李志祥 翟璐 宋佰芬
摘 要:牛皰疹病毒1型(Bovine Herpesviruses 1,BHV-1)和牛皰疹病毒5型(Bovine Herpesviruses,BHV-5)是引起牛呼吸道疾病的2大重要病原,其膜糖蛋白D(Glycoprotein D,gD)是病毒粒子的重要組成成分,在皰疹病毒的發(fā)病機(jī)理方面發(fā)揮重要作用。該文介紹了BoHV-1、BoHV-5的結(jié)構(gòu)和功能特征,以及它在病毒進(jìn)入宿主細(xì)胞和與宿主細(xì)胞受體相互作用中的作用,討論了gD與宿主免疫系統(tǒng)的相互作用,并結(jié)合該蛋白的結(jié)構(gòu)和功能特點(diǎn),闡述了其在新疫苗設(shè)計(jì)中的應(yīng)用。
關(guān)鍵詞:皰疹病毒;gD糖蛋白;結(jié)構(gòu)特點(diǎn);應(yīng)用
中圖分類號(hào) S855.3 文獻(xiàn)標(biāo)識(shí)碼 A 文章編號(hào) 1007-7731(2020)07-0016-07
Abstract: The viral envelope glycoprotein D from bovine herpesviruses 1 and 5 BoHV-1 and-5), two important pathogens of cattle, is a major component of the virion and plays a critical role in the pathogenesis of herpesviruses. Glycoprotein D is essential for virus penetration into permissive cells and thus is a major target for virus neutralizing antibodies during infection. This review highlights the structural and functional characteristics of BoHV-1, BoHV-5 and where appropriate, Human herpesvirus gD, as well as its role in viral entry and interactions with host cell receptors. Furthermore, the interactions of gD with the host immune system are discussed. Finally, the application of this glycoprotein in new vaccine design is reviewed, taking its structural and functional characteristics into consideration.
Key words: Herpesviruses; Envelope glycoprotein D; Structural Characteristics; Applications
皰疹病毒是一類具有包膜、多樣的病毒家族,分為α、β和γ3個(gè)亞科。α皰疹病毒亞科具有幾個(gè)共同的特征,包括快速的繁殖周期,具有入侵神經(jīng)元的能力和在感覺(jué)神經(jīng)節(jié)中建立潛伏性感染的特性[1]。該家族還包括人類皰疹病毒,如人類皰疹病毒(HHV)1型和2型,以及動(dòng)物皰疹病毒。感染反芻動(dòng)物的α皰疹病毒以牛皰疹病毒1型(BoHV-1)較為常見(jiàn),而牛皰疹病毒5型(BoHV-5)在獸醫(yī)學(xué)中也具有重要意義[2]。
