陳霞 崔凡 林昭春
(1.瀘州醫(yī)學(xué)院,瀘州 646000;2.四川省醫(yī)學(xué)科學(xué)院&四川省人民醫(yī)院皮膚病性病研究所,成都 610031)
白念珠菌是一種機(jī)會(huì)性致病菌,通常寄生在正常機(jī)體的皮膚和粘膜表面。當(dāng)機(jī)體正常防御機(jī)制受損時(shí),白念珠菌可成為致病菌,導(dǎo)致皮膚黏膜或系統(tǒng)性感染[1]。雖然目前抗真菌藥物不斷發(fā)展,由于耐藥及早期明確診斷困難,系統(tǒng)性白念珠菌感染病死率居高不下[2]。目前認(rèn)為先天免疫反應(yīng)在宿主抗系統(tǒng)性白念珠菌感染中起主導(dǎo)作用。熟悉宿主抗系統(tǒng)性白念珠菌感染的免疫機(jī)制可為新的免疫預(yù)防及治療措施提供理論依據(jù)。
病原體感染機(jī)體后,首先通過固有免疫細(xì)胞表面的模式識(shí)別受體 (pattern recognition receptor,PRR)與病原微生物表面的病原相關(guān)分子模式(pathogen associated molecular pattern,PAMP)結(jié)合并啟動(dòng)先天免疫,誘導(dǎo)相關(guān)細(xì)胞因子的產(chǎn)生,募集吞噬細(xì)胞殺滅病原體,同時(shí)誘導(dǎo)適應(yīng)性免疫細(xì)胞的活化,通過體液免疫和細(xì)胞免疫發(fā)揮抗菌作用。介導(dǎo)宿主識(shí)別白念珠菌的PRR主要包括Toll樣受體(Toll-1ike receptors,TLRs)和C型凝集素受體 (C-type lectin receptors,CLRs)[3]。
Toll樣受體是參與先天免疫的主要模式識(shí)別受體,廣泛表達(dá)于天然免疫細(xì)胞,與許多微生物病原體的PAMP結(jié)合后誘導(dǎo)先天免疫反應(yīng)以及參與繼發(fā)的適應(yīng)性免疫。目前在哺乳動(dòng)物中已發(fā)現(xiàn)存在著13種Toll樣受體,其中10種Toll樣受體在人類中發(fā)揮作用[3]。參與白念珠菌與宿主間相互識(shí)別的主要是TLR2和TLR4,并在宿主免疫反應(yīng)發(fā)展中起重要作用[4]。這兩種TLRs主要通過髓樣分化蛋白MyD88依賴型信號(hào)通路進(jìn)行轉(zhuǎn)導(dǎo),啟動(dòng)相關(guān)靶基因轉(zhuǎn)錄,表達(dá)炎性細(xì)胞因子,介導(dǎo)炎癥反應(yīng)。
TLR2識(shí)別白念珠菌細(xì)胞壁磷脂甘露聚糖和β-葡聚糖成分,激活信號(hào)傳導(dǎo)通路,產(chǎn)生細(xì)胞因子,如 IL-1β、IL-10、IL-4等,但不能誘導(dǎo) Th1型細(xì)胞 因 子,如 IL-12 和 IFN-γ 的 產(chǎn) 生[5-7]。Villamon[8]等證明TLR2在巨噬細(xì)胞對(duì)白念珠菌感染的免疫反應(yīng)中起主要作用,激發(fā)趨化因子等細(xì)胞因子生成,在體內(nèi)抗感染免疫中是必不可少的。Blasi[9]等證實(shí)TLR2的缺失增強(qiáng)了巨噬細(xì)胞的吞噬活性,同時(shí)Netea[10]等也證明TLR2介導(dǎo)信號(hào)通路的缺乏,宿主抵抗播散性念珠菌感染能力增強(qiáng)。