sentence: After this time , 1x10 lymphocytes were washed twice to remove IL-2 , and plated in RPMI 1640 with 12 % FBS ( negative control ) , with BKPyV peptide pool ( 2 μ g / mL ) , or PMA ( 1 μ g / μ L ) and Ionomycin ( 5 μ g / μ L ) as positive control , at 37 ° C for 6 hours .