Signal transduction mediated by the T-cell antigen receptor

Direct AKT activation in tumor-infiltrating lymphocytes markedly increases interferon-γ (IFN-γ) for the regression of tumors resistant to PD-1 checkpoint blockade

PD-1 immune checkpoint blockade against inhibitory receptors such as receptor programmed cell death-1 (PD-1), has revolutionized cancer treatment. Effective immune reactivity against tumour antigens requires the infiltration and activation of tumour-infiltrating T-cells (TILs). In this context, ligation of the antigen-receptor complex (TCR) in combination with the co-receptor CD28 activates the intracellular mediator AKT (or PKB, protein kinase B) and its downstream targets. PD-1 inhibits the activation of AKT/PKB. Given this, we assessed whether the direct activation of AKT might be effective in activating the immune system to limit the growth of tumors that are resistant to PD-1 checkpoint blockade. We found that the small molecule activator of AKT (SC79) limited growth of a B16 tumor and an EMT-6 syngeneic breast tumor model that are poorly responsive to PD-1 immunotherapy. In the case of B16 tumors, direct AKT activation induced (i) a reduction of suppressor regulatory (Treg) TILs and (ii) an increase in effector CD8+ TILs. SC79 in vivo therapy caused a major increase in the numbers of CD4+ and CD8+ TILs to express interferon-γ (IFN-γ). This effect on IFN-γ expression distinguished responsive from non-responsive anti-tumor responses and could be recapitulated ex vivo with human T-cells. In CD4+FoxP3+Treg TILs, AKT induced IFN-γ expression was accompanied by a loss of suppressor activity, the conversation to CD4⁺ helper Th1-like TILs and a marked reduction in phospho-SHP2. In CD8+ TILs, we observed an increase in the phospho-activation of PLC-γ. Further, the genetic deletion of the transcription factor T-bet (Tbx21) blocked the increased IFN-γ expression on all subsets while ablating the therapeutic benefits of SC79 on tumor growth. Our study shows that AKT activation therapy acts to induce IFN-γ on CD4 and CD8 TILs that is accompanied by the intra-tumoral conversation of suppressive Tregs into CD4⁺Th1-like T-cells and augmented CD8 responses.

How the Discovery of the CD4/CD8-p56lck Complexes Changed Immunology and Immunotherapy

The past 25 years have seen enormous progress in uncovering the receptors and signaling mechanisms on T-cells that activate their various effecter functions. Until the late 1980s, most studies on T-cells had focused on the influx of calcium and the levels of cAMP/GMP in T-cells. My laboratory then uncovered the interaction of CD4 and CD8 co-receptors with the protein-tyrosine kinase p56^lck which are now widely accepted as the initiators of the tyrosine phosphorylation cascade leading to T-cell activation. The finding explained how immune recognition receptors expressed by many immune cells, which lack intrinsic catalytic activity, can transduce activation signals via non-covalent association with non-receptor tyrosine kinases. The discovery also established the concept that a protein tyrosine phosphorylation cascade operated in T-cells. In this vein, we and others then showed that the CD4- and CD8-p56^lck complexes phosphorylate the TCR complexes which led to the identification of other protein-tyrosine kinases such as ZAP-70 and an array of substrates that are now central to studies in T-cell immunity. Other receptors such as B-cell receptor, Fc receptors and others were also subsequently found to use src kinases to control cell growth. In T-cells, p56^lck driven phosphorylation targets include co-receptors such as CD28 and CTLA-4 and immune cell-specific adaptor proteins such as LAT and SLP-76 which act to integrate signals proximal to surface receptors. CD4/CD8-p56^lck regulated events in T-cells include intracellular calcium mobilization, integrin activation and the induction of transcription factors for gene expression. Lastly, the identification of the targets of p56^lck in the TCR and CD28 provided the framework for the development of chimeric antigen receptor (CAR) therapy in the treatment of cancer. In this review, I outline a history of the development of events that led to the development of the “TCR signaling paradigm” and its implications to immunology and immunotherapy.

