Myocarditis is an inflammatory disease of the cardiovascular system with a predilection for adolescents and children that may eventually lead to DCM and heart failure. Chronic inflammation caused by virus infection or autoimmune reaction is the main cause of DCM[1, 22]. MCP-1 is an important inflammatory factor in vivo, which could mediate the infiltration of mononuclear cells. . For example, in patients with bronchial asthma, MCP-1 can chemotaxis monocytes and macrophages to the alveoli, leading to airway inflammation and remodeling[24–25]. Our previous study also found that MCP-1 can lead to a large number of inflammatory cell aggregation through its receptor CCL2 in viral myocarditis. Therefore, it is speculated that reducing the expression of MCP-1 is of great significance for the occurrence and development of myocarditis.
In previous studies, we found that IL-17 could up-regulate MCP-1 and aggravate cardiomyocyte injury. In this study, we explored the relationship between IL-17, MCP-1, and myocarditis. We performed IHC of mouse hearts by constructing an EAM mouse model. The results showed that with the development of EAM, both IL-17 and MCP-1 levels increased and peaked on 21 day, and decreased on 28 day. Meanwhile, we also measured its mRNA expression and its content in serum, and consistent results were obtained. It is suggested that IL-17 and MCP-1 are involved in the inflammatory infiltration process of EAM.
Th17 cells are helper T cells that differentiate in response to stimulation by IL-6 and IL-23. Evidence has proved that Th17 cells are associated with many autoimmune diseases, including Sjogren's syndrome, rheumatoid arthritis, Scleroderma, and so on[28–29]. Conversely, Treg cells, as indispensable cells for immune homeostasis, can play suppressive roles in a variety of autoimmune diseases[29–30]. From our analysis, we found that the proportion of Th17 cells in EAM group increased greatly, while the opposite result was found for Treg cells. We further performed a correlation between the proportion of Th17 cells, Treg cells and the degree of inflammatory infiltration. The analysis showed that the severity of EAM was closely related to the proportion of Th17 cells and Treg cells. Those results suggested that the Th17 cells/Treg cells ratio may be used as a new therapeutic target for EAM to ameliorate myocardial inflammatory infiltration, and also further proved that IL-17 plays a key role in the inflammatory infiltration of EAM. But in EAM, by which signal IL - 17 up-regulates MCP ‐ 1 expression in myocardial tissue, it remains unclear.
It has been documented that the intracellular region of IL-17R contains a SEFIR domain. Interestingly, Act1 contains both TRAF6 domains and a SEFIR domain. And Act1 is also the only one to contain a SEFIR domain in addition to the IL-17R family. Act1 can bind to a variety of TRAFs, and at the same time, it acts on cell receptors containing the SEFIR domain, such as IL-17R, through the interaction of the SEFIR-SEFIR domain[11,31−32]. Liao et al reported that TRAF6 could aggravate cardiac hypertrophy through TAK1 dependent signaling. Tadashi et al found that IL-17A could activate the Act1/TRAF6/TAK1 signaling pathway to regulate cardiomyocyte contractility and death. Therefore, we speculated that IL-17 regulates MCP-1 expression by activating Act1/TRAF6/TAK1 axis in EAM. In the present study, our data also support the hypothesis that the up-regulation of MCP-1 by IL-17 is dependent on the Act1/TRAF6/TAK1 signaling pathway. Promoters are DNA sequences that play an initiating role during transcription. Our previous studies revealed that MCP-1 promoter sequence contains NF-κB, AP-1 binding site[16,35−36]. Therefore, we further explored whether TAK1 activates NF-κB and AP-1 to up-regulate MCP-1 expression. We found that the levels of NF-κB and AP-1 increased significantly in EAM group, and the elevation was correlated with the degree of inflammatory infiltration. This suggests that in EAM, IL-17 may regulate MCP-1 expression through the Act1/TRAF6/TAK1 cascade to mediate inflammatory cell infiltration.
In the present study, we investigated the effect of IL-17 in the pathogenesis of EAM by regulating MCP-1 expression via Act1/TRAF6/TAK1 cascade. Other proinflammatory factors, such as TNF-α, IL-1, and IL-6, are also a key contributor to the occurrence of inflammatory diseases. Studies have shown that TNF-α can activate NF-κB, AP-1 to up-regulate MCP-1 of glomerular endothelial cells. Meanwhile, Yan et al found that IL-1β may activate IKK / NF-κB, JNK/c-Jun, C/EBPβ to regulate MCP-1 expression in alveolar epithelial cells. In addition, IL-6 can also stimulate MCP-1 secretion by activating downstream pathways through binding to IL-6R. Interestingly, IL-6 can be regulated through RORγt to promote Th17 cell differentiation synergistically enhance MCP-1 expression. It follows that IL-17, TNF-α, IL-1, and IL-6 synergistically promote MCP-1 expression.
In conclusion, this study shows that IL-17 regulates the inflammatory infiltration of EAM by regulating the expression of MCP-1 through Act1 / TRAF6 / TAK1, which will help to clarify the pathological mechanism of myocarditis and provide a new direction for clinical practice.