IL-21在多發(fā)性硬化發(fā)病中的意義①

汪鴻浩 胡學(xué)強(qiáng) (中山大學(xué)附屬第三醫(yī)院神經(jīng)科,廣州 510630)

多發(fā)性硬化(Multiple sclerosis,MS)是一種中樞神經(jīng)系統(tǒng)(Central nervous system,CNS)脫髓鞘性疾病,其真正病因至今尚未明確。國內(nèi)外有大量文獻(xiàn)證實(shí)其與細(xì)胞免疫、體液免疫均有關(guān)聯(lián)。自從2000年白細(xì)胞介素21(IL-21)和IL-21受體(IL-21R)被發(fā)現(xiàn)以來[1],一直是研究的熱點(diǎn),近年來又有很多新的作用被發(fā)現(xiàn)。IL-21對CD8+T細(xì)胞、CD4+Th細(xì)胞、調(diào)節(jié)性T細(xì)胞(regulatory T lymphocyte,Treg)均有作用,因此深入研究IL-21在MS發(fā)病中的意義具有重要的價(jià)值。

1 IL-21及IL-21R的生物學(xué)特點(diǎn)

人IL-21基因定位于4q26-q27[2],共642個(gè)核苷酸,其中自42位至535位核苷酸為編碼區(qū)。鼠定位于3號染色體上,兩者有57%的同源性[1]。IL-21是一個(gè)四螺旋簇Ⅰ型細(xì)胞因子,其結(jié)構(gòu)與γ c家族其他成員如 IL-2、IL-4、IL-15高度同源[3]。在體內(nèi)主要是活化的CD4+T細(xì)胞分泌IL-21[1]。IL-21R是一個(gè)由獨(dú)特的IL-21R亞單位和γ c組成的異二聚體,其中IL-21R亞單位為配體識(shí)別結(jié)合部位,γ c為信號傳導(dǎo)單位[4],與IL-2/IL-15Rβ鏈有高度同源性,由胞外和胞內(nèi)兩個(gè)部分組成,胞外區(qū)有細(xì)胞因子信號識(shí)別模塊,胞內(nèi)區(qū)有典型的Box1、Box2信號傳導(dǎo)區(qū)。IL-21分子信號傳遞途徑尚未完全清楚,但 JAK/STAT、MAPK和P13這三個(gè)途徑研究相對較多。IL-21可以通過激活STAT1和STAT3途徑,激活T細(xì)胞、B細(xì)胞和NK細(xì)胞,對固有免疫階段發(fā)揮相應(yīng)生物調(diào)節(jié)效應(yīng)[5];當(dāng)MAPK和PI3通路被特異抑制因子阻斷后IL-21介導(dǎo)的免疫細(xì)胞增殖即被阻斷[6]。另外IL-21R有STAT5錨定位點(diǎn),可激活STAT5途徑,作用于B細(xì)胞[4]。進(jìn)一步研究發(fā)現(xiàn),IL-21R胞質(zhì)區(qū)酪氨酸510(Y510)可以有效的介導(dǎo)STAT1和STAT3磷酸化,從而激活CD4+T細(xì)胞分化,而不依賴STAT5途徑。

2 IL-21在多發(fā)性硬化發(fā)病中的作用

2.1 多發(fā)性硬化發(fā)病中的免疫學(xué)機(jī)制 多發(fā)性硬化的發(fā)病機(jī)制中涉及體液免疫和細(xì)胞免疫。以前認(rèn)為,MS的發(fā)病和Th1/Th2分化的偏移有關(guān)。一般認(rèn)為MS患者CD4+Th1細(xì)胞分泌的TNF-α和IFN-γ是導(dǎo)致MS發(fā)病的重要細(xì)胞因子,另外CD4+Th2可以下調(diào)MS的自身免疫反應(yīng)[7]。近年來通過對MS患者腦組織免疫組化研究發(fā)現(xiàn),急性病灶內(nèi)浸潤的T細(xì)胞表達(dá)IL-17及IL-17mRNA均顯著增多,高于靜止病灶、慢性病灶及正常組織[8],表明MS的發(fā)病和IL-17相關(guān),而IL-17主要由活化的 Th17產(chǎn)生,故Th17與MS的病理機(jī)制之間的關(guān)系引起了關(guān)注。動(dòng)物實(shí)驗(yàn)也證明人體來源的Th17細(xì)胞能破壞小鼠血腦屏障,在體外對人類神經(jīng)元細(xì)胞有強(qiáng)烈的細(xì)胞毒性作用[9]。另外CD4+CD25+Foxp3+Treg功能低下是MS發(fā)病的另一重要原因。MS患者體內(nèi)CD4+CD25+CD45RA+初始 Treg比例減少,CD4+CD25+CD45RO+記憶性Treg比例相對增高,表達(dá)轉(zhuǎn)錄因子Foxp3減少,抑制T細(xì)胞活化能力減弱[10-13]。早在上世紀(jì)就發(fā)現(xiàn)了MS患者IgG1、IgG3、IgG4的鞘內(nèi)合成[14],通過腦脊液(Cerebrospinal fluid,CSF)蛋白電泳可見IgG寡克隆帶(Oligoclonal bands,OCB)。IgG抗原識(shí)別呈多樣性,有的識(shí)別髓鞘蛋白成份[15];有的識(shí)別病毒如EB病毒[16],據(jù)此認(rèn)為B細(xì)胞免疫在MS發(fā)病中有重要作用。

