小膠質(zhì)細(xì)胞在多發(fā)性硬化中的兩面性作用

吳鑒今

第二軍醫(yī)大學(xué)研究生管理大隊(duì)臨床一隊(duì) 上海 200433

1 MS

MS是一種以炎癥性脫髓鞘和神經(jīng)變性為主的CNS疾病。臨床癥狀主要包括視覺喪失、眼運(yùn)動障礙、痙攣、共濟(jì)失調(diào)和膀胱功能障礙等[1]。EAE是研究 MS的動物模型。MS/EAE的病理學(xué)特點(diǎn)是CNS白質(zhì)炎癥性多發(fā)性脫髓鞘病變伴軸突變性,導(dǎo)致慢性多發(fā)性硬化斑塊形成。該病確切發(fā)病機(jī)制未明,目前存在很多假說:如免疫啟始疾病假說(immune initiated disease hypothesis)、神經(jīng)起始疾病假說(neural initiated disease hypothesis)[2]、星形膠質(zhì)細(xì)胞免疫抑制假說(astrocytes immunosuppressive hypothesis)[3]等。主流的免疫啟始疾病假說認(rèn)為,MS/EAE是自身反應(yīng)性T細(xì)胞引起CNS炎癥級聯(lián)反應(yīng)導(dǎo)致的神經(jīng)組織損傷。Th0分化而來的Th1和Th7是自身反應(yīng)性T細(xì)胞的一種,可通過分泌干擾素-γ(IFN-γ)和腫瘤壞死因子-α(TNF-α)等細(xì)胞因子分別激活小膠質(zhì)細(xì)胞和炎癥反應(yīng)。所有假說中,小膠質(zhì)細(xì)胞在MS/EAE發(fā)病中都起到重要作用。

2 小膠質(zhì)細(xì)胞

小膠質(zhì)細(xì)胞是CNS主要免疫效應(yīng)細(xì)胞,盡管確切功能未完全闡明,但目前普遍認(rèn)為小膠質(zhì)細(xì)胞同時具有神經(jīng)毒性和神經(jīng)保護(hù)作用。Raivich等觀察到小膠質(zhì)細(xì)胞在體外激活后,同時在形態(tài)和表型上接近外周巨噬細(xì)胞[4]。活化的小膠質(zhì)細(xì)胞通過T LR識別并結(jié)合病原相關(guān)分子模式(pathogenassociated molecular patterns,PAMPs)啟動胞內(nèi)信號轉(zhuǎn)導(dǎo),促進(jìn)M HCⅠ、MHCⅡ、B7-1、B7-2和 CD40的表達(dá)以及TNF-α、IL-1、IL-6、IL-12、IL-18的分泌[5]。不同 PAMP對TLR的刺激作用會使小膠質(zhì)細(xì)胞分別向抗原提呈細(xì)胞(antigen presenting cell,APC)或巨噬細(xì)胞亞型分化[6]。APC亞型的小膠質(zhì)細(xì)胞可分泌TNF-α和IL-6等促炎細(xì)胞因子并上調(diào)細(xì)胞表面活性標(biāo)志物,而巨噬細(xì)胞亞型的小膠質(zhì)細(xì)胞可分泌IL-4和IL-10等抗炎細(xì)胞因子并表現(xiàn)出較高細(xì)胞吞噬能力。

3 小膠質(zhì)細(xì)胞產(chǎn)物在MS/EAE中的作用

活化的小膠質(zhì)細(xì)胞可產(chǎn)生多種效應(yīng)分子,這些產(chǎn)物在MS/EAE發(fā)病過程中起到重要神經(jīng)毒性或神經(jīng)保護(hù)作用。

3.1 IFN-γ與MS/EAE

3.1.1 IFN-γ在MS/EAE中的神經(jīng)毒性作用:IFN-γ通過正反饋回路(positive feedback loop)活化小膠質(zhì)細(xì)胞,導(dǎo)致CNS炎癥呈指數(shù)樣增加。活化的CD4+T細(xì)胞產(chǎn)生IFN-γ, IFN-γ隨即活化小膠質(zhì)細(xì)胞并使之對Th0行使抗原提呈作用。此外,活化的小膠質(zhì)細(xì)胞分泌IL-12、IL-18、IL-23和IL-27,促進(jìn)初始Th0向Th1分化。成熟的CD4+T細(xì)胞又產(chǎn)生IFN-γ活化小膠質(zhì)細(xì)胞。此外,IFN-γ還可刺激小膠質(zhì)細(xì)胞分泌大量細(xì)胞因子和趨化因子。

