Selective and Antigen-Dependent Effects of Myelin Degeneration on Central Nervous System Inflammation

Iron Heterogeneity in Early Active Multiple Sclerosis Lesions

OBJECTIVE

Multiple sclerosis (MS) is a heterogeneous inflammatory demyelinating disease. Iron distribution is altered in MS patients' brains, suggesting iron liberation within active lesions amplifies demyelination and neurodegeneration. Whether the amount and distribution of iron are similar or different among different MS immunopatterns is currently unknown.

METHODS

We used synchrotron X-ray fluorescence imaging, histology, and immunohistochemistry to compare the iron quantity and distribution between immunopattern II and III early active MS lesions. We analyzed archival autopsy and biopsy tissue from 21 MS patients.

RESULTS

Immunopattern II early active lesions contain 64% more iron (95% confidence interval [CI] = 17-127%, p = 0.004) than immunopattern III lesions, and 30% more iron than the surrounding periplaque white matter (95% CI = 3-64%, p = 0.03). Iron in immunopattern III lesions is 28% lower than in the periplaque white matter (95% CI = -40 to -14%, p < 0.001). When normalizing the iron content of early active lesions to that of surrounding periplaque white matter, the ratio is significantly higher in immunopattern II (p < 0.001). Microfocused X-ray fluorescence imaging shows that iron in immunopattern II lesions localizes to macrophages, whereas macrophages in immunopattern III lesions contain little iron.

INTERPRETATION

Iron distribution and content are heterogeneous in early active MS lesions. Iron accumulates in macrophages in immunopattern II, but not immunopattern III lesions. This heterogeneity in the two most common MS immunopatterns may be explained by different macrophage polarization, origin, or different demyelination mechanisms, and paves the way for developing new or using existing iron-sensitive magnetic resonance imaging techniques to differentiate among immunopatterns in the general nonbiopsied MS patient population. ANN NEUROL 2021;89:498-510.

Aquaporin 4-specific T cells and NMO-IgG cause primary retinal damage in experimental NMO/SD

Neuromyelitis optica/spectrum disorder (NMO/SD) is a severe, inflammatory disease of the central nervous system (CNS). In the majority of patients, it is associated with the presence of pathogenic serum autoantibodies (the so-called NMO-IgGs) directed against the water channel aquaporin 4 (AQP4), and with the formation of large, astrocyte-destructive lesions in spinal cord and optic nerves. A large number of recent studies using optical coherence tomography (OCT) demonstrated that damage to optic nerves in NMO/SD is also associated with retinal injury, as evidenced by retinal nerve fiber layer (RNFL) thinning and microcystic inner nuclear layer abnormalities. These studies concluded that retinal injury in NMO/SD patients results from secondary neurodegeneration triggered by optic neuritis.However, the eye also contains cells expressing AQP4, i.e., Müller cells and astrocytes in the retina, epithelial cells of the ciliary body, and epithelial cells of the iris, which raised the question whether the eye can also be a primary target in NMO/SD. Here, we addressed this point in experimental NMO/SD (ENMO) induced in Lewis rat by transfer of AQP4268-285-specific T cells and NMO-IgG.We show that these animals show retinitis and subsequent dysfunction/damage of retinal axons and neurons, and that this pathology occurs independently of the action of NMO-IgG. We further show that in the retinae of ENMO animals Müller cell side branches lose AQP4 reactivity, while retinal astrocytes and Müller cell processes in the RNFL/ganglionic cell layers are spared. These changes only occur in the presence of both AQP4268-285-specific T cells and NMO-IgG.Cumulatively, our data show that damage to retinal cells can be a primary event in NMO/SD.

