mAb-PN9cx3 blocks the binding of PRRSV virions to PAMs. A Sandwich ELISA for measuring the virion numbers in cell culture supernatants (100 μL) of PRRSV-infected MARC-145 cells (SD16 strain, 106.5 TCID50/mL) or an equal volume of fresh DMEM containing 10% FBS. Either mAb-PN9cx3 or IgG isotype control (mAb-2G8) was used as the coating antibody for virion capture. PRRSV convalescent-phase swine serum (SD16 strain) or swine serum negative for anti-PRRSV antibodies was used for the detection of captured PRRSV virions. All the experiments were repeated at least three times. The significant differences between different groups are marked by asterisks: ** (p < 0.01). B mAb-PN9cx3 inhibited PRRSV virion attachment to PAMs. A mixture of PRRSV virions (SD16 strain) at an MOI of 0.1 and 1 μM mAb-PN9cx3 was incubated for 1 h at 37 °C, prechilled on ice and added to PAMs (1 × 106), and the cells were then incubated for 2 h at 4 °C, washed three times with cooled PBS, and harvested for RT-qPCR analysis to determine the PRRSV-RNA levels. PAMs inoculated with virus alone (without mAb-PN9cx3) were included as a control. The experiment was repeated at least three times. The significant differences between two groups are marked by asterisks: * (p < 0.05).