Fas-FasL interactions: a common pathogenetic mechanism in organ-specific autoimmunity

V I E W P O IN T IMMUNOLOGY

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Fas-FasL interactions: common pathogenetic mechanism in organ-specific autoimmunity Ruggero De Maria and Roberto Testi Organ-specific autoimmunity is characterized by the accelerated loss of selected cell types, resulting in

soluble form, which is released by metalloas (CD95/APO-1) is a widely specific tissue destruction and protease-mediated proteolytic shedding 14. expressed 45 kDa membrane disease. The rote of different genetic This functional soluble form is responsible receptor, responsible for confor killing Fas-sensitive cells through either trolling tissue homeostasis and or environmental factors in autocrine suicide or paracrine death of immune responses 1,2. Several tissues coninitiating the autoimmune neighboring cells, whereas the membrane stitutively express Fas, including spleen, reactivity is still unclear. form requires direct cell contact to mediate lymph nodes, liver, lung, kidney and cytolysis. ovary 3. Its expression and function in However, novel mechanisms hematopoietic cells directly correlates with responsible for tissue destruction the rate of growth and self-renewal, sugUnconventional T-cell attack in have recently been revealed. gesting a potential role for Fas in the reguautoimmune diabetes lation of cell accumulation 4. Targeted muHere, Ruggero De Maria and Insulin-dependent diabetes mellitus (IDDM) tation of the gene encoding Fas in mice has Roberto Testi propose that Fas is a chronic autoimmune disease resulting provided evidence for the role played by from T-cell-mediated destruction of pancreFas in the homeostasis of the immune sysligand may represent a common atic 13cells ~5. The importance of T cells in the tem. These mice suffer from generalized weapon during the destructive pathogenesis of IDDM has been demonlymphadenopathy, massive lymphocytosis, phase of organ-specific strated by using two animal models of this splenomegaly and hepatomegaly 5. Moredisease: the nonobese diabetic (NOD) over, Fas-deficient mice in the absence of autoimmunity. mouse and the biobreeding diabetic-prone T cells develop lethal B-cell lymphoma, sugrat 16. Islet-specific T cells are very efficient gesting a potential contribution of Fas interactions with its ligand (FasL) in suppression of lymphoid tumors 6. in transferring the disease, whereas vaccination with autoreactive A cascade of biochemical events generated by Fas crosslinking has T-cell-specific peptides provides extremely effective protection recently been characterized. This cascade involves the recruitment against diabetes 17. Moreover, T cells constitute the major compoof adaptor molecules to the receptor 'death domain', and the subse- nent of the insulitis infiltrates both in human and in experimental quent activation of caspases and sphingomyelinases, eventually diabetes16,18. leading to apoptotic cell death 7,~. However, crosslinking of Fas is not always able to induce apoptosis, since molecular coupling with its apoptotic machinery is required to propagate the death signal, following the interaction with FasL (Refs 9, 10). FasL is a 40 kDa type II membrane protein belonging to the tumor necrosis factor (TNF) family 11. Although its expression was initially believed to be confined to activated T cells, several other cell types have subsequently been shown to produce and release this cytokine: FasL is constitutively expressed in neutrophils, neurons, thyrocytes, stroma cells of the retina, acinar cells in salivary glands and Sertoli cells in the mouse testis 12. Moreover, a variety of cell types can express FasL in response to different stimulatory conditions, including macrophages infected with human immunodeficiency virus, hepatocytes treated with ethanol, leukemia cells exposed to chemotherapy drugs, and various cell types upon tumor transformation ]2,]3. In humans, FasL can induce cytolysis of Fasexpressing cells, either as a membrane-bound form or as a 17 kDa PI SOl 67-~6~9 97)0 !202-q

