劉晨晨, 金靜怡, 徐兆軍, 張振玉
南京醫(yī)科大學(xué)附屬南京醫(yī)院(南京市第一醫(yī)院)消化科,江蘇 南京 210006
幽門螺桿菌與炎癥性腸病的關(guān)系
劉晨晨, 金靜怡, 徐兆軍, 張振玉
南京醫(yī)科大學(xué)附屬南京醫(yī)院(南京市第一醫(yī)院)消化科,江蘇 南京 210006
炎癥性腸病(inflammatory bowel disease, IBD)是腸道慢性炎癥性疾病,主要分為克羅恩病(Crohn’s disease,CD)和潰瘍性結(jié)腸炎(ulcerative colitis, UC);近年來IBD的發(fā)病率急劇增高,尤其是在發(fā)展中國家;雖然關(guān)于IBD的研究取得了很大進(jìn)展,但其確切發(fā)病機(jī)制仍不明確,目前IBD尚不能治愈,只能緩解。研究顯示免疫、感染、遺傳等因素是其可能的致病因素。幽門螺桿菌(Helicobacter pylori,H.pylori)是人類常見的致病菌,是引起胃炎、消化性潰瘍和胃癌等疾病明確病原菌;近年來,不斷有證據(jù)表明H.pylori可以通過誘導(dǎo)免疫耐受、降低炎癥反應(yīng)等作用對IBD產(chǎn)生一定的保護(hù)作用。這一現(xiàn)象對研究IBD的發(fā)病機(jī)制及尋找特異性的治療手段有重大意義,本文就H.pylori感染與IBD的關(guān)系及其機(jī)制作一概述。
幽門螺桿菌;炎癥性腸病;免疫耐受
炎癥性腸病(inflammatory bowel disease, IBD)是慢性消化道炎癥性疾病,是世界范圍內(nèi)沉重的醫(yī)療負(fù)擔(dān)[1],在過去的一個(gè)世紀(jì)里,許多發(fā)展中國家IBD的發(fā)病率呈現(xiàn)了急劇增加的態(tài)勢,這一趨勢可能與飲食結(jié)構(gòu)的改變、抗生素的濫用、環(huán)境因素以及腸道菌群的改變等有關(guān)。近年來,雖然對IBD的認(rèn)識(shí)不斷深入,但其確切病因仍不明確。幽門螺桿菌(Helicobacter pylori,H.pylori)是人類常見的致病菌,被列為Ⅰ類致癌因子,其慢性感染與胃炎、消化性潰瘍、胃癌等疾病密切相關(guān);近年來,流行病學(xué)研究顯示[2-5],H.pylori感染與IBD呈負(fù)相關(guān),H.pylori對IBD可能起到一定的保護(hù)作用。這種現(xiàn)象引起臨床醫(yī)師的共同關(guān)注,為IBD的病因及制等研究提供了新思路,本文就H.pylori與IBD的關(guān)系及潛在機(jī)制作一概述。
H.pylori屬于革蘭陰性菌,呈螺旋狀,主要定植在人胃黏膜中,是感染人類的常見病原體。H.pylori在世界范圍內(nèi)的感染率是50%,發(fā)展中國家感染率明顯高于發(fā)達(dá)國家[6]。H.pylori的慢性、持續(xù)感染可導(dǎo)致慢性胃炎、消化性潰瘍、胃癌、胃黏膜相關(guān)淋巴組織瘤等疾病[7]。H.pylori的感染能力主要取決于細(xì)菌毒力、宿主因素、環(huán)境因素等;所有H.pylori菌株中都包含尿素酶和鞭毛等毒力因子,H.pylori定植在胃黏膜后通過產(chǎn)生尿素酶而降低胃內(nèi)酸度;H.pylori通過鞭毛在胃腔中移動(dòng)而進(jìn)入到黏膜層,從而持續(xù)定植在胃中。
與H.pylori菌體相關(guān)毒力因子包括膜蛋白如空泡毒素A(VacA)、細(xì)胞毒素相關(guān)蛋白(CagA)[8],所有H.pylori菌株都包含VacA,VacA通過干擾線粒體及內(nèi)質(zhì)網(wǎng)功能而促進(jìn)胃黏膜上皮細(xì)胞凋亡[9-10]。VacA可以結(jié)合CD4+T細(xì)胞,抑制了抗原依賴性T細(xì)胞的增殖;VacA通過誘導(dǎo)樹突細(xì)胞(dendritic cells, DCs)表達(dá)及釋放IL-10、IL-18等抗炎因子,促進(jìn)Treg (regulatory T-cells, Tregs) 細(xì)胞分化,進(jìn)而產(chǎn)生免疫抑制作用;這種免疫抑制效應(yīng)可協(xié)助H.pylori躲避宿主免疫系統(tǒng)的攻擊[10]。Cag毒力島中的基因只存在于部分H.pylori菌株中, 其編碼的蛋白形成Ⅳ型細(xì)菌分泌系統(tǒng),可將細(xì)菌成分注入宿主細(xì)胞中;CagA蛋白在CD44的協(xié)助下促進(jìn)胃黏膜上皮細(xì)胞增殖[11],同時(shí)可引起細(xì)胞排列不規(guī)則、極性喪失等改變,被列為細(xì)菌性致癌蛋白[12]。H.pylori及其產(chǎn)物通過各種細(xì)胞因子引起機(jī)體持續(xù)的炎癥反應(yīng),這種炎癥反應(yīng)與表面上皮和巨噬細(xì)胞/單核細(xì)胞中促炎因子的表達(dá)有關(guān)。