在TLR2-/-小鼠中,IL-10的產(chǎn)生受到嚴(yán)重影響,IL-10的下降與IFN-γ的增加、調(diào)節(jié)性T細(xì)胞減少及巨噬細(xì)胞抗念珠菌功能增強(qiáng)相關(guān)。因此,TLR2可通過IL-10和調(diào)節(jié)性T細(xì)胞抑制宿主對(duì)白念珠菌的免疫力。中和體內(nèi)IL-10可能利于疾病轉(zhuǎn)歸,如抗白介素10單克隆抗體,但臨床應(yīng)用需更多實(shí)驗(yàn)研究數(shù)據(jù)支持。
TLR4識(shí)別白念珠菌細(xì)胞壁甘露聚糖成分,可誘導(dǎo) Th1型細(xì)胞因子的產(chǎn)生,如 IFN-γ、IL-12等[5,11]。Th1 型細(xì)胞免疫反應(yīng)在抵御白念珠菌感染免疫中起重要作用。至今TLR4在宿主抗系統(tǒng)性念珠菌感染的防御反應(yīng)中的作用存在爭論。Netea[6]證明TLR4缺陷的C3H/HeJ小鼠更易受到播散性念珠菌感染,可能與組織中巨噬細(xì)胞產(chǎn)生的趨化因子減少及中性粒細(xì)胞募集受損有關(guān)。Murciano等[12]證明與野生型小鼠相比,TLR4缺陷的小鼠念珠菌血性播散感染的易感性并未增加。有學(xué)者認(rèn)為上述不同結(jié)果可能與白念珠菌的菌株有關(guān),TLR4-/-小鼠僅對(duì)在體外實(shí)驗(yàn)中TLR4識(shí)別的菌株易感,而對(duì)不被 TLR4識(shí)別的菌株不易感[13]。TLR4只能識(shí)別白念珠菌的分生孢子,不能識(shí)別菌絲,而TLR2既能識(shí)別分生孢子又能識(shí)別菌絲。當(dāng)白念珠菌由孢子相轉(zhuǎn)變?yōu)榫z相時(shí),TLR4介導(dǎo)的信號(hào)通路缺失[7],TLR2介導(dǎo)的信號(hào)通路仍發(fā)揮作用,抑制宿主免疫力。因此白念珠菌菌絲的形成可能是一種重要的免疫逃逸方式。TLR4識(shí)別念珠菌后主要產(chǎn)生促炎性細(xì)胞因子,TLR2主要誘導(dǎo)抗炎性細(xì)胞因子的產(chǎn)生。TLR2和TLR4誘導(dǎo)信號(hào)間的平衡在免疫反應(yīng)調(diào)節(jié)中至關(guān)重要[1]。
CLRs是一種存在C型凝集素樣結(jié)構(gòu)域的模式受體。這些受體為跨膜蛋白,是主要的膜結(jié)合受體,共享一個(gè)或多個(gè)糖識(shí)別結(jié)構(gòu)域。它們中一些參與抗真菌免疫,如 Dectin-l、Dectin-2、甘露糖受體(MR)、Mincle、DC-SIGN 等[3]。
MR是一種I型跨膜受體,識(shí)別白念珠菌細(xì)胞壁上的N連接端甘露糖殘基[14],主要表達(dá)于巨噬細(xì)胞及樹突狀細(xì)胞 (DCs)表面。MR可介導(dǎo)吞噬細(xì)胞攝入白念珠菌后吞噬體的募集,并通過細(xì)胞內(nèi)信號(hào),導(dǎo)致細(xì)胞因子的產(chǎn)生[15]。MR識(shí)別念珠菌后刺激人單核細(xì)胞產(chǎn)生Th1型細(xì)胞因子,誘導(dǎo) Th1型免疫反應(yīng)[14]。Dectin-1屬于Ⅱ型跨膜受體,在抗真菌的先天免疫中起重要作用,主要表達(dá)于巨噬細(xì)胞、單核細(xì)胞、DCs和中性粒細(xì)胞表面[3]。Dectin-1識(shí)別白念珠菌細(xì)胞壁上的 β-葡聚糖。