Aquaporin-4 autoantibodies cause asymptomatic aquaporin-4 loss and activate astrocytes in mouse

T follicular helper cells contribute to pathophysiology in a model of neuromyelitis optica spectrum disorders

Neuromyelitis optica spectrum disorders (NMOSD) are inflammatory autoimmune disorders of the CNS. IgG autoantibodies targeting the aquaporin-4 water channel (AQP4-IgGs) are the pathogenic effector of NMOSD. Dysregulated T follicular helper (Tfh) cells have been implicated in loss of B cell tolerance in autoimmune diseases. The contribution of Tfh cells to disease activity and therapeutic potential of targeting these cells in NMOSD remain unclear. Here, we established an autoimmune model of NMOSD by immunizing mice against AQP4 via in vivo electroporation. After AQP4 immunization, mice displayed AQP4 autoantibodies in blood circulation, blood-brain barrier disruption, and IgG infiltration in spinal cord parenchyma. Moreover, AQP4 immunization induced motor impairments and NMOSD-like pathologies, including astrocytopathy, demyelination, axonal loss, and microglia activation. These were associated with increased splenic Tfh, Th1, and Th17 cells; memory B cells; and plasma cells. Aqp4-deficient mice did not display motor impairments and NMOSD-like pathologies after AQP4 immunization. Importantly, abrogating ICOS/ICOS-L signaling using anti-ICOS-L antibody depleted Tfh cells and suppressed the response of Th1 and Th17 cells, memory B cells, and plasma cells in AQP4-immunized mice. These findings were associated with ameliorated motor impairments and spinal cord pathologies. This study suggests a role of Tfh cells in the pathophysiology of NMOSD in a mouse model with AQP4 autoimmunity and provides an animal model for investigating the immunological mechanisms underlying AQP4 autoimmunity and developing therapeutic interventions targeting autoimmune reactions in NMOSD.

The role of complement and complement therapeutics in neuromyelitis optica spectrum disorders

INTRODUCTION

Neuromyelitis optica spectrum disorders (NMOSD) are characterized in the majority of cases by the presence of IgG1 autoantibodies against aquaporin 4 (AQP4) and myelin-oligodendrocyte glycoprotein (MOG), both capable of activating complement.

AREAS COVERED

We review evidence of complement involvement in NMOSD pathophysiology from pathological, in vitro, in vivo, human studies, and clinical trials.

EXPERT OPINION

In AQP4 NMOSD, complement deposition is a prominent pathological feature, while in vitro and in vivo studies have demonstrated complement-dependent pathogenicity of AQP4 antibodies. Consistent with these studies, the anti-C5 monoclonal antibody eculizumab was remarkably effective and safe in a phase 2/3 trial of AQP4-NMOSD patents leading to FDA-approved indication. Several other anti-complement agents, either approved or in trials for other neuro-autoimmunities, like myasthenia, CIDP, and GBS, are also relevant to NMOSD generating an exciting group of evolving immunotherapies. Limited but compelling in vivo and in vitro data suggest that anti-complement therapeutics may be also applicable to a subset of MOG NMOSD patients with severe disease. Overall, anticomplement agents, along with the already approved anti-IL6 and anti-CD19 monoclonal antibodies sartralizumab and inebilizumab, are rapidly changing the therapeutic algorithm in NMOSD, a previously difficult-to-treat autoimmune neurological disorder.

Low Frequency Ultrasound With Injection of NMO-IgG and Complement Produces Lesions Different From Experimental Autoimmune Encephalomyelitis Mice

Neuromyelitis optica spectrum disorder (NMOSD), a relapsing autoimmune disease of the central nervous system, mainly targets the optic nerve and spinal cord. To date, all attempts at the establishment of NMOSD animal models have been based on neuromyelitis optica immunoglobulin G antibody (NMO-IgG) and mimic the disease in part. To solve this problem, we developed a rodent model by opening the blood-brain barrier (BBB) with low frequency ultrasound, followed by injection of NMO-IgG from NMOSD patients and complement to mice suffering pre-existing neuroinflammation produced by experimental autoimmune encephalomyelitis (EAE). In this study, we showed that ultrasound with NMO-IgG and complement caused marked inflammation and demyelination of both spinal cords and optic nerves compared to blank control group, as well as glial fibrillary acidic protein (GFAP) and aquaporin-4 (AQP4) loss of spinal cords and optic nerves compared to EAE mice and EAE mice with only BBB opening. In addition, magnetic resonance imaging (MRI) revealed optic neuritis with spinal cord lesions. We further demonstrated eye segregation defects in the dorsal lateral geniculate nucleus (dLGN) of these NMOSD mice.

