李文靜,劉光輝
(華中科技大學(xué)附屬同濟(jì)醫(yī)院過敏反應(yīng)科,武漢 430030)
日本柳杉(Cryptomeriajaponica, Japanese cedar)(以下簡稱柳杉)為多年生常綠喬木,屬松柏門松柏綱松柏目柏科落羽杉亞科日本柳杉屬,是該屬的唯一樹種。柳杉廣泛分布于我國和日本,歐洲、北美、尼泊爾、印度等國也多有種植。在日本,柳杉覆蓋率可達(dá)國土面積的18%,每到花粉季節(jié),這些樹木所產(chǎn)生的大量花粉長距離播散(播散距離可100 km以上),導(dǎo)致日本國內(nèi)超過13%的人口產(chǎn)生過敏,出現(xiàn)全國范圍的花粉癥[1-2],且其發(fā)病率還有逐年上升的趨勢[3]。
柳杉與其他柏科植物一樣,花粉中含有大量的多糖成分及色素、苯酚等雜質(zhì),浸液蛋白產(chǎn)率僅為4.1‰[2]。其蛋白含量雖然很低,但變應(yīng)原蛋白成分卻很復(fù)雜,除了公認(rèn)的2種主要變應(yīng)原蛋白組分Cry j 1和Cry j 2外[4],還含有其他多種可與柳杉花粉癥患者血IgE結(jié)合的成分。目前,通過二維電泳及免疫印記等方法共發(fā)現(xiàn)了131種可與IgE結(jié)合的蛋白組分,其中IgE結(jié)合率高于Cry j 1 IgE結(jié)合力最低異構(gòu)體的有52種,相對分子量為35000~97000,等電點(diǎn)為3~10;高于Cry j 2 IgE反應(yīng)性最高異構(gòu)體的有31種,相對分子量為35000~97000,等電點(diǎn)為4~10[5]。隨著研究的深入,這些成分已被分離、純化并鑒別,如Cry j 3,CJP-6,CJP-4,CJP-8及CPA9等。
Cry j 1是最先被分離鑒別的柳杉花粉變應(yīng)原蛋白組分,發(fā)現(xiàn)初期時(shí)被稱為Sugi堿性蛋白(sugi basic protein,SBP)。SBP為熱敏感蛋白,等電點(diǎn)約為9,100 g干花粉可分離出25 mg SBP[2]。1993年,Cry j 1的cDNA及氨基酸序列被正式發(fā)布[6]。重組Cry j 1含有374個(gè)氨基酸殘基,相對分子量在異構(gòu)體間略有差異,為41000~46000。分子中推定含有4個(gè)糖基化位點(diǎn)和3個(gè)雙硫鍵。Cry j 1的部分氨基酸序列(RMPRARYGLVHVANNNY)與細(xì)菌果膠裂解酶的一致性較高,可達(dá)49%~57%[7];相關(guān)試驗(yàn)也表明,Cry j 1具有鈣離子依賴的果膠裂解酶活性[8];Cry j 1在花粉成熟期表達(dá)并定位于花粉外壁[9]。由此推測,Cry j 1的功能可能是在授粉過程中,通過降解雌花而促進(jìn)花粉管生長[10-11]。Cry j 1異構(gòu)體種類較多,目前共發(fā)現(xiàn)了12種[5,12]。異構(gòu)體間僅個(gè)別氨基酸有差異,分子量大小主要由N端糖鏈所決定。每種異構(gòu)體的IgE結(jié)合率不同,相對分子量42000、等電點(diǎn)為8.3的異構(gòu)體結(jié)合率最高,約為75%[5]。Cry j 1與其他植物變應(yīng)原蛋白組分間存在明顯交叉反應(yīng)性,與Amb a 1、Amb a 2的序列一致率分別為65%和53%[8];其與同科植物變應(yīng)原蛋白組分一致性更高,如Jun a 1(78.9%)、Cha o 1(79-80%)。這種高一致性,在柳杉與其他植物的交叉反應(yīng)方面發(fā)揮了重要作用[13-14]。與柳杉其他變應(yīng)原組分不同,Cry j 1的表達(dá)水平主要由植株克隆決定,氣象等因素影響較小[15]。
