AUTOCRINE PRODUCTION OF EXTRACELLULAR CATALASE PREVENTS APOPTOSIS OF THE HUMAN CEM T-CELL LINE IN SERUM-FREE MEDIUM

CCRF-CEM is a human T-cell line originally isolated from a child with acute lymphoblastic leukemia. At cell densities > 2 x 10(5) cells per ml, CEM cells grow in serum-free medium, but at lower cell densities the cultures rapidly undergo apoptosis, or programmed cell death. The viability of low-density CEM cells could be preserved by supplementing the serum-free medium with ''conditioned'' medium from high-density CEM cultures, but a variety of known growth factors and lymphokines were ineffective. Fractionation of conditioned medium by sequential chromatography on DEAE-cellulose, propyl agarose, chromatofocusing, and hydrophobic-interaction HPLC resulted in the isolation of a 60-kDa protein capable of sustaining CEM growth in the absence of serum. The active protein was identified as human catalase based on its amino acid sequence and composition and was subsequently shown to exhibit catalase activity and to be replaceable by human erythrocyte catalase or bovine liver catalase. Comparison of the level of intracellular catalase activity with the amount released into the culture medium demonstrated that the latter accounted for <3% of the total catalase activity present in the cell culture. These findings show that, despite its low amount, the catalase released by CEM cells, and perhaps by T cells in general, provides a critical first line of defense against hydrogen peroxide (H2O2) present in the extracellular milieu.