NK cells inhibit T cell proliferation via p21-mediated cell cycle arrest

NK cells have been shown to influence immune responses via direct interaction with cells of the adaptive immune system, such as dendritic cells, B cells, and T cells. A role for NK cells in down-regulation of T cell responses has been implicated in several studies; however, the underlying mechanism of this suppression has remained elusive. In this study we show that dark Agouti rat NK cells inhibit syngeneic T cell proliferation via up-regulation of the cell cycle inhibitor, p21, resulting in a G0/G1 stage cell cycle arrest. The inhibition is cell-cell contact dependent, reversible, and Ag nonspecific. Interestingly, NK cells do not inhibit IL-2 secretion or IL-2R up-regulation and do not induce T cell death. Thus, our results show that NK cells do not affect early T cell activation events, but specifically inhibit T cell proliferation by direct interaction with T cells. Our findings suggest that NK cells may play an important role in maintaining immune homeostasis by directly regulating clonal expansion of activated T cells. This novel mechanism of T cell regulation by NK cells provides insight into NK cell-mediated regulation of adaptive immunity and provides a mechanistic link between NK cell function and suppression of T cell responses.     

Indoleamine 2,3-dioxygenase pathways of pathogenic inflammation and immune escape in cancer

Genetic and pharmacological studies of indoleamine 2,3-dioxygenase (IDO) have established this tryptophan catabolic enzyme as a central driver of malignant development and progression. IDO acts in tumor, stromal and immune cells to support pathogenic inflammatory processes that engender immune tolerance to tumor antigens. The multifaceted effects of IDO activation in cancer include the suppression of T and NK cells, the generation and activation of T regulatory cells and myeloid-derived suppressor cells, and the promotion of tumor angiogenesis. Mechanistic investigations have defined the aryl hydrocarbon receptor, the master metabolic regulator mTORC1 and the stress kinase Gcn2 as key effector signaling elements for IDO, which also exerts a non-catalytic role in TGF-β signaling. Small-molecule inhibitors of IDO exhibit anticancer activity and cooperate with immunotherapy, radiotherapy or chemotherapy to trigger rapid regression of aggressive tumors otherwise resistant to treatment. Notably, the dramatic antitumor activity of certain targeted therapeutics such as imatinib (Gleevec) in gastrointestinal stromal tumors has been traced in part to IDO downregulation. Further, antitumor responses to immune checkpoint inhibitors can be heightened safely by a clinical lead inhibitor of the IDO pathway that relieves IDO-mediated suppression of mTORC1 in T cells. In this personal perspective on IDO as a nodal mediator of pathogenic inflammation and immune escape in cancer, we provide a conceptual foundation for the clinical development of IDO inhibitors as a novel class of immunomodulators with broad application in the treatment of advanced human cancer.     

In vitro suppression of CD8+ T cell function by Friend virus-induced regulatory T cells

Regulatory T cell (Treg)-mediated suppression of CD8+ T cells has been implicated in the establishment and maintenance of chronic viral infections, but little is known about the mechanism of suppression. In this study an in vitro assay was developed to investigate the suppression of CD8+ T cells by Friend retrovirus (FV)-induced Tregs. CD4+CD25+ T cells isolated from mice chronically infected with the FV suppressed the development of effector function in naive CD8+ T cells without affecting their ability to proliferate or up-regulate activation markers. In vitro restimulation was not required for suppression by FV-induced Tregs, correlating with their high activation state in vivo. Suppression was mediated by direct T cell-T cell interactions and occurred in the absence of APCs. Furthermore, suppression occurred irrespective of the TCR specificity of the CD8+ T cells. Most interestingly, FV-induced Tregs were able to suppress the function of CD8+ effector T cells that had been physiologically activated during acute FV infection. The ability to suppress the effector function of activated CTLs is likely a requisite role for Tregs in limiting immunopathology by CD8+ T cells during antiviral immune responses. Such activity may also have adverse consequences by allowing viruses to establish and maintain chronic infections if suppression of antiviral immune responses occurs before virus eradication.     

