism for signalling by dimeric cell-surface receptors.     

Dampening of death pathways by schnurri-2 is essential for T-cell development

Generation of a diverse and self-tolerant T-cell repertoire requires appropriate interpretation of T-cell antigen receptor (TCR) signals by CD4(+?) CD8(+) double-positive thymocytes. Thymocyte cell fate is dictated by the nature of TCR-major-histocompatibility-complex (MHC)-peptide interactions, with signals of higher strength leading to death (negative selection) and signals of intermediate strength leading to differentiation (positive selection). Molecules that regulate T-cell development by modulating TCR signal strength have been described but components that specifically define the boundaries between positive and negative selection remain unknown. Here we show in mice that repression of TCR-induced death pathways is critical for proper interpretation of positive selecting signals in vivo, and identify schnurri-2 (Shn2; also known as Hivep2) as a crucial death dampener. Our results indicate that Shn2(-/-) double-positive thymocytes inappropriately undergo negative selection in response to positive selecting signals, thus leading to disrupted T-cell development. Shn2(-/-) double-positive thymocytes are more sensitive to TCR-induced death in vitro and die in response to positive selection interactions in vivo. However, Shn2-deficient thymocytes can be positively selected when TCR-induced death is genetically ablated. Shn2 levels increase after TCR stimulation, indicating that integration of multiple TCR-MHC-peptide interactions may fine-tune the death threshold. Mechanistically, Shn2 functions downstream of TCR proximal signalling compenents to dampen Bax activation and the mitochondrial death pathway. Our findings uncover a critical regulator of T-cell development that controls the balance between death and differentiation.     

Regulatory properties of LFA-1 alpha and beta chains in human T-lymphocyte activation

Lymphocyte function-associated antigen-1 (LFA-1) is a heterodimer composed of an alpha and beta chain that is expressed on the surface of most leukocytes and is an essential molecule for adhesion reactions between cells participating in the immune response. A putative ligand for LFA-1 is the intercellular adhesion molecule ICAM-1 (refs 3-5). Leukocyte adhesion abnormality is found in patients with LFA-1 deficiency. It is not clear whether binding of ligand to the LFA-1 molecule merely spatially orientates cells towards each other or can also induce signals that regulate cell activation and differentiation. We have recently developed a T-cell proliferation assay which uses immobilized anti-CD3 monoclonal antibodies as stimulant and is independent of LFA-1-mediated cellular adhesion. As there is no interference by anti-LFA-1 monoclonal antibodies with the adhesion-dependent activation steps, this T-cell activation system allows us to investigate whether transmembrane signals are induced by binding of ligand to LFA-1 on T cells. Our data indicate that binding of ligand to LFA-1 results in the transduction of regulatory signal across the plasma membrane, rather like other molecules (CD2, CD4, CD8) (refs 8-11) with signal-modifying properties involved in the adhesion of T cells to target/stimulator cells. Indeed, adhesion molecules might generally be important in signal transduction, even in cells not belonging to the immune system.     

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

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

Signal for T-cell differentiation to a CD4 cell lineage is delivered by CD4 transmembrane region and/or cytoplasmic tail.

Mature T cells express either CD4 or CD8 on their surface. Most helper T cells express CD4, which binds to class II major histocompatibility complex (MHC) proteins, and most cytotoxic T cells express CD8, which binds to class I MHC proteins. In the thymus, mature CD4+CD8- and CD4-CD8+ T cells expressing alpha beta T-cell antigen receptors (TCR) develop from immature thymocytes through CD4+CD8+ alpha beta TCR+ intermediates. Experiments using mice transgenic for alpha beta TCR suggest that the specificity of the TCR determines the CD4/CD8 phenotype of mature T cells. These results, however, do not indicate how a T cell differentiates into the CD4 or CD8 lineage. Here we show that the CD4 transmembrane region and/or cytoplasmic tail mediates the delivery of a specific signal that directs differentiation of T cells to a CD4 lineage. We generated transgenic mice expressing a hybrid molecule composed of the CD8 alpha extracellular domains linked to the CD4 transmembrane region and cytoplasmic tail. We predicted that this hybrid molecule would bind to class I MHC proteins through the extracellular domains but deliver the intracellular signals characteristic of CD4. By crossing our transgenic mice with mice expressing a transgenic alpha beta TCR specific for a particular antigen plus class I MHC protein, we were able to express the hybrid molecule in developing thymocytes expressing the class I MHC-restricted TCR. Our results show that the signal transduced by the hybrid molecule results in the differentiation of immature thymocytes expressing a class I-restricted TCR into mature T cells expressing CD4.     

