Zeta phosphorylation without ZAP-70 activation induced by TCR antagonists or partial agonists

T cell anergy and costimulation

T lymphocytes play a key role in immunity by distinguishing self from nonself peptide antigens and regulating both the cellular and humoral arms of the immune system. Acquired, antigen-specific unresponsiveness is an important mechanism by which T cell responses to antigen are regulated in vivo. Clonal anergy is the term that describes T cell unresponsiveness at the cellular level. Anergic T cells do not proliferate or secrete interleukin (IL)-2 in response to appropriate antigenic stimulation. However, anergic T cells express the IL-2 receptor, and anergy can be broken by exogenous IL-2. Anergy can be induced by submitogenic exposure to peptide antigen in the absence of a costimulatory signal provided by soluble cytokines or by interactions between costimulatory receptors on T cells and counter-receptors on antigen-presenting cells. The molecular events that mediate the induction and maintenance of T cell anergy are the focus of this review. The molecular consequences of CD28-B7 interaction are discussed as a model for the costimulatory signal that leads to T cell activation rather than the induction of anergy.

Positive and negative regulation of T-cell activation through kinases and phosphatases

The sequence of events in T-cell antigen receptor (TCR) signalling leading to T-cell activation involves regulation of a number of protein tyrosine kinases (PTKs) and the phosphorylation status of many of their substrates. Proximal signalling pathways involve PTKs of the Src, Syk, Csk and Tec families, adapter proteins and effector enzymes in a highly organized tyrosine-phosphorylation cascade. In intact cells, tyrosine phosphorylation is rapidly reversible and generally of a very low stoichiometry even under induced conditions due to the fact that the enzymes removing phosphate from tyrosine-phosphorylated substrates, the protein tyrosine phosphatases (PTPases), have a capacity that is several orders of magnitude higher than that of the PTKs. It follows that a relatively minor change in the PTK/PTPase balance can have a major impact on net tyrosine phosphorylation and thereby on activation and proliferation of T-cells. This review focuses on the involvement of PTKs and PTPases in positive and negative regulation of T-cell activation, the emerging theme of reciprocal regulation of each type of enzyme by the other, as well as regulation of phosphotyrosine turnover by Ser/Thr phosphorylation and regulation of localization of signal components.

Role of TCR-induced extracellular signal-regulated kinase activation in the regulation of early IL-4 expression in naive CD4+ T cells

Although extracellular signal-regulated kinase (Erk) activation influences IL-4 production in various experimental systems, its role during Th differentiation is unclear. In this study, we show that Erk plays a critical role in IL-4 expression during TCR-induced Th differentiation of naive CD4(+) T cells. Stimulation of CD4(+) T cells with a high affinity peptide resulted in sustained Erk activation and Th1 differentiation. However, reduction of Erk activity led to a dramatic increase in IL-4 production and Th2 generation. Analysis of RNA and nuclear proteins of CD4(+) T cells 48 h after stimulation revealed that this was due to early IL-4 expression. Interestingly, transient Erk activation resulted in altered AP-1 DNA binding activity and the induction of an AP-1 complex that was devoid of Fos protein and consisted of Jun-Jun dimers. These data show that in the presence of a strong TCR signal, IL-4 expression can be induced in naive CD4(+) T cells by altering the strength of Erk activation. In addition, these data suggest that TCR-induced Erk activation is involved in the regulation of IL-4 expression by altering the composition of the AP-1 complex and its subsequent DNA binding activity.

TCR ligand discrimination is enforced by competing ERK positive and SHP-1 negative feedback pathways

Functional discrimination between structurally similar self and foreign antigens is a main attribute of adaptive immunity. Here we describe two feedback mechanisms in T lymphocytes that together sharpen and amplify initial signaling differences related to the quality of T cell receptor (TCR) engagement. Weakly binding ligands predominantly trigger a negative feedback loop leading to rapid recruitment of the tyrosine phosphatase SHP-1, followed by receptor desensitization through inactivation of Lck kinase. In contrast, strongly binding ligands efficiently activate a positive feedback circuit involving Lck modification by ERK, preventing SHP-1 recruitment and allowing the long-lasting signaling necessary for gene activation. The characteristics of these pathways suggest that they constitute an important part of the mechanism allowing T cells to discriminate between self and foreign ligands.

SWAP-70-like adapter of T cells, an adapter protein that regulates early TCR-initiated signaling in Th2 lineage cells

We describe the isolation of a protein, SWAP-70-like adapter of T cells (SLAT), which is expressed at high levels in thymocytes and differentiated Th2 cells. SLAT expression was upregulated in differentiating Th2 cells and downregulated in Th1 cells. Ectopic SLAT expression exerted positive or negative effects on IL-4 versus IFNgamma induction, respectively. TCR signaling induced translocation of SLAT to the immunological synapse and its association with ZAP-70 kinase. SLAT reduced the association of ZAP-70 with TCR-zeta and interfered with ZAP-70 but not Lck signaling. Consistent with these results, pharmacological inhibition of ZAP-70 also induced Th2 skewing. Thus, SLAT is a protein which plays a role in Th2 development and/or activation, perhaps by interfering with ZAP-70 signaling.

On the role of self-recognition in T cell responses to foreign antigen

The key role of the thymus in shaping the peripheral T cell receptor (TCR) repertoire has been appreciated for nearly a quarter of a century. For most of that time, a single model has dominated thinking about the physiological role of the positive selection process mediated by TCR recognition of self-peptides and major histocompatibility complex (MHC) molecules. This developmental filter was believed to populate secondary lymphoid tissues with T cells bearing receptors best able to recognize unknown foreign peptides associated with the particular allelic forms of the MHC molecules present in an individual. More recently, self-recognition has been suggested to regulate the viability of naïve T cells. Here we focus on new results indicating that a critical contribution of positive selection to host defense is insuring that each peripheral T cell can use self-recognition to (i) enhance TCR signaling sensitivity upon foreign antigen recognition and (ii) augment the clonal expansion that accompanies limiting foreign antigen display at early points in an infectious process. We also detail new insights into the intracellular signaling circuitry that underlies the effective discrimination between low- and high-quality ligands of the TCR and speculate on how this design might facilitate an additional contribution of self-recognition to T cell activation in the presence of foreign stimuli.

The formation and functions of the 21- and 23-kDa tyrosine-phosphorylated TCR zeta subunits

The interaction between the T cell receptor (TCR) and its cognate antigen/major histocompatibility complex (MHC) complex activates a cascade of intracellular protein phosphorylations within the T cell. The signals are initiated by the specific phosphorylation of two tyrosine residues located in a conserved sequence motif termed an ITAM (immune receptor-based tyrosine activation motif). There are 10 ITAMs in the TCR complex, and 6 of these ITAMs are present in the TCR zeta homodimer. Following TCR stimulation, the TCR zeta subunit forms two tyrosine-phosphorylated intermediates of 21- and 23-kDa, respectively. The dramatic and diverse biological responses of T cells are proposed to be partly regulated by the relative ratios of the 21- vs. 23-kDa phosphorylated forms of TCR zeta that are induced following TCR ligation. In this review, we describe a stepwise model of zeta phosphorylation required for the formation of these two phosphorylated derivatives. We describe the kinases and phosphatases controlling these phosphorylation processes. In addition, we present some preliminary findings from ongoing studies that discuss the contributions of each phosphorylated form of zeta on T cell development, TCR signaling, T cell anergy induction, and T cell survival.

