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，研究者们最近发现，在病原体与宿主“你捉我藏”游戏的过程之中，有一种病毒似乎能够在一种已知基因沉默机制的掩护下躲避免疫细胞的探测和攻击。
哈佛休斯医学研究中心(HHMI)的研究者在2005年6月2日的《自然》中发表了一份相关报告，这也许是科学家们首次能够说明病毒如何利用基因沉默机制来达到感染宿主的目的。
在人体、植物、以及寄生虫体内，只要数百个小分子RNA就能够通过干扰较大的信息核糖核酸(mRNAs)来使特定的基因保持沉默，而且这种干扰会使mRNAs无法产生蛋白质。科学家们还不知道这种小分子RNA究竟能够沉默那些基因，但是新的研究成果已经提供了越来越多的证据，表明这种小分子不仅在人体内起着重要的作用，而且还能够对细胞产生影响。
“人们普遍认为整个小核糖核酸（small RNAs）生成系统都是细胞为了抵抗细菌进攻而设立的防线。但是我们的研究显示，有一种病毒能够通过适应这一系统来来躲避免疫系统的攻击，” 来自美国加利福尼亚州立大学的Don Ganem说，他是哈佛休斯医学研究中心的研究人员之一。
Ganem已经对病毒如何感染人体并且引起疾病进行了研究。而他的同事Adam Grundhoff——博士后学者，同时也是该报告的作者之一——认为，当科学家们发现RNA的干扰可能是一种基本而普遍的基因调节机制时， “那么有一个事实很明显，那就是这样一种基本调节途径难免会被病毒所盗用。”
来自洛克菲勒大学的Thomas Tuschl是新任哈佛休斯医学研究中心的研究员，之前他已经发现了由EB病毒（Epstein-Barr virus）编译成的几种小分子RNA，但是并没有发现它们的具体功能。Grundhoff 与首席作者之一的Christopher Sullivan——Ganem实验室的博士后研究人员之一——合作开始进行研究来寻找这些受到一种被称为SV40的小病毒所含有的病毒性小分子RNA，他们相信这种病毒微小的体积有助于他们研究任何可能会发现的小分子RNA的功能。
SV40是一种相对无害的猴子病毒，它能够感染猿类宿主的肾脏器官。但是对于啮齿动物来说，它却能够引发癌症。虽然SV40的基因组也在某些人类的肿瘤中也发现过，但是对于它是否会引发人类癌症这一问题仍在讨论之中。这种病毒一般被作为模式系统，并且对“基因如何作用”的科学研究做出过极大的贡献。
为了开始进行研究，Grundhoff编写了一个计算机程序将SV40基因屏蔽，来避免可能出现的小分子RNA前体细胞。小分子RNA是带有特殊hairpin folds的信息RNA分子。这种发夹状的结构被分成小分子RNA段，然后与另外一组染色体共同产生影响，然后沉默另外一种带有互补序列的信息核糖核酸。
在科学家们预测的几十种小分子RNA中，表现最为丰富的RNA出现在那些被SV40感染的人类细胞中。
很快，Sullivan发现受到SV40感染的小分子RNA的目标——一种被称为T抗原（T antigen）的信息RNA，并且使其分裂。“SV40也许是人类科学家研究的最多的病毒，” Sullivan说。“而T抗原是科学家在对这种病毒研究中接触最多的东西。”
当SV40病毒进入细胞时会产生T抗原，它的作用是触发病毒DNA繁殖。对于这种病毒来说不幸的是，T抗原同样也是免疫（T）细胞的攻击对象，也就是说，免疫细胞将摧毁受到感染的细胞来防止病毒进一步扩展。
以T 抗原为攻击目标的小分子RNA在感染周期的最后环节才会产生，而这时T 抗原已经不再是病毒繁殖的必须条件。进一步的试验显示，细胞毒素免疫细胞更有可能去摧毁那些无法产生小RNA的变异病毒而不是普通的病毒。因此，小RNA引发T抗原表达减少，从而使病毒避免受到抗病毒T细胞的攻击，并且得以继续发展。
“核糖核酸宿主的干扰机制长期以来都被认为是由抗病毒机制的形式进化而来的，而病毒居然能够利用这种机制产生小分子RNA来达到自身的目的——”研究者在《自然》刊登的报告中称。
