THP-1 monocytic leukemia cells express Fas ligand constitutively and kill Fas-positive Jurkat cells

Normal human monocytes express Fas and are susceptible to Fas ligand (FasL)-induced apoptosis. Because the myeloid leukemia cell lines HL-60 and THP-1 can be differentiated into functional monocytes and macrophages, we studied their expression of Fas and Fas ligand (FasL) to determine whether there were differentiation-associated changes in these proteins. THP-1, HL-60 and HCW-2, both before and after treatment with PMA, expressed high levels of Fas ligand (FasL), but did not express Fas. The FasL expressed by THP-1 cells was functional as measured by their ability to kill Jurkat T-cells by apoptosis. The THP-1 Fas gene appears to be silent, because bacterial lipopolysaccharide (LPS) induced Fas expression in fully differentiated THP-1 cells. Our results suggest that FasL expression by leukemia cells may account in part for the pathophysiology of myeloid leukemia, and that PMA-differentiated THP-1 cells, while possessing many of the functional properties of normal macrophages, are abnormal with respect to a major apoptotic pathway.