Cbl-b regulates the CD28 dependence of T-cell activation

Whereas co-stimulation of the T-cell antigen receptor (TCR) and CD28 triggers T-cell activation, stimulation of the TCR alone may result in an anergic state or T-cell deletion, both possible mechanisms of tolerance induction. Here we show that T cells that are deficient in the adaptor molecule Cbl-b (ref. 3) do not require CD28 engagement for interleukin-2 production, and that the Cbl-b-null mutation (Cbl-b(-/-)) fully restores T-cell-dependent antibody responses in CD28-/- mice. The main TCR signalling pathways, such as tyrosine kinases Zap-70 and Lck, Ras/mitogen-activated kinases, phospholipase Cgamma-1 and Ca2+ mobilization, were not affected in Cbl-b(-/-) T cells. In contrast, the activation of Vav, a guanine nucleotide exchange factor for Rac1/Rho/CDC42, was significantly enhanced. Our findings indicate that Cbl-b may influence the CD28 dependence of T-cell activation by selectively suppressing TCR-mediated Vav activation. Mice deficient in Cbl-b are highly susceptible to experimental autoimmune encephalomyelitis, suggesting that the dysregulation of signalling pathways modulated by Cbl-b may also contribute to human autoimmune diseases such as multiple sclerosis.     

CD2-mediated adhesion facilitates T lymphocyte antigen recognition function

The CD2 T lymphocyte-surface glycoprotein serves to mediate adhesion between T lymphocytes and their cognate cellular partners which express the specific ligand LFA-3. In addition, CD2 by itself or in conjunction with T-cell receptor stimulation, transduces signals resulting in T-lymphocyte activation. One or both of these functions seems to be physiologically important, given that certain anti-CD2 monoclonal antibodies block T-cell activation and that antigen-responsive memory T cells express a high level of CD2 relative to virgin T cells, which are largely antigen-unresponsive. Nevertheless, the contribution of the individual CD2 functions in T-cell responses has not been independently examined. To this end, human CD2 complementary DNAs encoding an intact LFA-3-binding adhesion domain, but lacking a functional cytoplasmic signal transduction element (CD2trans-), were introduced into an ovalbumin-specific, I-Ad restricted murine T-cell hybridoma. The antigen-specific response of T hybridoma cells expressing human CD2trans- protein was enhanced up to 400% when the human LFA-3 ligand was introduced into the I-Ad expressing murine antigen-presenting cells. In contrast, no augmentation was observed if human LFA-3 was absent or expressed on a third-party cell lacking the I-Ad restriction element. These results directly demonstrate the functional significance of adhesion events mediated between CD2 on the antigen-responsive T lymphocyte and LFA-3 on the presenting cell in optimizing antigen-specific T-cell activation.     

The duration of antigen receptor signalling determines CD4+ versus CD8+ T-cell lineage fate

Signals elicited by binding of the T-cell antigen receptor and the CD4/CD8 co-receptor to major histocompatibility complex (MHC) molecules control the generation of CD4+ (helper) or CD8+ (cytotoxic) T cells from thymic precursors that initially express both co-receptor proteins. These precursors have unique, clonally distributed T-cell receptors with unpredictable specificity for the self-MHC molecules involved in this differentiation process. However, the mature T cells that emerge express only the CD4 (MHC class II-binding) or CD8 (MHC class I-binding) co-receptor that complements the MHC class-specificity of the T-cell receptor. How this matching of co-receptor-defined lineage and T-cell-receptor specificity is achieved remains unknown, as does whether signalling by the T-cell receptors, co-receptors and/or general cell-fate regulators such as Notch-1 contributes to initial lineage choice, to subsequent differentiation processes or to both. Here we show that the CD4 versus CD8 lineage fate of immature thymocytes is controlled by the co-receptor-influenced duration of initial T-cell receptor-dependent signalling. Notch-1 does not appear to be essential for this fate determination, but it is selectively required for CD8+ T-cell maturation after commitment directed by T-cell receptors. This indicates that the signals constraining CD4 versus CD8 lineage decisions are distinct from those that support subsequent differentiation events such as silencing of co-receptor loci.     