The relationship between predicted peptide-MHC class II affinity and T-cell activation in a HLA-DRbeta1*0401 transgenic mouse model

The HLA-DRB1*0401 MHC class II molecule (DR4) is genetically associated with rheumatoid arthritis. It has been proposed that this MHC class II molecule participates in disease pathogenesis by presenting arthritogenic endogenous or exogenous peptides to CD4+ T cells, leading to their activation and resulting in an inflammatory response within the synovium. In order to better understand DR4 restricted T cell activation, we analyzed the candidate arthritogenic antigens type II collagen, human aggrecan, and the hepatitis B surface antigen for T-cell epitopes using a predictive model for determining peptide-DR4 affinity. We also applied this model to determine whether cross-reactive T-cell epitopes can be predicted based on known MHC-peptide-TCR interactions. Using the HLA-DR4-IE transgenic mouse, we showed that both T-cell proliferation and Th1 cytokine production (IFN-gamma) correlate with the predicted affinity of a peptide for DR4. In addition, we provide evidence that TCR recognition of a peptide-DR4 complex is highly specific in that similar antigenic peptide sequences, containing identical amino acids at TCR contact positions, do not activate the same population of T cells.

Administration of different antigenic forms of altered peptide ligands derived from HIV-1 RTase influences their effects on T helper cell activation

Genetic hypervariability of viruses such as HIV-1 facilitates appearance of escape mutants for immune response. HIV-1 isolates display variant epitopes, which may fail to stimulate T-lymphocyte responses or act as natural T-cell receptor antagonists, contributing to viral persistence. We evaluated the effect on epitope specific T-cell reactions of different amino acid substitutions in a residue of the 248-262 sequence of HIV-1 reverse transcriptase (peptide 23), showing variability in different viral isolates. Responses against such a determinant have been detected in long-term nonprogressive patients. The modified antigenic determinant was administered either as synthetic peptide or as recombinant protein. Our results show that certain amino acid substitutions abolished peptide binding to major histocompatibility complex (MHC); other modifications, although not affecting the formation of the MHC/peptide complex, either abrogated T-cell proliferation or exhibited an antagonistic effect. The results suggest that residue 11 of peptide 23 exhibits a double function; its alteration affects both the peptide affinity for the MHC and the MHC/peptide complex affinity for the T-cell receptor. Furthermore, we demonstrated that synthetic ligands and recombinant proteins may produce distinct functional effects, providing evidence that synthetic peptides, compared with corresponding epitopes generated by intracellular processing of recombinant proteins, may bind to the MHC groove in a different conformation.

Inefficient clustering of tyrosine-phosphorylated proteins at the immunological synapse in response to an antagonist peptide

Interactions of T cells with MHC plus peptide in the peripheral lymphoid system are important for their survival. In this study we investigated further the molecular consequences of such interactions using F5 TCR transgenic mice and peptides previously shown to induce either negative or positive selection in the thymus. Following TCR ligation with the negatively selecting agonist peptide, mature CD8(+) cells proliferated and up-regulated the activation marker CD69. Interestingly, ligation of this TCR with MHC molecules loaded with high concentrations of the positively selecting peptide also resulted in the aforementioned changes, but with slower kinetics. Analysis of the biochemical changes that occur following stimulation with these peptides showed that phosphorylation of key signaling molecules, such as ZAP-70, CD3zeta, Vav, SLP-76, LAT, and ERK-1 and 2, could be detected after exposure to agonist but not antagonist peptide. Confocal microscopy, however, revealed infrequent phosphorylation 'patches' at the site of contact between T cells and APC presenting the antagonist peptide. Our data suggest that peptides capable of inducing positive selection in the thymus can be recognized by mature T cells and cause proliferation, up-regulation of CD69 and accumulation of phosphorylated proteins at the immunological synapse with low efficiency; however no phosphorylation of signaling molecules can be detected using conventional biochemical assays.

Hierarchical signaling thresholds determine the fates of naíve T cells: partial priming leads nai;ve T cells to unresponsiveness

Differing conditions of antigen priming varying either the concentration or affinity of T cell receptor (TCR) ligands greatly alter T cell responses. Here, we demonstrate that antigen-specific CD4(+) nai;ve T cells primed with either altered peptide ligands (APLs) or a minimal concentration of antigen peptide become anergic without observable cell divisions. Transforming growth factor-beta1 (TGF-beta1) expression was induced 24h following in these stimulation conditions producing anergic cells. Productively stimulated nai;ve T cells expressed IL-2 to differentiate into T helper 1 (Th1) cells, secreting interferon-gamma (IFN-gamma) upon secondary antigen stimulation; T cells primed with an APL did not secrete either interleukin-4 (IL-4) or IFN-gamma, but expressed TGF-beta1 and Tob, a member of the anti-proliferative gene family. Therefore, T cell responses are regulated by TCR signaling depending on the extent of TCR engagement. These results suggest that partial antigen stimulation in the periphery can induce nai;ve CD4(+)T cell unresponsiveness.

Reduced self-reactivity of an autoreactive T cell after activation with cross-reactive non-self-ligand

Autoreactive CD4(+) T lymphocytes are critical to the induction of autoimmune disease, but because of the degenerate nature of T cell receptor (TCR) activation such receptors also respond to other ligands. Interaction of autoreactive T cells with other non-self-ligands has been shown to activate and expand self-reactive cells and induce autoimmunity. To understand the effect on the autoreactivity of naive cross-reactive T cells of activation with a potent nonself ligand, we have generated a TCR transgenic mouse which expresses a TCR with a broad cross-reactivity to a number of ligands including self-antigen. The activation of naive transgenic recombination activating gene (Rag)2(-)(/)(-) T cells with a potent non-self-ligand did not result in a enhancement of reactivity to self, but made these T cells nonresponsive to the self-ligand and anti-CD3, although they retained a degree of responsiveness to the non-self-ligand. These desensitized cells had many characteristics of anergic T cells. Interleukin (IL)-2 production was selectively reduced compared with interferon (IFN)-gamma. p21(ras) activity was reduced and p38 mitogen-activated protein kinase (MAPK) was relatively spared, consistent with known biochemical characteristics of anergy. Surprisingly, calcium fluxes were also affected and the anergic phenotype could not be reversed by exogenous IL-2. Therefore, activation with a hyperstimulating non-self-ligand changes functional specificity of an autoreactive T cell without altering the TCR. This mechanism may preserve the useful reactivity of peripheral T cells to foreign antigen while eliminating responses to self.

