FUNCTIONAL CONSEQUENCES OF APO-1 FAS (CD95) ANTIGEN EXPRESSION BY NORMAL AND NEOPLASTIC HEMATOPOIETIC-CELLS/

Murine monoclonal antibody (mAb) 7C11 binds to the same cell surface e pitope as anti-APO-1 and anti-Fas and reacts specifically with cells t ransfected with a cDNA encoding the human Fas antigen. Furthermore, in cubation with 7C11 causes death of hematopoietic cell lines that expre ss APO-1/Fas but not APO-1/Fas-negative cell lines. 7C11 therefore rec ognizes the human APO-1/Fas (CD95) antigen, a 40 to 50 kDa cell surfac e glycoprotein that can trigger apoptosis or programmed cell death, Ex pression of APO-1/Fas antigen by normal and neoplastic hematopoietic c ells was determined by flow cytometry using 7C11. APO-1/Fas is express ed by similar to 30 to 40% of resting peripheral blood T cells, B cell s, and monocytes and by similar to 5% of resting NK cells and thymocyt es. It was not detected on granulocytes, erythrocytes, or platelets. A pproximately 80 to 90% of activated T cells, B cells, and thymocytes e xpress APO-1/Fas, as do the majority of activated NK cells. Perturbati on of APO-1/Fas by 7C11 does not affect the viability of resting lymph ocytes or monocytes. In contrast, activated T cells and NK cells under go apoptosis within 3 hours of exposure to 7C11. Other mAb that stimul ate T cells or NK cells do not cause rapid induction of programmed cel l death. APO-1/Fas antigen is expressed by many cell lines of lymphoid and myeloid lineage. However, this antigen was detected on neoplastic cells from only one of 69 patients with acute myeloid leukemia, acute lymphoblastic leukemia, chronic myelogenous leukemia, chronic lymphoc ytic leukemia, or multiple myeloma. Only 3 out of 25 tumor samples fro m patients with non-Hodgkin's lymphoma were found to express APO-1/Fas . All three of these lymphomas harbored the bcl-2-Ig fusion gene assoc iated with the chromosomal translocation t(14;18). Conversely, only 27 % of lymphomas that possessed the bcl-2-Ig gene were found to express the APO-1/Fas antigen. Like normal activated lymphocytes, leukemia and lymphoma cells that expressed APO-1/Fas antigen were found to undergo apoptosis in vitro after incubation with 7C11. The APO-1/Fas antigen appears to regulate the growth of normal hematopoietic cells, and the marked upregulation of this antigen on activated normal lymphocytes co ntrasts sharply with the absence of APO-1/Fas on neoplastic cells of h ematopoietic lineage. Defects in the apoptotic signal delivered throug h this antigen might contribute to the pathogenesis of hematopoietic n eoplasms. Thus, the gene encoding APO-1/Fas can be considered a novel type of tumor suppressor gene, just as bcl-2 can be considered a cellu lar proto-oncogene.