The Influenza-Induced Pulmonary Inflammatory Exudate in Susceptible Tpl2-Deficient Mice Is Dictated by Type I IFN Signaling.

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 Ifnar1Tpl2 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 Ifnar1Tpl2 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.