Human MSCs express the immunoregulatory molecule HO-1. HO-1 expression was assessed in 9 human bone marrow–derived MSCs by flow cytometry as shown in the representative (A) dot plot and (B) histogram depicting quantitatively an interindividual variability. (C) Prior MLR HO-1 was blocked in 6 different MSCs (in a minimum of 2 independent 5-day MLRs for every investigated MSC) using 50μM of the protoporphyrin SnPP. The ability of MSCs to suppress T-cell alloreactions was assessed as (i) representatively shown for MSC103 by measuring the thymidine incorporation in responder PBLs. RLC indicates responder PBLs alone. (ii) Suppression by individual untreated MSCs was compared with their SnPP-pretreated counterparts. (D) Expression levels of HO-1 in the MSCs defined as the mean fluorescence intensity (MFI) were correlated with their ability to suppress alloactivated T cells. (E) The relative basal gene expression levels (mRNA) of Nrf2 and HO-1 were correlated in the MSCs from different donors (n = 8). All experimental settings were conducted with MSCs derived from at least 3 individual donors, and n refers to the number of repeated experiments. Bar represents SD.