2.1 免疫因素 機(jī)體健康狀態(tài)的維持與T細(xì)胞調(diào)節(jié)網(wǎng)絡(luò)的平衡狀態(tài)密不可分,這種平衡的紊亂會(huì)引起慢性炎癥;FoxP3+CD4+T細(xì)胞通過刺激IL-10、轉(zhuǎn)化生長因子-β(transforming growth factor beta, TGF-β)的分泌有效地抑制致病性T細(xì)胞反應(yīng)[13],從而促進(jìn)腸道黏膜穩(wěn)態(tài)。UC患者中的炎癥表現(xiàn)主要是腸道Th2細(xì)胞通過適應(yīng)性免疫反應(yīng)產(chǎn)生IL-4和IL-13因子,而CD患者中炎癥表現(xiàn)主要是通過腸道Th1細(xì)胞產(chǎn)生γ-干擾素(interferon gamma, IFN-γ)、腫瘤壞死因子-α(tumor necrosis factor alpha, TNF-α)、IL-12因子[14]:二者都是以IL-17的積聚為特征,這種CD4+T稱為 Th17細(xì)胞。研究[15]顯示,在小鼠結(jié)腸炎模型中,IL-23通過直接作用于T細(xì)胞誘導(dǎo)其增殖及積聚而促進(jìn)腸道炎癥反應(yīng),同時(shí)IL-23可進(jìn)一步促進(jìn)產(chǎn)生免疫原性IL-17A+INF-γ+的CD4+T細(xì)胞,而抑制FoxP3+Treg細(xì)胞的分化,提示IL-23/Th17軸在IBD發(fā)病過程中起主要調(diào)節(jié)作用。IL-23R的免疫調(diào)節(jié)作用在臨床試驗(yàn)中也得到了證實(shí),IBD患者中檢測到IL-23的高表達(dá),IL-23受體基因突變可增加個(gè)體CD和UC的易感性[16]。
2.2 感染因素 感染因素作為IBD可能的病因一直備受爭議,其中有很多病原菌都與IBD的發(fā)病有關(guān),但目前尚不明確其致病菌。研究[17-18]顯示IBD患者腸道內(nèi)菌群的數(shù)量增加而種類減少,這種菌群失調(diào)是IBD的關(guān)鍵病理改變。很多研究報(bào)道了IBD繼發(fā)于某些感染性疾病,可能的原因是消化道內(nèi)的感染導(dǎo)致腸道內(nèi)菌群失調(diào)進(jìn)而引起IBD[19]。Kullberg等[20]研究顯示,肝螺桿菌在IL-10敲除的免疫缺陷小鼠中可以誘發(fā)腸道炎癥發(fā)生;Kuehl[17]和Whary[18]等研究都顯示,IBD小鼠模型中感染了螺桿菌之后,腸道菌群發(fā)生變化,螺桿菌迅速成為腸道的主導(dǎo)菌群,腸道內(nèi)菌群多樣性降低。近年來尋找IBD致病菌的關(guān)注點(diǎn)是腸腔內(nèi)菌群,研究[21]顯示,腸肝螺桿菌(EHS)包括肝螺桿菌、膽螺桿菌、嚙齒類螺桿菌。目前已從患結(jié)腸炎的絹毛猴和恒河猴中成功分離培養(yǎng)出EHS,研究顯示肝螺桿菌及膽螺桿菌可以在嚙齒類動(dòng)物中引起類似IBD的病變[22];這些細(xì)菌定植在黏液層,距離腸道上皮很近,因此可能在IBD的病變中有一定作用。目前關(guān)于人類IBD中EHS的作用,各種研究報(bào)道的結(jié)果不一致;Bell等[23]應(yīng)用PCR的方法未能在IBD患者中檢測到螺桿菌,而Bohr等[24]在CD患者中檢測到了EHS-DNA,檢測率為12%,而對照組只有4%。有研究通過PCR技術(shù)證實(shí)兒童CD患者的糞便中存在肝腸型和胃型螺桿菌,同時(shí)在1例CD患者的腸道黏膜中發(fā)現(xiàn)了H.pylori[25]。但由于檢測方法及研究設(shè)計(jì)等不同,各種研究報(bào)道的螺桿菌的種類差異很大。對于IBD的發(fā)病,可能的機(jī)理是遺傳易感因素、扳機(jī)事件(可能是感染因素)以及菌群失調(diào)都參與了其發(fā)病[26]。
