Mice lacking the transcription factor Foxp3 (Foxp3(-)) lack regulatory T (T(reg)) cells and develop fatal autoimmune pathology. In Foxp3(-) mice, many activated effector T cells express self-reactive T cell receptors that are expressed in T(reg) cells in wild-type mice. Thus, in wild-type mice, most self-reactive thymocytes escaping negative selection are diverted into the T(reg) lineage, and whether T(reg) cells are critical in self-tolerance in wild-type mice remains unknown. Here, acute in vivo ablation of T(reg) cells demonstrated a vital function for T(reg) cells in neonatal and adult mice. We suggest that self-reactive T cells are continuously suppressed by T(reg) cells and that when suppression is relieved, self-reactive T cells become activated and facilitate accelerated maturation of dendritic cells.

Programmed cell death–1 (PD-1) is a coinhibitory receptor that suppresses T cell activation and is an important cancer immunotherapy target. Upon activation by its ligand PD-L1, PD-1 is thought to suppress signaling through the T cell receptor (TCR). By titrating PD-1 signaling in a biochemical reconstitution system, we demonstrate that the co-receptor CD28 is strongly preferred over the TCR as a target for dephosphorylation by PD-1–recruited Shp2 phosphatase. We also show that CD28, but not the TCR, is preferentially dephosphorylated in response to PD-1 activation by PD-L1 in an intact cell system. These results reveal that PD-1 suppresses T cell function primarily by inactivating CD28 signaling, suggesting that costimulatory pathways play key roles in regulating effector T cell function and responses to anti–PD-L1/PD-1 therapy.

In the course of infection or autoimmunity, particular transcription factors orchestrate the differentiation of T H 1, T H 2 or T H 17 effector cells, the responses of which are limited by a distinct lineage of suppressive regulatory T cells (T reg ). T reg cell differentiation and function are guided by the transcription factor Foxp3, and their deficiency due to mutations in Foxp3 results in aggressive fatal autoimmune disease associated with sharply augmented T H 1 and T H 2 cytokine production [1][2][3] . Recent studies suggested that Foxp3 regulates the bulk of the Foxp3-dependent transcriptional program indirectly through a set of transcriptional regulators serving as direct Foxp3 targets 4,5 . Here we show that in mouse T reg cells, high amounts of interferon regulatory factor-4 (IRF4), a transcription factor essential for T H 2 effector cell differentiation, is dependent on Foxp3 expression. We proposed that IRF4 expression endows T reg cells with the ability to suppress T H 2 responses. Indeed, ablation of a conditional Irf4 allele in T reg cells resulted in selective dysregulation of T H 2 responses, IL4-dependent immunoglobulin isotype production, and tissue lesions with pronounced plasma cell infiltration, in contrast to the mononuclear-cell-dominated pathology typical of mice lacking T reg cells. Our results indicate that T reg cells use components of the transcriptional machinery, promoting a particular type of effector CD4 + T cell differentiation, to efficiently restrain the corresponding type of the immune response.T reg cell deficiency results in activation and expansion of CD4 + and CD8 + T cells, dendritic cells, granulocytes and macrophages, and greatly increased production of a wide range of cytokines including interleukin (IL)-2, T H 1 and T H 2 cytokines 6,7 . Expression of Foxp3 is required for the establishment and maintenance of T reg lineage identity and suppressor function [8][9][10][11] . Our recent study suggested that in T reg cells Foxp3 might regulate expression of IRF4 (refs 12 -14) a transcription factor that is indispensable for T H 2 effector cell differentiation 15,16 . Furthermore, a recent study suggested a prominent role for IRF4 in T H 17Correspondence and requests for materials should be addressed to A.Y.R. (rudenska@mskcc.org). † Present address: Department of Immunology, Memorial Sloan-Kettering Cancer Center, New York, New York 10021, USA.Supplementary Information is linked to the online version of the paper at www.nature.com/nature.Author Information Reprints and permissions information is available at www.nature.com/reprints. Full Methods and any associated references are available in the online version of the paper at www.nature.com/nature. Fig. 1a, b). Irf4 messenger RNA was increased in thymic and peripheral Foxp3 + T reg cells in comparison to CD25 − Foxp3 − CD4 + T cells (data not shown) 8 . Furthermore, Foxp3 knockdown using a retrovirally encoded Foxp3-specific short hairpin RNA resulted in a marked diminution in Irf4 mRNA ( Supplementary Fig. 1c 1...

Summary Regulatory T (Treg) cells, whose identity and function are defined by the transcription factor Foxp3, are indispensable for immune homeostasis. It is unclear whether Foxp3 exerts its Treg lineage specification function through active modification of the chromatin landscape and establishment of new enhancers or by exploiting a pre-existing enhancer landscape. Analysis of the chromatin accessibility of Foxp3-bound enhancers in Treg and Foxp3-negative T cells showed that Foxp3 was bound overwhelmingly to pre-accessible enhancers occupied by its cofactors in precursor cells or a structurally related predecessor. Furthermore, the bulk of Foxp3- bound Treg cell enhancers lacking in Foxp3− CD4+ cells became accessible upon T cell receptor activation prior to Foxp3 expression with only a small subset associated with several functionally important genes being exclusively Treg cell-specific. Thus, in a late cellular differentiation process Foxp3 defines Treg cell functionality in an “opportunistic” manner by largely exploiting the preformed enhancer network instead of establishing a new enhancer landscape.

Targeted inhibition of mitogen-activated protein kinase (MAPK) kinase (MEK) can induce regression of tumors bearing activating mutations in the Ras pathway but rarely leads to tumor eradication. Although combining MEK inhibition with T-cell-directed immunotherapy might lead to more durable efficacy, T cell responses are themselves at least partially dependent on MEK activity. We show here that MEK inhibition did profoundly block naive CD8(+) T cell priming in tumor-bearing mice, but actually increased the number of effector-phenotype antigen-specific CD8(+) T cells within the tumor. MEK inhibition protected tumor-infiltrating CD8(+) T cells from death driven by chronic TCR stimulation while sparing cytotoxic activity. Combining MEK inhibition with anti-programmed death-ligand 1 (PD-L1) resulted in synergistic and durable tumor regression even where either agent alone was only modestly effective. Thus, despite the central importance of the MAP kinase pathway in some aspects of T cell function, MEK-targeted agents can be compatible with T-cell-dependent immunotherapy.