2. 湖南中医药大学 中药粉体与创新药物省部共建国家重点实验室培育基地，湖南 长沙 410208;
3. 澳门科技大学中药质量研究国家重点实验室，澳门 氹仔 999078
2. Institute of Innovation and Applied Research in Chinese Medicine, Hunan University of Chinese Medicine, Changsha 410208, China;
3. State Key Lab for Quality Research in Chinese Medicine, Macau University of Science and Technology, Taipa Macau 999078, China
类风湿关节炎(rheumatoid arthritis，RA)是一种慢性炎症性自身免疫疾病，其主要病理特征为进行性关节滑膜炎症，软骨和骨破坏，最终造成关节畸形和功能丧失[1]。研究表明，RA是以大量CD4+ T细胞浸润为主的慢性滑膜炎症反应，CD4+ T细胞作为效应T细胞的重要成分，参与免疫应答过程的各个阶段，其介导的免疫反应异常被认为是RA主要发病机制之一[2]。根据其产生的细胞因子及其功能，通常将CD4+ T细胞分为辅助性T细胞1 (helper T 1，Th1)、Th2、Th17和调节性T细胞(regulatory T cell，Treg)四大亚群。Th17细胞主要分泌IL-17、IL-21等炎性细胞因子，具有促进炎性反应和促进关节软骨和骨破坏进程等多重效应。因此，本文拟收集近年来Th 17细胞参与RA发病的相关研究进展并进行综述，以期为RA病理机制研究提供参考。
1 Th17细胞概述在免疫应答的不同阶段，因抗原性质、局部微环境等因素的综合效应，CD4+ T细胞会朝不同方向分化，进而发挥其特定的生物学效应。Th17细胞是CD4+ T细胞新亚群，其主要分泌白介素IL-17A、IL-17F、IL-21等细胞因子，并表达转录因子维甲酸相关孤核受体(retinoid acid-related orphan receptorγ，RORγt)和RORα，而在自身免疫疾病中发挥重要作用。
在树突细胞、单核细胞等抗原呈递细胞作用下，IL-6协同低浓度的转化生长因子-β(transforming growth factor β，TGF-β)，激活JAK1，进而招募信号传导及转录激活因子-3 (signal transducer and activator of transcription-3，STAT3)，并使其磷酸化，活化的JAK1/STAT3通路激活RORγt，并促进IL-21产生[3]。在IL-6和IL-21协同作用下，产生IL-23R、RORα和RORγt，使Naive CD4+ T细胞向Th17细胞分化[4]，并抑制转录因子叉头蛋白3 (forkhead box protein 3，Foxp3)的负向调节作用。Th17细胞被转录因子STAT4和STAT6调控，生成IL-17，同时可产生IL-23、IL-6和肿瘤坏死因子-α (tumor necrosis factorα，TNF-α)等炎性细胞因子，IL-23可促进激活的记忆细胞产生IL-17，使Th17细胞得以存活和维持功能[5]。见Fig 1。
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| Fig 1 Pathway of differentiation of naive CD4+ T cells into Th17 cells In the presence of antigen-presenting cells such as dendritic cells and monocytes, IL-6 is synergized with low-concentration TGF-β to activate JAK1, and then activated JAK1 recruits and phosphorates STAT3. JAK1/STAT3 signaling activates RORγt and promotes production of IL-21.Under the circumstance of abundant IL-6 and IL-21, naive CD4+ T cells differentiate into Th17 cells, and simultaneously negative regulation of Foxp3 to this differentiation is inhibited. Transcription factors STAT4 and STAT6 regulate Th17 cells to produce pro-inflammatory cytokines IL-23, IL-6, and TNF-α.IL-23 promotes activated memory CD4+ T cells to produce IL-17 which in turn maintain the survival and function of Th17 cells. |
RA早期主要表现为大量CD4+ T细胞浸润为主的慢性滑膜炎症反应。IL-17是RA早期重要细胞之一，具有强大的促炎效应，大量、广泛表达于在RA患者关节滑液、滑膜组织、外周血、外周淋巴结等处[6]。RA患者发病前外周血IL-17表达水平显著高于发病后，RA患者(病程 < 12月)外周血IL-17表达量相当高，且与C-反应蛋白(C-reactive protein，CRP)呈正相关[7]。RA患者血清Th17水平显著升高，且与疾病活动性评分28 (disease activity score 28，DAS28)呈正相关[8]。RA患者外周血Th 17、IL-6、IL-17、IL-21表达量随疾病活动程度增加而增加[9]。RA患者外周血单个核细胞Th17细胞中趋化因子受体6(chemokine receptors 6，CCR6)和RORγt表达量显著增加，且关节滑液单核细胞Th17细胞比例显著高于外周血单个核细胞[10]。树突细胞可分泌IL-23促进IL-17的增殖，而参与RA风湿结节的形成[5]。胶原性关节炎(collagen induced arthritis, CIA)大鼠关节滑膜及外周血IL-17显著升高，外周血RORγt显著增加[11]。CIA小鼠血浆IL-17、外周血和脾脏Th17细胞比例，CD4+ T细胞IL-17 mRNA表达水平均显著增加[12]。IL-17可协同IL-1β和TNF-α共同诱导T细胞和树突细胞的趋化[13]。IL-17协同TNF-α促进成纤维样滑膜细胞(fibroblast-like synoviocytes，FLSs)和上皮细胞大量分泌IL-6、IL-8、前列腺素E2(prostaglandin E，PGE2)和中性粒细胞化学引诱物[13]。上述研究表明，Th17细胞在外周血、关节滑液等部位均有表达，不仅参与了RA发病，而且与疾病活动程度及临床检测指标密切相关。
