Mesenchymal stem cells inhibit lipopolysaccharide-induced inflammatory responses of BV2 microglial cells through TSG-6
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Microglia are the primary immunocompetent cells in brain tissue and microglia-mediated inflammation is associated with the pathogenesis of various neuronal disorders. Recently, many studies have shown that mesenchymal stem cells (MSCs) display a remarkable ability to modulate inflammatory and immune responses through the release of a variety of bioactive molecules, thereby protecting the central nervous system. Previously, we reported that MSCs have the ability to modulate inflammatory responses in a traumatic brain injury model and that the potential mechanisms may be partially attributed to upregulated TNF-α stimulated gene/protein 6 (TSG-6) expression. However, whether TSG-6 exerts an anti-inflammatory effect by affecting microglia is not fully understood. In this study, we investigated the anti-inflammatory effects of MSCs and TSG-6 in an in vitro lipopolysaccharide (LPS)-induced BV2 microglial activation model. We found that MSCs and TSG-6 significantly inhibited the expression of pro-inflammatory mediators in activated microglia. However, MSC effects on microglia were attenuated when TSG-6 expression was silenced. In addition, we found that the activation of nuclear factor (NF)-κB and mitogen-activated protein kinase (MAPK) pathways in LPS-stimulated BV2 microglial cells was significantly inhibited by TSG-6. Furthermore, we found that the presence of CD44 in BV2 microglial cells was essential for MSC- and TSG-6-mediated inhibition of pro-inflammatory gene expression and of NF-κB and MAPK activation in BV2 microglial cells. The results of this study suggest that MSCs can modulate microglia activation through TSG-6 and that TSG-6 attenuates the inflammatory cascade in activated microglia. Our study indicates that novel mechanisms are responsible for the immunomodulatory effect of MSCs on microglia and that MSCs, as well as TSG-6, might be promising therapeutic agents for the treatment of neurotraumatic injuries or neuroinflammatory diseases associated with microglial activation.
KeywordsMesenchymal Stem Cell Electrophoretic Mobility Shift Assay Central Nervous System Injury Primary Microglia CD44 Knockdown
Mesenchymal stem cell
Traumatic brain injury
Central nervous system
TNF-α stimulated gene/protein 6
electrophoretic mobility shift assay
Microglia, which are derived from primitive myeloid progenitor cells, are the resident immune cells of the central nervous system (CNS) . As CNS-specific macrophages, they play a critical role in immune surveillance and homeostatic maintenance of the CNS . They are highly responsive to stress and injury and become immediately and focally activated in response to brain injuries, systemic infections, and chronic diseases [3, 4]. Furthermore, microglia are involved in initiating inflammatory responses in the brain through secreting a variety of inflammatory mediators, including tumor necrosis factor (TNF)-α, interleukin (IL)-1β, IL-6, and nitric oxide (NO) [5, 6]. Many recent studies have attributed neuronal damage to the inflammatory responses of the microglia rather than to a direct neurotoxic insult [7, 8]. Reactive microglia are a common feature of numerous types of brain pathology . Therefore, modulating microglia function and activity appears to be an attractive approach to treating CNS injuries.
Mesenchymal stem cells (MSCs) are a population of heterogeneous multipotent cells that reside primarily in bone marrow but can be found in various postnatal organs and tissues, such as adipose tissue , umbilical cord blood , and amniotic fluid . These cells are relatively easy to isolate, expand rapidly in culture, and differentiate into several cellular phenotypes in vitro and in vivo. Due to their self-renewal and potential multilineage properties, MSCs appear to be an ideal cellular source for the repair of CNS injuries. Previous studies have tested the therapeutic potential of MSCs in treating CNS injuries, including traumatic brain injury (TBI), stroke, and spinal cord injury in animal models. The results of these studies suggested that transplanted MSCs have beneficial effects on CNS injuries. For instance, MSCs may promote functional neurological recovery, decrease apoptosis levels, foster endogenous neurogenesis, improve angiogenesis, and reduce lesion size .
