A message of peace

Credit: NPG/S.Bradbrook

Regulatory T (T_Reg) cells use a variety of strategies to suppress inflammation and protect the host from immune-mediated pathology. Okoye et al. now describe an additional skill in the T_Reg cell repertoire; they have found that T_Reg cells can suppress effector T cells by delivering microRNAs (miRNAs) via exosomes.

The authors measured exosome release by different populations of activated lymphocytes in vitro and found that T_Reg cells produce substantially more exosomes compared with B cells or other T cell populations. Detailed analysis of T_Reg cell-derived exosomes showed that they contained premature and mature miRNAs, and were particularly enriched in miRNAs with pro-apoptotic or anti-proliferative functions. To examine whether T_Reg cells can deliver miRNAs to effector T cells, the authors developed a flow cytometry-based system that enabled the tracking of T_Reg cell-derived double-stranded RNA. T_Reg cells were shown to transfer RNA to co-cultured T cells, B cells and dendritic cells, even when the cells were physically separated in transwell plates, suggesting that the RNA transfer was mediated by extracellular microvesicles. Experiments in which wild-type T_Reg cells were co-cultured with conventional T cells lacking the endoribonuclease Dicer (which processes miRNAs into their mature form) further supported the idea that T_Reg cells transfer suppressive miRNAs to neighbouring T cells; mature miRNAs could be isolated from the co-cultured Dicer^−/− T cells and these cells showed downregulation of several pro-inflammatory genes.

The authors next examined whether this T_Reg cell-mediated suppressive mechanism operates in vivo. Using a T cell transfer model of colitis, they found that the transfer of Dicer^−/− effector T cells into T cell-deficient mice caused a systemic wasting disease that could be prevented by the co-transfer of wild-type but not Dicer^−/− T_Reg cells. Furthermore, when they compared Dicer^−/− effector T cells that had been transferred alone (so-called 'pathogenic' T cells) with Dicer^−/− effector T cells that were co-transferred with T_Reg cells ('regulated' T cells), they found that the regulated Dicer^−/− T cells expressed lower levels of mRNAs encoding interferon-γ (IFNγ) and tumour necrosis factor, and that they contained the miRNAs miR-155, let-7b and let-7d. By contrast, these miRNAs were not present in the pathogenic Dicer^−/− effector T cells that were transferred alone, indicating that their delivery is T_Reg cell dependent.

Finally, to definitively show that the miRNAs in T_Reg cell-derived exosomes are immunosuppressive, the authors purified exosomes from wild-type and Dicer^−/− T_Reg cells. They found that exosomes from wild-type but not Dicer^−/− T_Reg cells suppressed proliferation and IFNγ production in T helper 1 (T_H1) cell cultures. Comparison of the properties of individual miRNAs from T_Reg cell-derived exosomes suggested that let-7d inhibits Ptgs2 (which encodes cyclooxygenase 2) and is particularly important for suppression of T_H1 cells. Indeed, T_Reg cell-derived exosomes lacking let-7d failed to suppress effector T cells in both in vitro and in vivo systems.

exosome-mediated delivery of miRNAs [is] ... an additional suppressive mechanism used by T_Reg cells

In summary, this study identifies exosome-mediated delivery of miRNAs as an additional suppressive mechanism used by T_Reg cells. By delivering miRNAs that can inhibit Ptgs2, this mechanism may be particularly important for limiting T_H1 cell-associated responses.