Discussion
Immune tolerance is an important physiological mechanism by which the body reduces or eliminates an immune response to particular agents. The current view is that Th17 cells have a more important role in the pathogenesis of autoimmune diseases and in mediating chronic inflammation [18], while Tregs maintain immune tolerance by inhibiting the activation and proliferation of CD4 + T cells [19]. The thymus is the organ in which T lymphocytes develop and mature. Thymoma is one of the important causes of MG. Therefore, the study of T lymphocyte characteristics can reveal the possible pathogenesis mechanisms of thymoma-related MG. This has already been shown in many studies, and an increase in Th17 cells and IL-17 was observed in PBMCs from MG patients with thymomas or thymic hyperplasia [15,20,21]. Dysfunction of Tregs and loss of FOXP3 expression are important mechanisms leading to autoimmune MG and are related to the severity of MG [12,14]. There are also reports of decreased Tregs and FOXP3 expression in thymoma patients with MG [16,22]. However, recent reports have focused on Th17 and Treg changes in peripheral blood and have not studied the changes in thymoma.
In our study, we found similar changes in Th17 and Treg numbers in both thymoma tissues and peripheral blood. This first indicates that the changes in Th17 and Tregs not only exist outside the thymus but also within it. Abnormal Th17 and Treg cells in thymoma may affect the differentiation of these cells in peripheral blood. Another possible explanation is that abnormal Th17 and Treg cells in peripheral blood migrate back to the thymus. In addition, our study showed that Th17 were increased and Tregs decreased in both the thymoma and peripheral blood of MG patients. This may be one of the causes of MG in thymoma, and may be a target for the future evaluation and treatment of MG.
The expression of RORγt and FOXP3, which are transcription factors in Th17 and Tregs, was also studied. The changes in RORγt and FOXP3 expression were consistent with the changes in Th17 and Treg. That is to say, the expression of RORγt was increased and the expression of FOXP3 was decreased in thymoma-related MG patients. However, the mRNA and protein expression of RORγt and FOXP3 were not detected in peripheral blood, possibly because their levels in peripheral blood are too low to be detected by our method. Some studies have shown that FOXP3 can directly affect RORγt and inhibit the differentiation of Th17 cells by reducing the binding of RORγt to the IL17A promoter region, specifically blocking the transcription of IL17A and leading to a significant reduction in IL-17A secretion [6,7].
Th17 and Treg cells are closely related and are mutually inhibited during their differentiation. IL-6 plays a critical role in regulating the balance of these two cell type cells. It can induce the differentiation of Th17 by promoting sequential engagement of the IL-21 and IL-23 pathways [23]. TGF-β can induce the differentiation of T cells into Th17 or Treg cells, while the differentiation of Tregs induced by TGF-β can be inhibited in the presence of IL-6 [24]. However, a large number of Th17 cells are produced by the co-induction of IL-6 and TGF-β, and abnormal regulation or overproduction of IL-6 can lead to the occurrence of autoimmune diseases [24,25]. An in vitro study showed that the expression of IL-17 mediated by RORγt increased and FOXP3 was blocked completely after the increase of TGF-β and IL-6 in cell culture medium [26]. Souroujon et al. reported that IL-6 has a crucial role in controlling the imbalance of Th17/Tregs in EAMG rats, and that treatment of myasthenic rats with neutralizing anti-IL-6 antibodies shifted this equilibrium in favor of Tregs and led to suppression of EAMG [27].
IL-6, IL-21, IL-23, and TGF-β levels in peripheral blood were detected in the current study. We found that only IL-6 was elevated in patients with MG compared with patients with thymoma alone, and there was no significant difference among the other three. We also found that the thy0517 cells can secrete IL-6 and IL-21, consume TGF-β by detecting the content of cytokines in culture supernatant. Furthermore, in order to explain the effect of thymoma cells on the differentiation of Th17 and Tregs, we used the coculture of Thy0517 cells and healthy CD4+ T cells. The results showed that thymoma could promote the differentiation of Th17 and inhibit the differentiation of Tregs. These results suggested that the elevation of IL-6 was associated with myasthenia gravis, and at the same time, it may be an important reason for the increase in Th17 cells and the decrease in Tregs. The mechanism may be related to the ability of the thymoma to secrete IL6 and inhibit TGF-β.
In conclusion, our results suggest that an imbalance in Th17/Tregs and RORγt/FOXP3 and increased expression of IL-6 are possible mechanisms leading to MG in patients with thymoma. The development of therapeutic drugs regulating various molecular targets in the Th17/Treg balance axis may become a new direction for the treatment of thymoma-related MG.