Confocal photomicrographs illustrating PRV(89V87) plaques at 20 h pi in (a) mock-treated and (b) Soybean Type I-S trypsin inhibitor-treated porcine nasal respiratory mucosa explants. Green fluorescence visualizes PRV antigens in virus plaques using FITC-conjugated polyclonal PRV-specific antibodies. Collagen IV (component of basement membrane (BM), black dashed line) is visualized using goat anti-collagen IV antibodies, stained afterwards with TexasRed. Figure (a) shows a representative image of two closely apposed viral plaques both invading through the BM. Figure (b) illustrates the blocked viral invasion underneath the BM in the presence of a trypsin-like protease inhibitor. Bar, 20 μm.