030000 太原,山西醫(yī)科大學(xué)第一醫(yī)院呼吸內(nèi)科
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·綜述·
程序性死亡分子配體-2在哮喘氣道高反應(yīng)中的作用
郭東瑾張煥萍
030000 太原,山西醫(yī)科大學(xué)第一醫(yī)院呼吸內(nèi)科
【關(guān)鍵詞】支氣管哮喘;程序性死亡分子配體-2;氣道高反應(yīng)性
最近幾十年哮喘的發(fā)病率大大增加,成為世界最普遍的慢性疾病之一[1-2]。過敏性哮喘是吸入過敏原后引發(fā)慢性Th2免疫反應(yīng),導(dǎo)致以氣道非特異刺激、慢性嗜酸性細(xì)胞氣道炎癥、杯狀細(xì)胞增生、氣道結(jié)構(gòu)改變,引起氣道高反應(yīng)為特點(diǎn)及Th2依賴的血清抗原特異性免疫球蛋白E(immunoglobulin E, IgE)和IgG1增加的慢性氣道疾病[3-4]。過敏性哮喘的氣道高反應(yīng)被認(rèn)為是依賴于CD4+T細(xì)胞,并且與肺部增加的Th2細(xì)胞因子有關(guān),而T淋巴細(xì)胞的激活需要共刺激分子參與才能最終實(shí)現(xiàn)。程序性死亡分子配體-2(programmed death ligand-2, PD-L2)作為抑制性共刺激分子——程序性死亡分子1(programmed death-1, PD-1)的第二配體,通過與多種已知或未知的受體相互作用參與調(diào)節(jié)支氣管哮喘氣道高反應(yīng)(asthma airway high reaction, AHR)及呼吸免疫耐受?,F(xiàn)就PD-L2的結(jié)構(gòu)及分布、對(duì)哮喘氣道高反應(yīng)的調(diào)節(jié)作用進(jìn)行總結(jié)。
一、 PD-L2來源及分布
PD-1(又稱CD279)最初是在死亡的T細(xì)胞上發(fā)現(xiàn)的[5],屬于免疫球蛋白超家族B7,CD28成員。PD-1是一個(gè)相對(duì)分子量為(50-55)×103的Ⅰ型跨膜受體,其胞外區(qū)只有一個(gè)IgV樣區(qū),缺乏細(xì)胞毒性T淋巴細(xì)胞抗原4(cytotoxic T lymphocyte antigen-4, CTLA-4)和CD28結(jié)合B7-1和B7-2的MYPPPY基序;胞質(zhì)區(qū)有兩個(gè)酪氨酸信號(hào)基序,其一組成免疫受體酪氨酸抑制模體(immune receptor tyrosine-based inhibitory motif, f ITIM),另一為免疫受體酪氨酸交換模體(immuno receptor tyrosine-based switchmoti, f ITSM)。ITSM募集磷酸酶SHP-1和SHP-2,使TCR或BCR傳遞的效應(yīng)信號(hào)發(fā)生去磷酸化,同時(shí),PD-1信號(hào)降低CD28介導(dǎo)的信號(hào),抑制Akt激酶磷酸化、糖代謝及Bc-lXL的表達(dá)。人PD-1基因定位于染色體2q37.3,表達(dá)于活化的T細(xì)胞、B細(xì)胞、髓系細(xì)胞和胸腺細(xì)胞中。
PD-1有PD-L1和PD-L2兩種配體, PD-L1較PD-L2的表達(dá)更廣泛,而PD-L2的親和力是PD-L1的2~6倍。PD-L1(也稱B7-H1或CD274)在鼠類T細(xì)胞、B細(xì)胞、樹突狀細(xì)胞(dendritic cells, DCs)、巨噬細(xì)胞、間充質(zhì)干細(xì)胞和培養(yǎng)的骨髓源肥大細(xì)胞組成性表達(dá),活化后表達(dá)增強(qiáng),人PD-L1的表達(dá)水平較鼠類低。PD-L2(也稱B7-DC或CD273)定位于人9p24.2,可編碼247個(gè)氨基酸,PD-L2表達(dá)譜較窄,僅在巨噬細(xì)胞及樹突細(xì)胞上表達(dá)[6-8]。在巨噬細(xì)胞上可被IL-4和IFN-γ誘導(dǎo)表達(dá),在樹突細(xì)胞上可被anti-CD40、GM-CSF、IL-4、IFN-γ、IL-12誘導(dǎo)表達(dá)[9]。
