Instruct: Given a scientific claim, retrieve relevant passages that support or refute the claim.
Query: T cell receptor /CD3 microdomains are required to induce the immunologic synapse.
T cell receptor (TCR) signaling is initiated and sustained in microclusters; however, it's not known whether signaling also occurs in the TCR-rich central supramolecular activation cluster (cSMAC). We showed that the cSMAC formed by fusion of microclusters contained more CD45 than microclusters and is a site enriched in lysobisphosphatidic acid, a lipid involved in sorting ubiquitinated membrane proteins for degradation. Calcium signaling via TCR was blocked within 2 min by anti-MHCp treatment and 1 min by latrunculin-A treatment. TCR-MHCp interactions in the cSMAC survived these perturbations for 10 min and hence were not sufficient to sustain signaling. TCR microclusters were also resistant to disruption by anti-MHCp and latrunculin-A treatments. We propose that TCR signaling is sustained by stabilized microclusters and is terminated in the cSMAC, a structure from which TCR are sorted for degradation. Our studies reveal a role for F-actin in TCR signaling beyond microcluster formation.
The mechanism of B cell–antigen encounter in lymphoid tissues is incompletely understood. It is also unclear how immune complexes are transported to follicular dendritic cells. Here, using real-time two-photon microscopy we noted rapid delivery of immune complexes through the lymph to macrophages in the lymph node subcapsular sinus. B cells captured immune complexes by a complement receptor–dependent mechanism from macrophage processes that penetrated the follicle and transported the complexes to follicular dendritic cells. Furthermore, cognate B cells captured antigen-containing immune complexes from macrophage processes and migrated to the T zone. Our findings identify macrophages lining the subcapsular sinus as an important site of B cell encounter with immune complexes and show that intrafollicular B cell migration facilitates the transport of immune complexes as well as encounters with cognate antigen.
Induction of tumor-specific immunity requires that dendritic cells (DCs) efficiently capture and present tumor antigens to result in the expansion and activation of tumor-specific cytotoxic T cells. The transition from antigen capture to T cell stimulation requires a maturation signal; in its absence tolerance, rather than immunity may develop. While immune complexes (ICs) are able to enhance antigen capture, they can be poor at inducing DC maturation, naive T cell activation and protective immunity. We now demonstrate that interfering with the inhibitory signal delivered by FcγRIIB on DCs converts ICs to potent maturation agents and results in T cell activation. Applying this approach to immunization with DCs pulsed ex-vivo with ICs, we have generated antigen-specific CD8+ T cells in vivo and achieved efficient protective immunity in a murine melanoma model. These data imply that ICs may normally function to maintain tolerance through the binding to inhibitory FcγRs on DCs, but they can be converted to potent immunogenic stimuli by selective engagement of activating FcγRs. This mechanism suggests a novel approach to the development of tumor vaccines.
Ligation of the CD28 receptor on T cells provides a critical second signal alongside T cell receptor (TCR) ligation for naive T cell activation. Here, we discuss the expression, structure, and biochemistry of CD28 and its ligands. CD28 signals play a key role in many T cell processes, including cytoskeletal remodeling, production of cytokines, survival, and differentiation. CD28 ligation leads to unique epigenetic, transcriptional, and post-translational changes in T cells that cannot be recapitulated by TCR ligation alone. We discuss the function of CD28 and its ligands in both effector and regulatory T cells. CD28 is critical for regulatory T cell survival and the maintenance of immune homeostasis. We outline the roles that CD28 and its family members play in human disease and we review the clinical efficacy of drugs that block CD28 ligands. Despite the centrality of CD28 and its family members and ligands to immune function, many aspects of CD28 biology remain unclear. Translation of a basic understanding of CD28 function into immunomodulatory therapeutics has been uneven, with both successes and failures. Such real-world results might stem from multiple factors, including complex receptor-ligand interactions among CD28 family members, differences between the mouse and human CD28 families, and cell-type specific roles of CD28 family members.