BoHV-1是引起牛傳染性鼻氣管炎(infectious bovine rhinotracheitis,IBR)和傳染性膿皰外陰陰道炎(infectious pustular vulvovaginitis, IPV)的病原體,也是牛呼吸系統(tǒng)疾病復(fù)合體(bovine respiratory disease complex,BRD)的病原之一,給養(yǎng)牛業(yè)帶來(lái)巨大的經(jīng)濟(jì)損失。BoHV-1感染可導(dǎo)致呼吸道嚴(yán)重?fù)p傷,為繼發(fā)性細(xì)菌感染創(chuàng)造了機(jī)會(huì)[3-4]。BoHV-5感染途徑和BoHV-1相同,即通過(guò)鼻黏膜,眼睛,口咽和生殖道黏膜感染而發(fā)生。病毒首先在入侵的上皮細(xì)胞中進(jìn)行增殖,然后擴(kuò)散到神經(jīng)元等部位[5]。雖然BoHV-5和BoHV-1在遺傳和抗原特性上有很大的關(guān)聯(lián)性,氨基酸序列也有82%的同源性[6],但它們?cè)谌肭稚窠?jīng)和神經(jīng)毒性方面的特性不同。BoHV-1的神經(jīng)侵犯通常不會(huì)超過(guò)三叉神經(jīng)節(jié)的一級(jí)神經(jīng)元,并在神經(jīng)元位置建立潛伏性感染,而BoHV-5則可以感染大腦的不同區(qū)域,尤其在青壯年動(dòng)物中引起致命的腦炎[7]。
疫苗接種是預(yù)防和控制這些病毒性疾病最有效的方式之一?,F(xiàn)有的BoHV-1減毒活疫苗和滅活疫苗雖有一定效果,但在安全性和有效性方面仍存在缺陷,該疫苗不適合接種一些特定的動(dòng)物,如懷孕的奶牛[8]。BoHV-1和BoHV-5疫苗開發(fā)的新戰(zhàn)略主要集中在標(biāo)記疫苗(也被稱為DIVA疫苗),以此來(lái)區(qū)分受感染動(dòng)物和已接種動(dòng)物。DIVA疫苗包括基因工程缺失病毒(如gE缺失病毒)以及基于病毒包膜糖蛋白如gD的亞單位疫苗或載體疫苗。BoHV-1gE缺失標(biāo)記活疫苗已經(jīng)研發(fā)出來(lái),并在小牛體內(nèi)進(jìn)行了測(cè)試,攻毒結(jié)果表明,該疫苗具有免疫保護(hù)作用,能夠減少BoHV-1病毒滴度且對(duì)免疫原性沒(méi)有任何影響[9-10]。在BoHV-1感染率高的國(guó)家,gE缺失標(biāo)記活疫苗,已被用于這一疾病的根除計(jì)劃[11],該疫苗的臨床試驗(yàn)也已開展[12]。2007年,在3個(gè)歐洲國(guó)家進(jìn)行的一項(xiàng)研究表明,接種疫苗的動(dòng)物血清轉(zhuǎn)化率有所下降,這表明了疫苗接種對(duì)遏制病毒的傳播是有效的[13]。然而,使用BoHV-1gE-標(biāo)記活疫苗對(duì)BoHV-5引起的腦炎的保護(hù)作用有限,當(dāng)免疫動(dòng)物受到BoHV-5病毒威脅時(shí),只能減輕臨床癥狀,但對(duì)隱藏在鼻腔內(nèi)的病毒及發(fā)生的腦炎病變無(wú)影響[14-15]。因此,在BoHV-5流行率較高的地區(qū)使用BoHV-1gE?標(biāo)記活疫苗可能是不合適的。
2007年,F(xiàn)ranco等[15]報(bào)道構(gòu)建了BoHV-5gI/gE/US9缺失毒株。2010年進(jìn)行了一系列以構(gòu)建病毒作為滅活疫苗的試驗(yàn),結(jié)果顯示,以高劑量的BoHV-5強(qiáng)毒株挑戰(zhàn)后,該疫苗能保護(hù)實(shí)驗(yàn)動(dòng)物免受腦炎的侵害,同時(shí)鼻腔內(nèi)隱藏的病毒量也顯著減少。雖然BoHV-1gE標(biāo)記活疫苗已成功應(yīng)用,但是這個(gè)疫苗仍然存在一系列問(wèn)題。甚至是一些批次的疫苗污染了牛病毒性腹瀉病毒(bovine viral diarrhea virus,BVDV),導(dǎo)致BVDV的爆發(fā)。
