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

Background. The induction of tolerance to pig antigens in primates may faci litate the development of successful clinical xenotransplantation protocols . The infusion of mobilized porcine peripheral blood leukocytes (PBPCs, com prised of approximately 2% peripheral blood progenitor cells) into splenect omized preconditioned (whole body irradiation (WBI)-based) baboons, intende d to induce mixed hematopoietic cell chimerism, however, results in a sever e 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 no w investigated the effects of cyclophosphamide (CPP) as an alternative to W BI 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 tra nsmission aggregometry, the extent of aggregation induced by platelet agoni sts (thrombin, adenosine diphosphate (ADP), collagen, ristocetin, and arach idonic acid) was determined in vitro. PRP was also prepared from untreated baboons and from baboons receiving CPP, PBPCs were added, and platelet aggr egation was measured in the absence of exogenous platelet agonists. Results, CPP markedly inhibited platelet aggregation induced by all standar d agonists. In vitro addition of PBPCs to PRP stimulated platelet aggregati on in the absence of any agonists. Prior treatment of baboons with CPP, how ever, inhibited this effect by 55% to 65%. TM was not evident in baboons re ceiving a conditioning regimen that included CPP instead of WBI. Conclusions. Aggregation of baboon platelets and TM is directly induced by PBPCs. CPP has direct antiaggregatory properties and may provide an alterna tive strategy to WBI in this pig-to-primate model intended to induce mixed hematopoietic cell chimerism.