Epithelial Toll-Like Receptor 9 Upregulation by Influenza Virus Impairs Pulmonary Antibacterial Immunity 2303048

Abstract

Abstract Introduction Influenza is often complicated by bacterial coinfections. Among the most common is methicillin-resistant S. aureus (MRSA). Viruses and bacteria are recognized by pattern recognition receptors (PRRs) such as Toll-like receptor 9 (TLR9). Most studies have focused on TLR9 function in immune cells. In contrast, its role in structural cells remains less understood. Our laboratory found that TLR9 knockout mice exhibit improved outcomes during influenza and MRSA coinfection, prompting us to explore the role of TLR9 in structural cells during coinfection. Methods To study TLR9 in structural cells, we generated mice that lack TLR9 in lung alveolar type II epithelial cells or in mouse lung fibroblasts. We measured bacterial burden and viral load following coinfection with influenza virus and MRSA. We performed primary isolation of alveolar type II cells and alveolar macrophages for in vitro supernatant studies and phagocytosis of MRSA. Results Influenza infection upregulates TLR9 through a MyD88-dependent pathway in fibroblasts, and we are investigating the signaling cascade that leads to TLR9 upregulation in epithelial cells. Epithelial TLR9 expression is detrimental during coinfection: mice lacking epithelial TLR9 have reduced bacterial burden without changes in immune recruitment or lung injury. Lack of TLR9 in alveolar epithelial type II cells does not affect responses to single infections with influenza virus or MRSA. Additionally, alveolar macrophage phagocytosis of MRSA was significantly enhanced by supernatant from influenza-infected TLR9-knockout epithelial type II cells compared with supernatant from influenza-infected wild type epithelial type II cells. We are further exploring epithelial-derived factors that modulate macrophage function. Conclusion These novel findings reveal that epithelial TLR9 regulates antibacterial immunity and highlight its potential as a target for improving coinfection outcome. Funding Source Beth Moore: NHLBI R35HL144481 Helen Rich: NHLBI T32HL007517, Michigan Postdoctoral Pioneer Program Francina Gonzalez De Los Santos: Immunology Graduate Program AI007413 T32, University of Michigan; Herman & Dorothy Miller Award, Immunology Program, University of Michigan Topic Categories Viral Immunology (VIR)