title: Modulation of platelet aggregation in baboons: implications for mixed chimerism in xenotransplantation. II. The effects of cyclophosphamide on pig peripheral blood progenitor cell-induced aggregation.
abstract: BACKGROUND
  The induction of tolerance to pig antigens in primates may facilitate the development of successful clinical xenotransplantation protocols. The infusion of mobilized porcine peripheral blood leukocytes (PBPCs, comprised of approximately 2% peripheral blood progenitor cells) into splenectomized preconditioned (whole body irradiation (WBI)-based) baboons, intended to induce mixed hematopoietic cell chimerism, however, results in a severe thrombotic microangiopathy (TM) that includes pronounced thrombocytopenia. Previous studies have indicated that the infused PBPCs initiate platelet aggregation, but that the various individual components of the conditioning regimen are not associated with the development of aggregation. We have now investigated the effects of cyclophosphamide (CPP) as an alternative to WBI on platelet aggregation.
  
  
  METHODS
  Splenectomized baboons (n=3) were treated with CPP. Blood samples were collected and platelet-rich plasma (PRP) was prepared. Using light transmission aggregometry, the extent of aggregation induced by platelet agonists (thrombin, adenosine diphosphate (ADP), collagen, ristocetin, and arachidonic acid) was determined in vitro. PRP was also prepared from untreated baboons and from baboons receiving CPP, PBPCs were added, and platelet aggregation was measured in the absence of exogenous platelet agonists.
  
  
  RESULTS
  CPP markedly inhibited platelet aggregation induced by all standard agonists. In vitro addition of PBPCs to PRP stimulated platelet aggregation in the absence of any agonists. Prior treatment of baboons with CPP, however, inhibited this effect by 55% to 65%. TM was not evident in baboons receiving a conditioning regimen that included CPP instead of WBI.
  
  
  CONCLUSIONS
  Aggregation of baboon platelets and TM is directly induced by PBPCs. CPP has direct anti-aggregatory properties and may provide an alternative strategy to WBI in this pig-to-primate model intended to induce mixed hematopoietic cell chimerism.
authors.affiliation: Transplantation Biology Research Center, Massachusetts General Hospital / Harvard Medical School, Boston, Massachusetts 01129, USA.
authors.forename: J Z
authors.initials: JZ
authors.lastname: Appel
authors.forename: I P
authors.initials: IP
authors.lastname: Alwayn
authors.forename: L E
authors.initials: LE
authors.lastname: Correa
authors.forename: D K
authors.initials: DK
authors.lastname: Cooper
authors.forename: S C
authors.initials: SC
authors.lastname: Robson
mesh_terms: D000818:Animals; D003520:Cyclophosphamide / Q000494:pharmacology*; D018380:Hematopoietic Stem Cell Transplantation / Q000009:adverse effects*; D010215:Papio; D010974:Platelet Aggregation / Q000187:drug effects*; D018183:Transplantation Chimera; D019172:Transplantation Conditioning / Q000379:methods; D014183:Transplantation, Heterologous
chemical_list: D003520:Cyclophosphamide