Antagonistic variant virus prevents wild-type virus-induced lethal immunopathology

Altered peptide ligands (APLs) and their antagonistic or partial agonistic character-influencing T cell activation have mainly been studied in vitro Some studies have shown APLs as a viral escape mechanism from cytotoxic CD8(+) T cell responses in vivo. However, whether infection or superinfection with a virus displaying an antagonistic T cell epitope can alter virus-host relationships via inhibiting T cell-mediated immunopathology is unclear. Here, we evaluated a recently described CD4(+) T cell escape lymphocytic choriomeningitis virus (LCMV) variant that in vitro displayed antagonistic characteristics for the major histocompatibility complex class II-restricted mutated epitope. Mice transgenic for the immunodominant LCMV-specific T helper epitope that usually succumb to wild-type LCMV-induced immunopathology, survived if they were simultaneously coinfected with antagonistic variant but not with control virus. The results illustrate that a coinfecting APL-expressing virus can shift an immunopathological virus-host relationships in favor of host survival. This may play a role in poorly cytopathic long-lasting virus carrier states in humans.

P59(fyn) is upregulated in anergic CD8+ T cells

The ultimate goal in clinical transplantation is achievement of graft tolerance. Despite long-term immunosuppression, alloantigens on transplants elicit alloresponses that can initiate organ rejection. Acute rejection is mediated by CD8(+) cytotoxic T cells, whereas chronic rejection is a result of many factors including non-immunological events. The aim of this study was to examine the molecular requirements of T cell anergy, a cellular state that is an integral component of tolerance in vivo. In vitro, the tolerant state is usually best represented by T cell anergy, which is defined by loss of the ability of T cells to produce and secrete interleukin-2 upon restimulation. In the literature, molecular changes in anergic CD4(+) T cells have been studied in great detail, but only little is known about functional and biochemical characteristics of anergic CD8(+) T lymphocytes. In this study, we demonstrate, that CD8(+) T cells are rendered anergic by TCR stimulation without costimulation. They exhibit impaired interleukin-2 production and tyrosine-phosphorylation, but markedly upregulated p59(fyn) expression, which could be shown to be an early event during anergization. Anergic CD8(+) T lymphocytes show elevated surface expression of early activation markers as well as costimulatory molecules, especially that of CTLA4. These results, are an important component for the discovery of potential molecular targets, which contribute to the development and maintenance of tolerance.

In the immune synapse, ZAP-70 controls T cell polarization and recruitment of signaling proteins but not formation of the synaptic pattern

Recognition by T cells of their ligands at the surface of antigen-presenting cells (APCs) leads to T cell activation, polarization of the T cell toward the APC, and formation of an immune synapse. Using ZAP-70-deficient T cells expressing zeta-GFP, we show that ZAP-70 signaling drives the TCR-dependent reorientation of the microtubule-organizing center thus leading to relocation of a zeta-GFP(+) intracellular compartment close to the APC. ZAP-70 is also necessary to supply the synapse with the signaling molecules PKC-theta and LAT. In contrast, ZAP-70 is not required for clustering of zeta-GFP and CD2 or exclusion of CD45 and CD43 from the synapse. These data show that ZAP-70-dependent signaling is required for formation of a functional immune synapse.

Strong induction of tyrosine phosphorylation, intracellular calcium, nuclear transcription factors and interferongamma, but weak induction of IL-2 in naïve T cells stimulated by bacterial superantigen

The outcome of T cell receptor (TCR) engagement is controlled by the differential recruitment of a variety of pathways, depending on the nature of the TCR ligand. Studies on superantigens (SAGs) were among the first describing such differential signaling; however, reported results are inconsistent. We took a quantitative approach to reinvestigate this question. Using nai;ve T cells from TCR transgenic mice, we found that compared to the antigenic peptide from pigeon cytochrome c, the SAG staphylococcal enterotoxin A very efficiently (100-2000-fold more sensitive on a weight basis) induced tyrosine kinase activity, intracellular calcium increase, and interferon (IFN)gamma production. Up-regulation of CD25 and CD69 and proliferation were less efficiently induced (20-30-fold more sensitive), and interleukin (IL)-2 production was induced least efficiently (only 2-fold more sensitive). This differential activation profile that varies with the activation event analyzed is discussed with respect to the propensity for SAG to induce anergy.

Truncation of the neuritogenic peptide bP2(60-70) results in the generation of altered peptide ligands with the potential to interfere with T cell activation

Due to the central role of T cells in the pathogenesis of inflammatory diseases of the peripheral nervous system like the Guillain-Barré syndrome, specific immunotherapies aim at modifying T cell responses. Use of truncated mutants of the neuritogenic peptide of myelin basic protein (MBP) has been shown to anergize autoreactive T cells and to reverse experimental autoimmune encephalitis (EAE). To establish a rationale basis for the use of altered peptide ligands (APLs) in the treatment of autoimmune diseases we designed a set of N- and C-terminally truncated mutants of the minimal experimental autoimmune neuritis (EAN) inducing bovine P2 (bP2) (60-70) peptide and compared them for the ability to induce immune responses and T cell receptor (TCR) cell signaling. Truncated peptides bound to MHC class II molecules and induced TCR internalization and expression of interferon gamma (IFN-gamma) and tumor necrosis factor alpha (TNF-alpha) with decreasing potency. None of the shortened mutants elicited a proliferative response in P2-specific T cells. Stimulation of these antigen-specific T cells with peptide bP2(62-69) using antigen presenting cells (APCs) prepulsed with bP2(60-70) resulted in a significant decrease of the proliferative response. In agreement with the observed effects on T cell activation, analysis of TCR signaling demonstrated a lack of CD3 epsilon phosphorylation and MAPK activation. Moreover, repeated injection of bP2(62-69) significantly slowed progression of adoptive transfer EAN (AT-EAN). Taken together, these findings strongly suggest that peptide bP2(62-69) can favorably modulate the antigen-induced response of neuritogenic T cells.

CD4-Lck through TCR and in the absence of Vav exchange factor induces Bax increase and mitochondrial damage

In the present study, we aimed to demonstrate that CD4 may represent a critical turning point that governs the apoptotic and survival programs in T cells, without modifying the physical association with the TCR-CD3 complex. To address this issue, we have explored the possibility that the activation of CD4 may transduce apoptotic signals unless signaling effectors neutralize them. Our data show that in Jurkat T cells CD4 engagement by Leu3a mAb results in a rapid and strong increase of Lck kinase activity, subsequent alterations of mitochondrial membrane potential, and apoptosis. Critical parameters are coassociation of CD4/Lck with TCR/CD3 and up-regulation of the proapoptotic protein Bax. Indeed, Leu3a-mediated Lck activation failed to induce apoptotic features in Jurkat cells either defective for TCR/CD3 or overexpressing the antiapoptotic protein Bcl-2. Furthermore, we demonstrate that Leu3a treatment of Jurkat cells overexpressing Vav results in the inhibition of mitochondrial damage and apoptosis; this rescue effect is accompanied with a significant decrease of Bax expression observed in apoptotic cells. Our evidence that the activation of Lck activates in T cells apoptotic pathways which are counteracted by Vav, a signaling molecule that cooperates with CD28 to boost TCR signals, suggests a novel role for costimulation in protecting T cells from CD4-mediated cell death.

