Cellular immune reactivity within the CNS

Infection of human T-lymphotropic virus type I to astrocytes in vitro with induction of the class II major histocompatibility complex

To clarify the pathogenesis of human T-lymphotropic virus type I (HTLV-I)-associated myelopathy (HAM), we examined whether HTLV-I infects normal human glial cells in vitro with induction of the major histocompatibility complex (HMC) class II antigen by immunofluorescence method. It was found that about 10% of astrocytes were infected with HTLV-I with induction of class II MHC antigen. Fluorescence-conjugated HTLV-I was adsorbed to 10% of astrocytes. On the contrary, there was no class II MHC antigen expression and very few HTLV-I infection on oligodendrocytes. We speculated that in patients with HAM, HTLV-I-specific, MHC class II antigen restricted, activated CD4+ cells could damage the MHC class II antigen + HTLV-I-infected astrocytes, leading to the disturbance of blood-brain barrier and to the destructive lesion in the central nervous system.

Primary rat astroglial cultures can generate leukotriene B4

Astrocytes are important elements of cellular immune responses in the central nervous system. Eicosanoids exhibit immunomodulatory properties. We studied whether astroglial cells have the capacity to convert arachidonic acid via the lipoxygenase pathway to leukotriene B4 (LTB4). Calcium ionophore A23187 evoked a dose-related release of LTB4 into supernatants of primary culture rat astrocytes. This stimulatory effect was abrogated by the addition of the lipoxygenase inhibitors BW755c and nordihydroguaretic acid. Astrocyte production of the phlogistic and immunoregulatory mediator LTB4 may be one mechanism by which these cells influence immunity and inflammation in the central nervous system.

Immunologic and tissue culture approaches to the neurobiology of aging

The neurobiology of aging continues to attract scientists and techniques from the more fundamental disciplines, as witness the great strides now being made from molecular genetic approaches to Alzheimer's disease. The present report is a commentary on reviews of immune mechanisms and tissue culture methods applied to investigations of aging and age-related cognitive dysfunction.

Expression of immune system-associated antigens by cells of the human central nervous system: relationship to the pathology of Alzheimer's disease

HLA-DR is a class II major histocompatibility complex antigen which in the periphery confers antigen presenting capability. We have previously shown that this marker is profusely expressed in cortex of elderly and Alzheimer's disease (AD) patients, as is the receptor for the lymphokine interleukin-2. We now report presence of additional immune-related antigens in AD, and distributional differences from normal elderly controls. In gray matter, HLA-DR immunoreactivity is normally sparse, except in AD where it co-localizes with virtually all neuritic plaques. HLA-DR positive T cells can be demonstrated in Alzheimer's disease brain tissue, as can instances of apposition between putative brain microglia and T cells. In addition, cells with the morphologic characteristics of astrocytes label for natural killer cell antigen (Leu-11), and apparent lymphocytes bearing T helper and T cytotoxic/suppressor cell antigens are observed. These and other data suggest that the glial proliferation and scavenger activity characteristic of Alzheimer's disease may occur in an immune context and may play an important role in the pathogenesis of the disorder.

Cellular immune reactions and blood cerebrospinal fluid barrier dysfunction in guinea pigs

Both young and adult strain 13 guinea pigs have been treated with complete Freund's adjuvant (CFA). In young animals CFA induced an increase in the lymphocyte cell count in blood and a reversible blood CSF barrier impairment. Both these effects could be suppressed by the immunosuppressant drug Cyclosporin A. In contrast to the young animals treatment with CFA in adult strain 13 guinea pigs influenced neither the lymphocyte cell count nor blood CSF barrier function. In young and adult CFA-treated animals the number of polymorphonuclear cells in blood were increased and this increase was not reversed by Cyclosporin A. We discuss the influence of a systemic immune stimulation and suppression on blood CSF barrier function for proteins and its relevance to the pathogenesis of inflammatory diseases of the central nervous system.

