The transplantation of allogenic skin as a live saving therapy after massive burn injuries is an important issue for the application of immuno-modulating strategies. The utilization of apoptosis-inducing properties of some members of the TNF ligand family for the development of immune-privileged transplants is a fascinating, yet highly controversial means of allogenic transplantation. In a mixed lymphocyte-endothelial cell culture model, Cappellesso et al. showed that FasL transfected endothelial cells were effective in inducing apoptosis in Jurkat T cells. The authors conclude that expression of FasL in endothelial cells could be interesting to convey a death signal and induce hyporesponsiveness of allo reactive T cells for organ transplantation . The precise role of the FasL//Fas system for the so-called tumor counter attack has not been clarified yet [17–19]. Various data from in-vitro and in-vivo studies suggest an association between the expression of FasL, which is present in numerous tumours [20–23], and a significant localized reduction of tumor-infiltrating lymphocytes (TILs) secondary to apoptosis of sensitive T-cells [23, 25]. FasL acts in an analogous function to establish an immune-privileged status for special tissues, thereby protecting delicate organs like the eye against potential damage from immunological reactions . Similar findings were noticed for TRAIL, which shares numerous characteristics with FasL. The functional expression of TRAIL was demonstrated in human colon adenocarcinoma cell lines , possibly representing a biochemical mechanism of tumor escape in analogy to the FasL expression. FasL and TRAIL conjointly contribute to the immune-privileged status of the placenta . In order to evaluate the potential capability of TRAIL gene expression to induce tolerance through the initiation of apoptosis in a localized T cell population, we produced a keratinocyte clone expressing TRAIL which would trigger apoptosis in co-cultured Jurkat cells.
We showed how the stable transfection of a complete cDNA of TRAIL with a strong, constituitive promoter will lead to a lasting expression of membrane-bound, functional TRAIL on the cell surface of the transfected keratinocyte cell line. Jurkat cells are sensitive for the cytotoxicity mediated by TRAIL because of the expression of death-inducing receptors like TRAIL-R2/DR5 [2, 4]. Inoue et al. showed how apoptosis in Jurkat cells can be triggered by tumor cells using a TRAIL dependant signal pathway . In analogy to FasL, TRAIL can be expressed in many different tumor cells, ranging from breast- and braintumors to myeloid, lymphoid and colon carcinoma cells. Although the significance of FasL and TRAIL for the establishment of tumors has not been clarified in vivo, the induction of apoptosis represents an effective mechanism for the selective limitation of TILs, which can possibly be applied in transplants. The findings of our study show a pronounced cytotoxicity of complete TRAIL on TRAIL sensitive Jurkat cells and freshly isolated CD4+ and CD8+ cells. The cells will die due to a programmed cell death mediated by caspase. The cytotxic effect on Jurkat cells can be amplified by previous activation and incubation with ConA in a so-called activation induced cell death. The cell clone JB6pcDNA5/FRT TRAIL can induce apoptosis in T cells on direct contact and can thus effect a localized reduction of the total number of T cells.
There was no detectable secretion of sTRAIL in the cell culture media with the culturing conditions chosen in this study. The exposure of TRAIL sensitive Jurkat cells to the cell culture supernatant did not produce a cytotoxic effect, however, direct cell contact in the co-culture led to apoptosis in the target cells. An increase of the content of sTRAIL in humans can possibly produce hepatotoxicity and must be carefully considered before the application of specific gene therapy [28, 29]. Although TRAIL represents a type II transmembrane-protein, it can also exist in a soluble condition following processing and it will exhibit tumoricidal activity [15, 30, 31]. sTRAIL can mediate apoptotic signals into cells through interaction with the receptors TRAIL-R1/DR4 and TRAIL-R2/DR5. sTRAIL, however, is not as effective as membrane-bound TRAIL [32, 33], which is probably due to conformational differences [34, 35]. A gene-therapeutic strategy with foreign gene expression in the target cells should be considered as an alternative option in cancer therapy as well as for the induction of immunological tolerance in allogenic transplants.
Further studies using animal models are required to investigate if TRAIL expression in transplanted keratinocytes can induce immunological tolerance. The long-term effects of TRAIL expression on the inflammatory phase of wound healing as well as the interaction with other dermal cells are currently under investigation.