Bacterial colonization dampens influenza-mediated acute lung injury via induction of M2 alveolar macrophages

Area of Science:

• Immunology
• Microbiology
• Respiratory Medicine

Background:

• Airway bacteria are linked to better immunity against influenza virus.
• The exact mechanisms of how the body's own microbes affect antiviral immunity are not fully understood.

Purpose of the Study:

• To investigate how airway microbiota influences the immune response to influenza virus.
• To elucidate the specific mechanisms by which Staphylococcus aureus colonization protects against influenza-induced lung injury.

Main Methods:

• Used specific pathogen-free mice and mice from a natural environment to compare influenza sensitivity.
• Administered Staphylococcus aureus priming to mice and assessed protection against influenza-mediated lung injury.
• Investigated the roles of Toll-like receptor 2 (TLR2) and CCR2(+)CD11b(+) monocytes in the protective mechanism.

Main Results:

• Mice in a natural environment were less susceptible to lethal influenza than specific pathogen-free mice.
• Priming with Staphylococcus aureus significantly reduced influenza-induced lung immune injury.
• Protection was dependent on Toll-like receptor 2 signaling and the recruitment of peripheral monocytes that differentiated into M2 alveolar macrophages.
• M2 macrophages suppressed lethal inflammation through anti-inflammatory cytokines and inhibitory ligands.

Conclusions:

• Airway microbiota, specifically Staphylococcus aureus, can protect against lethal influenza by modulating the immune response.
• A novel mechanism involves TLR2-mediated recruitment and polarization of monocytes into M2 alveolar macrophages, which dampen excessive inflammation.
• These findings highlight a previously unrecognized role of the airway microbiome in host defense against viral respiratory infections.