The indisputable role played by specific T cells in the pathogenesis of IDDM conflicts with the polyclonal cellular and humoral autoimmune responses found in the disease. Such reactivities are not 13-cell specific but are directed against multiple islet and nonislet antigens 19. Thus, it has never been completely clear why the outcome of this pleiotropic reaction is the specific destruction of pancreatic ~ cells, while c~ and ~ cells are essentially spared. These findings promoted the search for other pathogenetic mechanisms in IDDM, such as the preferential vulnerability of 13 cells to products of inflammation. Indeed, islet cells are susceptible to the toxic action of oxygen radicals and different combinations of cytokines. Thus, it has been proposed that the lysis of 13 cells could be mediated by activated macrophages through the production of interleukin 113 (IL-I~) and nitric oxide (NO) 2°. This mechanism may be responsible for initiating and amplifying the autoimmune process; however, it seems rather unlikely that, at physiological doses, IL-I~ and NO are directly responsible for the dramatic f3-cell destruction observed in © 1998 Elsevle' Science Lic

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directly recognized by CD4' T cells. These findings argue against a mechanism of 13-cei1 destruction requiring direct effectortarget recognition and cognate interaction by effector cells24.

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Fig. I. Proposed mechanism for [3-cell depletion in IDDM. (a) Normal islet cells do not express Fas. (b) Following an initial specific reaction causing limited damage to the [3-cell mass, NO-induced Fas expression primes [3 cells for destruction through the secondary polyclonal T-cell response. CTLs an" responsible for specific death of bystander Fas * [3 cells by membrane-bound or released FasL, sparing Fas a and 6 cells. Abbreviations: APC, antigen-presenting cell; CTL, cytotoxic T lymphocyte; FasL, Fas ligand; IDDM, insulin-dependent diabetes mellitus; IL-l fl, interleukin 113; MHC, major histocompatibility complex; NO, nitric oxide; TCR, T-cell receptor. IDDM. Therefore, T-cell-mediated cytotoxicity remains the most convincing mechanism of triggering 13-cell death.

Mechanism

or cytotoxicity

Two independent lytic pathways mediate T-cell cytotoxicity: (1) the exocytosis of perforin-containing granules on cognate target cells; and (2) the engagement of Fas on cognate or neighboring target cells by membrane-bound or released FasL (Refs 21, 22). Cytotoxic T lymphocyte (CTL) degranulation and perforin-induced cell lysis are part of a highly specific killing process, which requires interaction between the T-cell receptor and the major histocompatibility complex (MHC)-peptide for target recognition. This process has classically been regarded as the mechanism used by autoreactive T-cell clones for target destruction. Accordingly, perforin-deficient NOD mice show reduced incidence and delayed onset of diabetes, suggesting that perforin-mediated CTL-induced cytotoxicity participates in [3-cell destruction 23. However, although CD4 + T ceils can transfer diabetes in the absence of CD8 + T cells, mouse [3 cells do not express MHC class II molecules, and thus they are not

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The interaction between Fas and FasL mediates the nonspecific, non-MHCrestricted, T-cell cytotoxicity observed following polyclonal activation. Normal pancreatic 13 cells do not express Fas; however, during the insulitis process, different cytokines and inflammation mediators may prime [3 cells for Fas-induced destruction. indeed, the exposure of islet cells to IL-1[3 induces a selective and functional Fas expression on 13 cells through the production of NO (Ref. 25). Moreover, during the insulitis process, several Fas t t3 cells located in proximity to FasL + T cells display an apoptotic phenotype, suggesting Fasinduced apoptosis as a possible new effector mechanism for [3-cell destruction 2s. The potential role of FasL-based cytotoxicity in the pathogenesis of autoimmune diabetes is further supported by the absence of hyperglycemia and by complete protection from both spontaneous and T-celltransferred diabetes in Fas-defective NOD#,~/I~ mice26,27.

The involvement of Fas-mediated 13-cell destruction in IDDM may explain how a polyclonal reactivity against multiple and non-[3-cell-specific antigens is eventually responsible for [3-cell destruction. In this view, an unconventional mechanism for [3-cell destruction can be envisioned. Following an initial specific response against a restricted number of antigens, the resulting 13-cell damage promotes local inflammation and a consequent cascade of pathogenetic events. IL-1 and NO may upregulate Fas on 13cells and prime pancreatic [3 cells for Fas-mediated destruction. Autoreactive T cells producing FasL could then exert a massive noncognate destruction of Fas' bystander 13cells, sparing Fas c~ and 8 cells (Fig. 1). In summary, the selective expression of Fas on [3 cells, and the absence of Fas on and 8 cells, during the insuIitis process eventually confers a specific vulnerability to (3 cells, replacing the absence of specificity in effectar T cells. In the mouse, ~xcells constitutively express FasL and are strategically located at the periphery of pancreatic islets; these cells might even be involved in protection from attacking T cells, as suggested by recent data in the NOD model 28.

