Murder by proxy

[Role of apoptosis in pathogenesis of thyroiditis]

INTRODUCTION

Apoptosis is a highly regulated mechanism of cell death that differs from necrosis and plays an important role in normal tissue development, homeostasis and immune regulation.

DISEASES AND APOPTOSIS

Apoptosis is involved in many diseases. Defective apoptosis can cause systemic autoimmunity by allowing the survival of autoreactive lymphocytes. It may also be involved in the pathogenesis of organ-specific autoimmune diseases such as Hashimoto's thyroiditis and in the pathogenesis of tumors.

MECHANISM PROGRAMMED DEATH CELL

Apoptosis is regulated at multiple levels, including death receptor and ligand expression, adapter protein, cascades of caspases, mitochondria and the expression of anti apoptotic and pro apoptotic proteins. Apoptotic cell death can occur by two different pathways. Type I is initiated by the activation of death receptors (Fas, TNF-receptor-family) on the plasma membrane followed by activation of caspase 8. Type II involves changes in mitochondrial integrity initiated by various effectors, leading to the release of cytochrome c and activation of caspase 9.

MECHANISM OF APOPTOSIS IN HASHIMOTO THYROIDITIS

The thyroid cell destruction characteristic of autoimmune thyroiditis can be seen as the consequence of inappropriate expression of Fas or TRAIL death pathway molecules and down-regulation of the apoptosis controlling protein Bcl-2, which may be induced by cytokines released locally by infiltrating lymphocytes. Thyroid cell destruction in autoimmune hypothyroidism is dependent on T cell-mediated cytotoxicity with the likely additional effect of death receptor-mediated apoptosis. Modulation of apoptosis-related proteins by T helper 1 and T helper 2 cytokines controls thyrocyte survival in thyroid autoimmunity.

IL-10-producing CD4+CD25+ regulatory T cells play a critical role in granulocyte-macrophage colony-stimulating factor-induced suppression of experimental autoimmune thyroiditis

Our earlier study showed that GM-CSF has the potential not only to prevent, but also to suppress, experimental autoimmune thyroiditis (EAT). GM-CSF-induced EAT suppression in mice was accompanied by an increase in the frequency of CD4(+)CD25(+) regulatory T cells that could suppress mouse thyroglobulin (mTg)-specific T cell responses in vitro, but the underlying mechanism of this suppression was not elucidated. In this study we show that GM-CSF can induce dendritic cells (DCs) with a semimature phenotype, an important characteristic of DCs, which are known to play a critical role in the induction and maintenance of regulatory T cells. Adoptive transfer of CD4(+)CD25(+) T cells from GM-CSF-treated and mTg-primed donors into untreated, but mTg-primed, recipients resulted in decreased mTg-specific T cell responses. Furthermore, lymphocytes obtained from these donors and recipients after adoptive transfer produced significantly higher levels of IL-10 compared with mTg-primed, untreated, control mice. Administration of anti-IL-10R Ab into GM-CSF-treated mice abrogated GM-CSF-induced suppression of EAT, as indicated by increased mTg-specific T cell responses, thyroid lymphocyte infiltration, and follicular destruction. Interestingly, in vivo blockade of IL-10R did not affect GM-CSF-induced expansion of CD4(+)CD25(+) T cells. However, IL-10-induced immunosuppression was due to its direct effects on mTg-specific effector T cells. Taken together, these results indicated that IL-10, produced by CD4(+)CD25(+) T cells that were probably induced by semimature DCs, is essential for disease suppression in GM-CSF-treated mice.

Expression of chemokine receptor CXCR3 and its ligand, mig, in gastric and thyroid marginal zone lymphomas. Possible migration and autocrine mechanism

Chemokine receptors mediate the migration of lymphocytes through the binding of ligands, and the expression is differentially regulated in lymphocyte subsets. CXCR3 is usually expressed in Th1 T cells, however, recently is reported to be expressed in B cell chronic lymphocytic leukemia, mucosa-associated lymphoid tissue type lymphoma (MALT) (extranodal marginal zone lymphoma), and other B cell non-Hodgkin lymphomas. Our study was designed to investigate the expression of CXCR3 and its ligand Mig, and their relationships in MALT using immunohistochemistry. In addition, CCR4, which is characteristic Th2 helper phenotype, and its ligand thymus and activation-regulated chemokine (TARC), were compared with CXCR3, as Th1 phenotype. We studied 14 cases of gastric B cell lymphoma [low-grade MALT, 5 cases; high-grade MALT, 5 cases; and diffuse large (DL), 4 cases] and 16 cases of thyroid B cell lymphoma [low-grade MALT, 4 cases; high-grade MALT, 5 cases; and DL, 7 cases]. CXCR3-expressing lymphoid cells were detected in all cases. In double immunostaining (CXCR3-CD20), gastric and thyroid low/high MALT showed CXCR3-positive neoplastic B cells, but DL, except two cases, did not. In DL, CXCR3-positive lymphoid cells were mainly reactive T-cells (CD3-positive cells). Mig was expressed mainly in stromal cells (histiocytes, macrophages, fibroblasts, and endothelial cells). In gastric lymphoma, low-grade MALT contained abundant Mig-strongly expressing cells, while staining in high-grade MALT and DL was mild. In thyroid lymphoma, staining was strong in low- and high-grade MALT, but moderate in DL. In double-staining, CXCR3-Mig-coexpressing lymphoma cells were abundant in high MALT of the stomach and thyroid, but rare in other subtypes. TARC-positive cells and CCR4-positive cells were rarely encountered in all cases. Our results indicate a tendency for low-grade MALT to contain CXCR3(+)Mig- lymphoma cells, high-grade to contain CXCR3(+)Mig+ and DL to contain CRCR3(-)Mig- lymphoma cells. We speculate that CXCR3 is associated with migration of lymphoma cells in low-grade MALT, and autocrine function in high-grade MALT, and not associated with any function in DL.

