陳麗萍(綜述),趙 文,張 曉(審校)
(1.深圳市福田區(qū)第二人民醫(yī)院腎內(nèi)科,廣東 深圳 518000; 2.廣東省人民醫(yī)院風(fēng)濕科,廣州 510030)
系統(tǒng)性紅斑狼瘡(solid liquid equilibrium,SLE)是常見的系統(tǒng)性自身免疫性疾病,具體的發(fā)病機制尚未明確,目前認(rèn)為免疫系統(tǒng)功能紊亂是其重要致病因素。樹突狀細(xì)胞(dendritic cells,DCs)是SLE發(fā)病機制中免疫系統(tǒng)異常的關(guān)鍵因素,是新近研究的熱點[1-3],其中淋巴系來源的DC即漿細(xì)胞樣樹突細(xì)胞(plasmacytoid dendritic cells,pDCs)與SLE的發(fā)生、維持及發(fā)展有密切關(guān)系,并且與狼瘡性腎炎嚴(yán)重性相關(guān)。是免疫紊亂的關(guān)鍵點,可能是干預(yù)SLE的靶點。
DCs屬抗原呈遞細(xì)胞(antigen presenting cell,APC),其主要可分為髓系來源的DC,即髓樣樹突細(xì)胞(myeloid dendritic cells,mDCs)和淋巴系來源的DC,即pDCs。pDCs在機體抵抗細(xì)菌、病毒感染、自身免疫性疾病等方面有重要的免疫調(diào)節(jié)作用[4]。
pDCs在很多方面與mDCs不同。首先,pDCs可特異表達(dá)Toll樣受體(toll-like receptors,TLRs)7、9及分泌大量Ⅰ型干擾素(interferon,IFN);而人類mDCs表達(dá)TLRs1-6、-8和-10。其次,pDCs表面分子FcγRⅡα(CD32)可結(jié)合免疫復(fù)合物的Fc部位。CD32介導(dǎo)細(xì)胞內(nèi)吞免疫復(fù)合物,激活pDCs;mDCs通過吞噬和胞飲作用呈遞外源性抗原。再次,pDCs活化后,可誘導(dǎo)輔助性T細(xì)胞(Th細(xì)胞)向Th2細(xì)胞分化,引發(fā)Th2反應(yīng),并且激活B細(xì)胞,使其分泌大量自身抗體[5]。mDCs活化后可促使Th細(xì)胞向Th1細(xì)胞分化,引發(fā)Th1反應(yīng)。
SLE的特征是自身免疫反應(yīng)與多種細(xì)胞核成分直接相關(guān),特別是核小體是主要的自身抗原[6]。自身抗原性核酸物質(zhì)主要來自凋亡或壞死細(xì)胞,或中性粒細(xì)胞胞外攜帶物,即激活的中性粒細(xì)胞產(chǎn)生的富含核酸物質(zhì)的產(chǎn)物[7]。SLE患者血漿中含有自身核酸抗原的免疫復(fù)合物(immune complexes,ICs),可以激活pDCs,誘導(dǎo)IFN-α生成。IFN-α可促進激活的B細(xì)胞分化為漿細(xì)胞,分泌自身抗體,與ICs結(jié)合,形成正反饋環(huán)路,增加IFN-α產(chǎn)生;促進自身反應(yīng)性CD8+T細(xì)胞成熟,導(dǎo)致組織損傷,產(chǎn)生新的自身抗原,進一步促進循環(huán)ICs驅(qū)動的IFN-α的分泌。
2.1pDCs分泌的Ⅰ型IFN與SLE的關(guān)系 微生物激活的pDCs分泌Ⅰ型IFN的量約為其他類型免疫細(xì)胞的200~1000倍。Ⅰ型IFN有免疫調(diào)節(jié)的作用,與多種自身免疫性疾病的發(fā)病有關(guān),尤其與SLE關(guān)系密切[8-9]。長療程的IFN-α治療非自身免疫性疾病,部分患者出現(xiàn)嚴(yán)重的SLE樣癥狀[10]。部分SLE患者血清IFN-α水平的升高,與抗單鏈DNA自身抗體水平、疾病活動指數(shù)及嚴(yán)重性呈高度相關(guān)[11-12]。