Multiple killers in multiple sclerosis Multiple sclerosis (MS) is an inflammatory demyelinating disease of the central nervous system, characterized by neurological

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(a) Normal white matter symptoms due to impaired nerve conduction 29'3°. Histological analysis of active plaque lesions reveals myelin breakdown associated with abundant inflammatory infiltrates comprising macrophages, lymphocytes and plasma cells. Although axons are essentially preserved within plaques, the myelin-sustaining oligodendrocytes are massively depleted 29.3°. Most of the knowledge on the pathogenesis of MS derives from work on experimental allergic encephalomyelitis (EAE), a demyelinating autoimmune disease induced in a number of animal species by immunization with myelin components or by adoptive transfer of myelin-reactive CD4 + T cells29,31. Although experimental evidence suggests that myelin destruction in MS is mediated by T cells, other mechanisms may contribute to the effector phase of the process. Indeed, administration of antibodies against myelin has been shown to increase the disease severity in acute EAE in rats. Moreover, infiltrating macrophages may participate in oligodendrocyte damage through either antibody-dependent cell cytotoxicity or the release of inflammatory cytokines and NO (Refs 31, 32).

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Reactivity against several myelin proteins has been demonstrated in MS and EAE, the immunodominant peptides of myelin basic protein (MBP) and proteolipid protein (PLP) being the major antigenic determinants 31. Immunization with MBP and PLP peptides induces a massive CD4 + T-cell response and Fig. 2. Multiple potential effector cells in MS. (a) Fas and FasL are not expressed in normal white a severe acute EAE (Ref. 30). Treatment with matter. (b) By contrast, astrocytes, macrophages and oligodendrocytes have been implicated as a anti-MHC class II antibodies prevents EAE, possible source for FasL in MS white matter. However, activated CTLs and microglia show the highsuggesting the requirement of cognate inter- est FasL expression and are the most likely candidates for triggering Fas-induced oligodendrocyte actions between antigen-presenting cells and cell death. Abbreviations: Ast, astrocyte; CTL, cytotoxic T lymphocyte; M ~ , macrophage; Mi, CD4 + T cells for initiating the autoimmune microglia; MS, multiple sclerosis; Od, oligodendrocyte. process. The pathogenetic relevance of CD4 + T cells in EAE is further demonstrated by the ability of anti-CD4 revealed a diffuse and intense Fas reactivity among the oligoantibodies to reverse the disease both in rats and in mice2q. dendrocytes located along the lesion margin and in the adjacent As in IDDM, direct effector-target recognition does not seem to white matter. Moreover, FasL-expressing cells have been found in occur during the effector phase leading to myelin destruction, as proximity to apoptotic Fas + oligodendrocytes:33,34. For instance, in oligodendrocytes in the active lesions do not express MHC class II both acute and chronic plaques, FasL might be expressed by T cells, molecules. Moreover, polyclonally activated or myelin-reactive macrophages, microglial cells, astrocytes and oligodendrocytes34. CD4 ÷ T cells are able to induce non-MHC-restricted and TNF- Microglia and infiltrating lymphocytes show the most prominent independent oligodendrocyte lysis33. These data suggest the possi- reactivity for FasL (Ref. 33) and therefore represent the most bility that Fas-FasL interactions might act as an alternative likely killers in oligodendrocyte destruction (Fig. 2). Accordingly, mechanism for myelin destruction in MS. Fas-defective or FasL-defective mice raise a normal T helper 1 In normal white matter, Fas expression is weak and scarce, and (Thl)-cell response upon immunization with encephalomyelogenic only a few oligodendrocytes seem to express detectable amounts of peptides, yet they are substantially resistant to EAE Fas. However, analysis of acute and chronic MS plaques has induction35,36.

