Cathepsin S inhibitor- or saline-treated Tregs did not affect mouse bladder tumor growth. Tumor mice received inhibitor-treated Tregs had reduced splenic and tumor Treg number and total cell proliferation. Inhibitor-treated Tregs affected the proliferation and apoptosis of tumor cells and splenic lymphocytes differently from saline-treated Treg. Tumor cell conditioned media affected inhibitor-treated Treg activity in regulating lymphocyte proliferation and apoptosis.