Highly encephalitogenic aquaporin 4-specific T cells and NMO-IgG jointly orchestrate lesion location and tissue damage in the CNS

In neuromyelitis optica (NMO), astrocytes become targets for pathogenic aquaporin 4 (AQP4)-specific antibodies which gain access to the central nervous system (CNS) in the course of inflammatory processes. Since these antibodies belong to a T cell-dependent subgroup of immunoglobulins, and since NMO lesions contain activated CD4⁺ T cells, the question arose whether AQP4-specific T cells might not only provide T cell help for antibody production, but also play an important role in the induction of NMO lesions. We show here that highly pathogenic, AQP4-peptide-specific T cells exist in Lewis rats, which recognize AQP4_268–285 as their specific antigen and cause severe panencephalitis. These T cells are re-activated behind the blood–brain barrier and deeply infiltrate the CNS parenchyma of the optic nerves, the brain, and the spinal cord, while T cells with other AQP4-peptide specificities are essentially confined to the meninges. Although AQP4_268–285-specific T cells are found throughout the entire neuraxis, they have NMO-typical “hotspots” for infiltration, i.e. periventricular and periaqueductal regions, hypothalamus, medulla, the dorsal horns of spinal cord, and the optic nerves. Most remarkably, together with NMO-IgG, they initiate large astrocyte-destructive lesions which are located predominantly in spinal cord gray matter. We conclude that the processing of AQP4 by antigen presenting cells in Lewis rats produces a highly encephalitogenic AQP4 epitope (AQP4_268–285), that T cells specific for this epitope are found in the immune repertoire of normal Lewis rats and can be readily expanded, and that AQP4_268–285-specific T cells produce NMO-like lesions in the presence of NMO-IgG.

Routes of administration and dose optimization of soluble antigen arrays in mice with experimental autoimmune encephalomyelitis

Soluble antigen arrays (SAgAs) were developed for treating mice with experimental autoimmune encephalomyelitis (EAE), a mouse model of multiple sclerosis. SAgAs are composed of hyaluronan with grafted EAE antigen and LABL peptide (a ligand of ICAM-1). SAgA dose was tested by varying injection volume, SAgA concentration, and administration schedule. Routes of administration were explored to determine the efficacy of SAgAs when injected intramuscularly, subcutaneously, intraperitoneally, intravenously, or instilled into lungs. Injections proximal to the central nervous system (CNS) were compared with distal injection sites. Intravenous dosing was included to determine if SAgA efficiency results from systemic exposure. Pulmonary instillation (p.i.) was included as reports suggest T cells are licensed in the lungs before moving to the CNS. Decreasing the volume of injection or SAgA dose reduced treatment efficacy. Treating mice with a single injection on day 4, 7, and 10 also reduced efficacy compared with injecting on all three days. Surprisingly, changing the injection site did not lead to a significant difference in efficacy. Intravenous administration showed efficacy similar to other routes, suggesting SAgAs act systemically. When SAgAs were delivered via p.i., however, EAE mice failed to develop any symptoms, suggesting a unique lung mechanism to ameliorate EAE in mice.

Thymic stromal lymphopoietin is expressed in the intact central nervous system and upregulated in the myelin-degenerative central nervous system

Thymic stromal lymphopoietin (TSLP) is an epithelial cytokine expressed at barrier surfaces of the skin, gut, nose, lung, and the maternal/fetal interphase. At these sites, it is important for the generation and maintenance of non-inflammatory, tissue-resident dendritic cell responses. We show here that TSLP is also expressed in the central nervous system (CNS) where it is produced by choroid plexus epithelial cells and astrocytes in the spinal cord. Under conditions of low-grade myelin degeneration, the numbers of TSLP-expressing astrocytes increase, and microglia express transcripts for the functional TSLP receptor dimer indicating that these cells are targets for TSLP in the myelin-degenerative CNS.

Pathogenic T cell responses against aquaporin 4

Inflammatory lesions in the central nervous system of patients with neuromyelitis optica are characterized by infiltration of T cells and deposition of aquaporin-4-specific antibodies and complement on astrocytes at the glia limitans. Although the contribution of aquaporin-4-specific autoantibodies to the disease process has been recently elucidated, a potential role of aquaporin-4-specific T cells in lesion formation is unresolved. To address this issue, we raised aquaporin-4-specific T cell lines in Lewis rats and characterized their pathogenic potential in the presence and absence of aquaporin-4-specific autoantibodies of neuromyelitis optica patients. We show that aquaporin-4-specific T cells induce brain inflammation with particular targeting of the astrocytic glia limitans and permit the entry of pathogenic anti-aquaporin-4-specific antibodies to induce NMO-like lesions in spinal cord and brain. In addition, transfer of aquaporin-4-specific T cells provoked mild (subclinical) myositis and interstitial nephritis. We further show that the expression of the conformational epitope, recognized by NMO patient-derived aquaporin-4-specific antibodies is induced in kidney cells by the pro-inflammatory cytokine gamma-interferon. Our data provide further support for the view that NMO lesions may be induced by a complex interplay of T cell mediated and humoral immune responses against aquaporin-4.