糖蛋白D(gD)是病毒囊膜表面抗原之一,它參與病毒進(jìn)入細(xì)胞的過(guò)程,被認(rèn)為是牛和人皰疹病毒疫苗研發(fā)的主要靶標(biāo),這主要是因?yàn)樗軌虼碳C(jī)體產(chǎn)生體液免疫和細(xì)胞免疫反應(yīng)[16]?;谶@一糖蛋白的亞單位疫苗,DNA疫苗和載體疫苗已經(jīng)研制成功,并輔以不同的佐劑進(jìn)行了評(píng)價(jià)。本文從牛皰疹病毒的結(jié)構(gòu)、功能和免疫特性等方面綜述了其在病毒發(fā)病機(jī)制中的作用及其在牛皰疹病毒疫苗中的應(yīng)用。
1 gD的結(jié)構(gòu)
皰疹病毒科是復(fù)合病毒粒子結(jié)構(gòu)病毒的典型代表。病毒粒子由二十面體衣殼包裹大的雙鏈DNA基因組成,衣殼又由脂質(zhì)雙層膜所包裹,大量的糖蛋白鑲嵌在脂質(zhì)雙層膜中[17]。α皰疹病毒進(jìn)入細(xì)胞是一個(gè)復(fù)雜的過(guò)程,至今仍沒(méi)有清晰的結(jié)論。這些病毒作為一個(gè)自由粒子感染細(xì)胞,然后通過(guò)細(xì)胞間的接觸向鄰近的細(xì)胞擴(kuò)散。關(guān)于病毒進(jìn)入宿主細(xì)胞的過(guò)程目前認(rèn)為有2種機(jī)制:一是病毒囊膜的融合,二是細(xì)胞內(nèi)吞作用[18-19]。已知被鑒定的12種包膜糖蛋白中,至少有6種(gC、gB、gD、gH、gK和gL)參與病毒的附著和進(jìn)入宿主細(xì)胞的過(guò)程。α皰疹病毒和宿主細(xì)胞的結(jié)合是由于gC或gB與宿主細(xì)胞表面的硫酸乙酰肝素蛋白聚糖可逆性吸附[20]。事實(shí)上,雖然gC缺失突變體也能感染細(xì)胞,但是只有當(dāng)gC附著在細(xì)胞表面受體上時(shí),α皰疹病毒的感染性才會(huì)增強(qiáng)[21],這表明這種附著本身不足以使病毒進(jìn)入宿主細(xì)胞,而是需要其他輔助因子才能進(jìn)入細(xì)胞。病毒包膜和宿主細(xì)胞的細(xì)胞質(zhì)膜融合需要其他4種糖蛋白(gD,gB和gHgL復(fù)合體),其中g(shù)D結(jié)合到細(xì)胞表面受體之一是病毒進(jìn)入細(xì)胞所必須的,然后gB和gH-gL之間會(huì)發(fā)生一系列相互作用,并伴隨有膜融合的發(fā)生[22]。融合機(jī)制的另一個(gè)組成部分是gK,它也與gB相互作用,通過(guò)結(jié)合其氨基末端,并改變?cè)撎堑鞍捉閷?dǎo)“病毒包膜到細(xì)胞膜融合”的能力[22]。盡管該蛋白編碼基因的缺失對(duì)細(xì)胞培養(yǎng)中的復(fù)制并不是致命的,但卻降低了病毒的效價(jià),這表明gK缺陷的病毒粒子進(jìn)入細(xì)胞的過(guò)程受損[23]。
BoHV-1和BoHV-5的糖蛋白D由417個(gè)氨基酸組成,其中79.9%的氨基酸結(jié)構(gòu)已經(jīng)鑒定,它們是N、O連接的寡糖,分子量約為71 kDa[24,25]。它屬于I型膜糖蛋白,含有信號(hào)肽序列,BoHV-1gD的裂解位點(diǎn)在18和19位氨基酸之間,BoHV-5 gD的裂解位點(diǎn)在19和20位氨基酸之間[26]。信號(hào)肽被裂解后產(chǎn)生的成熟蛋白是399個(gè)氨基酸。gD的氨基端包含胞外結(jié)構(gòu)域,羧基端由疏水跨膜錨定序列和胞質(zhì)尾部組成,BoHV-1gD的胞質(zhì)尾部約28個(gè)氨基酸、BoHV-5gD的胞質(zhì)尾部約35個(gè)氨氨基酸[25,27]。該糖蛋白基因核苷酸序列的GC含量為70%[25],和豬皰疹病毒1型(SuHV-1)(75%)及HHV-1(65%)的gD同源物相似[28]。BoHV-1和BoHV-5gD氨基酸序列在其胞外區(qū)保留了6個(gè)半胱氨酸殘基,表明這些殘基之間可能是由二硫鍵連接,可能在維持適當(dāng)?shù)?D結(jié)構(gòu)及其功能中發(fā)揮作用[25]。2種gDs比對(duì)結(jié)果顯示,BoHV-1和BoHV-5[26]gD的氨基端2/3(1-282位氨基酸)相對(duì)保守。