Updated estimate of AQP4-IgG serostatus and disability outcome in neuromyelitis optica

Abstract

Objective: To 1) determine, using contemporary recombinant antigen-based assays, the aquaporin-4 (AQP4)-immunoglobulin G (IgG) detection rate in sequential sera of patients assigned a clinical diagnosis of neuromyelitis optica (NMO) but initially scored negative by tissue-based indirect immunofluorescence (IIF) assay; and 2) evaluate the impact of serostatus on phenotype and outcome.

Methods: From Mayo Clinic records (2005-2011), we identified 163 patients with NMO; 110 (67%) were seropositive by IIF and 53 (33%) were scored seronegative. Available stored sera from 49 "seronegative" patients were tested by ELISA, AQP4-transfected cell-based assay, and in-house fluorescence-activated cell sorting assay. Clinical characteristics were compared based on final serostatus.

Results: Thirty of the 49 IIF-negative patients (61%) were reclassified as seropositive, yielding an overall AQP4-IgG seropositivity rate of 88% (i.e., 12% seronegative). The fluorescence-activated cell sorting assay improved the detection rate to 87%, cell-based assay to 84%, and ELISA to 79%. The sex ratio (female to male) was 1:1 for seronegatives and 9:1 for seropositives (p < 0.0001). Simultaneous optic neuritis and transverse myelitis as onset attack type (i.e., within 30 days of each other) occurred in 32% of seronegatives and in 3.6% of seropositives (p < 0.0001). Relapse rate, disability outcome, and other clinical characteristics did not differ significantly.

Conclusions: Serological tests using recombinant AQP4 antigen are significantly more sensitive than tissue-based IIF for detecting AQP4-IgG. Testing should precede immunotherapy; if negative, later-drawn specimens should be tested. AQP4-IgG-seronegative NMO is less frequent than previously reported and is clinically similar to AQP4-IgG-seropositive NMO.

Figure 1. Flowchart of the NMO cohort

Of 163 patients whose serum was tested (one or more sequential specimens) by tissue-based IIF, 53 were scored negative. For 49 with available stored specimens, 30 (61%) were found to be AQP4-IgG positive when retested by recombinant antigen–based assays. AQP4 = aquaporin-4; CBA = cell-based assay; FACS = fluorescence-activated cell sorting assay; IgG = immunoglobulin G; IIF = indirect immunofluorescence; NMO = neuromyelitis optica; QNS = insufficient serum for further evaluation.

Figure 2. Kaplan-Meier estimates of time to motor and visual disability by AQP4-IgG serostatus

(A) Years from onset to use of a cane (p = 0.43): at 5 years after onset, 28% of seronegative patients and 22% of seropositive patients were expected to need a cane to walk (EDSS score 6). (B) Years from onset to need for a wheelchair (p = 0.10): at 5 years after onset, no seronegative and 8% of seropositive patients were expected to be unable to walk and to need a wheelchair (EDSS score 8). (C) Years from onset to legal blindness in at least one eye (p = 0.34): at 5 years after onset, 57% of seronegative and 41% of seropositive patients were expected to be legally blind in at least one eye. (D) Years from onset to legal blindness in both eyes (p = 0.64): at 5 years after onset, 5% of seronegative and 9% of seropositive patients were expected to be legally blind in both eyes. (E) Years from onset to use of a cane for seropositive NMO (p = 0.07): at 5 years after disease onset, 36% TM-onset vs 14% ON-onset patients were expected to need a cane. (F) Years from onset to legal blindness in at least one eye for seropositive NMO (p = 0.09): at 5 years after disease onset, 39% of TM-onset vs 55% of ON-onset patients were expected to be legally blind in at least one eye. EDSS score 6 = intermittent or unilateral assistance (canes, crutches, or braces) required to walk 100 m with or without resting; EDSS 8 = restricted to bed or chair or perambulated in wheelchair but may be out of bed much of day, retains many self-care functions, generally has effective use of arms; legal blindness = sustained visual acuity of 20/200 or less with best correction possible for more than 6 months. AQP4 = aquaporin-4; EDSS = Expanded Disability Status Scale; IgG = immunoglobulin G; NMO = neuromyelitis optica; ON = optic neuritis; TM = transverse myelitis.