Variable Effects of Different Serotonin Receptor Subtype Antagonists on the Development of Transplant Vasculopathy in Murine Aortic Allografts

Previous studies suggest an important role of platelets on developing transplant vasculopathy, the hallmark feature of chronic rejection. The signaling molecule serotonin (5-HT) is stored within platelets until cell activation and is involved in proliferation of vascular smooth muscle cells. Therefore, the aim of this study was to investigate the effect of 5-HT_2A and 5-HT_2B receptor antagonists on transplant vasculopathy (TV).

Fully allogeneic C57BL/6 (H2^b) donor aortas were transplanted into CBA (H2^k) recipients. Recipient mice were treated with the specific 5-HT_2A antagonist sarpogrelate (10 mg/kg/d), the specific 5-HT_2B antagonist SB204741 (10 mg/kg/d), and the non-selective 5-HT₂ receptor antagonist terguride (0.6 mg/kg/BD). Grafts were analyzed by morphometry and immunohistology on day 30 after transplantation. Intragraft gene expression was measured using quantitative RT-PCR on day 14 after transplantation.

In mice treated with sarpogrelate, transplant vasculopathy was reduced as compared with untreated controls (intima proliferation 43% ±  8% versus 54% ±  13% [control]/p< 0.05; n = 7). Daily application of the 5-HT2_B antagonist SB204741 did not reduce the development of TV (intima proliferation of 49% ± 9% versus 41% ± 13% [control]/p = n.s.; n = 8). Interestingly, the non-selective 5-HT₂ receptor antagonist terguride did not reduce the formation of TV either (intima proliferation: 56% ± 13% versus 54% ± 13% [control]/p = n.s.; n = 10). In addition experimental groups revealed a down regulation of ICAM-1, PDGF, VEGF, IFNγ, MCP1, CD40L, P-Selectin, and TGF-β, and upregulation of eNOS after sarpogrelate treatment. Terguride treatment significantly reduced PDGF, IFNγ, and TGF-β expression within the graft. Immunohistochemical analysis of 5-HT_2A receptor distribution revealed a significantly decreased expression within the neointima of sarpogrelate treated animals in contrast to SB204741 or terguride treated mice. Furthermore, sarpogrelate treatment resulted in a lower amount of infiltrating macrophages within the media.

These results demonstrate that antagonists of the serotonin subtype receptor 5-HT_2A can dramatically reduce the development of TV in a mouse aortic allograft model. Antagonisation of 5-HT_2B receptors alone or in combination did not result in reduced intima proliferation, suggesting that specific 5-HT_2A antagonism appears to be a very attractive strategy for prevention of TV that deserves further experimental and clinical evaluation.