The physiological functions of human TCRVγ9Vδ2+ γδ lymphocytes reactive to non-peptide phosphoantigens contribute to cancer immunosurveillance and immunotherapy. However, their regulation by mesenchymal stem cells (MSC), multipotent and immunomodulatory progenitor cells able to infiltrate tumors, has not been investigated so far. By analyzing freshly isolated TCRVγ9Vδ2+ lymphocytes and primary cell lines stimulated with synthetic phosphoantigen or B-cell lymphoma cell lines in the presence of MSC, we demonstrated that MSC were potent suppressors of γδ-cell proliferation, cytokine production and cytolytic responses in vitro. This inhibition was mediated by the COX-2-dependent production of prostaglandin E2 (PGE2) and by MSC through EP2 and EP4 inhibitory receptors expressed by Vγ9Vδ2 T lymphocytes. COX-2 expression and PGE2 production by MSC were not constitutive, but were induced by IFN-γ and TNF-α secreted by activated Vγ9Vδ2 T cells. This regulatory cross-talk between MSC and Vγ9Vδ2 T lymphocytes involving PGE2 could be of importance for the antitumor and antimicrobial activities of γδ T cells.