CD107a expression in patients with HAM/TSP and in HVCs after coculture with immunodominant Tax peptide. To determine the functional reactivity of HTLV-1 Tax–specific CD8⁺ T cells, we performed a CD107a mobilization assay. PBMCs derived from HLA-A*02–positive HTLV-1–infected patients (patients with HAM/TSP and HVCs) were stained with anti–CD8-PC5 and HLA-A*02/Tax11-19 tetramer after incubation with Tax11-19 peptide and anti-CD107a monoclonal antibody for 4 hours. (A) Left panel: Dot plots of HLA-A*02/Tax11-19 tetramer-PE (x axis) versus CD8-PC5 (y axis) fluorescence within the lymphocyte gate based on forward versus side scatter. Significant anti-CD107a staining was observed on the Tax-tetramer–positive population after coculture with Tax11-19 peptide (left panel), whereas only background staining was seen without peptide (center panel). (B) Comparison of degranulation marker CD107a expression in Tax-tetramer–positive cells between 10 patients with HAM/TSP and 14 HVCs. The percentage of CD107a⁺ Tax11-19 tetramer–positive cells/ Tax11-19 tetramer–positive cells was significantly decreased in patients with HAM/TSP compared with HVCs (P = .003, Mann Whitney). (C) The CD107a staining was still significantly lower in patients with HAM/TSP than in HVCs compared between PVL-matched groups (P = .006, Mann-Whitney). Horizontal bars represent the median value of percent CD107a⁺ Tax11-19 tetramer–positive cells/Tax11-19 tetramer–positive cells.