The initial assumption in exploring the mechanisms by which MSCs ameliorate CNS injury was that they migrated to the injured tissues and transdifferentiated to replace damaged neural cells [13, 14]. However, recent studies showed that transplanted MSCs exert their therapeutic effect without evidence of engraftment [15, 16], which indicated that their regenerative and differentiating abilities may not play a role in enhancing tissue repair or limiting tissue destruction. Instead, MSCs that are stimulated via an inflammatory signal secrete a variety of bioactive molecules, such as trophic factors and anti-inflammatory molecules, to modulate the host microenvironment , which may be the main mechanism responsible for their therapeutic effects .
We recently showed that MSCs can modulate inflammation-associated cytokine release and immune cell activation during TBI-induced cerebral inflammatory responses. We also showed that the beneficial effects of MSCs may be partially explained by the effect of TSG-6 on the nuclear factor (NF)-κB pathway . However, whether TSG-6 exerts an anti-inflammatory effect by directly affecting the resident inflammatory cells of the CNS (mainly microglial cells) remains unclear.
To understand the mechanism of the effects of MSCs on neuroinflammation induced by microglia, we used the in vitro lipopolysaccharide (LPS)-induced BV2 microglial activation model. Changes in the expression of pro-inflammatory mediators and in members of intracellular signaling pathways by microglia in response to the LPS stimulus were evaluated. The results indicated that TSG-6 expression by MSCs plays a vital role in suppressing LPS-induced overactivation of BV2-microglial cells. Moreover, CD44 expression by microglial cells is essential for the TSG-6-induced downregulation of LPS and TLR4-mediated signal transduction.
Materials and methods
Six-to-eight-week-old female BALB/c mice were purchased from the Laboratory Animal Center of the Southern Medical University (Guangzhou, China). All animal experiments were conducted in accordance with the National Institutes of Health Guide for the Care and Use of Laboratory Animals (National Institutes of Health publication number 80-23, 1996 revision). All experimental procedures were approved by the Southern Medical University Ethics Committee.
Mesenchymal stem cells isolation and culture
MSCs were prepared from mouse bone marrow (BM) cells, as previously described , with minor modifications. MSCs were isolated from the BM of the tibias and femurs of male BALB/c mice. The cells were plated in 25-cm2 culture flasks containing Dulbecco’s modified Eagle medium (DMEM; Gibco, Grand Island, New York, United States) supplemented with 10% heat-inactivated fetal bovine serum (FBS; Gibco), 10 nM GlutaMAX-I supplement (Invitrogen, Raritan, New Jersey, United States), 100 U/ml penicillin, 100 μg/ml streptomycin (Hyclone, Logan, Utah, United States), and 10 μg/ml gentamycin (Hyclone) and were grown at 37°C in a humidified 5% CO2 atmosphere. After a 48-hour incubation, the non-adherent cells were removed and fresh culture medium was added to the flasks. One-half of the medium was replaced twice a week. When the cells reached 80 to 90% confluence, adherent cells were trypsinized, harvested, and expanded. MSC were assessed using flow cytometry to detect cells that expressed the typical markers CD106, CD44, and Sca-1, and that were negative for CD11b, CD45, and MHC-II (Additional file 1: Figure S1). The MSCs used in the experiments were from passages 6 to 8.
Activation of mesenchymal stem cells using TNF-α
Before being utilized in co-cultures, MSCs were activated using TNF-α, as described previously . In brief, MSCs were plated at 2 × 105 cells per well in 6-well plates containing 2 mL of DMEM with 10% heat-inactivated FBS and were incubated for 1 day. The medium was then changed to DMEM containing 2% heat-inactivated FBS and 10 ng/mL TNF-α (R&D Systems, Minneapolis, Minnesota, United States). After incubation for 18 hours, the cells were trypsinized using 0.25% trypsin (Gibco) for 3 minutes at 37°C. To confirm the increased expression of TSG-6, RNA was extracted from cell aliquots (TRIzol™, Invitrogen) and assayed for TSG-6 expression using real-time RT-PCR (Additional file 2: Figure S2).
Transfection of mesenchymal stem cells with TSG-6 siRNA
A total of 2 × 105 MSCs were plated in 6-well dishes and cultured for 24 hours. The cells were then transfected with TSG-6 siRNA or control siRNA (sc-39820, sc-37007; Santa Cruz Biotechnology Inc, Paso Robles, California, United States) using Lipofectamine™ 2000 according to the manufacturer’s instructions (Invitrogen). To confirm the silencing effect of the TSG-6 siRNA, RNA was extracted from aliquots of the cells after 48 hours and analyzed for TSG-6 expression using real-time RT-PCR (Additional file 3: Figure S3).