3.1.2 IFN-γ在MS/EAE中的神經(jīng)保護(hù)作用:近年來Takeuchi等發(fā)現(xiàn)IFN-γ可通過負(fù)反饋回路(negative feedback loop)單獨(dú)促進(jìn)小膠質(zhì)細(xì)胞凋亡,從而發(fā)揮CNS抗炎作用。他們發(fā)現(xiàn)IFN-γ可促使小膠質(zhì)細(xì)胞發(fā)生活化誘導(dǎo)的細(xì)胞死亡(activation-induced cell death),這與促凋亡蛋白(proapoptosis proteins)的上調(diào)表達(dá)有關(guān)。他們認(rèn)為凋亡的根本原因是促凋亡蛋白表達(dá)量高于抑制凋亡蛋白(anti-apoptosis proteins)[7]。

由此可見,IFN-γ在脫髓鞘病變中既發(fā)揮促炎作用,也具有抗炎作用,這可能與IFN-γ的劑量有關(guān)。而IFN-γ正、負(fù)反饋回路理論也可用來解釋MS病情反復(fù)惡化—緩和的發(fā)病機(jī)制。

3.2 TNF-α與MS/EAE

3.2.1 TNF-α在MS/EAE中的神經(jīng)毒性作用:TNF-α在CNS的促炎作用是在將EAE過繼轉(zhuǎn)移至缺乏TNF受體-1 (TNF receptor-1,TNFR-1)的小鼠研究中發(fā)現(xiàn)的。Gimenez等發(fā)現(xiàn),盡管在TNFR-1足量和缺乏的小鼠中均有等量致腦炎T細(xì)胞(encephalitogenic T cells),但在TNFR-1缺乏的小鼠中致腦炎 T細(xì)胞進(jìn)入CNS受阻。從而證明 TNF-α與TNFR-1結(jié)合有利于T細(xì)胞透過血腦屏障,在中樞神經(jīng)系統(tǒng)炎癥反應(yīng)中起促進(jìn)作用[8]。TNF-α還可促進(jìn)小膠質(zhì)細(xì)胞釋放大量谷氨酸,通過谷氨酸的興奮性毒性作用損傷CNS神經(jīng)細(xì)胞[9]。少突膠質(zhì)細(xì)胞對谷氨酸的攝取作用被認(rèn)為在炎癥反應(yīng)中起到神經(jīng)保護(hù)作用,Pitt等發(fā)現(xiàn),TNF-α在體外會下調(diào)少突膠質(zhì)細(xì)胞上的谷氨酸轉(zhuǎn)運(yùn)子,從而抑制神經(jīng)保護(hù)作用[10]。可見,TNF-α主要通過促炎作用和誘導(dǎo)谷氨酸的興奮性毒性作用對損害CNS神經(jīng)細(xì)胞。

3.2.2 TNF-α在MS/EAE中的神經(jīng)保護(hù)作用:研究顯示, TNF-α缺乏的小鼠髓鞘再生作用明顯放緩。受損神經(jīng)細(xì)胞修復(fù)上的障礙,與少突膠質(zhì)前體細(xì)胞減少引起的成熟少突膠質(zhì)細(xì)胞減少有關(guān)。Arnett等通過對TNFR-1缺乏或TNFR-2缺乏的小鼠進(jìn)行研究,證明TNF-α通過與TNFR-2受體結(jié)合促進(jìn)少突膠質(zhì)前體細(xì)胞的聚集和增殖,從而發(fā)揮神經(jīng)保護(hù)作用。該神經(jīng)保護(hù)作用很可能與鉀電流降低和抗凋亡磷酸肌醇3-激酶/Akt激酶通路(anti-apoptotic phosphoinositide 3-kinase/Akt kinase pathway)激活有關(guān)[11]。他們還發(fā)現(xiàn), MHC-Ⅱ阻斷的小鼠表現(xiàn)出髓鞘再生明顯放慢,而MHC-Ⅰ阻斷的小鼠則無變化。這顯示由T NF-α誘導(dǎo)的MHC-Ⅱ在髓鞘再生中起著積極作用[12]。

3.3 白介素(interleukin,IL)與MS/EAE

3.3.1 IL-6、IL-1β、IL-23、IL-12和IL-17在MS/EAE中的神經(jīng)毒性作用:通過分泌IL-6、IL-1β、IL-23、IL-12等細(xì)胞因子,小膠質(zhì)細(xì)胞可促進(jìn)Th0向Th1或Th17分化,而Th1和Th17均參與MS/EAE發(fā)病。Kroenke等將Th1或Th17過繼轉(zhuǎn)移至小鼠后,均引起臨床評分相似的嚴(yán)重EAE[13]。IL-12可促進(jìn)Th0向Th1分化,而IL-6、IL-1β和IL-23在Th0向Th17分化過程中起到關(guān)鍵作用。主要由 Th1分泌的IFN-γ,還可刺激小膠質(zhì)細(xì)胞分泌IL-23。而IL-23對EAE中Th17細(xì)胞增殖和炎癥作用有重要促進(jìn)作用[14]。

