Fig. 1. Protein, activity and mRNA levels of cathepsin X in stably transfected Jurkat T lymphocytes (A) Protein levels of cathepsin X in the cytosol of resistant clones as determined by ELISA. The highest cathepsin X productivity was observed in clone 17, which was selected for further studies. (B) The molar level of cathepsin X in stably transfected Jurkat T lymphocytes with the highest productivity was 24-fold higher compared with wild-type cells. The mean levels obtained for wild-type Jurkat T lymphocytes (1.5 ng of cathepsin X per mg of total cell proteins) have been normalised to a relative concentration of 1.0. (C) Cathepsin X activity was determined by DCG-04 active-site labelling and the identity of cathepsin X in the cytosol was confirmed by immunoblotting. Activity of cathepsin X was increased in cathepsin-X-overexpressing T lymphocytes compared with wild-type cells. (D) Cathepsin X expression on the cell surface was unchanged after cathepsin X overexpression, as measured by flow cytometry. The geometric mean was 4.24 for wild-type and 4.23 for cathepsin-X-overexpressing T lymphocytes.