Heparinized blood was obtained from healthy donors under a protocol approved by the University of Rochester Medical Center Research Subjects Review Board. Written, informed consent was obtained from all subjects. PBMC were isolated by Ficoll-Hypaque (Cellgro, Herndon, VA) density gradient centrifugation. Cells were suspended in complete RPMI-8 (RPMI-1640 medium containing 100U penicillin/streptomycin (Invitrogen) supplemented with 8% heat-inactivated fetal calf serum (FCS, HyClone, Logan, UT)). In the experiment treating cells with adenosine, serum-free medium X-VIVO TM 20 (Lonza, Walkersville, MD) was used.