CD4+CD25+调节性T细胞的外周维持

CD4 CD25 调节性T细胞作为一种抑制性T细胞功能亚群,在维持机体的免疫自稳和免疫耐受方面发挥了关键作用。该作用的发挥与其外周细胞库的维持密切相关。新近的研究显示CD4 CD25 调节性T细胞主要通过两种机制来维持其外周细胞库,一些功能分子参与其中。     

Cutting edge: estrogen drives expansion of the CD4+CD25+ regulatory T cell compartment

CD4(+)CD25(+) regulatory T cells are crucial to the maintenance of tolerance in normal individuals. However, the factors regulating this cell population and its function are largely unknown. Estrogen has been shown to protect against the development of autoimmune disease, yet the mechanism is not known. We demonstrate that estrogen (17-beta-estradiol, E2) is capable of augmenting FoxP3 expression in vitro and in vivo. Treatment of naive mice with E2 increased both CD25(+) cell number and FoxP3 expression level. Further, the ability of E2 to protect against autoimmune disease (experimental autoimmune encephalomyelitis) correlated with its ability to up-regulate FoxP3, as both were reduced in estrogen receptor alpha-deficient animals. Finally, E2 treatment and pregnancy induced FoxP3 protein expression to a similar degree, suggesting that high estrogen levels during pregnancy may help to maintain fetal tolerance. In summary, our data suggest E2 promotes tolerance by expanding the regulatory T cell compartment.     

Uncoupling of IL-2 signaling from cell cycle progression in naive CD4+ T cells by regulatory CD4+CD25+ T lymphocytes

Prior reports have shown that CD4(+)CD25(+) regulatory T cells suppress naive T cell responses by inhibiting IL-2 production. In this report, using an Ag-specific TCR transgenic system, we show that naive T cells stimulated with cognate Ag in the presence of preactivated CD4(+)CD25(+) T cells also become refractory to the mitogenic effects of IL-2. T cells stimulated in the presence of regulatory T cells up-regulated high affinity IL-2R, but failed to produce IL-2, express cyclins or c-Myc, or exit G(0)-G(1). Exogenous IL-2 failed to break the mitotic block, demonstrating that the IL-2 production failure was not wholly responsible for the proliferation defect. This IL-2 unresponsiveness did not require the continuous presence of CD4(+)CD25(+) regulatory T cells. The majority of responder T cells reisolated after coculture with regulatory cells failed to proliferate in response to IL-2, but were not anergic and proliferated in response to Ag. The mitotic block was also dissociated from the antiapoptotic effects of IL-2, because IL-2 still promoted the survival of T cells that had been cocultured with CD4(+)CD25(+) T cells. IL-2-induced STAT5 phosphorylation in the cocultured responder cells was intact, implying that the effects of the regulatory cells were downstream of receptor activation. Our results therefore show that T cell activation in the presence of CD4(+)CD25(+) regulatory T cells can induce an alternative stimulation program characterized by up-regulation of high affinity IL-2R, but a failure to produce IL-2, and uncoupling of the mitogenic and antiapoptotic effects of IL-2.     

Autologous regulation of naive T cell homeostasis within the T cell compartment

Naive T cells undergo spontaneous slow proliferation on adoptive transfer into syngeneic T cell (T)-deficient hosts. Recent work has shown that such "homeostatic" T cell proliferation is driven by MHC molecules loaded with self-peptides rather than foreign peptides. Because naive T cells in normal T-sufficient hosts remain in interphase despite continuous contact with self-MHC/peptide ligands, T cells apparently inhibit homeostatic proliferation of neighboring T cells. To address this, we have investigated the requirements necessary for "bystander" T cells to inhibit homeostatic proliferation of other T cells. Three key findings are reported. First, homeostatic proliferation of T cells only occurs in specific microenvironments, namely the T cell compartment of the secondary lymphoid tissues. Second, direct entry into T cell compartments is also required for bystander inhibition of homeostatic proliferation. Third, bystander inhibition is mediated largely by naive rather than activated/memory T cells and does not require proliferation or TCR ligation. These findings suggest that homeostasis of naive T cells is unlikely to be regulated through competition for systemic soluble factors or for specific stimulatory self-MHC/peptide ligands. Rather, the data favor mechanisms that involve competition for local non-MHC stimulatory factors or direct cell-to-cell interactions between the T cells themselves within the T cell compartment.     

