MODULATION OF CD95 OR BCL-2 EXPRESSION INFLUENCE GRAFT REJECTION INDUCED BY DENDRITIC CELLS.

Abstract

817 In this study we investigated how modulation of apoptotic pathways influence graft rejection induced by dendritic cells (DCs). To explore this question we took advantage of a previously described transplant model in which graft rejection is selectively induced by DCs. In this model tail skin grafts from B6Bm7 (Bm7) mice are permanently accepted by B6 (H-2b) mice across a limited MHC Class I difference. However, small numbers of Bm7 DCs readily induce rejection of stable tail grafts after i.p. or i.v. injection, while other antigen presenting cells are ineffective. Thus, this model selectively allows us to explore the role of DCs in graft rejection. Programmed cell death (apoptosis) is thought to be an important mechanism in T cell mediated rejection. Expression of Fas ligand (fasL) promotes apoptosis by ligation with the Fas receptor (CD95). Conversely, overexpression of Bcl-2 inhibits activa-tion of the apoptotic cascade and promotes cell survival. To further examine the importance of fas mediated apoptosis in DC induced graft rejection, we examined the survival of Bm7 tail skin grafts transplanted onto Fas deficient B6 (B6.MRL-lpr, H-2b) mice. In the first set of studies we challenged established Bm7 tail grafts on B6lpr mice with donor-derived DCs. Interestingly, our results showed that challenge with as many as 2×106 donor DCs did not induce rejection in the CD95 deficient mice (1/4), whereas, 4/4 wildtype B6 rejected their transplants. These results suggest that rejection induced by DCs is dependent on Fas mediated apoptotic activity of the recipient. To test this hypothesis we used, as graft recipient, the transgenic bcl-2 mouse (H-2b) which overexpresses Bcl-2 but still express CD95. Bm7 tail skin was grafted onto transgenic mice having the bcl-2 gene expressed from either the human (high expressor) or chicken (low expressor) β-actin promoter. The results showed that without DC challenge, a late rejection ocurred at days 22-27 in 5/5 high expressing bcl-2 mice and at days 31-40 in 4/5 low expressing bcl-2 mice. In comparison, only 1/4 of the wildtype B6 mice with Bm7 tail skin rejected within this observation time (p<0.05). In conclusion, these results suggest that an abrogation of the CD95 apoptosis pathway blocks DC mediated allograft rejection in our mouse model. Furthermore, we see enhanced rejection with conditions that may stimulate recipient T cell survival, thus enhancing interactions between recipient T-cells and donor DCs. Because injection of recipient DCs can not induce a rejection we hypothesize that direct stimulation by donor DCs determine the graft survival.