Peripheral nervous system lesions in experimental allergic encephalomyelitis. Ultrastructural distribution of T cells and Ia-antigen

The origin, fate and function of macrophages in the peripheral nervous system—an update

The field of macrophage biology has made enormous progress over recent years. This was triggered by the advent of several new techniques such as the establishment of Cre/loxP-based transgenic mouse models that allowed for the first time delineation of the ontogeny and function of specific macrophage populations across many tissues. In addition, the introduction of new high-throughput technologies like bulk RNA sequencing and later single-cell RNA sequencing as well as advances in epigenetic analysis have helped to establish gene expression profiles, enhancer landscapes and local signaling cues that define and shape the identity of diverse macrophage populations. Nonetheless, some macrophage populations, like the ones residing in the peripheral nervous system (PNS), have not been studied in such detail yet. Here, we discuss recent studies that shed new light on the ontogeny, heterogeneity and gene expression profiles of resident macrophages in peripheral nerves and described differential activation of macrophage subsets during and after acute sciatic nerve injury.

Acquired immunodeficiency syndrome and the blood-brain barrier

The blood-brain barrier (BBB) plays a critical role in normal physiology of the central nervous system by regulating what reaches the brain from the periphery. The BBB also plays a major role in neurologic disease including neuropathologic sequelae associated with infection by human immunodeficiency virus (HIV) in humans and the closely related simian immunodeficiency virus (SIV) in macaques. In this review, we provide an overview of the function, structure, and components of the BBB, followed by a more detailed discussion of the subcellular structures and regulation of the tight junction. We then discuss the ways in which HIV/SIV affects the BBB, largely through infection of monocytes/macrophages, and how infected macrophages crossing the BBB ultimately results in breakdown of the barrier.

Central nervous system perivascular cells are immunoregulatory cells that connect the CNS with the peripheral immune system

Perivascular cells are a heterogeneous population found in the central nervous system (CNS) and the peripheral nervous system (PNS). Several terms are used for these cells, including perivascular cells, perivascular macrophages, perivascular microglia, fluorescent granular perithelial cells (FGP), or Mato cells. Different terminology used may reflect subpopulations of perivascular cells within different anatomic regions and experimental paradigms, neuropathological conditions, and species studied. Different terminology also points to the lack of clear consensus of what cells are perivascular cells in different disease states and models, especially with breakdown of the blood-brain barrier (BBB). Despite this, there is consensus that perivascular cells, although a minor component of the CNS, are important immunoregulatory cells. Perivascular cells are bone marrow derived, continuously turn over in the CNS, and are found adjacent to CNS vessels. Thus, they are potential sensors of CNS and peripheral immune system perturbations; are activated in models of CNS inflammation, autoimmune disease, neuronal injury and death; and are implicated as phagocytic and pinocytotic cells in models of stroke and hypertension. Recent evidence from our laboratory implicate perivascular cells as primary targets of human immunodeficiency virus (HIV) and simian immunodeficiency virus (SIV) infection in the CNS of humans and macaques. This article reviews current knowledge of perivascular cells, including anatomic location and nomenclature and putative immunoregulatory roles, and discusses new data on the infection of these cells by SIV, their accumulation after SIV infection, and a possible role of the immune system in SIV encephalitis.

T-cell receptor V beta-element expression in peripheral nerves of Lewis rats suffering from experimental autoimmune neuritis

In experimental autoimmune neuritis (EAN), peripheral nerves are infiltrated by T-lymphocytes and macrophages. By RT-PCR and sequence analysis we characterized TCR V beta-element usage in sciatic nerve tissue of Lewis rats suffering from EAN induced by immunization with peripheral myelin antigens. Several TCR V beta-chain sequences were detected, which did not show homology to sequences of P2-reactive T cells published so far. In EAN induced with peripheral nerve myelin, but not with P2-protein or P2 peptide aa 53-78, TCR V beta 8.2 sequences identical to sequences of encephalitogenic myelin basic protein (MBP) reactive T-cells were identified. These results provide further evidence for a contribution of MBP-directed T-cell reactivity to the pathogenesis of myelin induced EAN and may have implications for the pathogenesis of human demyelinating neuropathies.

