TLR7 is required for optimal immune defense against bacterial infection in tongue sole (Cynoglossus semilaevis)

• Tongue sole TLR7 (CsTLR7) was expressed in high levels in kidney, spleen and liver.
• CsTLR7 expression was upregulated by bacteria and downregulated by virus.
• Knockdown of CsTLR7 significantly enhanced bacterial dissemination in fish tissues.
• Activation of CsTLR7 enhanced host resistance against bacterial infection.
• Inhibition of CsTLR7 reduced host resistance against bacterial infection.

In mammals as well as in teleost, toll-like receptor 7 (TLR7) is known to be involved in antiviral immunity by recognizing viral RNA. However, the antibacterial potential of fish TLR7 is unclear. In this study, we analyzed the TLR7 of tongue sole (Cynoglossus semilaevis), CsTLR7, and examined its potential involvement in antibacterial immunity. CsTLR7 is composed of 1052 amino acid residues and shares 64.0%–75.9% overall sequence identities with known teleost TLR7. CsTLR7 possesses a toll/interleukin-1 receptor domain and six leucine-rich repeats. Constitutive expression of CsTLR7 occurred in relatively high levels in kidney, spleen and liver. Bacterial infection upregulated CsTLR7 expression, whereas viral infection downregulated CsTLR7 expression. Knockdown of CsTLR7 significantly enhanced bacterial dissemination in the tissues of tongue sole. Treatment of tongue sole with the imidazoquinoline compound R848 (TLR7 activator) and the endosomal acidification inhibitor chloroquine (TLR7 inhibitor) caused enhanced and reduced resistance against bacterial infection respectively. These results indicate that CsTLR7 plays an essential role in the antibacterial immunity of tongue sole.