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CLASS I MEDIATED CELL SURVIVAL SIGNALING IN ENDOTHELIAL CELLS.
Yi-Ping Jin M.D. , Michael C. Fishbein, M.D. , Jonathan W. Said, M.D. , Peter T. Jindra, B.S and Elaine F. Reed, Ph.D. . Los Angeles CA, USA, David Geffen School of Medicine at UCLA, 90095, Pathology .
Anti-HLA antibodies (Ab) have been implicated in the process of chronic rejection and transplant arteriosclerosis (TA) yet their mechanism of action is not understood. We hypothesize that Ab ligation of MHC class I molecules on EC promotes cell survival and furthers the development of TA by activating anti-apoptotic proteins in the ECs of the graft. To explore this possibility, we evaluated the effects of anti-HLA class I Ab on the PI3K/Akt signaling pathway, components of the cell death apparatus, including Bad and 14-4-3-3, and the anti-apoptotic proteins Bcl-2 and Bcl-xL. Treatment of EC with anti-class I Ab stimulated increased phosphorylation of PI3 kinase (PI3K) and its downstream target AKT. Ligation of class I molecules also stimulated prominent increases in Bcl-2 and Bcl-xL levels in EC. Interestingly, exposure of EC to low concentrations of anti-class I Ab stimulated the highest levels of Bcl-2 and Bcl-xL and may explain differences in outcome in patients producing anti-donor Ab post-transplant. The involvement of Bcl-2 as a downstream target of PI3K/Akt pathway was investigated using the PI3K inhibitor wortmannin. Treatment with wortmannin abolished class I mediated increases in Bcl-2 expression. Class I signaling induced phosphorylation of Bad and stimulated complex formation between 14-3-3 and Bad. In vivo evidence that anti-class I Ab initiate a pro-survival signaling cascade was provided by immunostaining of cardiac transplant biopsies with evidence of acute humoral rejection (HR). Grafts with evidence of HR showed specific up-regulation of Bcl-2. Promotion of EC growth and survival through the activation of anti-apoptotic proteins within the cell may contribute to the vasculopathy characteristic of chronic rejection.