AIMS: To confirm the recent data obtained in mice, showing that the Fas ligand (FasL) is involved in the phenomenon of "immune privilege" (the apparent defect of the immune system in specific anatomical sites) and to extend this finding to humans. METHODS: The expression of FasL was analysed in a panel of histologically normal human tissues by reverse transcriptase polymerase chain reaction and Western blotting. The tissues sampled were brain, breast, bone marrow, oesophagus, kidney, liver, lung, lymph node, ovary, pancreas, pituitary gland, prostate, spleen, stomach (antrum and fundus), striated muscle, testis, thyroid, and uterus. These were obtained from patients with various neoplastic and non-neoplastic disorders; placental tissue was obtained after normal obstetric delivery, and spontaneous or voluntary abortion. RESULTS: Strong FasL expression was detected in testis and placenta. FasL expression was also detectable, although it was seen to a lesser extent, in oesophagus, prostate, lung, and uterus, which also coexpressed variable amounts of Fas mRNA or protein or both. The other organs tested for FasL expression were all negative. CONCLUSIONS: FasL in humans is expressed predominantly in immune "sanctuaries" such as testis and placenta, suggesting that, similar to mice, this expression may contribute to the immune privileged status of these organs, by preventing dangerous inflammatory responses. The coexpression of FasL and Fas in particular epithelia suggests that the physiological cell turnover of some tissues may be regulated by the Fas-FasL apoptotic pathway.
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