FasL is functional in mature erythroblasts and able to kill immature erythroblasts in the absence of high levels of Epo. (A) Cytotoxic activity of day-7 and day-14 erythroblasts toward Fas-sensitive (sens) and Fas-resistant (res) EBV-transformed lymphoblastoid cell lines. Effector and target cells were spun down together and incubated for 8 hours. In some experiments anti-Fas antagonist MoAb ZB4 (10 μg/mL) was added to Fas-sensitive cells (sens + ZB4). Data show a representative experiment out of three performed with cells from different donors. (B) Cytotoxic activity of day 14 erythroblasts toward day-7 erythroblasts pretreated or not with 10 μg/mL ZB4, in the presence of low (30 mU/mL) or high (3 U/mL) amounts of Epo. Effector and target cells were spun down together and incubated for 36 hours. Data show mean ± SD of three experiments performed with cells from different donors.