Use of Cell and Genome Modification Technologies to Generate Improved "Off-the-Shelf" CAR T and CAR NK Cells

The broad success of adoptive immunotherapy to treat human cancer has resulted in a paradigm shift in modern medicine. Modification of autologous and allogenic immune cells with chimeric antigen receptors (CAR) designed to target specific antigens on tumor cells has led to production of CAR T and CAR NK cell therapies, which are ever more commonly introduced into cancer patient treatment protocols. While allogenic T cells may offer advantages such as improved anti-tumor activity, they also carry the risk of adverse reactions like graft-versus-host disease. This risk can be mitigated by use of autologous immune cells, however, the time needed for T and/or NK cell isolation, modification and expansion may be too long for some patients. Thus, there is an urgent need for strategies to robustly produce “off-the-shelf” CAR T and CAR NK cells, which could be used as a bridging therapy between cancer diagnosis or relapse and allogeneic transplantation. Advances in genome modification technologies have accelerated the generation of designer cell therapy products, including development of “off-the-shelf” CAR T cells for cancer immunotherapy. The feasibility and safety of such approaches is currently tested in clinical trials. This review will describe cell sources for CAR-based therapies, provide background of current genome editing techniques and the applicability of these approaches for generation of universal “off-the-shelf” CAR T and NK cell therapeutics.

Nongenomic glucocorticoid effects and their mechanisms of action in vertebrates

Glucocorticoids (GC) act on multiple organ systems to regulate a variety of physiological processes in vertebrates. Due to their immunosuppressive and anti-inflammatory actions, glucocorticoids are an attractive target for pharmaceutical development. Accordingly, they are one of the most widely prescribed classes of therapeutics. Through the classical mechanism of steroid action, glucocorticoids are thought to mainly affect gene transcription, both in a stimulatory and suppressive fashion, regulating de novo protein synthesis that subsequently leads to the physiological response. However, over the past three decades multiple lines of evidence demonstrate that glucocorticoids may work through rapid, nonclassical mechanisms that do not require alterations in gene transcription or translation. This review assimilates evidence across the vertebrate taxa on the diversity of nongenomic actions of glucocorticoids and the membrane-associated cellular mechanisms that may underlie rapid glucocorticoid responses to include potential binding sites characterized to date.

Crk adaptor proteins regulate CD3ζ chain phosphorylation and TCR/CD3 down-modulation in activated T cells

T cell receptor (TCR) recognition of a peptide antigen in the context of MHC molecules initiates positive and negative cascades that regulate T cell activation, proliferation and differentiation, and culminate in the acquisition of effector T cell functions. These processes are a prerequisite for the induction of specific T cell-mediated adaptive immune responses. A key event in the activation of TCR-coupled signaling pathways is the phosphorylation of tyrosine residues within the cytoplasmic tails of the CD3 subunits, predominantly CD3ζ. These transiently formed phosphotyrosyl epitopes serve as docking sites for SH2-domain containing effector molecules, predominantly the ZAP70 protein tyrosine kinase, which is critical for signal propagation. We found that CrkI and CrkII adaptor proteins also interact with CD3ζ in TCR activated-, but not in resting-, T cells. Crk binding to CD3ζ was independent of ZAP70 and also occurred in ZAP70-deficient T cells. Binding was mediated by Crk-SH2 domain interaction with phosphotyrosine-containing motifs on CD3ζ, via a direct physical interaction, as demonstrated by Far-Western blot. CrkII binding to CD3ζ could also be demonstrated in a heterologous system, where coexpression of a catalytically active Lck was used to phosphorylate the CD3ζ chain. TCR activation-induced Crk binding to CD3ζ resulted in increased and prolonged phosphorylation of CD3ζ, as well as ZAP70 and LAT, suggesting a positive role for CrkI/II binding to CD3ζ in regulation of TCR-coupled signaling pathways. Furthermore, Crk-dependent increased phosphorylation of CD3ζ coincided with inhibition of TCR downmodulation, supporting a positive role for Crk adaptor proteins in TCR-mediated signal amplification.

Grb2 functions at the top of the T-cell antigen receptor-induced tyrosine kinase cascade to control thymic selection

Grb2 is an adaptor molecule that mediates Ras-MAPK activation induced by various receptors. Here we show that conditional ablation of Grb2 in thymocytes severely impairs both thymic positive and negative selections. Strikingly, the mutation attenuates T-cell antigen receptor (TCR) proximal signaling, including tyrosine phosphorylation of multiple signaling proteins and Ca(2+) influx. The defective TCR signaling can be attributed to a marked impairment in Lck activation. Ectopic expression of a mutant Grb2 composed of the central SH2 and the C-terminal SH3 domains in Grb2(-/-) thymocytes fully restores thymocyte development. Thus, Grb2 plays a pivotal role in both thymic positive and negative selection. It amplifies TCR signaling at the top end of the tyrosine phosphorylation cascade via a scaffolding function.

