Extracellular Ubiquitin Enhances the Suppressive Effects of Regulatory T cells on Effector T cell Responses

Background: CD4(+)CD25(+) FoxP3(+) Regulatory T (Treg) cells have been proven to play important roles in immune homeostasis, especially immune tolerance, mainly by regulating the function of CD4(+)CD25(-) effector T (Teff) cells. As the reduced activity of Treg cells is closely related with autoimmune diseases, development of strategies for modulating the activity of Treg cells has become the focus of recent studies. Methods: We first determined the binding efficiency of Treg cells in human peripheral blood mononuclear cells (PBMCs) to FITC-labeled ubiquitin by flow cytometry. Using transwell assays, we evaluated the effect of ubiquitin on chemotactic migration of primary Treg cells. By multi-parametric flow cytometry analysis, we analyzed the influence of extracellular ubiquitin on the regulatory function and the apoptosis of Treg cells. Results: We observed that primary Treg cells and FITC-labeled extracellular ubiquitin show affinity for each other. In addition, we found that ubiquitin exhibited an enhancing effect on chemotactic migration of Treg cells. The addition of ubiquitin to co-cultures of Treg and Teff cells enhanced the inhibitory effect of Treg cells on Teff cell proliferation. Moreover, the apoptosis of Treg cells could be reduced after ubiquitin treatment. Conclusions: We proved that extracellular Ub could significantly affect the regulatory function of Treg cells by binding directly to them and, thus, modulate the immune response probably through the indirect control of Teff cells.