A comprehensive review of Tripterygium wilfordii hook. f. in the treatment of rheumatic and autoimmune diseases: Bioactive compounds, mechanisms of action, and future directions

Rheumatic and autoimmune diseases are a group of immune system-related disorders wherein the immune system mistakenly attacks and damages the body’s tissues and organs. This excessive immune response leads to inflammation, tissue damage, and functional impairment. Therapeutic approaches typically involve medications that regulate immune responses, reduce inflammation, alleviate symptoms, and target specific damaged organs. Tripterygium wilfordii Hook. f., a traditional Chinese medicinal plant, has been widely studied in recent years for its application in the treatment of autoimmune diseases, including rheumatoid arthritis, systemic lupus erythematosus, and multiple sclerosis. Numerous studies have shown that preparations of Tripterygium wilfordii have anti-inflammatory, immunomodulatory, and immunosuppressive effects, which effectively improve the symptoms and quality of life of patients with autoimmune diseases, whereas the active metabolites of T. wilfordii have been demonstrated to inhibit immune cell activation, regulate the production of inflammatory factors, and modulate the immune system. However, although these effects contribute to reductions in inflammatory responses and the suppression of autoimmune reactions, as well as minimize tissue and organ damage, the underlying mechanisms of action require further investigation. Moreover, despite the efficacy of T. wilfordii in the treatment of autoimmune diseases, its toxicity and side effects, including its potential hepatotoxicity and nephrotoxicity, warrant a thorough assessment. Furthermore, to maximize the therapeutic benefits of this plant in the treatment of autoimmune diseases and enable more patients to utilize these benefits, efforts should be made to strengthen the regulation and standardized use of T. wilfordii.


Introduction
Rheumatic and autoimmune diseases are chronic conditions that affect millions of people worldwide.These diseases are characterized by inflammation, pain, and damage to the affected organs and tissues, and thus significantly affect the quality of life of patients (Schwenzer et al., 2010;Ma et al., 2020).Several treatment modalities are currently available for these diseases, including the administration of non-steroidal anti-inflammatory drugs, diseasemodifying antirheumatic drugs, and biological agents (Tsitrouli et al., 2021).However, these therapeutic approaches do not always fully satisfy the clinical needs of patients and may be associated with potential side effects.Additionally, patients may exhibit an inadequate response to these strategies, and the occurrence of adverse events, coupled with their high cost, imposes a substantial burden on clinical healthcare (Shi et al., 2020).Consequently, there is a compelling need to identify effective novel drugs that could be applied in the treatment of rheumatic and autoimmune diseases.
Tripterygium wilfordii Hook.f. is a herb used in traditional Chinese medicine that was first recorded in the Compendium of Materia Medica (Ben Cao Gang Mu Shi Yi), and has been widely used to treat various diseases, including rheumatic and autoimmune disorders, for centuries (Jia et al., 2021).T. wilfordii is used as a herbal medicine in traditional Chinese medicine to treat joint pain, promote blood circulation, relieve rheumatism, and alleviate pain and swelling (Shen et al., 2021).In recent decades, the assessment of the therapeutic potential of T. wilfordii has attracted increased interest, which has led to extensive research on its pharmacological properties and potential clinical applications.Owing to its strong anti-inflammatory and immunomodulatory properties, T. wilfordii is considered a promising candidate for the treatment of rheumatic diseases and autoimmune disorders (Hou et al., 2017).
Preliminary clinical studies have indicated that T. wilfordii and its active metabolites have beneficial effects in the treatment of a range of rheumatic and autoimmune disorders, including rheumatoid arthritis (RA), systemic lupus erythematosus (SLE), and psoriasis (Gonçalves et al., 2020).For example, in a mouse model of collagen-induced arthritis, treatment with triptolide was found to significantly reduce disease severity and inhibit proinflammatory cytokine production and immune cell activation (Yang et al., 2016).Similarly, in a mouse model of lupus nephritis, treatment with celastrol was demonstrated to significantly reduce disease severity and reduce the levels of autoantibodies and pro-inflammatory cytokines (Chen et al., 2022).Clinical studies have also reported promising outcomes with the use of T. wilfordii for the treatment of rheumatic diseases and autoimmune disorders (Yu et al., 2020).For example, in a randomized, double-blind, placebo-controlled trial, patients with RA who received T. wilfordii extract showed significant improvements in disease severity and joint function compared with those who received a placebo (Jiang et al., 2017).In a similar trial, patients with psoriasis who received a T. wilfordii extract showed significant improvements in disease severity and skin symptoms compared with those who received a placebo (Lv et al., 2018).
Considering its potent anti-inflammatory and immunomodulatory properties, T. wilfordii has shown significant potential as a therapeutic agent for the treatment of rheumatic diseases and autoimmune disorders (Song CY. et al., 2020).Although further research is needed to gain a more comprehensive understanding of the mechanisms underlying the effects of T. wilfordii and its active metabolites, evidence obtained to date indicates that preparations of this plant may offer a safe and effective alternative to currently available treatment modalities (Lv et al., 2018).However, more studies will be required to determine the optimal dosages and treatment regimens for T. wilfordii, as well as to identify any potential side effects or drug interactions.In this review, we discuss the evidence supporting the use of T. wilfordii in the treatment of rheumatic diseases and autoimmune disorders, with a focus on key findings from preclinical and clinical studies.

