Increased T-cell immunity against aquaporin-4 and proteolipid protein in neuromyelitis optica

In neuromyelitis optica (NMO), B-cell autoimmunity to aquaporin-4 (AQP4) has been shown to be essential. However, the role of T cells remains ambiguous. Here, we first showed an increase in CD69+ activated T cells in PBMCs during NMO relapses. Next, T-cell responses to AQP4 and myelin peptides were studied in 12 NM0 patients, 10 multiple sclerosis (MS) patients and 10 healthy subjects (HS). Four hours after adding 1 of 28 overlapping AQP4 peptides, a mixture of AQP4 peptides (AQP4-M) or one of six distinct myelin peptides to 2-day cultured PBMC, CD69 expression on CD4+ T cells was examined. Data were analyzed by paired t-test, frequency of samples with 3-fold increase of CD69 on CD4+ cells (fSI3) and mean stimulation index (mSI). The T-cell response to AQP4-M was significantly increased in NMO (fSI3 = 10/12, mSI = 5.50), with AQP4 (11-30) and AQP4 (91-110) representing the two major epitopes (AQP4 (11-30), fSI3 = 11/12, mSI = 16.0 and AQP4 (91-110), fSI3 = 11/12, mSI = 13.0). Significant but less extensive responses to these two epitopes were also observed in MS and HS. Significant reactivities against AQP4 (21-40), AQP4 (61-80), AQP4 (101-120), AQP4 (171-190) and AQP4 (211-230) were exclusively found in NMO. In addition, responses to AQP4 (81-100) were higher and more frequently detected in NMO, without reaching statistical significance. Interestingly, among the six myelin peptides studied, proteolipid protein (95-116) induced a significant T-cell response in NMO (fSI3 = 7/12, mSI = 4.60). Our study suggests that cellular as well as humoral responses to AQP4 are necessary for NMO development and that the immune response to myelin protein may contribute to disease pathogenesis.