Showing papers on "Cytotoxic T cell published in 2009"

IL-17 and Th17 Cells.

Abstract: CD4+ T cells, upon activation and expansion, develop into different T helper cell subsets with different cytokine profiles and distinct effector functions. Until recently, T cells were divided into Th1 or Th2 cells, depending on the cytokines they produce. A third subset of IL-17-producing effector T helper cells, called Th17 cells, has now been discovered and characterized. Here, we summarize the current information on the differentiation and effector functions of the Th17 lineage. Th17 cells produce IL-17, IL-17F, and IL-22, thereby inducing a massive tissue reaction owing to the broad distribution of the IL-17 and IL-22 receptors. Th17 cells also secrete IL-21 to communicate with the cells of the immune system. The differentiation factors (TGF-β plus IL-6 or IL-21), the growth and stabilization factor (IL-23), and the transcription factors (STAT3, RORγt, and RORα) involved in the development of Th17 cells have just been identified. The participation of TGF-β in the differentiation of Th17 cells places ...

Coregulation of CD8+ T cell exhaustion by multiple inhibitory receptors during chronic viral infection

Abstract: T cell exhaustion often occurs during chronic infection and prevents optimal viral control. The molecular pathways involved in T cell exhaustion remain poorly understood. Here we show that exhausted CD8+ T cells are subject to complex layers of negative regulation resulting from the coexpression of multiple inhibitory receptors. Exhausted CD8+ T cells expressed up to seven inhibitory receptors. Coexpression of multiple distinct inhibitory receptors was associated with greater T cell exhaustion and more severe infection. Regulation of T cell exhaustion by various inhibitory pathways was nonredundant, as blockade of the T cell inhibitory receptors PD-1 and LAG-3 simultaneously and synergistically improved T cell responses and diminished viral load in vivo. Thus, CD8+ T cell responses during chronic viral infections are regulated by complex patterns of coexpressed inhibitory receptors.

T cell activation.

Abstract: This year marks the 25th anniversary of the first Annual Review of Immunology article to describe features of the T cell antigen receptor (TCR). In celebration of this anniversary, we begin with a brief introduction outlining the chronology of the earliest studies that established the basic paradigm for how the engaged TCR transduces its signals. This review continues with a description of the current state of our understanding of TCR signaling, as well as a summary of recent findings examining other key aspects of T cell activation, including cross talk between the TCR and integrins, the role of costimulatory molecules, and how signals may negatively regulate T cell function.

mTOR regulates memory CD8 T-cell differentiation

Abstract: Memory CD8 T cells are a critical component of protective immunity, and inducing effective memory T-cell responses is a major goal of vaccines against chronic infections and tumours. Considerable effort has gone into designing vaccine regimens that will increase the magnitude of the memory response, but there has been minimal emphasis on developing strategies to improve the functional qualities of memory T cells. Here we show that mTOR (mammalian target of rapamycin, also known as FRAP1) is a major regulator of memory CD8 T-cell differentiation, and in contrast to what we expected, the immunosuppressive drug rapamycin has immunostimulatory effects on the generation of memory CD8 T cells. Treatment of mice with rapamycin following acute lymphocytic choriomeningitis virus infection enhanced not only the quantity but also the quality of virus-specific CD8 T cells. Similar effects were seen after immunization of mice with a vaccine based on non-replicating virus-like particles. In addition, rapamycin treatment also enhanced memory T-cell responses in non-human primates following vaccination with modified vaccinia virus Ankara. Rapamycin was effective during both the expansion and contraction phases of the T-cell response; during the expansion phase it increased the number of memory precursors, and during the contraction phase (effector to memory transition) it accelerated the memory T-cell differentiation program. Experiments using RNA interference to inhibit expression of mTOR, raptor (also known as 4932417H02Rik) or FKBP12 (also known as FKBP1A) in antigen-specific CD8 T cells showed that mTOR acts intrinsically through the mTORC1 (mTOR complex 1) pathway to regulate memory T-cell differentiation. Thus these studies identify a molecular pathway regulating memory formation and provide an effective strategy for improving the functional qualities of vaccine- or infection-induced memory T cells.