Origins and functions of B-1 cells with notes on the role of CD5

Whether B-1a (CD5+) cells are a distinct lineage derived from committed fetal/neonatal precursors or arise from follicular B-2 cells in response to BCR ligation and other, unknown signals remains controversial. Recent evidence indicates that B-1a cells can derive from adult precursors expressing an appropriate specificity when the (self-) antigen is present. Antibody specificity determines whether a B cell expressing immunoglobulin transgenes has a B-2, B-1a or marginal zone (MZ) phenotype. MZ cells share many phenotypic characteristics of B-1 cells and, like them, appear to develop in response to T independent type 2 antigens. Because fetal-derived B cell progenitors fail to express terminal deoxynucleotidyl transferase (TdT) and for other reasons, they are likely to express a repertoire that allows selection into the B-1a population. As it is selected by self-antigen, the B-1 repertoire tends to be autoreactive. This potentially dangerous repertoire is also useful, as B-1 cells are essential for resistance to several pathogens and they play an important role in mucosal immunity. The CD5 molecule can function as a negative regulator of BCR signaling that may help prevent inappropriate activation of autoreactive B-1a cells.

T-cell activation through the antigen receptor. Part 2: role of signaling cascades in T-cell differentiation, anergy, immune senescence, and development of immunotherapy

Part 2 of this review on cellular activation by the T-cell antigen receptor (TCR) will highlight how TCR signaling pathways are adapted to achieve specific biologic outcomes, including different states of T-cell differentiation and the induction of T-cell tolerance. We will also explore how treatment with altered peptide ligands affects TCR signaling to change T-cell differentiation or to induce an anergy state. These changes are accomplished through alteration of protein tyrosine kinase activity, the stoichiometry of phosphorylation of immunoreceptor tyrosine-based activation motifs, intracellular free ionized calcium flux, mitogen-activated protein kinase activity, and transcriptional activation of key cytokine promoters. The CTLA-4 plays an important role in the induction and maintenance of anergy. The second theme will highlight how altered TCR signal transduction, including changes in the compartmentalization of signaling components at the TCR synapse, contributes to decreased T-cell activation during immune senescence. Finally, we will illustrate how the molecular details of TCR activation can be used to modify the function of the immune system. This includes a description of the mechanism of action of altered peptide ligands, CTLA-4Ig, and pharmacologic inhibitors of mitogen-activated protein kinases, nuclear factor kappaB, and protein kinase C cascades.

T-cell tolerance and autoimmune diabetes

Herein we describe the major signaling events that occur in T-cells upon T-cell receptor (TCR) engagement, and the mechanisms responsible for the induction of T-cell anergy that may ultimately lead to the development of immunospecific therapies in T-cell mediated autoimmune diseases. A new type of antigen presenting molecule (dimeric MHC class-II/peptide, DEF) endowed with antigen-specific immunomodulatory effects such as induction of Th2 polarization and T-cell anergy is also described as a potential antidiabetogenic agent. According to our preliminary results, the MHC II/peptide-based approach may provide rational grounds for further development of antigen-specific immunotherapeutic agents such as human-like MHC lI/peptide chimeras endowed with efficient down-regulatory effects in CD4 T-cell-mediated autoimmune diseases such as Type 1 diabetes, multiple sclerosis, primary biliary cirrhosis, and rheumatoid arthritis.

Very low affinity B cells form germinal centers, become memory B cells, and participate in secondary immune responses when higher affinity competition is reduced

To understand the relationship between the affinity of the B cell antigen receptor (BCR) and the immune response to antigen, two lines of immunoglobulin H chain transgenic (Tg) mice were created. H50Gmu(a) and T1(V23)mu(a) mice express mu H chain transgenes that associate with the lambda1 L chains to bind the (4-hydroxy-3-nitrophenyl)acetyl hapten with association constants (K(a)s) of only 1.2 x 10(5) M(-1) and 3 x 10(4) M(-1), respectively. Both lines mounted substantial antibody-forming cell (AFC) and germinal center (GC) responses. H50Gmu(a) Tg mice also generated memory B cells. T1(V23)mu(a) B cells formed AFC and GCs, but were largely replaced in late GCs by antigen-specific cells that express endogenous BCRs. Thus, B lymphocytes carrying BCRs with affinities previously thought to be irrelevant in specific immune responses are in fact capable of complete T cell-dependent immune responses when relieved of substantial competition from other B cells. The failure to observe such B cells normally in late primary responses and in memory B cell populations is the result of competition, rather than an intrinsic inability of low affinity B cells.

T-cell activation through the antigen receptor. Part 1: signaling components, signaling pathways, and signal integration at the T-cell antigen receptor synapse

Part 1 of this review will highlight the basic components and signaling pathways by which the T-cell antigen receptor (TCR) activates mature extrathymic T cells. TCR signaling commences with an early wave of protein tyrosine kinase activation, which is mediated by the Src kinases Lck and Fyn, the 70-kd zeta-associated protein kinase, and members of the Tec kinase family. This early wave of protein tyrosine phosphorylation leads to the activation of downstream signaling pathways, including an increase in intracellular free calcium, protein kinase C, nuclear factor kappaB and Ras-mitogen-activated protein kinase activation. These pathways activate transcription factors, such as activator protein 1, nuclear factor of activated T cells, and Rel proteins, which ultimately lead to the expression of genes that control cellular proliferation, differentiation, anergy, or apoptosis. This review also describes how costimulatory receptors assist in signal transduction and assembly of macromolecular complexes at the TCR contact site with the antigen-presenting cell, also known as the immune synapse. These basic concepts of TCR signal transduction will be used in part 2 to explain how T-cell function can be altered by therapeutic targeting of TCR signaling components, as well as to explain modification of TCR signaling during T(H)1/T(H)2 differentiation, tolerance, and immune senescence.

The potency of TCR signaling differentially regulates NFATc/p activity and early IL-4 transcription in naive CD4+ T cells

The potency of TCR signaling can regulate the differentiation of naive CD4(+) T cells into Th1 and Th2 subsets. In this work we demonstrate that TCR signaling by low-affinity, but not high-affinity, peptide ligands selectively induces IL-4 transcription within 48 h of priming naive CD4(+) T cells. This early IL-4 transcription is STAT6 independent and occurs before an increase in GATA-3. Furthermore, the strength of the TCR signal differentially affects the balance of NFATp and NFATc DNA binding activity, thereby regulating IL-4 transcription. Low-potency TCR signals result in high levels of nuclear NFATc and low levels of NFATp, which are permissive for IL-4 transcription. These data provide a model for how the strength of TCR signaling can influence the generation of Th1 and Th2 cells.

How and why does the immunological synapse form? Physical chemistry meets cell biology

During T lymphocyte (T cell) recognition of an antigen, a highly organized and specific pattern of membrane proteins forms in the junction between the T cell and the antigen-presenting cell (APC). This specialized cell-cell junction is called the immunological synapse. It is several micrometers large and forms over many minutes. A plethora of experiments are being performed to study the mechanisms that underlie synapse formation and the way in which information transfer occurs across the synapse. The wealth of experimental data that is beginning to emerge must be understood within a mechanistic framework if it is to prove useful in developing modalities to control the immune response. Quantitative models can complement experiments in the quest for such a mechanistic understanding by suggesting experimentally testable hypotheses. Here, a quantitative synapse assembly model is described. The model uses concepts developed in physical chemistry and cell biology and is able to predict the spatiotemporal evolution of cell shape and receptor protein patterns observed during synapse formation. Attention is directed to how the juxtaposition of model predictions and experimental data has led to intriguing hypotheses regarding the role of null and self peptides during synapse assembly, as well as correlations between T cell effector functions and the robustness of synapse assembly. We remark on some ways in which synergistic experiments and modeling studies can improve current models, and we take steps toward a better understanding of information transfer across the T cell-APC junction.