Suppression of experimentally induced autoimmune encephalomyelitis by cytolytic T-T cell interactions

Down-regulatory phenomena have been described in several experimental models of tissue-specific, T-cell-mediated autoimmunity. For example, resistance to active induction of experimental autoimmune encephalomyelitis (EAE) can be induced by pretreating animals with non-pathogenic inocula of autoantigen or effector cells. Moreover, animals that have recovered from one EAE episode are resistant to subsequent induction of EAE. In some models, resistance to EAE has been transferred with immune cells to naive recipients. These experiments, which were based on transfers of unseparated immune cell populations, are difficult to interpret. Immune suppression circuits are known to be complex and involve various distinct cellular subsets. To further complicate the issue, resistance to EAE can be transferred not only by suppressor cells, but also by encephalitogenic effector cells injected in 'subclinical' doses. We describe now the isolation of homogeneous T lymphocyte lines from the spleens of Lewis rats that had recovered from T-cell-mediated EAE (tEAE) caused by the MBP-specific T cell line S1. These spleen-derived T line cells express the CD8 phenotype and specifically respond to determinants on the inducing S1 line, but not to the autoantigen MBP. Furthermore, the anti-S1 cells selectively lyse the encephalitogenic S1 T line in vitro and efficiently neutralize their encephalitogenic capacity in vivo.

Immunogenetic analysis of cellular interactions governing the recruitment of T lymphocytes and monocytes in lymphocytic choriomeningitis virus-induced immunopathology

The Lyt2+ class I major histocompatibility complex (MHC)-restricted virus-immune T cells that induce murine lymphocytic choriomeningitis (LCM) are targeted onto radiation-resistant cells in the central nervous system of virus-infected mice. The use of appropriate bone marrow radiation chimeras as LCM virus-infected, (immunosuppressed recipients for immune T-cell transfer has established that, though bone marrow-derived cells can stimulate virus-specific cytotoxic T lymphocytes (CTL) in spleen, they do not reconstitute the barrier to T-cell recruitment from blood to cerebrospinal fluid. This is true for chimeras made up to 8 months previously, even though the inflammatory monocytes and macrophages in such chimeras are all of donor bone marrow origin. Radiation-resistant cells in the spleens of these chimeras are also still able to further stimulate virus-immune CTL. There is no requirement for H-2 compatibility between virus-immune T lymphocytes and secondarily recruited monocytes, or T cells of an inappropriate specificity. The key event in LCM immunopathology may thus be localization of T cells to the antigen-presenting endothelium in brain, leading to the secretion of mediators that promote the nonspecific recruitment of monocytes and other T cells.

Immunotherapy of multiple sclerosis

Based on the assumption that multiple sclerosis is an autoimmune disease, a number of clinical trials designed to suppress the immune system or to restore immune balance in multiple sclerosis have been attempted. Depending on the disease category, the clinical goals of immunotherapy differ. Therapeutic goals include improving recovery from acute attacks, preventing or decreasing the number of relapses, and halting the disease in its progressive stage. The ultimate goal of multiple sclerosis therapy is the early treatment of patients in an attempt to halt the onset of progression. Specific strategies of immunotherapy include generation of a suppressor influence, removal of helper/inducer cells, manipulation of activated T cells, manipulation of class II major histocompatibility complex-bearing cells, alteration of lymphocyte traffic, extracorporeal removal of serum factors or cells, and manipulation of antigen-specific cells. Present treatment modalities are beginning to show some efficacy of nonspecific immunosuppression, but these treatments are limited by their toxicities. As the immunotherapy of multiple sclerosis moves to the next stage in the coming years, patients at an earlier stage of their disease will have to be treated, nontoxic forms of therapy developed, clinical trials lengthened, and a laboratory monitor of the disease developed. Given the positive effects of immunotherapy seen thus far in the disease, it is possible that appropriate immunotherapeutic intervention may provide effective treatment for the disease in the future.