Nonlymphoid Fas ligand in peptide-induced peripheral lymphocyte deletion

Peripheral lymphocyte deletion is required for reduction of lymphocyte numbers after expansion in response to antigen. Peripheral deletion is mediated in part by the activation of apoptosis by engagement of the death receptor, Fas (CD95), by its ligand, Fas ligand (FasL; CD95L), among other mechanisms. Here we used T cell receptor (TCR) transgenic animals to examine the role of inducible expression of nonlymphoid FasL in response to peptide antigen. Antigenic challenge of TCR transgenic mice resulted in increased expression of FasL in a number of nonlymphoid tissues including the epithelium of the small intestine. Similar results were obtained in an adoptive transfer system in which TCR transgenic T cells were transferred into recipient animals. The functional relevance of nonlymphoid FasL in peripheral deletion is supported by the observation that FasL-deficient gld animals showed a significantly reduced rate of clearance of transferred antigen-specific lymphocytes, although the lymphocytes themselves were wild type for FasL. These observations were supported further by studies in a transgenic mouse model where lacZ was expressed under the control of the proximal promoter of the FasL gene. Using these transgenic mice, we observed induced activity of the FasL promoter in intestinal epithelial cells throughout the crypts and villi, where we also observed infiltration of activated T cells. These data demonstrate that nonlymphoid FasL is expressed in response to peripheral T cell activation and participates in the regulation of T cells that infiltrate peripheral tissues.

Tumor necrosis factor alpha up-regulates non-lymphoid Fas-ligand following superantigen-induced peripheral lymphocyte activation

Members of the tumor necrosis factor (TNF) and TNF receptor families play important roles in inducing apoptosis and mediating the inflammatory response. Activated T lymphocytes can trigger the expression of Fas-ligand on non-lymphoid tissue, such as intestinal epithelial cells (IEC), and this, in turn, can induce apoptosis in the T cells. Here, we examine the role of TNFalpha in this feedback regulation. Injection of TNFalpha into mice caused a rapid up-regulation of Fas-ligand mRNA in IEC. TNFalpha-induced activation of the Fas-ligand promoter in IEC requires NF-kappaB as this was blocked by an I-kappaBalphaM super-repressor and by mutation of an NF-kappaB site in the Fas-ligand promoter. Activation of T cells by antigen induced Fas-ligand expression in IEC in vivo in wild type, but not in TNFalpha-/- or TNFR1-/- mice. These results define a novel pathway wherein TNFalpha, produced by activated T cells in the intestine, induce Fas-ligand expression in IEC. This is the first observation that one member of the TNF superfamily mediates the regulation of another family member and represents a potential feedback mechanism controlling lymphocyte infiltration and inflammation in the small intestine.

Regulation of apoptosis in endocrine autoimmunity: insights from Hashimoto's thyroiditis and Graves' disease

Dysregulation of apoptosis is associated with the pathogenesis of organ-specific autoimmune diseases, through altered target organ susceptibility. Apoptosis signaling pathways can be initiated through activation of death receptors such as Fas. A comparative analysis of the expression of Fas and FasL, the antiapoptotic molecule Bcl-2, and apoptosis in both thyrocytes and thyroid-infiltrating lymphocytes (TILs) from patients with either Graves' disease (GD) or Hashimoto's thyroiditis (HT) was performed. GD thyrocytes expressed less Fas than HT thyrocytes, whereas GD TILs had higher levels of Fas and FasL than HT TILs. GD thyrocytes expressed higher levels of Bcl-2 compared with HT thyrocytes. The opposite pattern was observed in GD (low Bcl-2) and HT (high Bcl-2) TILs. Consistently, thyrocyte apoptosis was marked in HT and poor in GD thyroids, and TIL apoptosis was marked in GD and poor in HT. Our findings suggest that in GD thyroid the regulation of Fas/FasL/Bcl-2 favors apoptosis of infiltrating lymphocytes. Moreover, the reduced levels of Fas/FasL and increased levels of Bcl-2 should favor thyrocyte survival and hypertrophy associated with stimulatory thyroid-stimulating hormone receptor antibodies. In contrast, the regulation of Fas/FasL/Bcl-2 expression in HT can promote thyrocyte apoptosis via homophylic Fas-FasL interactions, and a gradual reduction in thyrocyte numbers leading to hypothyroidism. Fas-mediated apoptosis may be a general mechanism of cell damage in destructive organ-specific autoimmunity.

Humoral immunodeficiency in chronic lymphocytic leukemia: role of CD95/CD95L in tumoral damage and escape

B cell chronic lymphocytic leukemia (B-CLL) is the most frequent hematological neoplasm in the human adult life. This tumor is often associated with hypogammaglobulinemia and infections. This review focuses on recently reported mechanisms underlying the humoral immunodeficiency occurring in B-CLL. Tumoral cells provoke several alterations to normal regulatory T cells, which, in turn, can impair the correct maturation of B cells. In addition, the direct inhibitory effect of B-CLL cells on immunoglobulin (Ig)-secreting plasma cells (PC) may account for the humoral immunodeficiency. This phenomenon is mediated by the interaction of CD95L molecules expressed by B-CLL cells with the death receptor CD95 that is up-regulated on patients' PC, leading to PC apoptosis and subsequently to hypogammaglobulinemia. Furthermore, the presence of CD95L on B-CLL cells could play an additional role in tumoral immune-escape, as reported in other neoplasms.