In the present study, we established a cellular and molecular relationship among IL-1, IL-10, and Th17 cell development in inflammatory disease models in humans and mice. This link may be important in the regulation of immune pathogenesis of human chronic inflammatory conditions, including CD.
Th17 cells play a role in the inflammatory response associated with multiple human autoimmune diseases [4, 41–43] and cancer [8, 9]. Th17 cells and/or IL-17 are detected in CD patients [35–37, 44–48]. However, the generation and functional relevance of Th17 cells remains poorly understood in CD patients. We have tested the functionality of fresh CD Th17 cells, and found that these T cells induce the production of IL-1, IL-6, and IL-8 by colon tissue cells through IL-17 in vitro. It suggests that these cells may mediate or/and accelerate local inflammation by inducing inflammatory cytokine production. In line with this, elevated inflammatory cytokines are detected in the freshly isolated colon environment from patients with CD. It has been reported that recombinant IL-17 induces IL-6 expression in other systems [49–52]. As IL-1, IL-6, and IL-8 play crucial roles in CD [53–57], it is likely that Th17 cells promote the production of inflammatory cytokines and contribute to the immunopathogenesis of CD in patients. Notably, although IL-17, as a signature gene for Th17 cells, importantly attributes to Th17 cell biology, it is well appreciated that Th17 cell biology may depend on the synergistic effects between Th17-associated cytokines, rather than IL-17 alone [8, 58]. For example, IL-17 and IFNγ synergistically induce β-defensin expression in patients with psoriasis  and Th1-type chemokine production in patients with cancer . This may partially explain why IL-17 signaling blockade generates variable clinical benefits in patients with psoriasis, rheumatoid arthritis, and uveitis  and minimal clinical response in patients with CD.
We have demonstrated a cellular and molecular link among IL-10, IL-1, and Th17 cells in patients with CD and in IL-10-/- mice. In CD patients, the levels of intestinal IL-10 are negatively associated with IL-17 and IL-1. Both IL-10-/- mouse DCs and human Crohn's DCs are superior inducers of Th17 cells via their increased IL-1 production. Blockade of the IL-1 signaling pathway reduces Th17 cell development both in vitro and in vivo. In agreement with these observations, we and others have shown that IL-1 is crucial for inducing Th17 cells in humans and mice [16, 59]. In patients with psoriasis, psoriatic DCs potently induce Th17 cells in an IL-1-dependent manner . Human tumor-associated macrophages also promote Th17 cell development through IL-1 . IL-1 has been shown to induce gastric inflammation and is associated gastric carcinoma . Notably, IL-10 suppresses IL-1 production [60, 61] and that IL-1 is involved in controlling Th17 cells in the mouse model of experimental autoimmune encephalomyelitis (EAE) . Exogenous IL-10 can suppress the in vitro development of Th17 cells from CD4+ T cells in patients with rheumatoid arthritis . However, our study is the first to demonstrate a role for IL-10 in Th17 development through the control of IL-1 expression by DCs in both mouse and human systems, including CD patients. In support of our studies in patients with CD, one recent report demonstrates that mouse Th17 cells expressed interleukin-10 receptor α (IL-10Rα) in vivo. Importantly, T cell specific blockade of IL-10 signaling led to a selective increase of Th17 cells during intestinal inflammation in the small intestine in mice. Furthermore, in this mouse model, Treg cells were able to control Th17 cells in an IL-10-dependent manner in vivo. Thus IL-10 signaling directly suppresses Th17 cells . However, high levels of Treg cell infiltration are detected in patients with CD and ulcerative colitis . Although Treg cells inhibit Th17 cells in patients with cancer , it appears that human Treg cells failed doing so in the microenvironments of chronic graft-versus-host disease (GVHD), ulcerative colitis, and inflammation-associated colon cancer [9, 64]. It is possible that human Th17 cells have stem cell features and are resistant to apoptosis in the chronic inflammatory microenvironments [9, 64]. Nonetheless, our data indicate that IL-1 plays a key role in Th17 cell development in human autoimmune disease, and support the notion that IL-1 signaling blockade is a potential strategy to treat patients with these conditions. IL-10, via its downregulation of IL-1, is thus able to limit development of Th17 cells in mice and humans, and in doing so executes some of its anti-inflammatory effects.
The next logical step is to investigate how IL-10 controls IL-1 production by APCs. IL-10 dampens MyD88-dependent signaling in DCs and leads to LPS hyporesponsiveness . Because IL-1 signaling can be mediated by MyD88, this may explain how IL-10 controls endotoxin-induced IL-1 production. It is also possible that IL-10 controls IL-1 expression machinery, such as IL-1 converting enzyme (ICE) and components of the inflammasome [66, 67]. However, it remains to be determined if IL-10 suppresses IL-1 production induced by other stimuli, including the necrotic tissue often found in a chronic inflammatory environment. The key question remaining is why IL-10 production is reduced in some CD patients. A nucleotide-binding oligomerization domain containing 2 (NOD2) mutation commonly observed in CD patients may lead to inhibition of IL-10 transcription . However, we have not examined the gene profile of NOD expression in our patient populations. Since 30% of CD patients have NOD mutations, it is likely that alterations in NOD2 transcription may at least partially contribute to the reduced IL-10 production in our patient tissues. The data further suggests that IL-10 therapy or IL-1 signaling blockade may not be generally meaningful for all the CD patients.
In summary, we have demonstrated that IL-10 targets APCs, and suppresses Th17 cell development in mice and humans through modulation of IL-1 production. The data document a cellular and molecular link among IL-10, IL-1, and Th17 cells, and suggest that IL-10 may inhibit inflammation via control of Th17 cell development.