Early T cell differentiation lessons from T-cell acute lymphoblastic leukemia

T cells develop from bone marrow-derived self-renewing hematopoietic stem cells (HSC). Upon entering the thymus, these cells undergo progressive commitment and differentiation driven by the thymic stroma and the pre-T cell receptor (pre-TCR). These processes are disrupted in T-cell acute lymphoblastic leukemia (T-ALL). More than 70% of recurring chromosomal rearrangements in T-ALL activate the expression of oncogenic transcription factors, belonging mostly to three families, basic helix-loop-helix (bHLH), homeobox (HOX), and c-MYB. This prevalence is indicative of their importance in the T lineage, and their dominant mechanisms of transformation. For example, bHLH oncoproteins inhibit E2A and HEB, revealing their tumor suppressor function in the thymus. The induction of T-ALL, nonetheless, requires collaboration with constitutive NOTCH1 signaling and the pre-TCR, as well as loss-of-function mutations for CDKN2A and PTEN. Significantly, NOTCH1, the pre-TCR pathway, and E2A/HEB proteins control critical checkpoints and branchpoints in early thymocyte development whereas several oncogenic transcription factors, HOXA9, c-MYB, SCL, and LYL-1 control HSC self-renewal. Together, these genetic lesions alter key regulatory processes in the cell, favoring self-renewal and subvert the normal control of thymocyte homeostasis.

Molecular-genetic insights in paediatric T-cell acute lymphoblastic leukaemia

Paediatric T-cell acute lymphoblastic leukaemia (T-ALL) is an aggressive malignancy of thymocytes that accounts for about 15% of ALL cases and for which treatment outcome remains inferior compared to B-lineage acute leukaemias. In T-ALL, leukemic transformation of maturating thymocytes is caused by a multistep pathogenesis involving numerous genetic abnormalities that drive normal T-cells into uncontrolled cell growth and clonal expansion. This review provides an overview of the current knowledge on onco- and tumor suppressor genes in T-ALL and suggests a classification of these genetic defects into type A and type B abnormalities. Type A abnormalities may delineate distinct molecular-cytogenetic T-ALL subgroups, whereas type B abnormalities are found in all major T-ALL subgroups and synergize with these type A mutations during T-cell pathogenesis.

The three-dimensional structure of the ZAP-70 kinase domain in complex with staurosporine: implications for the design of selective inhibitors

The ZAP-70 tyrosine kinase plays a critical role in T cell activation and the immune response and therefore is a logical target for immunomodulatory therapies. Although the crystal structure of the tandem Src homology-2 domains of human ZAP-70 in complex with a peptide derived from the zeta subunit of the T cell receptor has been reported (Hatada, M. H., Lu, X., Laird, E. R., Green, J., Morgenstern, J. P., Lou, M., Marr, C. S., Phillips, T. B., Ram, M. K., Theriault, K., Zoller, M. J., and Karas, J. L. (1995) Nature 377, 32-38), the structure of the kinase domain has been elusive to date. We crystallized and determined the three-dimensional structure of the catalytic subunit of ZAP-70 as a complex with staurosporine to 2.3 A resolution, utilizing an active kinase domain containing residues 327-606 identified by systematic N- and C-terminal truncations. The crystal structure shows that this ZAP-70 kinase domain is in an active-like conformation despite the lack of tyrosine phosphorylation in the activation loop. The unique features of the ATP-binding site, identified by structural and sequence comparison with other kinases, will be useful in the design of ZAP-70-selective inhibitors.

Grap negatively regulates T-cell receptor-elicited lymphocyte proliferation and interleukin-2 induction

Grb-2-related adaptor protein (Grap) is a Grb2-like SH3-SH2-SH3 adaptor protein with expression restricted to lymphoid tissues. Grap(-/-) lymphocytes isolated from targeted Grap-deficient mice exhibited enhanced proliferation, interleukin-2 production, and c-fos induction in response to mitogenic T-cell receptor (TCR) stimulation, compared to wild-type cells. Ectopic expression of Grap led to a suppression of Elk-1-directed transcription induced by the Ras/Erk pathway, without having effects on gene expression mediated by Jnk and p38 mitogen-activated protein kinases. Together, these data suggest that Grap, unlike Grb2, acts as a negative regulator of TCR-stimulated intracellular signaling by downregulating signal relay through the Ras/Erk pathway.

