Optic neuritis (ON) is characterized by acute or subacute painful loss of central vision that may progress over a period of 7–10 days (1). ON may be associated with acute inflammatory demyelinating diseases, such as multiple sclerosis (MS) or neuromyelitis optica (NMO) (1–3).
NMO-IgG, a serum immunoglobulin autoantibody (NMO-Ab) directed against aquaporin-4 (AQP4), is a pathological effector in NMO. It binds to astrocytic transmembrane protein AQP4, the predominant water channel in the central nervous system (CNS) (4–6). Expression of NMO-Ab in the brain, spinal cord, and optic nerve is associated with astrocyte membranes that closely appose endothelial cell basal membranes (7). NMO-Ab titers correlate with disease activity in NMO relapse in some patients followed longitudinally (8). In patients with recurrent ON, NMO-Ab seropositivity predicts poor visual outcome and development of other neurologic involvement, particularly transverse myelopathy (9).
Myelin oligodendrocyte glycoprotein (MOG) is a minor myelin antigen and constitutes only 0.05% of CNS myelin proteins compared to 30% for myelin basic protein (MBP) and 50% for proteolipid protein (PLP) (10). Administration of anti-MOG antibodies (MOG-Ab) into CNS tissue leads to extensive demyelination in animal models (11). Humoral immunity against MOG correlates with demyelinating activity in chronic relapsing experimental autoimmune encephalomyelitis (EAE) (12). The relevance of MOG as a T-cell autoantigen was investigated by comparing the response to MOG of peripheral blood cells from MS patients and control individuals, and the results implicated the possibility of MOG as a primary target myelin antigen in the pathogenesis of MS (13). The pathophysiological significance of MOG-specific autoantibodies in MS remains controversial, but study in a primate model of MOG-induced EAE showed that demyelination is mediated by MOG-specific antibodies directed against conformational rather than linear MOG epitopes, suggesting functional segregation of pathogenic vs nonpathogenic anti-MOG antibodies in human disease (14).
We investigated the relationship between NMO-Ab and MOG-Ab in ON patients to determine if there was any relationship between the presence of these antibodies and visual outcome.
Patients and Blood Samples
Patients with a diagnosis of ON were recruited who fulfilled the following criteria: 1) tested for NMO-Ab and MOG-Ab between 2007 and 2010 at the Tokyo Medical University; 2) had at least 2 documented episodes of ON separated by 1 month before NMO-Ab and MOG-Ab measurements were obtained; and 3) were not being treated with corticosteroid or other medications known to affect the immune system. Diagnosis of ON was based on acute or subacute onset vision loss, sluggish pupillary response, visual field abnormality, and contrast enhancement of the affected optic nerve on MRI. Venous heparinized blood samples were obtained from patients and healthy volunteer control subjects. The ON group comprised 33 eyes of 23 consecutive outpatients (3 males and 20 females; mean age, 45.7 ± 12.8 years), and the control group consisted of 8 healthy adults. The study was approved by the Tokyo Medical University Institutional Review Board under numbers 1026 and 1262.
Treatment of ON
The patients were treated with 2 courses of corticosteroid pulse therapy. This consisted of 1,000 mg per day of methylprednisolone administered intravenously for 3 days given at a 1-week interval, followed by oral prednisone. Plasma exchange was performed by double-filtration plasmapheresis and only in patients who did not respond to pulse steroid therapy.
NMO-Ab was measured using the AQP4 antigen-presenting cells established by Tanaka et al (15). Total RNA was extracted from an adult human cerebellum from a donor bank, and DNA-encoding human aquaporin-4 (AQP4 M23 isoform; GenBank accession number U63623) was cloned by reverse transcription–polymerase chain reaction technique. Full-length complementary DNA was inserted into the XbaI site of a pEFBOS expression vector and used to transfect HEK 293 cells. The transfected HEK 293 cells were then fixed in 4% paraformaldehyde in 0.1 mM phosphate buffered saline (PBS), pH 7.4. Nonspecific binding was blocked with 10% goat serum in PBS. The cells were incubated with the patient serum for 60 minutes at room temperature and then incubated with fluorescein isothiocyanate–conjugated rabbit anti-human IgG (BD Biosciences, San Jose, CA). A SlowFade Gold antifade reagent (Molecular Probes, Carlsbad, CA) was then applied to the slide, and reactivity was verified under a fluorescence microscope.
Anti-human MOG1–125 IgG was measured by the SensoLyte Quantitative ELISA Kit (AnaSpec, Fremont, CA) (16–18). Anti-human MOG1–125 antibody concentration of the serum sample was calculated by 4-parameter logistic curve fit based on the average absorbance values. A serum MOG-Ab concentration higher than 4 standard deviations of the mean concentration in healthy controls is considered seropositive for MOG-Ab (19).