BV2 cell culture
The BV2 murine microglial cell line was obtained from Xiehe Medical University (Beijing, China). The cells were cultured in a humidified incubator at 37°C with 5% CO2 in DMEM supplemented with 10% heat-inactivated FBS, 100 μg/ml streptomycin, 100 U/ml penicillin (Hyclone), and 2 mmol/l glutamine (Invitrogen) at 37°C in a humidified atmosphere containing 5% CO2. A 6-well transwell system (0.3-mm pore size membrane; Corning, Cambridge, Massachusetts, United States) was used to assess the effect of MSCs on BV2 cells that were stimulated using LPS. A total of 5 × 105 BV2 cells were placed in the lower chamber and stimulated with or without 100 ng/mL of LPS in the presence of one of the following treatments in the upper chamber (recombinant mouse TSG-6 protein(rmTSG-6) was added directly to the microglial cultures during the LPS treatment): 1) control medium, 2) 1.0 × 105 TNF-α-activated MSCs, 3) 1.0 × 105 activated MSCs transfected with TSG-6 siRNA, 4) 1.0 × 105 activated MSCs transfected with control siRNA, 5) rmTSG-6 at 1 ng/ml, 6) rmTSG-6 at 10 ng/ml, or 7) rmTSG-6 at 100 ng/ml.
RNA isolation and quantification real-time polymerase chain reaction (qRT-PCR)
After 6 hours of treatment, the expression levels of TNF-α, IL-1β, IL-6, and induced nitric oxide synthase (iNOS) mRNA in BV2 cells were evaluated using real-time PCR. In brief, total RNA was isolated from BV2 cells using TRIzol™ reagent (Invitrogen) according to the manufacturers’ instructions. Total RNA from each sample was reverse transcribed using oligo dT and SuperScript III RT (Invitrogen). qRT-PCR was conducted using an ABI 7500HT rapid real-time PCR system (Applied Biosystems, Foster City, California, United States). Glyceraldehyde-3-phosphate dehydrogenase (GAPDH) was employed as the endogenous control. The sequence-specific primers used for the above genes are listed in Additional file 4: Table S1. The 2-ΔΔCt method was employed to calculate relative expression levels.
Western blotting analysis
Western blotting was performed to assess whether TSG-6 affected the downstream TLR4 signaling pathways of the BV2 cells. LPS-activated BV2 cells were cultured in the presence of 10 ng/ml rmTSG-6 (R&D Systems) for 15, 30, or 60 minutes, and total proteins were extracted using RIPA lysis buffer (sc-24948; Santa Cruz Biotechnology, California, United States). Protein concentrations were estimated using a bicinchoninic acid (BCA) protein assay kit (Thermo Scientific, Pierce, Rockford, Illinois, United States). The proteins were separated by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and transferred to polyvinylidene difluoride membranes (Millipore, Billerica, Massachusetts, United States). The membranes were blocked using 5% non-fat dry milk in Tris-Buffered Saline and Tween 20 (TBST) and then incubated with primary antibodies directed against JNK, p38, Erk, phospho-JNK, phospho-p38 or phospho-Erk (Cell Signaling Technology, Danvers, Massachusetts, United States) at a 1:1000 dilution at 4°C overnight. The secondary antibodies were applied for 1 hour at room temperature. The immunoblots were visualized using enhanced chemiluminescence (ECL; Thermo Scientific). The cellular expression levels were normalized to GAPDH (Cell Signaling Technology).
Immunofluorescence and laser-scanning confocal microscopy
Immunofluorescence staining and laser-scanning confocal microscopy were performed as previously described. BV2 cells were grown on poly-L-lysine- (Sigma, St Louis, MO, USA) coated glass slides for 12 hours. BV2 cells were stimulated using LPS in the presence of 10 ng/ml rmTSG-6 for 1 hour. After treatment, the cells were washed using PBS and fixed using 4% paraformaldehyde at room temperature. The cells were exposed to a primary antibody directed against NF-κB p65 (rabbit monoclonal NF-κB p65 antibody, 1:200) at 4°C overnight. Following primary antibody incubation, the slides were incubated with the secondary antibody (Alexa Fluor 594 goat anti-rabbit IgG (1:200)) for 1 hour at room temperature. After three washes using PBS, the slides were incubated with 5 μg/ml FITC-Phalloidin (Sigma) for 20 minutes at room temperature. The slides were mounted using ProLong® Gold Antifade Reagent containing 4', 6-diamidino-2-phenylindole (DAPI, Invitrogen). Images were obtained using an Olympus FV1000 confocal microscope (Olympus, Tokyo, Japan) and analyzed using FV10-ASW 3.0 Viewer software (Olympus). The percentage of cells with NF-κB p65 staining localized to the nucleus was determined by examining at least 100 cells per slide.