DCs上的 Dectin-1通過 Syk激酶和CARD9適配器誘導(dǎo)產(chǎn)生IL-2和IL-10[16]。有研究表明在體內(nèi)外 Dectin-1對(duì)于觸發(fā)白念珠菌的吞噬、酵母菌侵襲后的呼吸爆發(fā)是必不可少的[17]。Taylor等[18]發(fā)現(xiàn) Dectin-1-/-小鼠更易患播散性念珠菌病。而CARD9-/-小鼠患播散性念珠菌病易感性增加更加支持上述觀點(diǎn)[19]。β-葡聚糖位于白念珠菌酵母相細(xì)胞壁的最外層。當(dāng)白念珠菌轉(zhuǎn)變?yōu)榫z相時(shí),β-葡聚糖位于細(xì)胞壁內(nèi)層,無法被Dectin-1識(shí)別,從而逃逸宿主的免疫系統(tǒng)[20]。亞抑菌劑量的卡泊芬凈可使β-葡聚糖暴露在外,提高免疫活性,利于Dectin-1對(duì)白念珠菌的識(shí)別,且在體內(nèi)外卡泊芬凈優(yōu)先作用于菌絲相細(xì)胞壁而暴露 β-葡聚糖[21]。使真菌細(xì)胞壁上PAMP暴露在外的抗真菌藥物的研究將為抗真菌感染治療提供新途徑。Dectin-2也屬于Ⅱ型跨膜受體,主要表達(dá)于巨噬細(xì)胞和DCs上,識(shí)別白念珠菌細(xì)胞壁上的高甘露糖結(jié)構(gòu)。與Dectin-1相比,Dectin-2不能通過自身誘導(dǎo)信號(hào)通路。它優(yōu)先與白念珠菌菌絲相結(jié)合,識(shí)別后通過與FcRγ受體結(jié)合,激活NF-kB誘導(dǎo)產(chǎn)生IL-1受體拮抗劑 (IL-1Ra)和TNF-α,最終導(dǎo)致吞噬作用[22]。Dectin-2 可誘導(dǎo)Th17細(xì)胞分化,在宿主抗白念珠菌免疫中起重要作用[3]。DC-SIGN特異性表達(dá)于 DCs和巨噬細(xì)胞[23]。人類 DCs上的 DC-SIGN識(shí)別白念珠菌N-連接端甘露糖,誘導(dǎo)產(chǎn)生促炎性細(xì)胞因子IL-6[24]。Mincle主要表達(dá)于巨噬細(xì)胞上,但不是吞噬白念珠菌的受體。最近在人和小鼠中均發(fā)現(xiàn)Mincle可識(shí)別白念珠菌并刺激細(xì)胞因子的產(chǎn)生[25]。Mincle缺陷的小鼠更易患系統(tǒng)性念珠菌病[26]。
先天免疫反應(yīng)是宿主防御的第一道防線,負(fù)責(zé)立即識(shí)別病原微生物并對(duì)抗其入侵。過去認(rèn)為先天性免疫反應(yīng)在宿主防御反應(yīng)中雖然有效,但屬于非特異性、原始的反應(yīng)。后來發(fā)現(xiàn)先天免疫系統(tǒng)不僅能特異性識(shí)別不同類別病原體將其吞噬殺滅,還可啟動(dòng)和調(diào)節(jié)后繼的適應(yīng)性免疫反應(yīng)。在系統(tǒng)性白念珠菌感染中,先天免疫反應(yīng)是機(jī)體清除病原菌的主要機(jī)制[27-28]。
