Date Published: August , 2012
Publisher: Blackwell Publishing Inc
Author(s): J Hester, A Schiopu, S N Nadig, K J Wood.
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 mammalian target of rapamycin (mTOR), a serine/threonine protein kinase, is inhibited by the immunosuppressive drug rapamycin. mTOR plays a key role in the regulation of cell proliferation, adhesion and survival by integrating information from the cell’s environment 1–3). By targeting mTOR, rapamycin inhibits the proliferation of many cell types including T cells, one of the key cellular mediators of rejection following transplantation. T cells are a heterogenous population of lymphocytes with different subsets having different functional capabilities. Moreover, T cells exhibit plasticity enabling some populations to change their functional properties depending on the environmental cues they receive both as they differentiate, as well as when they function in vivo (4). Each T-cell subset demonstrates a differential sensitivity to mTOR inhibition (5). Thus, the impact of rapamycin therapy in vivo may be different depending on the composition of the T-cell compartment in the host, the microenvironment in which a T cell is functioning, and the duration and dose of rapamycin therapy.
Side branches of human IMAs, collected from patients undergoing cardiac bypass surgery, were transplanted as infrarenal interposition grafts into immunodeficient BALB/cRag2−/−IL2rg−/− mice. The following day, recipient mice were reconstituted i.p. with 10 × 106 human PBMC isolated from healthy blood donors. The PBMC donors were allogeneic to the vessel donors, as determined by HLA typing (not shown). The arterial grafts were harvested 30 days after transplantation for analysis (Figure 1A). Transplantation of a human vessel into an otherwise immunodeficient mouse reconstituted with allogeneic human PBMC results in transplant rejection as evidenced by the development of intimal expansion or TA within the graft (Figure 1B).
The mTOR inhibitor rapamycin promotes Treg expansion both in vitro and in vivo (15–17). We have previously shown that ex vivo expanded CD127loTreg are able to prevent the development of TA in a humanized mouse model of arterial transplantation. However, the effect of the treatment was dose-dependent and efficiency was lost when using suboptimal numbers of Treg (10). Here, we hypothesized that short treatment with low-dose rapamycin would promote the ability of Treg to inhibit TA. We demonstrate that Treg and rapamycin inhibit the proliferation of CD4+ and CD8+ T cells both in vivo and in vitro and induce apoptosis of CD4+ T cells in vitro. Combination treatment of transplant recipients with low-dose rapamycin and subtherapeutic numbers of CD127loTreg led to significantly reduced intimal expansion in the arterial allografts compared to subtherapeutic Treg treatment alone. The inflammatory cytokine IFNG, which plays an important role in the pathogenesis of TA, was inhibited to almost undetectable levels in the serum of mice receiving the combination of Treg and rapamycin.