Foxp3 is the master transcription factor for T regulatory (Treg) cell differentiation and function. This study aimed to test the therapeutic potential of cell penetrating recombinant Foxp3 protein in arthritis. Recombinant Foxp3 protein was fused to a cell penetrating polyarginine (Foxp3-11R) tag to facilitate intracellular transduction. In vitro Foxp3-11R treated CD4(+) T cells showed a 50% increase in suppressive function compared with control protein treated cells. Severity of arthritis in Foxp3-11R treated mice was significantly reduced compared with those treated with a control protein. CD4(+) T cells of lymph nodes and spleen from Foxp3-11R treated mice showed increased levels of Foxp3 expression compared with those of a control protein treated. These results demonstrated that Foxp3-11R can enhance T cell suppressive function and ameliorate experimental arthritis and suggest that cell penetrating recombinant Foxp3 is a potentially useful agent in therapy of arthritis. Published by Elsevier Inc.