Shikonin Prolongs Allograft Survival via Induction of CD4 + FoxP3 + Regulatory T Cells

A transplanted organ is usually rejected without any major immunosuppressive treatment because of vigorous alloimmune responsiveness. However, continuous global immunosuppression may cause severe side effects, including nephrotoxicity, tumors, and infections. Therefore, it is necessary to seek novel immunosuppressive agents, especially natural ingredients that may provide sufficient efficacy in immunosuppression with minimal side effects. Shikonin is a bioactive naphthoquinone pigment, an ingredient originally extracted from the root of . Previous studies have shown that shikonin regulates immunity and exerts anti-inflammatory effects. In particular, it can ameliorate arthritis in animal models. However, it is unclear whether shikonin inhibits alloimmunity or allograft rejection. In this study and for the first time, we demonstrated that shikonin significantly prolonged the survival of skin allografts in wild-type mice. Shikonin increased the frequencies of CD4 Foxp3 regulatory T cells (Tregs) post-transplantation and induced CD4 Foxp3 Tregs as well. Importantly, depleting the Tregs abrogated the extension of skin allograft survival induced by shikonin. It also decreased the frequencies of CD8 CD44 CD62L effector T cells and CD11c CD80 /CD11c CD86 mature DCs after transplantation. Moreover, we found that shikonin inhibited the proliferation of T cells and suppressed their mTOR signaling. It also reduced the gene expression of pro-inflammatory cytokines, including IFNγ, IL-6, TNFα, and IL-17A, while increasing the gene expression of anti-inflammatory mediators IL-10, TGF-β1, and indoleamine-2, 3-dioxygenase (IDO) in skin allografts. Further, shikonin downregulated IDO protein expression in skin allografts and DCs . Taken together, shikonin inhibits allograft rejection via upregulating CD4 Foxp3 Tregs. Thus, shikonin is a novel immunosuppressant that could be potentially used in clinical transplantation.