TLR9 and bacterial infectious disease: Noteworthy, the Legionella flagellin effectors and peptidoglycan-associated lipoprotein (PAL) are recognized by host cells toll-like receptor 2 (TLR2), TLR5, and TLR9, which activate the downstream nuclear factor NF-κB, inflammasome, and caspase-1 to induce pro-inflammatory cytokine production and restrict bacterial infection (Liu and Shin, 2019).