The human leukocyte antigen-independent immune-modulatory potential of human mesenchymal stem cells (hMSC) makes them a promising candidate for clinical cell therapy. A better understanding of their "immune-privileged" status is therefore of high priority. Here we used Ki-67-antigen staining to estimate T-cell alloreactivity in mixed lymphocyte cultures in the presence of hMSC as second or third party. We found that the allostimulatory activity of mesenchymal stem cells (MSC) leading to an increased T-cell proliferation and interleukin-2 secretion is measurable only at low MSC/effector ratios (<or =0.1:1). Moreover, this stimulating effect could be efficiently suppressed by MSC-conditioned medium. This suggests that the "immune-privileged" status of MSC exists only when MSC-mediated downregulation of immune cell activation can overrule their own allostimulatory potential. Thus the "immune-privileged" state of MSC represents a sensitive balance of suppressing and activating effects, which should be considered in a clinical setting with limited cell amounts.
