Peptide affinity assay using NHS-activated Sepharose microcolumn was already described elsewhere [36, 37]. The peptides were added onto BTLA-Sepharose column equilibrated in ammonium hydrogen carbonate (NH4HCO3, pH 7.4) and incubated for 2 h at 25°C with gentle shaking. After this time the unbound peptide was removed and the column was washed with 100 ml NH4HCO3 and 20 ml of H2O. Dissociation of the protein—peptide complex was performed using 0.1% TFA in H2O (2 x 0.5 ml) by 15 minutes. All fractions were analyzed using mass spectrometry (MALDI TOF/TOF™ 5800, AB SCIEX).