Complement expression on astrocytes and astrocytoma cell lines: failure of complement regulation at the C3 level correlates with very low CD55 expression

Classical Complement Pathway Inhibition in a "Human-On-A-Chip" Model of Autoimmune Demyelinating Neuropathies

Chronic autoimmune demyelinating neuropathies are a group of rare neuromuscular disorders with complex, poorly characterized etiology. Here we describe a phenotypic, human-on-a-chip (HoaC) electrical conduction model of two rare autoimmune demyelinating neuropathies, chronic inflammatory demyelinating polyneuropathy (CIDP) and multifocal motor neuropathy (MMN), and explore the efficacy of TNT005, a monoclonal antibody inhibitor of the classical complement pathway. Patient sera was shown to contain anti-GM1 IgM and IgG antibodies capable of binding to human primary Schwann cells and induced pluripotent stem cell derived motoneurons. Patient autoantibody binding was sufficient to activate the classical complement pathway resulting in detection of C3b and C5b-9 deposits. A HoaC model, using a microelectrode array with directed axonal outgrowth over the electrodes treated with patient sera, exhibited reductions in motoneuron action potential frequency and conduction velocity. TNT005 rescued the serum-induced complement deposition and functional deficits while treatment with an isotype control antibody had no rescue effect. These data indicate that complement activation by CIDP and MMN patient serum is sufficient to mimic neurophysiological features of each disease and that complement inhibition with TNT005 was sufficient to rescue these pathological effects and provide efficacy data included in an investigational new drug application, demonstrating the model's translational potential.

C5a-Preactivated Neutrophils Are Critical for Autoimmune-Induced Astrocyte Dysregulation in Neuromyelitis Optica Spectrum Disorder

Neuromyelitis optica spectrum disorder (NMOSD) is an autoimmune neuroinflammatory disease. In contrast to multiple sclerosis, autoantibodies against aquaporin-4 (AQP4) expressed on astrocytic end-feet have been exclusively detected in sera of NMOSD patients. Several lines of evidence suggested that anti-AQP4 autoantibodies are pathogenic, but the mechanism triggering inflammation, impairment of astrocyte function, and the role of neutrophils presented in NMOSD cerebrospinal fluid remains unknown. In this study, we tested how human neutrophils affect astrocytes in the presence of anti-AQP4 Ab-positive serum derived from NMOSD patients. An in vitro model of inflammation consisted of human astrocyte line, NMOSD serum, and allogenic peripheral blood neutrophils from healthy individuals. We showed evidence of pathogenicity of NMOSD serum, which by consecutive action of anti-AQP4 Abs, complement system, and neutrophils affected astrocyte function. Anti-AQP4 Ab binding astrocytes initiated two parallel complementary reactions. The first one was dependent on the complement cytotoxicity via C5b-9 complex formation, and the second one on the reverse of astrocyte glutamate pump into extracellular space by C5a-preactivated neutrophils. As a consequence, astrocytes were partially destroyed; however, a major population of astrocytes polarized into proinflammatory cells which were characterized by pathological glutamate removal from extracellular space.

Longitudinally extensive NMO spinal cord pathology produced by passive transfer of NMO-IgG in mice lacking complement inhibitor CD59

Spinal cord pathology with inflammatory, demyelinating lesions spanning three or more vertebral segments is a characteristic feature of neuromyelitis optica (NMO). NMO pathogenesis is thought to involve binding of immunoglobulin G anti-aquaporin-4 autoantibodies (NMO-IgG) to astrocytes, causing complement-dependent cytotoxicity (CDC) and secondary inflammation, demyelination and neuron loss. We investigated the involvement of CD59, a glycophosphoinositol (GPI)-anchored membrane protein on astrocytes that inhibits formation of the terminal C5b-9 membrane attack complex. CD59 inhibition by a neutralizing monoclonal antibody greatly increased NMO-IgG-dependent CDC in murine astrocyte cultures and ex vivo spinal cord slice cultures. Greatly increased NMO pathology was also found in spinal cord slice cultures from CD59 knockout mice, and in vivo following intracerebral injection of NMO-IgG and human complement. Intrathecal injection (at L5-L6) of small amounts of NMO-IgG and human complement in CD59-deficient mice produced robust, longitudinally extensive white matter lesions in lumbar spinal cord. Pathology was most severe at day 2 after injection, showing loss of AQP4 and GFAP, C5b-9 deposition, microglial activation, granulocyte infiltration, and demyelination. Hind limb motor function was remarkably impaired as well. There was partial remyelination and recovery of motor function by day 5. Our results implicate CD59 as an important modulator of the immune response in NMO, and provide a novel animal model of NMO that closely recapitulates human NMO pathology. Up-regulation of CD59 on astrocytes may have therapeutic benefit in NMO.