Cry j 1是一種糖蛋白,分子中有4個(gè)糖基化位點(diǎn)。糖基成分研究顯示,Cry j 1的糖基是一系列結(jié)合于共同核心的α-甘露糖殘基上具有雙觸角結(jié)構(gòu)的葡萄糖胺分支,共同核心為Manα1-6(Manα1-3)(Xylβ1-2)Manβ1-4GlcNAcβ1-4(Fucαl-3)GlcNAc。變異只出現(xiàn)于葡萄糖胺分支的巖藻糖化和半乳糖苷化[16]。此外,Cryj 1還含有(Galβ1-3(Fucα1-4)GlcNAcβ1-)這種Lewis a表位,該糖基表位可能與細(xì)胞間的信息交流相關(guān)[17]。針對糖基免疫性的研究則顯示,Cry j 1的N端糖基只具有免疫原性,不具有變應(yīng)原性,但可能對蛋白部分構(gòu)象表位的形成具有一定的意義[16]。其與外周血單個(gè)核細(xì)胞(peripheral blood mononuclear cells,PBMCs)抗原特異性輔助T細(xì)胞(T helper cell,Th)2反應(yīng)相關(guān),可能參與了變應(yīng)性炎性反應(yīng),并能影響柳杉花粉癥的嚴(yán)重程度[18]。但花粉過敏患者對糖基部分的反應(yīng)并不一致,糖基部分也不包含主要的T細(xì)胞表位,因此這些糖基可能是通過與抗原提呈細(xì)胞(antigen presenting cells,APCs)的糖類特異性受體結(jié)合,加強(qiáng)抗原的呈遞,從而促進(jìn)患者的Th2反應(yīng)[19-20]。
Cry j 2是第2個(gè)被分離和鑒別的主要變應(yīng)原蛋白組分,其與Cry j 1之間無明顯交叉反應(yīng)[12]。天然Cry j 2相對分子量為37000,等電點(diǎn)8.6~9.2[2,21-22]。重組Cry j 2前體蛋白有514個(gè)氨基酸殘基,相對分子量56600,而成熟的重組Cry j 2蛋白有只有388個(gè)氨基酸殘基,相對分子量42200。推測是由前體蛋白切割下45個(gè)N端氨基酸殘基和81個(gè)C端氨基酸殘基而形成成熟Cry j 2[23]。Cry j 2也是糖蛋白,分子中有2個(gè)糖基化位點(diǎn)。Cry j 2的cDNA序列與番茄多聚半乳糖醛酸酶的一致性高[24],經(jīng)試驗(yàn)證實(shí)Cry j 2也確有聚甲基半乳糖醛酸酶活性。此外,這種蛋白主要存在于花粉的造粉體中。造粉體也稱造粉質(zhì)體,主要分布于儲(chǔ)藏組織如子葉、胚乳、塊莖和塊根等,是貯存淀粉的質(zhì)體,是植物細(xì)胞內(nèi)碳水化合物的臨時(shí)“倉庫”。因此,Cry j 2的生物功能可能是與Cry j 1共同在授粉過程中通過降解雌花而促進(jìn)花粉管的生長[24]。與Cry j 1一樣,Cry j 2也有異構(gòu)體存在,且目前已發(fā)現(xiàn)了3種[14,25]。這3種異構(gòu)體與患者血IgE的結(jié)合率并不一致,波動(dòng)于32.5%~40%。Cry j 2的表達(dá)在不同地區(qū)及不同克隆植株間都有差異,這種差異主要來自于轉(zhuǎn)錄水平的變化[25],但具體影響和調(diào)節(jié)因素目前尚不清楚。