Response of endogenous glial cells to motor neuron degeneration induced by toxic ricin

The injection of toxic lectin from Ricinus communis into the rat facial nerve resulted in suicide transport and rapid degeneration of facial motor neurons. The reaction of glial cells to neuronal death in comparison with nerve crush lesions was studied by using lectin-HRP conjugates derived from Griffonia simplicifolia for the selective staining of microglial cells at both light and electron microscopic levels. In addition, the proliferative activity of microglia was assessed by quantification of 3H-thymidine incorporation. The astrocytic response was evaluated by light microscopic immunocytochemistry for glial fibrillary acidic protein. In the degenerating facial nucleus local microglial cells responded by rapid proliferation and phagocytosis of neuronal debris. After nerve crush, no phagocytes were observed, but microglial proliferation and perineuronal satellitosis were prominent. The astrocytic expression of glial fibrillary acidic protein in response to nerve crush proceeded gradually over a period of several weeks after which it declined, contrasting with accelerated astrocytic hypertrophy and permanent glial scarring after neuronal degeneration. These results show that the expression of glial fibrillary acidic protein by fibrous astrocytes is intensified after lethal neuronal injury compared to sublethal insults. In the absence of any observations indicating participation of hematogenous elements, it is proposed that local microglial cells transform into brain macrophages.

Cell recruitment during glial repair: the role of exogenous cells

Selective disruption of the neuroglia in penultimate abdominal connectives of the cockroach nerve is followed by a rapid accumulation of cells in the perineurial layer of the lesion. Subsequently, there is an abrupt, secondary, rise in cell numbers in the undamaged perineurial tissues, anterior to the lesion and adjacent to the 4th abdominal ganglia. By 7 days the increased cell numbers are again effectively confined to the original lesion zone. The initial rise in cell numbers is postulated to result from an invasion by blood-borne haemocytes and the subsequent increase, in undamaged perineurial tissues, from the mobilization of endogenous reactive cells. Recruitment of the endogenous cells is inhibited if the haemocytes are excluded from the lesion. There is a slower mobilization of sub-perineurial cells, which, again, is inhibited following exclusion of haemocytes from the lesion zone. It is postulated that the recruitment of the endogenous reactive cells is initiated by the invading haemocytes which transform to granule-containing cells and release diffusible morphogenic and/or mitogenic factors.

Astrocytes as eicosanoid-producing cells

A variety of prostaglandins and leukotrienes, together with thromboxane and prostacyclin metabolites, can be detected in central nervous tissues and in cerebrospinal fluid. Defined cultures of astrocytes have revealed these cells to be a major source of eicosanoids. In common with other eicosanoid-producing cells, agents such as calcium ionophores and phorbol esters are potent stimuli for promoting release. While in other tissues agonists for receptors linked to calcium mobilisation prompt eicosanoid release, this does not seem to be the case in astrocytes, though a range of such receptors are present. The notable exceptions to this observation are adenosine triphosphate and adenosine diphosphate, presumably acting through P2 purinergic receptors. Many cell types in the CNS are targets for eicosanoids, possessing receptors linked to adenylate cyclase or phospholipase C. An appreciation of the functional significance of activation of these receptors is just now beginning. Eicosanoids have effects in the CNS that involve not only the vascular supply but also synaptic modulation and immune regulation.

Cyclosporine versus azathioprine in the long-term treatment of multiple sclerosis--results of the German multicenter study

In a double-blind controlled trial of 194 patients with clinically definite active multiple sclerosis, 98 were randomized to treatment with cyclosporine (CyA, 5 mg/kg/day), and 96 to treatment with azathioprine (Aza, 2.5 mg/kg/day). Eighty-five patients in the CyA group and 82 in the Aza group completed a treatment period of 24 to 32 months in accordance with the study protocol. No significant differences could be detected between the two treatment groups at the end of the trial. Assessment was done by serial quantitative neurological examinations and Kurtzke's Expanded Disability Status Scale. Frequency of relapse and patient self-evaluation also failed to show significant differences. Overall deterioration observed in both groups during the trial was only minor. The incidence of side effects in the CyA group was more than two times that in the Aza group. We conclude that CyA as a single agent cannot be the drug of final choice in long-term immunosuppressive treatment of relapsing-remitting and relapsing-progressive multiple sclerosis.

