隨著精準醫療概念的提出,更加嚴謹地對隱匿性乙型肝炎病毒感染人群進行定義、診療及管理,將對HBV的防控有重要的意義。今天小編為大家推出魯鳳民,廖昊,劉永振,竇曉光在中華預防醫學雜誌2019年第5期發表的題為《隱匿性乙型肝炎病毒感染再認識》的文章,筆者就隱匿性乙型肝炎病毒感染的發生機制、流行趨勢和定義的演變進行了詳細探討,並提出對其定義的理解以及檢測的改進方法。
題目:隱匿性乙型肝炎病毒感染再認識
111212中國醫科大學附屬盛京醫院感染科,瀋陽 1100216通信作者:魯鳳民,Email:[email protected],電話:010?82805136
摘要:
關鍵詞 :乙型肝炎病毒;隱匿性感染;病毒複製;整合
DOI:10.3760/cma.j.issn.0253?9624.2019.05.002
乙型肝炎病毒(hepatitis B virus,HBV)感染是我國病毒性肝炎及相關肝臟疾病的主要原因之一。乙型肝炎病毒表面抗原(hepatitis B surface antigen,HBsAg)是HBV感染的主要血清學標誌物,血清HBsAg檢出陽性即可診斷為HBV感染,持續檢測陽性超過6個月則可被診斷為慢性HBV感染。慢性HBV感染除了HBsAg陽性外,血清中往往還能檢測到乙型肝炎病毒e抗原(hepatitis B e antigen,HBeAg)或其抗體,病毒基因組DNA,以及乙型肝炎病毒核心抗體(hepatitis B core antibody,抗-HBc)。其中抗-HBc在感染病毒被清除後仍可長期存在,單獨抗-HBc陽性往往被認為是既往感染。隨著檢測技術的完善、特別是核酸檢測靈敏度的提高,人們在臨床實踐過程中發現,有些個體的血清用商品化的試劑盒不能檢測出HBsAg,但其肝臟或血液中卻可以檢測出低水準的HBV DNA,這一現象稱為隱匿性HBV感染(occult hepatitis B virus infection,OBI)。OBI不僅可以造成臨床診斷失誤,與之相關的持續性的輕微的肝細胞炎症壞死,可導致慢性肝病進展,長期積累可導致肝硬化和肝癌發生。在免疫機能下降或進行免疫抑製治療時可能導致OBI患者HBV的復燃;而且,OBI患者是潛在的HBV傳染源,可通過輸血、器官移植和母嬰密切接觸等方式進行傳播。
一、OBI的可能發生機制和流行現狀
對於OBI發生機制,人們提出了如下幾種可能:(1)HBV基因突變:首先S區突變可導致抗原決定簇構象的改變,從而導致檢測的失敗,尤其是主要親水區中"α"決定簇的突變,如G145R、Q129R和M133T,均可導致蛋白構象的改變
一般OBI患者都曾經有既往HBV感染、甚至慢性乙型肝炎史,可能由於病毒變異、治療或宿主因素導致HBsAg無法檢出。一項研究發現,在HBV較低流行的法國,OBI的檢出率是1.2%(47/3 966),所有OBI檢出抗-HBc陽性
二、OBI認識的演變
自1978年Hoofnagle等[三、HBV的複製周期及基因組形式
HBV基因組結構是由長鏈L(負鏈)和短鏈S(正鏈)組成的不完全雙鏈鬆弛環狀DNA(relaxed circular DNA,rcDNA),基因組全長約3.2 kb,是目前已知的感染人類的最小DNA病毒。HBV DNA序列有四個開放讀碼框(open reading frame,ORF):Pre-S/S、Pre-C/C、Pol和X,分別編碼HBsAg(大蛋白,中蛋白,小蛋白)、HBeAg和核心蛋白(hepatitis B core antigen,HBcAg)、P蛋白和X蛋白。在病毒感染過程中,進入細胞內的HBV核衣殼脫去衣殼,HBV rcDNA進入細胞核並在細胞DNA聚合酶等的作用下,修復形成完整的雙鏈超螺旋的共價閉合環狀DNA(covalently closed circular DNA,cccDNA)。HBV病毒複製存在獨特的逆轉錄過程,轉錄自cccDNA的超基因組全長的3.5 kb前基因組RNA(pregenomic RNA,pgRNA)既能翻譯合成HBcAg和P蛋白,又可作為病毒逆轉錄合成病毒負鏈DNA的模板。在核衣殼內P蛋白以pgRNA為模板,通過其逆轉錄酶活性逆轉錄出全長的病毒負鏈DNA,與此同時,pgRNA在P蛋白RNA酶H活性作用下被降解;之後病毒以新合成的負鏈DNA為模板,在P蛋白DNA聚合酶活性作用下再合成互補的正鏈DNA。成熟的核衣殼即可獲得包含HBsAg的外包膜,裝配成完整子代病毒顆粒,以丹氏顆粒的形式釋放至肝細胞外形成子代病毒,這些新合成的rcDNA也可重新進入細胞核補充cccDNA池。在過去很長的一段時間內,人們一直在嘗試建立可靠的cccDNA特異的檢測方法,這些方法主要基於酶切消化rcDNA加跨雙缺口的PCR擴增策略四、HBV整合對OBI診斷的干擾
目前已知,HBV的DNA有多種存在形式。在具有完全感染活性丹氏顆粒中,病毒基因組以rcDNA形式存在,在感染肝細胞核內則以cccDNA存在。cccDNA既是病毒複製的模板,其持續存在也是HBV慢性感染的病毒學基礎。HBV雙鏈線性DNA(double stranded linear DNA,dslDNA)則是病毒發生整合的主要形式,但由於其線性特徵,整合的HBV DNA失去了形成超過基因組長度的pgRNA的能力,不能作為子代病毒複製的模板。而rcDNA既是子代病毒丹氏顆粒基因組的存在形式,也是補充和維持cccDNA池的來源。據此,我們認為,只要能夠將失去病毒DNA複製能力的整合DNA片段與具有產生子代HBV基因組能力的cccDNA或rcDNA區分開來即可。近年來,人們逐漸意識到,HBV DNA的宿主基因組整合是病毒感染早期發生的事件RcDNA:鬆弛環狀雙鏈DNA;cccDNA:共價閉合環狀DNA;
HBV:乙型肝炎病毒;dslDNA:雙鏈線性DNA;DR:直接重複
序列;「-」:HBV負鏈DNA;「+」:HBV正鏈DNA
儘管依賴於肝組織的rcDNA或cccDNA的OBI診斷準確可靠,但由於肝穿是有創檢查,而OBI人群癥狀較輕,在臨床實踐過程中難以廣泛通過肝穿來診斷OBI。單獨抗-HBc陽性是OBI人群最主要血清學表現形式之一,約佔OBI人群的80%七、小結
總之,隨著核酸檢測技術以及新血清學指標的發展,對OBI的認識和檢測手段有了新的變化。血清HBsAg陰性,肝組織中HBV DNA陽性的OBI定義略微模糊,隨著精準醫療概念的提出,我們建議OBI的定義也應該更加嚴謹的被定義為:排除HBV窗口期感染,按照現有血清學檢測技術檢測HBsAg陰性,但肝組織中HBV rcDNA或cccDNA陽性,對應血清HBV DNA低於檢測下限或低值陽性(<200 IU/ml)。OBI人群的診療及管理,對於HBV的防控有重要的意義。
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