The CD2 antigen associates with the T-cell antigen receptor CD3 antigen complex on the surface of human T lymphocytes

T lymphocytes can be activated by various stimuli directed either against the T-cell antigen receptor-CD3 antigen complex (Ti-CD3) or the CD2 molecule; see ref. 1 for a review. Activation signals generated by antigen binding to the antigen-specific alpha/beta heterodimer (Ti) are thought to be transduced via the invariant CD3 gamma, epsilon and delta chains, and the associated zeta and eta subunits. The physiological role of the interaction of CD2 with its homologous cell-surface associated ligand LFA-3 remains to be fully elucidated. It has been suggested that CD2 regulates an antigen-independent pathway of activation or that signals delivered via CD2 are an integral part of the antigen-specific pathway. Several recent studies have indicated a requirement for the Ti-CD3 complex in CD2 signalling. Thus, mutant T-cell lines expressing CD2, but not Ti-CD3, on the cell surface cannot be activated via the CD2 molecules. Functional interaction between the Ti-CD3 complex and the CD2 antigen suggests that these T-lymphocyte cell-surface structures are physically associated. Here we use a digitonin-based solubilization procedure to explore this possibility and show that 40% of the cell-surface CD2 molecules can be specifically co-precipitated in association with the Ti-CD3 complex.     

Normal development and function of CD8+ cells but markedly decreased helper cell activity in mice lacking CD4

T cells express T-cell antigen receptors (TCR) for the recognition of antigen in conjunction with the products of the major histocompatibility complex. They also express two key surface coreceptors, CD4 and CD8, which are involved in the interaction with their ligands. As CD4 is expressed on the early haemopoietic progenitor as well as the early thymic precursor cells, a role for CD4 in haemopoiesis and T-cell development is implicated. Thymocytes undergo a series of differentiation and selection steps to become mature CD4+8- or CD4-8+ (single positive) T cells. Studies of the role of CD4+ T cells in vivo have been based on adoptive transfer of selected or depleted lymphocytes, or in vivo treatment of thymectomized mice with monoclonal antibodies causing depletion of CD4+ T cells. In order to study the role of the CD4 molecule in the development and function of lymphocytes, we have disrupted the CD4 gene in embryonic stem cells by homologous recombination. Germ-line transmission of the mutation produces mutant mouse strains that do not express CD4 on the cell surface. In these mice, the development of CD8+ T cells and myeloid components is unaltered, indicating that expression of CD4 on progenitor cells and CD4+ CD8+ (double positive) thymocytes is not obligatory. Here we report that these mice have markedly decreased helper cell activity for antibody responses, although cytotoxic T-cell activity against viruses is in the normal range. This differential requirement for CD4+ helper T cells is important to our understanding of immune disorders including AIDS, in which CD4+ cells are reduced or absent.     

Negative regulation of T-cell activation and autoimmunity by Mgat5 N-glycosylation

T-cell activation requires clustering of a threshold number of T-cell receptors (TCRs) at the site of antigen presentation, a number that is reduced by CD28 co-receptor recruitment of signalling proteins to TCRs. Here we demonstrate that a deficiency in beta1,6 N-acetylglucosaminyltransferase V (Mgat5), an enzyme in the N-glycosylation pathway, lowers T-cell activation thresholds by directly enhancing TCR clustering. Mgat5-deficient mice showed kidney autoimmune disease, enhanced delayed-type hypersensitivity, and increased susceptibility to experimental autoimmune encephalomyelitis. Recruitment of TCRs to agonist-coated beads, TCR signalling, actin microfilament re-organization, and agonist-induced proliferation were all enhanced in Mgat5-/- T cells. Mgat5 initiates GlcNAc beta1,6 branching on N-glycans, thereby increasing N-acetyllactosamine, the ligand for galectins, which are proteins known to modulate T-cell proliferation and apoptosis. Indeed, galectin-3 was associated with the TCR complex at the cell surface, an interaction dependent on Mgat5. Pre-treatment of wild-type T cells with lactose to compete for galectin binding produced a phenocopy of Mgat5-/- TCR clustering. These data indicate that a galectin-glycoprotein lattice strengthened by Mgat5-modified glycans restricts TCR recruitment to the site of antigen presentation. Dysregulation of Mgat5 in humans may increase susceptibility to autoimmune diseases, such as multiple sclerosis.     

