The pathogenic relationship between neuromyelitis optica (NMO) and multiple sclerosis (MS) continues to be debated despite mounting evidence that these are distinct entities.1- 3 The NMO-IgG, which targets the water channel protein aquaporin-4 (AQP4), is the first confirmed serum biomarker for any form of central nervous system inflammatory demyelinating disease and reliably distinguishes NMO from MS and other neurological diseases. Four immunopathological patterns (IP I-IV) have been described in early active MS lesions.4 Some investigators have interpreted humoral MS IP II as having an immunopathogenic link with NMO because both share complement and immunoglobulin deposition and have a greater likelihood than other forms of MS to respond favorably to plasma exchange therapy.4,5 Here, we describe the NMO-IgG status of a cohort of patients with biopsy-proven early active lesions consistent with MS.
All patients had biopsy-proven central nervous system inflammatory demyelination consistent with MS (n = 85; 43 women), were classified immunohistochemically (IP I-IV),4 were enrolled into the MS lesion project (National Multiple Sclerosis Society RG 3185), and, at the last visit, were bled to test serum for NMO-IgG using indirect immunofluorescence.
Enrolled patients had a spectrum of disease courses (relapsing remitting, 57; monophasic, 20; primary progressive, 0; secondary progressive, 7; and uncertain, 1 patient). The median age at biopsy was 38 years (interquartile range, 27-46 years), expanded disability status scale score at biopsy was 2.0 (interquartile range, 1.0-3.5), and disease duration prior to biopsy was 1.4 months (interquartile range, 0.6-4.9 months). At the last follow-up (median, 3.6 years from disease onset), 85% of patients were defined as having definite (n = 65) or probable MS (n = 7). The remaining 13 patients (15%) were classified as having a clinically isolated syndrome.
The distribution of immunopathological patterns for the patients was as follows: IP I, n = 15; IP II, n = 53; IP III, n = 17; IP IV, n = 0. All were seronegative for NMO-IgG. Eighteen of 85 patients (22%) had received 1 or more immunosuppressant medication or plasma exchange in the 3-month period prior to blood draw (intravenous methylprednisolone, 6; short-term prednisone, 6; long-term prednisone, 5; methotrexate, 1; cyclophosphamide, 1; mitoxantrone, 2; and plasma exchange, 2). A minority (45%) were receiving immunomodulatory medications (interferon beta or glatiramer acetate) in the 3-month period prior to blood draw.
This study shows a lack of detectable NMO-IgG in pathologically confirmed patients with MS including those with evidence of humoral MS pathology (IP II), supporting the idea that MS and NMO are distinct clinicopathologic entities. The low percentage of patients receiving immunosuppressant therapy at the time of blood draw minimizes the potential for treatment to have been a determinant of seronegativity. Although immune complex deposition is a characteristic immunopathological finding for both NMO and MS IP II, the distribution of immune complexes is distinct.3 In MS IP II lesions, complement is deposited at sites of active myelin destruction with increased AQP4 expression, even in regions of immune complex deposition (Figure, A and B). In active NMO lesions, complement is deposited in a unique rim and rosettelike pattern corresponding to sites of concentrated AQP4 expression on the astrocytic foot process, and AQP4 expression is notably absent at sites of immune complex deposition (Figure, C and D). In vitro, NMO-IgG binds selectively to target cell membranes expressing AQP4 and causes complement activation or rapid downregulation of AQP4 expression,6 implicating a complement-activating AQP4-specific autoantibody in the pathogenesis of NMO lesions. Thus, NMO-IgG is not an epiphenomenon of antibody-mediated pathology, but rather a specific biomarker that reliably distinguishes NMO-related disorders from MS and plausibly allows the classification of NMO as an autoimmune channelopathy selectively targeting the central nervous system.
Comparison of C9neo and aquaporin 4 (AQP4) immunoreactivity in multiple sclerosis pattern II and neuromyelitis optica active lesions. A and B, Pattern II active multiple sclerosis brain lesion is shown; immunoreactivity to C9neo antigen is noted within myelin-laden macrophages (arrows), but absent around blood vessels (arrowhead) (A). This corresponds to a region of increased AQP4 immunoreactivity, with prominent staining of the cytoplasmic surface and processes of reactive astrocytes (B). C and D, Active neuromyelitis optica spinal cord lesion is shown. The C9neo antigen is deposited in a vasculocentric rim and rosette pattern similar to the normal distribution of AQP4 (C); C9neo reactivity is associated with a complete loss of AQP4 immunoreactivity in this same region (D). Immunocytochemistry for C9neu antigen is red; AQP4, brown.
Correspondence: Dr Lucchinetti, Department of Neurology, Mayo Clinic College of Medicine, 200 First St SW, Rochester, MN 55905 (email@example.com).
Author Contributions:Study concept and design: Pittock and Lucchinetti. Acquisition of data: Kale, Pittock, McKeon, and Lucchinetti. Analysis and interpretation of data: Kale, Pittock, Lennon, Thomsen, Roemer, and Lucchinetti. Drafting of the manuscript: Pittock. Critical revision of the manuscript for important intellectual content: Kale, Pittock, Lennon, Thomsen, Roemer, McKeon, and Lucchinetti. Statistical analysis: Thomsen. Obtained funding: Pittock. Administrative, technical, and material support: Pittock. Study supervision: Pittock and Lucchinetti.
Financial Disclosure: All coauthors have seen and agreed with the contents of the manuscript. In accordance with the Bayh-Dole Act of 1980 and Mayo Foundation policy, Drs Lennon and Lucchinetti report that they stand to receive royalties for intellectual property related to the aquaporin-4 autoantigen. This intellectual property is licensed to a commercial entity for development of a simple antigen-specific assay to be made available worldwide for patient care. The test will not be exclusive to Mayo Clinic. To date, the authors have received a total of less than $10 000 in royalties. Mayo Clinic offers the test as an indirect immunofluorescence assay to aid the diagnosis of neuromyelitis optica, but the authors do not benefit personally from the performance of the test. In addition, Drs Pittock, Lennon, and Lucchinetti are named inventors on a patent application filed by Mayo Foundation for Medical Education and Research that relates to neuromyelitis optica (aquaporin-4) antibody and its application to functional assays. The remaining authors report no financial disclosures.
Funding/Support: This study was supported by grants RO1-NS049577-01-A2 from the National Institutes of Health and NMSS RG 3185-B-3 from the National Multiple Sclerosis Society (Dr Lucchinetti); the Ralph C. Wilson Medical Research Foundation (Dr Lennon); and the Guthy Jackson Foundation (Drs Kale, Pittock, and Lucchinetti).
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