Experimental mouse model of optic neuritis with inflammatory demyelination produced by passive transfer of neuromyelitis optica-immunoglobulin G

Background

Although optic neuritis (ON) is a defining feature of neuromyelitis optica (NMO), appropriate animal models of NMO ON are lacking. Most NMO patients are seropositive for immunoglobulin G autoantibodies (NMO-IgG) against the astrocyte water channel aquaporin-4 (AQP4).

Methods

Several approaches were tested to develop a robust, passive-transfer mouse model of NMO ON, including NMO-IgG and complement delivery by: (i) retrobulbar infusion; (ii) intravitreal injection; (iii) a single intracranial injection near the optic chiasm; and (iv) 3-days continuous intracranial infusion near the optic chiasm.

Results

Little ON or retinal pathology was seen using approaches (i) to (iii). Using approach (iv), however, optic nerves showed characteristic NMO pathology, with loss of AQP4 and glial fibrillary acidic protein immunoreactivity, granulocyte and macrophage infiltration, deposition of activated complement, demyelination and axonal injury. Even more extensive pathology was created in mice lacking complement inhibitor protein CD59, or using a genetically modified NMO-IgG with enhanced complement effector function, including significant loss of retinal ganglion cells. In control studies, optic nerve pathology was absent in treated AQP4-deficient mice, or in wild-type mice receiving control (non-NMO) IgG and complement.

Conclusion

Passive transfer of NMO-IgG and complement by continuous infusion near the optic chiasm in mice is sufficient to produce ON with characteristic NMO pathology. The mouse model of NMO ON should be useful in further studies of NMO pathogenesis mechanisms and therapeutics.

AQP4 antibodies in neuromyelitis optica: diagnostic and pathogenetic relevance

Antibodies to aquaporin-4 (also known as AQP4-Ab or NMO-IgG) are sensitive and highly specific serum markers of autoimmune neuromyelitis optica (NMO). Second-generation recombinant diagnostic assays can detect AQP4-Ab in >or=80% of patients with NMO, and a role for AQP4-Ab in the pathophysiology of this condition was corroborated by a series of in vitro studies that demonstrated disruption of the blood-brain barrier, impairment of glutamate homeostasis and induction of necrotic cell death by AQP4-Ab-positive serum. Additional evidence for such a role has emerged from clinical observations, including the demonstration of a correlation between serum levels of AQP4-Ab and disease activity. The finding of NMO-like CNS lesions and clinical disease following passive transfer of AQP4-Ab-positive serum in several independent animal studies provided definitive proof for a pathogenic role of AQP4-Ab in vivo. Together, these findings provide a strong rationale for the use of therapies targeted against B cells or antibodies in the treatment of NMO. In this Review, we summarize the latest evidence in support of a direct involvement of AQP4-Ab in the immunopathogenesis of NMO, and critically appraise the diagnostic tests currently available for the detection of this serum reactivity.

Effector functions of antiaquaporin-4 autoantibodies in neuromyelitis optica

NMO-IgG autoantibody is now considered a useful serum biomarker of neuromyelitis optica (NMO). A series of clinical and pathological observations suggest that NMO-IgG may play a central role in NMO physiopathology. Although the exact role and function of NMO-IgG in vivo remain to be determined, recent reports suggest that this antibody that targets the aquaporin-4 (AQP-4) water channel expressed by astrocytes is able to destabilize the blood-brain barrier and contribute to astrocyte and oligodendrocyte damage. This review summarizes the latest molecular effector functions attributed to the NMO-IgG/anti-AQP4 antibody and discusses their potential role in NMO pathology.

Neurological autoimmunity targeting aquaporin-4

Neuromyelitis optica (NMO) is the first inflammatory autoimmune demyelinating disease of the CNS for which a specific tissue target molecule has been identified--the astrocytic water channel aquaporin-4 (AQP4). Immunological insights have propelled significant advances in understanding the clinical, radiologic and immunopathologic characteristics of the disease in the last 5 years. In this review, we describe features distinguishing CNS AQP4 autoimmunity from classical multiple sclerosis (MS). In NMO, disease attacks preferentially involve the optic nerves and spinal cord (hence the name), but neurological signs in the initial attack of AQP4 autoimmunity in children commonly involve the brain. A clinically validated serum biomarker, NMO-IgG, distinguishes relapsing CNS inflammatory demyelinating disorders related to NMO from MS. The NMO-IgG autoantibody is AQP4-specific. Clinical, radiological and immunopathological data support its role in the pathogenesis of NMO spectrum disorders. Lesions characteristic of NMO are distinct from MS: AQP4 and its coupled glutamate transporter, excitatory amino acid transporter 2 (EAAT2), are lost, with and without associated myelin loss, IgG, IgM and complement are deposited in a vasculocentric pattern, edema and inflammation are prominent. In vitro studies demonstrate that binding of NMO-IgG to astrocytic AQP4 initiates multiple potentially neuropathogenic mechanisms: complement activation, AQP4 and EAAT2 downregulation with disruption of water and glutamate homeostasis, enhanced blood-brain barrier permeability, plasma protein and granulocyte influx, and antibody-dependent cell-mediated cytotoxicity. Development of effective, and potentially curative, therapies requires validated models of the disease, in animals and cell culture systems.

