Thymocyte sensitivity and supramolecular activation cluster formation are developmentally regulated: a partial role for sialylation

Sialic acid in the regulation of blood cell production, differentiation and turnover

Sialic acid is a unique sugar moiety that resides in the distal and most accessible position of the glycans on mammalian cell surface and extracellular glycoproteins and glycolipids. The potential for sialic acid to obscure underlying structures has long been postulated, but the means by which such structural changes directly affect biological processes continues to be elucidated. Here, we appraise the growing body of literature detailing the importance of sialic acid for the generation, differentiation, function and death of haematopoietic cells. We conclude that sialylation is a critical post-translational modification utilized in haematopoiesis to meet the dynamic needs of the organism by enforcing rapid changes in availability of lineage-specific cell types. Though long thought to be generated only cell-autonomously within the intracellular ER-Golgi secretory apparatus, emerging data also demonstrate previously unexpected diversity in the mechanisms of sialylation. Emphasis is afforded to the mechanism of extrinsic sialylation, whereby extracellular enzymes remodel cell surface and extracellular glycans, supported by charged sugar donor molecules from activated platelets.

Tumor glucose metabolism and the T cell glycocalyx: implication for T cell function

The T cell is an immune cell subset highly effective in eliminating cancer cells. Cancer immunotherapy empowers T cells and occupies a solid position in cancer treatment. The response rate, however, remains relatively low (<30%). The efficacy of immunotherapy is highly dependent on T cell infiltration into the tumor microenvironment (TME) and the ability of these infiltrated T cells to sustain their function within the TME. A better understanding of the inhibitory impact of the TME on T cells is crucial to improve cancer immunotherapy. Tumor cells are well described for their switch into aerobic glycolysis (Warburg effect), resulting in high glucose consumption and a metabolically distinct TME. Conversely, glycosylation, a predominant posttranslational modification of proteins, also relies on glucose molecules. Proper glycosylation of T cell receptors influences the immunological synapse between T cells and tumor cells, thereby affecting T cell effector functions including their cytolytic and cytostatic activities. This review delves into the complex interplay between tumor glucose metabolism and the glycocalyx of T cells, shedding light on how the TME can induce alterations in the T cell glycocalyx, which can subsequently influence the T cell's ability to target and eliminate tumor cells.

Hyper α2,6-Sialylation Promotes CD4+ T-Cell Activation and Induces the Occurrence of Ulcerative Colitis

α2,6-sialylation, catalyzed by α2,6-sialyltransferase (ST6GAL1), plays a pivotal role in immune responses. However, the role of ST6GAL1 in the pathogenesis of ulcerative colitis (UC) remains unknown. ST6GAL1 mRNA is highly expressed in UC tissues compared with the corresponding adjacent normal tissues, and α2,6-sialylation is significantly increased in the colon tissues of patients with UC. The expression of ST6GAL1 and proinflammatory cytokines, such as interleukin (IL)-2, IL-6, IL-17, and interferon-gamma, is also increased. The number of CD4+ T cells increases in UC patients. St6gal1 gene knockout (St6gal1-/- ) rats are established by clustered regularly interspaced short palindromic repeats (CRISPR)-associated gene knockout system. St6gal1 deficiency reduces the levels of pro-inflammatory cytokines and alleviates colitis symptoms in UC model rats. Ablation of α2,6-sialylation inhibits the transport of the TCR to lipid rafts and suppresses CD4+ T-cell activation. The attenuation of TCR signaling downregulates the expression of NF-κB in ST6GAL1-/- CD4+ T-cells. Moreover, NF-κB could bind to the ST6GAL1 promoter to increase its transcription. Ablation of ST6GAL1 downregulates the expression of NF-κB and reduces the production of proinflammatory cytokines to relieve UC pathogenesis, which is a potential novel target for the clinical treatment of UC.

In Vitro Analysis of Thymocyte Signaling

When a developing thymocyte expresses a TCR, it is subjected to numerous interactions with self-peptide/MHC complexes that determine its fate. These include death by neglect, negative selection (apoptosis and lineage deviation), positive selection, and lineage commitment. Identifying signals that govern these unique cell fates requires the ability to assess the activity, level of expression, subcellular location, and molecular associations between numerous proteins within the developing T cell. Given the unique, temporal, and developmental changes that occur during development, isolating and analyzing small populations of thymocytes are necessary to get a complete picture of the development process. Thus, this chapter describes methods designed to analyze thymocyte signaling under various types of peptide-based stimulation in vitro.

Dynamics of the Coreceptor-LCK Interactions during T Cell Development Shape the Self-Reactivity of Peripheral CD4 and CD8 T Cells

Overtly self-reactive T cells are removed during thymic selection. However, it has been recently established that T cell self-reactivity promotes protective immune responses. Apparently, the level of self-reactivity of mature T cells must be tightly balanced. Our mathematical model and experimental data show that the dynamic regulation of CD4- and CD8-LCK coupling establish the self-reactivity of the peripheral T cell pool. The stoichiometry of the interaction between CD8 and LCK, but not between CD4 and LCK, substantially increases upon T cell maturation. As a result, peripheral CD8⁺ T cells are more self-reactive than CD4⁺ T cells. The different levels of self-reactivity of mature CD8⁺ and CD4⁺ T cells likely reflect the unique roles of these subsets in immunity. These results indicate that the evolutionary selection pressure tuned the CD4-LCK and CD8-LCK stoichiometries, as they represent the unique parts of the proximal T cell receptor (TCR) signaling pathway, which differ between CD4⁺ and CD8⁺ T cells.

