Dynamic tuning of T cell reactivity by self-peptide-major histocompatibility complex ligands

Immunotherapy to CD5, a T-cell antigen having roles from development to peripheral function: Future prospective and challenges

CD5 is a pan T-cell marker expressed by all T-cells and a subset of B-cells, i.e., B1a cells. The significance of CD5 is evident from its functions, starting from T-cell development, antigen priming, activation, and effector response to the maintenance of tolerance. Varying CD5 expression and signaling in response to TCR-pMHC complex avidity is associated with thymic selection, competency, and effector response. Altered CD5 expression is associated with immunological and diseased conditions such as CD5-/low infiltrating T-cells in solid tumors, CD5hi T-cells in anergy conditions, CD5-/low phenotype of leukemic T-cells, high CD5 expression by regulatory T-cells, CD5lowphenotype of autoreactive T-cells, etc. A low CD5 expression triggers activation-induced cell death upon antigenic stimulation. There are three forms of CD5: membrane CD5 (mCD5), intracellular CD5 (cCD5) and soluble CD5 (sCD5). mCD5 and cCD5 are generated from conventional and non-conventional mRNA variants, i.e., E1A and E1B, respectively. E1B variant encoding cCD5 is derived from a human endogenous retrovirus segment inserted 8.2 kb upstream to conventional E1A exon. Various conditions, such as leukemia, exposure to hydrocarbon, hypoxia, etc., can trigger E1B transcription and, thus, cCD5 expression. Blocking mCD5 with mAb can restore immune response, effectively targeting cancer. Understanding cCD5, linked to leukemogenesis, can offer new avenues of immunotherapy.

Cbl-b mitigates the responsiveness of naive CD8+ T cells that experience extensive tonic T cell receptor signaling

Naive T cells experience tonic T cell receptor (TCR) signaling in response to self-antigens presented by major histocompatibility complex (MHC) in secondary lymphoid organs. We investigated how relatively weak or strong tonic TCR signals influence naive CD8+ T cell responses to stimulation with foreign antigens. The heterogeneous expression of Nur77-GFP, a transgenic reporter of tonic TCR signaling, in naive CD8+ T cells suggests variable intensities or durations of tonic TCR signaling. Although the expression of genes associated with acutely stimulated T cells was increased in Nur77-GFPHI cells, these cells were hyporesponsive to agonist TCR stimulation compared with Nur77-GFPLO cells. This hyporesponsiveness manifested as diminished activation marker expression and decreased secretion of IFN-γ and IL-2. The protein abundance of the ubiquitin ligase Cbl-b, a negative regulator of TCR signaling, was greater in Nur77-GFPHI cells than in Nur77-GFPLO cells, and Cbl-b deficiency partially restored the responsiveness of Nur77-GFPHI cells. Our data suggest that the cumulative effects of previously experienced tonic TCR signaling recalibrate naive CD8+ T cell responsiveness. These changes include gene expression changes and negative regulation partially dependent on Cbl-b. This cell-intrinsic negative feedback loop may enable the immune system to restrain naive CD8+ T cells with higher self-reactivity.

CD5 Suppresses IL-15–Induced Proliferation of Human Memory CD8+ T Cells by Inhibiting mTOR Pathways

IL-15 induces the proliferation of memory CD8+ T cells as well as NK cells. The expression of CD5 inversely correlates with the IL-15 responsiveness of human memory CD8+ T cells. However, whether CD5 directly regulates IL-15–induced proliferation of human memory CD8+ T cells is unknown. In the current study, we demonstrate that human memory CD8+ T cells in advanced stages of differentiation respond to IL-15 better than human memory CD8+ T cells in stages of less differentiation. We also found that the expression level of CD5 is the best correlate for IL-15 hyporesponsiveness among human memory CD8+ T cells. Importantly, we found that IL-15–induced proliferation of human memory CD8+ T cells is significantly enhanced by blocking CD5 with Abs or knocking down CD5 expression using small interfering RNA, indicating that CD5 directly suppresses the IL-15–induced proliferation of human memory CD8+ T cells. We also found that CD5 inhibits activation of the mTOR pathway, which is required for IL-15–induced proliferation of human memory CD8+ T cells. Taken together, the results indicate that CD5 is not just a correlative marker for IL-15 hyporesponsiveness, but it also directly suppresses IL-15–induced proliferation of human memory CD8+ T cells by inhibiting mTOR pathways.

CD5 Controls Gut Immunity by Shaping the Cytokine Profile of Intestinal T Cells

CD5 is constitutively expressed on all T cells and is a negative regulator of lymphocyte function. However, the full extent of CD5 function in immunity remains unclear. CD5 deficiency impacts thymic selection and extra-thymic regulatory T cell generation, yet CD5 knockout was reported to cause no immune pathology. Here we show that CD5 is a key modulator of gut immunity. We generated mice with inducible CD5 knockdown (KD) in the autoimmune-prone nonobese diabetic (NOD) background. CD5 deficiency caused T cell-dependent wasting disease driven by chronic gut immune dysregulation. CD5 inhibition also exacerbated acute experimental colitis. Mechanistically, loss of CD5 increased phospho-Stat3 levels, leading to elevated IL-17A secretion. Our data reveal a new facet of CD5 function in shaping the T cell cytokine profile.

CD5 Deficiency Alters Helper T Cell Metabolic Function and Shifts the Systemic Metabolome

Metabolic function plays a key role in immune cell activation, destruction of foreign pathogens, and memory cell generation. As T cells are activated, their metabolic profile is significantly changed due to signaling cascades mediated by the T cell receptor (TCR) and co-receptors found on their surface. CD5 is a T cell co-receptor that regulates thymocyte selection and peripheral T cell activation. The removal of CD5 enhances T cell activation and proliferation, but how this is accomplished is not well understood. We examined how CD5 specifically affects CD4+ T cell metabolic function and systemic metabolome by analyzing serum and T cell metabolites from CD5WT and CD5KO mice. We found that CD5 removal depletes certain serum metabolites, and CD5KO T cells have higher levels of several metabolites. Transcriptomic analysis identified several upregulated metabolic genes in CD5KO T cells. Bioinformatic analysis identified glycolysis and the TCA cycle as metabolic pathways promoted by CD5 removal. Functional metabolic analysis demonstrated that CD5KO T cells have higher oxygen consumption rates (OCR) and higher extracellular acidification rates (ECAR). Together, these findings suggest that the loss of CD5 is linked to CD4+ T cell metabolism changes in metabolic gene expression and metabolite concentration.