3.1H.pylori與IBD的關(guān)系 人類正常胃黏膜中有H.pylori定植,遺傳學(xué)證據(jù)表明H.pylori在人類中的定植有悠久的歷史;在長期共同進(jìn)化過程中,H.pylori形成了可以影響機(jī)體固有免疫和適應(yīng)性免疫的能力,這種能力可以保證H.pylori持續(xù)感染[27]。流行病學(xué)顯示人群中H.pylori帶菌狀態(tài)與過敏性疾病、哮喘、肥胖、酸反流性疾病、一些自身免疫性疾病呈負(fù)相關(guān)[28]。IBD在H.pylori感染率低的地區(qū)發(fā)病率高,而在應(yīng)用根除H.pylori療法治療消化性潰瘍的地區(qū),IBD的發(fā)病率呈現(xiàn)穩(wěn)定上升趨勢[1];而且,不斷增多的實(shí)驗(yàn)數(shù)據(jù)提示H.pylori感染后對免疫系統(tǒng)有一定的調(diào)節(jié)作用。
1994年,EL-Omar等[29]最先報(bào)道了H.pylori感染與IBD的負(fù)相關(guān)現(xiàn)象,隨后很多研究[2-5]顯示:IBD患者中的H.pylori感染率比正常人群低;一項(xiàng)納入了30項(xiàng)IBD和H.pylori關(guān)系研究的Meta分析結(jié)果顯示:H.pylori感染可能在一定程度上預(yù)防IBD,IBD患者中H.pylori的感染率是27%,而對照組中為41%[4]。關(guān)于H.pylori感染對IBD的這種“保護(hù)作用”的原因,目前仍沒有一致看法,有學(xué)者認(rèn)為這種感染率的差異與CD患者用藥及其社會(huì)經(jīng)濟(jì)學(xué)因素有關(guān)。但一項(xiàng)納入了74例IBD患者的研究顯示,生物制劑、免疫抑制劑、激素的使用并不影響IBD患者中H.pylori的感染率[30]。有試驗(yàn)檢測了IBD患者中H.pylori感染率隨時(shí)間變化的情況,分析了2002年和2012年H.pylori的感染率,2002年總體的陽性率是35.5%,2012年24%,盡管隨時(shí)間推移感染率有下降的趨勢,但此差異無統(tǒng)計(jì)學(xué)意義(P=0.156)[5]。
3.2H.pylori腸道保護(hù)作用可能的機(jī)制
3.2.1H.pylori可誘導(dǎo)免疫耐受:在小鼠中的研究[27]表明,H.pylori可誘導(dǎo)適應(yīng)性免疫反應(yīng)向免疫耐受方向發(fā)展;免疫耐受可以促進(jìn)H.pylori持續(xù)性感染,同時(shí)可抑制自身侵襲性和過敏性T細(xì)胞反應(yīng),對調(diào)節(jié)性T細(xì)胞產(chǎn)生抑制作用。H.pylori感染后可以通過激活炎癥體及刺激IL-18分泌而促進(jìn)DCs向抗原耐受方向分化。在人群和小鼠中的研究表明:調(diào)節(jié)性T細(xì)胞介導(dǎo)外周免疫耐受,是輔助持續(xù)感染的關(guān)鍵因子,同時(shí)可以保護(hù)機(jī)體免受過敏性疾病和自身免疫性疾病的侵襲。最新的研究表明H.pylori激活靶標(biāo)DCs來誘導(dǎo)耐受,很多研究報(bào)道了H.pylori感染人和動(dòng)物模型時(shí),Tregs和DCs在介導(dǎo)系統(tǒng)性免疫調(diào)節(jié)作用時(shí)有免疫耐受活性;清除DCs可打破免疫耐受,導(dǎo)致機(jī)體對感染的控制增強(qiáng),同時(shí)促進(jìn)了T細(xì)胞介導(dǎo)的免疫反應(yīng)[31]。
3.2.2H.pylori可局限炎癥反應(yīng):研究[32]表明H.pylori感染對鼠傷寒沙門桿菌引起的結(jié)腸炎有保護(hù)作用;在鼠傷寒桿菌和H.pylori混合感染時(shí),H.pylori可通過抑制盲腸部位沙門桿菌特異性Th17的反應(yīng)性而減弱炎癥反應(yīng);這種保護(hù)作用與腸系膜淋巴結(jié)中IL-10水平增高有關(guān),IL-10可以調(diào)控Th17細(xì)胞的分化和激活。有研究[32]認(rèn)為H.pylori對腸炎的保護(hù)作用是由細(xì)菌DNA免疫調(diào)節(jié)序列/免疫激活序列比值增高所致,最近有研究提示H.pylori定植后其基因組在慢性炎癥性疾病中有一定的抗炎作用[33]。