多數(shù)柳杉花粉癥患者對Cry j 1和Cry j 2的反應(yīng)既有特異性IgE升高[26],又有T細(xì)胞活化[27-28]。因此,2種蛋白的分子結(jié)構(gòu)中既存在T細(xì)胞表位也存在B細(xì)胞表位。目前,T細(xì)胞表位已發(fā)現(xiàn)7個(gè),Cry j 1有3個(gè)——p212-224、p235-247、p312-330,Cry j 2有4個(gè)——p77-89、p96-107、p192-204、p356-367[29]。B細(xì)胞表位只發(fā)現(xiàn)1個(gè),是Cry j 2的124KWVNGREI131,這個(gè)表位不僅能被人IgE識別,還能被猴和鼠IgE識別[30]。有學(xué)者將上述數(shù)個(gè)T細(xì)胞表位組合起來合成雜交多肽,這些肽段保留了各表位的免疫原性和致耐受性,有望下調(diào)柳杉特異性T細(xì)胞,抑制病理性T細(xì)胞反應(yīng),從而應(yīng)用于柳杉花粉癥的特異性免疫治療中[31-32]。
Cry j 3含有多種異構(gòu)體,目前已發(fā)現(xiàn)的有8種(Cry j 3.1~3.8),部分異構(gòu)體內(nèi)部又存在變異型,因此,Cry j 3實(shí)際上是一個(gè)小型的多基因蛋白家族。除Cry j 3.8為天然狀態(tài)下分離所得外,其余Cry j 3.1~3.7均由cDNA克隆發(fā)現(xiàn)[33-35]。Cry j 3的異構(gòu)體性質(zhì)各異,Cry j 3.1和3.2是堿性蛋白,3.4和3.6則是酸性蛋白。其異構(gòu)體各存在部位也不同,Cry j 3.4和3.6可能定位于液泡內(nèi),其他異構(gòu)體則可能與分泌途徑相關(guān)[34]。Cry j 3各種異構(gòu)體在植株不同部位的表達(dá)水平各異,Cry j 3.5多表達(dá)于花粉中,其余異構(gòu)體則多表達(dá)于成熟雌性球果,雄性球果中也有表達(dá)[34]。除此之外,植物根部也有Cry j 3蛋白表達(dá),其他部位如子葉、葉片、莖等則表達(dá)稀少[33]。植物激素對各種異構(gòu)體的調(diào)控作用也不同,如Cry j 3.4可由水楊酸、乙烯磷誘導(dǎo)表達(dá)增加,3.3和3.6則對這2種物質(zhì)無類似反應(yīng)[34]。
天然狀態(tài)下的Cry j 3(Cry j 3.8)相對分子量為27000,含有225個(gè)氨基酸殘基。Cry j 3.8與患者血清IgE結(jié)合率為27%,其IgE結(jié)合力為Cry j 1的115,是Cry j 2的110。但部分患者的IgE結(jié)合力也可與Cry j 2相當(dāng)[35]。Cry j 3各種異構(gòu)體均具有索馬甜結(jié)構(gòu)域及保守的半胱氨酸(Cys)殘基,為索馬甜樣蛋白(Thaumatin-like proteins, TLPs),屬于病程相關(guān)蛋白(pathogenesis-related protein,PR)-5家族[34]。TLPs在植物種子、果實(shí)、花朵等組織發(fā)育中發(fā)揮著多種作用,一些特殊的TLPs還可保護(hù)植物免受滲透壓變化、病原攻擊、低溫冷凍等多種理化刺激因素的影響[36-37]。同時(shí),其表達(dá)水平在理化環(huán)境等因素(如紫外線B輻射、氯化鈉、3-苯三唑、花生四烯酸等)的作用下還會(huì)出現(xiàn)升高,導(dǎo)致植物組織中Cry j 3含量隨時(shí)間及地域等因素變化[34-35]。TLPs廣泛存在于各種花粉及水果中[38-39],且多數(shù)具有變應(yīng)原性,同家族變應(yīng)原蛋白組分間交叉反應(yīng)性很高,如Cry j 3和Jun a 3,其序列一致性最高可達(dá)85.8%,最低也達(dá)到了42.5%。因此,這類蛋白包括Cry j 3成為了臨床花粉癥和口腔變態(tài)反應(yīng)綜合癥中重要的致敏泛變應(yīng)原[40]。
