Characterization of the recombinant PRRSV expressing the GM-CSF gene in MARC-145 cells. (A) Detection of the GM-CSF gene insertion in the recombinant virus. F was product of the parental virus; the numbers show the passages of the recombinant viruses; M was Marker DL2000. The recombinant viruses used in B, C and D were passaged in MARC-145 cells for 20 passages. (B) Detection of GM-CSF protein expression in recombinant virus-infected MARC-145 cells by IFA. Original Magnification 200×. (C) Growth kinetics comparison between rHuN4-GM-CSF and the parental virus in MARC-145 cells. The infection was done as mentioned above. The cell supernatants were harvested at the indicated time points and titrated in MARC-145 cells. (D) Plaque assays for the parental virus and rHuN4-GM-CSF. MARC-145 cells infected with viruses were stained with crystal violet at 4 days post infection.