The Journal of Experimental Medicine JEM © The Rockefeller University Press $8.00 Vol. 201, No. 5, March 7, 2005 737–746 www.jem.org/cgi/doi/10.1084/jem.20040685 ARTICLE 737 T cells that cannot respond to TGF- escape control by CD4  CD25  regulatory T cells Linda Fahlén, 1 Simon Read, 1 Leonid Gorelik, 2 Stephen D. Hurst, 3 Robert L. Coffman, 4 Richard A. Flavell, 2 and Fiona Powrie 1 1 Sir William Dunn School of Pathology, University of Oxford, Oxford OX1 3RE, England, UK 2 Section of Immunobiology, Howard Hughes Medical Institute, Yale University School of Medicine, New Haven, CT 06520 3 Department of Immunology, Genentech, South San Francisco, CA 94080 4 Dynavax Technologies Corporation, Berkeley, CA 94710 CD4  CD25  regulatory T (T reg) cells play a pivotal role in control of the immune response. Transforming growth factor- (TGF-) has been shown to be required for T reg cell activity; however, precisely how it is involved in the mechanism of suppression is poorly understood. Using the T cell transfer model of colitis, we show here that CD4  CD45RB high T cells that express a dominant negative TGF- receptor type II (dnTRII) and therefore cannot respond to TGF-, escape control by T reg cells in vivo. CD4  CD25  T reg cells from the thymus of dnTRII mice retain the ability to inhibit colitis, suggesting that T cell responsiveness to TGF- is not required for the development or peripheral function of thymic-derived T reg cells. In contrast, T reg cell activity among the peripheral dnTRII CD4  CD25  population is masked by the presence of colitogenic effector cells that cannot be suppressed. Finally, we show that CD4  CD25  T reg cells develop normally in the absence of TGF-1 and retain the ability to suppress colitis in vivo. Importantly, the function of TGF-1 / T reg cells was abrogated by anti–TGF- monoclonal antibody, indicating that functional TGF- can be provided by a non–T reg cell source. Naturally occurring regulatory T (T reg) cells mediate a nonredundant role in control of the immune response. Among these populations, the most well-characterized are those con- tained within the CD4  CD25  subset (1–3). Although initially identified for their ability to prevent autoimmune disease (4), CD4  CD25  T reg cells are now known to mediate a more general suppressive role, acting to limit immune pathology in the face of chronic immune stimu- lation (5–8). Even though the list of immune responses affected by CD4  CD25  T reg cells continues to expand, their mechanism of action remains obscure. Attention has focused on the role of cell contact–dependent mechanisms and the actions of immunoregulatory cytokines such as IL-10 and TGF- (1). The relative contribution of these different mechanisms to suppressor function is highly controversial and may vary depending on the nature of the immune re- sponse being regulated. For example, TGF- appears to play a nonredundant role in control of intestinal inflammation and diabetes (6, 9, 10), but not gastritis (11). A proportion of CD4  CD25  cells express TGF-1 on their surface (12) and this has been implicated in their suppressor function in vitro. However, TGF-1  CD4  CD25  cells retain T reg cell activity in vitro, indicating that TGF-1 synthesis by CD4  CD25  cells is not essential for cell contact–dependent suppression (11). The TGF-s, encompassing TGF-1, TGF-2, and TGF-3, are highly pleiotropic cytokines with diverse effects on many devel- opmental and physiological processes. TGF-1 is the most abundant form in lymphoid organs and has a number of effects on cells of the im- mune system, including inhibition of T cell proliferation and differentiation and negative effects on macrophage activation and DC mat- uration (for review see reference 13). It plays a pivotal role in immune regulation as TGF- 1  mice develop a multi-organ inflamma- tory disease (14, 15). Recently, TGF-1 has been shown to act as a costimulatory factor for expression of FoxP3 (16), leading to the differ- entiation of CD4  CD25  T reg cells from pe- ripheral CD4  CD25  progeny (17, 18). These L. Fahlén and S. Read contributed equally to this work. The online version of this article contains supplemental material. CORRESPONDENCE Fiona Powrie: fiona.powrie@path.ox.ac.uk Abbreviations used: dnTRII, dominant negative TGF- re- ceptor II; LP, lamina propria; Tg, transgenic; T reg, regulatory T.