2種gDs圖譜的不同之處在于接近跨膜區(qū)胞外分子的C端(280和330位氨基酸),該區(qū)域是一個(gè)富含甘氨酸的伸展部位[24,26]。在BoHV-1gD中,這個(gè)區(qū)域有1個(gè)親水峰,對(duì)于蛋白質(zhì)與病毒的其他分子、宿主細(xì)胞或離子之間[27]的相互作用非常重要。BoHV-5gD的這個(gè)區(qū)域在氨基酸序列上與BoHV-1gD序列有幾處不匹配,從281位氨基酸到295位氨基酸存在一系列帶負(fù)電荷的殘基,從而產(chǎn)生一系列的親水峰,這和BoHV-1gD的1個(gè)寬峰形成對(duì)比[26]。BoHV-1gD有4個(gè)抗原結(jié)構(gòu)域,5個(gè)表位,其中3個(gè)表位相互關(guān)聯(lián),2個(gè)表位相互獨(dú)立,據(jù)報(bào)道它們是中和抗體的靶點(diǎn)[27-28]。HHV-1和BoHV-1gD有3個(gè)N-CHO位點(diǎn)(BoHV-1gD氨基酸位點(diǎn)分別為41,102和411),而BoHV-5gD僅在胞質(zhì)尾部有2個(gè)N-CHO位點(diǎn)(BoHV-5gD氨基酸位點(diǎn)分別為102和411)[26]。N-CHO對(duì)gD的重要性在于其糖基化對(duì)蛋白質(zhì)結(jié)構(gòu)和抗原性質(zhì)的影響,而不是直接與細(xì)胞受體相互作用的影響[29]。BoHV-1gD的酶促脫糖基顯示:添加碳水化合物可能會(huì)掩蓋參與T細(xì)胞識(shí)別的表位。目前為止,已被證明是gD細(xì)胞表面受體的有連接蛋白-1、皰疹病毒進(jìn)入介導(dǎo)者(HVEM)和硫酸肝素。連接蛋白-1是gD在上皮細(xì)胞和神經(jīng)元細(xì)胞上的主要受體[30],可以介導(dǎo)HHV-1和-2以及SuHV-1(偽狂犬病病毒或PRV)、BoHV-1和BoHV-5進(jìn)入宿主細(xì)胞[31]。而表達(dá)硫酸肝素鹽修飾的3-o-磺酸轉(zhuǎn)移酶或HVEM受體的細(xì)胞對(duì)BoHV-1不敏感[32]。連接蛋白是免疫球蛋白(Ig)樣超家族中與鈣無(wú)關(guān)的親細(xì)胞黏附分子,聚集在上皮細(xì)胞的黏著連接、突觸和神經(jīng)元的連接點(diǎn)上[33]。所有的連接蛋白都有一個(gè)胞外結(jié)構(gòu)域,該結(jié)構(gòu)域由3個(gè)免疫球蛋白樣結(jié)構(gòu)域(V-C1-C2)、1個(gè)跨膜結(jié)構(gòu)和1個(gè)胞質(zhì)尾部區(qū)域組成。gD與連接蛋白的結(jié)合區(qū)位于V樣結(jié)構(gòu)域中[34]。研究發(fā)現(xiàn),從HHV-1、HHV-2和SuHV-1中提取可溶性的gD能有效地與被截?cái)嗟膬H保留V樣結(jié)構(gòu)域的連接蛋白-1結(jié)合,這種結(jié)合可被該區(qū)域表位的特異性單克隆抗體所阻斷[35]。
gD在BoHV-1和BoHV-5感染初期的作用及其在病毒包膜中的豐富度使其成為宿主免疫系統(tǒng)的靶標(biāo)。宿主針對(duì)這些病毒的免疫反應(yīng)可分為由中性粒細(xì)胞、巨噬細(xì)胞、自然殺傷細(xì)胞(NK細(xì)胞),自然殺傷T細(xì)胞(NKT細(xì)胞)和樹突細(xì)胞(DC細(xì)胞)所介導(dǎo)的先天行免疫反應(yīng)和由B和T淋巴細(xì)胞所介導(dǎo)的適應(yīng)性反應(yīng)。對(duì)病毒感染免疫反應(yīng)的第一步是避免病毒與敏感細(xì)胞之間的相互作用,然而,在初次感染期間,抗體無(wú)法干擾他們之間的相互作用。因此,免疫系統(tǒng)對(duì)BoHV-1感染的第一反應(yīng)將包括非特異性炎癥反應(yīng)和細(xì)胞介導(dǎo)的免疫反應(yīng),gD和gB是NK細(xì)胞[36]的主要靶點(diǎn)。