key: cord-0009362-hc3jqpql authors: Bauer, Jan; Wekerle, Hartmut; Lassmann, Hans title: Apoptosis in brain-specific autoimmune disease date: 2002-02-11 journal: Curr Opin Immunol DOI: 10.1016/0952-7915(95)80057-3 sha: 273816d7482ab2731a95c280bca9dea05b5f669d doc_id: 9362 cord_uid: hc3jqpql Recent neuropathological studies of experimental autoimmune encephalomyelitis have focused attention on the high number of cells in the lesions that show typical morphological features of apoptosis. Surprisingly, it has turned out that the vast majority of apoptotic cells are T lymphocytes and that they actually represent the antigen-specific T-cell population responsible for the induction of the disease. Taken together, these data suggest that clearance of autoimmune inflammation in the nervous system is accomplished by the destruction of the antigen-specific T-cell population within the lesions. This may explain the low level of central nervous system specific T-cell memory formation, as well as previously unexplained phenomena of ‘epitope spreading’, in autoimmune inflammation of the nervous system Cell death is a key event in physiological, as well as pathological, processes of the immune system. It can be mediated by two principally different mechanisms: apoptosis and necrosis. Apoptosis refers to a series of morphological changes that occur in dying cells and that are different from the changes seen in necrosis [1] . Unlike necrosis, which mediates cell death by disruption of the plasma membrane and lytic degradation of cytoplasmic organelles, a cell undergoing apoptosis loses volume, the cytoplasm condenses, and the nucleus shows condensation and clumping of the chromatin in parallel with DNA fragmentation. In the final stage, cell fragments pinch off forming so called 'apoptotic bodies'. Biochemically, activation ofintraceUular enzyme systems leads to primary DNA fragmentation, which is later followed by cell lysis [2] . Overall, the cell is lysed without liberation ofproinflammatory degradation products. Before total degradation of apoptotic cells occurs, these cells are taken up by phagocytes thereby limiting the spilling of proinflammatory cytoplasmic contents into the surrounding tissue [3] . To date, many studies have focused on the role of apoptosis in physiological conditions of the immune system, such as positive or negative selection of T cells in the thymus [4] . However, relatively little is known about the patterns and mechanisms of cell death in immune mediated inflammatory lesions in target organs. In this review, we will discuss recent data, obtained from the well defined model of experimental autoimmune encephalomyelitis (EAE), that suggest that local cell death may play a major role in the regulation of the inflammatory process. The central nervous system (CNS) is continuously surveyed by the innnune system. CD4 + T lymphocytes, which are activated in the circulation, can pass through the blood-brain barrier irrespective of their specific target antigens [5, 6 ]. Yet, most of these T cells will not find the specific antigen in the nervous system and will therefore be rapidly cleared from the CNS parenchyma [6]. However, when T cells that are directed against an autoantigen of the CNS, such as myelin basic protein (MBP), confront their antigen on antigen-presenting cells (APCs) in the perivascular space, they will home and will be further activated in the nervous tissue ( Fig. 1) . This leads to a cascade of secondary events, such as the upregulation of the production of cytokines and chemokines [7, 8°,9] , endothelial adhesion molecules [10] [11] [12] , and further induction of the expression of MHC class II antigens on local APCs [13, 14] . These events facilitate the entrance of a secondary wave of leukocytes into the CNS, leading to disease and local tissue damage. Acute EAE is a monophasic disease followed by spontaneous recovery, yet, in certain animal species, a chronic progressive or relapsing disease may develop. Although the individual steps that operate in immune surveillance and the induction of brain inflammation are well understood, relatively little is known about the local mechanisms responsible for clearance of inflammation and the subsequent induction of tolerance. Recent evidence from both in vitro and in vivo studies of T-cell apoptosis, however, suggests that cell death in the CNS, besides playing a role in target-tissue destruction, APC--antigen-presenting cell; CNS~central nervous system; EAE---experimental autoimmnue encephalomyelitis; IL--interleukin; MBP--myelin basic protein; TCR--T-cell receptor; TGF--transforming growth factor; V--variable. is an essential aspect of the local regulation of the inflammatory reaction. Ill 1991, Pender et al. [24] described that besides oligodendrocytes, in being target cells in the tissue destruction in EAE, T cells in the lesions were also destroyed by apoptosis. With this description, the authors raised, for