3.1  Determination of the Optimal Inoculation Dose for PDCoV
To obtain the optimal inoculation dose that could produce DEPs to the greatest extent, IPEC-J2 cells were inoculated with PDCoV at low, medium, and high MOIs (i.e., 0.01, 0.1, and 1 TCID50/cell, respectively) for 36 h before the formal experiment. By observing the CPE, we found that the high MOI resulted in a rapid synchronous infection of all the cells, causing majority of the cells detached and disintegrated within 12 hpi. This is not conducive to the interaction between the virus and the cells. By contrast, the cells inoculated with the low MOI only displayed minimal CPEs at the same time points, which might induce limited changes in the expression levels of diverse proteins. Therefore, the medium MOI of 0.1 was selected as the optimal dose for PDCoV inoculation in all subsequent experiments.