The endotoxin-induced pulmonary inflammatory response is enhanced during the acute phase of influenza infection

Influenza infections are often complicated by secondary infections, which are associated with high morbidity and mortality, suggesting that influenza profoundly influences the immune response towards a subsequent pathogenic challenge. However, data on the immunological interplay between influenza and secondary infections are equivocal, with some studies reporting influenza-induced augmentation of the immune response, whereas others demonstrate that influenza suppresses the immune response towards a subsequent challenge. These contrasting results may be due to the use of various types of live bacteria as secondary challenges, which impedes clear interpretation of causal relations, and to differences in timing of the secondary challenge relative to influenza infection. Herein, we investigated whether influenza infection results in an enhanced or suppressed innate immune response upon a secondary challenge with bacterial lipopolysaccharide (LPS) in either the acute or the recovery phase of infection.

Male C57BL/6J mice were intranasally inoculated with 5 × 10³ PFU influenza virus (pH1N1, strain A/Netherlands/602/2009) or mock treated. After 4 (acute phase) or 10 (recovery phase) days, 5 mg/kg LPS or saline was administered intravenously, and mice were sacrificed 90 min later. Cytokine levels in plasma and lung tissue, and lung myeloperoxidase (MPO) content were determined.

LPS administration 4 days after influenza infection resulted in a synergistic increase in TNF-α, IL-1β, and IL-6 concentrations in lung tissue, but not in plasma. This effect was also observed 10 days after influenza infection, albeit to a lesser extent. LPS-induced plasma levels of the anti-inflammatory cytokine IL-10 were enhanced 4 days after influenza infection, whereas a trend towards increased pulmonary IL-10 concentrations was found. LPS-induced increases in pulmonary MPO content tended to be enhanced as well, but only at 4 days post-infection.

An LPS challenge in the acute phase of influenza infection results in an enhanced pulmonary pro-inflammatory innate immune response. These data increase our insight on influenza-bacterial interplay. Combing data of the present study with previous findings, it appears that this enhanced response is not beneficial in terms of protection against secondary infections, but rather damaging by increasing immunopathology.