Cells isolation
Whole blood was collected from newborn piglets, 40-day-old pigs or 120-day-old pigs. Peripheral blood mononuclear cells (PBMCs) were isolated by a density centrifugation using a porcine peripheral blood lymphocyte separation kit (Solarbio) and cultured in RPMI 1640 medium with 10 % FBS (Gibco) and 1% penicillin/streptomycin (20 ng/mL) at 37°C with 5 % CO2. Red blood cells (RBCs) were separated as a centrifuged pellet after purification of PBMCs were washed three times in Dulbecco’s phosphate saline (lacking Ca2+and Mg2+, DPBS) and stored in Red Cell Storaging Solution (Solarbio) at 4 °C. To isolate T cells, PBMCs were labeled with APC-CD3 antibody (BD Biosciences), incubated with anti-APC microbeads and sorted using MiniMACS Starting kits. Vero E6 cells were kindly provided by the Veterinary Medicine Research Center of the Da Bei Nong Group.