T 1500 rpm for 6 min at 4 , washed twice with 2 mL PBS + 2 BSA, and resuspended in 50 of PBS + 2 BSA solution for blocking. Antibodies were added to corresponding tubes and incubated in the dark at 4 for 45 min. Cells were removed from incubation, washed again twice with PBS + 2 BSA, and resuspended in PBS containing 0.5 formaldehyde. Cell populations were gated on a minimum of 250,000 ce