A NOVEL ANTI-APOPTOSIS SERUM FACTOR THAT DOWN-REGULATES FAS-MEDIATED APOPTOSIS

The Fas antigen (CD95) is a receptor for apoptotic cell death signals, Human T Jurkat cells that express the Fas molecule on the cell surfac e are sensitive to cytotoxic anti-Fas monoclonal antibodies (mAbs), We report here that fetal calf serum (FCS) contains a novel factor (SAF: serum anti-apoptosis factor) that suppresses Fas-mediated apoptosis o f Jurkat cells, The apoptotic cell death of Jurkat cells induced by an anti-Fas mAb was accelerated when FCS was removed from the medium. De pletion of serum itself did not induce apoptosis without the mAb, The apoptosis-suppressing activity was not seen in neonatal calf or adult bovine serum, suggesting that the SAF activity was developmentally reg ulated. The anti-apoptosis activity was not detected in any fraction o f FCS obtained on gel filtration chromatography, However, the activity was recovered in a certain fraction on the addition of calf serum, su ggesting that the anti-apoptotic activity of SAF requires other serum factor(s), We purified SAF from FCS using an assay system reconstitute d with calf serum, and raised antibodies (Abs) to it in rabbits, The a nti-SAF Ab inhibited the apoptosis-suppressing activity, indicating th at SAF is a key factor for the anti-apoptotic activity of FCS.