Influenza-induced bacterial infection: reduced number of airway macrophages

Background: Influenza-induced bacterial superinfection causes morbidity and mortality associated with influenza epidemics. Certain bacterial species are commonly isolated from such infections, but it is unclear why certain bacterial species are preferentially affected. In this study, dual infection of influenza A virus (A/Qld/6/72) with various bacteria (H. influenzae, S. pneumoniae, S. aureus and P aeruginosa) was studied in a mouse model of acute respiratory infection to assess the degree of bacteria] superinfection, and effect on airway leukocytes. Methods: Mice were infected with A/Qld alone, bacteria alone, or a mixture of virus and bacteria. After 22 h mice were killed and broncho-alveolar lavage (BAL) fluid and lung tissue homogenates were assayed for live bacteria and BAL leukocyte subsets. Results: Influenza A/Qld caused a significant increase in H. influenzae and S. pneumoniae infection. Neutrophil (PMN) numbers were primarily related to the number of bacteria present. Macrophage numbers were reduced during mixed infection of A/Qld with H. influenzae or S. pneumoniae but were not affected by A/Qld plus P. aeruginosa or S. aureus infection. Conclusion: Differential effects of influenza/bacteria combinations on macrophages may partially account for preferential enhancement of particular bacterial infections during influenza infection. (C) 2001 Elsevier Science B.V. All rights reserved.