Background: Although acute graft rejection can be successfully controlled by immunosuppressive agents, chronic rejection (CR), which is characterized by arteriosclerosis in the donor organ vessels, is a major hurdle to long-term allograft survival. Sonic hedgehog (Shh), a morphogen critical in embryogenesis, also promotes peripheral immunity, which prompted us to investigate if inhibition of Shh signaling could reduce CR and thereby enhance allograft survival.
Methods: In a rat orthotopic small bowel transplantation model, FK506 prevented acute rejection; however, recipients eventually lost their grafts by CR. Anti-Shh antibody or isotype control were administered to animals at day 30 postoperatively. Graft survival, tissue fibrosis, vascular occlusion, and expression of vascular endothelial growth factor (VEGF) were investigated.
Results: Immunostaining revealed that Shh and the Hedgehog receptor Patched 1 (Ptc1) are strongly expressed in CR grafts and that Ptc1 expression partially overlapped with that of ED-1, a macrophage marker. In contrast, only minimal expression of Shh and Ptc1 was detected in syngeneic grafts. Grafts survival was significantly prolonged after anti-Shh antibody treatment compared with the immunoglobulin G control (116 vs. 77.5 days). Collagen deposition and vascular occlusion in the mesentery were markedly reduced in recipients of the anti-Shh antibody. Specific transcripts and protein expression for VEGF, which was present mainly in the blood vessels, were reduced.
Conclusion: In a rat small bowel transplantation model, anti-Shh antibody treatment reduced CR and prolonged graft survival. These beneficial effects of Shh treatment may occur partly by reducing VEGF expression in the blood vessels of the allografts.