The study of immune response to co-infection of influenza virus and Streptococcus pneumoniae

Abstract

Influenza is a leading cause of respiratory disease worldwide. During pandemic and seasonal influenza, secondary Streptococcus pneumoniae infection is a severe complication that contributes to morbidity and mortality. With the clinical significance of this co-infection, it is imperative to understand the disease mechanisms and how our immune system would be modulated in dealing with the dual infection.

First, in vivo co-infection model was established. Mice were sequentially infected with influenza virus and then Streptococcus pneumoniae. Co-infected mice lost their body weight significantly and had 100% mortality, whereas mice infected with either influenza virus or pneumococcus alone lost their body weight transiently and all recovered from the infection.

Then, lung inflammatory response during the co-infection was examined. Although it is a common phenomenon that co-infection enhances inflammation, the kinetic of, and the relative contribution of influenza virus or pneumococcus to the lung inflammation is not well defined. Therefore, this study characterized the general lung inflammatory environment after co-infection. It was found that influenza virus and pneumococcus differentially modulated inflammatory response in terms of kinetics, leukocyte infiltration and cytokine production. At the early time point after co-infection, pneumococcal infection contributed more than the influenza virus infection to enhance inflammatory cytokine and neutrophil infiltrating the lung. At the later time point after co-infection, both influenza virus and pneumococcus contributed to synergistically increase inflammatory cytokine and macrophage infiltrating the lung. Influenza virus infection induced IFN-γ that contributed to the elevated IFN-γ level in co-infected mice. Influenza virus and pneumococcus synergistically increased Th2 associated cytokine including IL-4, IL-5, and IL-10. These up-regulated immune responses might contribute to the severe lung pathology.

Next, adaptive immunity to co-infection was examined. Literature studying co-infection often reports how prior influenza virus infection impairs the immune response against subsequent bacterial infection. However, whether and how secondary pneumococcal infection would affect the immunity to the initial influenza virus is unknown. Therefore this study investigated the modulation of immunity to influenza virus by secondary pneumococcal infection. It was found that co-infection significantly enhanced virus titer in lung and depleted the number of cell in spleen. Secondary pneumococcal infection after influenza decreased influenza virus specific IgG in the lung and peripheral blood. The reduced level of virus specific IgG was associated with the decrease in the number and the percentage of follicular B cell and CD4 T follicular helper cell through both pneumococcal capsular polysaccharide dependent and independent manner. Treating co-infected mice with immune serum containing influenza virus specific IgG successfully improved survival, which suggested the important protective function of virus specific IgG to the co-infection. Taken together, these data suggested that secondary pneumococcal infection impairs the antibody response to influenza virus, which might enhance mortality after co-infection.

In conclusion, this study provides new insight to understand the pathogenesis of co-infection, reveals the general lung inflammatory environment, highlights the negative role of pneumococcus to impair virus control and explores novel treatment for the co-infection.