二 、PD-L2與AHR
PD-1及其配體可在T細(xì)胞活化、T細(xì)胞耐受及免疫介導(dǎo)的組織損傷中帶來不同的致病影響,并且在自身免疫性疾病的調(diào)節(jié)、各種微生物傳染病模型的免疫功能調(diào)節(jié)、腫瘤免疫及組織移植中起重要作用。多項(xiàng)研究表明,PD-L2在支氣管哮喘過敏性氣道反應(yīng)的發(fā)展中亦扮演著重要角色[10-12]。
1. PD-L2通過iNKT細(xì)胞調(diào)節(jié)AHR: 在對(duì)哮喘小鼠模型肺樹突細(xì)胞表面PD-L2基因敲除研究中發(fā)現(xiàn),PD-L2表達(dá)缺失導(dǎo)致肺部炎癥反應(yīng)及AHR的增加,暗示肺部樹突細(xì)胞PD-L2的表達(dá)在iNKT細(xì)胞依賴的氣道炎癥的發(fā)生發(fā)展中起到保護(hù)作用。肺部炎癥反應(yīng)及AHR加重是因?yàn)閕NKT細(xì)胞產(chǎn)生了大量的IL-4,即PD-L2可抑制iNKT細(xì)胞產(chǎn)生IL-4。研究還發(fā)現(xiàn),經(jīng)卵清蛋白刺激后,小鼠肺部所有樹突細(xì)胞表面PD-L2表達(dá)增加,從未經(jīng)抗原刺激小鼠中分離出肺樹突細(xì)胞與IL-4共同培養(yǎng),發(fā)現(xiàn)其表面PD-L2表達(dá)上調(diào),而與IFN-γ及LPS一同培養(yǎng)時(shí)PD-L2的表達(dá)則被抑制,提示肺部細(xì)胞因子微環(huán)境可決定PD-L2的表達(dá)形式。之前已提及,PD-L2可抑制IL-4的分泌及iNKT細(xì)胞調(diào)控的AHR,上述結(jié)果說明肺部PD-L2的表達(dá)可在TH2細(xì)胞因子主導(dǎo)的肺部炎癥反應(yīng)過程中形成一個(gè)反饋回路,即初期可被IL-4上調(diào)表達(dá),隨后抑制IL-4繼續(xù)產(chǎn)生,因此來調(diào)節(jié)哮喘的嚴(yán)重程度,在此PD-L2/IL-4循環(huán)中PD-L2的缺失可導(dǎo)致IL-4的增加從而加重AHR[11]。
2. PD-L2與TH9細(xì)胞分化及慢性AHR: 對(duì)于慢性過敏性哮喘,已有報(bào)道顯示IL-9基因與過敏性炎癥關(guān)系密切[13-15],在轉(zhuǎn)基因小鼠肺部選擇性高表達(dá)的IL-9基因可導(dǎo)致氣道炎癥反應(yīng)及嗜酸粒細(xì)胞、淋巴細(xì)胞浸潤(rùn)增加[14-16]。除此之外,IL-9被發(fā)現(xiàn)在慢性哮喘氣道重塑的發(fā)生發(fā)展中有直接和間接的作用,并且在T細(xì)胞擴(kuò)增分化、肥大細(xì)胞和嗜酸性細(xì)胞的激活、B細(xì)胞IgE生成及上皮細(xì)胞黏液分泌增加等哮喘發(fā)病機(jī)制方面均扮演著重要的角色[14,17-20]。但是可調(diào)節(jié)體內(nèi)TH9細(xì)胞分化的分子尚不知。國(guó)外一項(xiàng)研究通過將小鼠暴露在鼻劑量的曲霉菌溶菌產(chǎn)物中數(shù)個(gè)星期來誘發(fā)慢性AHR模型,觀察到在第六周時(shí),有相當(dāng)數(shù)量的Th9細(xì)胞在肺部沉積,并且進(jìn)一步探索了在PD-L2缺乏的小鼠中,PD-L2通路在調(diào)控Th9細(xì)胞反應(yīng)及慢性AHR發(fā)展中所發(fā)揮的作用[11]。實(shí)驗(yàn)結(jié)果表明,阻斷PD-L2通路后,在致敏小鼠肺部TGF-β及IL-1α水平明顯增加,從而誘導(dǎo)Th9細(xì)胞分化增加(與肺部炎癥反應(yīng)、黏液生成及AHR的嚴(yán)重程度直接相關(guān))。