Ⅰ型IFN對多種細(xì)胞有效應(yīng)作用。IFN-α可直接激活B細(xì)胞,使其分化為漿母細(xì)胞,在有白細(xì)胞介素6條件下,進一步分化為成熟的漿細(xì)胞,產(chǎn)生大量自身抗體;IFN-α可上調(diào)pDCs和單核細(xì)胞的TLR7/9表達(dá)和干擾素啟動子結(jié)合蛋白7表達(dá),使其對ICs的應(yīng)答增加,進一步促進IFN-α的合成,使疾病持續(xù)進展[13];SLE患者血清IFN-α可促進CD34+造血干細(xì)胞和單核細(xì)胞分化為mDCs,調(diào)節(jié)mDCs發(fā)育,誘導(dǎo)mDCs成熟,使其呈遞自身抗原,從而導(dǎo)致自身免疫耐受的異常,導(dǎo)致SLE;IFN-α可抑制淋巴細(xì)胞生成,這可能與臨床檢測到狼瘡患者外周血T細(xì)胞、B細(xì)胞的減少有關(guān)。
狼瘡小鼠模型的研究進一步證實Ⅰ型IFN在SLE中的致病作用,IFN-α作用于(NZB/W)-F1小鼠加重病情,死亡率升高;NZB/W狼瘡小鼠模型中,Ⅰ型IFN受體缺陷可改善疾病情況[14];而過度表達(dá)IFN-α病情加重[15]。另外,用TLR-9受體激動劑處理自身免疫性NZB小鼠,血清中可檢測到高水平IFN-α[16]。
2.2TLR-7、TLR-9與SLE的關(guān)系 TLRs是一種跨膜受體,可識別細(xì)菌或病毒,引發(fā)多種前炎性因子形成,參與抗感染過程;ICs亦可啟動TLRs信號途徑,導(dǎo)致自身免疫性疾病。目前發(fā)現(xiàn)人類細(xì)胞有11種TLRs。TLR-7、-9是內(nèi)質(zhì)網(wǎng)內(nèi)核酸感受器,在人類pDCs、巨嗜細(xì)胞和B細(xì)胞表達(dá)。TLR-7能特異識別富含鳥苷或尿苷的單鏈RNA,可以被病毒RNA、鳥苷類似物和抗病毒復(fù)合物激活。TLR-9特異性識別細(xì)菌、病毒DNA中含有低甲基化的CpG序列,還可結(jié)合機體內(nèi)凋亡或壞死細(xì)胞產(chǎn)生的低甲基化CpG DNA系列。自身免疫性疾病(如SLE)由于異常凋亡細(xì)胞增多,釋放的凋亡物質(zhì)不能被及時有效清除,通過TLR-7、-9途徑,激活pDCs,使其分泌大量Ⅰ型IFN,打破免疫耐受,誘發(fā)特異性免疫反應(yīng),導(dǎo)致SLE。TLR-7、-9介導(dǎo)的Ⅰ型IFN的反應(yīng)在巨噬細(xì)胞和B細(xì)胞未被檢測到。生物合成的寡脫氧核苷酸(CpG ODN)亦可通過TLR-9激活pDCs,模擬自身免疫反應(yīng)。配體激活TLR-7、-9后,通過MyD88轉(zhuǎn)導(dǎo)信號至細(xì)胞質(zhì),與腫瘤壞死因子受體相關(guān)因子信號途徑激活干擾素啟動子結(jié)合蛋白7,干擾素啟動子結(jié)合蛋白7是Ⅰ型IFN生成的主要調(diào)節(jié)物,可轉(zhuǎn)移至細(xì)胞核,促進Ⅰ型IFN基因轉(zhuǎn)錄[17-18]。pDCs持續(xù)高水平表達(dá)干擾素啟動子結(jié)合蛋白7,可短期內(nèi)產(chǎn)生大量的Ⅰ型IFN,是pDCs的重要特點[19]。另外TLR-7、-9配體還可通過激活核因子κВ和絲裂原激活蛋白激酶信號途徑,和干擾素啟動子結(jié)合蛋白5共同啟動前炎性因子和共刺激分子基因轉(zhuǎn)錄。糖皮質(zhì)激素治療SLE呈現(xiàn)患者部分耐藥或不敏感與pDCs的TLR-7、-9被自身抗原性核酸物質(zhì)激活有關(guān),但糖皮質(zhì)激素可以抑制經(jīng)TLR-7、-8激活的DCs的成熟[20-21]。