Resident microglia and hematogenous macrophages as phagocytes in adoptively transferred experimental autoimmune encephalomyelitis: an investigation using rat radiation bone marrow chimeras

Hematogenous macrophages are known to be involved in the induction of tissue damage in the central nervous system (CNS) as well as of clinical symptoms in experimental autoimmune encephalomyelitis (EAE). Although resident microglia can become phagocytic under certain circumstances, little is known about the role of these cells in brain inflammation in vivo. We thus studied EAE in the model of radiation bone marrow chimeras that allows us to distinguish donor-derived hematogenous cells from resident effector cells. Inflammation in the CNS was qualitatively and quantitatively similar in chimeras compared to fully histocompatible Lewis rats. Although activated resident microglial cells were outnumbered four- to sevenfold in EAE lesions by hematogenous macrophages, the number of resident microglia with ingested myelin was equal to that of macrophages containing myelin debris. Phagocytic resident microglia, expressing the macrophage activation marker ED1, showed ramified as well as amoeboid morphology. From our studies the following conclusions can be drawn. First, a considerable proportion of resident microglia upregulated ED1. Second, resident microglia provide a small but substantial source of brain macrophages in EAE as compared to the large influx of macrophages. Third, our results suggest that microglia, due to their strategic position within the CNS, are more effective in removal of myelin debris compared to hematogenous macrophages.

T cell immunity to myelin basic protein induces anterior uveitis in Lewis rats

Uveitis of unknown etiology is known to occur in association with various systemic disorders. We now report that anterior uveitis (AU) can be produced by T cell immunity to myelin basic protein (BP) and accompanies experimental autoimmune encephalomyelitis (EAE). EAE with AU was induced in Lewis rats by immunization to BP in CFA or by immunization to various BP peptides including the encephalitogenic 71-90 peptide. Slit-lamp biomicroscopy of BP-immunized Lewis rats revealed AU, characterised by inflammation of the iris, in 73% of the eyes. The onset of AU in actively immunized rats varied between days 12 and 26, often appearing after spontaneous remission of the paralysis, the hallmark of EAE. The course of AU was progressive, affecting more than 50% of the surface of the iris in 16 of 29 diseased eyes. Like the paralysis, the AU was self-limiting: within 2 weeks the disease remitted. In addition, AU could be adoptively transferred to naive and irradiated rats by a T cell clone specific for BP peptide 71-90. The present observations are compatible with the idea that AU may be triggered by BP-reactive T cells. The myelinated nerves present in the iris have been shown to contain BP. However, these peripheral nerves would now appear to be the only peripheral nerves susceptible to acute EAE.

Expression of CD45RC and Ia antigen in the spinal cord in acute experimental allergic encephalomyelitis: an immunocytochemical and flow cytometric study

We performed immunocytochemical studies to analyze the inflammatory infiltrate and major histocompatibility complex class II (Ia) antigen expression in the spinal cord of Lewis rats with acute experimental allergic encephalomyelitis (EAE) induced by inoculation with myelin basic protein and adjuvants. Using antibodies to lymphocyte markers and other monoclonal antibodies we found that during clinical episodes the inflammatory infiltrate was chiefly composed of T lymphocytes and macrophages. The majority of cells in the inflammatory infiltrate were stained by the W3/25 antibody to CD4 and a proportion was stained by OX22 which labels the high molecular weight form of the leucocyte common antigen (CD45RC). CD8+ T cells were sparse and B cells were not detected. There was minimal staining with the OX39 antibody to the interleukin-2 receptor. Presumptive microglia, identified by their dendritic morphology, expressed Ia antigen during the clinical episodes and after recovery. The prominence of Ia antigen expression after recovery could indicate that this Ia expression was associated with downregulation of the encephalitogenic immune response. We also performed flow cytometry studies on cells extracted from the spinal cord of rats before and during attacks of EAE. With flow cytometry, we found that in established disease a mean of 83(SD, 23)% of CD2+ cells were CD4+, and a mean of 27(SD, 12)% of CD2+ cells were CD45RC+. In rats sampled on the first day of signs, a mean of 43(SD, 22)% of CD2+ cells were CD45RC+. In the cells extracted from the spinal cord of rats with established disease a mean of 47(SD, 32)% of macrophages were CD45RC+. Our study has combined an immunocytochemical assessment of tissue sections with quantitative flow cytometry assessment of cells extracted from the spinal cord of rats with acute EAE. We have shown that the majority of T lymphocytes in the spinal cord are CD45RC-. We have also found prominent Ia expression on dendritic cells in acute EAE and after clinical recovery.

