Low‐Dose Rapamycin Treatment Increases the Ability of Human Regulatory T Cells to Inhibit Transplant Arteriosclerosis In Vivo

Regulatory T cells (Treg) are currently being tested in clinical trials as a potential therapy in cell and solid organ transplantation. The immunosuppressive drug rapamycin has been shown to preferentially promote Treg expansion. Here, we hypothesized that adjunctive rapamycin therapy might potentiate the ability of ex vivo expanded human Treg to inhibit vascular allograft rejection in a humanized mouse model of arterial transplantation. We studied the influence of combined treatment with low‐dose rapamycin and subtherapeutic Treg numbers on the development of transplant arteriosclerosis (TA) in human arterial grafts transplanted into immunodeficient BALB/cRag2−/−Il2rg−/− mice reconstituted with allogeneic human peripheral blood mononuclear cell. In addition, we assessed the effects of the treatment on the proliferation and apoptosis of naïve/effector T cells. The combined therapy efficiently suppressed T‐cell proliferation in vivo and in vitro. Neointima formation in the human arterial allografts was potently inhibited compared with each treatment alone. Interestingly, CD4+ but not CD8+ T lymphocytes were sensitive to Treg and rapamycin‐induced apoptosis in vitro. Our data support the concept that rapamycin can be used as an adjunctive therapy to improve efficacy of Treg‐based immunosuppressive protocols in clinical practice. By inhibiting TA, Treg and rapamycin may prevent chronic transplant dysfunction and improve long‐term allograft survival. The authors show that adjunctive low‐dose rapamycin therapy potentiates the ability of ex vivo expanded human regulatory T cells to inhibit allograft rejection in a humanized mouse model of arterial transplantation, and further demonstrate that this effect is mediated by suppression of T cell proliferation and induction of CD4+ cell apoptosis.