Historical uses of Tripterygium wilfordii in traditional medicine
T. wilfordii has a long history of use in traditional medicine.Natural populations of this vine are widely distributed in moist regions of the Northern Hemisphere, including China, Japan, North and South Korea, the United States, and Canada, with a total of 1,219 locations confirmed globally (Figure 1) (Cheng et al., 2021).In traditional Chinese medicine, T. wilfordii is considered a potent herbal medicine, which can be used to treat a range of diseases and disorders (Lu et al., 2022).The medicinal use of T. wilfordii can be traced back to the Han Dynasty (220 BC), at which time it was used to treat various ailments such as gout, edemas, and fever.Since then, the plant has also been used to treat rheumatic diseases and promote blood circulation.During the Tang Dynasty (618-907 AD), the use of T. wilfordii became even more widespread and was used to treat a wider range of conditions, including inflammation, swelling, and joint pain (Lu et al., 2022), and the use of this plant in traditional medicine has continued to the present day.Currently, T. wilfordii is commonly administered to treat RA, SLE, and other autoimmune diseases, and its anticancer properties have been recognized (Xue et al., 2012).Furthermore, the traditional use of T. wilfordii has been documented in South Korea and Japan.In South Korea, this plant is referred to as "jeokseong" and has been used in the treatment of RA, while in Japan, T. wilfordii is known as "raiji," and is recognized not only for its therapeutic properties in RA management (Cheng et al., 2021) but also for its use in the treatment of inflammatory diseases, such as Crohn's disease and ulcerative colitis (Wang et al., 2016).

Chemical composition of Tripterygium wilfordii
Recently, considering its potential use in the treatment of rheumatic and autoimmune diseases, T. wilfordii has attracted the attention of the conventional medical community (Oliner et al., 2016).To date, over 500 metabolites have been isolated and identified from T. wilfordii, including sesquiterpenes, diterpenes, triterpenes, alkaloids, flavonoids, lignans, and glycosides.Among these metabolites, notable compounds, such as triptolide, celastrol, and wilforlide, are believed to contribute to the diverse pharmacological activities of T. wilfordii.The identification and characterization of such compounds can provide valuable insights into the chemical composition and potential therapeutic properties of this plant (Luo et al., 2019;Xu et al., 2019).The following subsections provide an overview of the major metabolites isolated from T. wilfordii in modern medicine and their current status in terms of mechanistic research.(Table 1)

Triptolide
Triptolide, one of the key active compounds identified in T. wilfordii, is primarily obtained through extraction and isolation from various medicinal plants.First isolated by Kupchan et al., in 1972, triptolide has been established to have significant antileukemic activity (Kupchan et al., 1972) in addition to other important biological activities, including prominent anti-inflammatory, immunosuppressive, and anticancer effects (Gao et al., 2021).Triptolide exerts its anticancer effects by modulating different signaling pathways and gene expression at multiple levels, as well as by interacting with miRNAs.Specifically, these therapeutic effects are mediated via the modulation of cell apoptosis, senescence, proliferation, invasion, migration, and angiogenesis (Tong et al., 2021).For example, triptolide has been established to induce apoptosis in prostate cancer cells through the activation of caspases and poly-ADP-ribose polymerase cleavage (Zhao et al., 2013).Moreover, it inhibits nuclear factor kappa-B (NF-κB) activation and downregulates the expression of toll-like receptor 4 (TLR4) (Wen et al., 2012).Reportedly, triptolide can also contribute to the prevention of bone deterioration by reducing the number of osteoclasts in inflamed joints, reducing the expression of receptor activator of NF-κB (RANK) and its ligand RANKL, enhancing the expression of osteoprotegerin (OPG) at the mRNA and protein levels, and lowering the ratio of RANKL to OPG in the serum and inflamed joints of mice with collagen-induced arthritis (Liu C. et al., 2013).

Celastrol
The pentacyclic triterpenoid celastrol has been identified as a major bioactive metabolite of T. wilfordii that has diverse biological properties, including antitumor, antioxidant, anti-obesity, neuroprotective, and immunosuppressive effects (Hou et al., 2020;Xu et al., 2021).Moreover, celastrol has been identified as a natural proteasome inhibitor, the activity of which accordingly leads to an accumulation of ubiquitinated proteins and proteasome substrates, such as NF-κB (IκBα), Bax, and p27, and induces the ubiquitination of orphan nuclear receptor NR4A1 (Nur77) through its Lys63 linkage (Hernandes et al., 2017).Under inflammatory  conditions, ubiquitinated Nur77 accumulates within the mitochondria, thereby rendering these organelles sensitive to autophagy.This event involves an interaction between Nur77 and p62/SQSTM1, thereby contributing to an alleviation of inflammation (Liu et al., 2022).Celastrol has also been shown to target multiple signaling pathways, including those of NF-κB, endoplasmic reticulum Ca 2+ -ATPase, myeloid differentiation factor 2, TLR4, pro-inflammatory chemokines, DNA damage, cell cycle arrest, and apoptosis (Song X. et al., 2020).

Wilforlide A
Wilforlide A is a triterpene isolated from T. wilfordii that can serve as a quality control standard for tripterygium glycosides, a class of drugs derived from T. wilfordii.Wilforlide A also exhibits potent antiinflammatory and immunosuppressive activities (Xue et al., 2010), and has been used alone as a traditional botanical drug for the treatment of autoimmune diseases and is widely used to treat conditions such as RA, SLE, and psoriasis (Wang et al., 2018).Regarding its mechanism of action, it has been established that wilforlide A inhibits the secretion of pro-inflammatory cytokines (MCP1, GM-CSF, and M-CSF) and the M1 biomarker inducible nitric oxide synthase in synovial cells (Cao et al., 2022).In in vitro experiments, wilforlide A was found to inhibit the lipopolysaccharideand interferon-γ (IFN-γ)-induced upregulation of TLR4, degradation of IκBα, and activation of NF-κB p65 (Cao et al., 2022).Additionally, treatment with the TLR4 inhibitor TAK242 has been demonstrated to have inhibitory effects on M1 polarization similar to those of wilforlide, and the combined administration of TAK242 and wilforlide A has been found to enhance these inhibitory effects (Mao et al., 2021).