Electrophoretic mobility shift assay (EMSA)
To examine the binding of NF-κB to DNA, nuclear extracts were prepared using NE-PER Nuclear and Cytoplasmic Extraction Reagents (Thermo Scientific). Synthetic complementary NF-κB-binding oligonucleotides (5′-AGT TGAGGG GAC TTT CCC AGG C-3′) (Santa Cruz Biotechnology) were biotinylated using a biotin 3′-DNA labeling kit (Thermo Scientific) according to the manufacturer’s instructions. The binding reactions were conducted for 20 minutes at room temperature in the presence of 50 ng/ml poly(dI-dC), 0.05% Nonidet P-40, 5 mM MgCl2, 10 mM ethylene diamine tetraacetic acid (EDTA), and 2.5% glycerol in 1 × binding buffer (LightShift chemiluminescent EMSA kit, Thermo Scientific) with 20 fmol biotin-end-labeled target DNA and 10 μg of nuclear extract. The samples were loaded onto 6.5% non-denaturing polyacrylamide gels pre-electrophoresed in 0.5 × Tris/Boric Acid/EDTA (TBE) buffer at 100 V for 60 minutes and were electrophoresed in 0.5 × TBE buffer at 100 V for 45 minutes. Then, the samples were transferred to a positively charged nylon membrane (Millipore) in 0.5 × TBE on ice at 390 mA for 30 minutes. The transferred DNAs were cross-linked to the membrane at 120 mJ/cm2 and detected using horseradish peroxidase-conjugated streptavidin (Thermo Scientific) according to the manufacturer’s instructions.
Transient transfection of NF-κB reporter construct and luciferase assay
The level of transcriptional activity of the NF-κB gene was measured using an NF-κB luciferase reporter gene assay (Promega, Madison, Wisconsin, United States). Briefly, BV2 cells were seeded at a density of 1 × 105 in 24-well plates and co-transfected with 0.2 μg of the pNF-κB-luciferase reporter plasmid (Bayotime, Shanghai, China) and 0.02 μg of the pRL-TK control plasmid (Promega) using Lipofectamine™ LTX (Invitrogen) according to the manufacturer’s protocol. After 24 hours, the culture medium was replaced with DMEM, and LPS stimulation was conducted in the presence of MSCs or TSG-6. Luciferase activity levels were determined using the Dual-Luciferase reporter Assay System (Promega) and normalized to that of Renilla reniformis. The altered luciferase activity was described as the relative ratio to the control activity.
Transfection of BV2 cells with CD44 siRNA
To knockdown CD44 expression, BV2 cells were transfected with CD44 siRNA or control siRNA (sc-35534, sc-37007; Santa Cruz Biotechnology) using Lipofectamine™ 2000 (Invitrogen) according to the manufacturer’s instructions. After 48 hours, CD44 knockdown was assessed using western blotting and immunofluorescence assays.
All experiments were performed at least three times, and the data are expressed as the mean value ± SD. Statistical analyses were performed using SPSS 13.0 software (SPSS, Chicago, Illinois, United States). A one-way analysis of variance (ANOVA) was used to determine the significance of differences between the groups. Significance was set at a level of P <0.05.
