Contemporary seasonal influenza A (H1N1) virus infection primes for a more robust response to split inactivated pandemic influenza A (H1N1) Virus vaccination in ferrets

Abstract

Human influenza pandemics occur when influenza viruses to which the population has little or no immunity emerge and acquire the ability to achieve human-to-human transmission. In April 2009, cases of a novel H1N1 influenza virus in children in the southwestern United States were reported. It was retrospectively shown that these cases represented the spread of this virus from an ongoing outbreak in Mexico. The emergence of the pandemic led to a number of national vaccination programs. Surprisingly, early human clinical trial data have shown that a single dose of nonadjuvanted pandemic influenza A (H1N1) 2009 monovalent inactivated vaccine (pMIV) has led to a seroprotective response in a majority of individuals, despite earlier studies showing a lack of cross-reactivity between seasonal and pandemic H1N1 viruses. Here we show that previous exposure to a contemporary seasonal H1N1 influenza virus and to a lesser degree a seasonal influenza virus trivalent inactivated vaccine is able to prime for a higher antibody response after a subsequent dose of pMIV in ferrets. The more protective response was partially dependent on the presence of CD8(+) cells. Two doses of pMIV were also able to induce a detectable antibody response that provided protection from subsequent challenge. These data show that previous infection with seasonal H1N1 influenza viruses likely explains the requirement for only a single dose of pMIV in adults and that vaccination campaigns with the current pandemic influenza vaccines should reduce viral burden and disease severity in humans.

FIG. 1.

Depletion of CD8⁺ cells in the ferret model. Cells were collected and stained 1 day after the first injection of the depleting MAb (4 days prior to seasonal virus inoculation). (A) Representative fluorescence-activated cell sorter (FACS) plots showing percentages of peripheral blood mononuclear cells (PBMCs) stained with fluorescently labeled antibodies against CD8 and IgG prior to (left) or after (right) CD8⁺ cell depletion. (B) Mean percentages of CD8⁺ cells prior to and after CD8⁺ cell depletion. Error bars indicate standard errors.

FIG. 2.

Virus replication in the upper respiratory tract after H1N1pdm virus challenge. Nasal washes were collected on days 1, 2, 4, 7, and 9 after H1N1pdm virus challenge, and virus titers were determined by endpoint titration in MDCK cells. Arithmetic mean titers are given, and error bars indicate standard errors. *, P < 0.05; **, P < 0.001.

FIG. 3.

Priming by seasonal influenza virus infection protected ferrets from severe lung histopathology after H1N1pdm virus challenge. Illustrated are the histopathologic findings associated with the bronchioles and interbronchiolar alveoli and interstitium in the lungs of ferrets challenged with the H1N1pdm virus following various pretreatments (H&E staining, ×20 magnification). (A and B) Lung tissue from a ferret that belongs to the I/M group, which was primed by seasonal H1N1 virus infection prior to receiving a single dose of pMIV. (A) Neutrophils (arrows) and mucus are present in the lumens of two bronchioles. (B) The alveoli (a) are free of inflammatory cells, and the interstitial septa (arrows) are thin and narrow and have very few cells other than red blood cells. (C and D) Lung tissue from a ferret that belongs to the M/M group, which received two doses of pMIV 3 weeks apart prior to the H1N1pdm virus challenge. (C) Mononuclear inflammatory cells (m) fill the lumens of two bronchioles. Due to bronchiole epithelial necrosis and sloughing, one bronchiole is not lined with epithelium (thick arrows), whereas the other bronchiole is lined with some hypertrophied regenerating epithelial cells (thin arrows). There is also a mononuclear cell infiltrate surrounding the bronchioles. (D) Mononuclear inflammatory cells fill several of the alveoli (a) that make up the lung parenchyma between the bronchioles, and the interstitial septa (arrows) associated with the alveoli are thickened with inflammatory mononuclear cells. (E and F) Lung tissue from a control ferret that was challenged with the H1N1pdm virus following pretreatment with PBS. (E) Mononuclear inflammatory cells (m) fill the lumen of a bronchiole that is lined with hypertrophied regenerating epithelial cells (arrows). Alveolar pneumocyte hyperplasia is associated with the alveoli adjacent to the bronchiole, and the alveoli are filled with mononuclear inflammatory cells. (F) The alveolar and interstitium architecture of the lung parenchyma between bronchioles is effaced with infiltrates of mononuclear inflammatory cells and hyperplastic epithelial pneumocytes.