Introduction: Coagulation dysregulation is major limitation for xenotransplantation but it is resolved to some extent by making genetically engineered (GE) pigs, which express complement inhibitors and or coagulation activators. Thrombomodulin (TBM) is a coagulation regulatory protein, which binds thrombin and enhances its activity converting protein C to activated protein C (aPC) which inhibits activated factors V and VIII, leading to decreased thrombin formation and prevention of a hypercoagulable state. Lack of functional TBM has been suggested to lead to microthrombosis and premature loss of grafts. We have previously reported long-term cardiac xenograft survival with hTBM gene on GE pig hearts along with anti-CD40 antibody. In this study, we have used non-hTBM pig heart and determined the necessity or importance of hTBM in prolongation of cardiac XTx in non-human primates.

Materials and Methods: Heterotopic cardiac XTx were performed in SPF baboons from 3-gene (i.e.GTKO.CD46.hTBM; (n=5) and 2-gene (i.e.GTKO.CD46 only) (n=3) GE pigs. Recipients were treated with a short course of anti-CD20 Ab, CVF, ATG, and were maintained on the anti CD40 Ab (clone 2C10R4), MMF and tapering dose of steroids. Recipients also received continuous i.v. heparin. Graft survival was monitored with telemetry, echocardiography, and manual palpation. Blood work for CBC, chemistry, (including troponin and ACT) and coagulation parameter (aPTT, PT, fibrinogen and D-dimer) was performed at 1-2 week intervals.

Results and Conclusion: Cardiac XTx were performed without any difficulty and xenograft with hTBM expression (3 Gene) survived the long time (median 298 days) as compared to 2 gene pig (median 70 days). Recipients maintained their hemoglobin, hematocrit and RBC counts. Platelet numbers had a drop after the surgery but later maintained at a healthy level. aPTT, PT, fibrinogen and anti-thrombin (AT) were increased in recipients receiving non-hTBM (2 gene) vs hTBM (3 gene) xenograft. Non-hTBM xenograft had significant adhesions around at graft explantation that is consistent with inflammation. These results suggest that hTBM expression on xenograft maintained coagulation profile, prevented early dysregulation of coagulation and prolonged the xenograft survival.

CITATION INFORMATION: Singh A., DiChiacchio L., Chan J., Corcoran P., Lewis B., Thomas M., Ayares D., Horvath K., Mohiuddin M. Human Thrombomodulin Expression on Genetically Modified Donor Pigs is Required to Maintain the Anti-Coagulation Profile Am J Transplant. 2017;17 (suppl 3).

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