AQP4 and neuromyelitis optica: In general, antigen-specific T cells recognize linear peptide fragments of 10–15 amino acids in association with MHC (HLA) proteins expressed on APC.13 To identify AQP4-specific T cells in NMO patients, we initially tested proliferation of PBMC to a library of 32 synthetic overlapping 15-mer and 20-mer peptides encompassing the 323-amino acid sequence of full-length human AQP4 (M1 isoform).