MSC mediate immunosuppression of T and NK cells via different mechanisms . Soluble factors secreted by MSC such as iNOS, IDO, PGE2, sHLA-G5 can suppress T- and NK cell functions, whereas galectin-1 inhibits T cells but not NK cells. In addition, MSC can indirectly mediate immunosuppression by inhibiting dendritic cells and inducing the expansion of regulatory T cells (Tregs). Furthermore, MSC can directly interact with T and NK cells via cell to cell contact. However, receptors and ligands involved in the cell contact-dependent interaction mechanisms are still largely unknown.