Satish Devadas

Granulocyte-macrophage colony-stimulating factor (GM-CSF) and T-cell responses: what we do and don't know

Granulocyte-macrophage colony-stimulating factor (GM-CSF) is an important hematopoietic growth factor and immune modulator. GM-CSF also has profound effects on the functional activities of various circulating leukocytes. It is produced by a variety of cell types including T cells, macrophages, endothelial cells and fibroblasts upon receiving immune stimuli. Although GM-CSF is produced locally, it can act in a paracrine fashion to recruit circulating neutrophils, monocytes and lymphocytes to enhance their functions in host defense. Recent intensive investigations are centered on the application of GM-CSF as an immune adjuvant for its ability to increase dendritic cell (DC) maturation and function as well as macrophage activity. It is used clinically to treat neutropenia in cancer patients undergoing chemotherapy, in AIDS patients during therapy, and in patients after bone marrow transplantation. Interestingly, the hematopoietic system of GM-CSF-deficient mice appears to be normal; the most significant changes are in some specific T cell responses. Although molecular cloning of GM-CSF was carried out using cDNA library of T cells and it is well known that the T cells produce GM-CSF after activation, there is a lack of systematic investigation of this cytokine in production by T cells and its effect on T cell function. In this article, we will focus mainly on the immunobiology of GM-CSF in T cells.

Reciprocal expression of TRAIL and CD95L in Th1 and Th2 cells: role of apoptosis in T helper subset differentiation.

AbstractUpon activation, naïve T helper cells can differentiate into two major distinct subsets, T helper 1 (Th1) and T helper 2 (Th2), as defined by their effector functions and cytokine secretion patterns. Cytokine milieu and costimulatory molecules have been shown to play an essential role in determining T helper differentiation. However, it is still unclear how the effects of signals of costimulatory molecules and cytokines are exerted during T helper differentiation. We show evidence suggesting that while cytokine signals initiate the differentiation program, the selective action of death effectors determines the end point balance of differentiating T helper subsets. We examined the expression of TNF-related apoptosis-inducing ligand (TRAIL) and CD95L in cloned and in vitro differentiated Th1 and Th2 cells. We found that activation-induced expression of TRAIL is exclusively observed in Th2 clones and primary T helper cells differentiated under the Th2 condition, while the expression of CD95L is mainly in Th1 cells. Furthermore, these two subsets exhibit distinct susceptibilities to TRAIL- and CD95L-mediated apoptosis. Th2 cells are more resistant to either TRAIL- or CD95L-induced apoptosis than Th1 cells. More importantly, both Th1 and Th2 cells could induce apoptosis in labeled Th1 but not Th2 cells. Blocking TRAIL and CD95L significantly enhance IFN-γ production in vitro. Likewise, young MRL/MpJ-lpr/lpr mice also showed more Th1 response to ovalbumin immunization as compared to MRL/MpJ+/+. Therefore, apoptosis mediated by CD95L and TRAIL is critical in determining the fate of differentiating T helper cells.

Stressed to death: implication of lymphocyte apoptosis for psychoneuroimmunology.

Psychological and physical stressors best exemplify the intercommunication of the immune and the nervous systems. It has been shown that stress significantly impacts leukocyte cellularity and immune responses and alters susceptibility to various diseases. While acute stress has been shown to enhance immune responses, chronic stress often leads to immunosuppression. Among many criteria examined upon exposure to chronic stress, the reduction in lymphocyte mitogenic response and lymphocyte cellularity are commonly assessed. We have reported that chronic restraint stress could induce lymphocyte reduction, an effect dependent on endogenous opioids. Interestingly, the effect of endogenous opioids was found to be exerted through increasing the expression of a cell death receptor, Fas, and an increased sensitivity of lymphocytes to apoptosis. Stress-induced lymphocyte reduction was not affected by adrenalectomy. In this review, based on available literature and our recent data, we will discuss the role of the hypothalamic-pituitary-adrenal axis and endogenous opioids and examine the mechanisms by which chronic stress modulates lymphocyte apoptosis.

