The presence of free thiols was investigated using Ellman’s assay. DTNB (Sigma Aldrich) was first dissolved in reaction buffer (0.1 M sodium phosphate, pH 7.4, containing 1 mM EDTA) at a concentration of 4 mg/ml. Then peptides and recombinant BTLA-Fc, alone or mixed together (ratio 1:1) were added in DTNB reaction buffer. After 15 minutes of incubation, the SH content of protein samples was then assessed by the measurement of absorbance at 412 nm. For each sample, the absorbance value from the blank was subtracted.