Expansion of an Unusual Virtual Memory CD8+ Subpopulation Bearing Vα3.2 TCR in Themis-Deficient Mice

Deletion of the gene for Themis affects T cell selection in the thymus, which would be expected to affect the TCR repertoire. We found an increased proportion of cells expressing Vα3.2 (TRAV9N-3) in the peripheral CD8+ T cell population in mice with germline Themis deficiency. Analysis of the TCRα repertoire indicated it was generally reduced in diversity in the absence of Themis, whereas the diversity of sequences using the TRAV9N-3 V-region element was increased. In wild type mice, Vα3.2+ cells showed higher CD5, CD6 and CD44 expression than non-Vα3-expressing cells, and this was more marked in cells from Themis-deficient mice. This suggested a virtual memory phenotype, as well as a stronger response to self-pMHC. The Vα3.2+ cells responded more strongly to IL-15, as well as showing bystander effector capability in a Listeria infection. Thus, the unusually large population of Vα3.2+ CD8+ T cells found in the periphery of Themis-deficient mice reflects not only altered thymic selection, but also allowed identification of a subset of bystander-competent cells that are also present in wild-type mice.

Non-Genetically Encoded Epitopes Are Relevant Targets in Autoimmune Diabetes

Islet antigen reactive T cells play a key role in promoting beta cell destruction in type 1 diabetes (T1D). Self-reactive T cells are typically deleted through negative selection in the thymus or deviated to a regulatory phenotype. Nevertheless, those processes are imperfect such that even healthy individuals have a reservoir of potentially autoreactive T cells. What remains less clear is how tolerance is lost to insulin and other beta cell specific antigens. Islet autoantibodies, the best predictor of disease risk, are known to recognize classical antigens such as proinsulin, GAD65, IA-2, and ZnT8. These antibodies are thought to be supported by the expansion of autoreactive CD4⁺ T cells that recognize these same antigenic targets. However, recent studies have identified new classes of non-genetically encoded epitopes that may reflect crucial gaps in central and peripheral tolerance. Notably, some of these specificities, including epitopes from enzymatically post-translationally modified antigens and hybrid insulin peptides, are present at relatively high frequencies in the peripheral blood of patients with T1D. We conclude that CD4⁺ T cells that recognize non-genetically encoded epitopes are likely to make an important contribution to the progression of islet autoimmunity in T1D. We further propose that these classes of neo-epitopes should be considered as possible targets for strategies to induce antigen specific tolerance.

Eomes identifies thymic precursors of self-specific memory-phenotype CD8+ T cells

Unprimed mice harbor a substantial population of "memory-phenotype" CD8⁺ T cells (CD8-MP cells) that exhibit hallmarks of activation and innate-like functional properties. Due to the lack of faithful markers to distinguish CD8-MP cells from bona fide CD8⁺ memory T cells, the developmental origins and antigen specificities of CD8-MP cells remain incompletely defined. Using deep T cell antigen receptor (TCR) sequencing, we found that the TCRs expressed by CD8-MP cells are highly recurrent and distinct from the TCRs expressed by naive-phenotype CD8⁺ T cells. CD8-MP clones exhibited reactivity to widely expressed self-ligands. T cell precursors expressing CD8-MP TCRs upregulated the transcription factor Eomes during maturation in the thymus, prior to induction of the full memory phenotype, suggestive of a unique program triggered by recognition of self-ligands. Moreover, CD8-MP cells infiltrate oncogene-driven prostate tumors and express high densities of PD-1, suggesting a potential role in anti-tumor immunity and response to immunotherapy.

Dynamic Mitochondrial Migratory Features Associated with Calcium Responses during T Cell Antigen Recognition

A T cell clone is able to distinguish Ags in the form of peptide-MHC complexes with high specificity and sensitivity; however, how subtle differences in peptide-MHC structures translate to distinct T cell effector functions remains unknown. We hypothesized that mitochondrial positioning and associated calcium responses play an important role in T cell Ag recognition. We engineered a microfluidic system to precisely manipulate and synchronize a large number of cell-cell pairing events, which provided simultaneous real-time signaling imaging and organelle tracking with temporal precision. In addition, we developed image-derived metrics to quantify calcium response and mitochondria movement. Using myelin proteolipid altered peptide ligands and a hybridoma T cell line derived from a mouse model of experimental autoimmune encephalomyelitis, we observed that Ag potency modulates calcium response at the single-cell level. We further developed a partial least squares regression model, which highlighted mitochondrial positioning as a strong predictor of calcium response. The model revealed T cell mitochondria sharply alter direction within minutes following exposure to agonist peptide Ag, changing from accumulation at the immunological synapse to retrograde movement toward the distal end of the T cell body. By quantifying mitochondria movement as a highly dynamic process with rapidly changing phases, our result reconciles conflicting prior reports of mitochondria positioning during T cell Ag recognition. We envision applying this pipeline of methodology to study cell interactions between other immune cell types to reveal important signaling phenomenon that is inaccessible because of data-limited experimental design.

TCR tuning of T cell subsets

After selection in the thymus, the post-thymic T cell compartments comprise heterogenous subsets of naive and memory T cells that make continuous T cell receptor (TCR) contact with self-ligands bound to major histocompatibility complex (MHC) molecules. T cell recognition of self-MHC ligands elicits covert TCR signaling and is particularly important for controlling survival of naive T cells. Such tonic TCR signaling is tightly controlled and maintains the cells in a quiescent state to avoid autoimmunity. Here, we review how naive and memory T cells are differentially tuned and wired for TCR sensitivity to self and foreign ligands.

