Interleukin-6 induces the generation of IL-10-producing Tr1 cells and suppresses autoimmune tissue inflammation

Abstract

Compared with its pro-inflammatory function, the mechanisms underlying the anti-inflammatory effect of IL-6 are poorly understood. IL-6 can cooperate with TGF-β to induce IL-10 production in Th17 cells in vitro. However, the effect of IL-6 on generation of Tr1 cells and the in vivo importance of this effect are mostly uncharacterized. In this study, we showed that in vitro, IL-6 can induce the generation of IL-10-producing Tr1 cells from naïve CD4 T cells, independently of IL-27 and TGF-β. IL-6 induces IL-21 production in CD4 T cells and IL-10-inducing effect of IL-6 requires both IL-21 and IL-2. Although IL-6 cannot induce IL-10 production in CD8 T cells in a cell-autonomous manner, it can do so indirectly through promoting CD4 T cell IL-21 production. The IL-10-producing T cells induced by IL-6 have phenotypic, genetic and functional traits of Tr1 cells and can suppress LPS-induced in vivo inflammatory response in an IL-10-dependent fashion. Blockade of IL-6 in two autoimmune inflammation models, induced respectively by anti-CD3 antibody or Treg-depletion, led to reduction in IL-10-producing T cells and exacerbated inflammation of lung and intestine. Thus, we delineated critical pathways involved in IL-6-induced generation of Tr1 cells and demonstrated the importance of this event in restraining autoimmune tissue inflammation.

Introduction

Interleukin-10 (IL-10) is a key anti-inflammatory cytokine which down-modulates both innate and adaptive immune response elicited by pathogens or self-antigens and suppresses tissue inflammation and damage [1], [2]. IL-10 potently suppresses the maturation of antigen presenting cells and also inhibits the production of pro-inflammatory cytokines by these cells [1], [2], [3]. In addition, IL-10 also can directly suppress T cell differentiation into IL-17 and IFN-γ-producing effector subsets [4], [5]. The essential role of IL-10 in suppressing inflammatory immune response and autoimmune response has been clearly demonstrated in IL-10-deficient mice, which spontaneously develop autoimmune inflammatory bowel disease largely due to heightened T helper 1 (Th1) and T helper 17 (Th17) activities and defective regulatory T cell function. These mice also have increased susceptibility to immune-related tissue inflammation and pathology in response to various microbial infections and chemical irritations [3], [6], [7].

Although IL-10 can be produced by diverse lineages of immune cells [1], [2], [8], T cell-derived IL-10 is essential for suppressing T cell hyperactivity, intestinal inflammation [7] and influenza virus-elicited lung inflammation and mortality [9]. Almost all the major T cell subsets can produce IL-10 when instructed by appropriate signals [1], [2], [8]. Regulatory type 1 helper T (Tr1) cells, an inducible subset of regulatory T cells which lack Foxp3, produce high levels of IL-10 and depend on IL-10 for their differentiation and function [1], [10]. The Foxp3⁺ regulatory T cells (Tregs) also can produce IL-10 [2], [8]. Both types of regulatory T cells employ IL-10 as a critical mechanism to suppress intestinal inflammation. Furthermore, effector Th1, Th2, Th17 and CD8 T cells also produce various amount of IL-10, which helps to limit the magnitude of respective immune response against pathogens and minimize the consequent host tissue damage [2], [8], [11].

A number of signals and transcription factors that are important for induction of IL-10 gene expression in T cells have been identified. IL-27, an IL-12 family cytokine mainly derived from dendritic cells, plays a dominant role in IL-10 production in Tr1 cells by promoting the expression of transcription factors c-Maf and AhR, which cooperate to activate IL-10 promoter [10], [12], [13]. IL-27-induced IL-10 production is critically dependent on IL-27-induced IL-21 [14], [15]. In addition, IL-2 was reported to play a crucial role in promoting IL-10 production in concert with IL-27 or complement regulator CD46 [11], [16], [17]. However, the interplay of IL-2 with other cytokines or signals in the induction of T cell IL-10 production has not been defined.

IL-6 is a pleotropic cytokine that has both pro- and anti-inflammatory function [18]. It is one of the earliest inflammatory cytokines upregulated in many infectious and inflammatory conditions. By promoting the differentiation or function of Th17 cells, T follicular helper cells and B cells, IL-6 facilitates the pathogenesis of a number of autoimmune and inflammatory diseases including experimental autoimmune encephalomyelitis, rheumatoid arthritis and poriasis [19], [20], [21]. However, in quite a few inflammatory disease conditions, such as inflammatory bone destruction, type-1 diabetes, DSS-induced colitis and streptococcal septic shock syndrome, IL-6 can exert protective effects [22], [23], [24], [25]. Hence, IL-6 can either enhance or suppress tissue inflammation and damage depending the disease, tissues, environmental signals and cytokine milieu. Compared with its pro-inflammatory effect, the anti-inflammatory function and immune-suppressive effect of IL-6 and the underlying mechanisms are much less understood. Recent studies showed that IL-6 can cooperate with TGF-β to induce IL-10 production in Th17 cells but has only moderate effect by itself [10], [26]. However, the precise action of IL-6, especially that independently of TGF-β, on T cell IL-10 production and the characteristics of the IL-10-producing T cells induced by IL-6, are not delineated. More importantly, the in vivo role of IL-6 in T cell IL-10 production is not established. A number of reports have shown that IL-6 induces IL-21 production in CD4 T cells [27], [28], [29], which suggests that IL-6 may have the ability to induce Tr1 cells through promoting IL-21 production.

In the current work, we aimed to understand the precise function and in vivo importance of IL-6 in the induction of IL-10 production in both CD4 and CD8 T cells. We showed that IL-6 promotes the differentiation of IL-10-producing Tr1-like cells from naïve CD4 T cells in the absence of both IL-27 and TGF-β. IL-6 induces IL-21 production from CD4 T cells, which in turn cooperates with IL-2 to induce IL-10-production in both CD4 and CD8 T cells. IL-6-induced IL-10-producing T cells possess functional traits of Tr1 cells in that they can suppress LPS-induced innate immune cell-mediated inflammatory response in vivo. Moreover, in two in vivo models of autoimmune inflammation, we demonstrated that IL-6 is required for IL-10 production by T cells and for suppressing tissue inflammation. We thereby delineate a new immune-regulatory mechanism underlying the anti-inflammatory effect of IL-6 that has been reported in a number of autoimmune or inflammatory diseases.