Modulation of dendritic cell function by naive and regulatory CD4+ T cells

The consequences of interactions between dendric cells (DCs) and either naive CD4+ T cells or regulatory CD4+CD25+ T cells on the expression of proinflammatory IL-6 and anti-inflammatory IL-10 in DC were examined over a period of 12 h, spanning the time frame during which stable T cell-DC interactions shape the development of tolerance and immunity in vivo. We demonstrate that the basal production of IL-6 and IL-10, which is initiated following DC stimulation with LPS, is modified in distinctly different ways by interaction with the two T cell populations. Naive CD4 T cells skew DC cytokine production toward IL-6 and suppress IL-10, whereas CD4+CD25+ T cells have the opposite effect. CD8 T cells or memory CD4 T cells do not influence basal cytokine production by stimulated DC. The effect of CD4+CD25+ T cells is dominant in coculture with naive CD4 T cells as long as inflammatory LPS is absent; the addition of LPS abrogates the suppression of IL-6. However, the modulating influence of CD4+CD25+ T cells remains evident in the enhancement of IL-10 production. Thus, mutual interactions between DC and CD4+ T cell subpopulations following contact with pathogens are likely to influence the strength and quality of incipient immune responses in the local microenvironment.     

Deficient CD4+ T cell priming and regression of CD8+ T cell functionality in virus-infected mice lacking a normal B cell compartment

In this study, we investigate the state of T cell-mediated immunity in B cell-deficient (B(-/-)) mice infected with two strains of lymphocytic choriomeningitis virus known to differ markedly in their capacity to persist. In B(-/-) C57BL mice infected with the more persisting virus, virus-specific CD8(+) T cells are initially generated that are qualitatively similar to those in wild-type mice. However, although cell numbers are well sustained over time, the capacity to produce cytokines is rapidly impaired. In similarly infected B(-/-) BALB/c mice, virus-specific CD8(+) T cells are completely deleted, indicating that host genotype influences the severity of the T cell defect. In B(-/-) C57BL mice infected with the less persisting virus, CD8(+) T cell dysfunction was not as pronounced, although it was clearly present. Most importantly, the appearance of dysfunctional CD8(+) T cells clearly precedes recrudescence of detectable virus, indicating that the T cell defect is not simply a secondary event due to virus buildup resulting from the failure of B(-/-) mice to produce neutralizing Abs. In contrast with CD8(+) T cells, which initially respond almost as in wild-type mice, the priming of virus-specific CD4(+) T cells was markedly impaired in B(-/-) mice infected with either virus strain. Thus, our results indicate that B cells play an important role in antiviral immunity not only as Ab producers, but also in promoting an optimal and sustained T cell response. The T cell defects are likely to contribute to the chronic course of viral infection in B(-/-) mice.     

CD4+CD25+调节性T细胞的作用机制及临床应用

免疫应答通常是机体对各种异源物质的重要防御机制.但有些免疫应答会造成机体的损伤.近来,大量研究发现免疫系统内存在一类CD4 CD25 调节性T淋巴(CD4 CD25 regulatory T cell,CD4 CD25 TReg),在阻止大量免疫介导的疾病中起重要作用.该文从自身免疫耐受、维持T细胞自稳态、肿瘤免疫等方面介绍这类细胞的免疫调节作用.     

Thymocyte antigens do not induce tolerance in the CD4+ T cell compartment.