Data are presented as median values (25th to 75th percentiles). Median MOG-Ab concentration in healthy controls was 28.4 ng/mL (22.0–52.6 ng/mL). Since the 99th percentile reference value was used as the cutoff point for abnormal level, patients' samples higher than 73.5 ng/mL of anti-MOG-Ab were considered seropositive. Statistical analysis of enzyme-linked immunosorbent assay data was performed using the Mann–Whitney U test. The differences between seropositive and seronegative groups were analyzed using the Mann–Whitney U test for frequency data.
Our patient cohort consisted of 23 patients (see Table E-1, Supplemental Digital Content 1,http://links.lww.com/WNO/A26) :11 were seropositive and 12 were seronegative for NMO-Ab. Eight patients had seropositive levels of MOG-Ab while 15 did not. Our patient cohort was divided into 4 groups based on seropositivity: NMO-Ab(+)/MOG-Ab(+), NMO-Ab(+)/MOG-Ab(−), NMO-Ab(−)/MOG-Ab(+), and NMO-Ab(−)/MOG-Ab(−) (Table 1).
Median serum MOG-Ab concentration was 88.7 ng/mL (25th to 75th percentiles: 30.1–165.8 ng/mL) in NMO-Ab(+) patients and 36.3 ng/mL (23.6–57.3 ng/mL) in NMO-Ab(−) patients, with no significant difference between the 2 groups (Fig. 1). A significant difference was observed between NMO-Ab(+) patients and controls (P = 0.033) but not between NMO-Ab(−) patients and controls.
We examined the relationship between pretreatment and posttreatment visual acuity in the 4 groups of our ON patients. When comparing improvement in posttreatment visual acuity, a significant difference was observed in NMO-Ab(+)/MOG-Ab(+) patients vs NMO-Ab(−)/MOG-Ab(+) patients (P = 0.03) or vs NMO-Ab(−)/MOG-Ab(−) patients (P = 0.0003) but not vs NMO-Ab(+)/MOG-Ab(−) patients (P = 0.67). A significant difference was also found in NMO-Ab(+)/MOG-Ab(−) vs NMO-Ab(−)/MOG-Ab(+) patients (P = 0.03) or vs NMO-Ab(−)/MOG-Ab(−) patients (P = 0.0089). No significant difference was found in NMO-Ab(−)/MOG-Ab(+) vs NMO-Ab(−)/MOG-Ab(−) patients (P = 0.23).
Next, we investigated whether visual acuity outcome was improved by changing treatment for ON. The patients were treated with 2 courses of corticosteroid pulse therapy. When they did not respond to corticosteroid pulse therapy, plasmapheresis was conducted. We defined favorable visual outcome as improvement by 2 or more Snellen lines. Meaningful statistical analysis was not possible because of small number of patients, but there appeared to be a trend of better response to treatment in NMO-Ab(−)/MOG-Ab(−) patients compared to the NMO-Ab(+)/MOG-Ab(+) group.
NMO-Ab targets astrocytes of the spinal cord and optic nerves (20) and spares the cerebral hemispheres. While patients with NMO develop ON and longitudinal extensive transverse myelitis, the 2 conditions generally manifest at different points in time (21). The optic nerves are particularly more sensitive to volume changes induced by AQP4 dysfunction across the brain–blood barrier than other regions of the CNS (22). ON patients seropositive for NMO-Ab have a poor visual prognosis (23), and 12.5% of recurrent ON cases evolved to NMO (24).
The role of MOG as a T-cell autoantigen in the pathophysiology of demyelinating disease is uncertain. Peripheral blood lymphocytes of patients with MS show a greater proliferative response to MOG compared to other antigens, including MBP and PLP (13). Yet Lim et al (24) found that the presence of MOG-Ab and MBP-Ab in patients did not predict conversion from a clinically isolated syndrome to MS.
The cellular or molecular basis of the relationship between NMO-Ab and MOG-Ab remains unknown. Both MOG and AQP4 antibodies have been detected in cerebrospinal fluid and sera of NMO patients. The CNS cells that express MOG and those that express AQP4 differ. MOG- or AQP4-specific T cells enter the CNS and might recognize cognate antigen presented by endogenous APC at multiple locations. Microglial cells as antigen-presenting cells are capable of “cross-presenting” MOG and AQP4 (7). Thus, it is possible that these 2 antigens play an important role in the induction of NMO.
The reason why visual acuity of ON patients who are NMO-Ab(+)/MOG-Ab(+) have poor visual recovery may be due to involvement of both astrocytes and oligodendrocytes. Further study of this hypothesis requires recruitment and follow-up of a large group of patients with ON.
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