吞噬細(xì)胞是參與宿主先天免疫反應(yīng)的主要細(xì)胞,包括中性粒細(xì)胞、單核細(xì)胞、巨噬細(xì)胞。系統(tǒng)性念珠菌感染與中性粒細(xì)胞和單核細(xì)胞數(shù)量和功能異常相關(guān)。淋巴瘤、白血病或癌癥化療導(dǎo)致中性粒細(xì)胞減少的患者患播散性念珠菌病風(fēng)險(xiǎn)增加[28]。中性粒細(xì)胞通過產(chǎn)生細(xì)胞因子IL-10和IL-12影響Th細(xì)胞的發(fā)育參與免疫調(diào)節(jié)作用。人中性粒細(xì)胞識(shí)別白念珠菌甘露糖蛋白后產(chǎn)生具有生物活性的IL-12,誘導(dǎo)人外周血單核細(xì)胞表達(dá)產(chǎn)生Th1型細(xì)胞因子,從而發(fā)揮抗白念珠菌作用。有研究表明白念珠菌尤其是細(xì)胞壁的β-葡聚糖成分可通過Dectin-1/Raf-1通路誘導(dǎo)單核細(xì)胞遺傳外的重新編程,增強(qiáng)細(xì)胞因子的產(chǎn)生,這種遺傳外的先天免疫調(diào)節(jié)可能與宿主抗再感染相關(guān)[29]。巨噬細(xì)胞在白念珠菌感染中既能作為效應(yīng)細(xì)胞吞噬和殺滅入侵念珠菌,還能通過抗原提呈細(xì)胞及Th1細(xì)胞激活而調(diào)節(jié)免疫反應(yīng)。巨噬細(xì)胞激活后可誘導(dǎo)一些促炎性細(xì)胞因子如TNF-a、IL-6、IL-12等的釋放,誘導(dǎo)Th1細(xì)胞免疫反應(yīng)。
吞噬細(xì)胞吞噬病原體后主要通過氧依賴和氧非依賴殺菌系統(tǒng)殺傷病原體。氧依賴殺菌系統(tǒng)包括反應(yīng)性氧中間產(chǎn)物 (reactive oxygen intermediates,ROI)和反應(yīng)性氮中間產(chǎn)物 (reactive nitrogen intermediates,RNI)。ROI系統(tǒng)被認(rèn)為是吞噬細(xì)胞抗真菌防御機(jī)制的主要組成部分[30]。在中性粒細(xì)胞和單核細(xì)胞中過氧化氫能與鹵化物、髓過氧化物(MPO)組成MPO殺菌系統(tǒng),產(chǎn)生次氯酸和次碘酸殺真菌氧化劑。巨噬細(xì)胞不具備MPO殺菌系統(tǒng),但可以通過MRs清除MPO,并轉(zhuǎn)移至吞噬體發(fā)揮殺滅真菌作用。氧非依賴殺菌系統(tǒng)不需要氧分子參與。吞噬細(xì)胞特別是中性粒細(xì)胞,具有幾種非氧化性機(jī)制能有效地殺死細(xì)胞內(nèi)外的真菌或限制其生長,包括抗菌肽 (AMPs)、水解酶和限制營養(yǎng)攝取。在白念珠菌中AMPs與細(xì)胞膜特定位點(diǎn)結(jié)合誘導(dǎo)非溶解性細(xì)胞通透性增加,釋放出細(xì)胞內(nèi)ATP而殺滅白念珠菌[31]。營養(yǎng)物質(zhì)的限制也是一種有效的抗真菌機(jī)制,將吞噬的真菌遏制在吞噬體中并啟動(dòng)營養(yǎng)限制,如鐵的攝取,主要通過乳鐵蛋白的結(jié)合、轉(zhuǎn)鐵蛋白受體的下調(diào)、鋅封閉而減少鐵進(jìn)入吞噬體,抑制吞噬體中真菌的生長而發(fā)揮抗菌作用[30]。
NK細(xì)胞是先天免疫系統(tǒng)的重要組成部分,通過細(xì)胞毒性發(fā)揮抗感染作用,不需抗原預(yù)先致敏,不受MHC限制。NK細(xì)胞在早期防御真菌感染方面同樣起了重要的作用。在系統(tǒng)念珠菌感染的小鼠實(shí)驗(yàn)中通過藥物增強(qiáng)NK細(xì)胞活性后小鼠抗念珠菌能力增強(qiáng)[32]。而NK細(xì)胞缺陷的轉(zhuǎn)基因小鼠更容易患念珠菌病[33]。白念珠菌激活NK細(xì)胞后可誘導(dǎo)致炎性細(xì)胞因子 GM-CSF、TNF-ɑ、INF-γ 的釋放,通過上調(diào)吞噬細(xì)胞的殺菌活性來介導(dǎo)宿主的抗真菌免疫反應(yīng)。IFN-γ是吞噬細(xì)胞的最佳活化因子,能增強(qiáng)吞噬細(xì)胞吞噬和殺滅白念珠菌能力[5]?;罨腘K細(xì)胞,可通過產(chǎn)生INF-γ,在機(jī)體抗真菌感染方面起到一定的作用。