Diterpenes
Diterpenes are among the major active metabolites of T. wilfordii and can be further classified as abietane, tigliane, and ingenane diterpenes (Qu et al., 2022).Among these, triptolide was first isolated from T. wilfordii in 1972 by Kupchan, and exhibits potent pharmacological activities against various immune disorders, cancers, viruses, fibrosis, asthma, depression, and metabolic imbalances (Guo et al., 2016;Yuan et al., 2019).

Triterpenes
The triterpenes present in T. wilfordii are primarily classified into friedelanes, oleananes, and ursanes (Chang et al., 2016;Hu et al., 2017).Pentacyclic triterpenes are a class of representative plant compounds produced as secondary metabolites, which have been shown to possess diverse biological activities, including anticancer, anti-inflammatory, cardioprotective, and antidiabetic effects (Ghante and Jamkhande, 2019).Bevirimat, a triterpene compound extracted from T. wilfordii, has significant antiviral, antifibrotic, immunosuppressive, metabolic dysfunction-regulating, and spermicidal activities (Song X. et al., 2020).Notably, it is the only triterpene derivative with anti-human immunodeficiency virus properties that has been assessed to date in Phase II clinical trials (Timilsina et al., 2016).

Alkaloids
The alkaloids present in T. wilfordii can be classified into two main categories, sesquiterpenes and indole alkaloids, both of which are characterized by prominent anti-inflammatory, antiviral, immunosuppressive, and insecticidal properties (Fang et al., 2012).To date, four indole alkaloids have been isolated from T. wilfordii, namely, celabazine, celacinnine, celafurine, and celallocinnine (Horiuch et al., 2006), and the findings of relevant studies have indicated that celacinnine and celallocinnine interact with matrix metalloproteinase-9, thereby contributing to an alleviation of skin inflammation (Tang et al., 2017).

Glycosides
Glycosides extracted from the roots of T. wilfordii have been demonstrated to exert anti-inflammatory and immunosuppressive effects by inhibiting the release of inflammatory cytokines such as interleukin (IL)-6, IL-1β, and tumor necrosis factor-alpha (TNF-α) (Zhang and Chen, 2017).These glycosides have been used extensively in the treatment of inflammatory diseases (Zhang et al., 2021).Mechanistically, they can reduce the activity of NF-κB, suppress the gene expression of cyclooxygenase-2 and M1 biomarker inducible nitric oxide synthase, and reduce the production of prostaglandin E2 and nitric oxide, thereby exerting anti-inflammatory effects (Yang et al., 2020).

LLDT-8
5R-Hydroxytriptolide (LLDT-8) is a novel analog of T. wilfordiiderived triptolide characterized by low cytotoxicity and high immunosuppressive activity (Su et al., 2017), and numerous in vitro and in vivo studies have demonstrated the significant anti-inflammatory and immunosuppressive properties of this compound.LLDT-8 has been approved by the China Food and Drug Administration as an immunosuppressive drug in clinical trials for RA treatment (Cui et al., 2019), and is currently being investigated as a low-toxicity immunosuppressive agent in Phase I clinical trials for RA in China.The findings of early studies indicated that the activity of LLDT-8 may be associated with anti-inflammatory, antioxidant, and cytokine effects and that it can prevent bleomycin-induced pulmonary fibrosis in mice (Zhou et al., 2021).Recently, the first long non-coding RNAtranscription factor-mRNA co-expression network was constructed to further elucidate the changes in genome-wide long non-coding RNA and mRNA expression before and after LLDT-8 treatment, which indicated that long non-coding RNAs could serve as biomarkers and targets for LLDT-8 drug development (Guo et al., 2019).Additionally, LLDT-8 has been found to have potent antiinflammatory effects, including the amelioration of anti-glomerular basement membrane glomerulonephritis by modulating the Fcγ signaling pathway and suppression of renal chemokine expression to inhibit immune cell infiltration, thereby ameliorating lupus nephritis (Ma et al., 2017).The protective effects of LLDT-8 include the elimination of activated T cells by promoting the expression of the pro-apoptotic gene signal transducer and activator of signal transducer and activator of transcription 1 and interferon regulatory factor-1 in the spleen (Ziaei and Halaby, 2016).LLDT-8 has also been established to reduce the production of IFN-γ, IL-2, and TNF-α by peripheral blood mononuclear cells (Ibrahim et al., 2021), and has therapeutic potential for the treatment of neurodegenerative diseases by effectively inhibiting pro-inflammatory cytokines (TNF-α and IL-1β) and suppressing the NF-κB signaling pathway, thereby reducing neuroinflammation (Ziaei and Halaby, 2016).Furthermore, LLDT-8 has been demonstrated to reduce the levels of serum alanine aminotransferase and aspartate aminotransferase, liver balloon cell formation, and macrovesicular steatosis, thereby preventing liver injury (Dong et al., 2019).LLDT-8 also regulates the expression of stearoyl-CoA desaturase-1 and peroxisome proliferator-activated receptor alpha, which contribute to promoting significant degradation of lipids and suppression of lipid synthesis (Gao et al., 2021).