Dual role of B cells with accelerated onset but reduced disease activity in P0₁₀₆₋₁₂₅-induced experimental autoimmune neuritis of IgH ⁰(/)⁰ mice

The role of B cells in autoimmune-mediated diseases of the peripheral nervous system was studied in experimental autoimmune neuritis (EAN) in B cell deficient IgH⁰(/)⁰ C57BL/6J mice having been immunized with P0₁₀₆₋₁₂₅ peptide. Compared to coisogenic IgH(+/+) mice, onset of EAN was accelerated [100% disease incidence at day 9 post immunization (p.i.) vs. day 15 p.i.]. At day 9 p.i., numbers of P0₁₀₆₋₁₂₅-specific interferon (IFN)-γ-producing CD4(+) T cells were increased, while IL-10 mRNA and production were decreased in IgH⁰(/)⁰ mice. Beyond day 9 p.i., declining disease activity and a significant reduction of maximal disease activity were correlated with significantly reduced numbers of IFN-γ-producing CD4(+) T cells in IgH(0/0) mice as compared with IgH(+/+) mice. Correspondingly, neuropathology demonstrated only mild axonal damage, while demyelination and dying back axonopathy with spinal cord motor neuron apoptosis were absent. Thus, depending on the stage of EAN, B cells play a dual, i.e. suppressive and enhancing, role during induction and at height of EAN, respectively. The combined interaction of B cells as well as CD4(+) and CD8(+) T cells is required for the development of EAN.

Neuromyelitis optica: pathogenicity of patient immunoglobulin in vivo

OBJECTIVE

Severe inflammation and astrocyte loss with profound demyelination in spinal cord and optic nerves are typical pathological features of neuromyelitis optica (NMO). A diagnostic hallmark of this disease is the presence of serum autoantibodies against the water channel aquaporin-4 (AQP-4) on astrocytes.

METHODS

We induced acute T-cell-mediated experimental autoimmune encephalomyelitis in Lewis rats and confronted the animals with an additional application of immunoglobulins from AQP-4 antibody-positive and -negative NMO patients, multiple sclerosis patients, and control subjects.

RESULTS

The immunoglobulins from AQP-4 antibody-positive NMO patients are pathogenic. When they reach serum titers in experimental animals comparable with those seen in NMO patients, they augment clinical disease and induce lesions in the central nervous system that are similar in structure and distribution to those seen in NMO patients, consisting of AQP-4 and astrocyte loss, granulocytic infiltrates, T cells and activated macrophages/microglia cells, and an extensive immunoglobulin and complement deposition on astrocyte processes of the perivascular and superficial glia limitans. AQP-4 antibody containing NMO immunoglobulin injected into naïve rats, young rats with leaky blood-brain barrier, or after transfer of a nonencephalitogenic T-cell line did not induce disease or neuropathological alterations in the central nervous system. Absorption of NMO immunoglobulins with AQP-4-transfected cells, but not with mock-transfected control cells, reduced the AQP-4 antibody titers and was associated with a reduction of astrocyte pathology after transfer.

INTERPRETATION

Human anti-AQP-4 antibodies are not only important in the diagnosis of NMO but also augment disease and induce NMO-like lesions in animals with T-cell-mediated brain inflammation.

The "window of susceptibility" for inflammation in the immature central nervous system is characterized by a leaky blood-brain barrier and the local expression of inflammatory chemokines

Early in postnatal development, the immature central nervous system (CNS) is more susceptible to inflammation than its adult counterpart. We show here that this "window of susceptibility" is characterized by the presence of leaky vessels in the CNS, and by a global chemokine expression profile which is clearly distinct from the one observed in the adult CNS and has three important characteristics. First, it contains chemokines with known roles in the differentiation and maturation of glia and neurons. Secondly, these chemokines have been described before in inflammatory lesions of the CNS, where they are important for the recruitment of monocytes and T cells. Lastly, the chemokine profile is shaped by pathological changes like oligodendrocyte stress and attempts of myelin repair. Changes in the chemokine expression profile along with a leaky blood-brain barrier pave the ground for an accelerated development of CNS inflammation.