Mesenchymal stem cells and TSG-6 suppressed the lipopolysaccharide-induced pro-inflammatory gene expression of microglia
TSG-6 interfered with the lipopolysaccharide-induced activation of NF-κB signaling
TSG-6 attenuated the lipopolysaccharide-mediated phosphorylation of p38, JNK and Erk mitogen-activated protein kinases
The inhibitory effects of TSG-6 were dependent on CD44
The aims of this study were to determine whether TSG-6 exerted an anti-inflammatory effect by modulating microglia function and to examine the possible underlying mechanisms. Therefore, we used an in vitro LPS-stimulated BV2 microglial model to investigate changes that occurred in pro-inflammatory mediator expression and in members of inflammatory signaling pathways in the presence of MSCs or TSG-6. In this study, we found that MSCs and TSG-6 mediated the LPS-induced inflammatory responses of microglia. Our results clearly showed that MSCs and TSG-6 reduced the LPS-induced expression of TNF-α, IL-1β, IL-6, and iNOS. In experiments designed to elucidate the molecular mechanism underlying the anti-inflammatory function of TSG-6 in microglia, we found that TSG-6 attenuated the LPS-induced activation of NF-κB signaling and the phosphorylation of p38, JNK, and Erk Moreover, the anti-inflammatory effect of TSG-6 depended on CD44 expression on BV2 microglial cells.
Microglia are activated by various stresses and by brain injury in order to provide supportive signals that lead to neuronal recovery. However, excessive activation of microglia, characterized by robust production and secretion of a variety of pro-inflammatory mediators, results in neuronal injury which is associated with the pathogenesis of a number of neurological disorders such as multiple sclerosis, trauma, Parkinson’s disease, and Alzheimer’s disease [25, 26]. Thus, a therapeutic approach targeting activated microglia may be a promising treatment for these conditions.
Recently, many studies have demonstrated that MSCs possess immunomodulatory properties. MSCs can directly inhibit the proliferation of T and B lymphocytes [27, 28], inhibit dendritic cell maturation and function , and modulate the cytokine-secretion profile of monocytes and macrophages . MSCs are also known to inhibit the basal and formyl-methionyl-leucyl-phenylalanine-stimulated production of reactive oxygen species by neutrophils . Due to these immunosuppressive properties, MSC-based therapy has been successfully applied to various immune-related diseases, such as graft-versus-host disease (GvHD) , systemic lupus erythematosus (SLE) , experimental autoimmune encephalomyelitis , and multiple sclerosis (MS) .
Previously, we reported that MSCs have the ability to modulate inflammatory responses in a TBI model and that the potential mechanisms responsible for the anti-inflammatory effects of MSCs may be partially explained by upregulated TSG-6 expression. TSG-6, also known as TNAIP6, is a 30 kD glycoprotein that is expressed at substantially higher levels by many cell types (including MSCs) in response to stimulation by several pro-inflammatory mediators compared to normal physiological conditions . Several studies have shown that TSG-6 produces anti-inflammatory effects both in vivo and in vitro [15, 16, 37]. In the present study, we found that TSG-6 was expressed at a substantially higher level in MSCs stimulated by TNF-α. The TNF-α-activated MSCs were co-cultured with BV2 cells followed by LPS stimulation. MSCs significantly reduced the level of pro-inflammatory gene expression in BV2 cells. However, when we utilized MSCs that were transfected with siRNA directed against TSG-6 in the co-cultures with activated microglia, we detected a partial restoration of the expression level of those genes compared with that of LPS-stimulated BV2 cells in the presence of non-transfected MSCs. To identify the functional roles of TSG-6 in microglial inflammatory responses induced by LPS, we added recombinant TSG-6 protein directly to microglial cultures during LPS treatment. The recombinant TSG-6 protein also reduced the expression of the pro-inflammatory genes in a dose-dependent manner. These data demonstrated that TSG-6 has a key role in MSC-mediated modulation of LPS-activated microglia.
LPS induces microglial activation through the expression of TLR4, which triggers the association of myeloid differentiation factor 88 (MyD88)-independent and -dependent pathways. More importantly, the MyD88-dependent pathway sequentially activates a cascade of enzymes and transcription factors, including NF-κB and MAPKs, such as p38, JNK, and Erk .