Interleukin-17 and type 17 helper T cells.

Abstract: Type 17 helper T cells (Th17) — a third class of T cells — and their major cytokine, interleukin-17, are essential for the defense against certain fungi, extracellular bacteria, and Mycobacterium tuberculosis. Defective control of Th17 cells and production of interleukin-17 are prominent in chronic inflammation and several immunoinflammatory disorders. Monoclonal antibodies against cytokines produced by Th17 cells are being investigated in clinical trials.

Enhancing CD8 T-cell memory by modulating fatty acid metabolism.

Abstract: CD8 T cells, which have a crucial role in immunity to infection and cancer, are maintained in constant numbers, but on antigen stimulation undergo a developmental program characterized by distinct phases encompassing the expansion and then contraction of antigen-specific effector (T(E)) populations, followed by the persistence of long-lived memory (T(M)) cells. Although this predictable pattern of CD8 T-cell responses is well established, the underlying cellular mechanisms regulating the transition to T(M) cells remain undefined. Here we show that tumour necrosis factor (TNF) receptor-associated factor 6 (TRAF6), an adaptor protein in the TNF-receptor and interleukin-1R/Toll-like receptor superfamily, regulates CD8 T(M)-cell development after infection by modulating fatty acid metabolism. We show that mice with a T-cell-specific deletion of TRAF6 mount robust CD8 T(E)-cell responses, but have a profound defect in their ability to generate T(M) cells that is characterized by the disappearance of antigen-specific cells in the weeks after primary immunization. Microarray analyses revealed that TRAF6-deficient CD8 T cells exhibit altered expression of genes that regulate fatty acid metabolism. Consistent with this, activated CD8 T cells lacking TRAF6 display defective AMP-activated kinase activation and mitochondrial fatty acid oxidation (FAO) in response to growth factor withdrawal. Administration of the anti-diabetic drug metformin restored FAO and CD8 T(M)-cell generation in the absence of TRAF6. This treatment also increased CD8 T(M) cells in wild-type mice, and consequently was able to considerably improve the efficacy of an experimental anti-cancer vaccine.

Instability of the transcription factor Foxp3 leads to the generation of pathogenic memory T cells in vivo.

Abstract: Regulatory T cells (T(reg) cells) are central to the maintenance of immune homeostasis. However, little is known about the stability of T(reg) cells in vivo. In this study, we demonstrate that a substantial percentage of cells had transient or unstable expression of the transcription factor Foxp3. These 'exFoxp3' T cells had an activated-memory T cell phenotype and produced inflammatory cytokines. Moreover, exFoxp3 cell numbers were higher in inflamed tissues in autoimmune conditions. Adoptive transfer of autoreactive exFoxp3 cells led to the rapid onset of diabetes. Finally, analysis of the T cell receptor repertoire suggested that exFoxp3 cells developed from both natural and adaptive T(reg) cells. Thus, the generation of potentially autoreactive effector T cells as a consequence of Foxp3 instability has important implications for understanding autoimmune disease pathogenesis.

Identification of a human helper T cell population that has abundant production of interleukin 22 and is distinct from T H -17, T H 1 and T H 2 cells

Abstract: Interleukin 22 (IL-22) is a member of the IL-10 cytokine family that is involved in inflammatory and wound healing processes. Originally considered a T helper type 1 (T(H)1)-associated cytokine, IL-22 has since been shown to be produced mainly by IL-17-producing helper T cells (T(H)-17 cells). Here we describe a previously uncharacterized IL-22-producing human helper T cell population that coexpressed the chemokine receptor CCR6 and the skin-homing receptors CCR4 and CCR10. These cells were distinct from both T(H)-17 cells and T(H)1 cells. Downregulation of either the aryl hydrocarbon receptor (AHR) or the transcription factor RORC by RNA-mediated interference affected IL-22 production, whereas IL-17 production was affected only by downregulation of RORC by RNA-mediated interference. AHR agonists substantially altered the balance of IL-22- versus IL-17-producing cells. This subset of IL-22-producing cells may be important in skin homeostasis and pathology.