T cell receptor-mediated signal transduction controlled by the beta chain transmembrane domain: apoptosis-deficient cells display unbalanced mitogen-activated protein kinases activities upon T cell receptor engagement

The bases that support the versatility of the T cell receptor (TCR) to generate distinct T cell responses remain unclear. We have previously shown that mutant cells in the transmembrane domain of TCRbeta chain are impaired in TCR-induced apoptosis but are not affected in other functions. Here we describe the biochemical mechanisms by which this mutant receptor supports some T cell responses but fails to induce apoptosis. Extracellular signal-regulated protein kinase (ERK) is activated at higher and more sustained levels in TCRbeta-mutated than in wild type cells. Conversely, activation of both c-Jun N-terminal kinase and p38 mitogen-activated protein kinase is severely reduced in mutant cells. By attempting to link this unbalanced induction to altered upstream events, we found that ZAP-70 is normally activated. However, although SLP-76 phosphorylation is normally induced, TCR engagement of mutant cells results in lower tyrosine phosphorylation of LAT but in higher tyrosine phosphorylation of Vav than in wild type cells. The results suggest that an altered signaling cascade leading to an imbalance in mitogen-activated protein kinase activities is involved in the selective impairment of apoptosis in these mutant cells. Furthermore, they also provide new insights in the contribution of TCR to decipher the signals that mediate apoptosis distinctly from proliferation.

Tob is a negative regulator of activation that is expressed in anergic and quiescent T cells

During a search for genes that maintain T cell quiescence, we determined that Tob, a member of an anti-proliferative gene family, was highly expressed in anergic T cell clones. Tob was also expressed in unstimulated peripheral blood T lymphocytes and down-regulated during activation. Forced expression of Tob inhibited T cell proliferation and transcription of cytokines and cyclins. In contrast, suppression of Tob with an antisense oligonucleotide augmented CD3-mediated responses and abrogated the requirement of costimulation for maximal proliferation and cytokine secretion. Tob associated with Smad2 and Smad4 and enhanced Smad DNA-binding. The inhibitory effect of Tob on interleukin 2 (IL-2) transcription was not mediated by blockade of NFAT, AP-1 or NF-kappaB transactivation but by enhancement of Smad binding on the -105 negative regulatory element of the IL-2 promoter. Thus, T cell quiescence is an actively maintained phenotype that must be suppressed for T cell activation to occur.

Radically altered T cell receptor signaling in glycopeptide-specific T cell hybridoma induced by antigen with minimal differences in the glycan group

A T cell hybridoma raised against the synthetic glycopeptide T(72)(Tn) was used to study whether the initial TCR signaling events are markedly different when the hybridoma is stimulated with glycopeptides closely related to the cognate glycopeptide antigen. T(72)(Tn) has an alpha-D-GalNAc group O-linked to the central threonine in the decapeptide VITAFTEGLK, and the hybridoma is known to be highly specific for this carbohydrate group. T(72)(Tn)-pulsed APC induced tyrosine phosphorylation of the TCR-zeta 21- and 23-kDa proteins and the downstream p42/44 MAP kinase and strong IL-2 secretion. APC pulsed with T(72)(alpha-D-GlcNAc), which differs from T(72)(Tn) solely by the orientation of a hydroxy group in the carbohydrate structure, completely failed to induce detectable tyrosine phosphorylation and IL-2 secretion. APC pulsed with S(72)(Tn), which differs from T(72)(Tn) by not having a methyl group in the serine amino acid side chain to which the glycan is attached, induced partial tyrosine phosphorylation of the TCR-zeta 21-kDa protein, no tyrosine phosphorylation of the MAP kinases and no IL-2 production. Molecular modeling of the MHC/glycopeptide complex revealed that the dramatic difference between the stimulatory power of T(72)(Tn) and T(72)(alpha-D-GlcNAc) is mainly due to very small differences in the TCR exposed carbohydrate structure.

Zap-70-independent Ca(2+) mobilization and Erk activation in Jurkat T cells in response to T-cell antigen receptor ligation

The tyrosine kinase ZAP-70 has been implicated as a critical intermediary between T-cell antigen receptor (TCR) stimulation and Erk activation on the basis of the ability of dominant negative ZAP-70 to inhibit TCR-stimulated Erk activation, and the reported inability of anti-CD3 antibodies to activate Erk in ZAP-70-negative Jurkat cells. However, Erk is activated in T cells receiving a partial agonist signal, despite failing to activate ZAP-70. This discrepancy led us to reanalyze the ZAP-70-negative Jurkat T-cell line P116 for its ability to support Erk activation in response to TCR/CD3 stimulation. Erk was activated by CD3 cross-linking in P116 cells. However, this response required a higher concentration of anti-CD3 antibody and was delayed and transient compared to that in Jurkat T cells. Activation of Raf-1 and MEK-1 was coincident with Erk activation. Remarkably, the time course of Ras activation was comparable in the two cell lines, despite proceeding in the absence of LAT tyrosine phosphorylation in the P116 cells. CD3 stimulation of P116 cells also induced tyrosine phosphorylation of phospholipase C-gamma1 (PLCgamma1) and increased the intracellular Ca(2+) concentration. Protein kinase C (PKC) inhibitors blocked CD3-stimulated Erk activation in P116 cells, while parental Jurkat cells were refractory to PKC inhibition. The physiologic relevance of these signaling events is further supported by the finding of PLCgamma1 tyrosine phosphorylation, Erk activation, and CD69 upregulation in P116 cells on stimulation with superantigen and antigen-presenting cells. These results demonstrate the existence of two pathways leading to TCR-stimulated Erk activation in Jurkat T cells: a ZAP-70-independent pathway requiring PKC and a ZAP-70-dependent pathway that is PKC independent.

Multi-modal antigen specific therapy for autoimmunity

Peripheral tolerance, represents an attractive strategy to down-regulate previously activated T cells and suppress an ongoing disease. Herein, immunoglobulins (Igs) were used to deliver self and altered self peptides for efficient peptide presentation without costimulation to test for modulation of experimental allergic encephalomyelitis (EAE). Accordingly, the encephalitogenic proteolipid protein (PLP) sequence 139-151 (referred to as PLP1) and an altered form of PLP1 known as PLP-LR were genetically expressed on Igs and the resulting Ig-PLP1 and Ig-PLP-LR were tested for efficient presentation of the peptides and for amelioration of ongoing EAE. Evidence is presented indicating that Ig-PLP1 as well as Ig-PLP-LR given in saline to mice with ongoing clinical EAE suppresses subsequent relapses. However, aggregation of both chimeras allows crosslinking of Fcgamma receptors (FcgammaRs) and induction of IL-10 production by APCs but does not promote the up-regulation of costimulatory molecules. Consequently, IL-10 displays bystander suppression and synergizes with presentation without costimulation to drive effective modulation of EAE. As Ig-PLP1 is more potent than Ig-PLP-LR in the down-regulation of T cells, we conclude that peptide affinity plays a critical role in this multi-modal approach of T cell modulation.