T Cell's Sense of Self: a Role of Self-Recognition in Shaping Functional Competence of Naïve T Cells

Post-thymic naïve T cells constitute a key cellular arm of adaptive immunity, with a well-known characteristic of the specificity and robustness of responses to cognate foreign antigens which is presented as a form of antigen-derived peptides bound to major histocompatibility complex (MHC) molecules by antigen-presenting cells (APCs). In a steady state, however, these cells are resting, quiescent in their activity, but must keep full ranges of functional integrity to mount rapid and robust immunity to cope with various infectious pathogens at any time and space. Such unique property of resting naïve T cells is not acquired in a default manner but rather requires an active mechanism. Although our understanding of exactly how this process occurs and what factors are involved remains incomplete, a particular role of self-recognition by T cells has grown greatly in recent years. In this brief review, we discuss recent data on how the interaction of T cells with self-peptide MHC ligands regulates their functional responsiveness and propose that variable strength of self-reactivity imposes distinctly different levels of functional competence and heterogeneity.

Exonal switch down-regulates the expression of CD5 on blasts of acute T cell leukaemia

To date, CD5 expression and its role in acute T cell lymphoblastic leukaemia (T-ALL) have not been studied closely. We observed a significant reduction in surface expression of CD5 (sCD5) on leukaemic T cells compared to autologous non-leukaemic T cells. In this study, we have shown the molecular mechanism regulating the expression and function of CD5 on leukaemic T cells. A total of 250 patients suffering from leukaemia and lymphoma were immunophenotyped. Final diagnosis was based on their clinical presentation, morphological data and flow cytometry-based immunophenotyping. Thirty-nine patients were found to be of ALL-T origin. Amplification of early region of E1A and E1B transcripts of CD5 was correlated with the levels of surface and intracellular expression of CD5 protein. Functional studies were performed to show the effect of CD5 blocking on interleukin IL-2 production and survival of leukaemic and non-leukaemic cells. Lack of expression of sCD5 on T-ALL blasts was correlated closely with predominant transcription of exon E1B and significant loss of exon E1A of the CD5 gene, which is associated with surface expression of CD5 on lymphocytes. High expression of E1B also correlates with increased expression of cytoplasmic CD5 (cCD5) among leukaemic T cells. Interestingly, we observed a significant increase in the production of IL-2 by non-leukaemic T cells upon CD5 blocking, leading possibly to their increased survival at 48 h. Our study provides understanding of the regulation of CD5 expression on leukaemic T cells, and may help in understanding the molecular mechanism of CD5 down-regulation.

Enhanced cytotoxic T-cell function and inhibition of tumor progression by Mst1 deficiency

Mammalian ste-20 like kinase Mst1 plays important roles during apoptosis, proliferation, cell polarity, and migration. Here, we report a novel role of Mst1 for cytotoxic T-cell responses and tumor suppression. The defect of Mst1 caused decreased levels of FoxO, and promoted cytotoxicity in vitro. Mst1(-/-) cytotoxic T cells also exhibited enhanced T-bet expression that was associated with elevated expression levels of IFNγ and granzyme B. Moreover, Mst1(-/-) cytotoxic T cells suppressed tumor growth in vivo. The data suggest that Mst1 inhibits cytotoxicity via T-bet suppression by FoxO1 and FoxO3a. Thus, Mst1 is a potential therapeutic target for tumor immunotherapy.

A critical role for alpha-synuclein in development and function of T lymphocytes

Alpha-synuclein is highly expressed in the central nervous system and plays an important role in pathogenesis of neurodegenerative disorders such as Parkinson's disease and Lewy body dementia. Previous studies have demonstrated the expression of α-synuclein in hematopoietic elements and peripheral blood mononuclear cells, although its roles in hematopoiesis and adaptive immunity are not studied. Using an α-synuclein knock out (KO) mouse model, we have recently shown that α-synuclein deficiency is associated with a mild defect in late stages of hematopoiesis. More importantly, we demonstrated a marked defect in B lymphocyte development and IgG, but not IgM production in these mice. Here we show a marked defect in development of T lymphocytes in α-synuclein KO mice demonstrated by a significant increase in the number of CD4 and CD8 double negative thymocytes and significant decreases in the number of CD4 single positive and CD8 single positive T cells. This resulted in markedly reduced peripheral T lymphocytes. Interestingly, splenic CD4(+) and CD8(+) T cells that developed in α-synuclein KO mice had a hyperactivated state with higher expression of early activation markers and increased IL-2 production. Moreover, splenic CD4(+) T cells from α-synuclein KO mice produced lower levels of IL-4 upon antigenic stimulation suggesting a defective Th2 differentiation. Our data demonstrate an important role for α-synuclein in development of T lymphocytes and regulation of their phenotype and function.

The Molecular Control of Regulatory T Cell Induction

Regulatory T cells (Tregs) are characterized by the expression of the master transcription factor forkhead box P3 (Foxp3). Although Foxp3 expression is widely used as a marker of the Treg lineage, recent data show that the Treg fate is determined by a multifactorial signaling pathway, involving cytokines, nuclear factors, and epigenetic modifications. Foxp3 expression and the Treg phenotype can be acquired by T cells in the periphery, illustrating that the Treg fate is not necessarily conferred during thymic development. The two main Treg populations in vivo, thymic Tregs and peripheral Tregs, differ in the pathways followed for their maturation. This chapter discusses the molecular control of Treg induction, in the thymus as well as the periphery.

Indoctrinating T cells to attack pathogens through homeschooling

Adaptive immunity is predicated on the ability of the T cell repertoire to have pre-existing specificity for the universe of potential pathogens. Recent findings suggest that T cell receptor (TCR)-self-major histocompatibility protein (pMHC) interactions limit autoimmune responses while enhancing T cell response to foreign antigens. We review these findings here, placing them in context of the current understanding of how TCR-self-pMHC interactions regulate T cell activation thresholds, and suggest that TCR-self-pMHC interactions increase the efficiency of the T cell repertoire by giving a competitive advantage to peptide cross-reactive T cells. We propose that self-reactivity and peptide cross-reactivity are controlled by particular CDR3 sequence motifs, which would allow thymic selection to contribute to solving the feat of broad pathogen specificity by exporting T cells that are pre-screened by positive and negative selection for the ability to be 'moderately' peptide cross-reactive.