New insights into the regulation of T cells by |[gamma]|c family cytokines

Abstract: Common cytokine receptor gamma-chain (gamma(c)) family cytokines have crucial roles in the development, proliferation, survival and differentiation of multiple cell lineages of both the innate and adaptive immune systems. In this Review, we focus on our current understanding of the distinct and overlapping effects of interleukin-2 (IL-2), IL-7, IL-9, IL-15 and IL-21, as well as the IL-7-related cytokine thymic stromal lymphopoietin (TSLP), on the survival and proliferation of conventional alphabeta T cells, gammadelta T cells and regulatory T cells. This knowledge potentially allows for the therapeutic manipulation of immune responses for the treatment of cancer, autoimmunity, allergic diseases and immunodeficiency, as well as for vaccine development.

Control of large, established tumor xenografts with genetically retargeted human T cells containing CD28 and CD137 domains

Abstract: Mesothelin is a cell-surface molecule over-expressed on a large fraction of carcinomas, and thus is an attractive target of immunotherapy. A molecularly targeted therapy for these cancers was created by engineering T cells to express a chimeric receptor with high affinity for human mesothelin. Lentiviral vectors were used to express a single-chain variable fragment that binds mesothelin and that is fused to signaling domains derived from T-cell receptor zeta, CD28, and CD137 (4–1BB). When stimulated by mesothelin, lentivirally transduced T cells were induced to proliferate, express the antiapoptotic gene Bcl-XL, and secrete multiple cytokines, all features characteristic of central memory T cells. When transferred intratumorally or intravenously into NOD/scid/IL2rγ−/− mice engrafted with large pre-established tumors, the engineered T cells reduced the tumor burden, and in some cases resulted in complete eradication of the tumors at low effector-to-target ratios. Incorporation of the CD137 signaling domain specifically reprogrammed cells for multifunctional cytokine secretion and enhanced persistence of T cells. These findings have important implications for adoptive immunotherapy of cancer, especially in the context of poorly immunogenic tumors. Genetically redirected T cells have promise of targeting T lymphocytes to tumor antigens, confer resistance to the tumor microenvironment, and providing immunosurveillance.

In Situ Cytotoxic and Memory T Cells Predict Outcome in Patients With Early-Stage Colorectal Cancer

Abstract: Purpose Many patients who present with early-stage colorectal cancer (International Union Against Cancer TNM stages I and II) are nevertheless at high risk of relapse. We hypothesized that intratumoral immune reaction could influence their prognosis. Patients and Methods The intratumoral immune reaction was investigated in 29 tumors by large-scale real-time polymerase chain reaction. Cytotoxic (CD8) and memory (CD45RO) T cells were quantified by immunohistochemical analyses of tissue microarrays from the center (CT) and the invasive margin (IM) of the 602 tumors from two independent cohorts. The results were correlated with tumor recurrence and patient survival. Results Patients with a strong infiltration of CD45RO+ cells in the tumor exhibited an increased expression of T-helper 1 and cytotoxicity-related genes. Densities of CD45RO+ and CD8+ cells in tumor regions (CT/IM) classified the patients into four distinct prognostic groups based on the presence of high density of each marker in each tumor region...

Wnt signaling arrests effector T cell differentiation and generates CD8+ memory stem cells

Abstract: Self-renewing cell populations such as hematopoietic stem cells and memory B and T lymphocytes might be regulated by shared signaling pathways. The Wnt-beta-catenin pathway is an evolutionarily conserved pathway that promotes hematopoietic stem cell self-renewal and multipotency by limiting stem cell proliferation and differentiation, but its role in the generation and maintenance of memory T cells is unknown. We found that induction of Wnt-beta-catenin signaling by inhibitors of glycogen sythase kinase-3beta or the Wnt protein family member Wnt3a arrested CD8(+) T cell development into effector cells. By blocking T cell differentiation, Wnt signaling promoted the generation of CD44(low)CD62L(high)Sca-1(high)CD122(high)Bcl-2(high) self-renewing multipotent CD8(+) memory stem cells with proliferative and antitumor capacities exceeding those of central and effector memory T cell subsets. These findings reveal a key role for Wnt signaling in the maintenance of 'stemness' in mature memory CD8(+) T cells and have major implications for the design of new vaccination strategies and adoptive immunotherapies.