The role of activation-induced cell death in the differentiation of T-helper-cell subsets

Activation-induced cell death (AICD) has been demonstrated in T-cell hybridomas, immature thymocytes, and activated mature T cells. However, the molecular mechanisms of AICD and its physiological role in T-helper-cell differentiation remain uncertain. Recently, we have shown that Th1 and Th2 cells have distinct mechanisms of AICD. Our findings suggest that signaling from cytokines initiates the differentiation program, but that the selective action of death effectors determines the fate of differentiating T-helper cells, and thus, the ultimate balance between T-helper subpopulations. Among T cells, activation-induced expression of the tumor necrosis factor (TNF)-related apoptosis-inducing ligand (TRAIL) is observed exclusively in Th2 clones and primary T-helper cells differentiated under Th2 conditions, while the expression of CD95L (Fas ligand) occurs mainly in Th1 cells. Furthermore, Th1 cells are more susceptible than Th2 cells to apoptosis induced through either TRAIL or CD95L, and radiolabeled Th1 cells can be induced into apoptosis via fratricide by both Th1 and Th2 cells, while Th2 cells are spared. The pan-caspase inhibitor, z-VAD, prevents AICD in Th1 cells, but not Th2 cells, indicating different mechanisms of AICD in each T-helper subtype. Antibody blockade of TRAIL and CD95L significantly boosts interferon-γ (IFN-γ) production in vitro. Also, young mice with mutant CD95 (MRL/MpJ-lpr/lpr) have a stronger Th1 response to ovalbumin immunization than do controls. We conclude that apoptosis mediated by CD95L and TRAIL is critical in the selective removal of differentiating T helper cells.

Immune activation induces immortalization of HTLV-1 LTR-Tax transgenic CD4+ T cells

Infection with the human T-cell leukemia virus-1 (HTLV-1) results in a variety of diseases including adult T-cell leukemia/lymphoma (ATL). Although the pathogenesis of these disorders is poorly understood, it involves complex interactions with the host immune system. Activation of infected T cells may play an important role in disease pathogenesis through induction of the oncogenic HTLV-1 Tax transactivator protein. To test this hypothesis, we employed transgenic mice in which Tax is regulated by the HTLV-1 LTR. T-cell receptor stimulation of LTR-Tax CD4(+) T cells induced Tax expression, hyper-proliferation, and immortalization in culture. The transition to cellular immortalization was accompanied by markedly increased expression of the antiapoptotic gene, mcl-1, previously implicated as important in T-cell survival. Immortalized cells exhibited a CD4(+)CD25(+)CD3(-) phenotype commonly observed in ATL. Engraftment of activated LTR-Tax CD4(+) T cells into NOD/Shi-scid/IL-2Rγ null mice resulted in a leukemia-like phenotype with expansion and tissue infiltration of Tax(+), CD4(+) lymphocytes. We suggest that immune activation of infected CD4(+) T cells plays an important role in the induction of Tax expression, T-cell proliferation, and pathogenesis of ATL in HTLV-1-infected individuals.

A novel subset of helper T cells promotes immune responses by secreting GM-CSF.

Helper T cells are crucial for maintaining proper immune responses. Yet, they have an undefined relationship with one of the most potent immune stimulatory cytokines, granulocyte macrophage-colony-stimulating factor (GM-CSF). By depleting major cytokines during the differentiation of CD4+ T cells in vitro, we derived cells that were found to produce large amounts of GM-CSF, but little of the cytokines produced by other helper T subsets. By their secretion of GM-CSF, this novel subset of helper T cells (which we have termed ThGM cells) promoted the production of cytokines by other T-cell subtypes, including type 1 helper T cell (Th1), type 2 helper T cell (Th2), type 1 cytotoxic T cell (Tc1), type 2 cytotoxic T cell (Tc2), and naive T cells, as evidenced by the fact that antibody neutralization of GM-CSF abolished this effect. ThGM cells were found to be highly prone to activation-induced cell death (AICD). Inhibitors of TRAIL or granzymes could not block AICD in ThGM cells, whereas inhibition of FasL/Fas interaction partially rescued ThGM cells from AICD. Thus, ThGM cells are a novel subpopulation of T helper cells that produce abundant GM-CSF, exhibit exquisite susceptibility to apoptosis, and therefore play a pivotal role in the regulation of the early stages of immune responses.