Although TNF is a major proinflammatory cytokine, increasing evidence indicates that TNF also has immunosuppressive feedback effects. We have demonstrated in this study that, in both resting and activated states, mouse peripheral CD4(+)CD25(+) T regulatory cells (Tregs) expressed remarkably higher surface levels of TNFR2 than CD4(+)CD25(-) T effector cells (Teffs). In cocultures of Tregs and Teffs, inhibition of proliferation of Teffs by Tregs was initially transiently abrogated by exposure to TNF, but longer exposure to TNF restored suppressive effects. Cytokine production by Teffs remained continually suppressed by Tregs. The profound anergy of Tregs in response to TCR stimulation was overcome by TNF, which expanded the Treg population. Furthermore, in synergy with IL-2, TNF expanded Tregs even more markedly up-regulated expression of CD25 and FoxP3 and phosphorylation of STAT5, and enhanced the suppressive activity of Tregs. Unlike TNF, IL-1beta and IL-6 did not up-regulate FoxP3-expressing Tregs. Furthermore, the number of Tregs increased in wild-type mice, but not in TNFR2(-/-) mice following sublethal cecal ligation and puncture. Depletion of Tregs significantly decreased mortality following cecal ligation and puncture. Thus, the stimulatory effect of TNF on Tregs resembles the reported costimulatory effects of TNF on Teffs, but is even more pronounced because of the higher expression of TNFR2 by Tregs. Moreover, our study suggests that the slower response of Tregs than Teffs to TNF results in delayed immunosuppressive feedback effects.
The activation of T cells is the fundamental on switch for the adaptive immune system. Ca2+ signaling is essential for T cell activation and starts as initial, short-lived, localized Ca2+ signals. The second messenger nicotinic acid adenine dinucleotide phosphate (NAADP) forms rapidly upon T cell activation and stimulates early Ca2+ signaling. We developed a high-resolution imaging technique using multiple fluorescent Ca2+ indicator dyes to characterize these early signaling events and investigate the channels involved in NAADP-dependent Ca2+ signals. In the first seconds of activation of either primary murine T cells or human Jurkat cells with beads coated with an antibody against CD3, we detected Ca2+ signals with diameters close to the limit of detection and that were close to the activation site at the plasma membrane. In Jurkat cells in which the ryanodine receptor (RyR) was knocked down or in primary T cells from RyR1−/− mice, either these early Ca2+ signals were not detected or the number of signals was markedly reduced. Local Ca2+ signals observed within 20 ms upon microinjection of Jurkat cells with NAADP were also sensitive to RyR knockdown. In contrast, TRPM2 (transient receptor potential channel, subtype melastatin 2), a potential NAADP target channel, was not required for the formation of initial Ca2+ signals in primary T cells. Thus, through our high-resolution imaging method, we characterized early Ca2+ release events in T cells and obtained evidence for the involvement of RyR and NAADP in such signals.
Successful cancer treatment requires understanding host immune response against tumor cells. PD-1 belongs to the CD28 superfamily of receptors that work as “checkpoints” of immune activation. PD-1 maintains immune self-tolerance to prevent autoimmunity and controls T-cell reaction during infection to prevent excessive tissue damage. Tumor cells that arise from normal tissue acquire mutations that can be targeted by lymphocytes. Accumulating lines of evidence suggest that tumor cells evade host immune attack by expressing physiological PD-1 ligands and stimulating PD-1 on the lymphocytes. Based on this idea, researchers have successfully demonstrated that systemic administration of monoclonal antibodies that inhibit the binding of PD-1 to the ligands reactivated T cells and augmented the anti-cancer immune response. In this review, I summarize the basics of T-cell biology and its regulation by PD-1 and discuss the current understanding and questions about this multifaceted molecule.
Although memory T cells respond more vigorously to stimulation and they are more sensitive to low doses of antigen than naive T cells, the molecular basis of this increased sensitivity remains unclear. We have previously shown that the T cell receptor (TCR) exists as different-sized oligomers on the surface of resting T cells and that large oligomers are preferentially activated in response to low antigen doses. Through biochemistry and electron microscopy, we now showed that previously stimulated and memory T cells have more and larger TCR oligomers at the cell surface than their naive counterparts. Reconstitution of cells and mice with a point mutant of the CD3ζ subunit, which impairs TCR oligomer formation, demonstrated that the increased size of TCR oligomers was directly responsible for the increased sensitivity of antigen-experienced T cells. Thus, we propose that an "avidity maturation" mechanism underlies T cell antigenic memory.
