Apyrase Administration Prolongs Discordant Xenograft Survival1,2,3,4

Abstract

Platelet thrombi and vascular inflammation are prominent features of discordant xenograft rejection. The purinergic nucleotides ATP and ADP, which are secreted from platelets and released by injured endothelial cells (EC), are important mediators of these reactions. Quiescent EC express the ectoenzyme ATP-diphosphohydrolase (ATPDase; an apyrase), which exerts an important thromboregulatory function by hydrolyzing both ATP and ADP. We have shown that ATPDase activity is rapidly lost from the surface of the EC following ischemia-reperfusion injury and during xenograft rejection. The aim of this study was to supplement ATPDase activity within xenografts by infusion of soluble apyrases, and thereby validate the importance of local ATPDase activity in the modulation of xenograft rejection. Lewis rats underwent heterotopic cardiac xenografting from guinea pigs and apyrase was administered intravenously (200 U/kg) as a single dose to evaluate effects on hyperacute rejection (HAR). This initial dose was followed by a continuous apyrase infusion (8.0 U/kg/hr) directly into the graft aorta in combination with systemic cobra venom factor (CVF) administration to deplete complement when delayed xenograft rejection (DXR) was studied. Functional apyrase levels in vivo were assessed by the capacity of blood samples taken at the time of surgery and rejection to inhibit platelet aggregation in vitro. Apyrase administration significantly prolonged graft survival in HAR and DXR. Functional assays showed inhibition of platelet aggregation suggesting effective systemic antiaggregatory effects of the administered apyrases. Histologic studies showed that apyrase administration abrogated local platelet aggregation and activation in HAR and DXR. Our data demonstrate that local administration of apyrase prolonged discordant xenograft survival. These observations emphasize the potential importance of purinergic mediators in platelet activation during xenograft rejection.

Identification and Characterization of CD39/Vascular ATP Diphosphohydrolase

“…We speculate that this loss, and the resultant decreased capacity to degrade ADP, could play a significant role in the extensive platelet activation and vascular inflammation seen in graft rejection and other forms of vascular injury. Certainly, the intravenous administration of apyrases to experimental animals has been shown to prolong xenograft survival and abrogate the platelet activation and deposition seen in this setting (26).…”

Lipopolysaccharide Treatment Modifies pH- and Cation-dependent Ecto-ATPase Activity of Endothelial Cells

“…Although ecto-ATPases are active in the presence of both Ca 2 ϩ and Mg 2 ϩ , they might have some ion-dependent "territorial dominance" on the EC surface in addition to their different sensitivity for pH and stress by injury or inflammation. Recently, several studies have appeared on the loss of ecto-ATPase activity on vascular ECs after injury or oxidative stress (Candinas et al 1996;Koyamada et al 1996;Robson et al 1997;Gayle et al 1998). The loss of ecto-ATPase activity may result in platelet aggregation, thrombosis, and xenograft rejection.…”

Ecto-enzymes: physiology meets pathology

“…Because CD39 destroys extracellular ATP and ADP, ADP concentrations would therefore be expected to rise, favoring clotting [102]. It has therefore been suggested that externally administered apyrase might be a useful antithrombotic agent [103].…”