Alarming and calming: Dual functions of S100A9 on Mycoplasma gallisepticun infection in avian cells.

Mycoplasma gallisepticum (MG) is the primary causative agent of chronic respiratory disease (CRD) in chickens, characterized by respiratory inflammation. S100A9 plays a pivotal role in modulating the inflammatory response to microbial pathogens. Our prior investigation revealed a significant upregulation of S100A9 in the lungs of chickens following MG infection. This study delves into the immunomodulatory effects of S100A9 during MG infection, demonstrating a notable increase in S100A9 levels in the lungs, immune organs, alveolar epithelial type II cells (AECII), and macrophage HD11 cells of MG-infected chicks and embryos. In MG-infected AECII cells, S100A9 overexpression significantly enhanced MG proliferation and adhesion, suppressed AVBD1, NFκB, pro-inflammatory factors (IL1β and TNFα), and chemokines, reduced apoptosis, and promoted cell proliferation, thereby facilitating MG infection. Conversely, inhibiting S100A9 produced opposing effects. In MG-infected HD11 cells, S100A9 impeded MG proliferation and adhesion, increased AVBD1, NFκB, pro-inflammatory factors, and chemokines, and induced cell apoptosis while inhibiting proliferation. Additional results demonstrated that S100A9 facilitates MG infection by modulating the TLR7/NFκB/JAK/STAT pathway in AECII/HD11 cells. In summary, S100A9 exhibits a dual role in activating/inhibiting the natural immune response through TLR7/NFκB/JAK/STAT pathway regulation. This dual role promotes MG infection in AECII cells while enabling MG to evade immune surveillance by HD11 cells, ultimately enhancing the overall infection process. These findings advance our understanding of host-pathogen interactions during MG infection and underscore S100A9's potential as a therapeutic target for CRD in chickens.