Lung fibroblasts inhibit activation-induced death of T cells through PGE(2)-dependent mechanisms

Interrelation Between Fibroblasts and T Cells in Fibrosing Interstitial Lung Diseases

Interstitial lung diseases (ILDs) are a heterogeneous group of diseases characterized by varying degrees of inflammation and fibrosis of the pulmonary interstitium. The interrelations between multiple immune cells and stromal cells participate in the pathogenesis of ILDs. While fibroblasts contribute to the development of ILDs through secreting extracellular matrix and proinflammatory cytokines upon activation, T cells are major mediators of adaptive immunity, as well as inflammation and autoimmune tissue destruction in the lung of ILDs patients. Fibroblasts play important roles in modulating T cell recruitment, differentiation and function and conversely, T cells can balance fibrotic sequelae with protective immunity in the lung. A more precise understanding of the interrelation between fibroblasts and T cells will enable a better future therapeutic design by targeting this interrelationship. Here we highlight recent work on the interactions between fibroblasts and T cells in ILDs, and consider the implications of these interactions in the future development of therapies for ILDs.

T-cell apoptosis induced by intratumoral activated hepatic stellate cells is associated with lung metastasis in hepatocellular carcinoma

Profound T cell inhibitory activity of hepatic stellate cells (HSCs) in vitro has recently been described in hepatocellular carcinoma (HCC). In the present study, we investigated the immune inhibitory activity of HSCs in vivo in an orthotopic rat HCC model with lung metastasis. Rats (n=24) were randomly sacrificed on days 7, 14, 21 and 28 (n=4 each). Lung tissues were stained with hematoxylin and eosin. Liver sections were stained for immunofluorescence analysis. T-cell apoptosis was detected using double staining for terminal deoxynucleotidyl transferase-mediated dUTP nick-end labeling (TUNEL). Staining revealed marked and continuous accumulation of α-smooth muscle actin with tumor progression after orthotopic tumor implantation in rat liver. T lymphocyte numbers gradually increased following tumor progression, and subset analysis revealed an increase in the distribution of liver CD8+ and CD4+ T cells. Double staining for CD3 and TUNEL demonstrated T-cell apoptosis. Apoptotic T cells were more frequent in the HCC livers compared to the normal livers, and were spatially associated with intratumoral activated HSCs (tHSCs), suggesting a direct interaction. T-cell apoptosis was more frequently induced in the co-cultures of activated splenic T cells(aT)/tHSCs compared to aT/quiescent (q) HSCs or qT/tHSCs. tHSCs were positively correlated with T-cell apoptosis, and the percentage of T-cells undergoing apoptosis was positively correlated with the number of lung metastasis nodules. T-cell apoptosis may be promoted via an interaction with tHSCs, suggesting that tHSCs regulate T cells and contribute to lung metastasis in HCC.

Autophagy is required for IL-2-mediated fibroblast growth

Autophagy is an evolutionarily conserved pathway responsible for delivery of cytoplasmic material into the lysosomal degradation pathway to enable vesicular exocytosis. Interleukin (IL)-2 is produced by T-cells and its activity is important for immunoregulation. Fibroblasts are an immune competent cell type, playing a critical role in wound healing, chronic inflammation, and tumor development. Although autophagy plays an important role in each of these processes, whether it regulates IL-2 activity in fibroblasts is unknown. Here, we show that autophagy is required for IL-2-induced cell growth in fibroblasts. IL-2 significantly induced autophagy in mouse embryonic fibroblasts (MEFs) and primary lung fibroblasts. Autophagy inhibitors (e.g., 3-methylamphetamine and bafilomycin A1) or knockdown of ATG5 and beclin 1 blocked clinical grade IL-2-induced autophagy. Moreover, IL-2 induced HMGB1 cytoplasmic translocation in MEFs and promoted interaction between HMGB1 and beclin1, which is required for autophagy induction. Pharmacological and genetic inhibition of autophagy inhibited IL-2-induced cell proliferation and enhanced IL-2-induced apoptosis. These findings suggest that autophagy is an important pro-survival regulator for IL-2-induced cell growth in fibroblasts.

