Previous studies have shown the immunomodulatory properties of bone marrow mesenchymal stem cells (BM-MSCs), opening the possibility of using these cells for the treatment of graft-versus-host disease (GVHD) in patients transplanted with allogeneic hematopoietic grafts. Additionally, Phase I studies in patients with Crohn’s disease suggested the efficacy of adipose tissue-derived mesenchymal stem cells (Ad-MSCs) for the healing of Crohn’s fistulas. In the present study we have investigated in vitro and in vivo, the immunomodulatory effects of Ad-MSCs, compared to BM-MSCs. We observed that both BM-MSCs and Ad-MSCs were negative for CD34, CD45, CD14, CD31 and MHC class I expression, while positive for CD29, CD44, CD90 and CD105. When studying the immunomodulatory effects of these cells in vitro, we found that - as happened with BM-MSCs - Ad-MSCs did not induce proliferation of allogeneic lymphocytes and were not lysed by cytotoxic T cells or alloreactive natural killer cells, indicating that Ad-MSCs are non-immunogenic. Additionally, the presence of Ad-MSCs inhibited in a dose-dependent fashion, both the mixed lymphocyte reaction (MLR) and the T cell proliferation induced by mitogens. To determine whether cell-to-cell contact between Ad-MSCs and PBMNCs was required for immunosuppression, transwell experiments were conducted. Phytohaemagglutinin (PHA)-stimulated lymphocytes were cultured in the upper chamber of a transwell, while irradiated Ad-MSCs remained in the lower chamber. As observed with BM-MSCs, Ad-MSCs were also capable of suppressing the lymphocytes proliferation in this transwell assay. When conditioned medium from Ad-MSCs was added to the MLR, the immuno-suppressive effect persisted, although at a lower level than that observed in a cell-to-cell contact system. Next we studied whether our in vitro findings were of significance in an in vivo mouse model of haploidentical transplantation. In these experiments irradiated F1(C57Bl/Balbc) recipient mice received 1x107 bone marrow cells from C57Bl mice, together with 2x107 splenocytes from the donor, to induce GVHD. One cohort of recipient mice received additional i.v. infusions of 5x105 mouse Ad-MSCs, administered at periodic intervals for up to 28 days post-transplant. When compared to the control group, the severity of the GVHD was significantly reduced in mice receiving Ad-MSCs. Our results suggest that Ad-MSCs obtained from adipose tissue may constitute a new and readily available source of immunomodulatory cells for the prophylaxis and/or treatment of GVHD in patients transplanted with allogeneic grafts.