T-cell anergy and peripheral T-cell tolerance

The discovery that T-cell recognition of antigen can have distinct outcomes has advanced understanding of peripheral T-cell tolerance, and opened up new possibilities in immunotherapy. Anergy is one such outcome, and results from partial T-cell activation. This can arise either due to subtle alteration of the antigen, leading to a lower-affinity cognate interaction, or due to a lack of adequate co-stimulation. The signalling defects in anergic T cells are partially defined, and suggest that T-cell receptor (TCR) proximal, as well as downstream defects negatively regulate the anergic T cell's ability to be activated. Most importantly, the use of TCR-transgenic mice has provided compelling evidence that anergy is an in vivo phenomenon, and not merely an in vitro artefact. These findings raise the question as to whether anergic T cells have any biological function. Studies in rodents and in man suggest that anergic T cells acquire regulatory properties; the regulatory effects of anergic T cells require cell to cell contact, and appear to be mediated by inhibition of antigen-presenting cell immunogenicity. Close similarities exist between anergic T cells, and the recently defined CD4+ CD25+ population of spontaneously arising regulatory cells that serve to inhibit autoimmunity in mice. Taken together, these findings suggest that a spectrum of regulatory T cells exists. At one end of the spectrum are cells, such as anergic and CD4+ CD25+ T cells, which regulate via cell-to-cell contact. At the other end of the spectrum are cells which secrete antiinflammatory cytokines such as interleukin 10 and transforming growth factor-beta. The challenge is to devise strategies that reliably induce T-cell anergy in vivo, as a means of inhibiting immunity to allo- and autoantigens.

The human CD8 coreceptor effects cytotoxic T cell activation and antigen sensitivity primarily by mediating complete phosphorylation of the T cell receptor zeta chain

Recognition of antigen by cytotoxic T lymphocytes (CTL) is determined by interaction of both the T cell receptor and its CD8 coreceptor with peptide-major histocompatibility complex (pMHC) class I molecules. We examine the relative roles of these receptors in the activation of human CTL using mutations in MHC class I designed to diminish or abrogate the CD8/pMHC interaction. We use surface plasmon resonance to determine that point mutation of the alpha3 loop of HLA A2 abrogates the CD8/pMHC interaction without affecting the affinity of the T cell receptor/pMHC interaction. Antigen-presenting cells expressing HLA A2 which does not bind to CD8 fail to activate CTL at any peptide concentration. Comparison of CTL activation by targets expressing HLA A2 with normal, abrogated, or diminished CD8/pMHC interaction show that the CD8/pMHC interaction enhances sensitivity to antigen. We determine that the biochemical basis for coreceptor dependence is the activation of the 23-kDa phosphoform of the CD3zeta chain. In addition, we produce mutant MHC class I multimers that specifically stain but do not activate CTL. These reagents may prove useful in circumventing undesirable activation-related perturbation of intracellular processes when pMHC multimers are used to phenotype antigen-specific CD8+ lymphocytes.

Altered functional and biochemical response by CD8+ T cells that remain after tolerance

To further define the molecular basis of tolerance to a peripherally expressed antigen we have correlated differences in functional capacity with biochemical events in hemagglutinin (HA)-specific cytotoxic T lymphocyte (CTL) clones derived either from a conventional B10.D2 mouse that is not tolerant to HA (D2 Clone 6) or from an InsHA mouse that is tolerant to HA (InsHA Clone 12). D2 Clone 6, but not InsHA Clone 12, triggers diabetes following in vivo transfer into irradiated InsHA hosts. This diabetogenic clone shows complete and sustained phosphorylation of TCR zeta chain and ZAP-70 following stimulation with HA-pulsed antigen-presenting cells. In contrast, InsHA Clone 12 showed only partial phosphorylation of TCR zeta and no phosphorylation of ZAP-70. There was no defect in activation or recruitment of Lck to the TCR complex in both the clones following stimulation with the cognate antigen. This deficiency in the proximal signaling in the InsHA Clone 12 could be overcome by increasing the strength of signal through the CD3-TCR complex, indicating that the signaling machinery of InsHA Clone 12 was functional. These data demonstrate that the HA-responsive CD8(+) T cells that can be retrieved from InsHA mice after tolerance induction respond to HA as a partial agonist/antagonist.

The impact of duration versus extent of TCR occupancy on T cell activation: a revision of the kinetic proofreading model

The widely accepted kinetic proofreading theory proposes that rapid TCR dissociation from a peptide/MHC ligand allows for stimulation of early but not late T cell activation events, explaining why low-affinity TCR ligands are poor agonists. We identified a low-affinity TCR ligand which stimulated late T cell responses but, contrary to predictions from kinetic proofreading, inefficiently induced early activation events. Furthermore, responses induced by this ligand were kinetically delayed compared to its high-affinity counterpart. Using peptide/MHC tetramers, we showed that activation characteristics could be dissociated from TCR occupancy by the peptide/MHC ligands. Our data argue that T cell responses are triggered by a cumulative signal which is reached at different time points for different TCR ligands.

Generation and biochemical analysis of human effector CD4 T cells: alterations in tyrosine phosphorylation and loss of CD3zeta expression

Human effector T cells have been difficult to isolate and characterize due to their phenotypic and functional similarity to the memory subset. In this study, a biochemical approach was used to analyze human effector CD4 T cells generated in vitro by activation with anti-CD3 and autologous monocytes for 3 to 5 days. The resultant effector cells expressed the appropriate activation/differentiation markers and secreted high levels of interferon gamma (IFN-gamma) when restimulated. Biochemically, effector CD4 T cells exhibited increases in total intracellular tyrosine phosphorylation and effector-associated phosphorylated species. Paradoxically, these alterations in tyrosine phosphorylation were concomitant with greatly reduced expression of CD3zeta and CD3epsilon signaling subunits coincident with a reduction in surface T-cell receptor (TCR) expression. Because loss of CD3zeta has also been detected in T cells isolated ex vivo from individuals with cancer, chronic viral infection, and autoimmune diseases, the requirements and kinetics of CD3zeta down-regulation were examined. The loss of CD3zeta expression persisted throughout the course of effector T-cell differentiation, was reversible on removal from the activating stimulus, and was modulated by activation conditions. These biochemical changes occurred in effector T cells generated from naive or memory CD4 T-cell precursors and distinguished effector from memory T cells. The results suggest that human effector T-cell differentiation is accompanied by alterations in the TCR signal transduction and that loss of CD3zeta expression may be a feature of chronic T-cell activation and effector generation in vivo. (Blood. 2001;97:3851-3859)

Molecular mimicry and antigen-specific T cell responses in multiple sclerosis and chronic CNS Lyme disease