Anti-aquaporin-4 auto-antibodies orchestrate the pathogenesis in neuromyelitis optica

NMO-IgG, the auto-antibody specific to the aquaporin-4 (AQP4) water channel associated with the autoimmune inflammatory disease neuromyelitis optica (NMO), is considered to be an accurate serum biomarker and is thought to be an important contributor to NMO pathology. In this review, we summarize recent evidences from our group and others indicating that NMO-IgG can be implicated at several levels in the immuno-pathology of NMO. NMO-IgG/anti-AQP4 antibodies may compromise the integrity of the blood-brain barrier and consequently facilitate and enhance the perivascular inflammation characteristic of NMO. Lastly, NMO-IgG can induce astrocyte injury which may lead to the accumulation of excitatory/toxic molecules and accordingly damage oligodendrocytes and compromise myelin integrity.

Human oligodendroglial cells express low levels of C1 inhibitor and membrane cofactor protein mRNAs

BACKGROUND: Oligodendrocytes, neurons, astrocytes, microglia, and endothelial cells are capable of synthesizing complement inhibitor proteins. Oligodendrocytes are vulnerable to complement attack, which is particularly observed in multiple sclerosis. This vulnerability may be related to a deficiency in their ability to express complement regulatory proteins. METHODS: This study compared the expression level of complement inhibitor mRNAs by human oligodendrocytes, astrocytes and microglia using semi-quantitative RT-PCR. RESULTS: Semi-quantitative RT-PCR analysis showed that C1 inhibitor (C1-inh) mRNA expression was dramatically lower in oligodendroglial cells compared with astrocytes and microglia. The mRNA expression level of membrane cofactor protein (MCP) by oligodendrocytes was also significantly lower than for other cell types. CONCLUSION: The lower mRNA expression of C1-inh and MCP by oligodendrocytes could contribute to their vulnerability in several neurodegenerative and inflammatory diseases of the central nervous system.

Complement components of the innate immune system in health and disease in the CNS

The innate immune system and notably the complement (C) system play important roles in host defense to recognise and kill deleterious invaders or toxic entities, but activation at inappropriate sites or to an excessive degree can cause severe tissue damage. C has been implicated as a factor in the exacerbation and propagation of tissue injury in numerous diseases including neurodegenerative disorders. In this article, we review the evidence indicating that brain cells can synthesise a full lytic C system and also express specific C inhibitors (to protect from C activation and C lysis) and C receptors (involved in cell activation, chemotaxis and phagocytosis). We also summarise the mechanisms involved in the antibody-independent activation of the classical pathway of C in Alzheimer's disease, Huntington's disease and Pick's disease. Although the primary role of C activation on a target cell is to induce cell lysis (particularly of neurons), we present evidence indicating that C (C3a, C5a, sublytic level of C5b-9) may also be involved in pro- as well as anti-inflammatory activities. Moreover, we discuss evidence suggesting that local C activation may contribute to tissue remodelling activities during repair in the CNS.

The role of complement in Alzheimer's disease pathology

Complement proteins are integral components of amyloid plaques and cerebral vascular amyloid in Alzheimer brains. They can be found at the earliest stages of amyloid deposition and their activation coincides with the clinical expression of Alzheimer's dementia. This review will examine the origins of complement in the brain and the role of beta-amyloid peptide (Abeta) in complement activation in Alzheimer's disease, an event that might serve as a nidus of chronic inflammation. Pharmacology therapies that may serve to inhibit Abeta-mediated complement activation will also be discussed.

Expression of the murine complement regulatory protein crry by glial cells and neurons

The expression of the murine complement regulatory protein, Crry, in the CNS remains largely unexplored. In this study, we examined murine astrocytes and microglia purified from neonatal brain and sections of adult murine brain for the expression of Crry. Using RT-PCR, immunohistochemistry, in situ hybridization, flow cytometry, and Western blot analysis, we demonstrated that astrocytes and microglia express Crry protein and RNA. Crry expression is greater on microglia than astrocytes and, as determined by Western blot analysis, each cell type expresses a Crry protein of different molecular weight. Interestingly, neuronal expression of Crry was seen only at the RNA level. These data demonstrate Crry expression by astrocytes, microglia, and neurons in the murine CNS and suggest that Crry may play an important role in protecting the CNS against complement-mediated damage.