Dynamic tuning of lymphocytes: physiological basis, mechanisms, and function

Dynamic tuning of cellular responsiveness as a result of repeated stimuli improves the ability of cells to distinguish physiologically meaningful signals from each other and from noise. In particular, lymphocyte activation thresholds are subject to tuning, which contributes to maintaining tolerance to self-antigens and persisting foreign antigens, averting autoimmunity and immune pathogenesis, but allowing responses to strong, structured perturbations that are typically associated with acute infection. Such tuning is also implicated in conferring flexibility to positive selection in the thymus, in controlling the magnitude of the immune response, and in generating memory cells. Additional functional properties are dynamically and differentially tuned in parallel via subthreshold contact interactions between developing or mature lymphocytes and self-antigen-presenting cells. These interactions facilitate and regulate lymphocyte viability, maintain their functional integrity, and influence their responses to foreign antigens and accessory signals, qualitatively and quantitatively. Bidirectional tuning of T cells and antigen-presenting cells leads to the definition of homeostatic set points, thus maximizing clonal diversity.

Peripheral residence of naïve CD4 T cells induces MHC class II-dependent alterations in phenotype and function

BACKGROUND

As individual naïve CD4 T lymphocytes circulate in the body after emerging from the thymus, they are likely to have individually varying microenvironmental interactions even in the absence of stimulation via specific target recognition. It is not clear if these interactions result in alterations in their activation, survival and effector programming. Naïve CD4 T cells show unimodal distribution for many phenotypic properties, suggesting that the variation is caused by intrinsic stochasticity, although underlying variation due to subsets created by different histories of microenvironmental interactions remains possible. To explore this possibility, we began examining the phenotype and functionality of naïve CD4 T cells differing in a basic unimodally distributed property, the CD4 levels, as well as the causal origin of these differences.

RESULTS

We examined separated CD4hi and CD4lo subsets of mouse naïve CD4 cells. CD4lo cells were smaller with higher CD5 levels and lower levels of the dual-specific phosphatase (DUSP)6-suppressing micro-RNA miR181a, and responded poorly with more Th2-skewed outcomes. Human naïve CD4lo and CD4hi cells showed similar differences. Naïve CD4lo and CD4hi subsets of thymic single-positive CD4 T cells did not show differences whereas peripheral naïve CD4lo and CD4hi subsets of T cell receptor (TCR)-transgenic T cells did. Adoptive transfer-mediated parking of naïve CD4 cells in vivo lowered CD4 levels, increased CD5 and reactive oxygen species (ROS) levels and induced hyporesponsiveness in them, dependent, at least in part, on availability of major histocompatibility complex class II (MHCII) molecules. ROS scavenging or DUSP inhibition ameliorated hyporesponsiveness. Naïve CD4 cells from aged mice showed lower CD4 levels and cell sizes, higher CD5 levels, and hyporesponsiveness and Th2-skewing reversed by DUSP inhibition.

CONCLUSIONS

Our data show that, underlying a unimodally distributed property, the CD4 level, there are subsets of naïve CD4 cells that vary in the time spent in the periphery receiving MHCII-mediated signals and show resultant alteration of phenotype and functionality via ROS and DUSP activity. Our findings also suggest the feasibility of potential pharmacological interventions for improved CD4 T cell responses during vaccination of older people via either anti-oxidant or DUSP inhibitor small molecules.

The TCR's sensitivity to self peptide-MHC dictates the ability of naive CD8(+) T cells to respond to foreign antigens

The strength of self-peptide–major histocompatibility complex (MHC) recognition dictates naïve CD8⁺ T cell homeostasis, but its effect on foreign antigen reactivity is controversial. As CD5 expression correlates with self-recognition, we studied CD5^lo and CD5^hi naïve CD8⁺ T cells. Gene expression characteristics suggested CD5^hi cells were better poised for reactivity and differentiation compared to the CD5^lo population, and we found that the CD5^hi pool exhibited more efficient clonal recruitment and expansion, as well as enhanced reactivity to inflammatory cues, during recognition of foreign antigen. Yet foreign peptide–MHC recognition was similar for both subsets. Thus, CD8⁺ T cells with higher self-reactivity dominate the immune response against foreign antigens, with implications for T cell repertoire diversity and autoimmunity.

T-cell modulatory properties of CD5 and its role in antitumor immune responses

The destruction of tumor cells by the immune system is under the control of positive and negative receptors that tightly regulate T-cell effector functions. The T-cell receptor (TCR) inhibitory molecule CD5 critically contributes to the regulation of antitumor immune responses. Indeed, the modulation of CD5 within the tumor microenvironment corresponds to a strategy adopted by tumor-specific cytotoxic T lymphocytes (CTLs) to optimize their cytotoxic and cytokine secretion functions. In this review, we provide insights into the immunobiology of CD5 and its role in regulating antitumor CD8 T-cell responses, and suggest the possibility of targeting CD5 to improve the efficacy of current immunotherapeutic approaches against cancer.

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.

Qa-2 associated lipid rafts are indispensable in the final maturation of CD4⁺CD8⁻ thymocytes

Lipid rafts have been shown to play significant roles in thymocyte development. However, the exact role of lipid rafts in single positive (SP) thymocyte differentiation is poorly characterized. We previously defined a developmental program (SP1→SP4) for CD4SP thymocytes. In this study, we found that lipid raft components were up-regulated during CD4SP maturation. Qa-2, a unique marker for the most mature SP4 subset, was localized to lipid rafts and heterogeneously expressed in SP4 cells. Functional assays showed that the proliferation capacity of SP4 cells correlated with the expression of Qa-2. Raft-disruption on both CD4SP and epithelial cells by cholesterol extraction or cholesterol oxidation in a medullary thymic epithelial cell (mTEC)-supported co-culture system impaired the transition from SP3 to SP4. This result was further confirmed in fetal thymic organ culture system. Collectively, these studies suggest that raft-associated signaling between mTECs and thymocytes drives the differentiation of CD4SP thymocytes and lipid rafts are involved in the final maturation of SP4 thymocytes.