IL-17是MS/EAE發(fā)病中又一重要促炎細(xì)胞因子。由小膠質(zhì)細(xì)胞自分泌形式產(chǎn)生的IL-17[15],與其包膜上表達(dá)的IL-17R結(jié)合,可促進(jìn)該細(xì)胞分泌單核細(xì)胞炎癥蛋白-2(monocyte inflammatory protein-2,MIP-2)。活化的小膠質(zhì)細(xì)胞也可表達(dá)IL-23受體(IL-23R),IFN-γ可劑量依賴性協(xié)同IL-23誘導(dǎo)小膠質(zhì)細(xì)胞分泌巨噬細(xì)胞趨化蛋白-1(macrophage chemotactic protein-1,MCP-1)、RANTES和干擾素誘導(dǎo)蛋白-10(IP-10)等趨化因子[16]。以上研究結(jié)果表明,IL-17和IL-23通過結(jié)合小膠質(zhì)細(xì)胞上相應(yīng)受體,可促進(jìn)小膠質(zhì)細(xì)胞分泌趨化因子進(jìn)而增強(qiáng)MS/EAE中炎癥反應(yīng)。

3.3.2 IL-4在MS/EAE中的神經(jīng)保護(hù)作用:Butovsky等發(fā)現(xiàn)抗炎細(xì)胞因子IL-4,對高劑量IFN-γ產(chǎn)生的神經(jīng)破壞性起拮抗作用。在體外,IL-4可對抗由高濃度IFN-γ引起的小膠質(zhì)細(xì)胞對少突膠質(zhì)細(xì)胞形成的抑制作用。在體內(nèi),注射IL-4的實(shí)驗(yàn)組動物較對照組少突膠質(zhì)細(xì)胞形成明顯增強(qiáng),神經(jīng)細(xì)胞損傷明顯減少。這與IL-4能促進(jìn)小膠質(zhì)細(xì)胞分泌低量IFN-γ和下調(diào)TNF-α表達(dá)有關(guān)[17]。

3.4 前列腺素類化合物與M S/EAE 前列腺素類化合物通過環(huán)氧化酶(cyclooxygenase,COX)通路合成,它們中的一部分具有神經(jīng)毒性作用,可導(dǎo)致脫髓鞘病變;另一部分因其抗炎活性而具有神經(jīng)保護(hù)作用。

3.4.1 COX-2通路合成的前列腺素類化合物在MS/EAE中的神經(jīng)毒性作用:小膠質(zhì)細(xì)胞在EAE的發(fā)病中與COX-2的免疫反應(yīng)性有關(guān)[18]。Mirjany等發(fā)現(xiàn),COX-2可啟動PGD2、PGE2、PGF2α和血栓素A2(thromboxane A2,TxA2)等多種前列腺素(prostaglandins,PGs)的合成。該P(yáng)Gs直接作用于神經(jīng)元后,可增加神經(jīng)元對谷氨酸的敏感性,從而提高谷氨酸的興奮性毒性作用。他們還發(fā)現(xiàn),這些PGs還可抑制星形膠質(zhì)細(xì)胞攝取胞外基質(zhì)中的谷氨酸并促進(jìn)谷氨酸外排[19]。

3.4.2 環(huán)戊烯酮前列腺素(cyclopentenone PGs,cPGs)和PGE2在MS/EAE中的神經(jīng)保護(hù)作用:cPGs主要包括PGA1,PGA2和PGJ2,它們通過抑制編碼促炎因子TNF-α、iNOS、COX-2基因的激活發(fā)揮CNS抗炎作用。Storer等發(fā)現(xiàn),小鼠小膠質(zhì)細(xì)胞接受促炎因子LPS或IFN-γ和TNF-α的共同刺激后,PGA2和d15-PGJ2仍可抑制NO的產(chǎn)生[20]。進(jìn)一步研究發(fā)現(xiàn),這兩種cPGs還可抑制TNF-α、IL-1β、IL-6等促炎細(xì)胞因子和MCP-1等促炎趨化因子的表達(dá)。

PGE2具有良好的抗炎神經(jīng)保護(hù)作用。它不但可抑制巨噬細(xì)胞功能、下調(diào)Th1細(xì)胞應(yīng)答,還可抑制小膠質(zhì)細(xì)胞產(chǎn)生促炎細(xì)胞因子和NO,并可抑制小膠質(zhì)細(xì)胞表面表達(dá)M HC-Ⅱ[21]。Belmira等發(fā)現(xiàn),EP2作為PGE2的一種亞型,可通過與EP2受體在布他前列素活化作用下發(fā)生的明顯結(jié)合作用,抑制神經(jīng)毒性基因表達(dá)[22]。