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Coupling of CD8-LCK but not CD4-LCK increases upon T cell maturation

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Dynamics of coreceptor-LCK coupling stoichiometry establish T cell self-reactivity

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CD8⁺ T cells are more self-reactive than CD4⁺ T cells

T-cell receptor affinity in the age of cancer immunotherapy

The strength of the interaction between T-cell receptors (TCRs) and their ligands, peptide/major histocompatibility complex complexes (pMHCs), is one of the most frequently discussed and investigated features of T cells in immuno-oncology today. Although there are many molecules on the surface of T cells that interact with ligands on other cells, the TCR/pMHC is the only receptor-ligand pair that offers antigen specificity and dictates the functional response of the T cell. The strength of the TCR/pMHC interaction, along with the environment in which this interaction takes place, is key to how the T cell will respond. The TCR repertoire of T cells that interact with tumor-associated antigens is vast, although typically of low affinity. Here, we focus on the low-affinity interactions between TCRs from CD8+ T cells and different models used in immuno-oncology.

Human T cell glycosylation and implications on immune therapy for cancer

Glycosylation is an important post-translational modification, giving rise to a diverse and abundant repertoire of glycans on the cell surface, collectively known as the glycome. When focusing on immunity, glycans are indispensable in virtually all signaling and cell-cell interactions. More specifically, glycans have been shown to regulate key pathophysiological steps within T cell biology such as T cell development, thymocyte selection, T cell activity and signaling as well as T cell differentiation and proliferation. They are of major importance in determining the interaction of human T cells with tumor cells. In this review, we will describe the role of glycosylation of human T cells in more depth, elaborate on the importance of glycosylation in the interaction of human T cells with tumor cells and discuss the potential of cancer immunotherapies that are based on manipulating the glycome functions at the tumor immune interface.^1,2

CD8 Binding of MHC-Peptide Complexes in cis or trans Regulates CD8+ T-cell Responses

The coreceptor CD8αβ can greatly promote activation of T cells by strengthening T-cell receptor (TCR) binding to cognate peptide-MHC complexes (pMHC) on antigen presenting cells and by bringing p56^Lck to TCR/CD3. Here, we demonstrate that CD8 can also bind to pMHC on the T cell (in cis) and that this inhibits their activation. Using molecular modeling, fluorescence resonance energy transfer experiments on living cells, biochemical and mutational analysis, we show that CD8 binding to pMHC in cis involves a different docking mode and is regulated by posttranslational modifications including a membrane-distal interchain disulfide bond and negatively charged O-linked glycans near positively charged sequences on the CD8β stalk. These modifications distort the stalk, thus favoring CD8 binding to pMHC in cis. Differential binding of CD8 to pMHC in cis or trans is a means to regulate CD8⁺ T-cell responses and provides new translational opportunities.

Cosmc is required for T cell persistence in the periphery

T lymphocytes, a key arm of adaptive immunity, are known to dynamically regulate O-glycosylation during T cell maturation and when responding to stimuli; however, the direct role of O-glycans in T cell maturation remains largely unknown. Using a conditional knockout of the gene (C1GalT1C1 or Cosmc) encoding the specific chaperone Cosmc, we generated mice whose T cells lack extended O-glycans (T cell conditional Cosmc knock out or TCKO mice) and homogeneously express the truncated Tn antigen. Loss of Cosmc is highly deleterious to T cell persistence, with near-complete elimination of Cosmc-null T cells from spleen and lymph nodes. Total T cell counts are 20% of wild type (WT), among which only 5% express the truncated glycans, with the remaining 95% consisting of escapers from Cre-mediated recombination. TCKO thymocytes were able to complete thymic maturation but failed to populate the secondary lymphoid organs both natively and upon adoptive transfer to WT recipients. Our results demonstrate that extended O-glycosylation is required for the establishment and maintenance of the peripheral T cell population.

Ablation of core fucosylation attenuates the signal transduction via T cell receptor to suppress the T cell development

Precise glycosylation plays a crucial and distinctive role in thymic T cell development. The core fucosylation is dramatically up-regulated at the transition from CD4^-CD8^- (DN) to CD4⁺CD8⁺ (DP) in the thymic development. Ablation of core fucosylation in T cells did reduce the size of the thymus due to a significant loss of CD4⁺ SP, CD8⁺ SP and DP thymocytes in core fucosyltransferase (Fut8) knockout (Fut8^-/-) mice. T cell receptors (TCRs) are heavily core fucosylated glycoproteins. Loss of core fucosylation of TCR contributed to the reduced phosphorylation of ZAP70 (pZAP70) in Fut8^-/- DP cells was observed. Compare to the Fut8^+/+OT-II DP thymocytes, pZAP70 was significantly reduced in Fut8^-/- OT-II DP thymocytes with OVA_323-339 stimulation. Also, the pZAP70 of Fut8^+/+OT-I DP thymocytes with OVA_257-264 stimulation was remarkably attenuated by treatment of the fucosidase. Upon anti-CD3/CD28 Abs stimulation, the increased apoptosis was found in Fut8^-/- thymocytes compared with Fut8^+/+ thymocytes. Moreover, the TCR^hiCD69^hi (post-positive selection thymocytes) was markedly depleted in the Fut8^-/- thymus without any stimulation. The expression of CD5 was significantly down-regulated on the DP cells in the Fut8^-/- thymus. Our results therefore demonstrate that ablation of core fucosylation results in the abnormal T cell development due to the attenuated signaling via TCR.

Core Fucosylation of the T Cell Receptor Is Required for T Cell Activation

CD4⁺ T cell activation promotes the pathogenic process of systemic lupus erythematosus (SLE). T cell receptor (TCR) complex are highly core fucosylated glycoproteins, which play important roles in T cell activation. In this study, we found that the core fucosylation of CD4⁺ T cells was significantly increased in SLE patients. Loss of core fucosyltransferase (Fut8), the sole enzyme for catalyzing the core fucosylation of N-glycan, significantly reduced CD4⁺ T cell activation and ameliorated the experimental autoimmune encephalomyelitis-induced syndrome in Fut8^−/− mice. T cell activation with OVA_323–339 loaded major histocompatibility complex II (pMHC-II) on B cell was dramatically attenuated in Fut8^−/−OT-II CD4⁺ T cells compared with Fut8^+/+OT-II CD4⁺ T cells. Moreover, the phosphorylation of ZAP-70 was significantly reduced in Fut8^+/+OT-II CD4⁺ T cells by the treatment of fucosidase. Our results suggest that core fucosylation is required for efficient TCR–pMHC-II contacts in CD4⁺ T cell activation, and hyper core fucosylation may serve as a potential novel biomarker in the sera from SLE patients.