2.2 IL-21和Th17 IL-21是可由活化的Th17分泌,而且相對其他的CD4+T細(xì)胞亞群,Th17更加有選擇的通過STAT3途徑產(chǎn)生IL-21,轉(zhuǎn)錄因子STAT3直接和Th17上IL-21基因的啟動(dòng)子結(jié)合,調(diào)控IL-21基因的表達(dá)[17]。盡管由抗原呈遞細(xì)胞分泌的IL-6、TGF-β可以通過STAT3促進(jìn)Th17的分化[18],但是抗體封閉實(shí)驗(yàn)證明Th17細(xì)胞分泌的IL-21也是Th17分化的重要途徑,甚至在IL-6缺乏的動(dòng)物模型上IL-21仍可誘導(dǎo)Th17的分化[19]。IL-21聯(lián)合IL-6通過STAT3和孤核受體(orphan nuclear receptor,RORγ t)作用促進(jìn)Th17的分化,并能增加IL-23R的表達(dá),IL-23可以促進(jìn)Th17的分化,并能維持Th17的存活[20,21]。Th17細(xì)胞分泌的IL-21的功能有重要的生物學(xué)意義,因?yàn)橛?IL-6聯(lián)合TGF-β促進(jìn)分化的 Th17其IL-21/IL21R表達(dá)下降[19,22]。在用少突膠質(zhì)細(xì)胞髓鞘磷脂糖蛋白(Myelin oligodendrocyte glycoprotein,MOG)35-55進(jìn)行實(shí)驗(yàn)變態(tài)反應(yīng)性腦脊髓炎(Experimental autoimmune encephalomyelitis,EAE)模型造模之前給予IL-21注射會(huì)加重神經(jīng)系統(tǒng)損傷,而IL-21/IL-21R基因敲除的小鼠在同樣進(jìn)行造型后分泌IL-17的細(xì)胞減少,神經(jīng)系統(tǒng)損傷較輕[19]。

2.3 IL-21和Treg IL-21并不直接作用于Treg,但是可以干擾CD4+CD25+Treg對CD4+CD25-T細(xì)胞的免疫抑制作用[23],也有研究認(rèn)為IL-21與TGF-β聯(lián)合、IL-21和IL-6聯(lián)合可抑制Foxp3的表達(dá),減少CD4+初始T細(xì)胞分化形成Treg[19]。IL-21抑制Treg的免疫調(diào)節(jié)作用具體是通過什么信號傳導(dǎo)途徑來實(shí)現(xiàn)目前還不清楚,但是無論通過什么機(jī)制減少IL-21的產(chǎn)生,增強(qiáng)Treg免疫調(diào)節(jié)功能作用,可能是一種潛在的治療自身免疫性疾病的方法[24]。雖然理論上IL-21的水平升高可以抑制CD4+CD25+Foxp3+Treg的免疫調(diào)節(jié)功能,促進(jìn)自身免疫反應(yīng),但是Piao等[25]在EAE模型鼠試驗(yàn)中卻得出了相反的結(jié)果:封閉IL-21R卻導(dǎo)致CD4+CD25+Treg的減少,他認(rèn)為試驗(yàn)結(jié)果不同可能和EAE模型鼠的遺傳背景的差異有關(guān)。

