Figure 5.

Cell fusion induced by trypsin treatment. A, trypsin activated cell fusion in ST cells but not in IPI-2I cells. ST and IPI-2I cells were infected with PDCoV or transfected with a PDCoV S-expressing plasmid with or without trypsin, and virus infection was detected by IFA with a mouse anti-PDCoV N protein mAb. Untreated or uninfected cells were used as controls. B, cell fusion was semi-quantified by counting the number of nuclei in the syncytia in ST cells following infection or transfection with trypsin treatment. C, ST cells were pretreated with Baf-A1, then infected with PDCoV (m.o.i. = 0.1) in the presence 10 μg/ml trypsin or trypsin and 40 μg/ml SBTI (Sigma) for 24 h. PDCoV-positive cells were detected by IFA. D, ST cells were infected with PDCoV to express S protein in the cells. The cells were collected and lysed by sonication, and then incubated with various concentrations of trypsin followed by Western blot analysis with S polyclonal antibody. Two extra bands (150 and 50 kDa) were detected after trypsin treatment. Data are expressed as the mean ± S.D. for triplicate samples. Level of significance was determined by Student's t test. ns, p > 0.05; ***, p < 0.001.