virology blog

About viruses and viral disease

rhinovirus

Interfering with interferon

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During a discussion about blogging on the Coast to Coast Bio Podcast, it was suggested that science professors should spend more time writing about their research – by explaining what problems they are trying to solve, how they approach them, and why they are interesting. My goal here at virology blog is mainly to teach virology. But explaining what we do in my virology laboratory can be an effective instructional tool.

About five years ago I became very interested in the innate immune response to viral infections. The innate response is considered the first line of immune defense because it is active even before infection begins. Many viral infections are halted by the innate immune system, which responds very quickly – within minutes to hours after infection.

The key to innate defenses are cellular proteins – toll-like receptors and cytoplasmic molecules – that can detect viruses and turn on pathways that lead to the synthesis of the antiviral interferons (IFN).

In 2005, I attended a virology meeting in Italy where I learned about two cellular proteins that can sense the presence of viral nucleic acids. I found it absolutely amazing that these cytoplasmic proteins – called MDA-5 and RIG-I – can detect viral RNA as a foreign molecule. When these proteins sense viral RNA, they interact with a mitochondrial protein called IPS-1, which then initiates a signal transduction cascade.

When I returned from that meeting I immediately began experiments to understand how our favorite viruses – poliovirus, rhinovirus, and encephalomyocarditis virus (EMCV) – interact with the innate defense system. To our surprise, we found that RIG-I, MDA-5, and IPS-1 are all degraded in cells infected with these viruses. The following figure shows sensing of viral RNAs (green) by these proteins, and the pathways that lead to IFN synthesis. The red arrowheads show which proteins are cleaved in virus-infected cells.

interfering-ifn

What does this mean? By eliminating these cell proteins, viruses could prevent the innate immune system from functioning properly – they could block synthesis of IFN. But we don’t have any evidence to support this hypothesis. RIG-I, MDA-5, and IPS-1 disappear quite late in infection, perhaps too late to impact IFN production.

A few days after our paper on the cleavage of RIG-I was published, another group reported similar findings. They found that in cells infected with EMCV, RIG-I is cleaved. In mouse cells lacking the RIG-I protein, normal amounts of IFN is produced after EMCV infection. They postulated that the presence of MDA-5 allowed viral infection to be sensed by the innate immune system. Next, they infected MDA-5-deficient cells with EMCV. In this case, no IFN was produced. This result made sense, they reasoned, because not only was MDA-5 absent, but RIG-I was cleaved by viral infection. To support this hypothesis, they infected cells lacking MDA-5 with EMCV, then added back new RIG-I protein to these cells. The addition of RIG-I lead to the synthesis of IFN, supporting their idea that both MDA-5 and RIG-I are important for sensing EMCV infection.

These results are still not conclusive – they do not prove that cleavage of MDA-5 and RIG-I antagonize the innate immune response. To prove this point, it is necessary to produce altered forms of the proteins that cannot be degraded in virus-infected cells. If cleavage of these proteins benefits viral replication, then synthesis of cleavage-resistant forms should lead to production of higher levels of IFN and reduced viral yields in infected cells.

I’m not sure if the results of these experiments would be worth the 6-9 months required for completion. We already know dozens of ways that viruses antagonize the innate immune system. I don’t believe that adding a few more mechanisms will substantially advance our knowledge of how viruses block immune responses. More importantly, I don’t think that these experiments are ‘fundable’. That is, it would be hard to convince the NIH to provide the money to do the experiments. There are other more interesting experiments that I’d like to do. But we’ll save a description of those for another time.

Drahos J, & Racaniello VR (2009). Cleavage of IPS-1 in cells infected with human rhinovirus. Journal of Virology PMID: 19740998

Barral, P., Sarkar, D., Fisher, P., & Racaniello, V. (2009). RIG-I is cleaved during picornavirus infection Virology, 391 (2), 171-176 DOI: 10.1016/j.virol.2009.06.045

Barral, P., Morrison, J., Drahos, J., Gupta, P., Sarkar, D., Fisher, P., & Racaniello, V. (2007). MDA-5 Is Cleaved in Poliovirus-Infected Cells Journal of Virology, 81 (8), 3677-3684 DOI: 10.1128/JVI.01360-06

Papon L, Oteiza A, Imaizumi T, Kato H, Brocchi E, Lawson TG, Akira S, & Mechti N (2009). The viral RNA recognition sensor RIG-I is degraded during encephalomyocarditis virus (EMCV) infection. Virology PMID: 19733381

Is an effective treatment for the common cold at hand?