Characteristics of TCR/CD3 complex CD3{varepsilon} chains of regulatory CD4+ T (Treg) lymphocytes: role in Treg differentiation in vitro and impact on Treg in vivo

Tregs are anergic CD4(+)CD25(+)Foxp3(+) T lymphocytes exerting active suppression to control immune and autoimmune responses. However, the factors in TCR recognition underlying Treg differentiation are unclear. Based on our previous data, we hypothesized that Treg TCR/CD3 antigen receptor complexes might differ from those of CD4(+)CD25(-) Tconv. Expression levels of TCR/CD3, CD3ε,ζ chains, or other molecules involved in antigen signaling and the characteristics of CD3ε chains were analyzed in thymus or spleen Treg cells from normal mice. Tregs had quantitative and qualitatively distinct TCR/CD3 complexes and CD3ε chains. They expressed significantly lower levels of the TCR/CD3 antigen receptor, CD3ε chains, TCR-ζ chain, or the CD4 coreceptor than Tconv. Levels of kinases, adaptor molecules involved in TCR signaling, and early downstream activation pathways were also lower in Tregs than in Tconv. Furthermore, TCR/CD3 complexes in Tregs were enriched in CD3ε chains conserving their N-terminal, negatively charged amino acid residues; this trait is linked to a higher activation threshold. Transfection of mutant CD3ε chains lacking these residues inhibited the differentiation of mature CD4(+)Foxp3(-) T lymphocytes into CD4(+)Foxp3(+) Tregs, and differences in CD3ε chain recognition by antibodies could be used to enrich for Tregs in vivo. Our results show quantitative and qualitative differences in the TCR/CD3 complex, supporting the hyporesponsive phenotype of Tregs concerning TCR/CD3 signals. These differences might reconcile avidity and flexible threshold models of Treg differentiation and be used to implement therapeutic approaches involving Treg manipulation.

Maturation and emigration of single-positive thymocytes

T lymphopoiesis in the thymus was thought to be completed once it reaches the single positive (SP) stage, a stage when T cells are “fully mature” and waiting to be exported at random or follow a “first-in-first-out” manner. Recent evidence, however, has revealed that the newly generated SP thymocytes undergo a multistage maturation program in the thymic medulla. Such maturation is followed by a tightly regulated emigration process and a further postthymic maturation of recent thymic emigrants (RTEs). This review summarizes recent progress in the late stage T cell development. The regulation of this developmental process is discussed.

Individual MHCI-Restricted T-Cell Receptors are Characterized by a Unique Peptide Recognition Signature

Effective immunity requires that a limited TCR repertoire is able to recognize a vast number of foreign peptide-MHCI (peptide-major histocompatibility complex class I) molecules. This challenge is overcome by the ability of individual TCRs to recognize large numbers of peptides. Recently, it was demonstrated that MHCI-restricted TCRs can recognize up to 10⁶ peptides of a defined length. Astonishingly, this remarkable level of promiscuity does not extend to peptides of different lengths, a fundamental observation that has broad implications for CD8⁺ T-cell immunity. In particular, the findings suggest that effective immunity can only be achieved by mobilization of “length-matched” CD8⁺ T-cell clonotypes. Overall, recent findings suggest that every TCR is specific for a unique set of peptides, which can be described as a unique “peptide recognition signature” (PRS) and consists of three components: (1) peptide length preference, (2) number of peptides recognized; and, (3) sequence identity (e.g., self versus pathogen derived). In future, the ability to de-convolute peptide recognition signatures across the normal and pathogenic repertoire will be essential for understanding the system requirements for effective CD8⁺ T-cell immunity and elucidating mechanisms which underlie CD8⁺ T-cell mediated disease.

Peptide length determines the outcome of TCR/peptide-MHCI engagement

αβ-TCRs expressed at the CD8(+) T-cell surface interact with short peptide fragments (p) bound to MHC class I molecules (pMHCI). The TCR/pMHCI interaction is pivotal in all aspects of CD8(+) T-cell immunity. However, the rules that govern the outcome of TCR/pMHCI engagement are not entirely understood, and this is a major barrier to understanding the requirements for both effective immunity and vaccination. In the present study, we discovered an unexpected feature of the TCR/pMHCI interaction by showing that any given TCR exhibits an explicit preference for a single MHCI-peptide length. Agonists of nonpreferred length were extremely rare, suboptimal, and often entirely distinct in sequence. Structural analysis indicated that alterations in peptide length have a major impact on antigenic complexity, to which individual TCRs are unable to adapt. This novel finding demonstrates that the outcome of TCR/pMHCI engagement is determined by peptide length in addition to the sequence identity of the MHCI-bound peptide. Accordingly, the effective recognition of pMHCI Ag, which is a prerequisite for successful CD8(+) T-cell immunity and protective vaccination, can only be achieved by length-matched Ag-specific CD8(+) T-cell clonotypes.

