Lack of IFNγ signaling attenuates spread of influenza A virus in vivo and leads to reduced pathogenesis.

IFNγ is a key regulator of inflammatory responses but its role in influenza A virus (IAV) pathogenesis is unclear. Our studies show that infection of mice lacking the IFNγ receptor (IFNγR-/-) at a dose which caused severe disease in wild type 129 Sv/Ev (WT) mice resulted in milder clinical symptoms and significantly lower lung virus titers by 6 days post-infection (dpi). Viral spread was reduced in IFNγR-/- lungs at 2 and 4 dpi. Levels of inflammatory cytokines and chemokines were lower in IFNγR-/- mice at 2 dpi and there was less infiltration of monocyte/macrophage lineage cells than in WT mice. There was no difference in CD4+ and CD8+ T cells and alveolar macrophages in the bronchoalveolar lavage fluid (BALF) at 2 and 4 dpi but by 4 dpi IFNγR-/- mice had significantly higher percentages of neutrophils. Our data strongly suggest that IAV can use the inflammatory response to promote viral spread.

Susceptibility of bone marrow-derived macrophages to influenza virus infection is dependent on macrophage phenotype.

The role of the macrophage in influenza virus infection is complex. Macrophages are critical for resolution of influenza virus infections but implicated in morbidity and mortality in severe infections. They can be infected with influenza virus and consequently macrophage infection is likely to have an impact on the host immune response. Macrophages display a range of functional phenotypes, from the prototypical pro-inflammatory classically activated cell to alternatively activated anti-inflammatory macrophages involved in immune regulation and wound healing. We were interested in how macrophages of different phenotype respond to influenza virus infection and therefore studied the infection of bone marrow-derived macrophages (BMDMs) of classical and alternative phenotype in vitro. Our results show that alternatively activated macrophages are more readily infected and killed by the virus than classically activated. Classically activated BMDMs express the pro-inflammatory markers inducible nitric oxide synthase (iNOS) and TNF-α, and TNF-α expression was further upregulated following infection. Alternatively activated macrophages express Arginase-1 and CD206; however, following infection, expression of these markers was downregulated whilst expression of iNOS and TNF-α was upregulated. Thus, infection can override the anti-inflammatory state of alternatively activated macrophages. Importantly, however, this results in lower levels of pro-inflammatory markers than those produced by classically activated cells. Our results showed that macrophage phenotype affects the inflammatory macrophage response following infection, and indicated that modulating the macrophage phenotype may provide a route to develop novel strategies to prevent and treat influenza virus infection.