在體外試驗(yàn)中,得到了與體內(nèi)試驗(yàn)相一致的結(jié)果,即PD-L2可負(fù)性調(diào)控Th9細(xì)胞分化。這說明,無論是在體內(nèi)還是體外,PD-L2都在慢性AHR及Th9細(xì)胞分化調(diào)控中扮演著關(guān)鍵角色,這可能為炎癥及過敏反應(yīng)適應(yīng)性免疫調(diào)節(jié)提供新的思路。試驗(yàn)中還發(fā)現(xiàn),在PD-L2缺乏的致敏小鼠肺部IL-9的濃度水平及疾病的嚴(yán)重程度均增加,在慢性過敏原暴露過程中,PD-L2缺乏的小鼠死亡率大大增加。經(jīng)IL-9抗體治療后,足以防止死亡的發(fā)生,并且可大大緩解疾病癥狀,同時(shí)對(duì)照組小鼠體內(nèi)使用IL-9抗體后也可減少輕AHR、肺部炎癥反應(yīng)及黏液分泌。但是,與對(duì)照組相比,或許是因?yàn)镮L-17a的存在(可調(diào)節(jié)部分肺部炎癥及功能),中和IL-9并不能完全消除氣道炎癥及AHR。
最近有報(bào)道稱PD-L2在調(diào)節(jié)急性AHR中扮演著重要角色[10],為了證明PD-L2是否對(duì)過敏原引起的慢性AHR發(fā)展也起作用,作者比較了慢性曲霉菌暴露所致AHR的PD-L2-/-小鼠及對(duì)照小鼠的AHR發(fā)展情況,結(jié)果顯示在PD-L2-/-小鼠的肺泡灌洗液中巨噬細(xì)胞及淋巴細(xì)胞數(shù)量大大增加。為了評(píng)估肺部炎癥水平,作者通過量化氣道上皮的厚度及炎癥和杯狀細(xì)胞的數(shù)量,發(fā)現(xiàn)PD-L2-/-小鼠的炎癥程度、細(xì)胞浸潤(rùn)及黏液生成都明顯高于對(duì)照組。上述結(jié)果說明,在慢性過敏原暴露下,PD-L2可調(diào)節(jié)AHR及肺部炎癥的嚴(yán)重程度[11]。此研究初次揭示了在慢性過敏原暴露模型體內(nèi)Th9細(xì)胞的分化情況,證明了Th9細(xì)胞與病原體相關(guān)過敏性哮喘的發(fā)展有關(guān)。而且,進(jìn)一步證實(shí)共刺激分子PD-L2通常以負(fù)性調(diào)節(jié)的角色直接和間接影響Th9細(xì)胞的分化過程。這將為慢性過敏性哮喘治療策略的發(fā)展提供重要思路。
3. PD-L2與IL-12水平及AHR: 在體內(nèi),DCs是肺部提呈抗原的主力,在致病性或保護(hù)性T細(xì)胞反應(yīng)中都扮演著關(guān)鍵的角色[21]。有研究顯示,過敏原被髓樣樹突細(xì)胞(myeloid dendritic cells, mDCs)提呈后不僅可促使幼稚小鼠發(fā)生AHR[21-22],還可加重已致敏小鼠的AHR[23],而被類漿樣樹突細(xì)胞 (plasma cell like dendritic cells, pDCs)提呈后卻可預(yù)防AHR的發(fā)生[24],并限制已致敏小鼠的過敏性炎癥反應(yīng)[25]。除此之外,DCs還可以通過偏移T細(xì)胞因子的產(chǎn)生來影響過敏原誘導(dǎo)的AHR,而肺部mDCs也可產(chǎn)生高濃度水平的IL-23,并與Th2/Th17因子增殖有關(guān),上述結(jié)果最終導(dǎo)致嚴(yán)重AHR的發(fā)生[21,26]。DCs上所表達(dá)的共刺激分子的類型同樣也可以調(diào)控T細(xì)胞發(fā)展,例如共刺激分子CD80、CD86、OX40L的表達(dá)可促進(jìn)過敏性疾病的發(fā)展[27-29],而ICOS-L的表達(dá)可誘導(dǎo)調(diào)節(jié)性T細(xì)胞的產(chǎn)生,促進(jìn)吸入性免疫耐受[30]。因此,DCs可通過其亞型特點(diǎn)及細(xì)胞因子分泌、共刺激分子的表達(dá)來調(diào)控T細(xì)胞反應(yīng)。另一種DCs調(diào)節(jié)T細(xì)胞反應(yīng)的途徑是通過其表面表達(dá)的PD-1家族實(shí)驗(yàn)的。一般來講,PD-1可抑制T細(xì)胞激活,但PD-1/PD-L軸在調(diào)節(jié)過敏性哮喘中的角色仍不確定,一些研究顯示其有保護(hù)氣道的作用[10,25,31-33],而有的研究結(jié)果卻與之相反。
