Endothelial cell cytoprotection induced in vitro by allo- or xenoreactive antibodies is mediated by signaling through adenosine A2 receptors

Endothelial cell (EC) expression of proteins such as hemoxygenase-1 or Bcl-xL is associated with enhanced survival of vascularized allo- or xenografts. These grafts are resistant to humoral rejection, a phenomenon called accommodation. In vitro, low concentrations of allo- and xenoreactive antibody (XNA) induce a cytoprotective phenotype in EC similar to that seen in accommodated grafts. In this study we examine whether adenosine plays a role in antibody-induced cytoprotection. Porcine EC were incubated with human anti-pig XNA or specific alloantibody. EC expressed Bcl-xL and were protected from TNF-mediated apoptosis. Bcl-xL expression was inhibited by an adenosine A2 receptor antagonist. Human anti-pig XNA were shown to bind and induce cyclic adenosine 3',5'-monophosphate (cAMP) generation through these receptors. This activity was abolished by depletion of anti-galalpha(1-3)gal-specific XNA. In contrast, alloantibody caused adenosine production. Protection from TNF-mediated apoptosis was also mediated through A2 receptor but involved additional non-cAMP-dependent signaling. This study indicates a molecular mechanism common to both antibody-mediated cytoprotection and ischemic preconditioning and suggests a potential therapeutic avenue based on adenosine for improving the outcome of transplanted grafts in those patients with pre-existing anti-graft antibody.