Thymocytes fail to tolerize the developing T cell repertoire to self MHC class I (MHC I) Ags because transgenic (CD2Kb) mice expressing H-2Kb solely in lymphoid cell lineages reject skin grafts mismatched only for H-2Kb. In this study, we examined why thymocytes fail to tolerize the T cell repertoire to self MHC I Ags. The ability of CD2Kb mice to reject H-2Kb skin grafts was age dependent because CD2Kb mice older than 20 wk accepted skin grafts. T cells from younger CD2Kb mice proliferated, but did not develop cytotoxic functions in vitro in response to H-2Kb. Proliferative responses were dominated by H-2Kb-specific, CD4+ T cells rather than CD8+ T cells. Representative CD4+ T cell clones from CD2Kb mice were MHC II restricted and recognized processed H-2Kb. TCR transgenic mice were generated from one CD4+ T cell clone (361) to monitor development of H-2Kb-specific immature thymocytes when all thymic cells or lymphoid cell lineages only expressed H-2Kb. Thymocyte precursors were not eliminated and mice were not tolerant to H-2Kb when Tg361 TCR transgenic mice were intercrossed with CD2Kb mice. In contrast, all thymocyte precursors were eliminated efficiently in thymic microenvironments in which all cells expressed H-2Kb. We conclude that self MHC I Ags expressed exclusively in thymocytes do not induce T cell tolerance because presentation of processed self MHC I Ags on self MHC II molecules fails to induce negative selection of CD4+ T cell precursors. This suggests that some self Ags are effectively compartmentalized and cannot induce self-tolerance in the T cell repertoire.     

Lung neutrophils facilitate activation of naive antigen-specific CD4+ T cells during Mycobacterium tuberculosis infection

Initiation of the adaptive immune response to Mycobacterium tuberculosis occurs in the lung-draining mediastinal lymph node and requires transport of M. tuberculosis by migratory dendritic cells (DCs) to the local lymph node. The previously published observations that 1) neutrophils are a transiently prominent population of M. tuberculosis-infected cells in the lungs early in infection and 2) that the peak of infected neutrophils immediately precedes the peak of infected DCs in the lungs prompted us to characterize the role of neutrophils in the initiation of adaptive immune responses to M. tuberculosis. We found that, although depletion of neutrophils in vivo increased the frequency of M. tuberculosis-infected DCs in the lungs, it decreased trafficking of DCs to the mediastinal lymph node. This resulted in delayed activation (CD69 expression) and proliferation of naive M. tuberculosis Ag85B-specific CD4 T cells in the mediastinal lymph node. To further characterize the role of neutrophils in DC migration, we used a Transwell chemotaxis system and found that DCs that were directly infected by M. tuberculosis migrated poorly in response to CCL19, an agonist for the chemokine receptor CCR7. In contrast, DCs that had acquired M. tuberculosis through uptake of infected neutrophils exhibited unimpaired migration. These results revealed a mechanism wherein neutrophils promote adaptive immune responses to M. tuberculosis by delivering M. tuberculosis to DCs in a form that makes DCs more effective initiators of naive CD4 T cell activation. These observations provide insight into a mechanism for neutrophils to facilitate initiation of adaptive immune responses in tuberculosis.     

Regulatory CD4 T cells control the size of the peripheral activated/memory CD4 T cell compartment

The mechanisms leading to stable T cell numbers in the periphery of a healthy animal are, to date, not well understood. We followed the expansion of CD45RBhigh (naive) and CD45RBlow (activated/memory) CD4 T cells transferred from normal mice into syngeneic Rag-20/0 recipients and the dynamics of peripheral reconstitution when both populations were coinjected. Naive cells acquired an activated phenotype and showed a high proliferative capacity that was dependent on the environment in which the recipients were kept (specific pathogen-free vs conventional housing conditions), the age of the recipients, and the presence of CD45RBlow T cells in the injected cohort. CD45RBlow CD4 T cells protected the host from CD45RBhigh CD4 T cell-induced inflammatory bowel disease and showed a limited degree of expansion. CD45RBlow CD4 T cells isolated from GF mice also showed the ability to prevent inflammatory bowel disease, indicating that at least part of the natural regulatory T cells are self-reactive. The results indicate that 1) peripheral T cell expansion in lymphocyte-deficient recipients represent classical immune responses, which are mainly promoted by exogenous Ags and 2) natural regulatory T cells control the size of the activated/memory peripheral CD4 T cell compartment.     

