BACKGROUND AIMS. Because data on the immunosuppressive effect of different subsets of mesenchymal stromal cells (MSC) are sparse, we investigated the molecular and cellular mechanisms underlying the allosuppressive effect of MSC generated from bone marrow CD271(+) cells (CD271-MSC) and asked whether this potential is comparable with that of MSC generated through plastic adherence (PA-MSC). METHODS. The immunosuppressive effect of CD271-MSC on the allogeneic reaction was investigated by mixed lymphocyte reaction (MLR). RESULTS. CD271-MSC significantly suppressed the alloantigen-induced proliferation of mononuclear cells (MNC) of two HLA-disparate donors at all MSC:MNC ratios, 1:1, 1:2 and 1:10. They also demonstrated a significantly higher allosuppression than PA-MSC at an MSC:MNC ratio of 1:1. This inhibitory effect was associated with significantly elevated levels of prostaglandin E2 (PGE2) at ratios of 1:1 and 1:2 (about 4-fold), but not at a ratio of 1:10. Indomethacin, and inhibitor of cyclooxygenase-1 and 2 necessary for the biosynthesis of PGE2, mitigated suppressive effects of CD271-MSC only at a ratio of 1:1, indicating that PGE2 is not involved in MSC-mediated inhibition when allogeneic MNC are in excess. The increase of PGE2 was associated with a significant decrease of pro-inflammatory cytokine levels (interferon-gamma and tumor necrosis-alpha), while no changes in levels of interleukin-10, soluble HLA-G and nitric oxide were observed. In addition, CD271-MSC induced an expansion of highly suppressive naive CD4(+)CD25(high)CD45RA(+)CD62L(+) T-regulatory cells, which may extend their allosuppressive effect. CONCLUSIONS. Our data suggest that CD271-MSC exert potent allosuppressive properties and therefore can be used as a reasonable alternative to PA-MSC for the treatment of patients with graft-versus-host disease.