CD25+CD4+ regulatory T cells in normal animals are engaged in the maintenance of immunological self-tolerance. We show here that glucocorticoid-induced tumor necrosis factor receptor family–related gene (GITR, also known as TNFRSF18)—a member of the tumor necrosis factor–nerve growth factor (TNF-NGF) receptor gene superfamily—is predominantly expressed on CD25+CD4+ T cells and on CD25+CD4+CD8− thymocytes in normal naïve mice. We found that stimulation of GITR abrogated CD25+CD4+ T cell–mediated suppression. In addition, removal of GITR-expressing T cells or administration of a monoclonal antibody to GITR produced organ-specific autoimmune disease in otherwise normal mice. Thus, GITR plays a key role in dominant immunological self-tolerance maintained by CD25+CD4+ regulatory T cells and could be a suitable molecular target for preventing or treating autoimmune disease.
No direct evidence has been reported whether the spatial organization of ICAM-1 on the cell surface is linked to its physiological function in terms of leukocyte adhesion and transendothelial migration (TEM). Here we observed that ICAM-1 by itself directly regulates the de novo elongation of microvilli and is thereby clustered on the microvilli. However, truncation of the intracellular domain resulted in uniform cell surface distribution of ICAM-1. Mutation analysis revealed that the C-terminal 21 amino acids are dispensable, whereas a segment of 5 amino acids ((507)RKIKK(511)) in the NH-terminal third of intracellular domain, is required for the proper localization and dynamic distribution of ICAM-1 and the association of ICAM-1 with F-actin, ezrin, and moesin. Importantly, deletion of the (507)RKIKK(511) significantly delayed the LFA-1-dependent membrane projection and decreased leukocyte adhesion and subsequent TEM. Endothelial cells treated with cell-permeant penetratin-ICAM-1 peptides comprising ICAM-1 RKIKK sequences inhibited leukocyte TEM. Collectively, these findings demonstrate that (507)RKIKK(511) is an essential motif for the microvillus ICAM-1 presentation and further suggest a novel regulatory role for ICAM-1 topography in leukocyte TEM.
Cytotoxic T cell (CTL) activation by antigen requires the specific detection of peptide–major histo-compatibility class I (pMHC) molecules on the target-cell surface by the T cell receptor (TCR). We examined the effect of mutations in the antigen-binding site of a Kb-restricted TCR on T cell activation, antigen binding and dissociation from antigen. These parameters were also examined for variants derived from a Kd-restricted peptide that was recognized by a CTL clone. Using these two independent systems, we show that T cell activation can be impaired by mutations that either decrease or increase the binding half-life of the TCR-pMHC interaction. Our data indicate that efficient T cell activation occurs within an optimal dwell-time range of TCR-pMHC interaction. This restricted dwell-time range is consistent with the exclusion of either extremely low or high affinity T cells from the expanded population during immune responses.
The small GTPase Rac1 orchestrates actin-dependent remodeling essential for numerous cellular processes including synapse development. While precise spatiotemporal regulation of Rac1 is necessary for its function, little is known about the mechanisms that enable Rac1 activators (GEFs) and inhibitors (GAPs) to act in concert to regulate Rac1 signaling. Here, we identify a regulatory complex composed of a Rac-GEF (Tiam1) and a Rac-GAP (Bcr) that cooperate to control excitatory synapse development. Disruption of Bcr function within this complex increases Rac1 activity and dendritic spine remodeling, resulting in excessive synaptic growth that is rescued by Tiam1 inhibition. Notably, EphB receptors utilize the Tiam1-Bcr complex to control synaptogenesis. Following EphB activation, Tiam1 induces Rac1-dependent spine formation, whereas Bcr prevents Rac1-mediated receptor internalization, promoting spine growth over retraction. The finding that a Rac-specific GEF/GAP complex is required to maintain optimal levels of Rac1 signaling provides an important insight into the regulation of small GTPases.
