Briefly, ~2 × 107 cells were incubated with CFSE (Molecular Probes, Eugene, Oregon, USA) at the pre-tested nontoxic concentration of 1 mM for 10 min at 37 °C and washed twice with 10 mL of 5 % fetal calf serum (FCS; GIBCO-Invitrogen Corporation, Auckland, New Zealand).