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J. Exp. Med.,
Volume 186, Number 12, December 15, 1997 2045-2050
By


From the * Osaka Bioscience Institute, Department of Molecular Biology, Suita, Osaka 565, Japan; It has been believed that the Fas expressed on human peripheral blood T cells (PBT) is nonfunctional, because these cells are insensitive to agonistic anti-Fas/Apo-1 mAbs that efficiently kill in vitro-activated T cells and many Fas-expressing cell lines. Here, we demonstrate that
membrane-bound Fas ligand (FasL) kills both fresh and in vitro-activated PBT, indicating that
the Fas expressed on fresh PBT is functional. In contrast, soluble FasL kills only the latter. Naive T cells in umbilical cord blood do not express Fas, but can be induced to express Fas by
IFN-
Department of Orthopaedic Surgery, and § Department of Genetics, Osaka University Medical School
Suita, Osaka 565, Japan
or by a combination of IL-2 and anti-CD28 mAb, after which they acquire sensitivity
to membrane but not to soluble FasL. Soluble FasL inhibited the killing of fresh PBT by membrane FasL. These results indicate that the shedding of FasL from the membrane is a mechanism for downregulating at least part of its killing activity.
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