The significance of in-situ Ia antigen expression in the pathogenesis of autoimmune central nervous system disease

Exocytosis of large-diameter lysosomes mediates interferon γ-induced relocation of MHC class II molecules toward the surface of astrocytes

Astrocytes are the key homeostatic cells in the central nervous system; initiation of reactive astrogliosis contributes to neuroinflammation. Pro-inflammatory cytokine interferon γ (IFNγ) induces the expression of the major histocompatibility complex class II (MHCII) molecules, involved in antigen presentation in reactive astrocytes. The pathway for MHCII delivery to the astrocyte plasma membrane, where MHCII present antigens, is unknown. Rat astrocytes in culture and in organotypic slices were exposed to IFNγ to induce reactive astrogliosis. Astrocytes were probed with optophysiologic tools to investigate subcellular localization of immunolabeled MHCII, and with electrophysiology to characterize interactions of single vesicles with the plasmalemma. In culture and in organotypic slices, IFNγ augmented the astrocytic expression of MHCII, which prominently co-localized with lysosomal marker LAMP1-EGFP, modestly co-localized with Rab7, and did not co-localize with endosomal markers Rab4A, EEA1, and TPC1. MHCII lysosomal localization was corroborated by treatment with the lysosomolytic agent glycyl-L-phenylalanine-β-naphthylamide, which reduced the number of MHCII-positive vesicles. The surface presence of MHCII was revealed by immunolabeling of live non-permeabilized cells. In IFNγ-treated astrocytes, an increased fraction of large-diameter exocytotic vesicles (lysosome-like vesicles) with prolonged fusion pore dwell time and larger pore conductance was recorded, whereas the rate of endocytosis was decreased. Stimulation with ATP, which triggers cytosolic calcium signaling, increased the frequency of exocytotic events, whereas the frequency of full endocytosis was further reduced. In IFNγ-treated astrocytes, MHCII-linked antigen surface presentation is mediated by increased lysosomal exocytosis, whereas surface retention of antigens is prolonged by concomitant inhibition of endocytosis.

Distribution and temporal regulation of the immune response in the rat brain to intracerebroventricular injection of interferon-gamma

The response to intracerebroventricular administration of interferon (IFN)-gamma was examined in the adult Wistar rat brain: major histocompatibility complex (MHC) antigens class I and II, CD8 and CD4 antigens, and the macrophage/microglia antigen OX42 were analyzed in respect to saline-injected cases over 1 week. The glial cell type expressing MHC antigens was characterized with double labeling. IFN-gamma was thus found to induce MHC class I and II expression in microglia, identified by tomato lectin histochemistry, and not in GFAP-immunostained astrocytes. MHC antigen-expressing microglia was detected in the periventricular parenchyma, several fields of the cerebral cortex, cerebellum, major fiber tracts, and brainstem superficial parenchyma. Different gradients of density and staining intensity of the MHC-immunopositive elements were observed in these regions, in which MHC class I antigens persisted up to 1 week, when MHC class II induction had declined. Quantitative analysis pointed out the proliferation of OX42-immunoreactive cells in periventricular and basal brain regions. CD8+ T cells were observed in periventricular regions, basal forebrain, and fiber tracts 3 days after IFN-gamma injection and their density markedly increased by 7 days. CD4+ T cells were also seen and they were fewer than CD8+ ones. However, numerous CD4+ microglial cells were observed in periventricular and subpial regions, especially 1 week after IFN-gamma injection. Our data indicate that this proinflammatory cytokine mediates in vivo microglia activation and T cell infiltration in the brain and that the cells involved in this immune response display a regional selectivity and a different temporal regulation of antigen expression.

Cytokines and the immune response

Cytokines participate in many physiological processes including the regulation of immune and inflammatory responses. These effector molecules are produced transiently and locally controlling the amplitude and duration of the response. A variety of experiments has shown that excessive or insufficient production may significantly contribute to the pathophysiology of a range of diseases. Particularly cytokines released by CD4+ T cells at the onset of an immune response are thought to be decisive for pathological or physiological consequences. The meeting in Budapest was focussed on cytokines known to contribute to the pathophysiology of autoimmune diseases, infectious diseases and allograft rejection (e.g., IL-1, IL-4, IL-6, IL-10, IL-12, TNF-alpha and IFN-alpha, -beta, -gamma). A central role for IFN-gamma in autoimmunity was suggested by blocking experiments in vivo using monoclonal antibodies and soluble forms of the IFN-gamma receptor (IFN-gamma SR). These agents ameliorated disease development in a variety of experimental autoimmune diseases in rodents. In a mouse model for the human disease Myasthenia gravis, IFN-alpha was found to reduce both the incidence and progression of the disease. Treatment of R. aurantiacus-infected mice with anti-IL-4 monoclonal antibodies (mAbs) was reported to interfere with the regression of granulomas in spleen and liver, most likely through inadequate IL-4-mediated suppression of IFN-gamma production. In addition, it was shown that mice with disrupted IFN-gamma R genes died rapidly after infection with the BCG strain of M. bovis, whereas normal mice survived the infection. IL-12 was found to be the main inductor of IFN-gamma during the lethal Shwartzman reaction. TNF-alpha was identified as the principal cause of mortality after the second injection with LPS. In a variety of studies examining the role of cytokines in the pathogenesis of AIDS, much attention was given to the in vitro effects of HIV-1 and/or the HIV-1 viral membrane protein gp120 on triggering cytokine production by peripheral blood leukocytes (PBLs) and purified monocytes/macrophages (Mø) originating from healthy donors. Gp120 as a sole agent significantly suppressed IFN-gamma production by mitogen-stimulated PBLs and induced the production of IFN-alpha in cultures of normal human peripheral blood mononuclear cells (PBMCs). In a human macrophage cell line, TNF-alpha exerted a stimulatory effect on viral replication and programmed cell death induced by HIV-1 which was potentiated by the simultaneous incubation with IFN-gamma. Upon transfection of human PBLs and CD4+ T cells with a retroviral vector encoding human IFN-beta, a notable reduction in reverse transcriptase activity after HIV-1 challenge was observed. Gp120 was also found to induce both IL-6 and TNF-alpha expression and to induce morphological changes reminiscent for apoptosis in primary astrocytes and in a re-aggregated human brain cell model, suggesting a role for these cytokines in the neuropathology of AIDS dementia. Moreover, data were presented indicating that cytokine-induced expression of cell adhesion molecules (e.g., ICAM-1) in HIV-1 infected U 937 cells leads to high level incorporation of this molecule in the membrane of the viral progeny which may play a role in the attachment of such virions to CD4-negative cells.

