Examining effects of NS1 specific antibodies on sublethal influenza infection and secondary bacterial pneumonia in mice

Influenza is a highly contagious respiratory disease widespread throughout the world that causes disease in humans, birds and many mammalian species. Annually, around 20% of the global human gets sick with influenza so that more than 500,000 people die its various complications. Secondary bacterial pneumonia poses the peak threat during influenza infection, being most frequently caused by S. pneumoniae. Multiple studies in humans confirm the negative impact of influenza virus infection on subsequent outcome of bacterial pneumonia and provides insight into increased morbidity and mortality due to complicated influenza infection. In particular, the last 2009 influenza pandemic caused by H1N1 virus revealed that 25-56% cases of severe disease forms were associated with secondary pneumonia, among which 14-46% of them were fatal. Based on the aforementioned, it is of high priority to investigate a role of influenza virus proteins in developing of pathogen synergism in viral-bacterial pneumonia, particularly influenza virus non-structural protein NS1. The study objective was to examine effects of NS1-specific antibodies on course of influenza infection and secondary bacterial pneumonia in mice. For this, we used an experimental model of sublethal influenza infection followed by secondary Streptococcus pneumoniae bacterial pneumonia. Influenza A/Puerto Rico/8/34 (H1N1) virus and S. pneumoniae No. 3405 strain were used to simulate influenza infection. Rabbit serum containing antibodies against recombinant NS1 protein from A/Puerto Rico/8/34 virus and native rabbit serum (contain no specific antibodies) were used for vaccination. The study was carried out with female BALB/c mice, weighing 20-22 g. Protective activity of animal serum was assessed by using the three criteria: infection-related mortality, life expectancy and body weight change. The data obtained showed that passive transfer of antibodies specific to influenza virus NS1 protein did not lowered viral replication in sublethal murine model of influenza infection. Subsequent secondary bacterial pneumonia induced by S. pneumoniae revealed no protective effect of anti-NS1 protein antibodies assessed by measuring survival rate, lung viral and bacterial titers in treated vs. control mice.