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.