胡 琦 綜述，廖 偉 審校

(第三軍醫(yī)大學(xué)西南醫(yī)院兒科，重慶 400038)

·綜 述·

IDO、Treg/Th17分化與中性粒細(xì)胞性哮喘*

胡 琦 綜述，廖 偉△審校

(第三軍醫(yī)大學(xué)西南醫(yī)院兒科，重慶 400038)

哮喘；中性粒細(xì)胞；調(diào)節(jié)性T細(xì)胞；限速酶

支氣管性哮喘是一種氣道慢性炎癥性疾病，具有明顯的異質(zhì)性。按哮喘氣道炎癥類型的不同，Simpson等[1]把哮喘分為嗜酸性粒細(xì)胞性哮喘、中性粒細(xì)胞性哮喘、混合細(xì)胞(嗜酸性粒細(xì)胞和中性粒細(xì)胞)性哮喘及寡細(xì)胞性哮喘。皮質(zhì)激素聯(lián)合支氣管擴(kuò)張劑吸入是目前支氣管哮喘的主要治療手段，但在臨床治療中發(fā)現(xiàn)部分重癥或難治性哮喘出現(xiàn)激素抵抗，單用激素治療病情難以控制[2]。研究也發(fā)現(xiàn)，這些患者氣道內(nèi)有更多的中性粒細(xì)胞浸潤、更明顯的組織損傷及氣道重塑[3]。Moore等[4]進(jìn)一步發(fā)現(xiàn)按氣道炎癥表型分類的中性粒細(xì)胞性哮喘在臨床上往往是重癥哮喘或難治性哮喘。因此明確中性粒細(xì)胞性哮喘的發(fā)病機(jī)制，對于重癥及難治性哮喘的防治有重要意義。

過去認(rèn)為Th1/Th2細(xì)胞失衡，尤其是Th2細(xì)胞的活化是哮喘發(fā)病的免疫機(jī)制中的關(guān)鍵環(huán)節(jié)，調(diào)節(jié)性T細(xì)胞(regulatory T cell，Treg)具有抑制Th2細(xì)胞活性的功能，在哮喘的防治中占有重要地位[5]。近年研究證實(shí)Th17細(xì)胞類細(xì)胞因子在重癥或難治性哮喘發(fā)病中有重要作用，尤其與中性粒細(xì)胞性哮喘發(fā)病密切相關(guān)[6-8]。而色氨酸分解代謝的限速酶吲哚胺2,3雙加氧酶(indoleamine 2,3 dioxygenase，IDO)， 在一定條件下是Th17細(xì)胞向Treg轉(zhuǎn)換的分子“開關(guān)”[9-10]。因此IDO介導(dǎo)的色氨酸代謝可能參與Treg/Th17細(xì)胞分化。本文就IDO、Treg/Th17分化與中性粒細(xì)胞性哮喘可能關(guān)系綜述如下。

1 Treg、Th17細(xì)胞與中性粒細(xì)胞性哮喘

多種免疫細(xì)胞(Th1、Th2、Th17及Treg細(xì)胞)和細(xì)胞因子參與了哮喘發(fā)病。目前認(rèn)為，Th2細(xì)胞與嗜酸性粒細(xì)胞性哮喘密切相關(guān)，而Th17細(xì)胞在中性粒細(xì)胞性哮喘發(fā)病的免疫機(jī)制中起著重要作用[11-13]。Treg可抑制Th2及Th17細(xì)胞活化，F(xiàn)oxp3+是其主要的調(diào)控基因，CD4+CD25+Treg在外周免疫耐受中起著關(guān)鍵作用。變應(yīng)原特異性免疫治療(SIT)是目前唯一有效的針對哮喘病因的預(yù)防治療，SIT能有效防治哮喘也是由于變應(yīng)原的反復(fù)刺激誘導(dǎo)了體內(nèi)可誘導(dǎo)Treg(inducible Treg，iTreg)產(chǎn)生。目前較多文獻(xiàn)證實(shí)哮喘發(fā)生的免疫機(jī)制正是由于Treg功能受到抑制、Th2細(xì)胞活化導(dǎo)致機(jī)體產(chǎn)生嗜酸性粒細(xì)胞炎癥，以及Th17細(xì)胞活化導(dǎo)致機(jī)體產(chǎn)生中性粒細(xì)胞性炎癥[12,14-15]。因此Treg誘導(dǎo)氣道免疫耐受在哮喘防治中占有重要地位[16-17]，其作用機(jī)制可能與表達(dá)細(xì)胞毒性T淋巴細(xì)胞相關(guān)抗原-4(CTLA-4)分泌IL-10、轉(zhuǎn)化生長因了β(TGF-β)等細(xì)胞因子有關(guān)[18-19]。