Tissue-resident memory T (Trm) cells provide enhanced protection against infection at mucosal sites. Here we found that CD4(+) T cells are important for the formation of functional lung-resident CD8(+) T cells after influenza virus infection. In the absence of CD4(+) T cells, CD8(+) T cells displayed reduced expression of CD103 (Itgae), were mislocalized away from airway epithelia, and demonstrated an impaired ability to recruit CD8(+) T cells to the lung airways upon heterosubtypic challenge. CD4(+) T cell-derived interferon-γ was necessary for generating lung-resident CD103(+) CD8(+) Trm cells. Furthermore, expression of the transcription factor T-bet was increased in "unhelped" lung Trm cells, and a reduction in T-bet rescued CD103 expression in the absence of CD4(+) T cell help. Thus, CD4(+) T cell-dependent signals are important to limit expression of T-bet and allow for the development of CD103(+) CD8(+) Trm cells in the lung airways following respiratory infection.
CD46 is a ubiquitous human cell surface receptor for the complement components C3b and C4b and for various pathogens, including the measles virus and human herpes virus 6. Ligand binding to CD46 affects (i) protection of autologous cells from complement attack by breakdown of complement components, (ii) intracellular signals that affect the regulation of immune cell function, (iii) antigen presentation, and (iv) down-regulation of cell surface CD46. Recent evidence indicates that CD46 signaling can link innate and acquired immune function. The molecular mechanisms for these processes and the importance of intracellular trafficking of the receptor have not yet been elucidated. We demonstrate here that, in nonlymphoid cells, CD46 is constitutively internalized via clathrin-coated pits, traffics to multivesicular bodies, and is recycled to the cell surface. However, cross-linking of CD46 at the cell surface, by either multivalent antibody or by measles virus, induces pseudopodia that engulf the ligand in a process similar to macropinocytosis, and leads to the degradation of cell surface CD46. Thus, we have elucidated two pathways for CD46 internalization, which are regulated by the valence of cross-linking of CD46 and which utilize either clathrin-coated pits or pseudopodial extension. This has important implications for CD46 signaling, antigen presentation, CD46 down-regulation, and engulfment of pathogens.
The generation of tumor-directed cytotoxic T lymphocytes is considered crucial for the induction of antitumor immunity. To activate these CD8(+) T cells, antigen-presenting cells (APCs) must initially acquire tumor cell-associated antigens. The major source of tumor antigens is dead tumor cells, but little is known about how APCs in draining lymph nodes acquire and crosspresent these antigens. Here we show that CD169(+) macrophages phagocytose dead tumor cells transported via lymphatic flow and subsequently crosspresent tumor antigens to CD8(+) T cells. Subcutaneous immunization with irradiated tumor cells protects mice from syngenic tumor. However, tumor antigen-specific CD8(+) T cell activation and subsequent antitumor immunity are severely impaired in mice depleted with CD169(+) macrophages. Neither migratory dendritic cells (DCs) nor lymph node-resident conventional DCs are essential for the crosspresentation of tumor antigens. Thus, we have identified CD169(+) macrophages as lymph node-resident APCs dominating early activation of tumor antigen-specific CD8(+) T cells.
We generated a series of knockin mouse lines, in which the cytokine receptor gp130-dependent STAT3 and/or SHP2 signals were disrupted, by replacing the mouse gp130 gene with human gp130 mutant cDNAs. The SHP2 signal-deficient mice (gp130F759/F759 were born normal but displayed splenomegaly and lymphadenopathy and an enhanced acute phase reaction. In contrast, the STAT3 signal-deficient mice (gp130FXQ/FXXQ) died perinatally, like the gp130-deficient mice (gp130D/D). The gp130F759/F759 mice showed prolonged gp130-induced STAT3 activation, indicating a negative regulatory role for SHP2. Th1-type cytokine production and IgG2a and IgG2b production were increased in the gp130F759/F759 mice, while they were decreased in the gp130FXXQ/FXXQ immune system. These results indicate that the balance of positive and negative signals generated through gp130 regulates the immune responses.
The transmembrane protein CD33 is a sialic acid-binding immunoglobulin-like lectin that regulates innate immunity but has no known functions in the brain. We have previously shown that the CD33 gene is a risk factor for Alzheimer's disease (AD). Here, we observed increased expression of CD33 in microglial cells in AD brain. The minor allele of the CD33 SNP rs3865444, which confers protection against AD, was associated with reductions in both CD33 expression and insoluble amyloid beta 42 (Aβ42) levels in AD brain. Furthermore, the numbers of CD33-immunoreactive microglia were positively correlated with insoluble Aβ42 levels and plaque burden in AD brain. CD33 inhibited uptake and clearance of Aβ42 in microglial cell cultures. Finally, brain levels of insoluble Aβ42 as well as amyloid plaque burden were markedly reduced in APP(Swe)/PS1(ΔE9)/CD33(-/-) mice. Therefore, CD33 inactivation mitigates Aβ pathology and CD33 inhibition could represent a novel therapy for AD.
