This study demonstrates the protective role of the serine-threonine mitogen-activated protein kinase, Tpl2, in influenza virus pathogenesis and reveals that host Tpl2 deficiency is sufficient to convert a low-pathogenicity influenza A virus infection into severe influenza disease that resembles ARDS, both histopathologically and transcriptionally. The IAV-infected Tpl2 −/− mouse thereby represents a novel murine model for studying ARDS-like disease that could improve our understanding of this aggressive disease and assist in the design of better diagnostics and treatments.

The most prominent host response to viral infection is the production of type 1 interferons (T1 IFNs). One host regulator of the T1 IFNs is the serine-threonine kinase, tumor progression locus 2 (TPL2). We have previously demonstrated that Tpl2 −/− mice succumb to infection with a low-pathogenicity influenza A strain (x31), in association with with increased pulmonary levels of interferon-β (IFN-β), chemokine CCL2, and excessive monocyte and neutrophil pulmonary infiltration. TPL2-dependent overexpression of IFN-β has been implicated in enhanced susceptibility to Mycobacterium tuberculosis ; therefore, we examined the role of T1 IFNs in susceptibility of Tpl2 −/− mice to influenza. CCL2 overexpression and monocyte recruitment were normalized in Ifnar1 −/− Tpl2 −/− mice, confirming that TPL2 constrains inflammatory monocyte recruitment via inhibition of the T1 IFN/CCL2 axis. Unexpectedly, excessive neutrophil recruitment in Ifnar1 −/− strains was further exacerbated by simultaneous TPL2 genetic ablation in Ifnar1 −/− Tpl2 −/− by 7 dpi, accompanied by overexpression of neutrophil-regulating cytokines, CXCL1 and IFN-λ. Collectively, our data suggest that TPL2 and T1 IFNs synergize to inhibit neutrophil recruitment. However, treatment with the neutrophil-depleting anti-Ly6G antibody showed only a modest improvement in disease. Analysis of sorted innate immune populations revealed redundant expression of inflammatory mediators among neutrophils, inflammatory monocytes and alveolar macrophages. These findings suggest that targeting a single cell type or mediator may be inadequate to control severe disease characterized by a mixed inflammatory exudate. Future studies will consider TPL2-regulated pathways as potential predictors of severe influenza progression as well as investigate novel methods to modulate TPL2 function during viral infection. Supplementary Information The online version contains supplementary material available at 10.1007/s10753-022-01736-8.