T cells and stromal fibroblasts in human tumor microenvironments represent potential therapeutic targets

The immune system of cancer patients recognizes tumor-associated antigens expressed on solid tumors and these antigens are able to induce tumor-specific humoral and cellular immune responses. Diverse immunotherapeutic strategies have been used in an attempt to enhance both antibody and T cell responses to tumors. While several tumor vaccination strategies significantly increase the number of tumor-specific lymphocytes in the blood of cancer patients, most vaccinated patients ultimately experience tumor progression. CD4+ and CD8+ T cells with an effector memory phenotype infiltrate human tumor microenvironments, but most are hyporesponsive to stimulation via the T cell receptor (TCR) and CD28 under conditions that activate memory T cells derived from the peripheral blood of the cancer patients or normal donors. Attempts to identify cells and molecules responsible for the TCR signaling arrest of tumor-infiltrating T cells have focused largely upon the immunosuppressive effects of tumor cells, tolerogenic dendritic cells and regulatory T cells. Here we review potential mechanisms by which human T cell function is arrested in the tumor microenvironment with a focus on the immunomodulatory effects of stromal fibroblasts. Determining in vivo which cells and molecules are responsible for the TCR arrest in human tumor-infiltrating T cells will be necessary to formulate and test strategies to prevent or reverse the signaling arrest of the human T cells in situ for a more effective design of tumor vaccines. These questions are now addressable using novel human xenograft models of tumor microenvironments.

TLR4/MYD88-dependent, LPS-induced synthesis of PGE2 by macrophages or dendritic cells prevents anti-CD3-mediated CD95L upregulation in T cells

Antigen-presenting cells (APCs) control T-cell responses by multiple mechanisms, including the expression of co-stimulatory molecules and the production of cytokines and other mediators that control T-cell proliferation, survival and differentiation. Here, we demonstrate that soluble factor(s) produced by Toll-like receptor (TLR)-activated APCs suppress activation-induced cell death (AICD). This effect was observed in non-stimulated APCs, but it was significantly increased after lipopolysaccharide (LPS) treatment. Using different KO mice, we found that the LPS-induced protective factor is dependent on TLR4/MyD88. We identified the protective factor as prostaglandin E(2) (PGE(2)) and showed that both APC-derived supernatants and PGE(2) prevented CD95L upregulation in T cells in response to TCR/CD3 stimulation, thereby avoiding both AICD and activated T cell killing of target macrophages. The PGE(2) receptors, EP2 and EP4, appear to be involved since pharmacological stimulation of these receptors mimics the protective effect on T cells and their respective antagonists interfere with the protection induced by either APCs derived or synthetic PGE(2). Finally, the engagement of EP2 and EP4 synergistically activates protein kinase A (PKA) and exchange protein directly activated by cAMP pathways to prevent AICD. Taken together, these results indicate that APCs can regulate T-cell levels of CD95L by releasing PGE(2) in response to LPS through a TLR4/MyD88-dependent pathway, with consequences for both T cell and their own survival.

Elevated expression of prostaglandin receptor and increased release of prostaglandin E2 maintain the survival of CD45RO+ T cells in the inflamed human pleural space

Throughout the body, the distribution and differentiation of T-cell subsets varies in a way that optimizes host responses. The role of activation-induced cell death (AICD) in altering the distribution of T-lymphocyte subsets at an immune or inflammatory sites has been unexplored. The objective of this study was to assess whether pleural macrophages modulate AICD of specific pleural T-lymphocyte subsets. We found that pleural T-lymphocytes spontaneously undergo apoptosis, which is associated to increased expression of both FAS and FAS ligand, to decreased expression of Bcl 2 and to caspase 8 and 3 activation. While pleural T lymphocytes were partly protected from apoptosis, autologous peripheral blood T lymphocytes increased their apoptosis when cultured with exudative pleural fluids. Pleural CD45RO(+) T cells, in comparison to pleural CD45RA(+) T cells, were more susceptible to apoptosis, but were preferentially protected by exudative pleural fluids. Pleural prostaglandin E 2 (PGE(2)) was implicated in protecting T-lymphocytes from apoptosis because exudative pleural T lymphocytes highly express PGE(2) receptors, and because exudative pleural fluid contained high concentrations of PGE(2). Activated pleural macrophages released PGE(2) and reduced the spontaneous apoptosis of pleural T lymphocytes and depletion of PGE(2) from pleural fluids decreased this protective effect. This study demonstrates that PGE(2), released in the pleural fluids following pleural macrophage activation, prolongs the survival of specific T-cell subsets, resulting in differentiation of the T-cell repertoire within the inflamed pleural space.

