Cell contact,prostaglandin E2 and transforming growth factor beta 1 play non‐redundant roles in human mesenchymal stem cell induction of CD4+CD25Highforkhead box P3+ regulatory T cells

Adult human mesenchymal stromal or stem cells (MSC) can differentiate into a variety of cell types and are candidate cellular therapeutics in regenerative medicine. Surprisingly, these cells also display multiple potent immunomodulatory capabilities, including allosuppression, making allogeneic cell therapy a possibility. The exact mechanisms involved in regulatory T cell induction by allogeneic human MSC was examined, using purified CD4⁺ populations and well‐characterized bone marrow‐derived adult human MSC. Allogeneic MSC were shown to induce forkhead box P3 (FoxP3)⁺ and CD25⁺ mRNA and protein expression in CD4⁺ T cells. This phenomenon required direct contact between MSC and purified T cells, although cell contact was not required for MSC induction of FoxP3 expression in an unseparated mononuclear cell population. In addition, through use of antagonists and neutralizing antibodies, MSC‐derived prostaglandins and transforming growth factor (TGF)‐β1 were shown to have a non‐redundant role in the induction of CD4⁺CD25⁺FoxP3⁺ T cells. Purified CD4⁺CD25⁺ T cells induced by MSC co‐culture expressed TGF‐β1 and were able to suppress alloantigen‐driven proliferative responses in mixed lymphocyte reaction. These data clarify the mechanisms of human MSC‐mediated allosuppression, supporting a sequential process of regulatory T cell induction involving direct MSC contact with CD4⁺ cells followed by both prostaglandin E₂ and TGF‐β1 expression. Overall, this study provides a rational basis for ongoing clinical studies involving allogeneic MSC.