Notch signals are necessary for the functional outcomes of T cell receptor β-selection, including differentiation, proliferation and rescue from apoptosis. The mechanism underlying this requirement for T cell development is unknown. Here we show that Notch receptor and Delta-like 1 ligand interactions promoted the survival of CD4−CD8− pre–T cells through the maintenance of cell size, glucose uptake and metabolism. Furthermore, the trophic effects of Notch signaling were mediated by the pathway of phosphatidylinositol-3-OH kinase and the kinase Akt, such that expression of active Atk overcame the requirement for Notch in β-selection. Collectively, our results demonstrate involvement of Notch receptor–ligand interactions in the regulation of cellular metabolism, thus enabling the autonomous signaling capacity of the pre–T cell receptor complex.*Note: In the version of this article initially published online, in the fourth sentence of the abstract, the term "Atk" was a misspelling; this should be "Akt. " In the fourth sentence of the second paragraph of the introduction, the name of the second kinase mentioned, "PI(3)K-dependent kinase 1," was incorrect; this should read "phosphoinositide-dependent kinase 1. " These errors have been corrected for the HTML and print versions of the article.
Immune tolerance and activation depend on precise control over the number and function of immunosuppressive Foxp3(+) regulatory T (T reg) cells, and the importance of IL-2 in maintaining tolerance and preventing autoimmunity is clear. However, the homeostatic requirement for IL-2 among specific populations of peripheral T reg cells remains poorly understood. We show that IL-2 selectively maintains a population of quiescent CD44(lo)CD62L(hi) T reg cells that gain access to paracrine IL-2 produced in the T cell zones of secondary lymphoid tissues due to their expression of the chemokine receptor CCR7. In contrast, CD44(hi)CD62L(lo)CCR7(lo) T reg cells that populate nonlymphoid tissues do not access IL-2-prevalent regions in vivo and are insensitive to IL-2 blockade; instead, their maintenance depends on continued signaling through the co-stimulatory receptor ICOS (inducible co-stimulator). Thus, we define a fundamental homeostatic subdivision in T reg cell populations based on their localization and provide an integrated framework for understanding how T reg cell abundance and function are controlled by unique signals in different tissue environments.
The cytokines IL-4, IL-13, and IL-5 are markers for the Th2 subset of effector T cells and are often expressed together. These cytokine genes are organized within 140 kb of orthologous DNA in both mouse and human. Using IL-4-expressing CD4+ T cell clones derived from F1 mice, we identified allelic polymorphisms for each of these cytokines and assessed the parental identity of the cytokine mRNAs. Both monoallelic and biallelic expression occurred for each gene and for an additional gene, IL-3, that lies with GM-CSF over 450 kb telomeric on the same chromosome. When coexpressed in T cell clones, IL-4 was expressed from the same allele as IL-13 or IL-5 in 81% of instances. In contrast, there was only 52% concordance of these three cytokines at the allelic level among clones that expressed IL-3. Independent expression of the cytokine alleles occurs commonly in T cells, but the clustered locus encompassing IL-4, IL-13, and IL-5 is subject to coordinate regulation.
INTRODUCTION • • CELLULAR AND MOLECULAR REQUIREMENTS FOR T-CELL ACTIVATION . The T-Cell Antigen Receptor Complex . . . .. . . . ..... . . . . . . . . . . . . . . . . ...... . . . T-Cell Activation by Antibodies and Leetins . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Other Cell Surface Structures (Accessory Molecules) Involved in Antigen Recognition and Activation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Minimal Requirements/or T-Cell Activation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . CONSEQUE�CES o�, T-CELL AC::IV A TION ; . ExpressIOn of ActIVatIOn Anllgens . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Mechanisms of Signal Transmission via the TCR Complex . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . The Mode of Control of Gene Expression during T-Cell Activation . . . . . . . . . . . . . . . . . . . . . . . . . . The Mechanism of Action of IL-2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Acquisition of Cytolytic Activity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . ANALOGIES WITH IMMATURE T CELLS .
