After incubation,
the supernatant was replaced
with 100 ll of medium containing 10% of 5 mg/ml MTT (Sigma)
solution and incubated for an additional 4 h. The supernatant
was removed and 100 ll of solubilising solution (1% Triton X-
100, 0.1 N HCl in isopropanol) was added. The absorbance at
570 nm was measured by ELISA reader with a reference wavelength
at 690 nm. All experiments were performed in triplicate.