The rapid decrease of the b-gal activity in the ears of immunocompetent mice following intrapinnal transplantation of LacZ-transduced US11-hMSCs, and its prevention by NK cell depletion points to NK cell-mediated rejection of the MHC class I2 donor cells in this animal model. The kinetics of donor cell infiltration into the implanted pinnea, as observed in this study, differs from the acute infiltration of innate immune cells into solid organ implants or into sites of cutaneous inflammation. We propose that this delay is caused by the implanted hMSCs and in particular their ability to modulate the cytokine milieu near the site of cell injection. DAPT Gamma-secretase inhibitor implantation of hMSCs into ischemic kidneys XAV939 resulted, after 24 hours, in an increase in anti-inflammatory cytokine gene expression and in a decrease in mRNA levels for the pro-inflammatory cytokines IFN-c, TNF-a and IL-1b. Interestingly, in MSC-treated infarcted rat hearts pro/antiinflammatory cytokine gene expression ratios increased between 24 hours and 2 weeks after MSC implantation. This phenomenon may well explain the delayed leukocyte infiltration following hMSC implantation in our experiments. Alternatively, the observed decline may reflect an intrinsic property of implanted ����free cells���� that have been deposited in an unfavorable microenvironment. This perception is supported by results of Eliopoulos and colleagues, showing a limited persistence of murine MSCs following subcutaneous implantation into syngeneic recipients. Long-term survival of the implanted cells was significantly improved by embedding the MSCs into a matrix prior to implantation, emphasizing that the subcutis represents a suboptimal milieu for MSC transplantation. The observation that US11-hMSC numbers diminish in NK cell-depleted C57Bl mice with similar kinetics as in NOD/SCID mice suggests that in vivo these MHC-class I2 cells do not activate any other immune effector cells than theNKcells. This is especially interesting in view of our finding that like in US11-transduced tumor cell lines, in vitro exposure of US11-hMSCs to IFN-c alleviates the effect of the immunoevasin and upregulates cell surface MHC class II protein expression. Apparently, such upregulation does not occur to any significant extent in vivo. Rather, a strong inhibition of the expression of genes encoding pro-inflammatory cytokines including IFN-c was recorded in injured kidney tissue of rats following administration of MSCs. If the interpretation of our results is correct, endowing hMSCs with the US11 protein and an immunoevasin that efficiently blocks NK cell-mediated cytolysis would result in completely non-immunogenic hMSCs for universal application.