Functional plasticity of microglia: a review

The present review summarizes recently acquired data in vivo, which support a role of CNS microglia as a source of defense cells in the CNS capable of carrying out certain immune functions autonomously. We have kept the following discussion restricted to microglial cells and have not included work on the immunological functions of astrocytes, which has been recently reviewed elsewhere (Fontana et al.: Immunological Reviews 137:3521-3527, 1987). Resting microglia are scattered uniformly throughout the CNS forming a network of potential immunoeffector cells, which can be activated by stimuli ranging from peripheral nerve injury over viral infections to direct mechanical brain trauma. The term "activated microglia" is used here to describe proliferating cells that demonstrate changes in their immunophenotype but have not undergone transformation into brain macrophages. Such a transformation can be stimulated by neuronal death but not by sublethal neuronal injury. Microglia may function as antigen-presenting cells and may thus represent the effector cell responsible for the recruitment of lymphocytes to the brain resulting in an inflammatory reaction. The recent developments in the understanding of microglial cell function may lead to a redefinition of the often cited "immune privilege" of the brain.

Significance of lymphocytes and blood vessel changes for edema formation in polyradiculoneuritis

Human sural nerve biopsies of eight cases with acute, subacute and chronic polyradiculoneuritis were studied by means of immunohistochemistry to characterize the inflammatory infiltrates. In addition, the structural changes of the endoneurial blood vessels were examined by electron microscopy, since both factors are likely to contribute to disturbances of the blood-nerve barrier. By use of six monoclonal antibodies, it was shown that the inflammatory infiltrates in cases with more acute polyradiculoneuritis are predominantly recruited by Leu 3a- and Leu 4-positive T lymphocytes. In more chronic polyradiculoneuritis beside of few Leu 3a-positive and Leu 4-positive T lymphocytes also B cells occurred. Leu M3-positive macrophages were detected in all cases with fluoride myelin degeneration. Since immunoreactivity for antigens of the HLA-D-locus (Leu-HLA-DR and Leu 10) were present on the infiltrating mononuclear cells, it can be postulated that they represent active and immunocompetent cells. Ultrastructurally, the amount of pinocytotic vesicles in the endothelial cells of the endoneurial blood vessels was increased. Moreover, a prominent folding of the luminal and abluminal surface of vascular endothelial cells and diminution of the intercellular tight junctions were observed. These findings appear suitable to explain the increased leakage of serum proteins across the blood-nerve barrier in polyradiculoneuritis sharing general features of cell-mediated immunity.

Leukotriene production by cultured astroglial cells

Astrocytes metabolize arachidonic acid via the cyclooxygenase pathway to prostanoids. We examined whether primary culture astrocytes from neonatal rat brain can be induced to generate and release the lipoxygenase derivative leukotriene C4 (LTC4). While there was only minute constitutive production of immunoreactive LTC4 this metabolite was liberated by astroglial cells in response to calcium ionophore A23187. The phorbol diester 12-O-tetradecanoylphorbol 13-acetate (TPA) failed to precipitate leukotriene release. However, when threshold doses of A23187 were added to astrocyte cultures challenged with TPA, LTC4 was recovered from their supernatants. It is suggested that leukotriene generation by astrocytes bears relevance to immunoinflammatory responses in the central nervous system and may be involved in brain edema formation.

A quantitative immunohistochemical comparison of actively versus adoptively induced experimental allergic encephalomyelitis in the Lewis rat

A quantitative immunohistochemical comparison of actively and adoptively induced experimental allergic encephalomyelitis (EAE) in the Lewis rat was performed. Since the methods of EAE induction of these two systems and the kinetics of disease appearance are different, while the histopathology, disease manifestations, duration, and severity are similar, this study sought to identify any differences which exist at the level of the target organ. The number of cells expressing the T helper (W3/25) or suppressor/cytotoxic (OX-8) phenotypes and the number of Ia-positive cells found in the spinal cord of animals given EAE by one of the two methods were compared at two time points at which maximal similarities should exist. The results show that during acute adoptively induced EAE the inflammatory infiltrate contains a larger number of T helper (TH) cells per unit area than in acute active EAE. With the resolution of clinical signs of EAE, the disappearance of cells from the spinal cord is more rapid in adoptive EAE. In contrast, the inflammatory infiltrate and Ia-positive parenchymal cells persist in active EAE following recovery. These results suggest that actively and adoptively induced EAE may differ with respect to the effector mechanisms and/or the mechanisms of recovery at the level of the target organ.