NF-κB is a critical transcription factor that regulates the expression of various pro-inflammatory genes. NF-κB normally exists in the cytosol under unstimulated conditions, bound to the inhibitory IκB protein. LPS stimulation leads to an increase in the nuclear translocation of NF-κB and in its DNA-binding activity at specific promoter regions of a variety of pro-inflammatory genes such as iNOS, TNF-α, IL-1β, and IL-6, which modulates their expression . To elucidate the molecular mechanism underlying the anti-inflammatory function of TSG-6 in microglia, we first evaluated its effect on NF-κB activity. The results demonstrated that LPS-induced nuclear translocation, DNA-binding activity, and transcriptional activity of NF-κB in BV2 cells were substantially suppressed by TSG-6. In addition to the NF-κB pathway, the MAPK pathway was found to be associated with the LPS-induced activation of BV2 microglial cells [38, 39]. Some studies have suggested that MAPKs are crucial for the LPS-stimulated expression of iNOS, COX-2, and TNF-α . Our study showed that LPS increased the activation of p38, JNK, and Erk in BV2 cells. However, pretreatment with TSG-6 significantly decreased the level of activation of these kinases in LPS-stimulated microglia. More importantly, we found that the inhibitory effects of TSG-6 were dependent on CD44 expression on microglial cells. Because MSCs or TSG-6 inhibited pro-inflammatory gene expression, NF-κB and MAPK activation was significantly restored in CD44-knockdown BV2 cells after LPS stimulation. These results are consistent with previous observations that the inhibitory effects of TSG-6 on NF-кB signaling were dependent on CD44 expression on macrophages and that TSG-6 might regulate the activity of TLR pathways upstream of MyD88 . TSG-6 is capable of binding to hyaluronan (HA), moreover, TSG-6 and HA have been reported to form stable complexes that modulate the interaction of HA with the cell surface receptor CD44 , which is involved in TLR4 signaling through the formation of a TLR4-CD44 complex. One study has shown that the engagement of CD44 by HA suppressed TLR4 signaling . In addition, there is an increasing body of evidence showing that CD44 is a negative regulator of TLR2- and TLR4-mediated inflammation . Because crosslinking HA with TSG-6 can significantly alter the affinity of CD44 for HA, TSG-6 may stabilize the binding of CD44 to HA , which are in close proximity to TLR4, thus hindering the binding of TLR4 agonists and thereby blocking TLR4 signaling. However, our study does not definitively support such a mechanism, and thus this hypothesis is only speculative. Further research must be performed to determine the specific mechanisms of action. Another limitation of our article is the lack of demonstration of the findings in primary microglia, although BV2 microglial cells are commonly used as an alternative to primary microglia to study various microglial responses and interactions [20, 45, 46]. To fully understand the anti-inflammatory and immunomodulatory properties of TSG-6, further experiments should focus on the role of TSG-6 on primary microglia.
In summary, we have demonstrated that following stimulation with TNF-α, MSCs modulate microglia activity through TSG-6. TSG-6 may interact with CD44 on microglial cells to inhibit LPS-induced activation of TLR4 signaling, decreasing NF-κB activity and the activation of MAPK signaling, thus diminishing the levels of expression of neuroinflammatory cytokines in BV2 microglia. These results are consistent with the recent evidence that MSCs can inhibit inflammation through diffusible molecules and that stimulation with certain inflammatory cytokines is essential for MSC-mediated immunosuppression. Several soluble factors have been reported to be involved in MSC-mediated immunoregulation, such as PGE2, IDO , and CX3CL1 , which are released by MSCs following stimulation with inflammatory factors. Therefore, in addition to TSG-6, it is apparent that MSCs can produce a variety of anti-inflammatory factors and the results presented here do not rule out the effects of other anti-inflammatory factors. However, the data obtained in experiments using TSG-6 siRNA and rmTSG-6 demonstrates that TSG-6 might play a vital role in inhibiting LPS-stimulated microglial activation.
The results of this study suggest that MSCs mediate microglial activation and the production of inflammatory factors through TSG-6. Our data suggest novel mechanisms for the MSC-mediated immunosuppression of microglia. These findings can not only serve as the basis for future studies of the beneficial immunomodulatory effects of MSCs but may also promote the future clinical application of MSCs to neurotraumatic and/or neuroinflammatory disorders in which microglia may play a major pathogenic role.
This work was supported by grants from the funds for National Key Clinic Department, the Natural Science Fund of China (number: 81171179, number: 81272439), the Funds for Key Natural Science Foundation of Guangdong (number: .S2013020012754), the Educational Commission of Guangdong (number: 2013CXZDA008), and the Key Project of Health Collaborative Innovation of Guangzhou (number: 201400000003-2) to Professor Xiaodan Jiang.
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