Excreted/secreted products from Taenia solium metacestodes cultured in vitro were analyzed for peptidase activity using peptide substrates Z-Phe- Arg-AFC, Arg-AFC, and Z-Gly-Gly-Arg-AFC and zymography studies. Specific inhibitor profiles revealed mainly cysteine and metalloprotease activities. Hydrolysis of substrate Z-Phe-Arg-AFC was augmented by the addition of L- cysteine and acid pH, consistent with cysteine protease activity. Cysteine protease activity was more prominent in supernatants from living metacestodes cultured in PBS than in either RPMI or RPMI plus fetal calf serum and was proportional to the number of metacestodes. Flow cytometry analysis showed depletion of human T lymphocytes cultured with living T. solium metacestodes. CD4+ expression was significantly decreased when metacestode E/S products and L-cysteine were added to lymphocyte cultures (P = 0.027). This peptidase activity was inhibited by E-64 indicating that the depletion of CD4+ cells was due to cysteine protease activity. Thus, T. solium metacestodes produce excretory/secretory proteases. These enzymes may cleave molecules critical for the host immune response allowing the parasites to survive in the host tissues. (C) 2000 Academic Press.