[Background] Endothelial cell (EC) injury is central to cardiac allograft vasculopathy and premature graft loss. Osteopontin (OPN) is a multifunctional anti-apoptotic protein that is involved in cell homeostasis, cell death and inflammation. It is expressed as secreted (sOPN) and intracellular (iOPN) forms but their role in cardiac EC survival remains undefined. In this study, we tested the capacity of OPN to regulate cardiac EC apoptosis and graft survival in a mouse model. [Methods & Results] By immunohistochemistry, we found OPN was highly expressed in B6 wildtype (wt) but not B6 OPN^-/- (null) mouse heart endothelium. in vitro, sOPN and iOPN mRNA and protein were expressed by B6 wt cardiac EC. Using flow cytometry, OPN^-/- EC with or without sOPN had enhanced expression of Fas ((98.4±1.2)% and (97.2±5.1)%, respectively) compared to wt EC (34.1±4.7)%, (n=3, p<0.01). OPN^-/- EC had greater extrinsic apoptosis induced by anti-Fas antibody compared to wt EC (12.7±1.7 vs. 5.7±0.7 % Annexin V positivity, n=3, p<0.05) or by activated Bm12 CD4+ T-cells (26.6±4.6 vs. 10.8±2.7%, n=3, p<0.05). Additionally key inhibitors of intrinsic apoptosis were downregulated in OPN^-/- EC compared to wt EC (decreased Bcl2 by 24 fold, Bcl-xl by 314 fold, realtime PCR, n=3, p<0.01), and intrinsic apoptosis by TNFα+IFNγ+IL-1β treatment was significantly higher in OPN^-/- EC (n=3, p<0.05), which was not changed by manipulating sOPN in either (p= ns). In a B6 to Bm12 heterotopic cardiac allograft model, OPN^-/- grafts were rejected significantly faster than wt grafts (22.0±0.9 vs. 79.5±23.0 days, n=6, p<0.01) despite excessive exposure of the graft to the recipient-derived sOPN in the circulation. [Conclusion] These results demonstrate a key role for endogenous OPN in protecting cardiac EC and promoting graft survival. Importantly the capacity of OPN to inhibit apoptosis in vascular endothelium is confined to the intracellular form. While OPN plays a crucial protective role in heart transplantation, consideration of its potential therapeutic use to improve long term heart graft survival will need to consider this decisive separation of roles by cellular location.

CITATION INFORMATION: Su Y, Zhang Z.-X, Yin Z.-Q, Huang X.-Y, Jevnikar A. Intracellular OPN Is an Essential Protective Factor for Cardiac Endothelial Cell and the Long Term Heart Graft Survival. Am J Transplant. 2016;16 (suppl 3).

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