The concept of molecular mimicry provides and elegant framework as to how cross-reactivity between antigens from a foreign agent with self proteins may trigger autoimmune diseases. While it was previously thought that sequence and structural homology between foreign and self proteins or the sharing of T cell receptor (TCR) and MHC-binding motifs are required for molecular mimicry to occur, we have shown that even completely unrelated peptide sequences may lead to cross-recognition by T cells. The use of synthetic combinatorial peptide libraries in the positional scanning format (PS-SCL) together with novel biometric prediction approaches has allowed us to describe the recognition profiles of individual autoreactive T cell clones (TCC) with unprecedented accuracy. Through studies of myelin-specific TCC as well as clones from the nervous system of patients suffering from chronic central nervous (CNS) Lyme disease it has become clear that at least some T cells are more degenerate than previously anticipated. These data will not only help us to redefine what constitutes specific T cell recognition, but also allow us to study in more detail the biological role of molecular mimicry. A recent clinical trial with an altered peptide ligand (APL) of one of the candidate myelin basic protein (MBP) epitopes in MS (amino acids 83-99) has shown that such a modified MBP peptide may not only have therapeutic efficacy, but also bears the potential to exacerbate disease. Thus, we provide firm evidence that the basic principles of cross-recognition and their pathogenetic significance are relevant in MS.

Prolonged exposure of T cells to TNF down-regulates TCR zeta and expression of the TCR/CD3 complex at the cell surface

A role for TNF-alpha in the pathogenesis of chronic inflammatory disease is now firmly established. Paradoxically, TNF also has potent immunomodulatory effects on CD4(+) T lymphocytes, because Ag-specific proliferative and cytokine responses are suppressed following prolonged exposure to TNF. We explored whether TNF attenuated T cell activation by uncoupling proximal TCR signal transduction pathways using a mouse T cell hybridoma model. Chronic TNF exposure induced profound, but reversible, T cell hyporesponsiveness, with TNF-treated T cells requiring TCR engagement with higher peptide concentrations for longer periods of time for commitment to IL-2 production. Subsequent experiments revealed that chronic TNF exposure led to a reversible loss of TCRzeta chain expression, in part through a reduction in gene transcription. Down-regulation of TCRzeta expression impaired TCR/CD3 assembly and expression at the cell surface and uncoupled membrane-proximal tyrosine phosphorylation events, including phosphorylation of the TCRzeta chain itself, CD3epsilon, ZAP-70 protein tyrosine kinase, and linker for activation of T cells (LAT). Intracellular Ca(2+) mobilization was also suppressed in TNF-treated T cells. We propose that TNF may contribute to T cell hyporesponsiveness in chronic inflammatory and infectious diseases by mechanisms that include down-regulation of TCRzeta expression. We speculate that by uncoupling proximal TCR signals TNF could also interrupt mechanisms of peripheral tolerance that are dependent upon intact TCR signal transduction pathways.

Glucocorticoids attenuate T cell receptor signaling

Glucocorticoids (GCs) affect peripheral immune responses by inhibiting T cell immunity at several stages of the activation cascade, causing impaired cytokine production and effector function. The recent demonstration that the thymic epithelium and possibly thymocytes themselves produce steroids suggests that endogenous GCs also play a role in the control of T cell development. As both peripheral responsiveness and thymic differentiation appear to be regulated by the quantity and quality of intracellular signals issued by antigen-major histocompatibility complex-engaged T cell receptor (TCR) complexes, we investigated the effects of GCs on the signaling properties of T cells stimulated by anti-CD3 monoclonal antibodies or agonist peptides. We demonstrate in this work that dexamethasone, a synthetic GC, inhibits the early signaling events initiated upon TCR ligation, such as tyrosine phosphorylation of several TCR-associated substrates including the zeta chain, the ZAP70 kinase, and the transmembrane adapter molecule linker for activation of T cells. Hypophosphorylation was not a consequence of reduced kinase activity of src protein tyrosine kinases, but was correlated with an altered- membrane compartmentalization of these molecules. These observations indicate that in addition to their well-described ability to interfere with the transcription of molecules involved in peripheral responses, GCs inhibit T cell activation by affecting the early phosphorylating events induced after TCR ligation.

Changes in the T cell receptor macromolecular signaling complex and membrane microdomains during T cell development and activation

Initiation and propagation of T cell receptor signaling pathways involves the mobilization and aggregation of a variety of signaling intermediates with the T cell receptor and associated molecules into specialized signaling complexes. Accumulating evidence suggests that differential regulation of the formation and composition of the T cell receptor macromolecular signaling complex may affect the different biological consequences of T cell activation. The regulatory mechanisms involved in the assembly of these complexes remains poorly understood, but in part is affected by the avidity of the T cell receptor ligand, co-stimulatory signals, and by the differentiation state of the T cell.

Cross‐reactivity in T‐cell antigen recognition

The molecular interactions between the T-cell receptor (TCR) and peptide-MHC (pMHC) have been elucidated in recent years. Nevertheless, the fact that binding of only slightly different ligands by a TCR, or ligation of the same pMHC at different developmental stages of the T cell, can have opposing consequences, continues to pose intellectual challenges. Kinetic proofreading models, which have at their core the dissociation rates of pMHC from the TCR, are best suited to account for these observations. However, T cells can be triggered by peptides with often minimal homology to the primary immunogenic peptide. This cross-reactivity of the TCR is manifest at several levels, from positive selection of immature thymocytes to homeostasis and antigen-cross- reactive immune responses of mature peripheral T cells. The implications of the high cross-reactivity of T-cell antigen recognition for self-tolerance and T-cell memory are discussed.

HuM291(NUVION), A HUMANIZED Fc RECEPTOR-NONBINDING ANTIBODY AGAINST CD3, ANERGIZES PERIPHERAL BLOOD T CELLS AS PARTIAL AGONIST OF THE T CELL RECEPTOR1

BACKGROUND

Humanized Fc receptor (FcR)-nonbinding antibodies against CD3 are promising immunosuppressive agents that may overcome both the neutralizing response to and the cytokine release syndrome seen with conventional monoclonal antibodies against CD3. In addition, evidence from several murine models suggests that these recombinant antibodies may actively induce T cell unresponsiveness by a mechanism other than modulation of the T cell receptor (TCR) or T cell depletion. We hypothesized that FcR-nonbinding antibodies against CD3 could induce T cell unresponsiveness by acting as partial agonist ligands of the TCR and thus, inducing T cell anergy.

METHODS

To test this hypothesis, we examined the signaling and functional effects of HuM291 (Nuvion), a FcR-nonbinding humanized antibody against CD3, on primary human T cells.

RESULTS

Short exposure of human peripheral blood T lymphocytes to HuM291 caused a partial agonist type of signaling through the TCR characterized by incomplete phosphorylation of TCR zeta, failure to activate ZAP-70 and to phosphorylate LAT but activation of ERK-1/-2 and subsequent up-regulation of CD69 expression. These changes correlated with a dose-dependent induction of anergy in human, primary resting T cells, which was reversed by exogenous interleukin-2.

CONCLUSIONS

The tolerogenic properties of FcR-nonbinding monoclonal antibodies against CD3 correlate with its ability to reproduce the biochemical and functional effects of TCR partial agonist ligands. Thus, generation of engineered antibodies against CD3 with low TCR oligomerization potential may provide a clinically applicable partial agonist-based strategy for the prevention of polyclonal T cell responses.