Tripterygium wilfordii in the treatment of RA
The earliest studies on T. wilfordii were conducted in the 1970s, during which its efficacy in the treatment of RA was initially discovered.Since then, numerous studies have investigated the therapeutic effects of T. wilfordii in the treatment of other rheumatic diseases and autoimmune disorders, including SLE, psoriasis, and multiple sclerosis (Table 2) (Xu et al., 2008).A meta-analysis of randomized controlled trials conducted to assess the efficacy of T. wilfordii in the treatment of RA found that the use of T. wilfordii glycoside tablets alone could reduce the number of tender and swollen joints in patients with RA, with an efficacy comparable to that of methotrexate (MTX).Furthermore, a combination of T. wilfordii glycoside tablets and MTX was found to be superior to the administration of MTX alone in improving the clinical symptoms of patients with RA (Zhou et al., 2018).Similarly, a further systematic review and network meta-analysis evaluating the efficacy and safety of T. wilfordii in treating RA found that the glycosides of this plant administered in combination with MTX may have been the optimal treatment of choice based on an assessment of the ACR20 response.Moreover, among the different treatments assessed, the use of T. wilfordii glycosides alone ranked second only to the T. wilfordii glycoside + MTX combined treatment (Wang et al., 2022).one of the primary targets (Urits et al., 2019).Targets downstream of NF-κB include pro-inflammatory cytokines such as TNF-β, which mediate the inflammatory response via NF-κB signaling (Lin, 2006).T. wilfordii glycosides, such as triptolide, celastrol, and wilforlide A, can exert anti-inflammatory effects by inhibiting the p56 subunit of the NF-κB signaling pathway, as well as by downregulating the expression of inflammatory factors such as TNF and IFN (Li et al., 2021).Studies have shown that NOD-like receptor protein 3 (NLRP3)-induced inflammation plays a key role in RA pathogenesis, and treatment with T. wilfordii has been demonstrated to inhibit activation of the NLRP3 inflammasome by blocking the NF-κB signaling pathway, thereby contributing reductions in joint swelling, arthritis index score, inflammatory cell infiltration, and synovial hyperplasia induced by complete Freund's adjuvant in rats (Jing et al., 2023).Additionally, significant reductions in the secretion of IL-1β and IL-18 have been observed in rat serum and T helper 1 cell supernatant exposed to a T. wilfordii extract (Kim et al., 2019).Further research has revealed that celastrol can alleviate RA symptoms and suppress inflammation by inhibiting the reactive oxygen species (ROS)-NF-κB-NLRP3 axis.Moreover T. wilfordii glycosides have been established to upregulate the levels of superoxide dismutase and glutathione peroxidase, which induces the clearance of ROS and reactive nitrogen species, thereby reducing oxidative stress and inflammation (Figure 2) (Kim et al., 2019;Lv et al., 2022).

Tripterygium wilfordii and its derivatives inhibit RA-associated inflammation
Celastrol has been found to significantly inhibit the protein and mRNA expression of the pro-inflammatory cytokines IL-6 and IL-8 and monocyte chemoattractant protein-1 induced by lipopolysaccharide in human retinal pigment epithelial and adult retinal pigment epithelial-19 cells (Tao et al., 2016).Further mechanistic studies have revealed that celastrol significantly reduces the phosphorylation of IKKα/IKKβ and IκBα, leading to the inactivation of p65, and thereby alleviating lipopolysaccharideinduced inflammation by inhibiting the NF-κB signaling pathway (Wang et al., 2017).Administration of T. wilfordii glycosides has also been shown to reduce the expression of the inflammatory factors IL-6, IL-8, NF-κB, and TNF-α (Liu et al., 2014), and promote an upregulation of the levels of IL-10 and CD4 + CD25 + regulatory T cells (Tregs), increase the expression of forkhead box protein P3 (FoxP3), downregulate TNF-α and endothelin-1 levels to inhibit paw edema, and reduce the expression of high-mobility group protein 1 and IL-17 in the serum of rats with collagen-induced arthritis (Lei and Jian, 2012;Hao et al., 2022).In other studies, T. wilfordii glycosides have been demonstrated to inhibit the differentiation, maturation, and migration of immature dendritic cells, as well as the secretion of cytokines, thereby suppressing the activation of neutrophils and T cells through the transcriptional signal transducer and activator of STAT pathways.This leads to the downregulation of inducible cyclooxygenase-2, prostaglandins, and metalloproteinases, resulting in an attenuation of the inflammatory responses mediated by these cells (Tian et al., 2020;Kianmehr et al., 2023).Mitochondria play essential roles in cell death, autophagy, immunity, and inflammation, and as mentioned earlier, Nur77 can induce cell apoptosis by targeting mitochondria (Lith and de Vries, 2021).Studies have shown that T. wilfordii glycosides can interact with Nur77, thereby promoting its translocation from the nucleus to mitochondria.Within the mitochondria, Nur77 interacts with tolllike receptor 2, leading to its ubiquitination.The ubiquitinated Nur77 remains within the mitochondria, rendering these organelles sensitive to autophagy and facilitating the clearance of damaged mitochondria, thereby exerting anti-inflammatory effects (Sommer et al., 2021).Furthermore, glycosides of T. wilfordii have been shown to reduce the expression of CD80 and CD86 induced by IFN-γ and inhibit the expression of IL-23 and IL-1 in dendritic cells and T helper 1 cells (Wangchuk et al., 2018), thereby retarding the progression of inflammatory diseases.Furthermore, these glycosides have also been reported to inhibit RA-induced cell autophagy by suppressing activation of the toll-like receptor 2/high-mobility

