Down-regulation of human alloimmune responses by genetically engineered expression of CD95 ligand on stimulatory and target cells.

Transgenic expression of apoptosis-inducing molecules could be a strategy to protect cells and tissues from destruction by apoptosis-susceptible effector T cells. Some evidence for the potency of this approach has been obtained in mouse and rat transplantation models. However, limited data are available on the capacity of apoptosis-inducing molecules to modulate human alloimmune responses. In this study we analyzed the in vitro consequences of an interaction of human T cells with allogeneic 293 cells and 293 transfectants stably expressing high levels of the apoptosis-inducing CD95 ligand (CD95L). Both, CD95L(-) and CD95L(+) 293 cells were able to activate allogeneic T cells as demonstrated by comparable CD25 expression at day 2 of culture. The analysis of viable T cells at day 7, however, revealed anti-293 cytotoxic activity only in cultures that had been stimulated with CD95L(-) 293 cells. Alloactivated effector T cells lysed CD95L(-) and CD95L(+) 293 targets with similar efficiency when tested in a 4-h 51Cr-release assay. Prolongation of the effector phase to 20 h resulted in a further increase in the destruction of CD95L(-) target cells, whereas lysis of CD95L(+) targets remained low. These data suggest that genetically engineered expression of CD95L on cells or tissues could be an approach to control human T cell reactivity towards allografts. During the induction of an alloimmune response depletion of cytotoxic precursor cells may be obtained by overexpressing CD95L on stimulatory cells; CD95L expression on graft tissue might limit T cell-mediated destruction of the transplant during the effector phase of the response.

[The CD95/CD95L system as target for modulation of allogeneic immune responses]. / Das CD95/CD95L System als Zielstruktur für die Modulation von allogenen Immunantworten.

The effects of COS7 cells which constitutively express the apoptosis-inducing human CD95 ligand (COSh95L cells) on in vitro alloimmune responses were investigated. In the presence of COSh95L cells a clear-cut inhibition of alloantigen-induced proliferation was observed. In these cultures fully activated blastoid T cells were absent suggesting that deletion of activated T cells is one mechanism of COSh95L-mediated inhibition of proliferation. Non-fully activated cells were not eliminated in MLR by COSh95L cells but achieved a hyporesponsive state as shown by their failure to proliferate in response to restimulation with alloantigen or mitogen. These data indicate that the effects of CD95L on T cells are heterogeneous. Further analysis of the actions of CD95L in T cells of different activation states and subsets will be crucial to estimate the therapeutical potential of this molecule as target for immunomodulation after allo- and xenotransplantation.