Cell cycle regulator E2F1 modulates angiogenesis via p53-dependent transcriptional control of VEGF

The transcription factor E2F1 is known to regulate cell proliferation and has been thought to modulate tumorigenesis via this mechanism alone. Here we show that mice deficient in E2F1 exhibit enhanced angiogenesis. The proangiogenic phenotype in E2F1 deficiency is the result of overproduction of vascular endothelial growth factor (VEGF) and is prevented by VEGF blockade. Under hypoxic conditions, E2F1 down-regulates the expression of VEGF promoter activity by associating with p53 and specifically down-regulating expression of VEGF but not other hypoxia-inducible genes, suggesting a promoter structure context-dependent regulation mechanism. We found that the minimum VEGF promoter mediating transcriptional repression by E2F1 features an E2F1- binding site with four Sp-1 sites in close proximity. These data disclose an unexpected function of endogenous E2F1: regulation of angiogenic activity via p53-dependent transcriptional control of VEGF expression.

Interaction between human lung fibroblasts and T-lymphocytes prevents activation of CD4+ cells

Background

T lymphocytes are demonstrated to play an important role in several chronic pulmonary inflammatory diseases. In this study we provide evidence that human lung fibroblasts are capable of mutually interacting with T-lymphocytes leading to functionally significant responses by T-cells and fibroblasts.

Methods

Human lung fibroblast were co-cultured with PMA-ionomycin activated T-CD4 lymphocytes for 36 hours. Surface as well as intracellular proteins expression, relevant to fibroblasts and lymphocytes activation, were evaluated by means of flow cytometry and RT-PCR. Proliferative responses of T lymphocytes to concanavalin A were evaluated by the MTT assay.

Results

In lung fibroblasts, activated lymphocytes promote an increase of expression of cyclooxygenase-2 and ICAM-1, expressed as mean fluorescence intensity (MFI), from 5.4 ± 0.9 and 0.7 ± 0.15 to 9.1 ± 1.5 and 38.6 ± 7.8, respectively. Fibroblasts, in turn, induce a significant reduction of transcription and protein expression of CD69, LFA-1 and CD28 in activated lymphocytes and CD3 in resting lymphocytes. In activated T lymphocytes, LFA-1, CD28 and CD69 expression was 16.6 ± 0.7, 18.9 ± 1.9 and 6.6 ± 1.3, respectively, and was significantly reduced by fibroblasts to 9.4 ± 0.7, 9.4 ± 1.4 and 3.5 ± 1.0. CD3 expression in resting lymphocytes was 11.9 ± 1.4 and was significantly reduced by fibroblasts to 6.4 ± 1.1. Intracellular cytokines, TNF-alpha and IL-10, were evaluated in T lymphocytes. Co-incubation with fibroblasts reduced the number of TNF-alpha positive lymphocytes from 54,4% ± 6.12 to 30.8 ± 2.8, while IL-10 positive cells were unaffected. Finally, co-culture with fibroblasts significantly reduced Con A proliferative response of T lymphocytes, measured as MTT absorbance, from 0.24 ± 0.02 nm to 0.16 ± 0.02 nm. Interestingly, while the activation of fibroblasts is mediated by a soluble factor, a cognate interaction ICAM-1 mediated was demonstrated to be responsible for the modulation of LFA-1, CD28 and CD69.

Conclusion

Findings from this study suggest that fibroblasts play a role in the local regulation of the immune response, being able to modulate effector functions of cells recruited into sites of inflammation.

