Loss of ecto-5′ nucleotidase from porcine endothelial cells after exposure to human blood: Implications for xenotransplantation

The endothelial cell surface expression of ecto-5′-nucleotidase (E5′N, CD73) is thought to be essential for the extracellular formation of cytoprotective, anti-thrombotic and immunosuppressive adenosine. Decreased E5′N activity may play a role in xenograft acute vascular rejection, preventing accommodation and tolerance mechanisms. We investigated the extent of changes in E5′N activity and other enzymes of purine metabolism in porcine hearts or endothelial cells when exposed to human blood or plasma and studied the role of humoral immunity in this context. Pig hearts, wild type (WT, n=6) and transgenic (T, n=5) for human decay accelerating factor (hDAF), were perfused ex vivo with fresh human blood for 4 h. Pig aortic endothelial cells (PAEC) were exposed for 3 h to autologous porcine plasma (PP), normal (NHP) or heat inactivated human plasma (HHP), with and without C1-inhibitor. Enzyme activities were measured in heart or endothelial cell homogenates with an HPLC based procedure. The baseline activity of E5′N in WT and T porcine hearts were 6.60±0.33 nmol/min/mg protein and 8.54±2.10 nmol/min/mg protein respectively (P<0.01). Ex vivo perfusion of pig hearts with fresh human blood for 4 h resulted in a decrease in E5′N activity to 4.01±0.32 and 4.52±0.52 nmol/min/mg protein (P<0.001) in WT and T hearts respectively, despite attenuation of hyperacute rejection in transgenic pigs. The initial PAEC activity of E5′N was 9.10±1.40 nmol/min/mg protein. Activity decreased to 6.76±0.57 and 4.58±0.47 nmol/min/mg protein (P<0.01) after 3 h exposure of HHP and NHP respectively (P<0.05), whereas it remained unchanged at 9.62±0.88 nmol/min/mg protein when incubated with PP controls. C1-inhibitor partially preserved E5′N activity, similar to the effect of HHP. Adenosine deaminase, adenosine kinase and AMP deaminase (other enzymes of purine metabolism) showed a downward trend in activity, but none were statistically significant. We demonstrate a specific decrease in E5′N activity in pig hearts following exposure to human blood which impairs adenosine production resulting in a loss of a cytoprotective phenotype, contributing to xenograft rejection. This effect is triggered by human humoral immune responses, and complement contributes but does not fully mediate E5′N depletion.