Tripterygium wilfordii and its derivatives inhibit the progression of RA via immune cell modulation
Normal functioning of the immune system is essential for maintaining bodily homeostasis; consequently, immune dysregulation can have particularly detrimental effects.Many autoimmune diseases develop as a response to abnormal immune expression, in which the body's own tissues are mistakenly targeted, thus leading to the development of diseases such as RA, SLE, and psoriasis (Perl, 2017).In this context, T. wilfordii has been demonstrated to inhibit the proliferation of T and B cells, reduce the production of proinflammatory cytokines, and induce the apoptosis of activated lymphocytes (Lu et al., 2017).T. wilfordii glycosides have been shown to downregulate expression of the cell factors TNF-β, IL-6, IL-1, and IL-8 and upregulate the expression of IL-10, thereby inhibiting the activity of Th1 cells and promoting the transformation of Th1 to Th2 cells.These glycosides can also inhibit the synthesis and release of NF-κB, promote apoptosis, and reduce the occurrence and progression of immune reactions (Lin et al., 2001).In addition to these effects, T. wilfordii glycosides can contribute to lowering the levels of immunoglobulins IgE and IgA, enhance the function of CD8 + T cells, and inhibit the function of CD4 + T cells, thereby reducing the CD4 + /CD8 + T cell ratio and inhibiting further development of abnormal immune responses (Liu et al., 2019).
The immunosuppressive effects T. wilfordii glycosides can be attributed to an inhibition of STAT3 phosphorylation, thereby suppressing Th17 cell function.Whereas Th17 cells promote inflammation, Tregs have anti-inflammatory functions (Balkrishna et al., 2020); therefore, maintaining an appropriate balance between Th17 cells and Tregs is essential for the maintenance of immune homeostasis.In this regard, it has been found that T. wilfordii can inhibit the expression of mammalian target of rapamycin, hypoxia-inducible factor 1-alpha, c-Myc, and protein kinase B, leading to metabolic blockade of Th17 cells and thus inhibiting their differentiation (Tang et al., 2020).Conversely, by upregulating carnitine palmitoyltransferase 1A and adenosine 5ʹmonophosphate-activated protein kinase alpha, T. wilfordii promotes lipid fatty acid oxidation, thereby facilitating the generation of inducible Tregs (Marks, 2011).Moreover, it has been demonstrated that the administration of T. wilfordii significantly increases the Bax/Bcl-2 ratio, induces the release of mitochondrial cytochrome C into the cytoplasm, and activates caspase-3 and caspase-9, ultimately inducing T cell apoptosis via the mitochondrial apoptosis pathway (Jin et al., 2022).
Assisting cells, particularly dendritic cells, are professional antigen-presenting cells that have been established to be further targets of T. wilfordii glycosides (Zhang G. et al., 2013).These glycosides inhibit the differentiation, maturation, trafficking, and functioning of immature dendritic cells (Wangchuk et al., 2018).High concentrations (>20 ng/mL) of T. wilfordii glycosides have been observed to induce dendritic cell apoptosis via the sequential phosphorylation of p38 mitogen-activated protein kinase and activation of caspase-3 (Ziaei and Halaby, 2016).The chemotaxis of neutrophils and T cells mediated by dendritic cells has also been shown to be inhibited by a T. wilfordii glycoside-mediated reduction in STAT3 phosphorylation and activation of NF-κB (Zhang and Ma, 2010).Moreover, these glycosides prevent the differentiation of immature monocyte-derived dendritic cells by downregulating the expression of CD1a, CD40, CD80, CD86, and HLA-DR, as well as reduce the capacity of monocyte-derived dendritic cells to stimulate lymphocyte proliferation in allogeneic mixed lymphocyte reactions (Zhao et al., 2021).Recently, it has also been demonstrated that T. wilfordii glycosides inhibit the expression of IL-12/IL-23 in antigen-presenting cells by binding with CCAAT-enhancer-binding protein α, thus providing mechanistic insights into the immunomodulatory properties of these glycosides (Ziaei and Halaby, 2016).