Activation of HTLV-1 expression in chronically-infected CD4+ T cells: mechanisms and implications for pathogenesis

Infection with the human T-cell leukemia virus-1 (HTLV-1) is associated with a range of outcomes ranging from asymptomatic infection, to the development of HAM/TSP and adult T cell leukemia/lymphoma. Pathogenesis of these disorders involves complex interactions with the host immune system. Our previous studies showed that phorbol ester (PMA) and T cell receptor (TCR)-mediated activation of chronically infected CD4 T cells increased expression of HTLV-1 gene products [1]. We hypothesized that immune activation of infected T cells may play an important role in disease pathogenesis through induction of Tax expression resulting in increased survival and proliferation. To test this, we employed transgenic mice in which Tax is regulated by the HTLV-1 LTR. These mice develop neurofibromas, but do not express Tax in T cells and do not develop lymphomas [2]. TCR stimulation of transgenic LTR-Tax CD4+ T cells induced Tax expression, early hyper-proliferation, and long-term growth in culture. Survival of these cells was accompanied by transiently increased expression of mcl-1. Long-term surviving cells exhibited a CD4+CD25+CD3- phenotype commonly observed in ATL. Engraftment of immune-activated LTR-Tax CD4+ T-cells into NOD/Shi-scid/IL-2Rγ null mice resulted in a leukemia-like phenotype. Immune activated Tax CD4+ T cells express characteristics of several different CD4+ T cell subtypes, suggesting that HTLV-1 Tax induces changes in the normal pattern of CD4+ subtype specification. We also investigated mechanisms of HTLV-1 activation following PMA stimulation. Surprisingly, PMA treatment was associated with a rapid, marked stabilization of HTLV-1 tax/rex mRNA. Increased RNA stability represents a novel mechanism for increasing HTLV-1 gene expression in chronically infected cells.

Modeling HTLV-lymphomagenesis: immune activation induces immortalization and leukemogenicity of HTLV-1 LTR-Tax transgenic CD4+T-cells.

Address: 1Child Health Institute of New Jersey, University of Medicine and Dentistry of New Jersey (UMDNJ), Robert Wood Johnson Medical, School (RWJMS), 89 French Street, New Brunswick, New Jersey 08901, USA, 2Department of Molecular Genetics, Microbiology, and Immunology UMDNJ-RWJMS, 661 Hoes Lane, Piscataway, New Jersey 08854, USA and 3Cancer Institute of New Jersey UMDNJ-RWJMS, 179 Little Albany Street, New Brunswick, NJ 08903, USA * Corresponding author

Activation-Induced Cell Death and T Helper Subset Differentiation

Activation-induced cell death (AICD) has been demonstrated to occur in T cell hybridomas, immature thymocytes, and activated mature T cells. However, the molecular mechanisms and the physiological role of AICD in the differentiation of T helper cell subpopulations remain elusive. We have recently shown that activation-induced cell death in Thl and Th2 cells is executed via distinct mechanisms. Our results suggest that cytokine signals initiate the differentiation program, but it is the selective action of death effectors that determines the endpoint balance of differentiating T helper subsets. Activation-induced expression of TNF-related apoptosis-inducing ligand (TRAIL) is observed exclusively in Th2 clones and primary T helper cells differentiated under Th2 conditions, while the expression of CD95L (FasL) is mainly in Thl cells. Furthermore, Th2 cells are more resistant to either TRAIL- or CD95L-induced apoptosis than are Thl cells. Both Thl and Th2 cells can induce apoptosis in labeled Thl but not Th2 cells, and caspase inhibitor z-VAD inhibits AICD in Thl but not Th2 cells, implicating different mechanisms of AICD in these subpopulations. Blocking TRAIL CD95L significantly enhances IFN-g production in vitro. Likewise, young CD95-defective transgenic (MRL/MpJlpr/lpr) mice show a greater Thl response to ovalbumin immunization compared to control mice. Therefore, apoptosis mediated by CD95L and TRAIL is critical in determining the fate of differentiating T helper cells.