SLE小鼠模型研究證實通過TLR-7、-9途徑可誘導(dǎo)pDCs成熟及激活pDCs,分泌大量的Ⅰ型干擾素[22]。用TLR-7、-9抑制劑治療發(fā)病的SLE(NZB_NZW) F1小鼠,疾病癥狀明顯改善,提示TLR-7、-9可能是調(diào)節(jié)pDCs的關(guān)鍵點[23]。CpG DNA/TLR-9信號途徑在狼瘡性腎炎中發(fā)揮重要作用。SLE小鼠模型腎組織的浸潤細(xì)胞表達(dá)TLR-9和TLR-9 mRNA明顯增加;CpG ODN/TLR-9途徑活化后加重MRLlpr/lpr小鼠蛋白尿和腎臟損害[24-25]。羥氯喹治療SLE通過阻礙或抑制TLR-3、-7、-8、-9信號途徑起作用[36]。提示TLRs可能是治療免疫性疾病的關(guān)鍵點。
微RNA(microRNA,miRNA)介導(dǎo)轉(zhuǎn)錄后基因調(diào)節(jié),是機體控制幾乎各種細(xì)胞普遍的生理現(xiàn)象[26-27]。miRNA可在生理及病理方面調(diào)節(jié)免疫反應(yīng),包括自身免疫性疾病。迄今為止,在人類基因組中發(fā)現(xiàn)超過1000種miRNA,每一種在各種生理功能和進化過程調(diào)節(jié)多種基因功能。miR-155和其配體相互作用共同調(diào)節(jié)正常人pDCs分泌Ⅰ型IFN的功能[28]。miR-155配體在TLRs激活的早期階段表達(dá),促進IFN-α合成。miR-155在TLR激活的較遲階段表達(dá),抑制IFN-α表達(dá)。IFN-α激活誘導(dǎo)miR-155的表達(dá),但抑制miR-155配體表達(dá),體外過表達(dá)或敲除miR-155表達(dá)對IFN-α的生成有相反作用[29]。提示在pDCs激活過程中,miRNA調(diào)節(jié)受到嚴(yán)格的控制。其他miRNA在SLE患者的pDCs中的失調(diào)仍有待進一步證實。miRNA與SLE方面的研究目前仍處初始階段,有待進一步研究。
狼瘡性腎炎發(fā)展必須有自身抗體的存在,抗雙鏈DNA(doublestranded DNA,dsDNA)/核小體抗體與腎炎的關(guān)系密切[30]。致病性抗dsDNA抗體以ICs形式沉積,并且抗C1q抗體與抗dsDNA抗體同時出現(xiàn)時,腎臟損害更加嚴(yán)重[31-32]。形成腎小球ICs的可能機制:循環(huán)ICs形成;原位ICs沉積;新近研究發(fā)現(xiàn)抗dsDNA/核小體與抗體結(jié)合出現(xiàn)在腎小球間質(zhì),也是目前最受關(guān)注的[33]。由于電荷/電荷相互作用,循環(huán)中dsDNA/核小體可以在腎小球基膜沉積,作為抗原與自身抗體結(jié)合;另外腎小球dsDNA/核小體可來自壞死腎小球固有細(xì)胞[30]。實際上用新近的電子顯微鏡共聚焦方法檢測人類和狼瘡鼠模型的腎臟組織,證實沉積在腎小球電子致密物為核小體物質(zhì)[34]。腎臟的DNAseⅠ減少,使核小體物質(zhì)在腎小球蓄積,形成ICs進一步激活pDCs,激活免疫通路和(或)補體瀑布反應(yīng)。ICs通過TLR-7、-9,激活腎臟原位細(xì)胞、激活B細(xì)胞,固有細(xì)胞和炎性細(xì)胞的相互作用促成組織損害。局部的炎性因子生成導(dǎo)致炎性細(xì)胞的進一步浸潤和更多的前炎性因子生成,最終導(dǎo)致腎臟組織炎癥損傷和纖維化。抑制TLR-7、-9能有效治療SLE小鼠,但這種效應(yīng)是主要干預(yù)系統(tǒng)性自身免疫反應(yīng),還是通過阻礙特異的腎臟組織損害仍不清楚。
