TCR-independent functions of Th17 cells mediated by the synergistic actions of cytokines of the IL-12 and IL-1 families

Targeting Interleukin-17 as a Novel Treatment Option for Fibrotic Diseases

Fibrosis is the end result of persistent inflammatory responses induced by a variety of stimuli, including chronic infections, autoimmune reactions, and tissue injury. Fibrotic diseases affect all vital organs and are characterized by a high rate of morbidity and mortality in the developed world. Until recently, there were no approved antifibrotic therapies. In recent years, high levels of interleukin-17 (IL-17) have been associated with chronic inflammatory diseases with fibrotic complications that culminate in organ failure. In this review, we provide an update on the role of IL-17 in fibrotic diseases, with particular attention to the most recent lines of research in the therapeutic field represented by the epigenetic mechanisms that control IL-17 levels in fibrosis. A better knowledge of the IL-17 signaling pathway implications in fibrosis could design new strategies for therapeutic benefits.

The Pathogenicity and Synergistic Action of Th1 and Th17 Cells in Inflammatory Bowel Diseases

Inflammatory bowel diseases (IBDs), including ulcerative colitis and Crohn's disease, are characterized by chronic idiopathic inflammation of gastrointestinal tract. Although the pathogenesis of IBD remains unknown, intestinal immune dysfunction has been considered as the core pathogenesis. In the intestinal immune system, T helper 1 (Th1) and Th17 cells are indispensable for intestine homeostasis via preventing pathogenic bacteria invasion, regulating metabolism and functions of intestinal epithelial cells (IECs), and promoting IEC self-renewal. However, during the development of IBD, Th1 and Th17 cells acquire the pathogenicity and change from the maintainer of intestinal homeostasis to the destroyer of intestinal mucosa. Because of coexpressing interferon-γ and interleukin-17A, Th17 cells with pathogenicity are named as pathogenic Th17 cells. In disease states, Th1 cells impair IEC programs by inducing IEC apoptosis, recruiting immune cells, promoting adhesion molecules expression of IECs, and differentiating to epithelial cell adhesion molecule-specific interferon γ-positive Th1 cells. Pathogenic Th17 cells induce IEC injury by triggering IBD susceptibility genes expression of IECs and specifically killing IECs. In addition, Th1 and pathogenic Th17 cells could cooperate to induce colitis. The evidences from IBD patients and animal models demonstrate that synergistic action of Th1 and pathogenic Th17 cells occurs in the diseases development and aggravates the mucosal inflammation. In this review, we focused on Th1 and Th17 cell programs in homeostasis and intestine inflammation and specifically discussed the impact of Th1 and Th17 cell pathogenicity and their synergistic action on the onset and the development of IBD. We hoped to provide some clues for treating IBD.

Emerging role of bystander T cell activation in autoimmune diseases

Autoimmune disease is known to be caused by unregulated self-antigen-specific T cells, causing tissue damage. Although antigen specificity is an important mechanism of the adaptive immune system, antigen non-related T cells have been found in the inflamed tissues in various conditions. Bystander T cell activation refers to the activation of T cells without antigen recognition. During an immune response to a pathogen, bystander activation of self-reactive T cells via inflammatory mediators such as cytokines can trigger autoimmune diseases. Other antigen-specific T cells can also be bystander-activated to induce innate immune response resulting in autoimmune disease pathogenesis along with self-antigen-specific T cells. In this review, we summarize previous studies investigating bystander activation of various T cell types (NKT, γδ T cells, MAIT cells, conventional CD4⁺, and CD8⁺ T cells) and discuss the role of innate-like T cell response in autoimmune diseases. In addition, we also review previous findings of bystander T cell function in infection and cancer. A better understanding of bystander-activated T cells versus antigen-stimulated T cells provides a novel insight to control autoimmune disease pathogenesis.

Significance of bystander T cell activation in microbial infection

During microbial infection, pre-existing memory CD8⁺ T cells that are not specific for the infecting pathogens can be activated by cytokines without cognate antigens, termed bystander activation. Studies in mouse models and human patients demonstrate bystander activation of memory CD8⁺ T cells, which exerts either protective or detrimental effects on the host, depending on the infection model or disease. Research has elucidated mechanisms underlying the bystander activation of CD8⁺ T cells in terms of the responsible cytokines and the effector mechanisms of bystander-activated CD8⁺ T cells. In this Review, we describe the history of research on bystander CD8⁺ T cell activation as well as evidence of bystander activation. We also discuss the mechanisms and immunopathological roles of bystander activation in various microbial infections.

Bystander CD4+ T cells: crossroads between innate and adaptive immunity

T cells are the central mediators of both humoral and cellular adaptive immune responses. Highly specific receptor-mediated clonal selection and expansion of T cells assure antigen-specific immunity. In addition, encounters with cognate antigens generate immunological memory, the capacity for long-term, antigen-specific immunity against previously encountered pathogens. However, T-cell receptor (TCR)-independent activation, termed “bystander activation”, has also been found. Bystander-activated T cells can respond rapidly and secrete effector cytokines even in the absence of antigen stimulation. Recent studies have rehighlighted the importance of antigen-independent bystander activation of CD4⁺ T cells in infection clearance and autoimmune pathogenesis, suggesting the existence of a distinct innate-like immunological function performed by conventional T cells. In this review, we discuss the inflammatory mediators that activate bystander CD4⁺ T cells and the potential physiological roles of these cells during infection, autoimmunity, and cancer.