Regulation of immunity by anti-T-cell antibodies.

Current pharmacologic approaches to immune suppression leave much to be desired. The prevention of allograft rejection and the suppression of autoimmunity generally require treatment with corticosteroids or cytotoxic drugs, or both, which may not be sufficiently effective and which frequently cause serious immediate and long-term complications. With the advent of monoclonal antibody technology, it has become possible to identify and selectively inhibit distinct elements in the immune system that contribute to pathologic immune responses. This achievement has led to new therapeutic strategies that may be safer and more effective than the immunosuppressive therapies currently available. Many of these strategies focus on subsets of T cells because of the critical importance of T cells in immune responses. Monoclonal antibodies directed against CD4 + T cells, T-cell activation antigens, and T-cell receptor families have all shown promise in animal models and, in some cases, in preliminary human trials. The challenge now is to translate this promise into practical new forms of immunosuppressive therapy.     

Adaptive resistance to RAF inhibitors in melanoma

The discovery of activating mutations in BRAF at high frequency in cutaneous melanoma opened the door to new treatment options, which have resulted in significantly better patient outcomes. Treatments such as the FDA‐approved RAF inhibitor vemurafenib and the more recently approved dabrafenib and trametinib combination therapy are designed to target the ERK1/2 pathway. Initial success in targeting this pathway is evidenced by the high percentage of melanoma patients who undergo tumor remission. However, the beneficial effects of these targeted therapies are usually short‐lived due to the development of resistance, which leads to disease progression. As a result, studies have focused on the acquired forms of resistance that develop following continued exposure to therapy. Conversely, far fewer studies have investigated the adaptive forms of resistance, which activate rapidly, promote cell survival, and may underlie the development of acquired resistance by providing melanoma cells the time to develop additional mutations. We provide a detailed review of the known mechanisms of adaptive resistance in melanoma and relate them to similar responses to targeted therapies in other tumor types.     

Therapeutic efficacy of tumor-targeted IL2 in LTα−/− mice depends on conditioned T cells

An effective immunological eradication of tumors by the adaptive immune system depends on T cell priming, expansion of specific T cells and their effector function. It has been shown that either step may be impaired in the tumor-bearing host, and several strategies have been used to improve antitumor immune responses. In this regard, tumor-targeted IL2 therapy leads to the destruction of established melanoma metastases in fully immune competent mice as previously demonstrated. This effect has been attributed, but never directly confirmed, to the boost of antigen-experienced T cells. To this end, we demonstrate the absence of any antitumor effect of targeted IL2 in mice characterized by an impaired priming of T cell responses. Notably, in these animals tumor-targeted IL2 therapy induced tumor regression only after adoptive transfer of tumor-conditioned splenocytes. A detailed analysis revealed that T cells present within the transferred splenocytes were actively participating in the immune response as these were clonally expanded after targeted IL2 therapy. In summary, we demonstrate here that in LTα^−/− mice lacking sufficient numbers of tumor-specific T cells only the passive transfer of such cells prior to therapy restores the efficacy of tumor-targeted IL2 therapy. Thus, the antitumor effect of tumor-targeted IL2 is indeed based on the boost of pre-existing T cell responses.     

Cutting edge: activation of the aryl hydrocarbon receptor by 2,3,7,8-tetrachlorodibenzo-p-dioxin generates a population of CD4+ CD25+ cells with characteristics of regulatory T cells

Activation of the aryl hydrocarbon receptor (AhR) by its most potent ligand, 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), leads to immune suppression in mice. Although the underlying mechanisms responsible for AhR-mediated immune suppression are not known, previous studies have shown that activation of the AhR must occur within the first 3 days of an immune response and that CD4+ T cells are primary targets. Using the B6-into-B6D2F1 model of an acute graft-vs-host response, we show that activation of AhR in donor T cells leads to the generation of a subpopulation of CD4+ T cells that expresses high levels of CD25, along with CD62L(low), CTLA-4, and glucocorticoid-induced TNFR. These donor-derived CD4+ CD25+ cells also display functional characteristics of regulatory T cells in vitro. These findings suggest a novel role for AhR in the induction of regulatory T cells and provide a new perspective on the mechanisms that underlie the profound immune suppression induced by exposure to TCDD.     