Tripterygium wilfordii and its derivatives alleviate joint damage in RA
With respect to the treatment of RA, T. wilfordii glycosides have been demonstrated to inhibit the release of chemotactic factors from macrophages, thereby reducing their effects on synovial cells and chondrocytes, and thus inhibiting the abnormal proliferation of synovial cells (Baoqi et al., 2021).Furthermore, treatment with LLDT-8 has been found to suppress matrix metalloproteinase-13 production via the OPG/RANK/RANKL signaling pathway, increase OPG/RANKL expression, and attenuate collageninduced arthritis (Shen et al., 2015).Moreover, T. wilfordiiderived triptolide can inhibit the expression of vascular endothelial growth factor (VEGF), VEGF receptor 1, and chemokine receptor 5 in the joint tissues of rats with adjuvantinduced arthritis.This activity has the effects of reducing the infiltration of lymphocytes and peripheral blood mononuclear cells into the synovium, thereby suppressing damage and degradation of the synovial tissue.Triptolide also targets neutrophils in mice with RA, promotes neutrophil apoptosis, and inhibits neutrophil autophagy and neutrophil extracellular trap formation, thereby reducing tissue damage and inflammation (Zhang B. et al., 2013).Studies have also shown that T. wilfordii glycosides can significantly alleviate paw swelling in rats (as determined by performing traditional anti-inflammatory tests), downregulate the expression of VEGF and VEGF receptor 2, and reduce vascular permeability and angiogenesis (Ashrafizadeh et al., 2022).Furthermore, by inhibiting the expression of miR-4478, T. wilfordii upregulates WAKMAR2, which in turn promotes an upregulation of E2F transcription factor 1 and p53 expression, with the outcome of this cascade being a suppression of fibroblast-like synoviocyte expression (Zhou et al., 2021).
5 Tripterygium wilfordii treatment of other rheumatic diseases 5.1 Systemic lupus erythematosus SLE is a chronic autoimmune disease characterized by a wide range of clinical symptoms, notably the abnormal production of selfantibodies, such as antinuclear and anti-double-stranded DNA antibodies (Moulton et al., 2017).SLE predominantly affects women of childbearing age, with a male-to-female patient ratio of approximately 1:9, and is accordingly speculated to be influenced by female hormones (Ameer et al., 2022).Moreover, genetic and environmental factors have been established to contribute to the development of SLE (Li et al., 2010).T. wilfordii has a long history of use in the treatment of SLE and, when administered with appropriate care, patients can experience satisfactory alleviation.In therapeutic strategies for this disease, the use of selected immunosuppressive agents can contribute to reducing the cumulative steroid dose and the prevention of disease relapse.Similarly, in patients with refractory or relapsed SLE, the use of other immunosuppressive agents can contribute to a reduction in steroid dosage, control of disease activity, and improvements in the rates of clinical remission (Li and Yang, 2022).In a study of patients with SLE, treatment with a T. wilfordii extract combined with prednisone was demonstrated to increase the levels of CD4 + and CD25 + T cells, thus enhancing immune tolerance in these patients.Based on the findings of these studies, it can be concluded that the regulatory effects of T. wilfordii extract on T cells further enhance its protective effects in patients with SLE.Studies have shown that neutrophil extracellular traps, web-like structures released by neutrophils, are involved in driving autoimmunity and tissue damage in SLE (Georgakis et al., 2021), and in this regard, it has been found that T. wilfordii can downregulate the activation of spleen tyrosine kinase and phosphorylation of mitogen-activated protein kinase, extracellular signal-regulated kinase, and IκBα, as well as the guanine methylation of histones.By downregulating the spleen tyrosine kinase-mitogen-activated protein kinase kinase-extracellular signal-regulated kinase-NF-κB signaling cascade, T. wilfordii can effectively inhibit neutrophil oxidative bursts and the formation of neutrophil extracellular traps induced by diverse inflammatory stimuli, thereby inhibiting SLE development (Kato et al., 2013).

Psoriasis
Psoriasis, a chronic skin disease affecting over 100 million people worldwide (Griffiths and Barker, 2007), is characterized by the typical symptoms of erythema, itching, and scaling, leading to physical discomfort and psychological burden (Kamiya et al., 2019).Psoriasis is a disease involving both the innate and adaptive immune systems, in which keratinocytes, dendritic cells, and T cells play key roles in disease development (Li and Yang, 2022).It has been found that an extract of T. wilfordii can inhibit the excessive proliferation of human keratinocytes (HaCaT cells) and significantly reduce the mRNA levels of inflammatory cytokines such as TNF-α and IL-6 in HaCaT cells.By modulating the interactions between keratinocytes and downstream dendritic cells and T cells in the immune system, as well as reducing the expression levels of inflammatory cytokines in the skin and circulation, T. wilfordii has an inhibitory effect on systemic inflammation, thus achieving therapeutic effects in patients with psoriasis et al., 2019).Furthermore, T. wilfordii extract has been shown to regulate the activity of dendritic cells and T cells, suppress the generation of CD4 T and Th17 cells, inhibit the development of Th17 cells, and inhibit the proliferation of keratinocytes and epidermal cells, as well as the expression of inflammatory cytokines such as TNF-α and IL-17A (Guo et al., 2020).Similarly, in murine animal models, treatment with T. wilfordii extract has been shown to significantly reduce white scales, erythema, inflammatory cells, excessive keratinization, and incomplete keratinization.Furthermore, this treatment contributes to marked reductions in the release of inflammatory factors, such as IL-17, INF-γ, and IL-22, in the plasma at the mRNA and protein levels (Maqbool et al., 2021).

Crohn's disease
Crohn's disease is a chronic granulomatous inflammatory condition that affects the entire gastrointestinal tract, particularly the ileocecal and perianal areas (Baumgart and Sandborn, 2012).Although the exact etiology of Crohn's has yet to be sufficiently established, several specific factors are known to be directly associated with the disease, including genetic factors, intestinal immune dysregulation, and alterations in the gut microbiota (Mazal, 2014).Recent findings have emphasized the key role played by the gut microbiota in the development of this disease, which is in turn, closely associated with sustained adverse inflammatory responses (Rodríguez et al., 2020).Currently, the treatment options for patients with CD are rather limited, although the use of natural products, such as extracts of T. wilfordii, which may have fewer side effects than conventional medications, has clear appeal (Zhang et al., 2020).Studies have reported that an extract of T. wilfordii induces apoptosis in T lymphocytes and dendritic cells by inhibiting the NF-κB signaling pathway and reduces the production of proinflammatory cytokines such as TNF-α and IL-1, thereby alleviating intestinal inflammation in patients (Ren et al., 2013).It has also been found that FoxP3 Tregs play a vital role in maintaining intestinal immune balance via the actions of IL-10 and transforming growth factor-beta, and that T. wilfordii extract promotes mucosal healing by upregulating FoxP3 Tregs and IL-10, downregulating inflammatory cytokines such as TNF-α, and modulating the local production of other inflammatory cytokines (Li et al., 2014).