The structural basis for autonomous dimerization of the pre-T-cell antigen receptor

The pre-T-cell antigen receptor (pre-TCR), expressed by immature thymocytes, has a pivotal role in early T-cell development, including TCR β-selection, survival and proliferation of CD4(-)CD8(-) double-negative thymocytes, and subsequent αβ T-cell lineage differentiation. Whereas αβTCR ligation by the peptide-loaded major histocompatibility complex initiates T-cell signalling, pre-TCR-induced signalling occurs by means of a ligand-independent dimerization event. The pre-TCR comprises an invariant α-chain (pre-Tα) that pairs with any TCR β-chain (TCRβ) following successful TCR β-gene rearrangement. Here we provide the basis of pre-Tα-TCRβ assembly and pre-TCR dimerization. The pre-Tα chain comprised a single immunoglobulin-like domain that is structurally distinct from the constant (C) domain of the TCR α-chain; nevertheless, the mode of association between pre-Tα and TCRβ mirrored that mediated by the Cα-Cβ domains of the αβTCR. The pre-TCR had a propensity to dimerize in solution, and the molecular envelope of the pre-TCR dimer correlated well with the observed head-to-tail pre-TCR dimer. This mode of pre-TCR dimerization enabled the pre-Tα domain to interact with the variable (V) β domain through residues that are highly conserved across the Vβ and joining (J) β gene families, thus mimicking the interactions at the core of the αβTCR's Vα-Vβ interface. Disruption of this pre-Tα-Vβ dimer interface abrogated pre-TCR dimerization in solution and impaired pre-TCR expression on the cell surface. Accordingly, we provide a mechanism of pre-TCR self-association that allows the pre-Tα chain to simultaneously 'sample' the correct folding of both the V and C domains of any TCR β-chain, regardless of its ultimate specificity, which represents a critical checkpoint in T-cell development. This unusual dual-chaperone-like sensing function of pre-Tα represents a unique mechanism in nature whereby developmental quality control regulates the expression and signalling of an integral membrane receptor complex.     

Tyrosine phosphatase CD45 is essential for coupling T-cell antigen receptor to the phosphatidyl inositol pathway

Stimulation of T lymphocytes through their antigen receptor (T-cell receptor; TCR) results in the activation of a tyrosine kinase and the generation of phosphatidyl inositol (PtdIns)-derived second messengers. Several reports have indicated that CD45, a haematopoietic cell-specific surface glycoprotein with tyrosine phosphatase activity in its cytoplasmic domain, is important in lymphocyte activation. To examine the possibility that CD45 might influence proximal signal transduction events through the TCR, we have isolated a variant of the human T-cell leukaemic line, HPB-ALL, which fails to express this phosphatase. Unlike cells expressing CD45, stimulation of the TCR in the CD45-negative cell does not result in PtdIns-derived second messengers. Reconstitution of CD45 expression restored early signalling events through the TCR. To localize the site of CD45 action, the human muscarinic type 1 receptor, which also activates the PtdIns second messenger pathway, was transfected into the CD45-negative cell. Although stimulation of the TCR failed to generate PtdIns-derived second messengers, there was normal activity of the PtdIns pathway when human muscarinic receptor type 1 was stimulated, despite the absence of CD45. These data indicate that CD45 influences a cellular component that is essential for effective coupling of the TCR to the PtdIns second messenger pathway.     