MHC-positive, ramified macrophages in the normal and injured rat peripheral nervous system

Resident endoneurial macrophages form a prominent, but little recognized component of the PNS. We have studied immunocytochemically the distribution, morphology and immunophenotype of endoneurial macrophages in several normal peripheral nerves of the rat. In addition, we investigated the macrophage response following crush injury of the sciatic nerve. Resident endoneurial macrophages had a ramified morphology with processes oriented parallel to the long axis of nerve fibres. They were positive for several monocyte/macrophage markers such as ED1, ED2 and the recently-described MUC 101 and MUC 102 antibodies. They furthermore expressed the complement type three receptor, the CD4 antigen and MHC class I and II molecules. These results were consistent in all the peripheral nerves studied. In addition, 1000 rad of gamma-irradiation led to a strong reduction in the number of MHC class II-positive ramified cells in the peripheral nerves similar to that observed in other peripheral organs such as the heart. A considerable percentage of resident macrophages in the PNS and/or their precursor cells are therefore radiosensitive and could be related to the lineage of dendritic cells. Following crush injury, ED1-3-, OX-42-, MUC 101- and MUC 102-positive round macrophages were observed from 24 h postlesion onward at the site of trauma. In the distal part, they were observed to form strings of round, foamy macrophages probably involved in myelin phagocytosis. In contrast, the number of MHC class II-positive resident macrophages was only slightly increased at the site of trauma and in the distal part. These cells transformed from a ramified to a round morphology, but did not appear as typical strings of foamy macrophages. These results demonstrate that the PNS is provided with a resident macrophage population analogous in many respects to microglial cells in the CNS. These constitutively MHC class II-positive PNS microglial-like cells could act as the major antigen-presenting cells in the peripheral nerve. They may thus constitute a local immune defense system of the PNS with a function similar to that of microglial cells in the CNS.

Coronavirus induced primary demyelination: indications for the involvement of a humoral immune response

Coronavirus MHV-JHM infection of rodents can result in demyelinating encephalomyelitis. We analysed histological changes induced by coronavirus MHV-JHM infection in Lewis rats. Besides an acute disease (AE), chronic panencephalitis (CPE) and subacute demyelinating encephalomyelitis (SDE) were induced. These disease types were differentiated by the incubation period, the localization of lesions, the type of tissue damage and distribution of virus antigen. In AE and CPE, virus antigen was detected in neurons, astrocytes and oligodendrocytes, whereas in SDE neurons lacked virus antigen. Viral nucleocapsid protein (N) was present in the cytoplasm and the spike protein (S) was displayed on the surface of infected neural cells. However, expression of S protein relative to N protein was severely impaired in SDE lesions. Quantitative analysis of infiltrating inflammatory cells revealed that the number of macrophages and T cells were similar in lesions of AE, CPE and SDE. In contrast to that, SDE lesions contained a significantly higher number of IgG + B cells and plasma cells. In addition active demyelinating SDE lesions displayed an enhanced IgG content and deposits of complement C9. These results indicate that virus induced primary demyelination could be a consequence of antibody mediated cytotoxicity. Furthermore, a reduction in the number of cells producing spike protein in the chronic forms of the disease indicates down-regulation of this protein, possibly mediated by anti-S antibodies.

Neuropeptide Levels after Pentylenetetrazol Kindling in the Rat

Levels of several neuropeptides were measured in the frontal cortex, dorsal hippocampus, striatum, and amygdala/pyriform cortex in rats kindled for 5 weeks by daily injection of pentylenetetrazol (30 mg/kg, i.p.). Significantly increased concentrations (by 30 - 140%) were found in all examined brain areas for neuropeptide Y, somatostatin (except hippocampus) and neurokinin-like immunoreactivity 10 days after the last kindling session. Similar but less pronounced changes were also found 24 h after the last seizure. The increase in total neurokinin-like immunoreactivity was due to a marked increase in neurokinin B as revealed by HPLC analysis. Increases in peptide levels, however, were restricted to fully kindled animals. At the same time no changes in levels of substance P, vasoactive intestinal polypeptide and calcitonin gene-regulated peptide were observed. Cholecystokinin octapeptide was enhanced only in the hippocampus (by 46%). The increases in neuropeptide Y, somatostatin, and neurokinin-like immunoreactivity subsided after 3 months. A markedly decreased seizure threshold was observed 10 days and 2 months after the final kindling session. No nerve cell degeneration was observed in kindled rats 24 h or 10 days after the last pentylenetetrazol injection. Some animals (2 of 4), however, exhibited signs of blood - brain barrier damage when examined 24 h after the last kindling session which may reflect the preceding convulsions. No such changes were detected after 10 days. The increases in peptide levels may suggest increased activity of respective neurons which, at least to some degree, may be associated with gamma-aminobutyric acid. The changes in peptide levels may be more closely related to the kindling procedure itself than to the decreased seizure threshold of the animals.

Molecular mimicry and Lyme borreliosis: a shared antigenic determinant between Borrelia burgdorferi and human tissue

The pathogenesis of chronic manifestations in Lyme borreliosis, a disease induced by Borrelia burgdorferi, is at present unresolved. By testing monoclonal antibodies directed against various borrelia antigens, we found an antigenic determinant shared by the 41 kDa flagella protein and human tissue, especially prominent on myelinated fibers of human peripheral nerve, on nerve cells and axons of the central nervous system, as well as on certain epithelial cells (including joint synovia) and on heart muscle cells. Immune reactions against such a shared antigen could play a pathogenetic role in chronic organ manifestations of Lyme borreliosis.