The role of dendritic cells in tumor microenvironments and their uses as therapeutic targets

Dendritic cells (DC), which consist of several different subsets, specialize in antigen presentation and are critical for mediating the innate and adaptive immune responses. DC subsets can be classified into conventional, plasmacytoid, and monocyte-derived DC in the tumor microenvironment, and each subset plays a different role. Because of the role of intratumoral DCs in initiating antitumor immune responses with tumor-derived antigen presentation to T cells, DCs have been targeted in the treatment of cancer. By regulating the functionality of DCs, several DC-based immunotherapies have been developed, including administration of tumor-derived antigens and DC vaccines. In addition, DCs participate in the mechanisms of classical cancer therapies, such as radiation therapy and chemotherapy. Thus, regulating DCs is also important in improving current cancer therapies. Here, we will discuss the role of each DC subset in antitumor immune responses, and the current status of DC-related cancer therapies.     

Tumor cell programmed death ligand 1-mediated T cell suppression is overcome by coexpression of CD80

Programmed death ligand 1 (PDL1, or B7-H1) is expressed constitutively or is induced by IFN-γ on the cell surface of most human cancer cells and acts as a "molecular shield" by protecting tumor cells from T cell-mediated destruction. Using seven cell lines representing four histologically distinct solid tumors (lung adenocarcinoma, mammary carcinoma, cutaneous melanoma, and uveal melanoma), we demonstrate that transfection of human tumor cells with the gene encoding the costimulatory molecule CD80 prevents PDL1-mediated immune suppression by tumor cells and restores T cell activation. Mechanistically, CD80 mediates its effects through its extracellular domain, which blocks the cell surface expression of PDL1 but does not prevent intracellular expression of PDL1 protein. These studies demonstrate a new role for CD80 in facilitating antitumor immunity and suggest new therapeutic avenues for preventing tumor cell PDL1-induced immune suppression.     

Interleukin-21 Is a Critical Regulator of CD4 and CD8 T Cell Survival during Priming under Interleukin-2 Deprivation Conditions

Optimal T cell activation and expansion require binding of the common gamma-chain (γc) cytokine Interleukin-2 (IL-2) to its cognate receptor that in turn engages a γc/Janus tyrosine kinase (Jak)3 signaling pathway. Because of its restricted expression by antigen-activated T cells and its obligatory role in promoting their survival and proliferation, IL-2 has been considered as a selective therapeutic target for preventing T cell mediated diseases. However, in order to further explore IL-2 targeted therapy, it is critical to precisely understand its role during early events of T cell activation. In this study, we delineate the role of IL-2 and other γc cytokines in promoting the survival of CD4 and CD8 T cells during early phases of priming. Under IL-2 inhibitory conditions (by neutralizing anti-IL-2 mAbs), the survival of activated CD8⁺ T cells was reduced, whereas CD4⁺ T cells remained much more resistant. These results correlated with reduced Bcl-2 expression, and mitochondrial membrane potential in CD8⁺ T cells in comparison to CD4⁺ T cells. However, using transwell co-culture assays we have found that CD4⁺ T cells could rescue the survival of CD8⁺ T cells even under IL-2 deprived conditions via secretion of soluble factors. A cytokine screen performed on CD8⁺ T cells cultured alone revealed that IL-21, another γc cytokine, was capable of rescuing their survival under IL-2 deprivation. Indeed, blocking the IL-21 signaling pathway along with IL-2 neutralization resulted in significantly reduced survival of both CD4⁺ and CD8⁺ T cells. Taken together, we have shown that under IL-2 deprivation conditions, IL-21 may act as the major survival factor promoting T cell immune responses. Thus, investigation of IL-2 targeted therapies may need to be revisited to consider blockade of the IL-21 signaling pathways as an adjunct to provide more effective control of T cell immune responses.     

