Relative expression of IFNα (A), IFNβ (B), IFNγ (C) and TNF-α (D) mRNA of SJPL and MARC-145 cells induced by PRRSV. MARC-145 and SJPL cells were infected at 0.5 MOI with PRRSV IAF-Klop strain or transfected with poly (I:C) as a positive control or treated with LPS as an IFNγ inducer. mRNA relative expression of IFNα (A), IFNβ (B), IFNγ (C) and TNF-α (D) was measure at 72 hrs by qRT-PCR in PRRSV infected or poly (I:C) treated cells. Values are presented as ± standard deviation (SD). When 2 sets of data within a cell type are labeled with superscripts of different letters or when only one set is labeled with a superscript, it indicates that these 2 sets of data are statistically different (P < 0.05). When 2 sets of data from different cell types are labeled with asterisk, it indicates that these 2 sets of data are statistically different (* P < 0.05, ** P < 0.01, *** P < 0.001).