在感染后期,先天性免疫反應(yīng)會(huì)引起適應(yīng)性免疫反應(yīng)。在牛體內(nèi),gC和gD是CD8+T淋巴細(xì)胞的主要靶標(biāo),gD所特有的多肽能刺激CD4+T淋巴細(xì)胞[37]。如上文所言,抗體反應(yīng)不能阻止病毒在細(xì)胞間的傳播,而且在初次感染的恢復(fù)階段也不能被檢測(cè)到抗體。但是,當(dāng)再次接觸病毒時(shí),抗體在預(yù)防感染方面則起著關(guān)鍵作用,因?yàn)樗鼈兡苤泻图?xì)胞外病毒,并阻止病毒傳播到相鄰的動(dòng)物。非中和抗體也可能與PMNs共同作用,通過(guò)抗體依賴性細(xì)胞毒性(ADCC)引起B(yǎng)oHV-1感染細(xì)胞裂解[38]。因此,主要糖蛋白不但參與誘導(dǎo)產(chǎn)生中和抗體,而且還刺激細(xì)胞介導(dǎo)的免疫反應(yīng),同樣它們也是CTL和ADCC作用的靶點(diǎn)。由于這些原因,針對(duì)BoHV-1和-5的新疫苗策略應(yīng)重點(diǎn)關(guān)注這些主要糖蛋白,特別是gD。
2 基于gD疫苗的設(shè)計(jì)
2.1 亞單位疫苗 亞單位疫苗由一種或多種純的或半純抗原組成。為了開發(fā)亞單位疫苗,鑒定保護(hù)性抗原的單個(gè)組分非常重要。這種疫苗可以通過(guò)常規(guī)技術(shù)生產(chǎn),如純化病原體產(chǎn)生的蛋白質(zhì)。為了開發(fā)有效對(duì)抗BoHV-1感染的安全疫苗,可以直接從病毒感染細(xì)胞中純化病毒包膜糖蛋白gB、gC和gD,保留它們的抗原活性,誘導(dǎo)牛產(chǎn)生中和抗體,并對(duì)強(qiáng)毒BoHV-1的感染產(chǎn)生保護(hù)作用。雖然這3種糖蛋白均能誘導(dǎo)產(chǎn)生中和抗體,但gD所誘導(dǎo)產(chǎn)生的中和抗體滴度最高,而且免疫動(dòng)物的ADCC水平也最高[39-40]。雖然從受感染細(xì)胞中純化的天然糖蛋白保留了它們的抗原特性,但這種方法成本較高。使用重組DNA技術(shù)大量生產(chǎn)蛋白質(zhì)用于疫苗生產(chǎn)可能滿足安全性和經(jīng)濟(jì)性的要求。重組BoHV-1gD已經(jīng)在多個(gè)表達(dá)系統(tǒng)中進(jìn)行表達(dá),包括原核表達(dá)系統(tǒng)和真核表達(dá)系統(tǒng)。原核系統(tǒng)雖然存在操作簡(jiǎn)便,成本低,有可能實(shí)現(xiàn)大量的蛋白質(zhì)生產(chǎn)等優(yōu)點(diǎn),但在病毒糖蛋白修飾方面有一些缺點(diǎn)。就像前面所提及的gD結(jié)構(gòu)構(gòu)象依賴于其正確的三維折疊,至少部分依賴于其插入內(nèi)質(zhì)網(wǎng)和添加碳水化合物。原核系統(tǒng)不具備gD折疊所需的翻譯后加工的細(xì)胞機(jī)制,這一點(diǎn)已經(jīng)得到證實(shí)。盡管大腸桿菌表達(dá)的重組gD(rgD)誘導(dǎo)的總抗體水平很高,但其中只有一小部分能夠中和病毒[41]。由于重組gD在原核生物中表達(dá)效率較低,一些研究者對(duì)酵母、哺乳動(dòng)物、植物和昆蟲細(xì)胞等真核表達(dá)系統(tǒng)進(jìn)行了嘗試:將BoHV-1 gD在畢赤酵母中進(jìn)行分泌表達(dá),或與牛白細(xì)胞介素(IL)-6嵌合表達(dá),均可誘導(dǎo)小鼠產(chǎn)生中和抗體[42,43]。此外,也有學(xué)者在畢氏酵母中表達(dá)了去除跨膜錨定序列的BoHV-5gD[44]。表達(dá)蛋白和油佐劑混合后免疫小鼠和牛,在小鼠和牛中均能誘導(dǎo)產(chǎn)生中和抗體。利用煙草花葉病毒(TMV)載體在植物細(xì)胞中表達(dá)了非糖基化的BoHV-1gD。將該蛋白用油佐劑混合后免疫小鼠和牛,也能誘導(dǎo)產(chǎn)生體液免疫反應(yīng)和細(xì)胞免疫反應(yīng),但當(dāng)用病毒感染后似乎只有部分動(dòng)物得到了保護(hù),但病毒中和試驗(yàn)沒(méi)有進(jìn)行評(píng)估[45]。