Blockade of CTLA-4 on both effector and regulatory T cell compartments contributes to the antitumor activity of anti–CTLA-4 antibodies

Abstract: Cytotoxic T lymphocyte–associated antigen 4 (CTLA-4) is a critical negative regulator of immune responses. Uniquely among known inhibitory receptors, its genetic ablation results in a fulminating and fatal lymphoproliferative disorder. This central regulatory role led to the development of antibodies designed to block CTLA-4 activity in vivo, aiming to enhance immune responses against cancer. Despite their preclinical efficacy and promising clinical activity against late stage metastatic melanoma, the critical cellular targets for their activity remains unclear. In particular, debate has focused on whether the effector T cell (Teff) or regulatory T cell (T reg cell) compartment is the primary target of antibody-mediated blockade. We developed a mouse expressing human instead of mouse CTLA-4, allowing us to evaluate the independent contributions of CTLA-4 blockade of each T cell compartment during cancer immunotherapy in an in vivo model of mouse melanoma. The data show that although blockade on effector cells significantly improves tumor protection, unicompartmental blockade on regulatory cells completely fails to enhance antitumor responses. However, concomitant blockade of both compartments leads to a synergistic effect and maximal antitumor activity. We conclude that the combination of direct enhancement of Teff cell function and concomitant inhibition of T reg cell activity through blockade of CTLA-4 on both cell types is essential for mediating the full therapeutic effects of anti–CTLA-4 antibodies during cancer immunotherapy.

CD56bright natural killer (NK) cells: an important NK cell subset

Abstract: Human natural killer (NK) cells can be subdivided into different populations based on the relative expression of the surface markers CD16 and CD56. The two major subsets are CD56(bright) CD16(dim/) (-) and CD56(dim) CD16(+), respectively. In this review, we will focus on the CD56(bright) NK cell subset. These cells are numerically in the minority in peripheral blood but constitute the majority of NK cells in secondary lymphoid tissues. They are abundant cytokine producers but are only weakly cytotoxic before activation. Recent data suggest that under certain conditions, they have immunoregulatory properties, and that they are probably immediate precursors of CD56(dim) NK cells. CD56(bright) NK cell percentages are expanded or reduced in a certain number of diseases, but the significance of these variations is not yet clear.

Highly tumorigenic lung cancer CD133+ cells display stem-like features and are spared by cisplatin treatment.

Abstract: The identification of lung tumor-initiating cells and associated markers may be useful for optimization of therapeutic approaches and for predictive and prognostic information in lung cancer patients. CD133, a surface glycoprotein linked to organ-specific stem cells, was described as a marker of cancer-initiating cells in different tumor types. Here, we report that a CD133+, epithelial-specific antigen-positive (CD133+ESA+) population is increased in primary nonsmall cell lung cancer (NSCLC) compared with normal lung tissue and has higher tumorigenic potential in SCID mice and expression of genes involved in stemness, adhesion, motility, and drug efflux than the CD133− counterpart. Cisplatin treatment of lung cancer cells in vitro resulted in enrichment of CD133+ fraction both after acute cytotoxic exposure and in cells with stable cisplatin-resistant phenotype. Subpopulations of CD133+ABCG2+ and CD133+CXCR4+ cells were spared by in vivo cisplatin treatment of lung tumor xenografts established from primary tumors. A tendency toward shorter progression-free survival was observed in CD133+ NSCLC patients treated with platinum-containing regimens. Our results indicate that chemoresistant populations with highly tumorigenic and stem-like features are present in lung tumors. The molecular features of these cells may provide the rationale for more specific therapeutic targeting and the definition of predictive factors in clinical management of this lethal disease.