The avidity of tumor‐specific T cells amplified by a plasmacytoid dendritic cell‐based assay can predict the clinical evolution of melanoma patients

The advent of immune checkpoint blockers and targeted therapies has changed the outcome of melanoma. However, many patients experience relapses, emphasizing the need for predictive and prognostic biomarkers. We developed a strategy based on plasmacytoid dendritic cells (pDCs) loaded with melanoma tumor antigens that allows eliciting highly efficient antitumor T‐cell responses. We used it to investigate antitumor T‐cell functionality in peripheral blood mononuclear cells and tumor‐infiltrating lymphocytes from melanoma patients. The pDCs elicited tumor‐specific T cells in different proportions and displaying diverse functional features, dependent upon the stage of the disease, but independent of the histological parameters at diagnosis. Strikingly, the avidity of the MelA‐specific T cells triggered by the pDCs was found to predict patient relapse time and overall survival. Our findings highlighted unexplored aspects of antitumor T‐cell responsiveness in melanoma, and revealed for the first time the structural avidity of tumor‐specific T cells as a crucial feature for predicting clinical evolution.     

The pro-metastatic role of bone marrow-derived cells: a focus on MSCs and regulatory T cells

Several bone marrow-derived cells have been shown to promote tumour growth and progression. These cells can home to the primary tumour and become active components of the tumour microenvironment. Recent studies have also identified bone marrow-derived cells—such as mesenchymal stem cells and regulatory T cells—as contributors to cancer metastasis. The innate versatility of these cells provides diverse functional aid to promote malignancy, ranging from structural support to signal-mediated suppression of the host immune response. Here, we review the role of mesenchymal stem cells and regulatory T cells in cancer metastasis. A better understanding of the bipolar nature of these bone marrow-derived cells in physiological and malignant contexts could pave the way for new therapeutics against metastatic disease.     

Contrasting effects of T cell growth factors on T cell responses to melanoma in vitro

 Determinants of T cell responses to tumor cells remain largely unknown. In the present study we have used long-term cultures of human melanoma cells and autologous peripheral blood lymphocytes to examine the influence of cytokines with T cell growth activity on the phenotype and cytotoxic and proliferative response of T cells to melanoma. It was found that addition of interleukin-4 (IL-4) inhibited the response of CD8⁺ T cells and promoted the response of the CD4 subset. IL-2 or IL-7 was effective in increasing melanoma-specific cytotoxic T lymphocyte (CTL) activity in cultures where CD8 T cells were predominant, whereas IL-4 followed by IL-2 was most effective in cultures where CD4 T cells predominated. IL-10 or IL-12 inhibited proliferation and CTL activity against melanoma in long-term cultures. The effects of IL-12 were reproduced in long-term cultures of T cells stimulated with mAb against CD3 and were shown to depend on prior exposure of T cells to IL-12 before IL-2. As yet unidentified factors, such as co-factor expression on melanoma, appear to be as important as exogenous cytokines in determining the nature of T cell responses to melanoma. These results suggest that analysis of responses in long-term culture may assist in defining the role of key cytokines and other determinants of immune responses to melanoma. Received: 4 June 1996 / Accepted: 12 November 1996     