Current understanding of the epidemiologic and clinical characteristics of optic neuritis

Optic neuritis is an ocular disorder whose pathogenesis has not been fully determined, although autoimmune mechanisms have been suggested to be involved in its development. In recent years, anti-aquaporin-4 antibody (AQP4-Ab) and anti-myelin oligodendrocyte glycoprotein antibody (MOG-Ab) have been shown to play major roles in the development of optic neuritis. Because these two antibodies target different tissues, optic neuritis can be classified by the type of antibody. AQP4-Ab-positive optic neuritis responds poorly to steroid therapy and has a poor prognosis in terms of visual acuity. On the other hand, MOG-Ab-positive optic neuritis responds favorably to steroid therapy but is likely to recur when the dosage of steroids is reduced or discontinued. We first present the high incidence of idiopathic optic neuritis and discuss these relatively newer disease concepts of AQP4-Ab-positive optic neuritis and MOG-Ab-positive optic neuritis.

Novel insights into pathophysiology and therapeutic possibilities reveal further differences between AQP4-IgG- and MOG-IgG-associated diseases

PURPOSE OF REVIEW

This review summarizes recent insights into the pathogenesis and therapeutic options for patients with MOG- or AQP4-antibodies.

RECENT FINDINGS

Although AQP4-IgG are linked to NMOSD, MOG-IgG-associated diseases (MOGAD) include a broader clinical spectrum of autoimmune diseases of the central nervous system (CNS). Details of membrane assembly of AQP4-IgG required for complement activation have been uncovered. Affinity-purified MOG-IgG from patients were shown to be pathogenic by induction of demyelination when the blood--brain barrier (BBB) was breached and by enhancement of activation of cognate T cells. A high-affinity AQP4-IgG, given peripherally, could induce NMOSD-like lesions in rats in the absence of BBB breach. Circulating AQP4-specific and MOG-specific B cells were identified and suggest differences in origin of MOG-antibodies or AQP4-antibodies. Patients with MOG-IgG show a dichotomy concerning circulating MOG-specific B cells; whether this is related to differences in clinical response of anti-CD20 therapy remains to be analyzed. Clinical trials of AQP4-IgG-positive NMOSD patients showed success with eculizumab (preventing cleavage of complement factor C5, thereby blocking formation of chemotactic C5a and membrane attack complex C9neo), inebilizumab (depleting CD19 + B cells), and satralizumab (anti-IL-6R blocking IL-6 actions).

SUMMARY

New insights into pathological mechanisms and therapeutic responses argue to consider NMOSD with AQP4-IgG and MOGAD as separate disease entities.

Experimental mouse model of NMOSD produced by facilitated brain delivery of NMO-IgG by microbubble-enhanced low-frequency ultrasound in experimental allergic encephalomyelitis mice

Although optic neuritis and myelitis are the core clinical characteristics of neuromyelitis optica spectrum disorders (NMOSD), appropriate animal models of NMOSD with myelitis and optic neuritis are lacking. we developed a mouse model of NMOSD by intravenously injecting 100 µg neuromyelitis optica immunoglobulin G antibody (NMO-IgG) and complement into experimental allergic encephalomyelitis (EAE) mice after reversible blood-brain barrier (BBB) opening by microbubble-enhanced low-frequency ultrasound (MELFUS). Animals were assessed by histopathology. We found noticeable inflammation and demyelination concomitant with the loss of aquaporin-4 (AQP4) and glial fibrillary acidic protein (GFAP) expression in the spinal cord, brain and optic nerve, as well as human IgG and C9neo deposition. Thus, with the help of MELFUS, we established an NMOSD mouse model with the core lesions of NMOSD by applying a considerably lower dose of human NMO-IgG, which may help identify the pathogenesis and facilitate the development of other neuroimmune disease models in the future.