PTPN22 alters the development of regulatory T cells in the thymus

PTPN22 encodes a tyrosine phosphatase that inhibits Src-family kinases responsible for Ag receptor signaling in lymphocytes and is strongly linked with susceptibility to a number of autoimmune diseases. As strength of TCR signal is critical to the thymic selection of regulatory T cells (Tregs), we examined the effect of murine PTPN22 deficiency on Treg development and function. In the thymus, numbers of pre-Tregs and Tregs increased inversely with the level of PTPN22. This increase in Tregs persisted in the periphery and could play a key part in the reduced severity observed in the PTPN22-deficient mice of experimental autoimmune encephalomyelitis, a mouse model of multiple sclerosis. This could explain the lack of association of certain autoimmune conditions with PTPN22 risk alleles.

Regulatory T cells: mechanisms of differentiation and function

The immune system has evolved to mount an effective defense against pathogens and to minimize deleterious immune-mediated inflammation caused by commensal microorganisms, immune responses against self and environmental antigens, and metabolic inflammatory disorders. Regulatory T (Treg) cell-mediated suppression serves as a vital mechanism of negative regulation of immune-mediated inflammation and features prominently in autoimmune and autoinflammatory disorders, allergy, acute and chronic infections, cancer, and metabolic inflammation. The discovery that Foxp3 is the transcription factor that specifies the Treg cell lineage facilitated recent progress in understanding the biology of regulatory T cells. In this review, we discuss cellular and molecular mechanisms in the differentiation and function of these cells.

Signal integration and crosstalk during thymocyte migration and emigration

The thymus produces self-tolerant functionally competent T cells. This process involves the import of multipotent haematopoietic progenitors that are then signalled to adopt the T cell fate. Expression of T cell-specific genes, including those encoding the T cell receptor (TCR), is followed by positive and negative selection and the eventual export of mature T cells. Significant progress has been made in elucidating the signals that direct progenitor cell trafficking to, within and out of the thymus. These advances are the subject of this Review, with a particular focus on the role of reciprocal cooperative and regulatory interactions between TCR- and chemokine receptor-mediated signalling.

T cell anergy as a strategy to reduce the risk of autoimmunity

Some self-reactive immature T cells escape negative selection in the thymus and may cause autoimmune diseases later. In the periphery, if T cells are stimulated insufficiently by peptide-major histocompatibility complex, they become inactive and their production of cytokines changes, a phenomenon called "T cell anergy". In this paper, we explore the hypothesis that T cell anergy may function to reduce the risk of autoimmunity. The underlying logic is as follows: Since those self-reactive T cells that receive strong stimuli from self-antigens are eliminated in the thymus, T cells that receive strong stimuli in the periphery are likely to be non-self-reactive. As a consequence, when a T cell receives a weak stimulus, the likelihood that the cell is self-reactive is higher than in the case that it receives a strong stimulus. Therefore, inactivation of the T cell may reduce the danger of autoimmunity. We consider the formalism in which each T cell chooses its response depending on the strength of stimuli in order to reduce the risk of autoimmune diseases while maintaining its ability to attack non-self-antigens effectively. The optimal T cell responses to a weak and a strong stimulus are obtained both when the cells respond in a deterministic manner and when they respond in a probabilistic manner. We conclude that T cell anergy is the optimal response when a T cell meets with antigen-presenting cells many times in its lifetime, and when the product of the autoimmunity risk and the number of self-reactive T cells has an intermediate value.

Mapping the landscape of host-pathogen coevolution: HLA class I binding and its relationship with evolutionary conservation in human and viral proteins

The high diversity of HLA binding preferences has been driven by the sequence diversity of short segments of relevant pathogenic proteins presented by HLA molecules to the immune system. To identify possible commonalities in HLA binding preferences, we quantify these using a novel measure termed "targeting efficiency," which captures the correlation between HLA-peptide binding affinities and the conservation of the targeted proteomic regions. Analysis of targeting efficiencies for 95 HLA class I alleles over thousands of human proteins and 52 human viruses indicates that HLA molecules preferentially target conserved regions in these proteomes, although the arboviral Flaviviridae are a notable exception where nonconserved regions are preferentially targeted by most alleles. HLA-A alleles and several HLA-B alleles that have maintained close sequence identity with chimpanzee homologues target conserved human proteins and DNA viruses such as Herpesviridae and Adenoviridae most efficiently, while all HLA-B alleles studied efficiently target RNA viruses. These patterns of host and pathogen specialization are both consistent with coevolutionary selection and functionally relevant in specific cases; for example, preferential HLA targeting of conserved proteomic regions is associated with improved outcomes in HIV infection and with protection against dengue hemorrhagic fever. Efficiency analysis provides a novel perspective on the coevolutionary relationship between HLA class I molecular diversity, self-derived peptides that shape T-cell immunity through ontogeny, and the broad range of viruses that subsequently engage with the adaptive immune response.

Fas (CD95/APO-1) limits the expansion of T lymphocytes in an environment of limited T-cell antigen receptor/MHC contacts

Fas-deficient mice (Fas(lpr/lpr)) and humans have profoundly dysregulated T lymphocyte homeostasis, which manifests as an accumulation of CD4(+) and CD8(+) T cells as well as an unusual population of CD4(-)CD8(-)TCRαβ(+) T cells. To date, no unifying model has explained both the increased T-cell numbers and the origin of the CD4(-)CD8(-)TCRαβ(+) T cells. As Fas(lpr/lpr) mice raised in a germ-free environment still manifest lymphadenopathy, we considered that this process is primarily driven by recurrent low-avidity TCR signaling in response to self-peptide/MHC as occurs during homeostatic proliferation. In these studies, we developed two independent systems to decrease the number of self-peptide/MHC contacts. First, expression of MHC class I was reduced in OT-I TCR transgenic mice. Although OT-I Fas(lpr/lpr) mice did not develop lymphadenopathy characteristic of Fas(lpr/lpr) mice, in the absence of MHC class I, OT-I Fas(lpr/lpr) T cells accumulated as both CD8(+) and CD4(-)CD8(-) T cells. In the second system, re-expression of β(2)m limited to thymic cortical epithelial cells of Fas(lpr/lpr) β(2)m-deficient mice yielded a model in which polyclonal CD8(+) thymocytes entered a peripheral environment devoid of MHC class I. These mice accumulated significantly greater numbers of CD4(-)CD8(-)TCRαβ(+) T cells than conventional Fas(lpr/lpr) mice. Thus, Fas shapes the peripheral T-cell repertoire by regulating the survival of a subset of T cells proliferating in response to limited self-peptide/MHC contacts.