Epidermal growth factor receptor (EGFR) is known to be critically involved in tissue development and homeostasis as well as in the pathogenesis of cancer. Here we showed that Foxp3(+) regulatory T (Treg) cells express EGFR under inflammatory conditions. Stimulation with the EGF-like growth factor Amphiregulin (AREG) markedly enhanced Treg cell function in vitro, and in a colitis and tumor vaccination model we showed that AREG was critical for efficient Treg cell function in vivo. In addition, mast cell-derived AREG fully restored optimal Treg cell function. These findings reveal EGFR as a component in the regulation of local immune responses and establish a link between mast cells and Treg cells. Targeting of this immune regulatory mechanism may contribute to the therapeutic successes of EGFR-targeting treatments in cancer patients.
Regulatory T (T reg) cells are critical regulators of immune tolerance. Most T reg cells are defined based on expression of CD4, CD25, and the transcription factor, FoxP3. However, these markers have proven problematic for uniquely defining this specialized T cell subset in humans. We found that the IL-7 receptor (CD127) is down-regulated on a subset of CD4+ T cells in peripheral blood. We demonstrate that the majority of these cells are FoxP3+, including those that express low levels or no CD25. A combination of CD4, CD25, and CD127 resulted in a highly purified population of T reg cells accounting for significantly more cells that previously identified based on other cell surface markers. These cells were highly suppressive in functional suppressor assays. In fact, cells separated based solely on CD4 and CD127 expression were anergic and, although representing at least three times the number of cells (including both CD25+CD4+ and CD25−CD4+ T cell subsets), were as suppressive as the “classic” CD4+CD25hi T reg cell subset. Finally, we show that CD127 can be used to quantitate T reg cell subsets in individuals with type 1 diabetes supporting the use of CD127 as a biomarker for human T reg cells.
Cell surface receptors have been extensively studied because they initiate and regulate signal transduction cascades leading to a variety of functional cellular outcomes. An important class of immune receptors (e.g., T-cell antigen receptors) whose ligands are anchored to the surfaces of other cells remain poorly understood. The mechanism by which ligand binding initiates receptor phosphorylation, a process termed "receptor triggering", remains controversial. Recently, direct measurements of the (two-dimensional) receptor-ligand complex lifetimes at cell-cell interface were found to be smaller than (three-dimensional) lifetimes in solution but the underlying mechanism is unknown. At the cell-cell interface, the receptor-ligand complex spans a short intermembrane distance (15 nm) compared to long surface molecules (LSMs) whose ectodomains span >40 nm and these LSMs include phosphatases (e.g., CD45) that dephosphorylate the receptor. It has been proposed that size-based segregation of LSMs from a receptor-ligand complex is a mechanism of receptor triggering but it is unclear whether the mechanochemistry supports such small-scale segregation. Here we present a nanometer-scale mathematical model that couples membrane elasticity with the compressional stiffness and lateral mobility of LSMs. We find robust supradiffusive segregation of LSMs from a single receptor-ligand complex. The model predicts that LSM redistribution will result in a time-dependent tension on the complex leading to a decreased two-dimensional lifetime. Interestingly, the model predicts a nonlinear relationship between the three- and two-dimensional lifetimes, which can enhance the ability of receptors to discriminate between similar ligands.
T cells are activated by antigen (Ag)-bearing dendritic cells (DCs) in lymph nodes in three phases. The duration of the initial phase of transient, serial DC-T cell interactions is inversely correlated with Ag dose. The second phase, characterized by stable DC-T cell contacts, is believed to be necessary for full-fledged T cell activation. Here we have shown that this is not the case. CD8⁺ T cells interacting with DCs presenting low-dose, short-lived Ag did not transition to phase 2, whereas higher Ag dose yielded phase 2 transition. Both antigenic constellations promoted T cell proliferation and effector differentiation but yielded different transcriptome signatures at 12 hr and 24 hr. T cells that experienced phase 2 developed long-lived memory, whereas conditions without stable contacts yielded immunological amnesia. Thus, T cells make fate decisions within hours after Ag exposure, resulting in long-term memory or abortive effector responses, correlating with T cell-DCs interaction kinetics.