Microvascular Loss in Experimental Murine Tracheal Transplantation can be Prevented by Anti-Platelet Therapy and in Combination with mTOR Inhibitor

Objectives: Maintenance of a functional microvasculature is of utmost importance in solid organ transplantation. Tissue hypoxia due to microvascular injury-associated ischemia can occur after thrombus formation in microvessels mediated by activated platelets. Therefore the current study sought to investigate if treatment with the platelet inhibitor clopidogrel has an impact on the microvasculature of transplanted tracheal grafts.

Methods: Tracheal grafts were extracted from C57BL/6 (H-2b) or CBA/J (H-2k) mice and transplanted orthotopically into CBA/J (H-2k) (n = 6 per treatment group and per time point of analysis). Mice received clopidogrel alone or in combination with a mTOR inhibitor. Grafts were analyzed by serial tissue PO2 monitoring by a fluorescence quenching technique. Blood flow monitoring was performed by laser Doppler flowmetry and a Lectin-binding assay to analyze the function of the microvasculature on postoperative days 4, 10 and 28. Intra-graft cytokine mRNA production was analyzed by RT-PCR on day 18 after transplantation.

Results: Isografts showed a stable tissue pO2 and blood flow during the first time points after transplantation. In contrast, allografts showed a steady decline in tissue pO2 and blood flow in rejecting airway allografts until the pO2 nadirs 10 days after transplantation. VEGF expression as inducer of angiogenesis was elevated in all treated animals after 18 days. HO-1 and NOS as protector against hypoxia/reoxygenation injury were upregulated in Everolimus and combined treated animals. Continuous administration of clopidogrel or Everolimus alone and in combination significantly improved tissue oxygenation, limited microvascular leakiness, and prevented airway ischemia.

Conclusion: These data demonstrate that clopidogrel alone and in combination with everolimus ameliorates microvascular injury during acute airway rejection and subsequently reduces post-transplant obliterative bronchiolitis.