IFN-β differentially regulates the function of T cell subsets in MS and EAE

Multiple sclerosis (MS) is considered as a T cell mediated autoimmune disease of the CNS, although a pathogenic role has also been attributed to other immune cell types as well as to environmental and genetic factors. Considering that T cells are interesting from an immunopathogenic point of view and consequently from a therapeutic perspective, various T cell targeted therapies have been approved for MS. Interferon beta (IFN-β) is widely used as first-line intervention for modulating T cell responses, although its pleiotropic and multifaceted activities influence its effectiveness on the disease development, with mechanisms that are not yet fully understood. Since different T cell populations, including pro-inflammatory and regulatory T cells, might affect the course of MS, the effects of IFN-β become even more complex.This review will summarize recent findings regarding the T cell targeted effect of IFN-β in MS and its animal model EAE, with emphasis on the direct actions of endogenous and exogenous IFN-β on each T cell subpopulation involved in CNS autoimmunity.Delineating how IFN-β exerts its action on different T cell types may eventually contribute to the designing of therapeutic strategies aiming to improve the effectiveness of this drug for MS treatment.     

Ultraviolet radiation-induced immunosuppression of delayed-type hypersensitivity in mice

Ultraviolet (UV) radiation present in sunlight plays a critical role in the initiation and promotion of nonmelanoma skin carcinogenesis and immune suppression. The immune suppressive effects of UV have been identified as a risk factor for skin cancer induction. For these reasons, scientists have focused on elucidating the mechanisms of UV-induced immune suppression to better understand the pathogenesis of skin cancer induction. A hallmark of UV-induced immune suppression is the generation of antigen-specific suppressor T cells. These suppressor cells have been shown to suppress antitumor immunity as well as other cell-mediated responses such as delayed-type hypersensitivity (DTH) reactions. Due to the excessive cost and time involved in traditional UV carcinogenic experiments, scientists have opted to use UV-induced suppression of DTH reactions as a surrogate model. DTH has been, and continues to be, a widely used assay system to measure in vivo immune function. Although somewhat unsophisticated by today's standards, this assay has great advantages because it presents a fast, inexpensive, and reliable model system to help dissect the mechanisms involved in UV-induced immune suppression. Furthermore, the murine model of DTH enables scientists to perform additional procedures, such as adoptive transfer studies with suppressor T cells, which are currently unavailable with human subjects.     

Innate IFN-γ is essential for programmed death ligand-1-mediated T cell stimulation following Listeria monocytogenes infection

Although best characterized for sustaining T cell exhaustion during persistent viral infection, programmed death ligand-1 (PDL-1) also stimulates the expansion of protective T cells after infection with intracellular bacterial pathogens. Therefore, establishing the molecular signals that control whether PDL-1 stimulates immune suppression or activation is important as immune modulation therapies based on manipulating PDL-1 are being developed. In this study, the requirement for PDL-1 blockade initiated before infection with the intracellular bacterium Listeria monocytogenes in reducing pathogen-specific T cell expansion is demonstrated. In turn, the role of proinflammatory cytokines triggered early after L. monocytogenes infection in controlling PDL-1-mediated T cell stimulation was investigated using mice with targeted defects in specific cytokines or cytokine receptors. These experiments illustrate an essential role for IL-12 or type I IFNs in PDL-1-mediated expansion of pathogen-specific CD8(+) T cells. Unexpectedly, direct stimulation by neither IL-12 nor type I IFNs on pathogen-specific CD8(+) cells was essential for PDL-1-mediated expansion. Instead, the absence of early innate IFN-γ production in mice with combined defects in both IL-12 and type I IFNR negated the impacts of PDL-1 blockade. In turn, IFN-γ ablation using neutralizing Abs or in mice with targeted defects in IFN-γR each eliminated the PDL-1-mediated stimulatory impacts on pathogen-specific T cell expansion. Thus, innate IFN-γ is essential for PDL-1-mediated T cell stimulation.     

自然杀伤T细胞抗胞内菌感染的研究进展

自然杀伤T细胞(NKT细胞)是免疫细胞中一类具有特定标记的T细胞亚群,能特异性识别南抗原呈递细胞表面CD1d分子所呈递的糖脂类抗原.活化后的NKT细胞能分泌多种细胞因子,参与机体的天然免疫和获得性免疫反应,还能直接杀伤靶细胞,具有效应细胞的功能.研究发现,NKT细胞在抗胞内菌感染中发挥着重要作用.