Docking protein Gab2 is phosphorylated by ZAP-70 and negatively regulates T cell receptor signaling by recruitment of inhibitory molecules

To maintain various T cell responses and immune equilibrium, activation signals triggered by T cell antigen receptor (TCR) must be regulated by inhibitory signals. Gab2, an adaptor protein of the insulin receptor substrate-1 family, has been shown to be involved in the downstream signaling from cytokine receptors. We investigated the functional role of Gab2 in TCR-mediated signal transduction. Gab2 was phosphorylated by ZAP-70 and co-precipitated with phosphoproteins, such as ZAP-70, LAT, and CD3zeta, upon TCR stimulation. Overexpression of Gab2 in Jurkat cells or antigen-specific T cell hybridomas resulted in the inhibition of NF-AT activation, interleukin-2 production, and tyrosine phosphorylation. The structure-function relationship of Gab2 was analyzed by mutants of Gab2. The Gab2 mutants lacking SHP-2-binding sites mostly abrogated the inhibitory activity of Gab2, but its inhibitory function was restored by fusing to active SHP-2 as a chimeric protein. A mutant with defective phosphatidylinositol 3-kinase binding capacity also impaired the inhibitory activity, and the pleckstrin homology domain-deletion mutant revealed a crucial function of the pleckstrin homology domain for localization to the plasma membrane. These results suggest that Gab2 is a substrate of ZAP-70 and functions as a switch molecule toward inhibition of TCR signal transduction by mediating the recruitment of inhibitory molecules to the TCR signaling complex.

Alterations of protein tyrosine phosphorylation in T cells of immunocompromised patients

Activation of the T-cell receptor (TCR) results in recruitment of tyrosine kinases and changes of tyrosine phosphorylation levels. We quantitatively analyzed protein phosphorylation of resting and TCR stimulated T cells for healthy donors and immunocompromised patients using two-colour flow cytometry. Stimulation of T cells of healthy persons by OKT3 antibody led to a biphasic increase of phosphotyrosine levels with the first peak after 15 s and the absolute maximum occurring after 3-5 min. Levels remained high up to 30 min and returned to baseline levels afterwards. Compared to healthy blood donors, the phosphotyrosine baseline levels were 20-30% increased in patients after bone marrow transplantation (BMT). Using OKT3 to stimulate T cells of BMT patients led to strong increases in phosphotyrosine levels comparable to those of controls. In contrast, the response of T cells of human immunodeficiency virus-infected patients with acquired immune deficiency syndrome was severely impaired (P = 0.01). In conclusion, this flow cytometric methodology enables analyses of changes in cellular phosphotyrosine levels following TCR stimulation. The increased baseline levels in BMT patients and the observed unresponsiveness of T cells in AIDS patients could be of interest for the study of predictors of graft-versus-host reactivity and the clinical analysis of immune functions in AIDS patients, respectively.

Hematopoietic Cell Phosphatase Negatively Regulates Erythropoietin-Induced Hemoglobinization in Erythroleukemic SKT6 Cells

In an increasing number of hematopoietic cytokine receptor systems (T-cell receptor, B-cell receptor, and macrophage colony-stimulating factor, stem cell factor, interleukin-3, and erythropoietin [EPO] receptors), inhibitory roles for the protein tyrosine phosphatase hematopoietic cell phosphatase (HCP; SHPTP1, PTP1C, and SHP1) have been defined in proliferative signaling. However, evidence exists to suggest that HCP also may exert important effects on blood cell differentiation. To investigate possible roles for HCP during late erythroid differentiation, effects of manipulating HCP expression or recruitment on EPO-induced hemoglobinization in erythroleukemic SKT6 cells have been investigated. No effects of EPO on levels of HCP, Syp, Stat5, the EPO receptor, or GATA-1 expression were observed during induced differentiation. However, the tyrosine phosphorylation of JAK2, the EPO receptor, and Stat5 was efficiently activated, and HCP was observed to associate constitutively with the EPO receptor in this differentiation-specific system. In studies of HCP function, inhibition of HCP expression by antisense oligonucleotides enhanced hemoglobinization, whereas the enforced ectopic expression of wild-type (wt) HCP markedly inhibited EPO-induced globin expression and Stat5 activation. Based on these findings, epidermal growth factor (EGF) receptor/EPO receptor chimeras containing either the wt EPO receptor cytoplasmic domain (EECA) or a derived HCP binding site mutant (EECA-Y429,431F ) were expressed in SKT6 cells, and their abilities to mediate differentiation were assayed. Each chimera supported EGF-induced hemoglobinization, but efficiencies for EECA-Y429,431F were enhanced 400% to 500%. Thus, these studies show a novel role for HCP as a negative regulator of EPO-induced erythroid differentiation. In normal erythroid progenitor cells, HCP may act to prevent premature commitment to terminal differentiation. In erythroleukemic SKT6 cells, this action also may enforce mitogenesis.