Immunohistochemical localisation of terminal complement component C9 in experimental allergic encephalomyelitis

The deposition of terminal complement component C9 within the central nervous system (CNS) has been studied immunohistochemically in three models of experimental allergic encephalomyelitis (EAE) in the rat; inflammatory EAE induced by the passive transfer of myelin basic protein-specific T cells (tEAE), antibody-mediated, demyelinating tEAE and a subacute/chronic model induced by active immunisation with guinea pig spinal cord tissue in adjuvant. Two distinct patterns of C9 reactivity were observed, a diffuse staining of the tissue adjacent to inflammatory lesions, similar to that seen for other extra-vasculated serum proteins, and also granular, sometimes fibrillar C9 deposits around some inflammed vessels and in areas of active demyelination. The latter staining pattern was most pronounced in animals with acute antibody-mediated demyelinating tEAE, in which extensive, but transient, subpial and perivascular granular deposits of C9 were associated with regions of acute demyelination. A similar pattern of granular C9 reactivity was also associated with demyelinating lesions in animals with actively induced chronic progressive EAE. However, these C9 deposits were not observed in rats with purely inflammatory, clinically mild tEAE, although C9 deposition was occasionally observed around a small number of inflammed vessels in animals with hyperacute, lethal tEAE. These observations demonstrate that deposition of C9, the major component of the cytolytic membrane attack complex, in EAE is related to myelin injury rather than CNS inflammation.

Role of endoneural cells in experimental allergic neuritis and characterisation of a resident phagocytic cell

Electrophysiological, clinical and histological techniques were used to monitor the time course of events related to experimental allergic neuritis (EAN) in 48 Lewis rats. The primary lesion was found to be paranodal demyelination without cellular infiltration. Endoneural phagocytes derive from hematogenous ED1+ED2- monocytes and possibly from resident ED1-ED2+ monocytic cells, not from Schwann cells and fibroblasts. We demonstrate a population of monocytic Ia-bearing, ED1-ED2+ spindle-shaped cells residing in normal peripheral nerve and provide evidence for their transformation into macrophages in the course of EAN.

Dynamics of IgG+, IgA+, and IgM+ plasma cells in the central nervous system of guinea pigs with chronic relapsing experimental allergic encephalomyelitis

IgG+, IgA+, and IgM+ plasma cells (PC) were investigated by quantitative immunocytochemistry in the central nervous system (CNS) of guinea pigs with chronic relapsing experimental allergic encephalomyelitis (crEAE) and in the CNS of controls. Frequencies of Ig+ PC in crEAE were, on average, 30-130 times higher than in controls. At all stages of crEAE, IgG was the dominant isotype of CNS PC. In the course of crEAE there was an increase in the proportion of IgG+ PC and a decrease in the proportions of IgM+ and, to a lesser extent, of IgA+ PC. Accumulation of IgM+ PC appears to be related to the acuteness of the disease. Our studies also indicate that IgA, in addition to IgG and IgM, is involved in the immunological response of crEAE in the CNS, and demonstrate that the time course of crEAE is characterized by isotype-specific differences in the immune responses of IgG+, IgM+, and IgA+ PC in the CNS.

Neuritis cordis due to the acute polyneuritis of the Guillain-Barré syndrome

Three patients with the Guillain-Barré syndrome which followed the course of Landry's acute ascending paralysis died a sudden cardiac death. Autonomic dysfunction had appeared clinically, consisting of sphincter disturbances in one patient and fluctuating blood pressure and bradycardia in the other. In a twenty-three year old female patient cardiac function had been inconspicuous, apart from tachycardia, but the ECG showed S-T segment depression and flat T waves. Postmortem examination revealed acute inflammatory demyelinating polyradiculoneuritis involving the peripheral autonomic nervous system and especially the nerves of the heart. Immunohistochemically, the inflammatory cell infiltrations of this neuritis cordis consisted of macrophages (MAC 387 positive) and T lymphocytes (UCHL1 positive). No indication of a direct viral infection of the inflamed cardiac nerves was detectable by immunohistochemistry (HSV, CMV, influenza virus) nor by electron microscopy. The neuritis cordis was classified as an inflammatory cardio-neuropathy secondary to a patchy acute polyneuritis of the Guillain-Barré syndrome, involving the autonomic nervous system. Myocarditis could be discounted, and the neuritis cordis was thought to be responsible for the sudden cardiac death.

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.