CD44 expression positively correlates with Foxp3 expression and suppressive function of CD4⁺ T_reg cells

Background: CD4⁺CD25⁺ regulatory T (T_reg) cells develop in the thymus and can suppress T cell proliferation, modulated by Foxp3 and cytokines; however, the relevance of CD44 in T_reg cell development is less clear. To address this issue, we analyzed Foxp3 expression in CD44⁺ T_reg cells by using multiple parameters, measured the levels of the immunoregulatory cytokine interleukin (IL)-10 in various thymocyte subsets, and determined the suppressor activity in different splenic T_reg cell populations. Results: Within mouse thymocytes, we detected T_reg cells with two novel phenotypes, namely the CD4⁺CD8^-CD25⁺CD44⁺ and CD4⁺CD8^-CD25⁺CD44^- staining features. Additional multi-parameter analyses at the single-cell and molecular levels suggested to us that CD44 expression positively correlated with Foxp3 expression in thymocytes, the production of IL-10, and T_reg activity in splenic CD4⁺CD25⁺ T cells. This suppressive effect of T_reg cells on T cell proliferation could be blocked by using anti-IL-10 neutralizing antibodies. In addition, CD4⁺CD25⁺CD44⁺ T_reg cells expressed higher levels of IL-10 and were more potent in suppressing effector T cell proliferation than were CD4⁺CD25⁺CD44^- cells. Conclusion: This study indicates the presence of two novel phenotypes of T_reg cells in the thymus, the functional relevance of CD44 in defining T_reg cell subsets, and the role of both IL-10 and Foxp3 in modulating the function of T_reg cells. Reviewers: This article was reviewed by Dr. M. Lenardo, Dr. L. Klein & G. Wirnsberger (nominated by Dr. JC Zungia-Pfluker), and Dr. E.M. Shevach.