CTL lysis efficiency and CD38/HLA-DR expression in HAM/TSP subjects treated with VPA. (A) Before and after VPA treatment (eg, days −6, +29, and +42 are illustrated), the rate of CD8⁺ cell-mediated lysis of HTLV-1–infected cells was estimated as described previously.⁹  CD8⁺ lymphocytes were selected by MACS and titrated back into the CD8-depleted fraction at different ratios. Reconstituted cell populations were cocultivated at 37°C for 18 hours, fixed, and analyzed by FACS for Tax, CD4, and CD8 expression. The proportion of Tax⁺ CD4⁺ cells surviving coculture was plotted against the proportion of CD8⁺ cells present. Two independent experiments were performed with 3 HAM/TSP patients 1 through 3. (B) As previously described, a mathematical model was then used to analyze the data.⁹  The model describes the onset of Tax expression in CD4⁺ cells and the lysis of Tax⁺CD4⁺ cells by CD8⁺ cells. The model was solved analytically and then fitted to the data using nonlinear regression. The rate of lysis of CD4⁺ Tax⁺ cell, “CD8⁺ cell lysis efficiency,” is estimated. CD8⁺ cell lysis efficiency (expressed as the proportion of Tax-expressing CD4⁺ cells killed per CD8⁺ cell per day) was calculated for each HAM/TSP patient tested. All assays were done in duplicate, and the results are presented as the mean CD8⁺ cell lytic efficiency. Indicated values result from experiments performed in duplicate at days −6, +10, +15, +29, +35, +42, and +56. (C) The median absolute change in lysis efficiency between consecutive time points was plotted against initial rate of lysis efficiency for control patients (♦) and VPA treated patients (□). (D) Expression of CD38 and HLA-DR in CD4⁺ cells before (at month −1: upper plots) and after (at month +3: lower plots) initiation of VPA treatment. PBMCs were labeled with the MultiTEST CD4 FITC/CD38 PE/CD3 peridinin chlorophyll protein/anti–HLA-DR allophycocyanin and analyzed with a FACSAria (BD Biosciences). After gating of CD3⁺ cells (left panels: peridinin chlorophyll protein), the percentages of FITC-labeled CD4⁺ cells expressing either CD38 (middle plots: PE) or HLA-DR (right panels: allophycocyanin) were determined. (E) FACS analysis of PBMCs isolated from 5 patients before (month −1: M-1) and after (at 3 months: M3) initiation of VPA treatment: percentages of CD4⁺ or CD8⁺ (CD3⁺ CD4⁻) cells expressing CD38 or HLA-DR.