中性粒細(xì)胞性哮喘是按哮喘氣道炎癥表型分類的一種哮喘，目前沒有統(tǒng)一定義，加拿大Nair等[20]將誘導(dǎo)痰中中性粒細(xì)胞計數(shù)持續(xù)(至少有2次)大于細(xì)胞總數(shù)的65%，或絕對計數(shù)大于5.0×106/mL的哮喘定義為中性粒細(xì)胞哮喘。這種類型哮喘臨床上往往多是重癥或激素抵抗性哮喘，并且氣道炎癥細(xì)胞以中性粒細(xì)胞為主，Th17細(xì)胞比例遠(yuǎn)遠(yuǎn)高于Th2細(xì)胞[21]。Th17細(xì)胞是2005年發(fā)現(xiàn)的CD4+T 細(xì)胞亞群，以分泌IL-17為特征，獨(dú)核受體-γt(ROR-γt)是其重要轉(zhuǎn)錄因子[22]。Nakagome等[23]在卵蛋白致敏小鼠哮喘動物模型中證實(shí)，反復(fù)給予抗IL-17抗體可以顯著減少骨髓外周血和肺泡灌洗液(BALF) 的中性粒細(xì)胞數(shù)量。除動物模型實(shí)驗(yàn)外，Al-Ramli等[24]發(fā)現(xiàn)在哮喘患者肺組織中IL-17大量表達(dá)，并且表達(dá)量與哮喘嚴(yán)重程度有關(guān)，在那些激素抵抗性哮喘患者這一現(xiàn)象更為明顯。Kerzel等[25]在兒童重癥哮喘患者的肺泡灌洗液中檢測到大量IL-17。IL-17可激活上皮細(xì)胞分泌CXCL8蛋白趨化募集中性粒細(xì)胞形成中性粒細(xì)胞炎癥。因此Th17 細(xì)胞介導(dǎo)的免疫反應(yīng)導(dǎo)致氣道內(nèi)中性粒細(xì)胞的增多和募集，在中性粒細(xì)胞哮喘形成占有重要地位[13,26]。此外，IL-17還參與了其他的哮喘發(fā)病機(jī)制，包括上皮細(xì)胞的結(jié)構(gòu)改變、黏液分泌增加、氣道高反應(yīng)性及平滑肌的收縮等[27-30]。因此，通過抑制Th17細(xì)胞活化來防治中性粒細(xì)胞哮喘，引起了研究者們極大的興趣。但遺憾的是，Busse等[31]在300多例中度哮喘患者中用3種劑量的抗IL-17A的單克隆抗體藥物(brodalumab)皮下注射，在為期10周的療程中并未發(fā)現(xiàn)該藥對哮喘治療有效，與安慰劑治療組相比較無顯著差異。但是將IL-17A的單克隆抗體藥物用于中性粒細(xì)胞性哮喘的研究，目前尚無研究涉及。

2 IDO與Treg/Th17分化

肺部樹突狀細(xì)胞(dendritic cells，DC)是氣道最有效的抗原呈遞細(xì)胞。機(jī)體初次接觸過敏原，樹突狀細(xì)胞攝取過敏原后被激和，提呈抗原，刺激初始型(Na?ve )CD4+T淋巴細(xì)胞增殖，產(chǎn)生特異記憶性CD4+T細(xì)胞；當(dāng)機(jī)體再次接觸該過敏原，特異記憶性CD4+T淋巴細(xì)胞分化為效應(yīng)性T細(xì)胞，并刺激其他細(xì)胞釋放炎性細(xì)胞因子，始動了哮喘的發(fā)生。健康人肺部在反復(fù)吸入過敏原后，體內(nèi)由于存在某種免疫耐受機(jī)制，CD4+T淋巴細(xì)胞分化成Treg，避免了哮喘發(fā)生。