H.pylori活菌和蛋白提取物可有效地改善慢性腸炎的臨床和組織學(xué)表現(xiàn);有研究[34]評估H.pylori及H.pylori提取物是否可以調(diào)節(jié)葡聚糖硫酸鈉(sodium dextran sulfate, DSS)誘導(dǎo)腸炎的嚴(yán)重程度,在DSS誘導(dǎo)的C57BL/6小鼠腸炎模型中分別給予H.pylori及H.pylori提取物處理,結(jié)果顯示H.pylori感染的小鼠腸道炎癥程度明顯較輕,并且很少出現(xiàn)實(shí)質(zhì)上皮細(xì)胞改變,在用H.pylori提取物處理的組中也觀察到類似表現(xiàn)。與對照組相比,活H.pylori和應(yīng)用H.pylori提取物處理組中小鼠的結(jié)腸鏡下評分較低,并且結(jié)腸黏膜中Th1、Th17、IFN-γ、IL-17水平較低;而且在T細(xì)胞轉(zhuǎn)移介導(dǎo)的結(jié)腸炎模型中應(yīng)用H.pylori提取物處理也觀察到了小鼠腸炎緩解結(jié)果,與DSS模型中結(jié)果一致[33]?;頗.pylori及其提取物對腸炎小鼠模型明顯作用的是引起黏液素增加,黏液素主要成分是由杯狀細(xì)胞分泌的MUC2;MUC2在腸道表面形成一種不可溶性的屏障而保護(hù)腸道上皮細(xì)胞免受腸道細(xì)菌的侵襲?;頗.pylori及其提取物對腸炎的保護(hù)作用伴隨MUC2的上調(diào),并與NLRP3炎癥體和IL-18信號通路有關(guān);在H.pylori及其提取物處理組中,與對照組相比MUC2基因轉(zhuǎn)錄水平明顯上調(diào)。腸道內(nèi)轉(zhuǎn)錄因子CDX2可以調(diào)控MUC2的產(chǎn)量[35],在實(shí)驗(yàn)組中其表達(dá)量也明顯增加。TGF-β的表達(dá)與MUC2和CDX2平行,提示TGF-β-CDX2-MUC2軸與這種保護(hù)作用關(guān)系密切;可以推測TGF-β可能在H.pylori對腸炎發(fā)揮的保護(hù)作用中有重要意義[36]。而在未用DSS處理的小鼠中應(yīng)用H.pylori提取物處理,MUC2表達(dá)也會(huì)明顯增加,提示H.pylori提取物誘導(dǎo)的MUC2表達(dá)量的增加不局限于DSS模型,MUC2表達(dá)增加是應(yīng)用H.pylori提取物處理的直接結(jié)果而與DSS誘導(dǎo)的腸炎無關(guān)[33]。
諸多研究報(bào)道了H.pylori可以激活炎癥體和Caspase-1來誘導(dǎo)IL-1β、IL-18的表達(dá)和分泌[37-38];IL-18參與了H.pylori及H.pylori提取物預(yù)防過敏性哮喘的保護(hù)機(jī)制,IL-18的基因多態(tài)性可以影響UC的易感性[39]。在NLRP3、IL-18、IL-18R、調(diào)試蛋白MyD88缺失的小鼠中應(yīng)用DSS誘導(dǎo)腸炎模型,然后給以H.pylori提取物處理。在所有小鼠中H.pylori提取物都沒有表現(xiàn)出對腸炎的保護(hù)作用,在H.pylori提取物發(fā)揮作用過程中NLRP3炎癥體的激活和后續(xù)的IL-18的分泌有重要作用?;頗.pylori或其提取物對IBD的多種實(shí)驗(yàn)?zāi)P捅憩F(xiàn)出了保護(hù)作用,同時(shí)表明活H.pylori對急性鼠傷寒沙門桿菌引起的盲腸結(jié)腸炎有保護(hù)作用[34]。H.pylori在隨后的模型中表現(xiàn)出來的腸道保護(hù)作用可能是其免疫調(diào)節(jié)序列下調(diào)了促炎因子的產(chǎn)量,同時(shí)誘導(dǎo)樹突狀細(xì)胞的免疫耐受[40],H.pylori特異性激活NLRP3炎癥體的激活及隨后IL-18的分泌對這種保護(hù)作用有重要意義