Subcellular distribution of Lck during CD4 T-cell maturation in the thymic medulla regulates the T-cell activation threshold

Mature peripheral T cells respond to foreign but not to self-antigens. During development in the thymus, deletion of high-affinity self-reactive immature thymocytes contributes to tolerance of mature T cells. However, double-positive thymocytes are positively selected to survive if they respond to self-peptide-MHC complexes; thus, there must be mechanisms to prevent overt reactivity to those same complexes in the periphery. "Developmental tuning" is the active process through which T-cell receptor (TCR)-associated signaling pathways of single-positive (SP) thymocytes are attenuated to respond appropriately to self-peptide-MHC complexes in the periphery. We previously showed that MHC class II expression in the thymic medulla was necessary to tune CD4(+) SP (CD4 SP) thymocytes. CD4 SP thymocytes from mice lacking medullary MHC class II expression had inappropriately enhanced proximal TCR signaling to low-affinity self-ligands that was associated with altered cellular distribution of the tyrosine kinase Lck. Now, we report that activation of both tuned and untuned CD4 SP thymocytes is Lck-dependent. Untuned CD4 SP cells contain a pool of Lck with increased basal phosphorylation that is not associated with the CD4 coreceptor. Phosphorylation of this pool of Lck decreases with tuning. Immunogold transmission electron microscopy of membrane sheets permitted direct visualization of Lck. In the absence of tuning, a significant proportion of Lck and the TCR subunit CD3ζ are expressed on the same protein island; this close association of Lck and the TCR probably explains the enhanced activation of untuned CD4 SP cells. Thus, changes in membrane topography during thymic maturation determine the set point for TCR responsiveness.

microRNAs at the regulatory frontier: an investigation into how microRNAs impact the development and effector functions of CD4 T cells

CD4 T cells are an integral part of adaptive immunity. microRNAs have been identified as fundamental regulators of post-transcriptional programs and to play roles in T lymphocytes' development, differentiation, and effector functions. To better understand the role of miRNAs in T cells and to identify potential therapeutic tools and targets, we have undertaken studies of miRNAs that modulate or are modulated by T-cell receptor signaling. We identified miR-181a as a key regulator of TCR signaling strength, and hence T-cell development, and the miR-17-92 cluster as an important player in CD4 T cells' response against antigens. These discoveries, coupled with work by other researchers, reveal the power and importance of miRNA-mediated regulation in T-cell responses and offer new insights into the burgeoning field of immunoregulation.

Identification of glycoproteins targeted by Trypanosoma cruzi trans-sialidase, a virulence factor that disturbs lymphocyte glycosylation

Trypanosoma cruzi, the agent of the American trypanosomiasis or Chagas disease, bypasses its lack of de novo synthesis of sialic acids by expressing a surface-anchored trans-sialidase. This enzyme transfers sialic acid residues from the host's sialylglycoconjugates to the parasite's galactosylglycoconjugates. In addition to carrying out a pivotal role in parasite persistence/replication within the infected mammal, the trans-sialidase is shed into the bloodstream and induces alterations in the host immune system by modifying the sialylation of the immune cells. A major obstacle to understand these events is the difficulty to identify the transferred sialic acid among all those naturally occurring on the cell surface. Here, we report the use of azido-modified unnatural sialic acid to identify those molecules that act as cell surface acceptors of the sialyl residue in the trans-sialidase-catalyzed reaction, which might then be involved in the immune alterations induced. In living parasites, we readily observed the transfer of azido-sialic acid to surface mucins. When evaluating mouse thymocytes and splenocytes as acceptors of the azido-sugar, a complex pattern of efficiently tagged glycoproteins was revealed. In both leukocyte populations, the main proteins labeled were identified as different CD45 isoforms. Disruption of the cell architecture increased the number and the molecular weight distribution of azido-sialic acid tagged proteins. Nevertheless, CD45 remained to be the main acceptor. Mass spectrometry assays allowed us to identify other acceptors, mainly integrins. The findings reported here provide a molecular basis to understand the abnormalities induced in the immune system by the trans-sialidase during T. cruzi infection.

Functional development of the T cell receptor for antigen

For over three decades now, the T cell receptor (TCR) for antigen has not ceased to challenge the imaginations of cellular and molecular immunologists alike. T cell antigen recognition transcends every aspect of adaptive immunity: it shapes the T cell repertoire in the thymus and directs T cell-mediated effector functions in the periphery, where it is also central to the induction of peripheral tolerance. Yet, despite its central position, there remain many questions unresolved: how can one TCR be specific for one particular peptide-major histocompatibility complex (pMHC) ligand while also binding other pMHC ligands with an immunologically relevant affinity? And how can a T cell's extreme specificity (alterations of single methyl groups in their ligand can abrogate a response) and sensitivity (single agonist ligands on a cell surface are sufficient to trigger a measurable response) emerge from TCR-ligand interactions that are so low in affinity? Solving these questions is intimately tied to a fundamental understanding of molecular recognition dynamics within the many different contexts of various T cell-antigen presenting cell (APC) contacts: from the thymic APCs that shape the TCR repertoire and guide functional differentiation of developing T cells to the peripheral APCs that support homeostasis and provoke antigen responses in naïve, effector, memory, and regulatory T cells. Here, we discuss our recent findings relating to T cell antigen recognition and how this leads to the thymic development of foreign-antigen-responsive alphabetaT cells.

The activation threshold of CD4+ T cells is defined by TCR/peptide-MHC class II interactions in the thymic medulla

Immature thymocytes that are positively selected based upon their response to self-peptide-MHC complexes develop into mature T cells that are not overtly reactive to those same complexes. Developmental tuning is the active process through which TCR-associated signaling pathways of single-positive thymocytes are attenuated to respond appropriately to the peptide-MHC molecules that will be encountered in the periphery. In this study, we explore the mechanisms that regulate the tuning of CD4(+) single-positive T cells to MHC class II encountered in the thymic medulla. Experiments with murine BM chimeras demonstrate that tuning can be mediated by MHC class II expressed by either thymic medullary epithelial cells or thymic dendritic cells. Tuning does not require the engagement of CD4 by MHC class II on stromal cells. Rather, it is mediated by interactions between MHC class II and the TCR. To understand the molecular changes that distinguish immature hyperactive T cells from tuned mature CD4(+) T cells, we compared their responses to TCR stimulation. The altered response of mature CD4 single-positive thymocytes is characterized by the inhibition of ERK activation by low-affinity self-ligands and increased expression of the inhibitory tyrosine phosphatase SHP-1. Thus, persistent TCR engagement by peptide-MHC class II on thymic medullary stroma inhibits reactivity to self-Ags and prevents autoreactivity in the mature repertoire.

