Ains-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 ofthe 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 nonneoplastic 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 ofFasL and Fas in particular epithelia suggests that the physiological cell turnover of some tissues may be regulated by the Fas-FasL apoptotic pathway.

Monoclonal antibodies (mAb) derived from BALB/c mice immunized with human anti-HLA-A2 cloned cytotoxic T lymphocytes (CTL) were screened for their ability to block, in the absence of complement, the cytolytic activity of the immunizing CTL clone. Eight cytolysis-inhibiting mAb have been derived. One of these was directed against a monomorphic determinant expressed on HLA-class I molecules and thus probably inhibited cytolysis via a target cell antigen-masking effect. The 7 other mAb recognized "CTL function-associated structures" and did not have to interfere with target cell antigens in order to inhibit cytolysis. F(ab')2 and Fab fragments of these 7 mAb were also inhibitory. Competitive inhibition of binding and preliminary biochemical analysis suggested that these 7 mAb defined on cloned CTL various epitopes of a structure of 30 kDa disulfide-bonded into several multimeric forms when analyzed without reduction. The inhibitory effect of these 7 mAb has been investigated on a series of short-term (1 month) and long-term (greater than 10 months) expanded cloned CTL lines derived in vitro from an in vivo allosensitized individual exhibiting various specificities. Unexpectedly, only 10% of these CTL clones were inhibited. Flow cytofluorimetric analysis further revealed that noninhibited and inhibited CTL clones expressed similar amounts of the 30-kDa structure. Consequently, the inhibition of CTL was heterogeneous when analyzed at the clonal level and not simply related to the presence or absence of this structure. Furthermore, the ability of CTL clones to be inhibited appeared to be unrelated to their HLA-A, B or C specificity or to their lytic activity. The connections between this mAb-defined structure and CTL function are discussed.

A panel of eight different CD28 mAbs was used to analyse the structure-function relationships of the CD28 molecule. The results of binding inhibition experiments show a complex and heterogeneous pattern of inhibition; however a subgroup of mAbs was identified, namely CD28.1, CD28.3, and CD28.5, which exhibited almost identical inhibition profiles. To test the hypothesis that the different binding specificities are related to functionally distinct subregions of the CD28 molecule, the ability of each mAb to (i) induce IL-2 release and (ii) increase intracellular calcium [(Ca2+)i] in Jurkat T cells was analysed. The results show that the mAbs CD28.1, CD28.3, and CD28.5 are almost totally unable to induce IL-2 release, and their ability to increase (Ca2+)i is relatively low. All other mAbs are able to induce a marked (Ca2+)i rise, however they strongly differ in their ability to induce IL-2 release. Such differences cannot be explained by differences in the isotypes or binding kinetics of the mAbs. These results imply the existence of functionally distinct subregions on the CD28 molecule. In addition, the (Ca2+)i rise may be associated with either high or low IL-2 secretion following CD28 triggering.

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