Cerebrospinal fluid biomarkers in multiple sclerosis

In patients with multiple sclerosis (MS) intensive efforts are directed at identifying biomarkers in bodily fluids related to underlying disease mechanisms, disease activity and progression, and therapeutic response. Besides MR imaging parameters cerebrospinal fluid (CSF) biomarkers provide important and specific information since changes in the CSF composition may reflect disease mechanisms inherent to MS. The different cellular and protein-analytical methods of the CSF and the recommended standard of the diagnostic CSF profile in MS are described. A brief update on possible CSF biomarkers that might reflect key pathological processes of MS such as inflammation, demyelination, neuroaxonal loss, gliosis and regeneration is provided.

Immunologic pathogenesis of multiple sclerosis

Multiple sclerosis (MS) is an autoimmune disease. The etiology and pathogenesis of MS remain unclear. At present, there are substantial evidences to support the hypothesis that genetics plays a crucial role. The people who have genetic predisposing genes easily develop immune-mediated disorder, probably in conjunction with environmental factors. The aim of this review is to describe recent observations regarding the immunologic pathogenesis of MS.

After injection into the striatum, in vitro-differentiated microglia- and bone marrow-derived dendritic cells can leave the central nervous system via the blood stream

The prototypic migratory trail of tissue-resident dendritic cells (DCs) is via lymphatic drainage. Since the central nervous system (CNS) lacks classical lymphatic vessels, and antigens and cells injected into both the CNS and cerebrospinal fluid have been found in deep cervical lymph nodes, it was thought that CNS-derived DCs exclusively used the cerebrospinal fluid pathway to exit from tissues. It has become evident, however, that DCs found in peripheral organs can also leave tissues via the blood stream. To study whether DCs derived from microglia and bone marrow can also use this route of emigration from the CNS, we performed a series of experiments in which we injected genetically labeled DCs into the striata of rats. We show here that these cells migrated from the injection site to the perivascular space, integrated into the endothelial lining of the CNS vasculature, and were then present in the lumen of CNS blood vessels days after the injection. Moreover, we also found these cells in both mesenteric lymph nodes and spleens. Hence, microglia- and bone marrow-derived DCs can leave the CNS via the blood stream.

Mast cell activation by myelin through scavenger receptor

A role for mast cells (MC) in the pathogenesis of multiple sclerosis (MS) has been suggested, based on the analysis of human lesions and on an animal model of the disease (EAE). What role MC play in the development of MS is not well understood. We hypothesized that the link connecting MC with demyelinating diseases may be represented by their interaction with myelin. Here we show that myelin can activate mast cells. This process could be a key event in the mast cell function required for inducing EAE in mice and possibly in MS in man.

[Update on pathophysiologic and immunotherapeutic approaches for the treatment of multiple sclerosis]

Multiple sclerosis (MS) is a chronic disabling disease with significant implications for patients and society. The individual disease course is difficult to predict due to the heterogeneity of clinical presentation and of radiologic and pathologic findings. Although its etiology still remains unknown, the last decade has brought considerable understanding of the underlying pathophysiology of MS. In addition to its acceptance as a prototypic inflammatory autoimmune disorder, recent data reveal the importance of primary and secondary neurodegenerative mechanisms such as oligodendrocyte death, axonal loss, and ion channel dysfunction. The deepened understanding of its immunopathogenesis and the limited effectiveness of currently approved disease-modifying therapies have led to a tremendous number of trials investigating potential new drugs. Emerging treatments take into account the different immunopathological mechanisms and strategies, to protect against axonal damage and promote remyelination. This review provides a compilation of novel immunotherapeutic strategies and recently uncovered aspects of known immunotherapeutic agents. The pathogenetic rationale of these novel drugs for the treatment of MS and accompanying preclinical and clinical data are highlighted.