Alpha 2-macroglobulin synthesis in an astrocyte subpopulation

The proteinase inhibitor alpha 2-macroglobulin (alpha 2-M) is an acute phase protein in the adult rat. During inflammatory events, it is synthesized in the liver and secreted into the bloodstream to remove proteases that are released on injury. Recently, its occurrence in fetal rat brain has been reported. Its cellular origin and biological function in the developing brain, however, remained obscure. In this article, it is shown that astroglial cells cultured from newborn rat brain synthesize and secrete alpha 2-M. Its synthesis markedly increases with time in culture. Immunocytochemical studies reveal that only a subpopulation of astrocytes is alpha 2-M positive, alpha 2-M synthesis in the developing brain by neuroectoderm-derived cells asks for a broader definition of its function in the body. Since interactions of proteases and protease inhibitors appear to play a crucial role in cell migration and neurite outgrowth, alpha 2-M expression in astrocytes is discussed not only in relation to its potential role in the acute phase response to injury in the adult brain but also in regard to its possible involvement in brain development.

Major histocompatibility antigens and lymphocyte subsets during experimental allergic neuritis in the Lewis rat

Major histocompatibility antigens were identified in frozen sections of normal Lewis rat peripheral nerve tissue with monoclonal antibodies and an avidin-biotin-peroxidase complex system. Class I antigen is normally required for cytotoxic/suppressor T lymphocyte function and class II antigen for activation of helper T lymphocytes. In the sciatic nerves class I antigen was expressed diffusely by most endoneurial and perineurial cells but class II antigen only by a minority. In the cauda equina class I antigen was expressed by all arachnoid and some endoneurial cells, while class II antigen was expressed by a smaller proportion of arachnoid cells in the endoneurium of spinal roots and interstitial cells surrounding dorsal root ganglion neurons. The endothelium of endoneurial, perineurial and meningeal vessels uniformly expressed class I but not class II antigen. Experimental allergic neuritis was induced in Lewis rats by immunisation with bovine intradural root myelin. Early lesions consisted of multifocal infiltration of the nerve roots by cells expressing leucocyte common antigen. Surrounding endoneurial cells showed markedly increased expression of major histocompatibility antigens. In inflammatory lesions about 10% of the cells were stained with pan T cell antibodies. T lymphocyte subsets were identified with antibody W3/25 for helper cells and MRC OX-8 for cytotoxic/suppressor cells. The W3/25 positive cells were usually slightly in excess of OX-8 positive cells and their relative proportions did not alter during the disease. The presence of class I antigen on normal endothelium and its increased expression on endoneurial cells in the early phase of inflammation suggest an important role for class I restricted lymphocytes in the pathogenesis of the early stages of experimental allergic neuritis.

Lymphocytic activation in peripheral blood and cerebrospinal fluid during the course of chronic relapsing experimental allergic encephalomyelitis

A monoclonal antibody against the human interleukin-2 receptor (anti-Tac) has been found to cross-react with an antigen on the surface of guinea pig leucocytes. Cells marking with anti-Tac and with an anti-pan T cell monoclonal antibody have been quantitated in the peripheral blood and cerebrospinal fluid (CSF) of guinea pigs with chronic relapsing experimental allergic encephalomyelitis (CR-EAE). T cells account for about 90% of peripheral blood leucocytes in all animals whilst in the CSF, T cells are the major contributor only when there is a pleocytosis. The proportion of T cells marking with anti-Tac, a measure of T cell activation, in blood and CSF of control animals is 12%, rising to 23% in blood in the post-acute phase of the disease. However, a fall in the blood Tac/T ratio to 13% occurs during the first 10 days of relapse with a subsequent rise to 30-35%. This change is related to the time after onset of relapse irrespective of the subsequent course of the disease. From first relapse onwards CSF lymphocytes show a greater level of activation than lymphocytes from paired peripheral blood samples but the proportion of Tac+ cells in CSF does not increase with increasing CSF pleocytosis. The data is consistent with migration of activated T cells from blood to CSF at the onset of relapse.