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2289334120_c27ed36206_mThe sequence of all known rhinovirus genomes reported in Science last week is an important advance for the field. Analyses of the sequences have revealed new relationships among the viruses, evidence for recombination, a new viral species, and conserved regions of the genome. These findings will be extremely valuable for those studying the biology, pathogenesis, and epidemiology of the common cold. But the press has over reacted to this work –  it was reported on the front page of the New York Times with the headline “Cure for the Common Cold? Not Yet, but Possible”.  Does the work deserve such fanfare?

The Times quoted Stephen Liggett, an asthma expert, as saying “We are now quite certain that we see the Achilles’ heel, and that a very effective treatment for the common cold is at hand.” He was apparently referring to the observation that a sequence within the 5′-noncoding region of the viral genome is highly conserved among the 99 rhinovirus sequences, in comparison with other regions of the viral RNA. He suggested that all 99 rhinovirus serotypes would therefore be susceptible to the same drug. But what kind of drug, and what function would it inhibit? The very 5′-end of the genome of enteroviruses and rhinoviruses binds viral and cellular proteins, and these interactions are essential for viral replication. So it might be possible to identify small molecules that block these protein-RNA interactions. But such drugs are very difficult to identify. Furthermore, if such a drug were identified, its efficacy would have to be tested against all rhinovirus serotypes. Therefore it is not clear that knowing that this sequence of the genome is conserved helps to identify drug targets and more readily than did the observations made years ago about the importance of RNA-protein interactions in this region. Clearly, many years of research are needed before such drugs are developed – not consistent with Dr. Liggett’s a treatment is ‘at hand’.

An even more crucial aspect of the problem was omitted from the Times article. Even if an antiviral drug could be identified that blocks essential RNA-protein interactions, it probably would not be useful in treating the common cold. As we discussed last week, rhinoviruses cause acute infections - characterized by rapid onset of disease, a relatively brief period of symptoms, and resolution within days. Most are complete by the time the patient feels ill, and the virus has already spread to another host. Antiviral therapy  must be given early in infection to be effective. There is little hope of treating most acute viral infections with antiviral drugs until rapid diagnostic tests are become available.

To be fair, some of the scientists quoted in the Times article were more realistic about the possibilities for rhinovirus treatments. One antiviral drug expert noted that it costs about $700 million to bring a drug to market. Because most rhinovirus infections are benign, who would pay for such an expensive drug, and would the Food and Drug Administration ever approve it? Ann Palmenberg, the lead author on the study, was even more realistic, admitting that a rhinovirus vaccine would not likely be made.

I cannot see how this new study identified a new or better target for therapeutic intervention. So why get the public excited by running a front page headline in the New York Times? It’s great to keep the public informed about scientific progress – but the press should not cry wolf. If this advance does not soon lead to a treatment for the common cold, the public will shake their heads and lose a bit more trust in science.

I’m not blaming the scientists for this over reaction to their study. I am sure that the journal Science engaged in strong pre-publication promotion: more publicity is better for their advertising revenues. And the newspapers are equally at fault: they should speak to a broader range of scientists to obtain a more balanced view. I particularly blame the author of the Times article, Nicholas Wade, for not sufficiently researching his article.

Perhaps Dr. Liggett and his colleagues would benefit from the lessons of history – specifically, the history of poliomyelitis and its conquest. On March 9, 1911, three years after the isolation of poliovirus, The Rockefeller Institute issued a press release, saying that it believed “that its search for a cure for infantile paralysis is about to be rewarded. Within six months, according to Dr. Simon Flexner, definite announcement of a specific remedy may be expected.” They quoted Dr. Flexner:  “We have already discovered how to prevent the disease, and the achievement of a cure, I may conservatively say, is not now far distant.” Dr. Flexner’s imminent ‘cure’ was a failure, and a successful poliovirus vaccine required another 44 years of research. Last week’s Times article seemed to have a similar overdose of hubris.

A. C. Palmenberg, D. Spiro, R. Kuzmickas, S. Wang, A. Djikeng, J. A. Rathe, C. M. Fraser-Liggett, S. B. Liggett (2009). Sequencing and Analyses of All Known Human Rhinovirus Genomes Reveals Structure and Evolution Science DOI: 10.1126/science.1165557