TWiV #74: Influenza with Professor Adolfo Garcia-Sastre

Hosts: Vincent Racaniello and Adolfo Garcia-Sastre

Vincent speaks with Adolfo Garcia-Sastre talk about the origin, pathogenesis, and prevention of the 2009 pandemic influenza H1N1 virus.

This episode is sponsored by Data Robotics Inc. Use the promotion code VINCENT to receive $50 off a Drobo or $100 off a Drobo S.

Win a free Drobo S! Contest rules here.

Click the arrow above to play, or right-click to download TWiV #74 (34 MB .mp3, 47 minutes)

Subscribe to TWiV (free) in iTunes , at the Zune Marketplace, by the RSS feed, or by email.

Links for this episode:

Send your virology questions and comments (email or mp3 file) to twiv@microbe.tv or leave voicemail at Skype: twivpodcast. You can also post articles that you would like us to discuss at microbeworld.org and tag them with twiv.

The D225G change in 2009 H1N1 influenza virus

sialic-acid-2Last year a mutation in the HA gene of theĀ 2009 H1N1 influenza virus was identified in isolates from patients with severe disease. At the time I concluded that the emergence of this change was not a concern. Recently the Norwegian Institute of Public Health reported that the mutation, which causes a change from the amino acid aspartic acid to glycine at position 225 of the viral HA protein (D225G), has been identified in 11 of 61 cases (18%) of severe or fatal influenza, but not in any of 205 mild cases. Have these observations changed my view of the importance of this mutation?

The cell receptor for influenza A virus strains is sialic acid. Human influenza A strainsĀ bind preferentially to sialic acids linked to galactose by an alpha(2,6) bond, while avian and equine strains prefer alpha(2,3) linked sialic acids (pictured). Alpha(2,6) linked sialic acids are dominant on epithelial cells in the human nasal mucosa, paranasal sinuses, pharynx, trachea, and bronchi. Alpha(2,3) linked sialic acids are found on nonciliated bronchiolar cells at the junction between the respiratory bronchiole and alveolus, and on type II cells lining the alveolar wall.

The 2009 swine-origin H1N1 influenza virus is known to bind both alpha(2,3) and alpha(2,6) linked sialic acids. This is consistent with the ability of the virus to cause lower respiratory tract disease. The D225G change might be expected to increase affinity for alpha(2,3) linked sialic acids. However, it is not known if increased binding affinity correlates with higher infectivity and pathogenicity. It’s equally likely that high affinity binding might restrict the movement of the virus in lung tissues by causing retention of the virus on nonsusceptible cells.

One view of the D225G mutation is that it is spreading globally and causing more severe disease. However there is no evidence in support of this hypothesis. According to WHO, viruses with the D225G change have been found in 20 countries since April 2009, but there has been no temporal or geographic clustering. As of January, the HA change has been identified in 52 sequences out of more than 2700. Furthermore, the authors of the Norwegian study write, “Our observations are consistent with an epidemiological pattern where the D225G substitution is absent or infrequent in circulating viruses, with the mutation arising sporadically in single cases where it may have contributed to severity of infection”.

One explanation for the sporadic emergence of influenza viruses with the D225G change is that they are selected for in the lower respiratory tract where alpha(2,3) sialic acids are more abundant than in the upper tract. Such selection might be facilitated in individuals with compromised lung function (e.g. asthmatics, smokers) or suboptimal immune responses, in whom the virus more readily reaches the lung. One way to address this hypothesis would be to compare the HA at amino acid 225 of viral isolates obtained early in infection, from the upper tract, with isolates obtained from the lower tract late in disease. However such paired isolates have not yet been obtained. But whether the presence of viruses with D225G increases viral virulence is unknown. Many H1N1 isolates from cases of fatal or severe disease do not contain this amino acid change.

There is an alternative explanation for the isolation of at least some influenza viruses with the D225G change: it is selected by propagation in embryonated chicken eggs. This selection occurs because cells of the allantoic cavity of chicken eggs have only alpha(2,3) linked sialic acids. A change in receptor specificity does not occur when viruses are propagated in MDCK (canine kidney) cells, which possess sialic acids with both alpha(2,3) and alpha(2,6) linkages. Consistent with this hypothesis, WHO reports (pdf) that the D225G substitution in 14 virus isolates occurred after growth in the laboratory.

Studies on the binding of influenza viruses to glycan arrays have shown that attachment is influenced not only by the linkage to the next sugar, but the type of sialic acid as well as the rest of the carbohydrate chain. The distribution of all the possible sialic acid containing sugars in the respiratory tract is unknown, as is the specific molecules that can support productive viral infection. The view that HA preferentially binds to either alpha(2,3) or alpha(2,6) linked sialic acids is likely to be overly simplistic: another casualty of reductionism.

