Discussion
In
this study, we demonstrated for the first time that SBI could alleviate
lupus-like manifestations and improve the efficacy of MSCs therapy in
R848-induced mice. SBI synergized with MSCs to restore immune balance,
particularly by suppressing aberrant Tfh cell production. On the
molecular level, SBI increased the ability of MSCs to down-regulate the
expression of Tfh
differentiation-related genes,
mainly through the inhibition of IL-6 and its downstream pathways.
So far, there is no study reported of SBI regulating Tfh cells. As the
main active ingredient of silymarin, SBI has been previously reported to
have immunomodulatory effects on Th1, Th17
cells[11[] Ribeiro, Vanessa Rocha et al.
“Silibinin downregulates the expression of the Th1 and Th17 profiles
by modulation of STATs and transcription factors in pregnant women
with preeclampsia.” International immunopharmacology vol. 109 (2022):
108807. doi:10.1016/j.intimp.2022.108807]. It
also increases the function of regulatory T cells through the
immunomodulatory effect of up-regulation of FOXP3 expression, and can be
used as an adjuvant therapy to alleviate the adverse effects associated
with interferon-beta treatment in patients with multiple sclerosis
(MS)[22[] Abbasirad, Faezeh et al.
“Significant immunomodulatory and hepatoprotective impacts of
Silymarin in MS patients: A double-blind placebo-controlled
clinicaltrial.” International immunopharmacology vol. 97 (2021):
107715. doi:10.1016/j.intimp.2021.107715]. Ourin vivo and in vitrostudy
suggests that SBI mainly regulates Tfh and Treg cells in lupus mice.
Among them, the inhibition of Tfh cell production was largely achieved
by suppressing its differentiation-related genes, while the effect on
cell proliferation and apoptosis was not so evident. Overall, SBI is a
promising immunomodulatory agent with a wide range of immunomodulatory
functions under different conditions.
IL-6/STAT3 axis is critical in the development of Tfh
cells[33[] Wan, Siyuan et al.
“Costimulation molecules differentially regulate the ERK-Zfp831 axis
to shape T follicular helper cell differentiation.” Immunity vol.
54,12 (2021): 2740-2755.e6. doi:10.1016/j.immuni.2021.09.018].
Consistent with literature reports[44[]
Zheng, Rongjuan et al. “Chemopreventive Effects of Silibinin on
Colitis-Associated Tumorigenesis by Inhibiting IL-6/STAT3 Signaling
Pathway.” Mediators of inflammation vol. 2018 1562010. 25 Oct. 2018,
doi:10.1155/2018/1562010], we experimentally
verified the inhibitory effect of SBI on the IL-6/STAT3 pathway. Besides
STAT3, IL-6 may also activate the downstream PI3K/mTOR pathway through
the action of signal transducer glycoprotein
(gp130)[55[] Heinrich PC, Behrmann I,
Haan S, Hermanns HM, Müller-Newen G, Schaper F. Principles of
interleukin (IL)-6-type cytokine signalling and its regulation.
Biochem J. 2003 Aug 15;374(Pt 1):1-20. doi: 10.1042/BJ20030407].
The role of SBI on the mTOR pathway remains controversial, as a study
examining the protective effects of SBI against cerebral ischemia showed
that SBI treatment activated Akt/mTOR signaling[66[] Wang, Chaohui et al. “Protection
by silibinin against experimental ischemic stroke: up-regulated pAkt,
pmTOR, HIF-1α and Bcl-2, down-regulated Bax, NF-κB
expression.” Neuroscience letters vol. 529,1 (2012): 45-50.
doi:10.1016/j.neulet.2012.08.078], and another
research on LPS stimulated porcine mammary epithelial cells showed SBI
increased the expression of mTOR and S6[77[]
Xu, Shengyu et al. “Silibinin Alleviates Lipopolysaccharide Induced
Inflammation in Porcine Mammary Epithelial Cells via mTOR/NF-κB
Signaling Pathway.” Molecular nutrition & food research vol. 67,14
(2023): e2200715. doi:10.1002/mnfr.202200715].
Our data indicate that SBI inhibits both STAT3 and mTOR pathways
downstream of IL-6, thus further clarifying the molecular mechanism of
SBI treatment.
A series of studies have shown a reduction in the dose of
immunosuppressive drugs and a significant decrease in mortality in
patients with SLE after MSC
treatment[8][88[] Wang, Dandan et
al. “Long-term safety of umbilical cord mesenchymal stem cells
transplantation for systemic lupus erythematosus: a 6-year follow-up
study.” Clinical and experimental medicine vol. 17,3 (2017): 333-340.
doi:10.1007/s10238-016-0427-0], yet there are
still many challenges to overcome before clinical application. MSCs have
strong immunomodulatory plasticity and are susceptible to
microenvironmental influences. Besides, MSCs can secrete cytokines with
strong pro-inflammatory effects, such as IL-6, which may reduce the
efficacy of MSC therapy in SLE patients[99[]
Li, Aifen et al. “Mesenchymal Stem Cell Therapy: Hope for Patients
With Systemic Lupus Erythematosus.” Frontiers in immunology vol. 12
728190. 30 Sep. 2021, doi:10.3389/fimmu.2021.728190].
In this study, we revealed that SBI synergized with MSCs to inhibit Tfh
cell production. SBI could counteract IL-6 and its downstream pathways,
thus may enhance the therapeutic effect of MSCs in lupus mice.
In summary, our data suggest that SBI is an effective and promising
agent for the treatment of lupus autoimmunity. Moreover, SBI could
improve the efficacy of MSCs in R848-induced lupus mice, which may
provide a theoretical basis for better clinical application of MSCs.
However, the prospective use of SBI in SLE patients still needs to be
validated by further experimental and clinical evidence.