Regulatory T cell suppression is potentiated by target T cells in a cell contact, IL-35- and IL-10-dependent manner

Regulatory T cells (Treg) are believed to suppress conventional T cell (Tconv) proliferation in vitro in a contact-dependent, cytokine-independent manner, based in part on experiments in which Treg and Tconv are separated by a permeable membrane. We show that the production of IL-35, a novel inhibitory cytokine expressed by natural Treg, increases substantially following contact with Tconv. Surprisingly, Treg were able to mediate potent suppression of Tconv across a permeable membrane when placed in direct contact with Tconv in the upper chamber of a Transwell plate. Suppression was IL-35 and IL-10 dependent, and Tconv activation was required for maximal potentiation of Treg suppression. These data suggest that it is the induction of suppression, rather than the function of Treg that is obligatorily contact dependent. The Journal of Immunology, 2009, 182: 6121–6128. A lthough several studies have assessed the mechanism ofregulatory T cell (Treg)3-mediated suppression, manydetails remain unknown. It is thought that Treg can sup-press through direct contact with target cells or APCs (1–3) and by using secreted cytokines (4–6). The conventional assay for Treg function in vitro tests the ability of Treg to suppress conventional T cell (Tconv) proliferation when cultured together in a tissue cul-ture plate. Experiments using Transwell inserts, in which Treg and Tconv are separated by a permeable membrane, have demon-strated that Treg are essentially unable to suppress Tconv prolifer