綜上所述,諸多流行病學(xué)和基礎(chǔ)研究提示H.pylori感染與IBD呈負(fù)相關(guān),但這種關(guān)系仍存在一定的爭議。H.pylori的腸道保護(hù)作用在動(dòng)物實(shí)驗(yàn)中已經(jīng)得到了一定的證實(shí),但H.pylori小鼠模型中這種保護(hù)作用的機(jī)制是否適用于人類有待進(jìn)一步探討。急性自限性感染是否足以使IBD發(fā)病,慢性感染是否為IBD炎癥慢性化的必要因素,目前仍不可知;大宗設(shè)計(jì)良好的臨床和基礎(chǔ)研究仍待進(jìn)行。免疫和感染因素是IBD致病過程中的重要環(huán)節(jié),H.pylori對IBD的潛在作用為闡明IBD的致病機(jī)制提供了新的思路和希望,為尋找IBD的特效治療開辟了新的途徑。
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(責(zé)任編輯:李 健)
Relationship between Helicobacter pylori and inflammatory bowel diseases
LIU Chenchen, JIN Jingyi, XU Zhaojun, ZHANG Zhenyu
Department of Gastroenterology, the Affiliated Nanjing Hospital of Nanjing Medical University (the First Hospital of Nanjing), Nanjing 210006, China
Inflammatory bowel disease (IBD) comprises two main conditions: Crohn’s disease (CD) and ulcerative coltis (UC). The incidence of IBD has increased at an alarming rate all over the world, especially in developing countries. Although great advances have been made in the understanding of its pathogenesis, clinically palliation rather than cure is very frequent due to a partial knowledge of its etiology. Studies show genetic, immunological and infective factors play roles in the etiology and pathogenesis of IBD.H.pyloriis the etiological agent for almost all cases of gastric and duodenal ulcers as well as gastric cancer. Recently, a series of epidemiological studies described an inverse association betweenH.pyloriinfection and IBD. The Intestinal protective effects ofH.pylorihave been attributed to its capacity to induce immune tolerance and inhibit inflammation. The prophylactic benefit ofH.pylorifor IBD has great implication in exploring its pathogenesis and effective therapeutic approach for IBD. The relationship betweenH.pyloriinfection and IBD as well as the possible mechanisms was reviewed in this article.
Helicobacter pylori; Inflammatory bowel diseases; Immune tolerance
10.3969/j.issn.1006-5709.2016.03.027
南京市衛(wèi)生局課題(YKK12079)
劉晨晨,在讀碩士研究生,研究方向:幽門螺桿菌。
E-mail: njliucc@163.com
張振玉,主任醫(yī)師,碩士生導(dǎo)師,研究方向:幽門螺桿菌。
E-mail: njzzy808@163.com
R574
A
1006-5709(2016)03-0333-04
2015-04-08