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(a) Normal thyrocyt infiltrating lymphocytes 4°. However, FasL expression might not be sufficient per se as transgenic expression of FasL on pancreatic cells does not confer immune privilege in the course of autoimmune diabetes or islet transplantation 27,4~.

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IP. __J Fig. 3. Autoimmune thyn~cyte suicide in Hashimoto's thyroiditis. (a) Normal thyrocytes produce FasL but express negligible amounts of Fas. (b) However, following inflammation, IL-1~8 induces inappropriate Fas upregulation and consequent apoptosis through autocrine or paracrine FasL production, leading to Hashimoto's thyroiditis. For abbreviations, see Fig. i legend. Autocrine Fas-FasL interactions in autoimrnune thyrocyte destruction Hashimoto's thyroiditis (HT) is a common disorder of the thyroid gland arising from a humoral and cellular autoimmune response against the thyroid follicular cells. Histological analysis of HT reveals the presence of an intense infiltrate of mononuclear cells, comprising T and B cells, macrophages and plasma cells37,38.Following thyrocyte destruction, infiltrating lymphocytes and abundant fibrosis replace the epithelial parenchyma. The accumulation of lymphocytes may form several lymphoid germinal centers: indeed, HT is also referred to as struma lymphomatosa or lymphadenoid goiter. Although HT was the first-described organ-specific autoimmune disease, attempts to define the mechanism responsible for thyrocyte destruction were unsuccessful for many years, as the search for effector cells converged on infiltrating lymphocytes 3.. Analysis of the possible pathogenetic role of autoantibodies revealed that some patients have anti-thyroid antibodies able to induce complementmediated thyrocyte lysis in vitro. However, the presence and titer of anti-thyroglobulin (Tg) and anti-thyroid peroxidase (TPO) antibodies in HT patients are variable. Moreover, due to the antigen location, these autoantibodies have limited access to their target in living cells, and they are also present in many euthyroid individuals 1~,3s.Thus, the pathogenetic value of anti-Tg and anti-TPO antibodies seems to be confined to their potential ability to intensify inflammation 37. Recently, attention has focused on the role of FasL expression in the determination of immune-privileged sites. In organs such as testis and eye, the constitutive expression of FasL may prevent immune attack, due to the induction of Fas-mediated apoptosis of

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Normal thyrocytes constitutively express functional FasL, suggesting the thyroid gland might be a site of immune privilege42. Nevertheless, FasL expression turns out to be a dangerous asset for thyrocytes. Fas expression is negligible in normal thyrocytes; however, following thyroid inflammation due to cellular and humoral autoreactivity, IL-16 produced by activated macrophages induces massive Fas upregulation in thyrocytes 42. The consequent simultaneous expression of Fas and FasL induces thyrocyte apoptosis (Fig. 3). Although T cells mav contribute to the effector phase of HT, FasL expression is low and scarce in infiltrating lymphocytes, whereas FasL is further upregulated in HT thyrocytes 42. Thus, the

autocrine interaction between Fas and FasL on thyrocytes may represent their major mechanism of autoimmune depletion.

Conclusions and perspectives The different pathogenetic mechanisms underlying IDDM, MS and HT seem to proceed through Fas-induced target destruction, resulting in organ-specific autoimmune diseases. Given the rapid expansion of basic and clinical research on Fas and FasL, future studies will probably describe the involvement of Fas-mediated apoptosis in other autoimmune conditions, A role for the Fas-FasL system in the destruction of acinar epithelial cells in salivary glands of patients with Sj6gren's syndrome has recently been proposed 43. Additional candidates that deserve particular attention are autoimmune hepatitis, ulcerative colitis, aplastic anemia and infertility. In this perspective, the ability to modulate the Fas-FasL system at selected areas of the body may yield new hopes for effective therapeutic intervention.

This work is supported in part by Associazione Italiana Ricerca sul Cancro, Telethon, Istituto Superiore di Sanita Progetto Sclerosi Multipla and European Commission Biomed 2 Program.

Ruggero De Maria ([email protected]) and Roberto Testi (tesrob@ flashnet.it) alv at the Dept of Experimental Medicine and Biochemical Sciences, University of Rome "Tor Vergata', 00133 Rome, Italy.

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