Mesenchymal stem cells (MSCs) display immunomodulatory properties mediated by various factors, including inducible nitric oxide synthase (iNOS). Since heme oxygenase-1 (HO-1) is a potent immunosuppressive enzyme, we tested the hypothesis that HO-1 could mediate the immunosuppressive effects of MSCs. We generated adult rat MSCs that inhibited T-cell proliferation in vitro. These MSCs expressed both HO-1 and iNOS. In vitro, whereas neither HO-1 nor iNOS inhibition alone could interfere with the immunosuppressive properties of rat MSCs, simultaneous inhibition of both enzymes restored T-cell proliferation. In vivo, injection of MSCs significantly delayed heart allograft rejection, and inhibition of either HO-1 or iNOS totally reversed the protective activity of MSCs, inducing rejection. Adult human MSCs also expressed HO-1; in these cells, HO-1 inhibition was sufficient to completely block their immunosuppressive capacity. In con IntroductionThe possibility that mesenchymal stem cells (MSCs) could modulate the immune response in vivo 1 was first suggested by skin graft experiments in nonhuman primates. Strong nonspecific immunosuppressive properties have been reported in vitro for both humans and rodent MSCs. [2][3][4] Various mechanisms have been proposed to explain such properties, including production of transforming growth factor beta (TGF␤), 5 hepatocyte growth factor, PGE2, 6 IL-10, 5 and indoleamine 2,3-dioxygenase (IDO), 7,8 as well as suppression of antigen-presenting cell (APC) maturation. 9,10 A recent study demonstrated that the suppressive activity of murine MSCs is dependent on inducible NO synthase (iNOS) expression. 11 However, the precise mechanism of action of these cells remains poorly understood. It is interesting to note, however, that MSCs are not intrinsically immunoprivileged, since allogeneic MSCs can induce a memory T-cell response. 12 Heme oxygenases (HOs) are the rate-limiting intracellular enzymes that degrade heme to biliverdin, CO, and free divalent iron 13 . The inducible form, HO-1, has been described as an anti-inflammatory 13 and immunosuppressive molecule. 14 Furthermore, HO-1 can mediate the effect of molecules such as IL-10 and NO. 15 We thus hypothesized that HO-1 could contribute to the immunosuppressive properties of adult rat and human MSCs. Materials and methodsThis study was approved by the Institutional Review Board of Nantes University. MSC cultureAdult rat MSCs were obtained from LEW.1A and LEW.1W (complete major histocompatibility complex [MHC] I and II mismatch) bone marrow cells collected by flushing femurs and tibias with alpha MEM medium supplemented with 20% fetal calf serum, penicillin, and streptomycin. Adult rat MSCs were regularly split using trypsin and used before passage 4. Culture of human MSCs was performed in the same conditions from bone marrow aspirates from healthy volunteer donors who had provided informed consent in accordance with the Declaration of Helsinki. Adherent cells displayed a morphology and phenotype AbCys, Paris, France typical of ...

y Both authors contributed equally.The angiotensin II type 1 receptor (AT 1 R) is an emerging target of functional non-HLA antibodies (Ab). We examined the potential of determining the degree of presensitization against AT 1 R as a risk factor for graft survival and acute rejection (AR). The study included 599 kidney recipients between 1998 and 2007. Serum samples were analyzed in a blinded fashion for anti-AT 1 R antibodies (AT 1 R-Abs) using a quantitative solid-phase assay. A threshold of AT 1 R-Ab levels was statistically determined at 10 U based on the time to graft failure. An extended Cox model determined risk factors for occurrence of graft failure and a first AR episode. AT 1 R-Abs >10 U were detected in 283 patients (47.2%) before transplantation. Patients who had a level of AT 1 R-Abs >10 U had a 2.6-fold higher risk of graft failure from 3 years posttransplantation onwards (p ¼ 0.0005) and a 1.9-fold higher risk of experiencing an AR episode within the first 4 months of transplantation (p ¼ 0.0393). Antibody-mediated rejection (AMR) accounted for 1/3 of AR, whereby 71.4% of them were associated with >10 U of pretransplant AT 1 R-Abs. Pretransplant anti-AT 1 R-Abs are an independent risk factor for long-term graft loss in association with a higher risk of early AR episodes.