Cytokine-secreting follicular T cells shape the antibody repertoire

Abstract: High-affinity antibodies are critical for host protection and underlie successful vaccines. The generation of such antibodies requires T cell-dependent help, which mediates germinal center reactions in which mutation and selection of B cells occurs. Using an interleukin 4-reporter system, we show here that CD4(+) follicular helper T cells constituted essentially all of the cytokine-secreting T cells in lymph nodes and were functionally distinct from T cells secreting the same cytokine in peripheral tissues. Follicular helper T cells with different cytokine profiles could be isolated as conjugates with B cells undergoing cytokine-specific immunoglobulin class switching with evidence of somatic hypermutation. Our findings support a model in which B cells compete for cytokines produced by follicular helper T cells that shape the affinity and isotype of the antibody response.

MicroRNA miR-326 regulates TH-17 differentiation and is associated with the pathogenesis of multiple sclerosis.

Abstract: Interleukin 17 (IL-17)-producing T helper cells (T H -17 cells) are increasingly recognized as key participants in various autoimmune diseases, including multiple sclerosis. Although sets of transcription factors and cytokines are known to regulate T H -17 differentiation, the role of noncoding RNA is poorly understood. Here we identify a T H -17 cell–associated microRNA, miR-326, whose expression was highly correlated with disease severity in patients with multiple sclerosis and mice with experimental autoimmune encephalomyelitis (EAE). In vivo silencing of miR-326 resulted in fewer T H -17 cells and mild EAE, and its overexpression led to more T H -17 cells and severe EAE. We also found that miR-326 promoted T H -17 differentiation by targeting Ets-1, a negative regulator of T H -17 differentiation. Our data show a critical role for microRNA in T H -17 differentiation and the pathogenesis of multiple sclerosis.

Cross-presentation of viral and self antigens by skin-derived CD103 + dendritic cells

Abstract: Skin-derived dendritic cells (DCs) include Langerhans cells, classical dermal DCs and a langerin-positive CD103(+) dermal subset. We examined their involvement in the presentation of skin-associated viral and self antigens. Only the CD103(+) subset efficiently presented antigens of herpes simplex virus type 1 to naive CD8(+) T cells, although all subsets presented these antigens to CD4(+) T cells. This showed that CD103(+) DCs were the migratory subset most efficient at processing viral antigens into the major histocompatibility complex class I pathway, potentially through cross-presentation. This was supported by data showing only CD103(+) DCs efficiently cross-presented skin-derived self antigens. This indicates CD103(+) DCs are the main migratory subtype able to cross-present viral and self antigens, which identifies another level of specialization for skin DCs.

1,25-Dihydroxyvitamin D3 and IL-2 Combine to Inhibit T Cell Production of Inflammatory Cytokines and Promote Development of Regulatory T Cells Expressing CTLA-4 and FoxP3

Abstract: The active form of vitamin D, 1,25-dihydroxyvitamin D(3) (1,25(OH)(2)D(3)), has potent immunomodulatory properties that have promoted its potential use in the prevention and treatment of infectious disease and autoimmune conditions. A variety of immune cells, including macrophages, dendritic cells, and activated T cells express the intracellular vitamin D receptor and are responsive to 1,25(OH)(2)D(3.) Despite this, how 1,25(OH)(2)D(3) regulates adaptive immunity remains unclear and may involve both direct and indirect effects on the proliferation and function of T cells. To further clarify this issue, we have assessed the effects of 1,25(OH)(2)D(3) on human CD4(+)CD25(-) T cells. We observed that stimulation of CD4(+)CD25(-) T cells in the presence of 1,25(OH)(2)D(3) inhibited production of proinflammatory cytokines including IFN- gamma, IL-17, and IL-21 but did not substantially affect T cell division. In contrast to its inhibitory effects on inflammatory cytokines, 1,25(OH)(2)D(3) stimulated expression of high levels of CTLA-4 as well as FoxP3, the latter requiring the presence of IL-2. T cells treated with 1,25(OH)(2)D(3) could suppress proliferation of normally responsive T cells, indicating that they possessed characteristics of adaptive regulatory T cells. Our results suggest that 1,25(OH)(2)D(3) and IL-2 have direct synergistic effects on activated T cells, acting as potent anti-inflammatory agents and physiologic inducers of adaptive regulatory T cells.