Immune cells that become activated in the absence of antigen stimulation could be harnessed in the fight against infection, autoimmunity, and cancer. Je-Min Choi and colleagues from Hanyang University in Seoul, South Korea, review how the immune system can deploy helper T cells through an unusual process called bystander activation. Most T cells become activated only after receptors on their surface bind to specific cognate antigen. In contrast, bystander T cells are activated non-specifically in response to cytokines and other pro-inflammatory mediators. Studies have shown that this cell population has a variety of protective and pathogenic functions, for example, guarding against multiple sclerosis, aggravating the symptoms of parasitic infections and promoting antitumor immunity. A better understanding of these immune cells could lead to new therapeutic options for these diseases.

Ontogeny of cytokine responses to PHA from birth to adulthood

BACKGROUND

Altered production of cytokines is believed to contribute to early childhood susceptibility to infection. The aim of this study was to get further insight into the developmental patterns of cytokine responses from birth to adulthood.

METHODS

The expression levels of 13 cytokines were compared in the supernatants of phytohemaggluttinin (PHA)-stimulated whole blood from healthy neonates (cord blood, n = 8), infants ( < 1-year-old, n = 20), and school-aged children (3-15 y; n = 20). Five adults were used as reference.

RESULTS

While Th1, Th2, and Th17 cytokine levels increased progressively from birth to childhood (Mann-Whitney, p < 0.003), high IL-10 secretion at birth dropped to low adult levels in infants (p < 0.004) such that a negative correlation between IL-10 and Th1, Th2, and Th17 cytokine levels at birth (Spearman's correlation, r < -0.70, p < 0.01) converted to a positive correlation in infants (r > 0.60, p < 0.001). Finally, high IL-2, IL-7, and Granulocyte-Colony Stimulating factor (G-CSF) cytokine levels at birth decreased steadily over the first year of life (Mann-Whitney, p ≤ 0.001).

CONCLUSION

The most noticeable result of the study is the rapid shift from enhanced IL-10 secretion capacity at birth toward balanced IL-10/Th1/Th2/Th17 cytokine levels early in life. This change appears an essential precondition to fight pathogens and at the same time to avoid overwhelming inflammatory reactions.

Transition from metal-DTH resistance to susceptibility is facilitated by NLRP3 inflammasome signaling induced Th17 reactivity: Implications for orthopedic implants

Metal hypersensitivity has been recognized as an adverse biologic reaction that can compromise total joint arthroplasty (TJA) performance. However, the etiology of metal hypersensitivity responses in TJAs remains unclear. Metal implant debris is known to act as a danger signal that drives NLRP3 inflammasome activation. It remains unknown if implant debris induced inflammasome activation regulates T cell lineage in TJA metal hypersensitivity responses. In this study, we show both in vivo and in vitro that the pathogenesis of metal hypersensitivity responses to implant debris are largely dependent on activation of the inflammasome/caspase-1 pathway and subsequent production of IL-17A/F by CD4+ T cells. Inhibiting either the inflammasome pathway or IL-17A bioactivity in vivo and in vitro (in vivo using NLRP3 and Caspase-1 deficient mice or in vitro using blocking agents such as Capase-1 inhibitor, IL-1Ra and anti-IL-17A), significantly (p<0.05) mitigated metal-DTH paw inflammation as well as lymphocyte cytokine (IFN-γ and IL-17) and proliferation responses in metal-sensitized mice and primary human PBMCs. This study provides mechanistic insight into how in vivo exposure to orthopedic implant debris, and metals in general, elicits NLRP3 inflammasome activation that mediates the generation of IL-17A/F producing CD4+ T cells, leading to metal-delayed type hypersensitivity reactions.

IL-1R signaling promotes STAT3 and NF-κB factor recruitment to distal cis-regulatory elements that regulate Il17a/f transcription

Interleukin (IL)-1β plays a critical role in IL-6β- and transforming growth factor β (TGFβ)-initiated Th17 differentiation and induction of Th17-mediated autoimmunity. However, the means by which IL-1 regulates various aspects of Th17 development remain poorly understood. We recently reported that IL-1β enhances STAT3 phosphorylation via NF-κB-mediated repression of SOCS3 to facilitate Il17 transcription and Th17 differentiation, identifying an effect of IL-1 signaling on proximal events of STAT3 signaling. Here, we show that IL-1β promotes STAT3 binding to key cis-elements that control IL-17 expression. Additionally, we demonstrate that the IL-1-induced NF-κB factor RelA directly regulates the Il17a/f loci in cooperation with STAT3. Our findings reveal that IL-1 impacts both proximal signaling events and downstream interactions between transcription factors and cis-regulatory elements to promote Il17a/f transcription and Th17 differentiation.