a nonamer peptide derived from PRSSV M protein at 20 μg/ml, or phorbol-12-myristate-13-acetate (PMA 10 ng/ mL) and ionomycin (500 ng/mL) as a positive control. Cells incubated in medium alone or with a peptide derived from the infectious bronchitis virus (IBV) H52 strain [17] were used as negative controls. Following a 12 h incubation at 37°C, 10 uM monensin (Sigma) was added and the splenocytes were incubated for an additional 4 h at 37°C before staining.