Multiple sclerosis
Multiple sclerosis, also known as demyelinating disease, is a multifocal chronic autoimmune inflammatory disease of the central nervous system (Doshi and Chataway, 2016).Although the etiology of multiple sclerosis has yet to be elucidated, the accumulation and activation of monocytes within the central nervous system have been established to play key roles in its pathogenesis.Chemokines are major players in leukocyte recruitment and activation at the sites of inflammation (Moreira et al., 2006), and studies have shown that neuroinflammation can have both detrimental and beneficial effects on the function of neurons and glial cells, with the NF-κB signaling pathway playing a pivotal role in controlling this process.These findings accordingly indicate that selective anti-NF-κB treatment strategies may contribute to minimizing damage during the acute and chronic inflammatory phases of this disease (Webb et al., 2017;Sanadgol et al., 2018).Experimental autoimmune encephalomyelitis is a T cell-mediated autoimmune disease of the central nervous system that serves as an animal model of human multiple sclerosis (Baecher-Allan et al., 2018), and it has been proposed that T. wilfordii may act as an active suppressor of experimental autoimmune encephalomyelitis, whereas LLDT-8 has been shown to inhibit T cell proliferation and activation (Fu et al., 2006).Specifically, it can attenuate the inflammatory response in the experimental autoimmune encephalomyelitis model by increasing the levels of heat shock protein 70 and stabilizing the NF-κB/IκB complex (Figure 3) (Zheng et al., 2013).

Toxic effects of Tripterygium wilfordii
In addition to its therapeutic efficacy, the safety of the medicinal use of T. wilfordii has been extensively studied.Although preparations of this plant are generally well tolerated, they can cause a rage of side effects, including gastrointestinal discomfort, rashes, alopecia, liver and kidney damage, reproductive toxicity, and myelosuppression (Xi et al., 2017).A recent meta-analysis of T. wilfordii in the treatment of renal disease indicated that approximately 11.7% of patients experienced adverse events after taking the T. wilfordii preparations.Briefly, the authors found that 273 studies reported gastrointestinal reactions, in which 4,843 (8.6%) patients had elevated levels of alanine transaminase (twice the normal level), 12.7% of female patients had menstrual disorders, approximately 4.9% of patients experienced cardiovascular events, and approximately 6.5% and 7.8% of patients experienced hematological and mucosal damage, respectively.Moreover, 43 studies reported renal damage in 193 patients and 159 studies reported alopecia, weight loss, and fatigue in 1,007 patients (Zhang et al., 2016).However, despite their potential severity, these adverse effects are generally reversible and can be prevented through careful monitoring and dose adjustment (Baoqi et al., 2021).

Hepatotoxic side effects
During its hepatic metabolism, metabolites of T. wilfordii undergo lipid peroxidation, which can lead to hepatocytic necrosis.Hepatotoxicity is among the major toxic reactions associated with the use of T. wilfordii, which manifests clinically as liver discomfort, hepatomegaly, and abnormal liver function.This is primarily characterized by elevated activities of alanine aminotransferase and aspartate aminotransferase, increased levels of total bile acids and total bilirubin, lower levels of prealbumin expression, hepatocellular steatosis, necrosis, and inflammatory cell infiltration (Zhong et al., 2013).Mechanistic studies that have examined the hepatotoxicity of T. wilfordii glycosides have revealed that when administered at high doses, these glycosides can inhibit the mRNA expression of key hepatic cytochrome P450 family members, including CYP27A1 and CYP8B1, as well as that of antioxidative enzymes such as SOD-1 and glutathione peroxidase 1.The consequent reduced protein expression of these enzymes results in bile stasis, mitochondrial damage, and a series of oxidative stress responses within the liver (Li et al., 2020).

Nephrotoxic side effects
The nephrotoxicity associated with T. wilfordii usage is mainly attributable to its diterpenoid, triterpenoid, and alkaloid metabolites, which can damage renal tubular epithelial cells.The clinical manifestations of T. wilfordii-induced nephrotoxicity include oliguria, edema, hematuria, hypotension, and hyperkalemia (Zhang Q. et al., 2018).Administration of a T. wilfordii extract to rats through gavage has be shown to result in elevated levels of blood creatinine, blood urea nitrogen, and uric acid, as well as causing renal glomerular atrophy, liquefaction, and necrosis of the local renal tubules (Shen et al., 2020).Further investigations have revealed that the molecular mechanisms underlying T. wilfordii-induced nephrotoxicity may involve an increase in the levels of the death receptor Fas and its ligand FasL through the mediation of triptolide.Additionally, cytochrome C, an intrinsic pathway mediator, is released into the cytoplasm, leading to alterations in electron transport, loss of mitochondrial membrane potential, and induction of apoptosis through multiple pathways (Feng et al., 2019).It has also been suggested that T. wilfordii-induced renal injury may be attributable to an impairment of angiotensin II receptor synthesis, resulting in elevated levels of ROS in renal tissue and the promotion of cell apoptosis by damaging the renal glomeruli (Zhang C. et al., 2018).