Neuroprotective effect of microglia against impairments of auditory steady-state response induced by anti-P IgG from SLE patients in naïve mice

OBJECTIVE

Autoantibodies against ribosomal P proteins (anti-P antibodies) are strongly associated with the neuropsychiatric manifestations of systemic lupus erythematosus (NPSLE). The present study was designed to assess whether anti-P antibodies can induce abnormal brain electrical activities in mice and investigate the potential cytopathological mechanism.

METHODS

Affinity-purified human anti-ribosomal P antibodies were injected intravenously into mice after blood-brain barrier (BBB) disruption. The auditory steady-state response (ASSR) was evaluated based on electroencephalography (EEG) signals in response to 40-Hz click-train stimuli, which were recorded from electrodes implanted in the skull of mice. Immunofluorescence staining was used to examine the morphology and density of neurons and glia in the hippocampus and cortex. The presence of apoptosis in the brain tissues was studied using the TUNEL assay. A PLX3397 diet was used to selectively eliminate microglia from the brains of mice.

RESULTS

Circulating anti-P antibodies caused an enhancement of the ASSR and the activation of microglia through the disrupted BBB, while no obvious neural apoptosis was observed. In contrast, when microglia were depleted, anti-P antibodies induced a serious reduction in the ASSR and neural apoptosis.

CONCLUSION

Our study indicates that anti-P antibodies can directly induce the dysfunction of auditory-evoked potentials in the brain and that microglia are involved in the protection of neural activity after the invasion of anti-P antibodies, which could have important implications for NPSLE.

Aquaporin-4 Autoantibodies From Neuromyelitis Optica Spectrum Disorder Patients Induce Complement-Independent Immunopathologies in Mice

Neuromyelitis optica spectrum disorders (NMOSD) are central nervous system inflammatory disorders causing significant morbidities and mortality. The majority of NMOSD patients have autoimmunity against aquaporin-4 (AQP4), evidenced by seropositivity for autoantibodies against aquaporin-4 (AQP4–IgG). AQP4–IgG is pathogenic with neuroinflammation initiated upon binding of AQP4–IgG to astrocytic AQP4. Complement activation contributes to astrocytic cytotoxicity, neuroinflammation, and tissue necrosis in NMOSD, but the role of complement-independent mechanisms is uncertain. We studied the complement-independent pathogenic effects of AQP4–IgG by passive transfer of IgG from NMOSD patients to mice with breached blood–brain barrier (BBB). Mice, pretreated with bacterial proteins, received daily intraperitoneal injections of IgG purified from AQP4–IgG-seropositive NMOSD patients [IgG_(AQP4+)], or IgG from AQP4–IgG-seronegative patients [IgG_(AQP4−)] or healthy subjects [IgG_(Healthy)] for 8 days. Motor function was tested by walking across narrow beams, and spinal cords were collected for immunofluorescent analysis. We found that human IgG infiltrated into cord parenchyma of mice with breached BBB without deposition of complement activation products. Spinal cord of mice that received IgG_(AQP4+) demonstrated loss of AQP4 and glial fibrillary acidic protein (suggestive of astrocyte loss), decrease in excitatory amino acid transporter 2, microglial/macrophage activation, neutrophil infiltration, patchy demyelination, and loss in axonal integrity. Mice that received IgG_(AQP4+) required longer time with more paw slips to walk across narrow beams indicative of motor slowing and incoordination. Our findings suggest that AQP4–IgG induces complement-independent cord pathologies, including astrocytopathy, neuroinflammation, demyelination, and axonal injuries/loss, which are associated with subtle motor impairments. These complement-independent pathophysiologies likely contribute to early NMOSD lesion development.

Neuromyelitis optica study model based on chronic infusion of autoantibodies in rat cerebrospinal fluid

Background

Devic’s neuromyelitis optica (NMO) is an autoimmune astrocytopathy, associated with central nervous system inflammation, demyelination, and neuronal injury. Several studies confirmed that autoantibodies directed against aquaporin-4 (AQP4-IgG) are relevant in the pathogenesis of NMO, mainly through complement-dependent toxicity leading to astrocyte death. However, the effect of the autoantibody per se and the exact role of intrathecal AQP4-IgG are still controversial.

