Virus Uses Tiny RNA to Evade the Immune System
In the latest version of the hide-and-seek game between pathogens and the hosts they infect, researchers have found that a virus appears to cloak itself with a recently discovered gene silencing device to evade detection and destruction by immune cells.
The report by Howard Hughes Medical Institute (HHMI) researchers in an article published in the June 2, 2005, issue of Nature may be the first to show how a virus uses the gene silencing machinery for its own infectious purposes.
"A popular notion is that the whole system of generating small RNAs was designed to be a defense by cells against viruses. Our study shows that a virus can also adapt it to evade the immune response." -- Donald Ganem
In people, plants, and worms, hundreds of tiny RNA molecules can silence specific genes by interfering with larger messenger RNAs (mRNAs). That interference prevents mRNAs from making proteins. Scientists do not know which genes are hushed by the microRNAs in people, but the new study bolsters growing evidence that the little molecules can play important roles not only in normal human cells but in infected cells as well.
“A popular notion is that the whole system of generating small RNAs was designed to be a defense by cells against viruses. Our study shows that a virus can also adapt it to evade the immune response,” said HHMI investigator Don Ganem, who is at University of California, San Francisco.
Ganem studies how viruses infect people and cause disease. When scientists found that RNA interference appeared to be a basic and widespread gene regulatory mechanism, “it became clear that such a fundamental pathway could of course be pirated by a virus,” said postdoctoral fellow Adam Grundhoff, co-first author of the paper.
Thomas Tuschl, a newly selected HHMI investigator at The Rockefeller University, had already reported the existence of several microRNAs encoded by Epstein-Barr virus, although their functions were unknown. Grundhoff and co-first author Christopher Sullivan, a postdoctoral fellow in Ganem's lab, started their search for viral microRNAs with a small virus, known as SV40, in the belief that its diminutive size would make it easier to understand the functions of any microRNAs they found.
SV40 is a relatively harmless monkey virus that can cause kidney infections in its natural simian host. In rodents, however, it can cause cancer. Although the SV40 genome has been found in some human tumors, its role in human cancer has been debated. The virus is better known as a model system that has greatly contributed to major scientific advances about how genes work.
To launch their study, Grundhoff wrote a computer program to screen the SV40 genome for possible microRNA precursors. MicroRNAs are made from messenger RNA molecules with distinctive hairpin folds. The hairpin structure is diced into a microRNA segment that works with another complex to disable other messenger RNAs with complementary sequences.
Among several dozen predicted microRNAs, the top candidate turned out to be abundantly expressed in human cells infected with SV40.
Sullivan soon found the target of the plentiful SV40 microRNA. It effectively targeted the messenger RNA for a protein known as T antigen, leading to its cleavage. “SV40 may be the world's most studied virus,” Sullivan said, “and T antigen is its most studied part.”
When SV40 enters a cell, it produces T antigen, which functions to trigger viral DNA replication. Unfortunately for the virus, T antigen also serves as a target for immune (T) cells, which can destroy infected cells and prevent the virus from spreading.
Conveniently, the microRNA that targets T antigen is made late in the infectious cycle, just when T antigen is no longer essential for virus replication. Further experiments showed that cytotoxic immune cells were more likely to kill cells infected with a mutant virus that cannot make the microRNA than the normal virus. Thus, microRNA-induced reductions in T antigen expression promote escape from antiviral T cells without affecting virus growth.
“Viruses can use the host RNA inference machinery, which is often speculated to have evolved as an antiviral mechanism, to generate small RNAs that serve their own purposes — the latest chapter in the long cat-and-mouse game known to virologists as host-virus coevolution,” the researchers conclude in their Nature article.