Hematopoietic Cell Phosphatase Negatively Regulates Erythropoietin-Induced Hemoglobinization in Erythroleukemic SKT6 Cells

In an increasing number of hematopoietic cytokine receptor systems (T-cell receptor, B-cell receptor, and macrophage colony-stimulating factor, stem cell factor, interleukin-3, and erythropoietin [EPO] receptors), inhibitory roles for the protein tyrosine phosphatase hematopoietic cell phosphatase (HCP; SHPTP1, PTP1C, and SHP1) have been defined in proliferative signaling. However, evidence exists to suggest that HCP also may exert important effects on blood cell differentiation. To investigate possible roles for HCP during late erythroid differentiation, effects of manipulating HCP expression or recruitment on EPO-induced hemoglobinization in erythroleukemic SKT6 cells have been investigated. No effects of EPO on levels of HCP, Syp, Stat5, the EPO receptor, or GATA-1 expression were observed during induced differentiation. However, the tyrosine phosphorylation of JAK2, the EPO receptor, and Stat5 was efficiently activated, and HCP was observed to associate constitutively with the EPO receptor in this differentiation-specific system. In studies of HCP function, inhibition of HCP expression by antisense oligonucleotides enhanced hemoglobinization, whereas the enforced ectopic expression of wild-type (wt) HCP markedly inhibited EPO-induced globin expression and Stat5 activation. Based on these findings, epidermal growth factor (EGF) receptor/EPO receptor chimeras containing either the wt EPO receptor cytoplasmic domain (EECA) or a derived HCP binding site mutant (EECA-Y429,431F ) were expressed in SKT6 cells, and their abilities to mediate differentiation were assayed. Each chimera supported EGF-induced hemoglobinization, but efficiencies for EECA-Y429,431F were enhanced 400% to 500%. Thus, these studies show a novel role for HCP as a negative regulator of EPO-induced erythroid differentiation. In normal erythroid progenitor cells, HCP may act to prevent premature commitment to terminal differentiation. In erythroleukemic SKT6 cells, this action also may enforce mitogenesis.

Interactions of TCRs with MHC-peptide complexes: a quantitative basis for mechanistic models

The activation of T lymphocytes is initiated by the binding of MHC-peptide complexes on antigen-presenting cells to MHC-restricted, peptide specific TCRs. Significant progress has recently been made in understanding the structure of the TCR and in the direct quantitative examination of the primary binding interactions between MHC-peptide complexes and the TCR. Attempts to develop quantitative models for the differential activation of T cells by MHC-peptide ligands that differ subtly in their structure have largely been based on either the affinity of the MHC-peptide complexes for the TCR in question or on the dissociation kinetics of the MHC-peptide complex from the T cell.

Identification and characterization of a substrate specific for the T cell protein tyrosine kinase ZAP-70

ZAP-70 is a protein tyrosine kinase (PTK) that plays a critical role in T cell activation. To study the role of ZAP-70 catalytic activity in this process, a substrate capable of distinguishing between the activities of ZAP-70 and other PTKs would be useful, especially since it has recently been shown that ZAP-70 interacts with another T cell PTK, Lck. We have thus identified a site of phosphorylation on the cytoplasmic fragment of the erythrocyte band 3 protein that is recognized by ZAP-70, but not Lck. A synthetic peptide based on this site has been demonstrated to be a good in vitro substrate for ZAP-70 and a poor substrate for the T cell PTKs Lck and Itk. This peptide molecule should thus prove useful to many investigators working in the field of T cell activation.