Regulatory T-cell differentiation versus clonal deletion of autoreactive thymocytes

The concept of clonal deletion of immune cells that carry an autoreactive antigen receptor was a central prediction of Burnet's clonal selection theory. A series of classical experiments in the late 1980s revealed that certain immature thymocytes upon encounter of 'self' are indeed removed from the T-cell repertoire before their release into the blood circulation. A second essential cornerstone of immunological tolerance, not anticipated by Burnett, has more recently surfaced through the discovery of Foxp3(+) regulatory T cells (Treg). Intriguingly, it appears that the expression of an autoreactive T-cell receptor is a shared characteristic of T cells that are subject to clonal deletion as well as of those deviated into the Treg lineage. This is all the more striking as Treg differentiation for the most part branches off from mainstream CD4T cell development during thymocyte maturation in the thymus, that is, it may neither temporally nor spatially be separated from clonal deletion. This raises the question of how an apparently identical stimulus, namely the encounter of 'self' during thymocyte development, can elicit fundamentally different outcomes such as apoptotic cell death on the one hand or differentiation into a highly specialized T-cell lineage on the other hand. Here, we will review the T-cell intrinsic and extrinsic factors that have been implicated in intrathymic Treg differentiation and discuss how these parameters may determine whether an autoreactive major histocompatibility complex class II-restricted thymocyte is deviated into the Treg lineage or subject to clonal deletion.

Attenuated T cell responses to a high-potency ligand in vivo

According to this study, the strongest T cell receptor ligands in vitro do not necessarily induce the strongest T cell responses in vivo, suggesting that vaccine designers may need to reconsider their strategies.

αβ T cell receptor (TCR) recognition of foreign peptides bound to major histocompatibility complex (pMHC) molecules on the surface of antigen presenting cells is a key event in the initiation of adaptive cellular immunity. In vitro, high-affinity binding and/or long-lived interactions between TCRs and pMHC correlate with high-potency T cell activation. However, less is known about the influence of TCR/pMHC interaction parameters on T cell responses in vivo. We studied the influence of TCR/pMHC binding characteristics on in vivo T cell immunity by tracking CD4⁺ T cell activation, effector, and memory responses to immunization with peptides exhibiting a range of TCR/pMHC half-lives and in vitro T cell activation potencies. Contrary to predictions from in vitro studies, we found that optimal in vivo T cell responses occur to ligands with intermediate TCR/pMHC half-lives. The diminished in vivo responses we observed to the ligand exhibiting the longest TCR/pMHC half-life were associated with attenuation of intracellular signaling, expansion, and function over a broad range of time points. Our results reveal a level of control over T cell activation in vivo not recapitulated in in vitro assays and highlight the importance of considering in vivo efficacy of TCR ligands as part of vaccine design.

As an important part of immune system, T cells fight infections by recognizing signs of foreign invaders. A molecule on the surface of these cells—called the T cell receptor—recognizes and binds to protein components (peptides) from bacteria, viruses, and other pathogens that are displayed on the surface of other cells. The T cells can then use this information to orchestrate the fight against infection. Vaccination involves injecting into the body foreign peptides that mimic a pathogen, therefore tricking it into raising a T cell response against that pathogen that will be protective in the event of a real infection. We studied T cell responses in mice injected with one of several peptides to which the T cell receptor binds more or less strongly. Contrary to expectations, we found that the peptide that interacted most strongly with the T cell receptor did not provoke the strongest T cell response. This may be nature's way of preventing harmful inflammatory damage due to excessively strong T cell activation. Our work shows that peptides that bind the T cell receptor with medium strength may be best to use for vaccines. Current vaccine strategies seeking to design peptides that bind with maximum strength to the T cell receptor may need to be reconsidered in light of our findings.

ITAM-mediated signaling by the T-cell antigen receptor

Signal transduction by the T-cell antigen receptor (TCR) is initiated by phosphorylation of conserved motifs (ITAMs) contained within the cytoplasmic domains of the invariant subunits. TCR complexes contain a total of 10 ITAMs and this unusual configuration has prompted studies of the role of specific ITAMs, or of ITAM multiplicity, in regulating TCR-directed developmental and effector responses. Here, we summarize data generated during the past two decades and discuss how these findings have in some cases resolved, and in others complicated, outstanding questions relating to the function of TCR ITAMs.

Membrane potential of CD4+ T cells is a subset specific feature that depends on the direct cell-to-cell contacts with monocytes

CD4(+) T cells can be divided into three subsets: naive (Tn), central memory (Tcm), and effector memory (Tem) lymphocytes. These subpopulations differ in phenotype, migratory capacity, pattern of secreted cytokines, and activation threshold. T-cell activation is associated with changes in membrane potential, which provide an electrical driving force for calcium entry into the cytosol. These phenomena were shown to precede lymphocyte proliferation, cytokine synthesis, migration, and apoptosis. Hence the aim of the study was the analysis of these early activation events in the subsets of CD4(+) T cells. We measured the membrane potential and intracellular calcium concentration ([Ca(2+)](i)) in CD4(+) Tn, Tcm, and Tem cells as well as the dependency of these parameters in CD4(+) T cells on their cell-to-cell contacts with other leukocyte subsets. The data indicate that membrane potential of CD4(+) T cells is a subset specific feature maintained by direct contact with monocytes. In addition, monocytes were found to control Ca(2+) influx in CD4(+) T cells.