Endosomal TLR signaling is required for anti-nucleic acid and rheumatoid factor autoantibodies in lupus

Using the Unc93b1 3d mutation that selectively abolishes nucleic acid-binding Toll-like receptor (TLR) (TLR3, -7, -9) signaling, we show these endosomal TLRs are required for optimal production of IgG autoAbs, IgM rheumatoid factor, and other clinical parameters of disease in 2 lupus strains, B6-Fas(lpr) and BXSB. Strikingly, treatment with lipid A, an autoAb-inducing TLR4 agonist, could not overcome this requirement. The 3d mutation slightly reduced complete Freund's adjuvant (CFA)-mediated antigen presentation, but did not affect T-independent type 1 or alum-mediated T-dependent humoral responses or TLR-independent IFN production induced by cytoplasmic nucleic acids. These findings suggest that nucleic acid-sensing TLRs might act as an Achilles' heel in susceptible individuals by providing a critical pathway by which relative tolerance for nucleic acid-containing antigens is breached and systemic autoimmunity ensues. Importantly, this helps provide an explanation for the high frequency of anti-nucleic acid Abs in lupus-like systemic autoimmunity.

Low ligand requirement for deletion and lack of synapses in positive selection enforce the gauntlet of thymic T cell maturation

Immature double-positive (CD4(+)CD8(+)) thymocytes respond to negatively selecting peptide-MHC ligands by forming an immune synapse that sustains contact with the antigen-presenting cell (APC). Using fluorescently labeled peptides, we showed that as few as two agonist ligands could promote APC contact and subsequent apoptosis in reactive thymocytes. Furthermore, we showed that productive signaling for positive selection, as gauged by nuclear translocation of a green fluorescent protein (GFP)-labeled NFATc construct, did not involve formation of a synapse between thymocytes and selecting epithelial cells in reaggregate thymus cultures. Antibody blockade of endogenous positively selecting ligands prevented NFAT nuclear accumulation in such cultures and reversed NFAT accumulation in previously stimulated thymocytes. Together, these data suggest a "gauntlet" model in which thymocytes mature by continually acquiring and reacquiring positively selecting signals without sustained contact with epithelial cells, thereby allowing them to sample many cell surfaces for potentially negatively selecting ligands.

Characterizing the glycome of the mammalian immune system

The outermost layer of all immune cells, the glycocalyx, is composed of a complex mixture of glycoproteins, glycolipids and lectins, which specifically recognize particular glycan epitopes. As the glycocalyx is the cell's primary interface with the external environment many biologically significant events can be attributed to glycan recognition. For this reason the rapidly expanding glycomics field is being increasingly recognized as an important component in our quest to better understand the functioning of the immune system. In this review, we highlight the current status of immune cell glycomics, with particular attention being paid to T- and B-lymphocytes and dendritic cells. We also describe the strategies and methodologies used to define immune cell glycomes.

A role for CD8 in the developmental tuning of antigen recognition and CD3 conformational change

TCR engagement by peptide-MHC class I (pMHC) ligands induces a conformational change (Deltac) in CD3 (CD3Deltac) that contributes to T cell signaling. We found that when this interaction took place between primary T lineage cells and APCs, the CD8 coreceptor was required to generate CD3Deltac. Interestingly, neither enhancement of Ag binding strength nor Src kinase signaling explained this coreceptor activity. Furthermore, Ag-induced CD3Deltac was developmentally attenuated by the increase in sialylation that accompanies T cell maturation and limits CD8 activity. Thus, both weak and strong ligands induced CD3Deltac in preselection thymocytes, but only strong ligands were effective in mature T cells. We propose that CD8 participation in the TCR/pMHC interaction can physically regulate CD3Deltac induction by "translating" productive Ag encounter from the TCR to the CD3 complex. This suggests one mechanism by which the developmentally regulated variation in CD8 sialylation may contribute to the developmental tuning of T cell sensitivity.

Biological applications of a chimeric probe for the assessment of galectin-3 ligands

Beta1-6 branching of N-linked oligosaccharides has been correlated with the progression of different cancers. The leukoagglutinins of Phaseolus vulgaris (L-PHA) have been used to study this pattern of glycosylation whose biological significance is incompletely understood. The animal lectin, galectin-3, also binds to structures recognized by L-PHA. To develop a functional tool for the in situ identification of this pattern of glycosylation, human galectin-3 was fused to bacterial alkaline phosphatase (gal3/AP). Gal3/AP recognized both A and B blood group saccharides (B>A) and lactosamine derivatives. Gal3/AP recognition depended at least in part on the N-linked oligosaccharides of different glycoproteins. The presence and distribution of galectin-3 ligands were analyzed in both murine and human normal and tumor samples. Loss of apical expression of galectin-3 ligands was commonly found in carcinomas. Endothelial and inflammatory cells were enriched in galectin-3 ligands as compared with tumor cells; thus, gal3/AP is a suitable tool for studying tumor microenvironments. Comparative analysis of both gal3/AP and L-PHA binding patterns indicated that although similar, these patterns are not identical. The probe developed was useful for several immunoenzymatic assays and will allow the physiological and clinical significance of the expression pattern of galectin-3 ligands to be established. This manuscript contains online supplemental material at http://www.jhc.org. Please visit this article online to view these materials.

Enhancement of CD8 T-cell function through modifying surface glycoproteins in young and old mice

Previous work from our laboratory has shown that modifying cell surface glycosylation with either a Clostridium perfringens-derived sialidase (CP-Siase), or an O-linked glycoprotein endopeptidase (OSGE) can enhance the function of CD4 T cells from both young and old mice at multiple levels. Here we have re-assessed the effect of age on CD8 T-cell function, and examined the outcome of enzymatic treatment with CP-Siase and OSGE on its different aspects. Pre-treatment of CD8 T cells with either CP-Siase or OSGE led to a significant increase in anti-CD3-mediated Ca2+ response in both young and old mice. Pre-treated CD8 T cells from both age groups also displayed a significant increase in activation-induced CD69 and CD25 expression, and produced significantly higher amounts of interleukin-2 and interferon-gamma in comparison to their untreated counterparts. Furthermore, pretreatment with either enzyme enhanced granzyme B expression in CD8 T cells, and increased their cytolytic activity in vitro. These data support the notion that glycosylated surface proteins hinder CD8 T-cell activation and function in both young and old mice, and raise the possibility of significantly improving CD8 T cell function in older individuals through enzymatic alteration of surface glycoproteins.