目前研究證實(shí)中性粒細(xì)胞性哮喘發(fā)生時，氣道CD4+T細(xì)胞通過CD40配基與氣道DC上的共刺激分子CD40結(jié)合，并在一定的細(xì)胞因子(IL-1β、TGF-β及IL-6)微環(huán)境下，上調(diào)ROR-γt轉(zhuǎn)錄基因，使CD4+T細(xì)胞向Th17細(xì)胞分化，產(chǎn)生IL-17，募集中性粒細(xì)胞至氣道，并釋放IL-23等炎性細(xì)胞因子，始動哮喘的發(fā)生[7,32-33]。因此中性粒細(xì)胞哮喘的發(fā)生是由于CD4+T細(xì)胞向Treg分化減弱，而向Th17細(xì)胞分化增強(qiáng)，即Th17/Treg分化失衡，如果能逆轉(zhuǎn)在中性粒細(xì)胞哮喘中的Th17/Treg分化失衡，有望對其有治療作用。然而，哮喘發(fā)病機(jī)制極為復(fù)雜，CD4+T淋巴細(xì)胞向何種細(xì)胞分化，除了與呈遞的抗原種類、數(shù)量及細(xì)胞因子微環(huán)境外，還可能與呈遞抗原的DC亞群有關(guān)。Vroman等[7]研究證實(shí)氣道CD4+T細(xì)胞向效應(yīng)性T細(xì)胞或Treg分化取決于提呈抗原的不同DC亞群，已證實(shí)氣道內(nèi)類漿細(xì)胞樣DC(plasmacytoid DC，pDC)亞群可誘導(dǎo)Treg產(chǎn)生，從而抑制Th1、Th2和Th17細(xì)胞的活化，形成免疫耐受；髓樣DC(myeloid DC，mDC) 亞群可誘導(dǎo)效應(yīng)性T細(xì)胞形成，產(chǎn)生炎癥反應(yīng)；但是氣道pDC誘導(dǎo)Treg分化的機(jī)理還不完全清楚。有趣的是，小鼠脾來源CD8α+DC及人外周血單核細(xì)胞來源的CD123+CCR6+DC 2種DC亞群能大量分泌IDO，通過抑制T細(xì)胞增殖及介導(dǎo)T細(xì)胞凋亡，誘導(dǎo)Treg產(chǎn)生，在外周誘導(dǎo)和維持T細(xì)胞耐受[34]。

IDO是色氨酸分解代謝的限速酶，在外周免疫耐受中有重要作用，其在妊娠、器官移植及腫瘤免疫耐受的作用已成為近年研究熱點(diǎn)，目前在對肺部真菌感染發(fā)現(xiàn)，IDO及其代謝產(chǎn)物犬尿酸可以促進(jìn)叉頭狀螺旋轉(zhuǎn)錄調(diào)節(jié)因子(FOXP3+)基因轉(zhuǎn)錄誘導(dǎo)Treg生成，而同時抑制RORγt 基因而抑制Th17細(xì)胞產(chǎn)生，從而導(dǎo)致Th17/Treg分化失衡而使氣道對真菌發(fā)生耐受使氣道炎癥遷延不愈[35]。Bettelli 等[10]在《Nature》雜志上發(fā)表文章認(rèn)為小鼠體內(nèi)Th17和iTreg產(chǎn)生于同一前體細(xì)胞-CD4+Foxp3+的T細(xì)胞，當(dāng)加入TGF-β時，此細(xì)胞轉(zhuǎn)化為Foxp3+的Treg細(xì)胞；但是當(dāng)加人TGF-β和IL-6時，此CD4+Foxp3-的T細(xì)胞轉(zhuǎn)化為Th17細(xì)胞，表明IL-6作為關(guān)鍵細(xì)胞因子決定CD4+Foxp3+T細(xì)胞是轉(zhuǎn)化為Treg還是Th17細(xì)胞。有趣的是，Baban等[36]報道在IL-6基因敲出的大鼠靜脈注射IDO誘導(dǎo)劑免疫刺激序列寡脫氧核苷酸(ISS-ODN)后，發(fā)現(xiàn)在脾臟Treg增多，而皮下注射IDO抑制劑1-甲基色氨酸(1-MT)后可使脾臟pDC表達(dá)IL-6，從而使Treg向Th17細(xì)胞轉(zhuǎn)化。因此在一定條件下IDO可作為Th17細(xì)胞向Treg轉(zhuǎn)換的分子“開關(guān)”，這為防治中性粒細(xì)胞哮喘帶來新的思路。An等[37]已證實(shí)轉(zhuǎn)IDO基因的未成熟DC可減輕致敏氣道的過敏性炎癥反應(yīng)。