hhmi.org6月2日消息,，研究者們最近發(fā)現(xiàn)，在病原體與宿主“你捉我藏”游戲的過程之中,，有一種病毒似乎能夠在一種已知基因沉默機制的掩護下躲避免疫細胞的探測和攻擊,。
哈佛休斯醫(yī)學研究中心(HHMI)的研究者在2005年6月2日的《自然》中發(fā)表了一份相關報告，這也許是科學家們首次能夠說明病毒如何利用基因沉默機制來達到感染宿主的目的,。
在人體,、植物、以及寄生蟲體內,，只要數百個小分子RNA就能夠通過干擾較大的信息核糖核酸(mRNAs)來使特定的基因保持沉默,，而且這種干擾會使mRNAs無法產生蛋白質?？茖W家們還不知道這種小分子RNA究竟能夠沉默那些基因,，但是新的研究成果已經提供了越來越多的證據，表明這種小分子不僅在人體內起著重要的作用,，而且還能夠對細胞產生影響,。
“人們普遍認為整個小核糖核酸（small RNAs）生成系統(tǒng)都是細胞為了抵抗細菌進攻而設立的防線。但是我們的研究顯示,，有一種病毒能夠通過適應這一系統(tǒng)來來躲避免疫系統(tǒng)的攻擊,，” 來自美國加利福尼亞州立大學的Don Ganem說，他是哈佛休斯醫(yī)學研究中心的研究人員之一,。
Ganem已經對病毒如何感染人體并且引起疾病進行了研究,。而他的同事Adam Grundhoff——博士后學者，同時也是該報告的作者之一——認為,，當科學家們發(fā)現(xiàn)RNA的干擾可能是一種基本而普遍的基因調節(jié)機制時,， “那么有一個事實很明顯，那就是這樣一種基本調節(jié)途徑難免會被病毒所盜用,。”
來自洛克菲勒大學的Thomas Tuschl是新任哈佛休斯醫(yī)學研究中心的研究員,，之前他已經發(fā)現(xiàn)了由EB病毒（Epstein-Barr virus）編譯成的幾種小分子RNA，但是并沒有發(fā)現(xiàn)它們的具體功能,。Grundhoff 與首席作者之一的Christopher Sullivan——Ganem實驗室的博士后研究人員之一——合作開始進行研究來尋找這些受到一種被稱為SV40的小病毒所含有的病毒性小分子RNA,，他們相信這種病毒微小的體積有助于他們研究任何可能會發(fā)現(xiàn)的小分子RNA的功能。
SV40是一種相對無害的猴子病毒,，它能夠感染猿類宿主的腎臟器官,。但是對于嚙齒動物來說,，它卻能夠引發(fā)癌癥。雖然SV40的基因組也在某些人類的腫瘤中也發(fā)現(xiàn)過,，但是對于它是否會引發(fā)人類癌癥這一問題仍在討論之中。這種病毒一般被作為模式系統(tǒng),，并且對“基因如何作用”的科學研究做出過極大的貢獻,。
為了開始進行研究，Grundhoff編寫了一個計算機程序將SV40基因屏蔽,，來避免可能出現(xiàn)的小分子RNA前體細胞,。小分子RNA是帶有特殊hairpin folds的信息RNA分子。這種發(fā)夾狀的結構被分成小分子RNA段,，然后與另外一組染色體共同產生影響,，然后沉默另外一種帶有互補序列的信息核糖核酸。
在科學家們預測的幾十種小分子RNA中,，表現(xiàn)最為豐富的RNA出現(xiàn)在那些被SV40感染的人類細胞中,。
很快，Sullivan發(fā)現(xiàn)受到SV40感染的小分子RNA的目標——一種被稱為T抗原（T antigen）的信息RNA,，并且使其分裂,。“SV40也許是人類科學家研究的最多的病毒，” Sullivan說,。“而T抗原是科學家在對這種病毒研究中接觸最多的東西,。”
當SV40病毒進入細胞時會產生T抗原，它的作用是觸發(fā)病毒DNA繁殖,。對于這種病毒來說不幸的是,，T抗原同樣也是免疫（T）細胞的攻擊對象，也就是說,，免疫細胞將摧毀受到感染的細胞來防止病毒進一步擴展,。
以T 抗原為攻擊目標的小分子RNA在感染周期的最后環(huán)節(jié)才會產生，而這時T 抗原已經不再是病毒繁殖的必須條件,。進一步的試驗顯示,，細胞毒素免疫細胞更有可能去摧毀那些無法產生小RNA的變異病毒而不是普通的病毒。因此,，小RNA引發(fā)T抗原表達減少,，從而使病毒避免受到抗病毒T細胞的攻擊，并且得以繼續(xù)發(fā)展,。
“核糖核酸宿主的干擾機制長期以來都被認為是由抗病毒機制的形式進化而來的,，而病毒居然能夠利用這種機制產生小分子RNA來達到自身的目的——”研究者在《自然》刊登的報告中稱。