Activation of murine CD4+ and CD8+ T lymphocytes leads to dramatic remodeling of N-linked glycans

Differentiation and activation of lymphocytes are documented to result in changes in glycosylation associated with biologically important consequences. In this report, we have systematically examined global changes in N-linked glycosylation following activation of murine CD4 T cells, CD8 T cells, and B cells by MALDI-TOF mass spectrometry profiling, and investigated the molecular basis for those changes by assessing alterations in the expression of glycan transferase genes. Surprisingly, the major change observed in activated CD4 and CD8 T cells was a dramatic reduction of sialylated biantennary N-glycans carrying the terminal NeuGcalpha2-6Gal sequence, and a corresponding increase in glycans carrying the Galalpha1-3Gal sequence. This change was accounted for by a decrease in the expression of the sialyltransferase ST6Gal I, and an increase in the expression of the galactosyltransferase, alpha1-3GalT. Conversely, in B cells no change in terminal sialylation of N-linked glycans was evident, and the expression of the same two glycosyltransferases was increased and decreased, respectively. The results have implications for differential recognition of activated and unactivated T cells by dendritic cells and B cells expressing glycan-binding proteins that recognize terminal sequences of N-linked glycans.

The sialyltransferase ST3Gal-I is not required for regulation of CD8-class I MHC binding during T cell development

The CD8 coreceptor plays a crucial role in thymocyte and T cell sensitivity by binding to class I MHC and recruiting downstream signaling molecules to the TCR. Previous studies reported considerable changes in TCR-independent CD8/class I MHC binding (i.e., CD8 noncognate interactions) during T cell development, changes that correlated with altered glycosylation of surface molecules. In particular, expression of the sialyltransferase ST3Gal-I has been proposed as a critical factor regulating the attenuation of CD8 avidity during the double-positive to CD8 single-positive progression. This hypothesis is strengthened by the fact that ST3Gal-I(-/-) animals show a profound disregulation of CD8 T cell homeostasis. In contrast to this model, however, we report in this study that ST3Gal-I deficiency had no detectable impact on CD8 noncognate binding to multimeric peptide/MHC class I ligands at any stage of thymocyte development. We also found that the susceptibility to CD8-induced cell death is not markedly influenced by ST3Gal-I deficiency. Thus, the profound effects of ST3Gal-I on CD8 T cell survival evidently do not involve a role for this enzyme in controlling CD8-class I binding.

Defective adhesion in tumor infiltrating CD8+ T cells

CD8(+) tumor-infiltrating lymphocytes (TIL) are defective in cytolysis due to tumor-induced inhibition of proximal TCR-mediated signaling, a defect that is relieved upon purification and brief culture. We show in this study that frequency of conjugation in vitro of nonlytic TIL with tumor cells is low in comparison with their lytic counterparts, and the strength of interaction and duration of conjugation are also reduced. Previous reports show that p56(lck) activation is required for TCR-initiated LFA-1 avidity up-regulation, raising the question: is low LFA-1 avidity the basis of reduced TIL conjugation frequency? When stimulated with phorbol ester, nonlytic TIL bind purified ICAM-1 equivalently as lytic TIL, suggesting that LFA-1 can be activated if proximal TCR signaling is bypassed. However, when treated with phorbol ester, the conjugation frequency of nonlytic TIL does not increase. CD2 and CD8 also mediate T cell adhesion to cognate target cells and are both expressed at lower levels in nonlytic TIL in addition to being excluded from the immune synapse formed upon conjugation. Collectively, these results imply that adhesion defects in nonlytic TIL result from a combination of decreased cell surface levels of adhesion molecules, deficient LFA-1 activation, and the failure to recruit essential adhesion receptors to the membrane contact site formed with cognate target cells.

Sialoadhesin-deficient mice exhibit subtle changes in B- and T-cell populations and reduced immunoglobulin M levels

Sialoadhesin (Sn, also called Siglec-1 or CD169) is a transmembrane receptor and the prototypic member of the Siglec family of sialic acid binding immunoglobulin-like lectins. It is expressed on specialized subsets of resident macrophages in hematopoietic and lymphoid tissues and on inflammatory macrophages. In order to investigate its function, we generated Sn-deficient mice and confirmed that these mice are true nulls by fluorescence-activated cell sorter analysis and immunohistochemistry. Mice deficient in Sn were viable and fertile and showed no developmental abnormalities. Analysis of cell populations revealed no differences in bone marrow, peritoneal cavity, and thymus, but there was a small increase in CD8 T cells and a decrease in B220-positive cells in spleens and lymph nodes of Sn-deficient mice. Furthermore, in spleen there was a slight decrease in follicular B cells with an increase in numbers of marginal zone B cells. B- and T-cell maturation as well as responses to stimulation with thioglycolate were only slightly affected by Sn deficiency. Immunoglobulin titers in Sn-deficient mice were significantly decreased for immunoglobulin M (IgM) but similar for IgG subclasses. These results suggest a role for sialoadhesin in regulating cells of the immune system rather than in influencing steady-state hematopoiesis.