雖然酵母、植物和昆蟲細(xì)胞能夠產(chǎn)生具有一定真實(shí)性的gD,但大多數(shù)報(bào)道都是MDBK細(xì)胞在誘導(dǎo)型牛熱休克70A基因啟動(dòng)子(HSP70啟動(dòng)子)控制下產(chǎn)生的分泌型的gD(也稱為tgD)[46]。據(jù)證明,當(dāng)BoHV-1gD在MDBK細(xì)胞中組成性表達(dá)時(shí),高水平的BoHV-1gD對(duì)細(xì)胞是有毒害作用的,只有當(dāng)BoHV-1gD置于誘導(dǎo)型啟動(dòng)子控制下或在基礎(chǔ)水平表達(dá)時(shí),才能建立穩(wěn)定的表達(dá)細(xì)胞系[47]。但這一問(wèn)題可以通過(guò)去除跨膜錨定序列得以解決,并能產(chǎn)生與全長(zhǎng)gD具有同等免疫原性的分泌型gD[48-49]。BoHV-1tgD與多種佐劑及輔助佐劑聯(lián)合應(yīng)用,通過(guò)不同途徑誘導(dǎo)全身及粘膜免疫應(yīng)答。其中最有效的方法之一是將含有CpG的寡脫氧核苷酸(ODN)與經(jīng)典佐劑相結(jié)合。細(xì)胞介導(dǎo)的免疫應(yīng)答對(duì)保護(hù)機(jī)體免受多種病原體的侵襲是至關(guān)重要的,盡管目前所有獲得許可的疫苗都能有效地誘導(dǎo)抗體應(yīng)答,但只有經(jīng)過(guò)修飾的活疫苗才能有效地誘導(dǎo)細(xì)胞介導(dǎo)的免疫反應(yīng)。但是,當(dāng)亞單位疫苗與適當(dāng)?shù)淖魟┡浞綍r(shí),由于交叉提呈可能會(huì)誘發(fā)細(xì)胞介導(dǎo)的免疫反應(yīng)。tgD與明礬和CpG ODN的配方(甚至單獨(dú)與CpG ODN的tgD)能在犢牛體內(nèi)誘導(dǎo)產(chǎn)生較強(qiáng)的中和抗體和細(xì)胞介導(dǎo)的免疫反應(yīng),從而使其抵抗BoHV-1的挑戰(zhàn)[50]。和常規(guī)佐劑相比,CpG ODN聯(lián)合油佐劑能誘導(dǎo)較強(qiáng)的Th1型免疫反應(yīng),增加IFN-γ的產(chǎn)生,或平衡免疫反應(yīng)[51]。另一方面,針對(duì)BoHV-5的實(shí)驗(yàn)疫苗,當(dāng)僅用油基佐劑配制重組的BoHV-5gD接種小鼠時(shí),觀察到混合的Th1/Th2免疫反應(yīng)。這誘導(dǎo)產(chǎn)生了IFN-γ以及促炎細(xì)胞因子,如IL-17和集落刺激因子(GM-CSF)[25]。
2.2 DNA疫苗 DNA疫苗又稱核酸疫苗或基因疫苗,是指將編碼某種蛋白質(zhì)抗原的重組真核表達(dá)載體直接注射到動(dòng)物體內(nèi),使外源基因在活體內(nèi)表達(dá),產(chǎn)生的抗原激活機(jī)體的免疫系統(tǒng),從而誘導(dǎo)特異性的體液免疫和細(xì)胞免疫應(yīng)答。DNA疫苗之所以受到關(guān)注,是因?yàn)樗a(chǎn)生的免疫反應(yīng)與自然感染引起的免疫反應(yīng)極為相似,是由病毒蛋白和糖蛋白內(nèi)源性產(chǎn)生的[52]。最早開發(fā)的BoHV-1DNA疫苗是基于pRSV質(zhì)粒設(shè)計(jì)的,該質(zhì)粒包含勞斯肉瘤病毒的轉(zhuǎn)錄控制序列,并有較廣的宿主細(xì)胞范圍[53]。有研究者將BoHV-1 3種糖蛋白(gD,gB or gC)以肌肉注射的方式免疫小鼠,并對(duì)小鼠體內(nèi)產(chǎn)生的抗體滴度進(jìn)行了測(cè)試,結(jié)果發(fā)現(xiàn)gD的中和抗體滴度遠(yuǎn)高于gB或gC[54]。用gD的DNA疫苗肌內(nèi)免疫后,體液免疫反應(yīng)較低。