Sascha Rutz

Interleukin-2 is essential for CD4+CD25+ regulatory T cell function

Constitutive expression of CD25, the IL‐2 receptor α‐chain, defines a distinct population of CD4+ T cells (Treg) with suppressive activity in vitro and in vivo. IL‐2 has been implicated in the generation and maintenance of Treg, however, a functional contribution of the IL‐2 receptor during suppression is thus far unknown. We show that IL‐2 is required for Treg function in vitro, since suppression is completely abrogated by selective blocking of the IL‐2 receptor on Treg during co‐culture with responder T cells. We demonstrate that Treg, which do not produce IL‐2, compete for IL‐2 secreted by responder T cells. In accordance with the idea of competition being part of the suppressive mechanism, in vitro neutralization of IL‐2 mimics all effects of Treg. Conversely, recombinant IL‐2 abrogates inhibition of IL‐2 production in responder T cells, the hallmark of Treg suppression. Finally, activation in the presence of IL‐2 primes Treg to produce IL‐10 upon secondary stimulation, indicating that IL‐2 uptake is also required to induce additional suppressive factors that might be more relevant for suppression in vivo. We propose the parakrine uptake of soluble mediators as a flexible mechanism to adapt Treg activity to the strength of the responder T cell reaction.

IL‐22, not simply a Th17 cytokine

Interleukin‐22 (IL‐22) has important functions in host defense at mucosal surfaces as well as in tissue repair. It is unique as a cytokine that is produced by immune cells, including T‐helper (Th) cell subsets and innate lymphocytes, but acts only on non‐hematopoietic stromal cells, in particular epithelial cells, keratinocytes, and hepatocytes. Although IL‐22 is beneficial to the host in many infectious and inflammatory disorders, depending on the target tissue it can be pathogenic due to its inherent pro‐inflammatory properties, which are further enhanced when IL‐22 is released together with other pro‐inflammatory cytokines, in particular IL‐17. To avoid pathology, IL‐22 and IL‐17 production have to be controlled tightly and independently. While common factors such as signal transducer and activator of transcription 3 (STAT3) and retinoid orphan receptor γt (RORγt) drive the expression of both cytokines, other factors, such as c‐Maf act specifically on IL‐22 and enable the separate expression of either cytokine. Here, we discuss the production of IL‐22 from various T‐cell populations as well as protective versus pathogenic roles of IL‐22. Finally, we focus on recent advances in our understanding of the molecular regulation of IL‐22 in T cells.

A Genomic Regulatory Element That Directs Assembly and Function of Immune-Specific AP-1–IRF Complexes

Helping T Helper Transcription Members of the interferon response family of transcription factors (IRFs) are specifically expressed in immune cells and are known to regulate their differentiation. IRF4 and IRF8 regulate gene expression by binding to other transcription factors, which results in their recruitment to composite motifs in the genome. Although the specific mechanism of how this regulation works in some immune cells is understood, how it occurs in T cells is not clear because the transcription factors that normally partner with IRFs are absent. Using genomic analysis, Glasmacher et al. (p. 975, published online 13 September; see the Perspective by Martinez and Rao) now identify IRF4–AP-1 composite elements in T helper 17 (TH17) cells and show that IRF4 and the AP-1 factor Batf cooperatively assemble on a large array of genes required for TH17 cell differentiation and function. Assembly of such heterodimers was also observed in TH2 cells, B cells, and dendritic cells, which suggests the general importance of this motif in immune cell differentiation. Cooperative binding of transcription factors to composite genomic elements regulates T helper 17 cell differentiation. Interferon regulatory factor 4 (IRF4) and IRF8 regulate B, T, macrophage, and dendritic cell differentiation. They are recruited to cis-regulatory Ets-IRF composite elements by PU.1 or Spi-B. How these IRFs target genes in most T cells is enigmatic given the absence of specific Ets partners. Chromatin immunoprecipitation sequencing in T helper 17 (TH17) cells reveals that IRF4 targets sequences enriched for activating protein 1 (AP-1)–IRF composite elements (AICEs) that are co-bound by BATF, an AP-1 factor required for TH17, B, and dendritic cell differentiation. IRF4 and BATF bind cooperatively to structurally divergent AICEs to promote gene activation and TH17 differentiation. The AICE motif directs assembly of IRF4 or IRF8 with BATF heterodimers and is also used in TH2, B, and dendritic cells. This genomic regulatory element and cognate factors appear to have evolved to integrate diverse immunomodulatory signals.