T cells use rafts for survival

T cell homeostasis must be tightly controlled. In this issue of Immunity, Cho et al. (2010) describe results that begin to define the roles of the T cell receptor, self-peptide-MHC ligands, cytokines, and membrane rafts in this dynamic process.

T cell receptor-dependent regulation of lipid rafts controls naive CD8+ T cell homeostasis

T cell receptor (TCR) contact with self ligands keeps T cells alive and is shown here to cause naive CD8(+), but not CD4(+), T cells to be hypersensitive to certain gamma(c) cytokines, notably interleukin (IL)-2, IL-15, and IL-7. Hypersensitivity of CD8(+) T cells to IL-2 was dependent on a low-level TCR signal, associated with high expression of CD5 and GM1, a marker for lipid rafts, and was abolished by disruption of lipid rafts. By contrast, CD4(+) T cells expressed low amounts of GM1 and were unresponsive to IL-2. Physiologically, sensitivity to IL-7 and IL-15 maintains survival of resting CD8(+) T cells, whereas sensitivity to IL-2 may be irrelevant for normal homeostasis but crucial for the immune response. Thus, TCR contact with antigen upregulates GM1 and amplifies responsiveness of naive CD8(+) T cells to IL-2, thereby making the cells highly sensitive to exogenous IL-2 from CD4(+) T helper cells.

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.

Control of regulatory T cell lineage commitment and maintenance

Foxp3-expressing regulatory T (Treg) cells suppress pathology mediated by immune responses against self and foreign antigens and commensal microorganisms. Sustained expression of the transcription factor Foxp3, a key distinguishing feature of Treg cells, is required for their differentiation and suppressor function. In addition, Foxp3 expression prevents deviation of Treg cells into effector T cell lineages and confers dependence of Treg cell survival and expansion on growth factors, foremost interleukin-2, provided by activated effector T cells. In this review we discuss Treg cell differentiation and maintenance with a particular emphasis on molecular regulation of Foxp3 expression, arguably a key to mechanistic understanding of biology of regulatory T cells.

Harnessing endogenous miR-181a to segregate transgenic antigen receptor expression in developing versus post-thymic T cells in murine hematopoietic chimeras

MicroRNAs (miRNAs) are small, noncoding RNAs that regulate gene expression by targeting complementary sequences, referred to as miRNA recognition elements (MREs), typically located in the 3' untranslated region of mRNAs. miR-181a is highly expressed in developing thymocytes and markedly downregulated in post-thymic T cells. We investigated whether endogenous miR-181a can be harnessed to segregate expression of chimeric antigen receptors (CARs) and TCRs between developing and mature T cells. Lentiviral-encoded antigen receptors were tagged with a miR-181a-specific MRE and transduced into mouse BM cells that were used to generate hematopoietic chimeras. Expression of a CAR specific for human CD19 (hCD19) was selectively suppressed in late double-negative and double-positive thymocytes, coinciding with the peak in endogenous miR-181a expression. Receptor expression was fully restored in post-thymic resting and activated T cells, affording protection against a subsequent challenge with hCD19+ tumors. Hematopoietic mouse chimeras engrafted with a conalbumin-specific TCR prone to thymic clonal deletion acquired peptide-specific T cell responsiveness only when the vector-encoded TCR transcript was similarly engineered to be subject to regulation by miR-181a. These results demonstrate the potential of miRNA-regulated transgene expression in stem cell-based therapies, including cancer immunotherapy.

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.

T cell homeostasis

The pool of mature T cells comprises a heterogeneous mixture of naive and memory CD4(+) and CD8(+) cells. These cells are long lived at a population level but differ markedly in their relative rates of turnover and survival. Here, we review how contact with exogenous stimuli, notably self MHC ligands and various gamma(c) cytokines, plays a decisive role in controlling normal T cell homeostasis.

Multistep pathogenesis of autoimmune disease

In the immune system, many tolerance checkpoints exist to prevent self-antigens from stimulating the relentless growth of self-reactive B and T lymphocytes. The genes and molecular pathways underpinning these checkpoints overlap with those involved in tumor suppression. As with an inherited predisposition to cancer, inherited defects in self-tolerance genes typically precipitate autoimmune disease stochastically after a latent phase. Multiple mutations, inherited and somatic, may be needed before a self-reactive clone bypasses sequential tolerance checkpoints resulting in the emergence of autoimmune disease.

CD83 influences cell-surface MHC class II expression on B cells and other antigen-presenting cells

CD83 is a member of the Ig superfamily expressed primarily by mature dendritic cells (DCs). In mice, CD83 expression by thymic stromal cells regulates CD4(+) T cell development, with CD83(-/-) mice demonstrating dramatic reductions in both thymus and peripheral CD4(+) T cells. In this study, CD83 expression was also found to affect MHC class II antigen expression within the thymus and periphery. CD83 deficiency reduced cell-surface class II antigen expression by 25-50% on splenic B cells and DCs, thymic epithelial cells and peritoneal macrophages. Reduced class II expression was a stable and intrinsic property that resulted from increased internalization of class II from the surface of CD83(-/-) B cells. Otherwise, class II antigen transcription, intracellular expression, heterodimer structure, antigen processing and antigen presentation were normal. Reduced class II antigen expression was not the primary cause of the CD83(-/-) phenotype since thymocyte and peripheral T cell development was normal in class II(+/-) mice. Comparable blocks in CD4(+) thymocyte development were also observed in CD83(-/-) and CD83(-/-)class II(+/-) littermates. TCR and CD69 expression patterns in CD83(-/-) mice further suggested that double-positive thymocytes proceed through the class II-dependent stages of positive selection in the absence of CD83. These studies further emphasize a role for CD83 in lymphocyte development and immune regulation and reveal an unexpected role for CD83 expression in influencing cell-surface MHC class II turnover.