Recovery phase of acute experimental autoimmune encephalomyelitis in rats corresponds to development of endothelial cell unresponsiveness to interferon gamma activation

Activation of the vascular endothelium is important in the development of inflammation. Activated endothelial cells (EC) express surface markers not expressed by quiescent EC. These surface markers augment adhesion reactions and leukocyte migration. We examined microvessel EC activation longitudinally in experimental autoimmune encephalomyelitis (EAE) in Lewis rats. CNS microvessels were isolated at 0, 3, 7, 12, 20, and 30 days post-inoculation (PI). Normal and CFA-injected rat microvessels do not express activation antigens (Ag). Increased expression of major histocompatibility complex (MHC) class II molecule and intercellular adhesion molecule-1 (ICAM-1) were detected on CNS microvessels from immunized rats at 7 days PI, prior to development of clinical signs, and at 12 days PI. Enhanced MHC class I molecule was seen only at 12 days. MHC class II molecule expression was focally expressed along microvessel fragments. By 20 days PI, EC did not exhibit increased levels of any of the markers tested. Perivascular cells (possibly pericytes), however, were found to express MHC class II molecule and ICAM-1 up to 30 days PI. During the recovery phase isolated CNS microvessels from MBP-immunized rats were unresponsive to IFN gamma-mediated endothelial activation. Unresponsiveness was independent of IFN gamma concentration. These results suggest that the endothelium is restored to functional quiescence during the recovery phase of acute EAE.

The role of macrophage subpopulations in autoimmune disease of the central nervous system

In this review the role of various subpopulations of macrophages in the pathogenesis of experimental autoimmune encephalomyetitis is discussed. Immunohistochemistry with macrophage markers shows that in this disease different populations of macrophages (i.e. perivascular cells, microglia and infiltrating blood-borne macrophages) are present in the central nervous system. These subpopulations partially overlap in some functional activity while other activities seem to be restricted to a distinct subpopulation, indicating that these subpopulations have different roles in the pathogenesis of encephalomyelitis. The studies discussed in this review reveal that immunocytochemical and morphological studies, combined with new techniques such as in situ nick translation and experimental approaches like the use of bone marrow chimeras and macrophage depletion techniques, give valuable information about the types and functions of cells involved in central nervous system inflammation. The review is divided in three parts. In the first part the experimental autoimmune encephalomyelitis model is introduced. The second part gives an overview of the origin, morphology and functions of the various subpopulations. In the third part the role of these subpopulations is discussed in relation to the various stages (i.e. preclinical, clinical and recovery) of the experimental disease.

Phagocytic activity of macrophages and microglial cells during the course of acute and chronic relapsing experimental autoimmune encephalomyelitis

The ED1 monoclonal antibody recognizes an antigen in lysosomal membranes of phagocytes. The expression of this antigen in cells increases during phagocytic activity. Here we describe the expression of ED1-immunoreactivity during the various stages of both acute (monophasic) and chronic relapsing experimental autoimmune encephalomyelitis (EAE) in the Lewis rat. During the first attack of acute and chronic relapsing EAE, ED1-immunoreactivity was present in macrophages and in cells which displayed morphologic features of activated microglial cells (i.e., cells with thick short processes). At the ultrastructural level these cells were seen to contain phagocytosed myelin structures in lysosomes. ED1-immunoreactivity in these cells was present in the cytoplasm near lysosomes. During the remission phase of acute EAE and the relapse phase of chronic relapsing EAE, ED1-positive cells with dendritic morphology not only were present in or nearby lesions, but were also found at sites distant from lesions throughout large parts of the brain. These cells had a morphology comparable to microglial cells in normal brain. A major difference between animals which were in remission and animals which on day 25 were suffering from a relapse, was that the latter showed the presence of lesions with darkly stained round ED1-positive macrophages and activated microglial cells. These results indicate that during a relapse, newly recruited blood-borne macrophages infiltrate the brain and, together with activated lymphocytes and microglial cells, recommence a new demyelination process.