Co-stimulation and counter-stimulation: lipid raft clustering controls TCR signaling and functional outcomes

T cell receptor (TCR) antigen recognition induces the formation of a specialized 'immunological synapse' at the T cell : antigen presenting cell (APC) junction. This junction is generated by the recruitment and exclusion of particular proteins from the contact area and is required for T cell activation. We and others have hypothesized that lipid raft/non-raft partitioning provides a molecular basis for protein sorting which organizes the TCR, co-stimulators, signal transducers and the actin cytoskeleton at the T cell : APC interface. Here we discuss the emerging paradigm that co-stimulators induce the directional transport and clustering of lipid rafts at the T cell : APC interface, thus generating platform(s) specialized for processive and sustained TCR signal transduction and T cell activation. We also discuss recent data implicating the involvement of 'counter-stimulators' and other negative regulators which prevent optimal raft clustering at the TCR contact site and, thus, facilitate T cell inactivation and tolerance induction.

Signal strength in thymic selection and lineage commitment

During development, alphabeta T cells undergo positive or negative selection and CD4(+)/CD8(+) lineage commitment-events that have a major impact on the functionality of the T cell repertoire. The precise mechanisms of these differentiative steps remain elusive. Research this year has focused on quantitative models of signaling. For positive selection, the timing and extent of ERK activation may be important. For lineage commitment, the extent of Lck recruitment and activation may be the decisive factor. Next, the search is on for the genes that commit the cell to the fate determined by these quantitative differences in signals.

Identification of a crucial energetic footprint on the alpha1 helix of human histocompatibility leukocyte antigen (HLA)-A2 that provides functional interactions for recognition by tax peptide/HLA-A2-specific T cell receptors

Structural studies have shown that class I major histocompatibility complex (MHC)-restricted peptide-specific T cell receptor (TCR)-alpha/betas make multiple contacts with the alpha1 and alpha2 helices of the MHC, but it is unclear which or how many of these interactions contribute to functional binding. We have addressed this question by performing single amino acid mutagenesis of the 15 TCR contact sites on the human histocompatibility leukocyte antigen (HLA)-A2 molecule recognized by the A6 TCR specific for the Tax peptide presented by HLA-A2. The results demonstrate that mutagenesis of only three amino acids (R65, K66, and A69) that are clustered on the alpha1 helix affected T cell recognition of the Tax/HLA-A2 complex. At least one of these three mutants affected T cell recognition by every member of a large panel of Tax/HLA-A2-specific T cell lines. Biacore measurements showed that these three HLA-A2 mutations also altered A6 TCR binding kinetics, reducing binding affinity. These results show that for Tax/HLA-A2-specific TCRs, there is a location on the central portion of the alpha1 helix that provides interactions crucial to their function with the MHC molecule.

Identification and modulation of a naturally processed T cell epitope from the diabetes-associated autoantigen human glutamic acid decarboxylase 65 (hGAD65)

T cell recognition of autoantigens is critical to progressive immune-mediated destruction of islet cells, which leads to autoimmune diabetes. We identified a naturally presented autoantigen from the human islet antigen glutamic acid decarboxylase, 65-kDa isoform (GAD65), by using a combination of chromatography and mass spectrometry of peptides bound by the type I diabetes (insulin-dependent diabetes mellitus, IDDM)-associated HLA-DR4 molecule. Peptides encompassing this epitope-stimulated GAD65-specific T cells from diabetic patients and a DR4-positive individual at high risk for developing IDDM. T cell responses were antagonized by altered peptide ligands containing single amino acid modifications. This direct identification and manipulation of GAD65 epitope recognition provides an approach toward dissection of the complex CD4(+) T cell response in IDDM.

CD4(+) T cells from lupus-prone mice are hyperresponsive to T cell receptor engagement with low and high affinity peptide antigens: a model to explain spontaneous T cell activation in lupus

Polyclonal CD4(+) T cell activation is characteristic of spontaneous lupus. As a potential explanation for this phenotype, we hypothesized that T cells from lupus-prone mice are intrinsically hyperresponsive to stimulation with antigen, particularly to those peptide ligands having a low affinity for the T cell receptor (TCR). To test this hypothesis, we backcrossed the alpha and beta chain genes of the AND TCR specific for amino acids 88-104 of pigeon cytochrome C (PCC) to the Fas-intact MRL/Mp(+)(Fas-lpr) and to the H-2(k)-matched control backgrounds B10.BR and CBA/CaJ (MRL.AND, B10.AND, and CBA.AND, respectively), and assessed naive CD4(+) TCR transgenic T cell activation in vitro after its encounter with cognate antigen and lower affinity altered peptide ligands (APLs). MRL.AND T cells, compared with control B10.AND and CBA.AND cells, proliferated more when stimulated with agonist antigen. More strikingly, MRL.AND T cells proliferated significantly more and produced more interleukin 2 when stimulated with the APLs of PCC 88-104, having lower affinity for the transgenic TCR. These results imply that one of the forces driving polyclonal activation of alpha/beta T cells in lupus is an intrinsically heightened response to peptide antigen, particularly those with low affinity for the TCR, independent of the nature of the antigen-presenting cell and degree of costimulation.

Altered T-cell receptor + CD28-mediated signaling and blocked cell cycle progression in interleukin 10 and transforming growth factor-beta-treated alloreactive T cells that do not induce graft-versus-host disease

The induction of anergy in T cells, although widely accepted as critical for the maintenance of tolerance, is still poorly understood at the molecular level. Recent evidence demonstrates that in addition to blockade of costimulation using monoclonal antibodies (mAbs) directed against cell surface determinants, treatment of mixed lymphocyte reaction (MLR) cultures with interleukin 10 (IL-10) and transforming growth factor-beta (TGF-beta) results in induction of tolerance, rendering alloreactive murine CD4(+) T cells incapable of inducing graft-versus-host disease (GVHD) after in vivo transfer to histoincompatible recipients. The present study, using these cells prior to adoptive transfer, determined that IL-10 + TGF-beta-tolerant CD4(+) T cells exhibit an altered pattern of T-cell receptor (TCR) + CD28-mediated signaling and are incapable of progressing out of the G(1) phase of the cell cycle during stimulation with HLA class II disparate antigen-presenting cells. TGFbeta + IL-10-tolerant cells were incapable of phosphorylating TCR-zeta, or activating ZAP-70, Ras, and MAPK, similarly to T-cell tolerized by blockade of B7/CD28 and CD40/CD40L pathways. Moreover, these cells were incapable of clonal expansion due to defective synthesis of cyclin D3 and cyclin A, and defective activation of cyclin-dependent kinase (cdk)4, cdk6, and cdk2. These cells also exhibited defective down-regulation of p27(kip1) cdk inhibitor and lack of cyclin D2-cdk4 activation, Rb hyperphosphorylation, and progression to the S phase of the cell cycle. These data link anergy-specific proximal biochemical alterations and the downstream nuclear pathways that control T-cell expansion and provide a biochemical profile of IL-10 + TGF-beta-tolerant alloreactive T cells that do not induce GVHD when transferred into MHC class II disparate recipients in vivo.