Methods

To explore the intrinsic effect of intrathecal AQP4-IgG, independent from additional inflammatory effector mechanisms, and to evaluate its clinical impact, we developed a new animal model, based on a prolonged infusion of purified immunoglobulins from NMO patient (IgG^AQP4+, NMO-rat) and healthy individual as control (Control-rat) in the cerebrospinal fluid (CSF) of live rats.

Results

We showed that CSF infusion of purified immunoglobulins led to diffusion in the brain, spinal cord, and optic nerves, the targeted structures in NMO. This was associated with astrocyte alteration in NMO-rats characterized by loss of aquaporin-4 expression in the spinal cord and the optic nerves compared to the Control-rats (p = 0.001 and p = 0.02, respectively).

In addition, glutamate uptake tested on vigil rats was dramatically reduced in NMO-rats (p = 0.001) suggesting that astrocytopathy occurred in response to AQP4-IgG diffusion. In parallel, myelin was altered, as shown by the decrease of myelin basic protein staining by up to 46 and 22 % in the gray and white matter of the NMO-rats spinal cord, respectively (p = 0.03). Loss of neurofilament positive axons in NMO-rats (p = 0.003) revealed alteration of axonal integrity. Then, we investigated the clinical consequences of such alterations on the motor behavior of the NMO-rats. In a rotarod test, NMO-rats performance was lower compared to the controls (p = 0.0182). AQP4 expression, and myelin and axonal integrity were preserved in AQP4-IgG-depleted condition. We did not find a major immune cell infiltration and microglial activation nor complement deposition in the central nervous system, in our model.

Conclusions

We establish a link between motor-deficit, NMO-like lesions and astrocytopathy mediated by intrathecal AQP4-IgG. Our study validates the concept of the intrinsic effect of autoantibody against surface antigens and offers a model for testing antibody and astrocyte-targeted therapies in NMO.

Electronic supplementary material

The online version of this article (doi:10.1186/s12974-016-0577-8) contains supplementary material, which is available to authorized users.

An association of Aquaporin-4 with the immunoregulation of liver pathology in mice infected with Schistosoma japonicum

Background

Schistosomiasis is a chronic parasitic disease that affects approximately 200 million people. In Schistosomiasis japonica and mansoni, parasite eggs were trapped in host liver and stimulated the CD4⁺T cell responses to regulate the formation of the granulomas. Subsequently, excessive granulomatous response in some heavily, and/or repeatedly infected individuals could result in chronic liver fibrosis and circulatory impairment. Thus, elucidation of the mechanisms of these responses will not only provide more information to better understand the mechanisms of the immunoregulation in schistosomiasis, but also help to design new therapies to control granuloma-associated immunopathology. The role of aquaporin-4 (AQP4) in water transport has been extensively investigated in the central nervous system (CNS). Recently, studies have shown that AQP4 expresses in immune system and lack of AQP4 in mice results in significantly less CD4⁺CD25⁺ T regulatory cells (Treg cells) under physiological condition, one of the subpopulations of CD4⁺T cells which restrains immunopathology in hosts with schistosomiasis. However, little information exists regarding the contribution of AQP4 to the immune regulation in schistosome infection.

Methods

The liver granulomatous response in S. japonicum-infected AQP4 knockout (KO) mice and its wild-type (WT) littermates were detected by staining liver sections with hematoxylin and eosin. The generation of various CD4⁺ T subsets, including Th1, Th2, Th17, and Treg cells were analyzed by flow cytometry. In addition, the levels of total IgG, IgG1, IgG2a in serum of infected mice were detected by ELISA assay.

Results

Our results showed an enhanced granulomatous response with increased accumulation of eosinophils and macrophages around eggs in the liver of AQP4 KO mice with Schistosomiasis japonica. In addition, our study demonstrated enhanced Th2 but reduced Th1 and Treg cells generation in AQP4 KO mice with Schistosomiasis japonica, which may, at least partly, account for the enhancement of the liver granuloma formation.

Conclusion

Our study for the first time provides evidences that AQP4 has an association with the immunoregulation of the liver granuloma formation, which may confer a new option for schistosomiasis treatment.

