Interaction of the human serine protease inhibitor alpha-1-antitrypsin with Cryptosporidium parvum.

The protozoan parasite Cryptosporidium parvum was studied for interaction with a human serine protease inhibitor (serpin), alpha-1-antitrypsin (AAT). A C. parvum homogenate (CPH) prepared from oocysts was incubated with purified human AAT and complexes formed between the serpin and CPH were detected using an enzyme-linked immunosorbent assay (ELISA). The optical density read at 450 nm of AAT:CPH reactivity was significantly increased (P < 0.001) relative to CPH in the absence of AAT treatment. Additionally, ELISA reactivity was blocked by incubating AAT with a cognate target enzyme, porcine pancreatic elastase (PPE), prior to treatment of the CPH. Incubation of a partially excysted sample of C. parvum with AAT (37 C x 60 min) demonstrated preferential fluorescence labeling of sporozoites by indirect immunofluorescence assay; AAT complexes were not detected on intact oocysts. Localization of AAT interactions with C. parvum sporozoites was visualized by transmission immunoelectron microscopy. Collectively, these data suggest that C. parvum sporozoites express a protease-like component that is recognized by human AAT. The ability to block ELISA reactivity with PPE suggests that the AAT interactions we detected are functionally similar to the serpin-enzyme complex AAT forms with a protease target.