On Blastocystis secreted cysteine proteases: a legumain-activated cathepsin B increases paracellular permeability of intestinal Caco-2 cell monolayers.

Blastocystis spp. pathogenic potential remains unclear as these anaerobic parasitic protozoa are frequently isolated from stools of both symptomatic and asymptomatic subjects. In silico analysis of the whole genome sequence of Blastocystis subtype 7 revealed the presence of numerous proteolytic enzymes including cysteine proteases predicted to be secreted. To assess the potential impact of proteases on intestinal cells and gut function, we focused our study on two cysteine proteases, a legumain and a cathepsin B, which were previously identified in Blastocystis subtype 7 culture supernatants. Both cysteine proteases were produced as active recombinant proteins. Activation of the recombinant legumain was shown to be autocatalytic and triggered by acidic pH, whereas proteolytic activity of the recombinant cathepsin B was only recorded after co-incubation with the legumain. We then measured the diffusion of 4-kDa FITC-labelled dextran across Caco-2 cell monolayers following exposition to either Blastocystis culture supernatants or each recombinant protease. Both Blastocystis culture supernatants and recombinant activated cathepsin B induced an increase of Caco-2 cell monolayer permeability, and this effect was significantly inhibited by E-64, a specific cysteine protease inhibitor. Our results suggest that cathepsin B might play a role in pathogenesis of Blastocystis by increasing intestinal cell permeability.