DC是專職抗原提呈細(xì)胞,其主要功能是攝取、加工處理和提呈抗原,誘導(dǎo)初始T細(xì)胞活化,啟動(dòng)特異性免疫應(yīng)答。正常情況下,體內(nèi)絕大多數(shù)DC處于未成熟DC狀態(tài),在外周組織中作為哨兵細(xì)胞監(jiān)測(cè)外來抗原并作出反應(yīng)。病原體刺激DC后,分化為成熟DC,分泌不同細(xì)胞因子,啟動(dòng)并調(diào)節(jié)機(jī)體免疫反應(yīng)。DC表面PRRS識(shí)別白念珠菌后通過吞噬作用吞噬白念珠菌并將抗原呈遞給T細(xì)胞誘導(dǎo)適應(yīng)性免疫應(yīng)答,是連接固有免疫和適應(yīng)性免疫的重要橋梁[34]。DC還可識(shí)別不同形式白念珠菌,并誘導(dǎo)相應(yīng)的免疫反應(yīng)。d'Ostiani等[35]用酵母和菌絲兩種形式的白念在體外與FSDC(胎鼠皮膚DC)和小鼠脾DC共同培養(yǎng),發(fā)現(xiàn)FSDC以不同的機(jī)制吞噬酵母和菌絲。酵母狀態(tài)的白念珠菌通過卷曲吞噬作用被攝入,而菌絲體狀態(tài)的白念珠菌通過拉鏈途徑被吞噬,但菌絲隨后脫離小體在胞漿中裂解。在體外脾DC攝取酵母細(xì)胞后誘導(dǎo)產(chǎn)生IL-12并啟動(dòng)Th1型細(xì)胞免疫反應(yīng),而攝取菌絲后誘導(dǎo)產(chǎn)生IL-4,可抑制IL-12的產(chǎn)生和Th1型細(xì)胞免疫反應(yīng)的啟動(dòng)。由此可知,脾DC通過產(chǎn)生不同的細(xì)胞因子介導(dǎo)機(jī)體針對(duì)酵母和菌絲相白念珠菌的免疫反應(yīng)。Romagnoli等[36]進(jìn)一步證實(shí)人DC亦能有效攝取并降解兩種形式的白念珠菌,兩種形式的白念珠菌均能誘導(dǎo)DC成熟和活化,進(jìn)而啟動(dòng)Th1型免疫反應(yīng)。
宿主抗系統(tǒng)性白念珠菌感染的免疫反應(yīng)機(jī)制復(fù)雜,雖然目前對(duì)宿主識(shí)別白念珠菌相關(guān)受體結(jié)構(gòu)及其機(jī)制有所認(rèn)識(shí),但PAMP與PRR間的相互作用非常復(fù)雜,如病原體如何通過PRR逃避宿主識(shí)別,宿主PRR如何區(qū)分識(shí)別白念珠菌菌絲相和酵母相,實(shí)驗(yàn)?zāi)P椭蠵AMP與PRR間的識(shí)別機(jī)制及信號(hào)通路對(duì)于人類是否同樣適用,需做更多實(shí)驗(yàn)研究工作。治療方面基于宿主抵抗白念珠菌的免疫反應(yīng)機(jī)制,通過輔助免疫治療增強(qiáng)宿主免疫反應(yīng)是治療的新選擇,如重組IFN-γ或G-CSF,雖已作為Ⅲ期臨床試驗(yàn)候選制劑,但無臨床實(shí)踐療效證明,也無確切臨床應(yīng)用指南,未來相關(guān)實(shí)驗(yàn)設(shè)計(jì)對(duì)于其臨床價(jià)值至關(guān)重要。
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