Identification of IL-17-producing FOXP3⁺ regulatory T cells in humans

Abstract: IL-17–producing CD4 + T helper (Th17) cells have recently been defined as a unique subset of proinflammatory helper cells whose development depends on signaling initiated by IL-6 and TGF-β, autocrine activity of IL-21, activation of STAT3, and induction of the orphan nuclear receptor RORγt. The maintenance, expansion, and further differentiation of the committed Th17 cells depend on IL-1β and IL-23. IL-17 was originally found produced by circulating human CD45RO + memory T cells. A recent study found that human Th17 memory cells selectively express high levels of CCR6. In this study, we report that human peripheral blood and lymphoid tissue contain a significant number of CD4 + FOXP3 + T cells that express CCR6 and have the capacity to produce IL-17 upon activation. These cells coexpress FOXP3 and RORγt transcription factors. The CD4 + FOXP3 + CCR6 + IL-17–producing cells strongly inhibit the proliferation of CD4 + responder T cells. CD4 + CD25 high -derived T-cell clones express FOXP3, RORγt, and IL-17 and maintain their suppressive function via a cell-cell contact mechanism. We further show that human CD4 + FOXP3 + CCR6 − regulatory T (Treg) cells differentiate into IL-17 producer cells upon T-cell receptor stimulation in the presence of IL-1β, IL-2, IL-21, IL-23, and human serum. This, together with the finding that human thymus does not contain IL-17–producing Treg cells, suggests that the IL-17 + FOXP3 + Treg cells are generated in the periphery. IL-17–producing Treg cells may play critical roles in antimicrobial defense, while controlling autoimmunity and inflammation.

Interactions between PD-1 and PD-L1 promote tolerance by blocking the TCR-induced stop signal.

Abstract: Programmed death 1 (PD-1) is an inhibitory molecule expressed on activated T cells; however, the biological context in which PD-1 controls T cell tolerance remains unclear. Using two-photon laser-scanning microscopy, we show here that unlike naive or activated islet antigen-specific T cells, tolerized islet antigen-specific T cells moved freely and did not swarm around antigen-bearing dendritic cells (DCs) in pancreatic lymph nodes. Inhibition of T cell antigen receptor (TCR)-driven stop signals depended on continued interactions between PD-1 and its ligand, PD-L1, as antibody blockade of PD-1 or PD-L1 resulted in lower T cell motility, enhanced T cell-DC contacts and caused autoimmune diabetes. Blockade of the immunomodulatory receptor CTLA-4 did not alter T cell motility or abrogate tolerance. Thus, PD-1-PD-L1 interactions maintain peripheral tolerance by mechanisms fundamentally distinct from those of CTLA-4.

Effector memory T cell responses are associated with protection of rhesus monkeys from mucosal simian immunodeficiency virus challenge

Abstract: The rapid onset of massive, systemic viral replication during primary HIV or simian immunodeficiency virus (SIV) infection and the immune evasion capabilities of these viruses pose fundamental problems for vaccines that depend upon initial viral replication to stimulate effector T cell expansion and differentiation. We hypothesized that vaccines designed to maintain differentiated effector memory T cell (TEM cell) responses at viral entry sites might improve efficacy by impairing viral replication at its earliest stage, and we have therefore developed SIV protein-encoding vectors based on rhesus cytomegalovirus (RhCMV), the prototypical inducer of life-long TEM cell responses. RhCMV vectors expressing SIV Gag, Rev-Tat-Nef and Env persistently infected rhesus macaques, regardless of preexisting RhCMV immunity, and primed and maintained robust, SIV-specific CD4+ and CD8+ TEM cell responses (characterized by coordinate tumor necrosis factor, interferon-gamma and macrophage inflammatory protein-1beta expression, cytotoxic degranulation and accumulation at extralymphoid sites) in the absence of neutralizing antibodies. Compared to control rhesus macaques, these vaccinated rhesus macaques showed increased resistance to acquisition of progressive SIVmac239 infection upon repeated limiting-dose intrarectal challenge, including four macaques who controlled rectal mucosal infection without progressive systemic dissemination. These data suggest a new paradigm for AIDS vaccine development--vaccines capable of generating and maintaining HIV-specific TEM cells might decrease the incidence of HIV acquisition after sexual exposure.