Stochasticity and spatial heterogeneity in T-cell activation

Stochastic and spatial aspects are becoming increasingly recognized as an important factor in T-cell activation. Activation occurs in an intrinsically noisy environment, requiring only a handful of agonist peptide-major histocompatibility complex molecules, thus making consideration of signal to noise of prime importance in understanding sensitivity and specificity. Furthermore, it is widely established that surface-bound ligands are more effective at activation than soluble forms, while surface patternation has highlighted the role of spatial relocation in activation. Here we consider the results of a number of models of T-cell activation, from a realistic model of kinetic segregation-induced T-cell receptor (TCR) triggering through to simple queuing theory models. These studies highlight the constraints on cell activation by a surface receptor that recruits kinases. Our analysis shows that TCR triggering based on trapping of bound TCRs in regions of close proximity that exclude large ectodomain-containing molecules, such as the phosphatases CD45 and CD148, can effectively reproduce known signaling characteristics and is a viable 'signal transduction' mechanism distinct from oligomerization and conformation-based mechanisms. A queuing theory analysis shows the interrelation between sensitivity and specificity, emphasizing that these are properties of individual cell functions and need not be, nor are likely to be, uniform across different functions. In fact, threshold-based mechanisms of detection are shown to be poor at ligand discrimination because, although they can be highly specific, that specificity is limited to a small range of peptide densities. Time integration mechanisms however are able to control noise effectively, while kinetic proofreading mechanisms endow them with good specificity properties. Thus, threshold mechanisms are likely to be important for rapidly detecting minimal signaling requirements, thus achieving efficient scanning of antigen-presenting cells. However, for good specificity, time integration on a scale of hours is required.

MHC class II deprivation impairs CD4 T cell motility and responsiveness to antigen-bearing dendritic cells in vivo

The role continuous contact with self-peptide/MHC molecules (self ligands) in the periphery plays in the function of mature T cells remains unclear. Here, we elucidate a role for MHC class II molecules in T cell trafficking and antigen responsiveness in vivo. We find that naïve CD4 T cells deprived of MHC class II molecules demonstrate a progressive and profound defect in motility (measured by real-time two-photon imaging) and that these cells have a decreased ability to interact with limiting numbers of cognate antigen-bearing dendritic cells, but they do not demonstrate a defect in their responsiveness to direct stimulation with anti-CD3 monoclonal antibody. Using GST fusion proteins, we show that MHC class II availability promotes basal activation of Rap1 and Rac1 but does not alter the basal activity of Ras. We propose that tonic T cell receptor signaling from self-ligand stimulation is required to maintain a basal state of activation of small guanosine triphosphatases critical for normal T cell motility and that T cell motility is critical for the antigen receptivity of naïve CD4 T cells. These studies suggest a role for continuous self-ligand stimulation in the periphery for the maintenance and function of mature naïve CD4 T cells.

Physiologic regulation of central and peripheral T cell tolerance: lessons for therapeutic applications

Immunologic tolerance is a state of unresponsiveness that is specific for a particular antigen. The immune system has an extraordinary potential for making T cell and B cell that recognize and neutralize any chemical entity and microbe entering the body. Certainly, some of these T cells and B cells recognize self-components; therefore, cellular mechanisms have evolved to control the activity of these self-reactive cells and achieve immunological self-tolerance. The most important in vivo biological significance of mechanisms regulating self-tolerance is to prevent the immune system from mounting an attack against the host's own tissues resulting in autoimmunity. This review summarizes recent developments in our understanding of T-helper cell tolerance and discusses how the new findings can be exploited to prevent and treat autoimmune diseases, allergy, cancer, and chronic infection, or establish donor-specific transplantation tolerance.

The effects of thymic selection on the range of T cell cross-reactivity

Based on the results of a computational model of thymic selection, we propose a mechanism that produces the observed wide range of T cell cross-reactivity. The model suggests that the cross-reactivity of a T cell that survives thymic selection is correlated with its affinity for self peptides. In order to survive thymic selection, a T cell with low affinity for all self peptides expressed in the thymus must have high affinity for major histocompatibility complex (MHC), which makes it highly cross-reactive. A T cell with high affinity for any self peptide must have low MHC affinity to survive selection, which makes it highly specific for its cognate peptide. Our model predicts that (1) positive selection reduces by only 17% the number of T cells that can detect any given foreign peptide, even though it eliminates over 95% of pre-selection cells; (2) negative selection decreases the average cross-reactivity of the pre-selection repertoire by fivefold; and (3) T cells responding to foreign peptides similar to self peptides will have a lower average cross-reactivity than cells responding to epitopes dissimilar to self.

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.

Impact of effector cell differentiation on CD4+ T cells that evade negative selection by a self-peptide

We have used a transgenic mouse system to examine how differing reactivities of TCRs expressed by naive versus effector cells can shape the functional potential of autoreactive CD4+ T cells. Transgenic mice expressing TCRs that exhibit either high (TS1) or low [TS1(SW)] reactivity toward the I-Ed-restricted determinant S1 from the influenza virus PR8 hemagglutinin (HA) were mated with transgenic mice expressing HA under the control of different promoters. HACII mice express HA driven by an MHC class II promoter, and both the TS1 and TS1(SW) TCRs underwent substantial deletion in this background. HA104 mice express HA driven by an SV40 promoter, and the highly reactive TS1 TCR was substantially deleted. By contrast, the less reactive TS1(SW) TCR underwent little or no deletion in TS1(SW) x HA104 mice, although CD5 up-regulation indicated that they had interacted with the S1 self-peptide. In adoptive transfer studies, naive CD4+ T cells expressing the TS1(SW) TCR failed to proliferate in response to the S1 peptide in HA104 mice, and were inefficient at providing help for HA-specific antibody responses. However, effector CD4+ T cells generated from TS1(SW) x HA104 mice acquired the ability to proliferate in response to the S1 peptide in HA104 mice, and were as efficient as CD4+ T cells expressing the high reactivity TS1 TCR in helping HA-specific antibody responses. Collectively, these studies demonstrate a basis by which CD4+ T cells expressing TCRs with low reactivity toward self-peptides can evade negative selection and acquire enhanced autoreactivity following activation by a cross-reactive antigen.