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

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

Regulatory CD4+CD25+Foxp3+ T cells selectively inhibit the spontaneous form of lymphopenia-induced proliferation of naive T cells

Regulatory CD4(+)CD25(+)Foxp3(+) T cells play a critical role in controlling autoimmunity and T cell homeostasis. However, their role in regulation of lymphopenia-induced proliferation (LIP), a potential mechanism for generation of autoaggressive T cells, has been poorly defined. Currently, two forms of LIP are recognized: spontaneous and homeostatic. Spontaneous LIP is characterized by fast, burst-like cell-cycle activity, and may allow effector T cell differentiation. Homeostatic LIP is characterized by slow and steady cell cycle activity and is not associated with the acquisition of an effector phenotype. In this study, we demonstrate that CD4(+)CD25(+)Foxp3(+) T cells suppress the spontaneous, but not homeostatic, LIP of naive CD8 and CD4 T cells. However, selective inhibition of spontaneous LIP does not fully explain the tolerogenic role of Tregs in lymphopenia-associated autoimmunity. We show here that suppression of LIP in the lymphoid tissues is independent of Treg-derived IL-10. However, IL-10-deficient Tregs are partially defective in their ability to prevent colitis caused by adoptive transfer of CD4 T cells into RAG(-/-) mice. We propose that Tregs may inhibit emergence of effector T cells during the inductive phase of the immune response in the secondary lymphoid tissues by IL-10-independent mechanisms. In contrast, Treg-mediated inhibition of established effector T cells does require IL-10. Both Treg functions appear to be important in control of lymphopenia-associated autoimmunity.     

Organ-specific CD4+ T cell response during Listeria monocytogenes infection

The immune response against the intracellular bacterium Listeria monocytogenes involves both CD4(+) and CD8(+) T cells. We used the MHC class II-presented peptide listeriolysin(189-201) to characterize the organ-specific CD4(+) T cell response during infection. Systemic listeriosis resulted in a strong peptide-specific CD4(+) T cell response with frequencies of 1/100 and 1/30 CD4(+) splenocytes at the peak of primary and secondary response, respectively. This response was not restricted to lymphoid organs, because we detected specific CD4(+) T cells in all tissues analyzed. However, the tissue distribution of the T cell response was dependent on the route of infection. After i.v. infection, the strongest CD4(+) T cell response and the highest levels of memory cells were observed in spleen and liver, the major sites of L. monocytogenes replication. After oral infection, we detected a strong response in the liver, the lamina propria, and the intestinal epithelium. These tissues also harbored the highest frequencies of listeriolysin(189-201)-specific CD4(+) memory T cells 5-8 wk post oral infection. Our results show that kinetics and magnitude of the CD4(+) T cell response and the accumulation of CD4(+) memory T cells depend on the route of infection and are regulated in a tissue-specific way.     