Reproductive toxicity
Toxic side effects of T. wilfordii have also been reported for the reproductive system, as exemplified by instances of amenorrhea in women and reduced sperm counts in men (Singla and Challana, 2014), and these have been found to be significantly correlated with patient age, drug dosage, and course of treatment.As a cytotoxic medication, T. wilfordii mainly acts on follicular cells, thereby reducing the level of estrogen secreted by these follicles (Deng et al., 2022).Additionally, preparations of this plant can reduce progesterone levels, thereby altering the levels of hormones in the hypothalamus-pituitary-gonadal axis and leading to abnormalities in the menstrual cycle and thus menstrual disorders or amenorrhea.Furthermore, T. wilfordii has been shown to reduce the expression of spermatogenesis-related genes and inhibit the morphological and functional development of spermatozoa, leading to reduced fertility (Ruan et al., 2018), whereas in animal experiments, T. wilfordii has been found to induce mitochondrial apoptosis of ovarian granulosa cells in NIH mice, leading to ovarian damage (Zeng et al., 2017).In the latter case, however, it was established that the perturbation of ovarian function caused by tretinoin was reversible, and that function recovered naturally after discontinuing administration of the drug.

Gastrointestinal side effects
Adverse reactions of the human gastrointestinal tract associated with the oral administration of different preparations of T. wilfordii primarily include a loss of appetite, dry mouth, nausea and vomiting, constipation, abdominal pain, and diarrhea (Liu et al., 2018).However, most patients tolerate these symptoms and recover after discontinuing administration of the drug.In laboratory studies, mice administered the LD 50 dosage of triptolide, either intraperitoneally and orally, showed significant congestion at the base of the stomach and irregularly scattered intestinal ulcers.In a study on the in vivo activity of triptolide, it was found that although this compound was readily absorbed by the gastrointestinal tract, the absorption was incomplete, which may account for the gastrointestinal tract irritation associated with triptolide usage (Liu Y. et al., 2013).

Other side effects
Abnormal skin-related symptoms are an additional frequently reported side effect of T. wilfordii usage, among which skin pigmentation, drug eruption, erythema nodosum, oral mucosal herpes, and skin allergic vasculitis are common reactions (Tao et al., 2002).However, most of these responses require no special treatment and resolve after a reduction in dosage or discontinued administration of the drug.Furthermore, celastrol has been shown to inhibit cell viability and migration, block the cell cycle at the G0/ G1 phase, promote apoptosis by enhancing the expression of caspase-3 and Bax, and reduce the expression of Bcl-2 in human biliary epithelial cell death, thereby inducing cholangiocytotoxicity (Y-JJCT and Drugs, 2020).Furthermore, triptolide has been found to inhibit the proliferation and viability of inner ear stem cells, and induces apoptosis by enhancing the expression of the DNA damage repair proteins γH2AX and 53BP1.Moreover, it has been speculated that triptolide-induced inner ear stem cell cytotoxicity may be associated with mitochondrial dysfunction caused by optic nerve atrophy and incision (Tang et al., 2019).

Discussion
T. wilfordii has a long-standing history and extensive application in the treatment of rheumatic and autoimmune diseases, often with notable clinical efficacy.With continual advances in clinical research and progress in the pharmacology and toxicology of T. wilfordii, a diverse range of bioactive metabolites within this plant have gradually been discovered and demonstrated to have notable antiinflammatory and immunomodulatory effects.In this review, we have systematically summarized the historical evolution of T. wilfordii in the treatment of rheumatic and autoimmune diseases, along with its global distribution.We also describe the effective metabolites produced by this plant and their derivatives, which have been shown to have notable anti-inflammatory, anticancer, immunosuppressive, and immunomodulatory properties (Table 3).Additionally, we present a comprehensive overview of the mechanistic actions of T. wilfordii in the treatment rheumatoid arthritis, including immune suppressive, anti-inflammatory, antiangiogenic, and bone and cartilage protective effects, and discuss the mechanisms underlying the efficacy of T. wilfordii treatment for other rheumatic and autoimmune diseases, namely, SLE, psoriasis, Crohn's disease, and multiple sclerosis.Moreover, we highlight the potential toxic effects of T. wilfordii preparations, primarily in terms of hepatorenal damage.However, although these effects have been observed in clinical settings and are clearly undesirable, many eventually resolve without intervention, or can be partially reversed or alleviated through dose adjustment.
Although significant progress has been made in the treatment of rheumatic and autoimmune diseases using T. wilfordii, particularly in the case of RA, limitations persist in advanced chemical and pharmacological methods, as well as in the accumulation of experience in clinical practice.Despite substantial accomplishments in clinical trials, meta-analyses, experimental studies, and guideline development, gaps remain in our understanding of the pathogenesis and etiology of rheumatic and autoimmune diseases, as well as the precise mechanisms of action of T. wilfordii preparations.Currently, these gaps present a substantial hurdle hampering the more widespread application of T. wilfordii preparations in countries other than China, as well as the realization of personalized treatment for patients.Moreover, T. wilfordii and the extracts thereof contain a diverse range of metabolites that may have synergistic or antagonistic effects, which consequently presents considerable challenges in establishing clear associations between these metabolites and their corresponding biological targets.Accordingly, elucidating the potential molecular mechanisms underlying the effects of T. wilfordii metabolites and delineating their functional and toxicological characteristics are of paramount importance.An integrated approach that combines clinical data with systems biology, network pharmacology, analytical chemistry, and molecular biology approaches is necessary to achieve these goals.Frontiers in Pharmacology frontiersin.org18 Shan et al. 10.3389/fphar.2023.1282610

TABLE 1
Chemical composition of Tripterygium wilfordii Hook F.

TABLE 2
As discussed in previous sections, triptolide, celastrol, and wilforlide A have been shown to significantly inhibit inflammatory responses, with NF-κB activation in B cells being Clinical trials on the active ingredients of Tripterygium wilfordii Hook F.

TABLE 2 (
Continued) Clinical trials on the active ingredients of Tripterygium wilfordii Hook F.

TABLE 3
Preclinical research on Tripterygium wilfordii Hook F.