天然CJP-6 相對分子量為30000,與患者血IgE結(jié)合率高達(dá)76%。重組CJP-6含有306個(gè)氨基酸殘基,相對分子量為33573,含有2個(gè)可能的糖基化位點(diǎn)——Nos21-23和208-210。CJP-6與植物源性異黃酮還原酶家族(isoflavone reductase,IFR)同源性高,與松樹苯基二氫香豆素芐基還原酶的序列一致性高達(dá)73%,與大豆及土豆中IFR家族蛋白的一致性也分別達(dá)到62%和60%[41]。此外,IFR家族中還含有多種植物花粉及水果變應(yīng)原,如Bet v 5 及Pyr c 5等[42],因此CJP-6也可能在植物種間交叉反應(yīng)方面發(fā)揮了一定作用。
CJP-4含有281個(gè)氨基酸殘基,相對分子量為29286,等電點(diǎn)為4.51,與患者血IgE結(jié)合率很高,變性狀態(tài)下為52.5%,天然狀態(tài)下高達(dá)100%,因此,CJP-4是一種非常重要的變應(yīng)原蛋白組分,具有很高的臨床意義[43]。CJP-4熱穩(wěn)定性好。與植物來源IV類殼多糖酶同源性高,序列一致率達(dá)43.1%~65.6%[44]。相關(guān)研究也證實(shí),CJP-4確實(shí)具有內(nèi)切殼多糖酶活性。此外,CJP-4還與乳膠變應(yīng)原具有交叉反應(yīng)。因此,CJP-4可能也是柳杉花粉中重要的泛變應(yīng)原成分[43],在柳杉花粉與其他植物變應(yīng)原的交叉反應(yīng)中發(fā)揮著重要作用。
CJP-8含有165個(gè)氨基酸殘基,相對分子量為17409,等電點(diǎn)為7.39。重組CJP-8相對分子量為20500,與患者血IgE結(jié)合率為37.5%。CJP-8序列中含有8個(gè)保守的半胱氨酸殘基,這些殘基形成4個(gè)二硫鍵,這些是脂質(zhì)轉(zhuǎn)運(yùn)蛋白(lipid transfer proteins, LTPs)的特征性結(jié)構(gòu),因此,CJP-8屬于LTPs家族[45]。植物非特異性LTPs是植物變應(yīng)原蛋白的重要組成部分,廣泛存在于食物、乳膠及花粉中[46-47],可導(dǎo)致各種植物種屬間交叉反應(yīng),誘發(fā)口腔變態(tài)反應(yīng)綜合征等臨床癥狀。
CPA9含有757個(gè)氨基酸殘基,相對分子量為80148.79,等電點(diǎn)為6.22。天然CPA9與患者血IgE結(jié)合率高達(dá)88.5%,提示其也是柳杉花粉的主要變應(yīng)原蛋白組分之一[48]。CPA9屬于枯草桿菌樣絲氨酸蛋白酶家族(亞家族S8A)[49],與同家族的甜瓜變應(yīng)原Cuc m 1同源性高[50],序列一致率40.1%,相似率達(dá)到55.1%。因此,CPA9及枯草桿菌樣絲氨酸蛋白酶家族在植物間的交叉反應(yīng)性方面應(yīng)該也發(fā)揮了一定的作用。
綜上所述,柳杉花粉蛋白含量雖然很低,但其變應(yīng)原蛋白成分復(fù)雜、種類多樣。目前分離鑒別的變應(yīng)原蛋白已有7種,但仍尚存多種未能鑒別出的成分。然而,即使是這7種變應(yīng)原蛋白,也在空間結(jié)構(gòu)、生物功能、具體發(fā)揮作用的肽段表位、糖基成分及功能等方面存在許多未解決的問題,需要繼續(xù)進(jìn)行更廣泛及深入的研究來明確。
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