Bispecific T-cell engaging antibodies for cancer therapy.

Abstract: There is increasing evidence that T cells are able to control tumor growth and survival in cancer patients, both in early and late stages of the disease. However, tumor-specific T-cell responses are difficult to mount and sustain in cancer patients, and are limited by numerous immune escape mechanisms of tumor cells selected during immunoediting. An alternative approach to engage T cells for cancer therapy are antibodies, which are bispecific for a surface target antigen on cancer cells, and for CD3 on T cells. These are capable of connecting any kind of cytotoxic T cell to a cancer cell, independently of T-cell receptor specificity, costimulation, or peptide antigen presentation. Here, we review the principle of a new class of bispecific antibodies called BiTE (for "bispecific T-cell engager") antibodies. Recent results from clinical studies with a CD19/CD3-bispecific BiTE antibody suggest that this therapeutic paradigm is finally showing promise for treatment of both bulky and minimal residual disease.

Complete but curtailed T-cell response to very low-affinity antigen

Abstract: After an infection, T cells that carry the CD8 marker are activated and undergo a characteristic kinetic sequence of rapid expansion, subsequent contraction and formation of memory cells. The pool of naive T-cell clones is diverse and contains cells bearing T-cell antigen receptors (TCRs) that differ in their affinity for the same antigen. How these differences in affinity affect the function and the response kinetics of individual T-cell clones was previously unknown. Here we show that during the in vivo response to microbial infection, even very weak TCR-ligand interactions are sufficient to activate naive T cells, induce rapid initial proliferation and generate effector and memory cells. The strength of the TCR-ligand interaction critically affects when expansion stops, when the cells exit lymphoid organs and when contraction begins; that is, strongly stimulated T cells contract and exit lymphoid organs later than weakly stimulated cells. Our data challenge the prevailing view that strong TCR ligation is a prerequisite for CD8(+) T-cell activation. Instead, very weak interactions are sufficient for activation, but strong TCR ligation is required to sustain T-cell expansion. We propose that in response to microbial challenge, T-cell clones with a broad range of avidities for foreign ligands are initially recruited, and that the pool of T cells subsequently matures in affinity owing to the more prolonged expansion of high-affinity T-cell clones.

Analysis of islet inflammation in human type 1 diabetes.

Abstract: The immunopathology of type 1 diabetes (T1D) has proved difficult to study in man because of the limited availability of appropriate samples, but we now report a detailed study charting the evolution of insulitis in human T1D. Pancreas samples removed post-mortem from 29 patients (mean age 11.7 years) with recent-onset T1D were analysed by immunohistochemistry. The cell types constituting the inflammatory infiltrate within islets (insulitis) were determined in parallel with islet insulin content. CD8(+) cytotoxic T cells were the most abundant population during insulitis. Macrophages (CD68(+)) were also present during both early and later insulitis, although in fewer numbers. CD20(+) cells were present in only small numbers in early insulitis but were recruited to islets as beta cell death progressed. CD138(+) plasma cells were infrequent at all stages of insulitis. CD4(+) cells were present in the islet infiltrate in all patients but were less abundant than CD8(+) or CD68(+) cells. Forkhead box protein P3(+) regulatory T cells were detected in the islets of only a single patient. Natural killer cells were detected rarely, even in heavily inflamed islets. The results suggest a defined sequence of immune cell recruitment in human T1D. They imply that both CD8(+) cytotoxic cells and macrophages may contribute to beta cell death during early insulitis. CD20(+) cells are recruited in greatest numbers during late insulitis, suggesting an increasing role for these cells as insulitis develops. Natural killer cells and forkhead box protein P3(+) T cells do not appear to be required for beta cell death.