Notch regulates IL-10 production by T helper 1 cells

T helper 1 (Th1) cells mediate powerful cellular immune responses. However, if unbalanced, Th1 immunity eventually may cause pathology. Recently, it has been shown that IL-10, an antiinflammatory cytokine strongly antagonizing Th1-mediated effects, can be produced by Th1 cells and is indeed essential for self-regulation of Th1 immunity. Here, we show that Notch induces IL-10 production in newly developing and already established Th1 cells via a signal transducer and activator of transcription 4 (STAT4)-dependent process. Notch signaling in the presence of the cytokines IL-12 or IL-27 induces Th1 cells to produce large amounts of IL-10 without diminishing IFN-γ production. Notch-modified Th1 cells completely lose their inflammatory capacity and instead are able to actively suppress a Th1 cell-induced delayed-type hypersensitivity (DTH) reaction in an IL-10-dependent fashion. IL-10 production can be elicited by active forms of all four mammalian Notch receptors but was found to be specific for the Delta-like family of Notch ligands. Dendritic cells (DC) selectively acquire Delta-like 4 expression upon stimulation with various Toll-like receptor (TLR) ligands and concomitantly induce IL-10 production by Th1 cells in vitro and in vivo. This effect can be selectively reversed by pharmacological inhibitors of Notch signaling (γ-secretase inhibitor). Our data suggest that Notch regulates IL-10 production in Th1 cells by a STAT4-dependent process that converts proinflammatory Th1 cells into T cells with regulatory activity. This pathway may provide unique opportunities for therapeutic intervention in Th1-driven immune diseases and for Th1-associated vaccination strategies.

Transcription factor c-Maf mediates the TGF-[beta]-dependent suppression of IL-22 production in TH17 cells

Interleukin 22 (IL-22), which is produced by cells of the TH17 subset of helper T cells and other leukocytes, not only enhances proinflammatory innate defense mechanisms in epithelial cells but also provides crucial protection to tissues from damage caused by inflammation and infection. In TH17 cells, transforming growth factor-β (TGF-β) regulates IL-22 and IL-17 differently. IL-6 alone induces T cells to produce only IL-22, whereas the combination of IL-6 and high concentrations of TGF-β results in the production of IL-17 but not IL-22 by T cells. Here we identify the transcription factor c-Maf, which is induced by TGF-β, as a downstream repressor of Il22. We found that c-Maf bound to the Il22 promoter and was both necessary and sufficient for the TGF-β-dependent suppression of IL-22 production in TH17 cells.

The IL-20 subfamily of cytokines — from host defence to tissue homeostasis

The interleukin-20 (IL-20) subfamily of cytokines comprises IL-19, IL-20, IL-22, IL-24 and IL-26. These cytokines are all members of the larger IL-10 family, but have been grouped together to form the IL-20 subfamily based on their usage of common receptor subunits and similarities in their target-cell profiles and biological functions. Members of the IL-20 subfamily facilitate the communication between leukocytes and epithelial cells, thereby enhancing innate defence mechanisms and tissue repair processes at epithelial surfaces. In this Review, we describe the cellular sources and targets of the IL-20 subfamily cytokines, and we detail how their expression is regulated. Much of our understanding of the unique biology of this group of cytokines is still based on IL-22, which is the most studied member of the IL-20 subfamily. Nevertheless, we attempt a broader discussion of the emerging functions of IL-20 subfamily cytokines in host defence, inflammatory diseases, cancer and metabolism.

Notch ligands Delta-like1, Delta-like4 and Jagged1 differentially regulate activation of peripheral T helper cells.

