Discussion
Fungi are widely distributed across the globe and have been identified
as a rich source of new pharmacologically active ingredients. The
members of the Aspergillus genus are already an important source
of novel pharmacological metabolites such as alkaloids, meroterpenoids,
and lignans. Secoemestrin C is an epitetrathiodioxopiperazine extracted
from Aspergillus nidulans . The biological activity of
secoemestrin C and its analog has only been reported in terms of
inhibiting tumor cell proliferation
(Onodera et al. , 2004). Here, we
first present evidence to show that secoemestrin C can inhibit Con
A-induced mouse splenocyte proliferation as well as hepatic MNC
activation and proliferation in vitro , and then we demonstrate
that this molecule has a protective effect in vivo on Con
A-induced autoimmune hepatitis in mice.
Accumulating evidence suggests that CD4+ T cells
(Mizuhara et al. , 1994;
Wang et al. , 2012),
CD8+ T cells (Watanabeet al. , 1996), and NKT cells
(Takeda et al. , 2000a;
Kumar, 2013) are critical components of
Con A-induced liver damage. After Con A challenge, NKT cells and
conventional T cells are activated and either directly participate in
perforin/granzyme- and/or Fas/Fas ligand-mediated cytotoxicity or
indirectly release large amounts of proinflammatory cytokines, thus
leading to hepatocyte death. Therefore, agents that can inhibit the
activation of T cells and/or NKT cells are most likely capable of
alleviating hepatitis induced by Con A. In our in vitroexperiments, we found that secoemestrin C can potently suppress the
proliferation of splenocytes and activation of hepatic MNCs induced by
Con A, indicating that it may have a significant protective effect on
Con A-induced liver damage. As predicted, pretreatment with secoemestrin
C markedly protected against liver injury in Con A-challenged mice, as
evidenced by decreased serum levels of liver enzymes and proinflammatory
cytokines, attenuated bridging coagulative necrosis of hepatocytes, and
a lessened apoptotic response in the livers. In addition, the levels of
serum transaminase and hepatocyte apoptosis in secoemestrin C-treated
mice were similar to those in normal control mice, indicating that
secoemestrin C has limited hepatic toxicity. Thus, secoemestrin C
appears to be a good prospect for clinical application in
immunosuppression.
In addition to aggravating inflammation and liver injury, activation of
NKT cells also leads to Fas/FasL-mediated apoptotic elimination of NKT
cells during Con A challenge (Takedaet al. , 2000b). Our results have confirmed the rapid
disappearance of activated NKT cells in Con A-challenged livers.
Although we did not detect any expression of FasL on NKT cells, our
observation of a lowered CD69 expression level, together with a restored
NKT cell ratio, suggest that the activation-induced cell death (AICD) of
NKT cells is largely abolished in the secoemestrin C-treated mice.
Given that the depletion of NKT cells begins as early as 4 h after Con A
administration (Takeda et al. ,
2000b), it appears that an NKT cell-derived proinflammatory cytokine
response, rather than NKT cell-mediated cytotoxicity, plays a
particularly important role in the latter phase of inflammation induced
by Con A. In the treatment of autoimmune disease, targeting the
production of proinflammatory cytokines has been approved as a potential
therapy (Danese et al. , 2019;
Salvi et al. , 2019). It has been
reported that SOCS3 suppresses IFN-γ production from NKT and T cells,
thereby protecting against Con A-induced liver injury
(Nakaya et al. , 2009). Our results
show that secoemestrin C can down-regulate IFN-γ production by NKT and
conventional T cells after Con A challenge. In addition, we have found
that secoemestrin C is able to suppress cytokine production by Con
A-stimulated hepatic MNCs. The regulatory mechanism(s) by which
secoemestrin C inhibits IFN-γ production remain to be further explored.
Overall, we have demonstrated for the first time that secoemestrin C can
inhibit NKT- and conventional T-cell activation stimulated by Con A and
thus can protect against Con A-induced autoimmune hepatitis in mice. Our
results provide evidence for the immunosuppressive activity of
secoemestrin C and offer novel insights regarding the development of
fungi-derived metabolites for the treatment of T cell-dependent
autoimmune diseases.
Author contributions: X. Tan and G. Chen conceived and designed
the experiments. X. Tan, L. Sun, C. Qi, C. Fu, X. Yang. H. Feng, and Y.
Li performed the experiments. Q. Li and Y. Zhang isolated and identified
secoemestrin C. X. Tan, L. Sun, Y. Zhang, and G. Chen analyzed the data.
X. Tan, L. Sun, C. Qi, Y. Zhang, and G. Chen wrote the manuscript.