Crystal structure of the B7-1/CTLA-4 complex that inhibits human immune responses

Optimal immune responses require both an antigen-specific and a co-stimulatory signal. The shared ligands B7-1 and B7-2 on antigen-presenting cells deliver the co-stimulatory signal through CD28 and CTLA-4 on T cells. Signalling through CD28 augments the T-cell response, whereas CTLA-4 signalling attenuates it. Numerous animal studies and recent clinical trials indicate that manipulating these interactions holds considerable promise for immunotherapy. With the consequences of these signals well established, and details of the downstream signalling events emerging, understanding the molecular nature of these extracellular interactions becomes crucial. Here we report the crystal structure of the human CTLA-4/B7-1 co-stimulatory complex at 3.0 A resolution. In contrast to other interacting cell-surface molecules, the relatively small CTLA-4/B7-1 binding interface exhibits an unusually high degree of shape complementarity. CTLA-4 forms homodimers through a newly defined interface of highly conserved residues. In the crystal lattice, CTLA-4 and B7-1 pack in a strikingly periodic arrangement in which bivalent CTLA-4 homodimers bridge bivalent B7-1 homodimers. This zipper-like oligomerization provides the structural basis for forming unusually stable signalling complexes at the T-cell surface, underscoring the importance of potent inhibitory signalling in human immune responses.     

The T lymphocyte glycoprotein CD2 binds the cell surface ligand LFA-3

CD2 (known also as T11 (ref. 1), LFA-2 (ref. 2) and the erythrocyte rosette receptor (ref. 3] is a functionally important T lymphocyte surface glycoprotein of relative molecular mass 50,000 to 58,000 (Mr 50-58 K) which appears early in thymocyte ontogeny and is present on all mature T cells. Monoclonal antibodies to CD2 inhibit cytotoxic T-lymphocyte (CTL)-mediated killing by binding to the T lymphocyte and blocking adhesion to the target cell. Such antibodies also inhibit T helper cell responses including antigen-stimulated proliferation, interleukin-2 (IL-2) secretion, and IL-2 receptor expression. Certain combinations of monoclonal antibodies to CD2 epitopes trigger proliferation of peripheral blood T lymphocytes, cytotoxic effector function and expression of IL-2 receptors by thymocytes, resulting in thymocyte proliferation in the presence of exogenous IL-2 (ref. 11). These findings suggest that CD2 can function in signalling as well as being an adhesion molecule. To understand the role of CD2 in T-cell adhesion and activation, it is essential to define its natural ligand. Our previous observation that purified CD2 inhibits rosetting of T lymphocytes with sheep erythrocytes and can be absorbed by sheep erythrocytes suggested it also might bind with detectable affinity to human cells. We now report that CD2 binds to a cell-surface antigen known as lymphocyte function-associated antigen-3 (LFA-3) with high affinity, and can mediate adhesion of lymphoid cells via interaction with LFA-3.     

RICK/Rip2/CARDIAK mediates signalling for receptors of the innate and adaptive immune systems

The immune system consists of two evolutionarily different but closely related responses, innate immunity and adaptive immunity. Each of these responses has characteristic receptors-Toll-like receptors (TLRs) for innate immunity and antigen-specific receptors for adaptive immunity. Here we show that the caspase recruitment domain (CARD)-containing serine/threonine kinase Rip2 (also known as RICK, CARDIAK, CCK and Ripk2) transduces signals from receptors of both immune responses. Rip2 was recruited to TLR2 signalling complexes after ligand stimulation. Moreover, cytokine production in Rip2-deficient cells was reduced on stimulation of TLRs with lipopolysaccharide, peptidoglycan and double-stranded RNA, but not with bacterial DNA, indicating that Rip2 is downstream of TLR2/3/4 but not TLR9. Rip2-deficient cells were also hyporesponsive to signalling through interleukin (IL)-1 and IL-18 receptors, and deficient for signalling through Nod proteins-molecules also implicated in the innate immune response. Furthermore, Rip2-deficient T cells showed severely reduced NF-kappaB activation, IL-2 production and proliferation on T-cell-receptor (TCR) engagement, and impaired differentiation to T-helper subtype 1 (TH1) cells, indicating that Rip2 is required for optimal TCR signalling and T-cell differentiation. Rip2 is therefore a signal transducer and integrator of signals for both the innate and adaptive immune systems.