The Notch pathway is involved in cell differentiation processes in various organs and at several developmental stages. The importance of Notch for early T lymphocyte development is well established. Recently, Notch has been implicated in directing naive T helper cell differentiation towards the Th1, Th2 or regulatory T cell lineages. However, the molecular events underlying these processes are poorly understood.We show that the Notch ligands Delta‐like1, Delta‐like4 and Jagged1 differentially affect early T cell activation and proliferation following T cell receptor cross‐linking. Delta‐like1 and Jagged1 induce a dose‐dependent inhibition of early activation markers CD69 and CD25, as well as inhibition of proliferation after anti‐CD3 stimulation of purified CD4+ T cells. Similarly, the rapid activation of transcription factors NF‐AT, AP‐1 and NF‐κB is suppressed. In contrast, triggering of Notch by Delta‐like4 enhances T cell activation and proliferation. The observed effects are dependent on simultaneous cross‐linking of TCR and Notch but independent of γ‐secretase‐mediated cleavage of Notch. These data suggest direct interference between Notch and early TCR signal transduction events, independent of the classical Notch pathway via release of the Notch intracellular domain. A Notch‐mediated alteration of TCR signaling strength may contribute to the recently described modulation of naïve T cell differentiation by Notch ligands.

NRROS negatively regulates reactive oxygen species during host defence and autoimmunity.

Reactive oxygen species (ROS) produced by phagocytes are essential for host defence against bacterial and fungal infections. Individuals with defective ROS production machinery develop chronic granulomatous disease. Conversely, excessive ROS can cause collateral tissue damage during inflammatory processes and therefore needs to be tightly regulated. Here we describe a protein, we termed negative regulator of ROS (NRROS), which limits ROS generation by phagocytes during inflammatory responses. NRROS expression in phagocytes can be repressed by inflammatory signals. NRROS-deficient phagocytes produce increased ROS upon inflammatory challenges, and mice lacking NRROS in their phagocytes show enhanced bactericidal activity against Escherichia coli and Listeria monocytogenes. Conversely, these mice develop severe experimental autoimmune encephalomyelitis owing to oxidative tissue damage in the central nervous system. Mechanistically, NRROS is localized to the endoplasmic reticulum, where it directly interacts with nascent NOX2 (also known as gp91phox and encoded by Cybb) monomer, one of the membrane-bound subunits of the NADPH oxidase complex, and facilitates the degradation of NOX2 through the endoplasmic-reticulum-associated degradation pathway. Thus, NRROS provides a hitherto undefined mechanism for regulating ROS prodution—one that enables phagocytes to produce higher amounts of ROS, if required to control invading pathogens, while minimizing unwanted collateral tissue damage.

Autoregulation of Th1-mediated inflammation by twist1

The basic helix-loop-helix transcriptional repressor twist1, as an antagonist of nuclear factor κB (NF-κB)–dependent cytokine expression, is involved in the regulation of inflammation-induced immunopathology. We show that twist1 is expressed by activated T helper (Th) 1 effector memory (EM) cells. Induction of twist1 in Th cells depended on NF-κB, nuclear factor of activated T cells (NFAT), and interleukin (IL)-12 signaling via signal transducer and activator of transcription (STAT) 4. Expression of twist1 was transient after T cell receptor engagement, and increased upon repeated stimulation of Th1 cells. Imprinting for enhanced twist1 expression was characteristic of repeatedly restimulated EM Th cells, and thus of the pathogenic memory Th cells characteristic of chronic inflammation. Th lymphocytes from the inflamed joint or gut tissue of patients with rheumatic diseases, Crohns disease or ulcerative colitis expressed high levels of twist1. Expression of twist1 in Th1 lymphocytes limited the expression of the cytokines interferon-γ, IL-2, and tumor necrosis factor-α, and ameliorated Th1-mediated immunopathology in delayed-type hypersensitivity and antigen-induced arthritis.

Regulation of epithelial immunity by IL-17 family cytokines

Cutaneous and mucosal epithelial cells function as both a physical barrier and as immune sentinels against environmental challenges, such as microbial pathogens, allergens and stress. The crosstalk between epithelial cells and leukocytes is essential for orchestrating proper immune responses during host defense. Interleukin (IL)-17 family cytokines are important players in regulating innate epithelial immune responses. Although IL-17A and IL-17F promote antibacterial and antifungal responses, IL-17E is essential for defense against parasitic infections. Emerging data indicate that another member of this family, IL-17C, specifically regulates epithelial immunity. IL-17C production serves as an immediate defense mechanism by epithelial cells, utilizing an autocrine mechanism to promote antibacterial responses at barrier surfaces.