Attenuated demyelination in the absence of the macrophage-restricted adhesion molecule sialoadhesin (Siglec-1) in mice heterozygously deficient in P0

Mouse mutants heterozygously deficient for the myelin component P0 mimic some forms of inherited neuropathies in humans. We have previously shown that both T lymphocytes and macrophages contribute to the demyelinating neuropathy. Both cell types appear to influence each other mutually, i.e., impaired T lymphocyte development in RAG-1-deficient P0 mutants leads to decreased macrophage numbers and retarded macrophage activation causes reduced T lymphocyte numbers in the peripheral nerves of P0(+/-) mice. In the present study, we investigated the possible role of the macrophage-restricted sialic acid-binding Ig-like lectin sialoadhesin (Sn, Siglec-1) in the pathogenesis of inherited demyelination in P0(+/-) mice. We found that most peripheral nerve macrophages express Sn in the mutants. Myelin mutants devoid of Sn show reduced numbers of CD8+ T lymphocytes and macrophages in peripheral nerves and less severe demyelination, resulting in improved nerve conduction properties. Our findings are potentially important in the development of future treatment strategies for inherited demyelinating neuropathies.

Developmental alterations in thymocyte sensitivity are actively regulated by MHC class II expression in the thymic medulla

Developing thymocytes are positively selected if they respond to self-MHC-peptide complexes, yet mature T cells are not activated by those same self-complexes. To avoid autoimmunity, positive selection must be followed by a period of maturation when the cellular response to TCR signals is altered. The mechanisms that mediate this postselection developmental tuning remain largely unknown. Specifically, it is unknown whether developmental tuning is a preprogrammed outcome of positive selection or if it is sensitive to ongoing interactions between the thymocyte and the thymic stroma. We probed the requirement for MHC class II-TCR interactions in postselection maturation by studying single positive (SP) CD4 thymocytes from K14/A(beta)(b) mice, in which CD4 T cells cannot interact with MHC class II in the thymic medulla. We report here that SP CD4 thymocytes must receive MHC class II signals to avoid hyperactive responses to TCR signals. This hyperactivity correlates with decreased expression of CD5; however, developmental tuning can occur independently of CD5, correlating instead with differences in the distribution of Lck. Thus, the maturation of postselection SP CD4 thymocytes is an active process mediated by ongoing interactions between the T cell and MHC class II molecules. This represents a novel mechanism by which the thymic medulla prevents autoreactivity.

Inefficient cell spreading and cytoskeletal polarization by CD4+CD8+ thymocytes: regulation by the thymic environment

CD4(+)CD8(+) double-positive (DP) thymocytes express a lower level of surface TCR than do mature T cells or single-positive (SP) thymocytes. Regulation of the TCR on DP thymocytes appears to result from intrathymic signaling, as in vitro culture of these cells results in spontaneous TCR up-regulation. In this study, we examined cell spreading and cytoskeletal polarization responses that have been shown to occur in response to TCR engagement in mature T cells. Using DP thymocytes stimulated on lipid bilayers or nontransgenic thymocytes added to anti-CD3-coated surfaces, we found that cell spreading and polarization of the microtubule organizing center and the actin cytoskeleton were inefficient in freshly isolated DP thymocytes, but were dramatically enhanced after overnight culture. SP (CD4(+)) thymocytes showed efficient responses to TCR engagement, suggesting that releasing DP thymocytes from the thymic environment mimics some aspects of positive selection. The poor translation of a TCR signal to cytoskeletal responses could limit the ability of DP thymocytes to form stable contacts with APCs and may thereby regulate thymocyte selection during T cell development.

Sialylation regulates peripheral tolerance in CD4+ T cells

Decreased binding by the 6C10 auto-antibody serves as a unique marker for CD4+ T cell unresponsiveness after the induction of T cell tolerance in Vbeta8.1 TCR transgenic mice. We further define the nature of the epitope recognized by the 6C10 antibody to be a subset of Thy-1 bearing incompletely sialylated N-linked glycans, and furthermore, we demonstrate that tolerant CD4+ T cells have an increased degree of cell-surface sialylation. To test the significance of the altered glycosylation state identified by the 6C10 auto-antibody in the tolerant CD4+ T cell population, surface sialic acid was cleaved enzymatically. Treatment of purified peripheral CD4+ T cells with Vibrio cholerae sialidase (VCS) leads to increased 6C10 binding, significantly enhances proliferation in the tolerant CD4+ population and corrects defects in phosphotyrosine signaling observed in the tolerant CD4+ T cell. Furthermore, in vivo administration of VCS enhances proliferation in both tolerant and naive CD4+ T cell subsets. These studies suggest that sialylation of glycoproteins on the surface of the CD4+ T cell contributes to the regulation of T cell responsiveness in the tolerant state.

Plasticity in the positive selection of T cells: affinity of the selecting antigen and IL-7 affect T cell responsiveness

The current study examines how responsiveness of T cells is affected by the avidity of the peptide/MHC engaged during positive selection of their thymocyte precursors. We used a thymus reaggregate culture system in which CD4(+)CD8(+) thymocytes from AND TCR transgenic mice were induced to undergo positive selection by pigeon cytochrome c (PCC) peptide or its analogs presented by I-E(k) class II MHC on a thymic epithelial cell line. When low-affinity peptide analogs drove positive selection, up to 100 microM was needed to produce >50% CD4(+) T cells, and these cells were highly responsive to PCC. In contrast, <0.2 microM high-affinity peptides was required to achieve similar selection efficiency, but the resultant cells failed to respond to PCC. However, these cells were not dead based on dye exclusion and capacity to respond to phorbal ester and to agonist if IL-2 was also present, supporting the view that non-responsiveness of cells selected on high-affinity peptides is a form of central T cell tolerance distinct from deletion. Cells selected on intermediate-affinity peptides showed variable responsiveness which was suppressed 5- to 10-fold by addition during reaggregate culture of antibody to the IL-7R. Similarly, supplementary IL-7 in the reaggregate culture produced CD4(+) T cells that were promiscuously responsive. Overall, this study demonstrates that the responsiveness of T cells is not rigidly controlled and that the presence of IL-7 during T cell development has the potential to negate central T cell tolerance and produce autoreactive T cells.