IL-21是Th17细胞分泌的促炎性细胞因子，大量表达于CD4+ T细胞及FLSs。RA患者关节滑液中CD4+ IL-21+T细胞比例显著增加，外周血CD4+ IL-21+T细胞及血清IL-21与DAS28、抗-CCP抗体、血沉和类风湿因子呈正相关[14, 15]。IL-21通过抑制Foxp3表达，上调RORC而促进Th17细胞分化与增殖[16]。研究显示，联合阻断IL-6和IL-21可有效抑制CIA小鼠脾脏CD4+ Th17细胞分化，进而减轻疾病严重程度[4]。IL-21/IL-21R可介导磷脂酰肌醇3-激酶/蛋白激酶B (phosphoinosmde-3-kinase/protein kinase B，PI3K/PKB) PI3K/Akt促进IL-23表达而促进RA-FLS异常增殖[17]。IL-21通过丝裂原活化蛋白激酶(mitogen-activated protein kinase，MAPK)、STAT3及PI3K/Akt通路介导FLS分泌TNF-α、IL-6等促炎性细胞因子[18]。
3 Th17细胞在RA骨破坏中的作用关节软骨的破坏及骨侵蚀是RA进展期重要组织学特征。研究表明，破骨细胞是介导RA骨破坏的关键细胞，核因子-κB受体活化因子配基(receptor activator of nuclear factor-κ B ligand，RANKL)是破骨细胞生成的启动信号。CD4+ T细胞介导的免疫应答在RA骨破坏中具有重要作用。IL-17A/F通过上调Runt相关转录因子-2 (runt-related transcription factor，RUNX2)蛋白，增强IL-6蛋白和基质金属蛋白酶-3 (matrix metalloproteinase-3，MMP-3)mRNA表达而抑制RA关节软骨的产生潜能[19]。骨桥蛋白可促进成骨细胞中IL-17表达而引起CIA小鼠骨质侵蚀[20]。IL-17通过活化IL-17/IL-17R/STATS-3信号通路促进RANKL表达，抑制骨保护素(osteoprotegerin，OPG)表达而介导佐剂性关节炎(adjuvant induced arthritis, AIA)大鼠FLS破骨细胞分化[21]。RA患者骨髓血浆中IL-17A/F及CD3+CD4+ IL-17+细胞百分比较骨关节炎患者显著升高，提示骨髓微环境可以促进Th17细胞和IL-17A/F过度产生而介导RA破坏[4]。IL-17促进K/BxN小鼠体内人分泌型卷曲相关蛋白1 (secreted frizzled related protein，sFRP1)分泌，减少sFRP3的分泌，进而抑Wnt/β-catenin/RUNX2通路，负调控成骨细胞生成[22]。IL-17通过诱导软骨MMP-3表达而抑制关节滑膜和软骨细胞COL2A1表达而促进骨破坏[23]。综上，IL-17主要通过以下途径参与RA骨破坏：(1)促进RANKL表达，进而促进破骨细胞分化；(2)促进MMPs表达，进而降解骨细胞外基质，引起骨破坏；(3)抑制成骨细胞生成。见Fig 2。
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| Fig 2 Bone destruction in RA promoted by Th17 cells Th17 cells produce IL-17 and IL-21 which play an important role in bone destruction of patients with RA. IL-17 promotes bone destruction in RA through the following pathways: (1) IL-17 promotes RANKL expression and osteoclast differentiation; (2) IL-17 promotes the expression of MMPs that degrade the extracellular matrix of bone; (3) IL-17 inhibits osteoblastogenesis.IL-21 promotes bone destruction in RA through the following pathways:(1) IL-21 promotes the expression of MMPs which degrade the extracellular matrix of bone; (2) IL-21 promotes the expression of bone resorption-related factors; (3) IL-21 promotes osteoclast differentiation |
IL-21促进FLS的RANKL表达而介导破骨细胞生成[24]。IL-21促进FLS分泌介导RA骨与关节软骨破坏的关键蛋白酶MMP1、MMP2、MMP3、MMP9、MMP13而介导，其中MMP3、MMP9的活化与PI3K、STAT-3及细胞外调节蛋白激酶(extracellular regulated protein kinase 1/2，ERK1/2)有关[25]。IL-21/IL-21R相互作用可活化PI3K/Akt通路介导活化T细胞核因子1蛋白(nuclear factor of activated T-cells，NFATc1)及骨吸收相关因子组织蛋白酶k (cathepsin K)和抗酒石酸酸性磷酸酶(tartrate-resistant acid phosphatase，TRAP)表达进而介导骨破坏[17]。综上，IL-21参与RA骨破坏的途径主要有：(1)促进MMPs表达，进而降解骨细胞外基质，引起骨破坏；(2)调节骨吸收相关因子的表达；(3)促进破骨细胞分化。见Fig 2。
4 总结与展望综上所述，Th17细胞及其分泌的IL-17、IL-21等促炎性细胞因子对RA早期的滑膜炎症反应和RA晚期的骨破坏等病理环节均发挥促进作用，但对于不同病理环节的作用强度是否有差异；对于不同环节的作用机制及其效应靶点是否异同，均尚有待于进一步深入研究。因此，深入研究Th17细胞对RA不同病理环节的影响，有助于进一步揭示其针对RA不同病理环节的病理机制及其效应靶点，进而开发基于Th17细胞的RA多靶向防治药物，具有重大的研究意义和广袤的应用开发前景。
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