狼瘡鼠模型及人類SLE患者腎臟病理研究均證實T細(xì)胞是促進腎炎進展的重要因素[35-36]。T細(xì)胞通過激活B細(xì)胞,使其分泌自身抗體,募集巨噬細(xì)胞及pDCs至腎臟組織,產(chǎn)生細(xì)胞因子,引發(fā)狼瘡性腎炎,在腎臟浸潤的T細(xì)胞被激活后表達(dá)大量的前炎性因子[37]。消除T細(xì)胞或限制T細(xì)胞的激活可減輕狼瘡鼠腎臟損害[38-39]。
致病性B細(xì)胞生成的各種自身抗體有多重效能導(dǎo)致狼瘡性腎炎。自身抗體可破壞細(xì)胞的功能,與補體的相互作用介導(dǎo)細(xì)胞毒效應(yīng)及釋放炎性介質(zhì)。狼瘡鼠模型研究證實,在腎臟浸潤的B細(xì)胞分泌抗體與各種特異性抗原結(jié)合,與局部的ICs形成有關(guān)。在狼瘡鼠模型研究中在疾病發(fā)生前、后消除B細(xì)胞可阻止或延遲狼瘡性腎炎的發(fā)生,并且可使部分患者完全或部分緩解[40-42]。
在狼瘡性腎炎腎臟組織的中性粒細(xì)胞、巨噬細(xì)胞和DC均與腎臟炎癥損害有關(guān)。中性粒細(xì)胞是中性粒細(xì)胞胞外攜帶物來源,含自身抗原(如組蛋白、DNA)以ICs形式沉積在SLE患者腎臟組織;中性粒細(xì)胞胞外攜帶物通過激活pDCs產(chǎn)生Ⅰ型IFN損傷腎組織。DCs和巨噬細(xì)胞產(chǎn)生Ⅰ型T輔助細(xì)胞前炎性細(xì)胞因子(如白細(xì)胞介素12和IFN-γ),表達(dá)細(xì)胞因子受體,與自身反應(yīng)性T細(xì)胞相互作用,可募集更多的炎癥細(xì)胞遷移至腎臟。
SLE患者外周血pDCs較健康對照者少,尤其是SLE活動期患者,但是在患者腎臟組織發(fā)現(xiàn)pDCs浸潤增加,并且pDCs在腎臟組織局部浸潤與狼瘡腎炎的活動呈正相關(guān),腎臟組織的pDCs浸潤與患者活動期腎炎腎臟組織高表達(dá)白細(xì)胞介素18有關(guān)[43]。
SLE是涉及多系統(tǒng)的自身免疫疾病,發(fā)病機制中參與自身免疫紊亂的細(xì)胞包括多種免疫細(xì)胞:DCs、mDCs、pDCs、T細(xì)胞、B細(xì)胞、巨噬細(xì)胞、Th細(xì)胞分化等,其中DC,尤其是pDCs與多種免疫細(xì)胞有關(guān)聯(lián),pDCs對T細(xì)胞、B細(xì)胞、巨噬細(xì)胞和DC有免疫調(diào)節(jié)或促進成熟的激活作用,可能是糾正免疫紊亂的關(guān)鍵點,而TLR-7、-9可能是調(diào)節(jié)pDCs的關(guān)鍵點。故抑制pDCs活性或減少pDCs的數(shù)量,可能是治療狼瘡性腎炎的治療靶點?,F(xiàn)有的臨床治療手段對pDCs的影響如何,是否干預(yù)pDCs是治療有效的關(guān)鍵,還有待于進一步的研究。
miRNA調(diào)節(jié)方面的研究為SLE傳統(tǒng)治療方案提供理論基礎(chǔ),可能為臨床實踐提供新思路,特異的miRNA可作為預(yù)測性檢測生物指標(biāo)。
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