3 IDO、Treg/Th17與中性粒細(xì)胞哮喘

IDO+pDC是一種體內(nèi)數(shù)量較少的具有分泌表達(dá)IDO能力的pDC亞群(IDO-competent pDCs)。目前有關(guān)IDO+pDC研究多集中在大鼠的肝、脾、淋巴結(jié)引流區(qū)中的DC。Kahler等[38]發(fā)現(xiàn)IDO+pDC除了高表達(dá)CD11c及B細(xì)胞特征性標(biāo)志CD19，并且在抗原刺激下通過分泌表達(dá)IDO介導(dǎo)Th細(xì)胞抑制及Treg活化。Shao等[39]在大鼠氣道炎癥模型證實(shí)，用Flt3配基處理肺部DC后發(fā)現(xiàn)：氣道高表達(dá)CD11c的CD11c高CD11b低DC亞群具有耐受原性可抑制Th2細(xì)胞產(chǎn)生，從而抑制氣道高反應(yīng)性。Taher等[40]在變應(yīng)原特異性免疫治療模型中證實(shí)用IDO抑制劑1-MT腹膜注射后可使氣道中的嗜酸性細(xì)胞減少及Th2類細(xì)胞因子增高，表明IDO及代謝產(chǎn)物可增強(qiáng)變應(yīng)原特異性免疫治療作用。氣道IDO主要表達(dá)在淋巴區(qū)域的巨噬細(xì)胞及DC上，同時DC是氣道最有效的抗原呈遞細(xì)胞(APC)，因此在哮喘發(fā)生時，機(jī)體除了與抗原攝入的種類及量，環(huán)境及個體差異有關(guān)外，還與體內(nèi)(特別是氣道)是否缺乏IDO+pDC亞群有關(guān)。以至于在中性粒細(xì)胞哮喘形成過程中，T淋巴細(xì)胞發(fā)生免疫應(yīng)答時，由于缺乏IDO，使TH0細(xì)胞向TH17細(xì)胞分化，而不向Treg分化，或者Th17細(xì)胞不能向Treg轉(zhuǎn)換，從而導(dǎo)致氣道以中性粒細(xì)胞炎癥為主。而目前關(guān)于這種IDO+pDC亞群與前述氣道CD11c高CD11b低DC亞群是否為同一DC亞群，有何關(guān)系，目前均不甚清楚。

4 展 望

綜上所述，Th17細(xì)胞與中性粒細(xì)胞性哮喘密切相關(guān)，中性粒細(xì)胞哮喘存在Treg/Th17失衡，IDO在一定條件下是Th17細(xì)胞向Treg轉(zhuǎn)換的分子“開關(guān)”，IDO+pDC是體內(nèi)數(shù)量較少的一種具有分泌表達(dá)IDO能力的pDC亞群。闡明IDO(包括IDO+pDC亞群)、Treg/Th17分化與中性粒細(xì)胞性哮喘三者的關(guān)系對于研究中性粒細(xì)胞哮喘的發(fā)生機(jī)制及哮喘防治有重要意義。

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10.3969/j.issn.1671-8348.2016.34.032

國家自然科學(xué)基金資助項(xiàng)目(81170034；81570022)；重慶市科委基礎(chǔ)與前沿研究項(xiàng)目(cstc2013jcyiA1021)。 作者簡介：胡琦(1977-)，主治醫(yī)師，碩士，主要從事小兒哮喘及新生兒的研究。△

，E-mail:liaowei01@163.com。

R725.6

A

1671-8348(2016)34-4852-03

2016-05-21

2016-08-09)