4 小膠質(zhì)細(xì)胞在MS/EAE中的神經(jīng)保護(hù)作用

如前文所述,盡管小膠質(zhì)細(xì)胞可通過分泌多種產(chǎn)物在MS/EAE中引起脫髓鞘、軸突變性等神經(jīng)毒性作用。但其亦從以下三方面在MS/EAE中表現(xiàn)出神經(jīng)保護(hù)作用:(1)產(chǎn)生神經(jīng)保護(hù)性產(chǎn)物。(2)促進(jìn)對髓磷脂碎片的清除作用。(3)招募干細(xì)胞和促進(jìn)神經(jīng)形成作用。

圖1 小膠質(zhì)細(xì)胞在MS/EAE中的神經(jīng)保護(hù)作用

4.1 小膠質(zhì)細(xì)胞產(chǎn)生神經(jīng)保護(hù)性產(chǎn)物 前文已述,小膠質(zhì)細(xì)胞產(chǎn)生的IFN-γ、IL-4、cPGs、PGE2等因子具有神經(jīng)保護(hù)作用,故不再贅述。

4.2 小膠質(zhì)細(xì)胞促進(jìn)對髓磷脂碎片的清除作用 MS/EAE的病變部位可聚集大量變性神經(jīng)細(xì)胞產(chǎn)生的髓磷脂碎片,這些碎片中包含的Nogo A蛋白對軸突再生具有抑制作用[23]。髓樣細(xì)胞受體-2(myeloidcells-2,TREM2)在介導(dǎo)小膠質(zhì)細(xì)胞對髓磷脂碎片的清除上發(fā)揮重要作用。Piccio等發(fā)現(xiàn)TREM2在發(fā)生EAE小鼠的小膠質(zhì)細(xì)胞上高表達(dá),并且通過抗體介導(dǎo)的方法阻斷TREM2后小鼠EAE明顯加重[24]。由于未找到TREM2的配體,故作用機(jī)制仍然未明。Takahashi等發(fā)現(xiàn),通過靜脈內(nèi)移植髓樣前體細(xì)胞可以使小膠質(zhì)細(xì)胞表達(dá)TREM2程度提高,從而促進(jìn)髓磷脂碎片的清除和軸突再生[25]。

4.3 小膠質(zhì)細(xì)胞招募干細(xì)胞和促進(jìn)神經(jīng)形成作用 成人神經(jīng)干細(xì)胞(neural stem cells)和神經(jīng)祖細(xì)胞(neural progenitor cells)平時處于相對靜止?fàn)顟B(tài),具有自我更新和分化為神經(jīng)元或膠質(zhì)細(xì)胞的潛力。Thored等通過對卒中動物模型的研究,發(fā)現(xiàn)卒中后大量表達(dá)胰島素樣生長因子-1(insulin-like growth factor-1,IGF-1)的小膠質(zhì)細(xì)胞具有減緩神經(jīng)細(xì)胞凋亡,促進(jìn)神經(jīng)干細(xì)胞增生、遷移和分化的能力[26]。此外,Choi的研究結(jié)果表明,小膠質(zhì)細(xì)胞可通過表達(dá)早老素-1(presenilin 1,PS1)促進(jìn)神經(jīng)元前體細(xì)胞(neural precursor cells, NPCs)在海馬區(qū)的神經(jīng)形成、富集誘導(dǎo)增殖(enrichment-induced proliferation)和神經(jīng)譜系定型(neuronal lineage commitment)[27]。該研究結(jié)果顯示小膠質(zhì)細(xì)胞在調(diào)節(jié)NPC功能上也發(fā)揮重要作用。

5 總結(jié)和展望

盡管小膠質(zhì)細(xì)胞在MS/EAE中的具體作用機(jī)制尚未明,但目前基本認(rèn)為小膠質(zhì)細(xì)胞可通過產(chǎn)生促炎產(chǎn)物發(fā)揮神經(jīng)毒性作用;亦可通過產(chǎn)生抗炎產(chǎn)物、促進(jìn)對髓磷脂碎片的清除以及招募干細(xì)胞和促進(jìn)神經(jīng)形成三方面發(fā)揮神經(jīng)保護(hù)作用。本文較其他相關(guān)文章的創(chuàng)新之處在于辯證地介紹了小膠質(zhì)細(xì)胞在小膠質(zhì)細(xì)胞的兩面性作用。

進(jìn)一步研究小膠質(zhì)細(xì)胞在MS/EAE中的作用機(jī)制不但有助于徹底了解MS/EAE的發(fā)病機(jī)制,更有助于進(jìn)MS新藥的開發(fā)。未來理想MS藥物應(yīng)在抑制小膠質(zhì)細(xì)胞促炎作用的同時,促進(jìn)其抗炎、神經(jīng)營養(yǎng)等神經(jīng)保護(hù)功能。相信隨著研究的不斷深入,對MS的治療終會得到滿意的進(jìn)步。

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