用人巨細(xì)胞病毒(HCMV)立即早期啟動(dòng)子/增強(qiáng)子取代RSV啟動(dòng)子后,將分泌表達(dá)的gD(tgD)進(jìn)行皮內(nèi)免疫,從而改善了免疫應(yīng)答。在2004年的另一項(xiàng)研究中,將編碼截?cái)嘈虰oHV-1gD的DNA疫苗以肌肉、皮內(nèi)和鼻內(nèi)3種不同的途徑免疫動(dòng)物,結(jié)果只有肌肉注射的才能產(chǎn)生中和抗體,并在感染早期清除病毒[55]。將編碼分泌型gD和gB的質(zhì)粒通過(guò)鼻內(nèi)途徑免疫小鼠,可誘導(dǎo)產(chǎn)生高水平的中和抗體[56]。將多個(gè)拷貝的CpG基序整合到tgD編碼的質(zhì)粒中,也引起淋巴細(xì)胞增殖和細(xì)胞免疫,在BoHV-1攻毒后,有較高的中和效價(jià)和較少的病毒排出,但臨床癥狀無(wú)明顯差異[57]。將編碼tgD的質(zhì)粒與牛CD154結(jié)合,經(jīng)皮內(nèi)免疫接種后,該質(zhì)粒能與皮膚內(nèi)的表達(dá)有CD40的樹突狀細(xì)胞結(jié)合。與單純的tgD相比,tgD- CD154并沒(méi)有增強(qiáng)免疫應(yīng)答或?qū)膊〉谋Wo(hù)[58]。
2.3 載體疫苗 載體疫苗是指利用基因工程的技術(shù),將病原的基因克隆到另一個(gè)安全無(wú)致病能力的病毒或細(xì)菌載體中,從而能將目的基因攜帶到細(xì)胞中并進(jìn)行表達(dá),最終產(chǎn)生很好免疫效果的疫苗。這種疫苗的主要優(yōu)點(diǎn)之一是有可能將抗原直接送到粘膜表面,并能誘導(dǎo)體液和細(xì)胞介導(dǎo)的免疫反應(yīng)。由于牛腺病毒3型(BAV-3)能夠在牛的呼吸道中復(fù)制,同時(shí)產(chǎn)生輕微或無(wú)臨床癥狀,并在細(xì)胞培養(yǎng)中生長(zhǎng)達(dá)到較高滴度[59],因此一些研究者將BAV-3作為載體來(lái)表達(dá)gD蛋白并免疫牛。經(jīng)鼻內(nèi)免疫后,具有復(fù)制能力的BAV-3表達(dá)了全長(zhǎng)gD或tgD,產(chǎn)生了gD特異性免疫應(yīng)答,包括鼻分泌物中的IgA。然而,免疫動(dòng)物在BoHV-1強(qiáng)毒株感染后雖得到部分保護(hù),但I(xiàn)gA反應(yīng)的強(qiáng)度仍不足以消除隱匿的BoHV-1[60]。通過(guò)氣管內(nèi)或皮下途徑免疫牛只也觀察到同樣的反應(yīng)模式[61]。人腺病毒5型(HAdV-5)也被用作病毒載體來(lái)傳遞BoHV-1糖蛋白。表達(dá)gD的HAdV-5以鼻內(nèi)途徑單獨(dú)或聯(lián)合表達(dá)gC的HAdV-5共同免疫動(dòng)物均可誘導(dǎo)中和抗體的產(chǎn)生,且gD抗體滴度遠(yuǎn)高于gC抗體滴度,肌肉注射表達(dá)gD的HAdV-5也觀察到同樣的結(jié)果,但中和抗體滴度低于鼻內(nèi)免疫所產(chǎn)生的抗體滴度[62]。這些實(shí)驗(yàn)都沒(méi)有進(jìn)行細(xì)胞免疫反應(yīng)的評(píng)估。最近,有學(xué)者將山羊皰疹病毒1型(CpHV-1)的gD在牛皰疹病毒4型(BoHV-4)的基因組中進(jìn)行表達(dá)。攜帶CpHV-1gD基因的重組BoHV-4皮下注射給實(shí)驗(yàn)動(dòng)物,用相應(yīng)的病毒進(jìn)行攻毒后可保護(hù)試驗(yàn)動(dòng)物不出現(xiàn)臨床癥狀,并減少感染后病毒的排出[63]。雖然這增強(qiáng)了gD疫苗刺激宿主免疫系統(tǒng)反應(yīng)、減少了病毒引起的臨床癥狀和病毒的排出,但在其他α皰疹病毒如HHV-1和HHV-2上并不一定成功。當(dāng)用HHV-1和HHV-2的gD抗原作為抗原疫苗時(shí),也能誘導(dǎo)免疫應(yīng)答,調(diào)節(jié)臨床保護(hù)和減少病毒的排出。