Kilander A, Rykkvin R, Dudman SG, & Hungnes O (2010). Observed association between the HA1 mutation D222G in the 2009 pandemic influenza A(H1N1) virus and severe clinical outcome, Norway 2009-2010. Euro surveillance : bulletin europeen sur les maladies transmissibles = European communicable disease bulletin, 15 (9) PMID: 20214869

Takemae N, Ruttanapumma R, Parchariyanon S, Yoneyama S, Hayashi T, Hiramatsu H, Sriwilaijaroen N, Uchida Y, Kondo S, Yagi H, Kato K, Suzuki Y, & Saito T (2010). Alterations in receptor-binding properties of swine influenza viruses of the H1 subtype after isolation in embryonated chicken eggs. The Journal of general virology, 91 (Pt 4), 938-48 PMID: 20007353

Garcia-Sastre, A. (2010). Influenza Virus Receptor Specificity. Disease and Transmission American Journal Of Pathology DOI: 10.2353/ajpath.2010.100066

TWiV 62: Persistence of West Nile virus

The_Persistence_of_MemoryHosts: Vincent Racaniello, Dickson Despommier, and Alan Dove

On episode #62 of the podcast This Week in Virology, Vincent, Dickson, and Alan discuss STEP HIV-1 vaccine failure caused by the adenovirus vector, presence of West Nile virus in kidneys for years after initial infection, adaptation of the influenza viral RNA polymerase for replication in human cells, and the significance of the D225G change in the influenza HA protein.

Click the arrow above to play, or right-click to download TWiV #62 (47 MB .mp3, 66 minutes)

Subscribe to TWiV (free) in iTunes , at the Zune Marketplace, by the RSS feed, or by email.

Links for this episode:

Weekly Science Picks
Dick Smallpox – The Death of a Disease by DA Henderson
Alan Olympus Bioscapes Digital Imaging Competition
Vincent Microbe Magazine

Send your virology questions and comments (email or mp3 file) to twiv@microbe.tv or leave voicemail at Skype: twivpodcast. You can also post articles that you would like us to discuss at microbeworld.org and tag them with twiv.

The D225G change in 2009 H1N1 influenza virus is not a concern

sialic-acid-2The Norwegian Institute of Public Health recently identified a mutation in 2009 H1N1 influenza virus isolated from two patients who died and one with severe disease. It has been suggested that this mutation, which causes a change from the amino acid aspartic acid to glycine at position 225 of the viral HA protein (D225G), could make the virus more likely to infect deeper in the airways and cause more severe disease. What is the basis for this concern and does it have merit?

Attachment of all influenza A virus strains to cells requires sialic acids. There are a number of chemically different forms of sialic acids, and influenza virus strains vary in their affinity for them. Human influenza A strains bind preferentially to sialic acids linked to galactose by an alpha(2,6) bond, while avian and equine strains prefer alpha(2,3) linked sialic acids.

The type of sialic acid preferred by influenza viruses is controlled by amino acids in the HA protein. Amino acids 190 and 225 are important determinants of receptor binding specificity of the 1918 H1 hemagglutinin. The HA of the 1918 strain A/South Carolina/1/18 prefers alpha(2,3) linked sialic acids; the New York variant, isolated in September 1918, binds both alpha(2,3) and alpha(2,6) sialic acids. These two H1 hemagglutinins differ only by a single amino acid, position 225, which is aspartic acid (D) in the South Carolina strain and glycine (G) in the NY strain. When amino acid 190, which is D in both strains, is changed to E in the NY HA, the virus (AV18) preferentially binds alpha(2,3) sialic acids. These findings are summarized in the table.

ha_specificity

Different isolates of the 2009 H1N1 influenza virus have D at HA at amino acid 190 and mostly D at amino acid 225. The virus prefers to bind to alpha(2,6) linked sialic acids. The amino acid change D225G would be expected to produce a virus with preference for both alpha(2,3) and alpha(2,6) linked sialic acids.

In the human respiratory tract, alpha(2,6) linked sialic acids are dominant on epithelial cells in the nasal mucosa, paranasal sinuses, pharynx, trachea, and bronchi. Alpha(2,3) linked sialic acids are found on nonciliated bronchiolar cells at the junction between the respiratory bronchiole and alveolus, and on type II cells lining the alveolar wall.

Based on these considerations, it could be hypothesized that the D225G change would allow the 2009 H1N1 virus to replicate deeper in the respiratory tract. But 2009 H1N1 virus without this amino acid change can already replicate deep in the respiratory tract of ferrets, and probably also in humans. Cells with alpha(2,6) linked sialic acids are present in the lower respiratory tract of humans. So it’s not clear if any effect on virulence would be conferred by the ability of the 2009 H1N1 strain to bind alpha(2,3) linked sialic acids.

An important consideration is that the D225G amino acid change has a negative impact on transmission. The change from D to G at amino acid 225 of the 1918 HA significantly impairs transmission among ferrets. When both D225G and D190E are present, transmission is abolished. These changes do not impair viral replication or virulence in the respiratory tract of inoculated animals.

Transmissibility is clearly a positive selection factor for viral evolution. There may be selection for increased virulence only if there is no negative impact on viral transmission. Given these considerations, the choice between an H1 HA amino acid at position 225 that allows efficient transmission (D225) or one that impairs transmission and might or might not allow multiplication deeper in the lung (D225G) seems obvious.

Tumpey, T., Maines, T., Van Hoeven, N., Glaser, L., Solorzano, A., Pappas, C., Cox, N., Swayne, D., Palese, P., Katz, J., & Garcia-Sastre, A. (2007). A Two-Amino Acid Change in the Hemagglutinin of the 1918 Influenza Virus Abolishes Transmission Science, 315 (5812), 655-659 DOI: 10.1126/science.1136212

Shen J, Ma J, & Wang Q (2009). Evolutionary Trends of A(H1N1) Influenza Virus Hemagglutinin Since 1918. PloS one, 4 (11) PMID: 19924230