Astrocytes differentially respond to inflammatory autoimmune insults and imbalances of neural activity

BACKGROUND

Neuronal activity intimately communicates with blood flow through the blood-brain barrier (BBB) in the central nervous system (CNS). Astrocyte endfeet cover more than 90% of brain capillaries and interact with synapses and nodes of Ranvier. The roles of astrocytes in neurovascular coupling in the CNS remain poorly understood.

RESULTS

Here we show that astrocytes that are intrinsically different are activated by inflammatory autoimmune insults and alterations of neuronal activity. In the progression of experimental autoimmune encephalomyelitis (EAE), both fibrous and protoplasmic astrocytes were broadly and reversibly activated in the brain and spinal cord, indicated by marked upregulation of glial fibrillary acidic protein (GFAP) and other astrocytic proteins. In early and remitting EAE, upregulated GFAP and astrocytic endfoot water channel aquaporin 4 (AQP4) enclosed white matter lesions in spinal cord, whereas they markedly increased and formed bundles in exacerbated lesions in late EAE. In cerebellar cortex, upregulation of astrocytic proteins correlated with EAE severity. On the other hand, protoplasmic astrocytes were also markedly activated in the brains of ankyrin-G (AnkG) and Kv3.1 KO mice, where neuronal activities are altered. Massive astrocytes replaced degenerated Purkinje neurons in AnkG KO mice. In Kv3.1 KO mice, GFAP staining significantly increased in cerebellar cortex, where Kv3.1 is normally highly expressed, but displayed in a patchy pattern in parts of the hippocampus.

CONCLUSIONS

Thus, astrocytes can detect changes in both blood and neurons, which supports their central role in neurovascular coupling. These studies contribute to the development of new strategies of neuroprotection and repair for various diseases, through activity-dependent regulation of neurovascular coupling.

Origins and significance of astrogliosis in the multiple sclerosis model, MOG peptide EAE

Astroglia, the most abundant cells in the human CNS, and even more prominent in multiple sclerosis patients, participate in CNS innate and adaptive immunity, and have been hypothesized to play an important role in multiple sclerosis progression. Experimental autoimmune encephalomyelitis elicited in mice by immunization with myelin oligodendrocyte glycoprotein peptide 35-55 provides a means by which to explore the genesis and disease significance of astrogliosis during a chronic immune-mediated CNS inflammatory/demyelinative disorder that, in its' pathological features, strongly resembles multiple sclerosis.

Analyses of haptoglobin level in the cerebrospinal fluid and serum of patients with neuromyelitis optica and multiple sclerosis

BACKGROUND

Neuromyelitis optica (NMO), which was previously considered a variant of multiple sclerosis (MS), is characterized by recurrent optic neuritis and longitudinally extensive spinal cord lesions. It has been shown that the level of haptoglobin in cerebrospinal fluid (CSF) is elevated in NMO. However, it is uncertain whether this change is specific to NMO, or is also seen in MS and other neurological diseases.

METHODS

We used an enzyme-linked immunosorbent assay (ELISA) to measure the haptoglobin levels in the CSF and serum in 25 NMO, 16 MS, and 15 Alzheimer's disease (AD) patients and 22 controls.

RESULTS

The CSF haptoglobin concentration of the NMO patients (0.309±0.074mg/dl, P<0.001) was significantly higher than that of MS patients (0.081±0.016mg/dl) and AD patients (0.058±0.011mg/dl), and the controls (0.060±0.009mg/dl), whereas the serum haptoglobin and albumin concentrations in the serum and CSF did not differ significantly across groups. NMO patients (0.59±0.15, P=0.001) demonstrated a higher haptoglobin index than MS patients (0.13±0.01), AD patients (0.12±0.03), and the controls (0.17±0.04). Furthermore, the haptoglobin concentration and haptoglobin index in the CSF correlated significantly with the expanded disability scale score (EDSS) in NMO patients.

CONCLUSIONS

The high CSF haptoglobin concentration in NMO may be explained by increased intrathecal haptoglobin synthesis. The correlation between CSF haptoglobin concentration/haptoglobin index and EDSS highlights the potential of haptoglobin as a biomarker of NMO.