the first time, the possibility that local apoptosis of T cells in the target tissue could be a way of downregulating inflammation and installing tolerance. This initial observation of the presence of apoptotic T cells in the CNS of animals with EAE was followed by a more detailed study from Schmied et al. [25] who used in situ nick translation in combination with immunocytochemistry to show that, at least in acute monophasic EAE, mainly T lymphocytes and not the cells of the CNS parenchyma were undergoing apoptosis. Furthermore, in this study, quantitation suggested that apoptosis is a very efficient mechanism of T-cell elimination in the lesions. At the time of recovery from the disease, up to 49% of all T cells present showed nuclear changes. These data also indicated that within a 24 hour period, two to sixfold more than the total T-cell population present in the inflammatory infiltrates were destroyed by apoptosis and, thus, that active inflammation could only been maintained by the recruitment of new T cells from the circulation or by local proliferation. More recently, it became clear that the local destruction of T cells is not restricted to EAE, but can also be found in other inflammatory diseases of the nervous system. Several mechanisms may operate in the elimination of T cells. During EAE, a strong corticosteroid response is generated, which leads to the generation of high serum levels of glucocorticoids at the onset of remission (Fig. 1) (Fig. 1) . Preliminary experiments pertbrmed in our laboratories studying the behaviour ofprelabeled encephalitogenic T cells in vivo indeed suggest that it is predominantly the antigen-specific T-cell population that is ehminated by apoptosis in situ. Liberation of MBP during the course of inflammatory mediated tissue damage may finally lead to a concentration of the specific antigen in the brain extraceUular space (Fig. 1 ) that is sufficient for the induction ofT-cell apoptosis in situ [36"]. Besides the antigen concentration, the kind of APC may also play a role in antigen-induced apoptosis. During EAE, perivascular cells, infiltrating macrophages, microglial cells and astrocytes may act as APCs. For the latter, in recent years, evidence has been obtained that points to a downregulatory role in EAE [37] . These in vivo observations are in line with the in vitro finding that apoptosis of T lymphocytes at late post-activation stage is pronounced when astrocytes are used as APCs, whereas apoptosis is marginal or absent when antigen presentation is accomplished by APCs from the thymus (R Gold, M Schmied, U Tontsch, H-P Hartung, H Wekerle, unpublished data). However, the rate of T-cell apoptosis during EAE in radiation bone marrow chimeric rats is similar to normal rats with EAE, in spite of the inability in these chimeras to present antigen to the transferred T lymphocytes by local astrocytes or microglial cells [38] . As part of the probable multifactorial induction of T-cell apoptosis, a third mechanism involved in T-cell elimination during the course of EAE may be cytokinemediated apoptosis. In particular TGF-[~ may be of importance (Fig. 1) . When administered during EAE, this cytokine has been shown to improve the clinical course of this disease [39, 40] . As seen in the in vitro studies by Weller et al. [21°], TGF-~ may also affect EAE by inducing apoptosis in T cells. TGF-~ in brain lesions is not only made by inflammatory cells, such as macrophages [41] , but can also be produced by astrocytes [42] . Recent evidence shows that the disposal ofautoaggresive T cells during EAE is a very effective mechanism for clearance of CNS inflammation. Apoptosis of T cells might occur through an increase in the production of corticosteroids, the presence or absence of cytokines, through antigen induction, or through combinations of these. Whatever the mechanisms are that finally lead to apoptosis of T lymphocytes in the lesions of EAE, the elimination of these cells in the target organ has profound immunological consequences and may explain characteristics of CNS autoimmunity, which so far have remained enigmatic. For instance, it may explain why Ohmori et al. [43] found that in CNS lesions, proliferation of T cells is discontinued rapidly. Moreover, it is generally known that rodents that have gone through one episode of EAE are protected against attempts to induce a relapse by a second immunization. This is against intuition because, in general, primary immunization leads to the expansion of antigen-specific lymphocyte clones and to differentiation of memory cells. Although a variety of phenomena (i.e. hormonal conditioning and immunological network interactions) play a role in the resistance to reinduction of EAE, deletion of primary encephalitogenic T cells is definitely an additional contributing factor. This has been best shown in Lewis rats and H-2 u mice where, initially, most encephalitogenic (MBP-specific) T cells use the V138.2 + gene for their antigen receptors. During the course of disease, however, these originally dominant T-cell clones are progressively lost [44] . This process is accompanied by the gradual loss of reactivity against the dominant encephalitogenic epitopes, MBP sequence 68-88 in Lewis rats [33, 45] and Acl-10 in H-2u mice [31,32], and a redirected response against additional secondary epitopes [46, 47] , a phenomenon termed (intramolecular) 'epitope spreading' [48] . We propose that both the low MBP-specific T-cell memory formation, as well as epitope spreading, are a direct result of local death (apoptosis) of the encephalitogenic T cells within the target parenchyma. Accordingly, in the Lewis rat, the first wave of V138.2 + (MBP peptide 68-88) specific T cells, which are responsible for the clinical EAE episode, would be eliminated from CNS by apoptosis. T cells specific for secondary, 'cryptic' epitopes would be expanded instead. It will have to be established in the future whether the effective elimination of T cells as seen in both EAE as well as in experimental autoimmune neuritis (EAN), is a unique mechanism in nervous system immune reactivity or if it is a general mechanism which also functions in autoimmune or infectious diseases of other organs. expression by interferon-y and tumor necrosis factor-(x Apoptosis in the nervous system in experimental allergic encephalomyelitis Apoptosis of T lymphocytes in experimental autolmmune encephalomyelitis. Evidence for programmed cell death as a mechanism to control inflammation in the brain Apoplotic cell death 36 T cell deletion in high antigen dose therapy of autoimune encephalomyelitis T cells die by apoptosis during IL-2 stimulated cell cycling and TCR engagement at high doses of antigens (including MBP and peptide Acl-11 ) $itu inactivation of infiltrating T cells in the central nervous system with autoimmune encephalomyelitis. The role of astrocytes Perivascular microglial ceils of the CNS are bone-marrow derived and present antigen in vivo Prevention and treatement of chronic relapsing experimental allergic encephalomyelitis by transforming growth factor Long-term treatment of chronic relapsing experimental allergic encepahlomyelitis by transforming growth factor-[32 Transforming growth factor-[31: a lesion-associated cytokine of the nervous system Macrophage-and aslrocyte-derived transforming growth factor-[3 as a mediator of central nervous system dysfunction in acquired immune deficiency syndrome In situ demonstration of proliferating cells in the rat central nervous system during autoimmune encephalomyelitis. Evidence suggesting that most infiltrating T cells do not proliferate in the target organ Where, when, and how to detect biased expression of disease-relevant V[3 genes in rats with experimental autoimmune encephalomyelitis Both rat and mouse T cell receplors specific for the encephalitogenic determinant of myelin basic protein use similar Vcc and V[3 chain genes even though the major histocompatibility complex and encephalitogenic determinants being recognized are different Sercarz hE: Spreading of T-cell autoimmunity to cryptic determinants of an autoantigen Shifts in the epitopes of myelin basic protein recognized by Lewis rat T cells before, during and after the induction of experimental autoimmune encephalomyelitis Determinant spreading and the dynamics of the autoimmune T cell repertoire A-1090 Wien, Austria. H Wekerle, Department of Neuroimmunology, Max-Planck Institute for Psychiatry • of T-lymphocytes in experimental autoimmune neuritis of the Lewis rat. Neurosci Lett 1994, 176:75-79. This study shows for the first time that apoptosis of inflammatory T cells is not unique to the CNS, but operates in the peripheral nervous system as well. Barac-Latas V, Wege H, Lassmann H: Apoptosis of T • lymphocytes in coronavirus induced encephalomyelitis. Reg Immuno/ 1995, 6:355-357. The study shows that, similar to autoimmune encephalomyelitis, apoptosis is also a major mechanism to eliminate T lymphocytes in brain inflammatory lesion encephalomyelitis induced by coronavirus. Ozawa G, Suchanek G, Breitschopf H, BrOck W, Budka H, • Jellinger K, Lassmann H: Pattems of oligodendroglia pathology in multiple sclerosis. Brain 1994, 117:1311-1322. This paper deals with patterns of inflammation, demyelination and cell death in multiple sclerosis. Of importance here is that, by combination of immunocytochemistry and in situ nick translation, it is shown that besides oligodendrocytes, T lymphocytes also undergo acute cell death. MacPbee IA, Antoni FA, Mason DW: Spontaneous recovery of rats from experimental allergic encephalomyelitis is dependent on regulation of the immune system by endogenous adrenal corticosteroids. 2 + cells in lymphoid organs, indicating that elimination of these autoagressive T cells in CNS by apoptosis is complete and that the decrease in number of these cells is not due to recirculation to lymphoid organs.