Activated T-cells and macrophages in the cerebrospinal fluid and the spinal meningeal exudate in chronic relapsing experimental allergic encephalomyelitis

An analysis has been made of the cell types which mark with monoclonal antibodies against T cells, macrophages and the IL-2 receptor (anti-Tac) in the blood, cerebrospinal fluid (CSF) and spinal meningeal exudates taken from guinea pigs in the relapse and remission stages of chronic relapsing experimental allergic encephalomyelitis (CR-EAE). Whilst the T-cell and macrophage content of blood remained unchanged throughout the course of CR-EAE, T cells accounted for the majority of the CSF pleocytosis associated with relapsing disease but both T cells and macrophages populated the meningeal exudate in substantial numbers. Activated T cells (Tac+) rose in number in blood only after the onset of relapse but formed a far higher proportion of the CSF pleocytosis or meningeal exudate than in paired blood samples. Meningeal exudate cells from Freund's adjuvant-inoculated, but not uninoculated animals, also showed an increase in Tac+ cell levels. In addition, the meningeal exudate contained a substantial number of cells which did not label with anti-T or anti-macrophage antibodies and which did not vary in absolute numbers throughout the course of disease.

Expression of apolipoprotein E by mouse brain astrocytes and its modulation by interferon-gamma

We have found that mouse brain astrocytes in culture synthesize and secrete apolipoprotein E (ApoE) and that its expression is modulated by interferon-gamma, which influences the functional state of astrocytes. Interferon-gamma, which induces expression of Ia determinants on the surface of the astrocyte, reduced secretion of ApoE while enhancing intracellular expression of ApoE. Our data suggest that the synthesis and secretion phases of the ApoE pathway are regulated independently. It is possible that, by the production of ApoE, astrocytes may perform functions that are usually ascribed to macrophages and that may be important in immune reactivity in the brain.

Induction of synchronized relapses in SJL/J mice with chronic relapsing experimental allergic encephalomyelitis

Chronic relapsing experimental allergic encephalomyelitis (CR-EAE) was induced in SJL/J mice by two injections of encephalitogenic emulsion. The majority of mice developed multiple relapses from day 22 to day 367 post injection. To induce synchronized relapses a third injection of the encephalitogenic emulsion was given. Almost all mice that received the third injection developed an acute, synchronized relapse with severe clinical signs within 7 to 11 days. Histologically, there was no difference between the lesions in the spontaneous versus the precipitated relapse. CR-EAE in SJL/J mice modified with the third injection offers an advantage of a reproducible and well-timed acute relapse, which allows precise dissection of the immunological events governing spontaneous relapses in chronic EAE.

Antigen presentation in the peripheral nervous system: Schwann cells present endogenous myelin autoantigens to lymphocytes

Schwann cells (SC) isolated from neonatal rat sciatic nerves are shown to immunogenically present foreign and exogenous autoantigen to antigen-specific syngeneic T line cells in vitro. The antigen-presenting SC express Ia antigens on their membranes upon treatment with interferon gamma and contact with syngeneic T line cells. Monoclonal antibodies against Ia block specific antigen presentation, but not polyclonal mitogenic T cell activation. The antigen-presenting SC bind antibodies specific for astrocytic glial fibrillary acidic protein and may thus be related to the nonmyelinating glia cells of the peripheral nerve. Furthermore, SC isolated from 6-day-old rats activate rat myelin basic protein (MBP)-specific syngeneic T line cells in the absence of exogenous MBP. In contrast, they activate purified protein derivative of tuberculin (PPD)-specific T cells only in the presence of PPD. Since the MBP-specific T line cells are not activated by syngeneic professional antigen-presenting cells in the absence of MBP, endogenous MBP produced in the 6-day-old sciatic nerves appears to be presented by autochthonous SC to the autoreactive T cells.