Epithelial TLR9 upregulation by respiratory viruses impairs pulmonary antibacterial immunity

Abstract While much is known about signaling downstream of the dsDNA sensor TLR9, regulation of its expression is underexplored and has significant consequences for innate immune responses to pathogens. Mice lacking TLR9 surprisingly fare better upon coinfection with influenza and Staphylococcus aureus, and other groups have shown protection from S. aureus alone as well. These TLR9KO mice display lower inflammatory cytokine production, reduced cellular infiltration into airspaces, and increased survival. While macrophages taken from influenza-infected TLR9KO mice display increased bacterial phagocytosis and killing, inhibition of TLR9 on wild-type murine macrophages ex vivo surprisingly decreased their ability to phagocytize and kill S. aureus. Bone marrow chimera experiments revealed that lack of TLR9 in non-hematopoietic cells was responsible for the protection from bacterial super-infection seen in whole-body TLR9KO mice. To assess which structural cell types were important for this protection, we bred epithelial- and endothelial-specific TLR9KO mice, which revealed a role for epithelial cells but not endothelium. Interestingly, we also found that influenza infection caused upregulation of TLR9 in epithelial cells. Moreover, infection with two other respiratory viruses also caused epithelial cells to increase TLR9 expression, as did treatment with the antiviral mediator interferon beta (IFNB). IFNB has long been known to regulate susceptibility to bacterial coinfection during influenza, and these results suggest that it does so in part by upregulating TLR9. Together, these data show a role for epithelial TLR9 upregulation, likely driven by IFNB, in the pathogenesis of respiratory viral/bacterial coinfection. Funding for this research was made possible in part by the Michigan Postdoctoral Pioneer Program at the University of Michigan Medical School, and by NIH grants NHLBI T32HL007517 (Helen Rich) and NHLBI R35HL144481 (Beth Moore).