How do mesenchymal stromal cells suppress T cells?

Accumulating information indicates that mesenchymal stem or stromal cells (MSCs) are immunomodulatory, but the data to explain the observations are frequently conflicting. In this issue of Cell Stem Cell, Ren et al. (2008) provide evidence for a possible underlying mechanism of MSC-mediated T cell suppression. A perspective for considering these interesting observations is discussed.