國(guó)外研究顯示,經(jīng)屋塵螨刺激的小鼠肺部多種細(xì)胞表面可檢測(cè)到PD-L2的表達(dá),包括肺泡巨噬細(xì)胞AMs、炎癥性DCs、髓樣DCs及中性粒細(xì)胞,但mDCs上PD-L2的表達(dá)率較其他細(xì)胞明顯增高,高達(dá)15%,而其他類型細(xì)胞僅為3%,說明肺部mDCs是PD-L2表達(dá)的主要載體[12]。該研究還探索了PD-L2表達(dá)是否與哮喘的嚴(yán)重程度有關(guān),結(jié)果顯示,與對(duì)照組相比,嚴(yán)重哮喘組PD-L2的表達(dá)大大提高(P<0.05),而且,PD-L2的表達(dá)在對(duì)照組及輕度哮喘組僅有升高的趨勢(shì)(P=0.07)。為了證明PD-L2在小鼠AHR中的作用,作者用屋塵螨制造小鼠AHR模型,經(jīng)使用PD-L2單抗后,AHR的病程發(fā)展大大緩解。鑒于PD-L2是PD-1的配體之一,但又無法排除PD-L2增加哮喘嚴(yán)重程度是通過與PD-1的相互作用,因此作者使用一種可阻斷PD-1/PD-L2通路的抗體作用于AHR小鼠模型,結(jié)果顯示此抗體對(duì)屋塵螨誘導(dǎo)的AHR沒有任何影響。說明PD-L2與PD-1以外的受體具有相互作用來調(diào)節(jié)AHR的嚴(yán)重程度。除此之外,作者還發(fā)現(xiàn)阻斷PD-L2后,對(duì)Th2/Th17因子(IL-4、IL-5、IL-13、IL-17A)的增殖無任何影響,但血清中IL-12的濃度水平顯著增加,因IL-12可限制過敏原誘導(dǎo)AHR的發(fā)展,所以血清中增加的IL-12或許可以解釋PD-L2-/-小鼠中緩解的AHR。為了證明這個(gè)假設(shè),作者在PD-L2-/-的小鼠中使用IL-12單抗,結(jié)果顯示使用IL-12單抗與未使用之前的結(jié)果相對(duì)抗,即同時(shí)給予PD-L2及IL-12單抗后的AHR與對(duì)照組相比無顯著差異,而單獨(dú)使用IL-12單抗對(duì)屋塵螨誘導(dǎo)的小鼠AHR無影響。說明PD-L2在不影響Th2/Th17因子的前提下通過調(diào)節(jié)IL-12的濃度水平來調(diào)控AHR[12]。
支氣管哮喘是由多種細(xì)胞成分及細(xì)胞組分參與的慢性氣道炎癥性疾病,目前主要認(rèn)為與Th1/Th2細(xì)胞分化失衡有關(guān)。PD-L2可通過調(diào)節(jié)多種細(xì)胞因子(IL-4、IL-9、IL-12)的生成來調(diào)控肺部氣道炎癥反應(yīng)及AHR,對(duì)哮喘發(fā)病機(jī)制的認(rèn)識(shí)邁上了一個(gè)新的臺(tái)階,也為哮喘的治療提供了新靶點(diǎn)。
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(本文編輯:王亞南)
郭東瑾,張煥萍. 程序性死亡分子配體-2在哮喘氣道高反應(yīng)中的作用[J/CD]. 中華肺部疾病雜志: 電子版, 2015, 9(1): 92-94.
(收稿日期:2015-05-16)
中圖法分類號(hào):R563
文獻(xiàn)標(biāo)識(shí)碼:A
通訊作者:張煥萍, Email: Zhp326@163.com
DOI:10.3877/cma.j.issn.1674-6902.2016.01.026