A sexual dimorphism in intrathymic sialylation survey is revealed by the trans-sialidase from Trypanosoma cruzi

Sialylation is emerging as an important issue in developing thymocytes and is considered among the most significant cell surface modifications, although its physiologic relevance is far from being completely understood. It is regulated by the concerted expression of sialyl transferases along thymocyte development. After in vivo administration of trans-sialidase, a virulence factor from the American trypanosomatid Trypanosoma cruzi that directly transfers the sialyl residue among macromolecules, we found that the alteration of the sialylation pattern induces thymocyte apoptosis inside the "nurse cell complex." This suggests a glycosylation survey in the development of the T cell compartment. In this study, we report that this thymocyte apoptosis mechanism requires the presence of androgens. No increment in apoptosis was recorded after trans-sialidase administration in females or in antiandrogen-treated, gonadectomized, or androgen receptor mutant male mice. The androgen receptor presence was required only in the thymic epithelial cells as determined by bone marrow chimeric mouse approaches. The presence of the CD43 surface mucin, a molecule with a still undefined function in thymocytes, was another absolute requirement. The trans-sialidase-induced apoptosis proceeds through the TNF-alpha receptor 1 deathly signaling leading to the activation of the caspase 3. Accordingly, the production of the cytokine was increased in thymocytes. The ability of males to delete thymocytes altered in their sialylation pattern reveals a sexual dimorphism in the glycosylation survey during the development of the T cell compartment that might be related to the known differences in the immune response among sexes.

Ligand-induced conformational change in the T-cell receptor associated with productive immune synapses

Triggering of the T-cell receptor (TCR) can produce very different responses, depending on the nature of the major histocompatibility complex/antigen peptide (MHCp) ligand. The molecular mechanisms that permit such fine discrimination are still unknown. We show here that an epitope in the cytoplasmic tail of the TCR CD3epsilon subunit, recognized by antibody APA1/1, is only detected when the TCR is fully activated. Exposure of the APA1/1 epitope is shown to be fast and independent of tyrosine kinase activity and that it takes place even when T cells are stimulated at 0 degrees C. These results suggest that APA1/1 detects a conformational change in the TCR. APA1/1 staining concentrates in a restricted area of the immunologic synapse. Most important, we show that full agonist, but not partial agonist, peptides induce exposure of the APA1/1 epitope, indicating a correlation between the induction of the conformational change in the TCR and full T-cell activation. Finally, the conformational change is shown to occur in T cells that are being stimulated by antigen in vivo. Therefore, these results demonstrate that the TCR undergoes a conformational change on MHCp binding in vitro and in vivo, and they establish a molecular correlate for productive TCR engagement.

Cutting edge: LFA-1 integrin-dependent T cell adhesion is regulated by both ag specificity and sensitivity

Ab stimulation of the TCR rapidly enhances the functional activity of the LFA-1 integrin. Although TCR-mediated changes in LFA-1 activity are thought to promote T cell-APC interactions, the Ag specificity and sensitivity of TCR-mediated triggering of LFA-1 is not clear. We demonstrate that peptide/MHC (pMHC) tetramers rapidly enhance LFA-1-dependent adhesion of OT-I TCR transgenic CD8(+) T cells to purified ICAM-1. Inhibition of src family tyrosine kinase or PI3K activity blocked pMHC tetramer- and anti-CD3-stimulated adhesion. These effects are highly specific because partial agonist and antagonist pMHC tetramers are unable to stimulate OT-I T cell adhesion to ICAM-1. The Ag thresholds required for T cell adhesion to ICAM-1 resemble those of early T cell activation events, because optimal LFA-1 activation occurs at tetramer concentrations that fail to induce maximal T cell proliferation. Thus, TCR signaling to LFA-1 is highly Ag specific and sensitive to low concentrations of Ag.

Alteration of cell surface sialylation regulates antigen-induced naive CD8+ T cell responses

The strength of interactions with APC instructs naive T cells to undergo programmed expansion and differentiation, which is largely determined by the peptide affinity and dose as well as the duration of TCR ligation. Although, most ligands mediating these interactions are terminally sialylated, the impact of the T cell sialylation status on Ag-dependent response remains poorly understood. In this study, by monitoring TCR transgenic CD8+ T cells, OT-I, we show that biochemical desialylation of naive OT-I T cells increases their sensitivity for agonist as well as partial agonist peptides. Desialylation enhances early activation and shortens the duration of TCR stimulation required for proliferation and differentiation, without increasing apoptosis. Moreover, desialylation of naive OT-I T cells augments their response to tumor-presented Ag. These results provide direct evidence for a regulatory role for sialylation in Ag-dependent CD8+ T cell responses and offer a new approach to sensitize or dampen Ag-specific CD8+ T cell responses.

Peanut Agglutinin High Phenotype of Activated CD8+ T Cells Results from de Novo Synthesis of CD45 Glycans

Following activation in the periphery, murine CD8+ T cells exhibit a characteristic increased binding of peanut agglutinin (PNA), reflecting an increased expression of hyposialylated O-linked glycans (Galbeta1-3GalNAcalpha-O-Thr/Ser) on the cell surface. In this report, we show that the majority of the PNA receptors expressed on activated CD8+ T cells are carried by CD45. Other glycoproteins (e.g. CD8) and the glycolipid asialo-GM1 also carry PNA receptors, although to a much lesser extent. Analysis of enzymes involved in the sialylation/de-sialylation pathways showed that generation of PNA receptors in activated CD8+ T cells is not due to up-regulation of endogenous sialidases. Instead, our results indicate that the PNA(high) phenotype results from de novo synthesis of CD45 carrying reduced sialylated core 1 O-glycans.

Real-time imaging of T-cell development

The cellular and molecular dynamics underlying intrathymic T-cell development are still poorly understood. Dynamic imaging techniques offer new opportunities for dissecting the complex journey of developing T cells in the thymus. In particular, two-photon laser scanning microscopy has recently demonstrated great promise in tracking lymphocyte behavior in tissue environments. Resolving thymocyte behavior at the single-cell level should help to clarify the mode of trafficking in the thymus and identify the spatio-temporal aspects of thymic selection events.