用哺乳動(dòng)物細(xì)胞中表達(dá)的HHV-1gD蛋白免疫豚鼠時(shí),誘導(dǎo)產(chǎn)生了針對(duì)HHV-1和HHV-2的中和抗體,用HHV-2經(jīng)陰道感染后并沒(méi)有出現(xiàn)臨床癥狀[64]。然而,在人體內(nèi)進(jìn)行試驗(yàn)時(shí)并沒(méi)有出現(xiàn)類似的結(jié)果[65]。HHV-2gD聯(lián)合gB和MF59佐劑雖然能誘導(dǎo)HHV-2特異性中和抗體的產(chǎn)生,但對(duì)HHV-2只能產(chǎn)生部分和短暫的臨床保護(hù)[66]。使用該gD加上明礬和MPL佐劑時(shí),該疫苗具有良好的耐受性,能保護(hù)預(yù)先沒(méi)有HHV-1抗體的女性免受感染,但未能保護(hù)男性和先前已經(jīng)感染HHV-1的女性[67-68]。2012年,有試驗(yàn)再次表明,注射2次疫苗后,對(duì)HHV-1和HHV-2引起的生殖系統(tǒng)疾病的保護(hù)效果不明顯,而注射3次后則對(duì)HHV-1有效,對(duì)HHV-2無(wú)效[69]。隨后對(duì)疫苗的給藥與HHV-1疾病預(yù)防的關(guān)聯(lián)性進(jìn)行了研究,顯示的結(jié)果可能是由于疫苗中使用的HHV-2gD區(qū)域與HHV-1gD區(qū)域具有同源性。雖然該疫苗刺激產(chǎn)生了中和抗體,但這些抗體未能保護(hù)HHV-2引起的臨床癥狀,正如學(xué)者所建議,這可能意味著需要開發(fā)新的疫苗策略,以刺激更高的抗體滴度來(lái)應(yīng)對(duì)HHV-2的感染[70]。最近的HHV-2gD試驗(yàn)也表明,需要適當(dāng)刺激細(xì)胞免疫以保護(hù)機(jī)體免受HHV引起的疾病。小鼠或豚鼠實(shí)驗(yàn)表明,與單獨(dú)的gD相比,HHV-2gD與立即早期蛋白如ICP27或ICP4聯(lián)合使用,更能加強(qiáng)T細(xì)胞免疫應(yīng)答,進(jìn)而增強(qiáng)臨床保護(hù)水平[71]。
3 結(jié)語(yǔ)
α-皰疹病毒gD在病毒與細(xì)胞的相互作用中起重要作用,具有很強(qiáng)的免疫原性。在過(guò)去幾十年中,gD一直作為候選疫苗來(lái)研究,只要融入一些新的免疫策略,就可以臨床應(yīng)用。了解gD的特性以及這種糖蛋白在疫苗研究中所取得的成就,對(duì)于進(jìn)一步開發(fā)新方法以減少BoHV-1和BoHV-5在世界范圍內(nèi)對(duì)養(yǎng)牛業(yè)造成的經(jīng)濟(jì)損失具有重要意義?,F(xiàn)已證明gD亞單位疫苗是非常有效的,但存在費(fèi)用昂貴的缺點(diǎn)。這可以通過(guò)添加有效佐劑等配方的方式來(lái)克服,特別是添加CpG ODN和苦杏仁酶等制品,從而降低抗原劑量,使疫苗更便宜。有文獻(xiàn)報(bào)道,雖然編碼gD的DNA疫苗適用于誘導(dǎo)細(xì)胞介導(dǎo)的免疫應(yīng)答,但仍需要改進(jìn)才能實(shí)現(xiàn)細(xì)胞免疫和體液免疫應(yīng)答之間的平衡。這意味著需要開發(fā)更好的傳遞系統(tǒng),以提高免疫動(dòng)物體內(nèi)的轉(zhuǎn)染效率和抗原表達(dá)水平。gD載體疫苗在粘膜免疫誘導(dǎo)方面具有特別的優(yōu)勢(shì),但也需要改進(jìn)。病毒中和抗體的產(chǎn)生是BoHV-1和BoHV-5臨床保護(hù)的關(guān)鍵因素之一。這表明,在目前階段,用佐劑配制的gD亞單位疫苗可促進(jìn)免疫反應(yīng)的平衡,能滿足誘導(dǎo)強(qiáng)的體液免疫和細(xì)胞免疫的要求。
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(責(zé)編:張 麗)