Effect of priming with H1N1 influenza viruses of variable antigenic distances on challenge with 2009 pandemic H1N1 virus

Abstract

Compared to seasonal influenza viruses, the 2009 pandemic H1N1 (pH1N1) virus caused greater morbidity and mortality in children and young adults. People over 60 years of age showed a higher prevalence of cross-reactive pH1N1 antibodies, suggesting that they were previously exposed to an influenza virus or vaccine that was antigenically related to the pH1N1 virus. To define the basis for this cross-reactivity, ferrets were infected with H1N1 viruses of variable antigenic distance that circulated during different decades from the 1930s (Alaska/35), 1940s (Fort Monmouth/47), 1950s (Fort Warren/50), and 1990s (New Caledonia/99) and challenged with 2009 pH1N1 virus 6 weeks later. Ferrets primed with the homologous CA/09 or New Jersey/76 (NJ/76) virus served as a positive control, while the negative control was an influenza B virus that should not cross-protect against influenza A virus infection. Significant protection against challenge virus replication in the respiratory tract was observed in ferrets primed with AK/35, FM/47, and NJ/76; FW/50-primed ferrets showed reduced protection, and NC/99-primed ferrets were not protected. The hemagglutinins (HAs) of AK/35, FM/47, and FW/50 differ in the presence of glycosylation sites. We found that the loss of protective efficacy observed with FW/50 was associated with the presence of a specific glycosylation site. Our results suggest that changes in the HA occurred between 1947 and 1950, such that prior infection could no longer protect against 2009 pH1N1 infection. This provides a mechanistic understanding of the nature of serological cross-protection observed in people over 60 years of age during the 2009 H1N1 pandemic.

Fig 1

Replication of CA/09 after priming with various influenza viruses. The lungs (A) and nasal turbinates (B) from groups of four ferrets primed with various influenza viruses, and subsequently challenged with CA/09, were harvested at day 1 (d1) and d3 postinfection. At necropsy, portions of the right and left lung were harvested for virus titration. Thus, there are two data points for each ferret (one for the right lobe and one for the left lobe) and, with four ferrets per group, there are eight data points for each group at d1 and d3 postinfection. Titers are expressed as the log₁₀ TCID₅₀/g tissue. Each data point represents the titer from an individual animal, and the mean is indicated by a bar. The limit of detection for this assay was 1.8 log₁₀ TCID₅₀/g tissue and is highlighted by the broken black bar in each graph. Virus titers were compared to B/Mal for statistical significance (*, P < 0.05).