Progenitor cells suppress t-cell proliferation via a paracrine mechanism

Abstract

Purpose: Limited treatment options are available for heart failure patients. Stem cell therapy has recently become a potential new way of achieving repair of injured cardiac tissue. Several different progenitor cell sources have been investigated, but most promising for cardiac therapy are mesenchymal stem cells (MSC) and cardiomyocyte progenitor cells (CMPC). Cardiac stem cell therapy using MSC or CMPC improved cardiac function, despite low engraftment of the cells. It is likely that paracrine factors, produced by the injected cells are causing these improvements. Many studies are performed on the paracrine effects, however, modulation of the immune response in cardiac stem cell therapy, especially the strong influence of T-lymphocytes on adverse remodeling, has not been explored extensively. Material and methods: Human fetal MSC and CMPC were characterized using flow cytometry and tested for multipotency by determining differentiation into all mesenchymal or cardiac lineages. The immunosuppressive properties of both cell types were tested in co-culture with freshly isolated, allogeneic peripheral blood mononuclear cells (PBMC) or T-lymphocytes stimulated with IL-2 and PMA. Proliferation was measured by CFSE-analysis using flow cytometry. Results: Proliferation of PBMC and T-lymphocytes was significantly reduced in the presence of MSC (65±8%) or CMPC (97±0.6%). In addition, the inflammatory cytokine panel of the cells in culture changed, with strong downregulation of IFN-gamma and TNF-alpha. This effect was observed in both direct cell contact as well as in transwell co-culture systems (MSC: 58±10%; CMPC: 62±9%). Transfer of conditioned medium from these co-cultures to unrelated, activated PBMC or T-lymphocytes abrogated proliferation in these cells to a similar extent as the original co-culture (MSC: 51±8%; CMPC: 97±0.7%). Interestingly, exosomes isolated from the conditioned medium of MSC and CMPC prevented T-lymphocyte proliferation in a dose-dependent fashion. At a concentration of 1.5μg, T-lymphocyte proliferation was significantly suppressed (MSC-exosomes: 73±12%; CMPC-exosomes: 77±10%). Conclusion: Both MSC and CMPC have a strong capacity for in vitro immunosuppression, which is mediated by paracrine factors. One potent immunosuppressive factor secreted by both MSC and CMPC are exosomes, which prevented T-lymphocyte proliferation in a dose-dependent fashion.