Endogenous shRNA in MTCL conferred natural resistance to FMDV infection. Eight cell lines and normal IBRS-2 cells were challenged with 20 TCID50 of FMDV O/HKN/2002 (A), 50 TCID50 of FMDV O/HKN/2002 (B) or 20 TCID50 of FMDV Asia 1/Jiangsu/2005 (C). At 24 hpc (left column) and 48 hpc (middle column), cells were observed under a microscope. Mock challenged cells from the same cell lines at 48 hpc were used as controls (right column). Normal IBRS-2 cells form an epithelial-like monolayer. These cells round up, separate, and finally undergo cell lysis (CPE) when infected by FMDV. The MTCL IB32-2 and IB32-3 exhibited significant antiviral capacity with considerable delayed CPE. IB32-6 had the strongest anti-FMDV capacity and was undamaged by all three challenges. The images were adjusted for contrast and dpi to improve their appearance, and the magnification for all images is 200 ×.