Dual T cell receptor T cells have a decreased sensitivity to physiological ligands due to reduced density of each T cell receptor

A considerable fraction of T cells express two distinct T cell receptors (TCR), mainly due to expression of two TCR alpha chains. It has been suggested that such dual-TCR cells could have a role in autoimmunity. However, as such cells express less of each TCR, they could be less sensitive to their physiological ligand, i.e. peptide plus major histocompatibility complex molecules (MHC). We tested this hypothesis in a transgenic TCR model in which most T cells express different amounts of the transgene-encoded TCR, due to expression of endogenous TCR alpha chains. Five Th1 clones derived from lambda2(315) immunoglobulin light chain-specific TCR-transgenic mice expressed different levels of the transgene-encoded TCR, ranging from approximately 10,000 to approximately 50,000 TCR per cell. Cytosolic Ca2+ mobilization in single T cells from these clones elicited by lambda2(315) peptide-pulsed, I-Ed-expressing antigen-presenting cells, correlated linearly with the relative transgene-encoded TCR expression. The peptide requirement for half-maximal T cell proliferation showed a similar correlation, with low TCR levels requiring higher peptide concentration. Corroborative evidence was obtained by deployment of short-term polyclonal CD4+ lines from TCR-transgenic mice. Such lines had reduced early (Ca2+ mobilization) and late (lymphokine and proliferation) responses, compared with T cell lines from recombination-deficient TCR-transgenic severe combined immunodeficiency mice (which express only a single transgene-encoded TCR). Taken together, the Ca2+ responses increase gradually with increasing TCR expression per cell, similar to the previously described analog Ca2+ signaling elicited by increasing amounts of peptide/MHC [Røtnes et al., Eur. J. Immunol. 1994. 24: 851]. Surprisingly small reductions in TCR expression per cell reduce T cell responsiveness. This suggests that dual-TCR T cells are immunologically less effective than single-TCR T cells.

Characterization of Cbl tyrosine phosphorylation and a Cbl-Syk complex in RBL-2H3 cells

Tyrosine phosphorylation of the Cbl protooncogene has been shown to occur after engagement of a number of different receptors on hematopoietic cells. However, the mechanisms by which these receptors induce Cbl tyrosine phosphorylation are poorly understood. Here we demonstrate that engagement of the high affinity IgE receptor (Fc epsilon R1) leads to the tyrosine phosphorylation of Cbl and analyze how this occurs. We show that at least part of Fc epsilon R1-induced Cbl tyrosine phosphorylation is mediated by the Syk tyrosine kinase, and that the Syk-dependent tyrosine phosphorylation of Cbl occurs mainly distal to the Cbl proline-rich region within the COOH-terminal 250 amino acids. Furthermore, we show by coprecipitation that Cbl is present in a complex with Syk before receptor engagement, that the proline-rich region of Cbl and a region of Syk comprised of the two SH2 domains and intradomain linker are required for formation of the complex, and that little or no tyrosine-phosphorylated Cbl is detected in complex with Syk. Overexpression of truncation mutants of Cbl capable of binding Syk has the effect of blocking tyrosine phosphorylation of endogenous Cbl. These results define a potentially important intramolecular interaction in mast cells and suggest a complex function for Cbl in intracellular signaling pathways.

Studying interactions involving the T-cell antigen receptor by surface plasmon resonance

T-lymphocyte activation is initiated by the interaction of the alpha beta TCR with a complex consisting of a class I or class II MHC-encoded molecule and an antigenic peptide, displayed on the surface of an antigen-presenting cell. Real-time binding measurements using surface plasmon resonance have revealed kinetic and equilibrium parameters for the interactions between purified MHC molecules and peptides, between TCR and MHC-peptide complexes, and between TRC and superantigens. The MHC-peptide interaction is characterized by its high affinity and long half-life, the TCR-MHC/peptide interaction by its low affinity and short half-life, and the TCR-superantigen interaction by its low-to-moderate affinity, which is dependent on the particular superantigen involved. The consistent finding is that both MHC-peptide complexes and superantigens interact with TCR with a low affinity attributable to rapid dissociation. That an MHC-peptide complex that encounters a single TCR only briefly can still deliver the necessary activation signals offers a mechanistic conundrum for which several solutions have been proposed.