2.4 IL-21對B細(xì)胞分化的影響 IL-21對B細(xì)胞的增殖分化起重要的作用,IL-21有促進(jìn)記憶性B細(xì)胞分化為CD138+漿細(xì)胞并促進(jìn)漿細(xì)胞分泌抗體[26],該作用主要通過影響 Bcl-6、Blimp-1(B-lymphocyte-induced maturation protein-1),Pax-5的表達(dá)實(shí)現(xiàn)[27]。IL-21可以上調(diào)Blimp-1、下調(diào)Pax-5的表達(dá)使記憶性B細(xì)胞分化為漿細(xì)胞,而上調(diào)Bcl-6的作用可以使生發(fā)中心的B細(xì)胞分化為同型記憶性B細(xì)胞[28-30]。在缺乏IL-21R的小鼠中,免疫刺激后IgG1和IgG2b、IgG3的含量較免疫刺激后的正常小鼠血清含量減少,但是IgG2a、IgM的水平幾乎未受影響。也有實(shí)驗(yàn)證明IL-21聯(lián)合CD40mb對刺激B細(xì)胞分泌抗體的作用是IL-10的100倍[31,32]。造模前給予IL-21注射相對于PBS注射的小鼠行EAE造模后外周血IgG含量明顯升高,而小鼠體內(nèi)受Th2產(chǎn)生的細(xì)胞因子如:IL-4調(diào)控的IgG2a卻未見明顯變化,并且B細(xì)胞缺乏的小鼠EAE造模成功后給予IL-21則對肢體殘疾程度未見明顯影響[33]。

2.5 IL-21和IFN-γ NK細(xì)胞在MS和EAE模型發(fā)病中的意義目前不肯定,但是研究證實(shí)NK細(xì)胞可以通過殺傷靶細(xì)胞、產(chǎn)生一系列的細(xì)胞因子在自身免疫性疾病的病理機(jī)制中發(fā)揮重要的作用[34]。將小鼠NK細(xì)胞轉(zhuǎn)移至IL-18基因敲除的NK細(xì)胞相對缺乏的小鼠體內(nèi),這些小鼠保持了對自身免疫性腦脊髓炎的易感性,而將IFN-γ缺乏的NK細(xì)胞轉(zhuǎn)移至同種小鼠體內(nèi)卻沒有保持這種特性[35]。NK細(xì)胞在MS發(fā)病中的作用和該細(xì)胞分泌的IFN-γ有關(guān),IFN-γ使初始T細(xì)胞向Th1分化,促進(jìn)EAE的發(fā)病[36,37]。在EAE模型上證實(shí)了在造模過程中注射IL-21可以提高NK細(xì)胞分泌IFN-γ的水平,促進(jìn)Th1的分化[35]。

3 展望

IL-21是重要的免疫平衡調(diào)節(jié)因子,對Th17、Treg、B細(xì)胞、NK細(xì)胞的分化和功能均有影響,在自身免疫性疾病的發(fā)病中有重要意義。以往MS的免疫學(xué)研究很大部分將細(xì)胞免疫和體液免疫分開研究,未找到兩者之間的聯(lián)系。隨著IL-21研究的深入,越來越多的證據(jù)表明,Th17細(xì)胞在MS病灶處的增多并分泌IL-21等細(xì)胞因子,一方面促進(jìn)CD4+T細(xì)胞的分化,另一方面促進(jìn)B細(xì)胞的分化可能是MS病理機(jī)制的一個(gè)重要因素,因此IL-21在MS發(fā)病中的作用值得去深入研究。但是IL-21在MS和EAE模型的發(fā)病機(jī)制中仍有很多問題懸而未決,如:IL-21可以誘導(dǎo)Th1免疫反應(yīng)相關(guān)基因IFN-γ、T-bet(T box expressed in T cells)、IL-2Rα、IL-12Rβ2 、IL-18R 、髓樣分化因子(myeloid differentiation factor88,MyD88)的表達(dá)[38],但是Wurster等[39]卻得出了相反的結(jié)論認(rèn)為IL-21主要是促進(jìn)Th2反應(yīng)的細(xì)胞因子,并抑制IFN-γ的分泌;另外,近年來一種新的CD4+細(xì)胞亞群被發(fā)現(xiàn),其表型為CD4+CXCR5+CD40L+ICOS+T細(xì)胞,稱之為濾泡輔助性T細(xì)胞(T follicular helper cells,Tfh),和Th17一樣其分化依賴IL-6、IL-21,而且分泌IL-21[40],該細(xì)胞亞群在MS發(fā)病中的作用仍有待進(jìn)一步探尋。

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