Bird flu virus: interfering with interferon

The virulent influenza A subtype H5N1, known colloquially as bird flu, has caused sporadic cases of human disease but has not yet become a pandemic strain. There are several things which still separate H5N1 from the kind of seasonal influenza infections that are a serious periodic public health threat to humans. A significant population immunity to the seasonal virus subtypes is probably a major factor preventing seasonal flu from being the pandemic monster that the 1918 flu became. But there are other differences, too.
H5N1 currently seems to prefer birds to humans, a second difference. We are only just beginning to unravel the reasons for species preference in influenza viruses. Even when the occasional person becomes infected with H5N1, there seems to be much less chance to transmit to other people. Seasonal influenza is easily transmitted between people. And while seasonal flu can be a nasty and sometimes fatal disease, the case fatality ratio is usually only about 0.1%. By contrast, the current H5N1 infections are highly virulent, meaning that infections with the virus are extremely serious. Current case fatality ratios are in excess of 60%. So there are a lot of things still to understand about H5N1.

A new paper in Nature (Zachary A. Bornholdt, B. V. Venkataram Prasad, X-ray structure of NS1 from a highly pathogenic H5N1 influenza virus, Nature, doi:10.1038/nature07444) looks at the virulence question. What makes H5N1 such a nasty bug once a human is infected? The H and N in the subtype designation H5N1 represent two viral proteins, hemagglutinin and neuraminidase, but there are another 9 proteins the virus makes. Most are not "seen" by the immune system but play important roles in viral infection. One is a protein designated NS1, for non-structural protein 1. We know that NS1 plays some roles in viral virulence and that one role involves interfering with the innate immune response to a viral infection. The objective of a virus is pretty simple: make copies of itself. It does this by gaining access to the cell's protein and genetic factories and tricks it into making copies of itself instead of proteins and genetic material the cell needs to function. Cell's have defenses against this kind of criminal activity by viruses, however. When a cell sees that viral genetic material is being manufactured (as indicated by the presence of double stranded RNA), it triggers a response to manufacture a natural antiviral cytokine, interferon. NS1 seems to aid viral infection by combatting interferon. But how?

The Nature paper by Bornholdt and Prasad at Baylor College of Medicine, succeeded in crystallizing H5N1's NS1 protein and revealing its probable three dimensional structure by x-ray crystallography. Proteins are characterized, first, by their amino acid sequences. The sequence of amino acids is determined by your cell's DNA or, for the influenza virus, its genetic material (which is RNA). But just knowing the sequence isn't enough. We know the amino acid sequence of NS1. But the long chain of amino acids also folds into a complicated three dimensional structure which determines its biology. Mutations may alter the amino acid sequence but it is the folding and 3-D changes that are produced that make the biological difference. By knowing what H5N1 and other flu virus NS1s look like in 3-D, we can begin to figure out how they work. NS1 has two subparts separated by a threadlike linker sequence of amino acids. The H5N1 NS1s have a 5 amino acid deletion in the linker thread that may be important for why they are more virulent. But the Bornhold/Prasad paper has much more to it. By examining the 3-D picture they discovered that NS1 might work by lining up in chains that form a tubular structure that encloses the virus's RNA when it is in its double stranded phase during replication so it can't be seen and therefore doesn't trigger the interferon response.

If this novel structure is a feature contributing to virulence (and this is not yet certain), it does not preclude NS1 from having other roles as well. That is still to be determined. By targeting with drugs the essential features of the NS1 structure we might be able to develop a prophylactic or treatment for H5N1. That won't solve the pandemic threat. You can't stop a pandemic with drugs. But drugs can help you treat people who are sick, and that's a good thing.

This is one more example of the elegant but very basic research that must be done if we are to bring influenza under control. We have an awful long way to go.