The larvae of Schistosoma mansoni invade their mammalian host by utilizing a
serine protease, cercarial
elastase (SmCE), to degrade macromolecular
proteins in host skin. The catalytic activity of
serine and
cysteine proteases can be regulated after activation by
serpins. SmSrpQ, one of two S. mansoni
serpins found in larval secretions, is only expressed during larval development and in the early stages of mammalian
infection. In vitro, (35)S-SmSrpQ was able to form an SDS-stable complex with a component of the larval lysate, but no complex was detected when (35)S-SmSrpQ was incubated with several mammalian host
proteases. Formation of a complex was sensitive to the
protease active site inhibitors PMSF, Z-AAPF-CMK, and Z-AAPL-CMK. Western blot analysis of parasite lysates from different life stages detected a complex of comparable size to SmCE bound to SmSrpQ using anti-SmSrpQ or anti-SmCE
antibodies. SmSrpQ and SmCE are located in adjacent but discrete compartments in the secretion glands of the parasite. Fluorescence immunohistochemical analysis of simulated
infection showed co-localization of SmCE and SmSrpQ in host tissue suggesting a post release regulation of parasite
protease activity during skin transversal. The results of this study suggest that cercarial
elastase degradation of skin tissue is carefully regulated by SmSrpQ.