Isolate and epitope variability in susceptibility of Giardia lamblia to intestinal proteases

The surface antigens of Giardia lamblia differ. To determine whether the unique surface antigens found in variants and isolates could differentially protect the parasite from digestion by intestinal protease, G. lamblia clones WB-2X (WB), GS/M-H7 (GS/M), and B6, each of which expresses a unique surface variant antigen, were exposed to alpha-chymotrypsin and trypsin at concentrations up to 20 mg/ml in culture medium. The number of surviving trophozoites and morphologic changes were assessed over time. After 24 h, there was a significant decrease in the number of surviving trophozoites of WB (80.5 and 94.2% for trypsin and alpha-chymotrypsin treatments, respectively, compared with controls) and B6 (78.9 and 95.5% for trypsin and alpha-chymotrypsin treatments, respectively, compared with controls) at 10 mg of enzyme per ml compared with culture medium alone. Cytotoxicity was prevented by the presence of soybean trypsin inhibitor, indicating the effects were due to protease activity. In contrast, there was no significant cytotoxicity after exposure of GS/M to either enzyme at the same enzyme concentration. After exposure to alpha-chymotrypsin, susceptible G. lamblia became rounded and then lysed, but after exposure to trypsin, G. lamblia appeared plastered onto the surface of the well and was intertwined and surrounded by finely granular material. Effects were concentration and time dependent; at least 6 h of treatment was required to observe changes 12 to 18 h later. Trophozoites surviving alpha-chymotrypsin or trypsin exposure became stably resistant to protease treatment. In vitro, the variant surface antigen of GS/M, but not those of WB or B6, resisted digestion by trypsin or alpha-chymotrypsin, suggesting that the variant surface antigens impart susceptibility or resistance to digestion. The initial surface variant antigens of WB and B6 were replaced in resistant cultures. Trophozoites differ in their ability to survive after exposure to intestinal proteases, which may enable certain G. lamblia isolates or isolates possessing certain surface variant antigens to survive in the small intestine.