Conventional, naive CD4+ T cells provide an initial source of IL-4 during Th2 differentiation

IL-4 is known to promote the differentiation of CD4+ T cells into IL-4-secreting Th2 cells. However, the cellular source of the early burst of IL-4 that drives Th2 responses in vivo has not been conclusively identified. Mice deficient for the IL-4 receptor alpha-chain (IL-4Ralpha-/-) retain the capacity to secrete IL-4 and can be used to identify those cell types that produce IL-4 without a requirement for prior IL-4-mediated stimulation. To address whether naive, conventional CD4+ T cells may act as initial producers of IL-4 in Ag-specific responses, we crossed the BALB/c IL-4Ralpha-/-mice to DO11.10/scid TCR transgenic mice. Lymph node cells from wild-type and IL-4Ralpha-/- DO11.10/scid mice secreted approximately 50 pg of IL-4 per10(6) cells within 48 h after peptide stimulation. This small amount of IL-4 was sufficient to cause the differentiation of wild-type CD4+ T cells into Th2 cells, particularly if IFN-gamma and IL-12 were neutralized during the priming cultures. CD4+ cells from the IL-4Ralpha-/- mice gave rise to a minor proportion (approximately 2%) of IL-4-producing cells upon stimulation in the presence of anti-IFN-gamma and anti-IL-12. These data show that conventional, naive CD4+ T cells may be considered as initial sources of IL-4 and, in the absence of IFN-gamma and IL-12, this IL-4 can induce Th2 polarization.     

CD4+ T cell effects on CD8+ T cell location defined using bioluminescence

T lymphocytes of the CD8+ class are critical in delivering cytotoxic function and in controlling viral and intracellular infections. These cells are "helped" by T lymphocytes of the CD4+ class, which facilitate their activation, clonal expansion, full differentiation and the persistence of memory. In this study we investigated the impact of CD4+ T cells on the location of CD8+ T cells, using antibody-mediated CD4+ T cell depletion and imaging the antigen-driven redistribution of bioluminescent CD8+ T cells in living mice. We documented that CD4+ T cells influence the biodistribution of CD8+ T cells, favoring their localization to abdominal lymph nodes. Flow cytometric analysis revealed that this was associated with an increase in the expression of specific integrins. The presence of CD4+ T cells at the time of initial CD8+ T cell activation also influences their biodistribution in the memory phase. Based on these results, we propose the model that one of the functions of CD4+ T cell "help" is to program the homing potential of CD8+ T cells.     

CD25+Foxp3+ regulatory T cells facilitate CD4+ T cell clonal anergy induction during the recovery from lymphopenia

T cell clonal anergy induction in lymphopenic nu/nu mice was found to be ineffective. Exposure to a tolerizing peptide Ag regimen instead induced aggressive CD4(+) cell cycle progression and increased Ag responsiveness (priming). Reconstitution of T cell-deficient mice by an adoptive transfer of mature peripheral lymphocytes was accompanied by the development of a CD25(+)Foxp3(+)CTLA-4(+)CD4(+) regulatory T cell population that acted to dampen Ag-driven cell cycle progression and facilitate the induction of clonal anergy in nearby responder CD25(-)CD4(+) T cells. Thus, an early recovery of CD25(+) regulatory T cells following a lymphopenic event can prevent exuberant Ag-stimulated CD4(+) cell cycle progression and promote the development of clonal anergy.     

Multiple paths for activation of naive CD8+ T cells: CD4-independent help

CD8(+) CTLs play a pivotal role in immune responses against many viruses and tumors. Two models have been proposed. The "three-cell" model focuses on the role of CD4(+) T cells, proposing that help is only provided to CTLs by CD4(+) T cells that recognize Ag on the same APC. The sequential "two-cell" model proposes that CD4(+) T cells can first interact with APCs, which in turn activate naive CTLs. Although these models provide a general framework for the role of CD4(+) T cells in mediating help for CTLs, a number of issues are unresolved. We have investigated the induction of CTL responses using dendritic cells (DCs) to immunize mice against defined peptide Ags. We find that help is required for activation of naive CTLs when DCs are used as APCs, regardless of the origin or MHC class I restriction of the peptides we studied in this system. However, CD8(+) T cells can provide self-help if they are present at a sufficiently high precursor frequency. The important variable is the total number of T cells responding, because class II-knockout DCs pulsed with two noncompeting peptides are effective in priming.