Antibody to aquaporin-4 in the long-term course of neuromyelitis optica

Abstract

Neuromyelitis optica (NMO) is a severe inflammatory CNS disorder of putative autoimmune aetiology, which predominantly affects the spinal cord and optic nerves. Recently, a highly specific serum reactivity to CNS microvessels, subpia and Virchow-Robin spaces was described in patients with NMO [called NMO-IgG (NMO-immunoglobulin G)]. Subsequently, aquaporin-4 (AQP4), the most abundant water channel in the CNS, was identified as its target antigen. Strong support for a pathogenic role of the antibody would come from studies demonstrating a correlation between AQP4-Ab (AQP4-antibody) titres and the clinical course of disease. In this study, we determined AQP4-Ab serum levels in 96 samples from eight NMO-IgG positive patients (median follow-up 62 months) in a newly developed fluorescence-based immunoprecipitation assay employing recombinant human AQP4. We found that AQP4-Ab serum levels correlate with clinical disease activity, with relapses being preceded by an up to 3-fold increase in AQP4-Ab titres, which was not paralleled by a rise in other serum autoantibodies in one patient. Moreover, AQP4-Ab titres were found to correlate with CD19 cell counts during therapy with rituximab. Treatment with immunosuppressants such as rituximab, azathioprine and cyclophosphamide resulted in a marked reduction in antibody levels and relapse rates. Our results demonstrate a strong relationship between AQP4-Abs and clinical state, and support the hypothesis that these antibodies are involved in the pathogenesis of NMO.

Fig. 1

AQP4-Ab levels during relapse and remission. (A) Median AQP4-Ab levels from 57 samples stratified according to disease activity (P < 0.0001; Mann Whitney test). (B) Maximum AQP4-Ab serum levels from 11 samples taken during relapses in six patients and Nadir values during subsequent remission (P = 0.001; Wilcoxon matched-pairs signed-rank test).

Fig. 2

AQP4-Ab levels, CD19 cell counts (% of total lymphocytes), relapses and immunosuppressive treatment over time in eight patients with NMO. Time points are selected to illustrate the relationship between AQP4-Ab, relapses and therapies. (A and J) Pat.1; (B) Pat.3; (C) Pat.2; (D and H) Pat. 7; (E) Pat. 8; (F) Pat. 6; (G) Pat. 5; (I) Pat. 4. See results section for details. ♦ = AQP4-Ab serum levels;  = CD19 cell counts;  = clinical relapse;  = intravenous methylprednisolone;  = prednisolone;  = azathioprine;  = dexamethasone;  = rituximab;  = cyclophosphamide;  = mitoxantrone;  = plasma exchange; eod = every other day.

Fig. 2

AQP4-Ab levels, CD19 cell counts (% of total lymphocytes), relapses and immunosuppressive treatment over time in eight patients with NMO. Time points are selected to illustrate the relationship between AQP4-Ab, relapses and therapies. (A and J) Pat.1; (B) Pat.3; (C) Pat.2; (D and H) Pat. 7; (E) Pat. 8; (F) Pat. 6; (G) Pat. 5; (I) Pat. 4. See results section for details. ♦ = AQP4-Ab serum levels;  = CD19 cell counts;  = clinical relapse;  = intravenous methylprednisolone;  = prednisolone;  = azathioprine;  = dexamethasone;  = rituximab;  = cyclophosphamide;  = mitoxantrone;  = plasma exchange; eod = every other day.

Fig. 3

Increase in AQP4-Ab levels during relapse is not paralleled by a rise in other autoimmune autoantibodies. ♦ = AQP4-Ab;  = AChR-Ab;  = TG-Ab;  = TPO-Ab;  = clinical relapse;  = intravenous methylprednisolone;  = prednisolone;  = azathioprine;  = cyclophosphamide;  = mitoxantrone.

Fig. 4

AQP4-Ab serum levels before and after (values refer to Nadir values) eight applications of rituximab in four patients with NMO/LETM (P = 0.0156; Wilcoxon matched-pairs rank sum test).