Intestinal Fibroblast-Derived IL-10 Increases Survival of Mucosal T Cells by Inhibiting Growth Factor Deprivation- and Fas-Mediated Apoptosis

Mucosal T cells are essential to immune tolerance in the intestine, an organ constantly exposed to large amounts of dietary and bacterial Ags. We investigated whether local fibroblasts affect mucosal T cell survival, which is critical for maintenance of immune tolerance. Coculture with autologous fibroblasts significantly increased viability of mucosal T cells by inhibiting IL-2 deprivation- and Fas-mediated apoptosis, an effect that was both contact- and secreted product-dependent. Investigation of anti-apoptotic factors in the fibroblast-conditioned medium (FCM) revealed the presence of IL-10 and PGE2, but not IFN-beta, IL-2, or IL-15. Although recombinant IFN-beta, but not PGE2, effectively prevented T cell apoptosis, neutralizing Ab studies showed that only IL-10 blockade significantly increased T cells apoptosis, whereas neutralizing IFN-beta or IFN-alpha failed to inhibit the anti-apoptotic effect of FCM. To confirm that fibroblast-derived IL-10 was responsible for preserving mucosal T cell viability, IL-10 mRNA was demonstrated in fibroblasts by Southern blotting and RT-PCR. When FCM was submitted to HPLC fractionation, only the peak matching rIL-10 contained the anti-apoptotic activity, and this was eliminated by treatment with an IL-10-neutralizing Ab. Finally, when fibroblasts were transiently transfected with IL-10 antisense oligonucleotides, the conditioned medium lost its T cell anti-apoptotic effect, whereas medium from fibroblasts transfected with IFN-beta antisense oligonucleotides displayed the same anti-apoptotic activity of medium from untransfected fibroblasts. These results indicate that local fibroblast-derived IL-10 is critically involved in the survival of mucosal T cells, underscoring the crucial importance of studying organ-specific cells and products to define the mechanisms of immune homeostasis in specialized tissue microenvironments like the intestinal mucosa.

Integrin receptors are crucial for the restimulation of activated T lymphocytes

Stimulation via the T-cell receptor results in proliferation of naive T cells and activation-induced death of activated T cells. The expression of Fas ligand and activation-induced cell death are major mechanisms by which immune responses are modulated in the lung. Although it is known that the binding of integrin receptors to extracellular matrix proteins provides co-stimulatory signals to naive T cells, it is not clear whether these signals are critical for activated T cells. The activation and differentiation of T cells is marked by significant changes in integrin expression and affinity. To determine the role of integrin signaling in restimulation of activated T cells, we blocked integrin receptors with RGD peptides. Using murine activated CD4+ T cells and the T-cell hybridoma DO11.10, we found that RGD peptides inhibit tyrosine phosphorylation of CD3 epsilon-chain and ZAP-70, clustering of T-cell receptors, extracellular signal-regulated kinase mitogen-activated protein-kinase activation, and Fas ligand expression and prevent activation-induced cell death. We demonstrate that activated T cells are sensitive to integrin co-stimulation and that integrin receptors are required for the successful restimulation of activated T cells. This indicates that matrix proteins may play a major role in regulating T-cell-mediated immune responses in the lung.

Apoptosis of T cells in the hepatic fibrotic tissue of the rat: a possible inducing role of hepatic myofibroblast-like cells

Apoptosis of T cells contributes to the immune homeostasis in inflamed organs. A prominent T-cell infiltration is usually seen in human chronic active hepatitis, being associated with liver fibrosis. In order to demonstrate T-cell apoptosis in the hepatic fibrotic tissue, we induced T-cell infiltration in the fibrotic liver of the rat by injecting concanavalin A (Con A), a T-cell mitogen. Lymphocytes increased in number with a peak at 1 day, preferentially distributing in the fibrotic tissue rather than the parenchyma. They consisted of CD4-positive and CD8-positive cells, and gave the feature of lymphoblasts. Double staining for CD3 and TUNEL demonstrated that T cells underwent apoptosis. Apoptotic cells were more frequent in the fibrotic livers than the normal livers, and were spatially associated with alpha-smooth muscle actin-positive myofibroblast-like cells that possibly derived from hepatic stellate cells (HSCs) and portal fibroblasts through activation. In vitro experiments demonstrated that lymphocyte apoptosis was more frequently induced in the co-culture of Con A-activated splenic T cells/activated HSCs compared to that induced in activated T cells/quiescent HSCs or resting T cells/activated HSCs. The present results indicate that T cells which have extravasated and infiltrated the hepatic fibrotic tissue undergo apoptosis probably through an interaction with myofibroblast-like cells, suggesting the regulatory role of the latter cells in T-cell accumulation in the fibrotic liver.