Efficient suilysin-mediated invasion and apoptosis in porcine respiratory epithelial cells after streptococcal infection under air-liquid interface conditions

Meng, Fandan; Wu, Nai-Huei; Seitz, Maren; Herrler, Georg; Valentin-Weigand, Peter

To find out whether suilysin was involved in the apoptosis induced by S. suis, we analyzed colocalization of cleaved caspase-3 with suilysin by confocal microscopy. As shown in Fig. 6B , in wt and complemented strain cS148 infected PBEC, suilysin (in green) was detected by immunostaining and colocalized with cells that had been also stained for cleaved caspase-3 (in red). High density of membrane-associated suilysin was detectable on apoptotic cells, which is consistent with the localization of suilysin-positive wt streptococci in invasion areas described above (Fig. 6A ). As expected, no suilysin and only few cleaved caspase-3 positive cells were detected in PBEC infected by the suilysin-deficient mutant 10Δ sly. Interestingly, accumulated membrane-associated suilysin was detected in the complemented strain cW461F infected PBEC, but this accumulated suilysin did not induce increased apoptotic cells (as shown in Fig. 6C ). Quantification of apoptotic cells revealed that suilysin-positive S. suis wt strain and complemented strain cS148 induced a 40-fold and 35-fold increase of apoptotic cells compared to mock infection of PBEC, respectively. Increases were significantly higher than those caused by the suilysin-deficient mutant 10Δ sly (5-fold) and complemented strain cW461F (8-fold). There was no difference in increase of apoptotic cells between the latter two strains. These results indicate that suilysin induces apoptosis in PBEC and suggest that this effect requires macropore-formation activity of suilysin.