Continued administration of ciliary neurotrophic factor protects mice from inflammatory pathology in experimental autoimmune encephalomyelitis

Multiple sclerosis is an inflammatory disease of the central nervous system that leads to loss of myelin and oligodendrocytes and damage to axons. We show that daily administration (days 8 to 24) of murine ciliary neurotrophic factor (CNTF), a neurotrophic factor that has been described as a survival and differentiation factor for neurons and oligodendrocytes, significantly ameliorates the clinical course of a mouse model of multiple sclerosis. In the acute phase of experimental autoimmune encephalomyelitis induced by myelin oligodendrocyte glycoprotein peptide 35-55, treatment with CNTF did not change the peripheral immune response but did reduce the number of perivascular infiltrates and T cells and the level of diffuse microglial activation in spinal cord. Blood brain barrier permeability was significantly reduced in CNTF-treated animals. Beneficial effects of CNTF did not persist after it was withdrawn. After cessation of CNTF treatment, inflammation and symptoms returned to control levels. However, slight but significantly higher numbers of oligodendrocytes, NG2-positive cells, axons, and neurons were observed in mice that had been treated with high concentrations of CNTF. Our results show that CNTF inhibits inflammation in the spinal cord, resulting in amelioration of the clinical course of experimental autoimmune encephalomyelitis during time of treatment.

Transient axonal injury in the absence of demyelination: a correlate of clinical disease in acute experimental autoimmune encephalomyelitis

Axonal degeneration contributes to the transient and permanent neurological deficits seen in multiple sclerosis, an inflammatory disease of the central nervous system. To study the immunological mechanisms causing axonal degeneration, we induced experimental autoimmune encephalomyelitis (EAE) in wildtype Lewis rats and Lewis rats with a slowly progressive myelin degeneration due to proteolipid protein (PLP) overexpression. EAE was triggered either by the transfer of encephalitogenic T-cells alone or by the co-transfer of T-cells with demyelinating antibodies. Inducible nitric oxide synthase (iNOS) expression in perivascular macrophages was associated with a transient functional disturbance of axons, reflected by the focal and reversible accumulation of amyloid precursor protein. Clinical disease correlated with the numbers of APP positive axon spheroids. Demyelination was associated with a further increase of iNOS expression in macrophages and with a higher degree of axonal injury. Our studies suggest that nitric oxide and its metabolites contribute to axonal pathology and possibly also to subsequent neurological dysfunction in EAE.

Pathogenesis of multiple sclerosis

PURPOSE OF REVIEW

The aim of this article is to describe recent observations regarding the basis for the initiation and disease evolution of multiple sclerosis.

RECENT FINDINGS

A current debate is where and what initiates the neuroinflammatory reaction that characterizes the acute multiple sclerosis lesion. Immune sensitization to neural antigens could develop within the systemic compartment consequent to exposure to cross-reacting, possibly viral derived, peptides (molecular mimicry). Although CD4 T cells are considered central to initiating central nervous system inflammation, the actual extent and specificity of tissue injury reflects the array of adaptive (CD8 T cells and antibody) and innate (microglia/macrophages) immune constituents present in the lesions. Neuropathologic studies indicate that lethal changes in neural cells (oligodendrocytes) could also be the initiating event, reflecting as yet unidentified acquired insults (e.g. exogenous virus or reactivated endogenous retrovirus) or intrinsic abnormalities ('neurodegenerative' hypothesis). Recurrence or persistence of the disease process can reflect events occurring at multiple sites including expansion of the immune repertoire in response to neural antigens transported to regional lymph nodes (determinant spreading), especially if immune regulatory mechanisms are defective; alterations in blood-brain barrier properties consequent to initial cellular transmigration; and participation of endogenous (microglia, astrocytes) or long lived infiltrating cells (macrophages, B cells in ectopic germinal centers) in regulating and effecting immune functions within the central nervous system. Accumulating neurologic deficit reflects the balance between injury and repair; the latter also being negatively or positively (trophic support and clearance of tissue debris) impacted by inflammatory processes.

SUMMARY

Understanding the full spectrum of multiple sclerosis presents a continuing challenge for both immunology and neurobiology.