Fish NF‐κB couples TCR and IL‐17 signals to regulate ancestral T‐cell immune response against bacterial infection

As fish constitute the first evolutionary group with primordial T cells, they are of importance for understanding the origin and evolution of adaptive immunity. Yet, the knowledge about how ancestral T cells function remains limited. Therefore, the teleost model Nile tilapia (Oreochromis niloticus) was used in this study to investigate the regulatory mechanisms of T-cell immunity in fish. We identified an evolutionarily conserved canonical NF-κB signaling pathway in Nile tilapia, which participates in primary adaptive immune response during Streptococcus agalactiae infection. Blockade of NF-κB activity severely impairs T-cell activation and expansion, rendering the animals more vulnerable to pathogen attack. Meanwhile, NF-κB signaling is indispensable for fish T cells to produce IL-17A during the antibacterial immune response. Moreover, IL-17A binds its receptor IL-17RA, initiates the ACT1–TRAF6–TAK1 axis, and triggers NF-κB-dependent T-cell activation, thus forming a positive feedback loop of T-cell immunity in Nile tilapia. Furthermore, IL-17A seems to promote innate immunity by regulating pro-inflammatory cytokines via TRAF6–NF–κB axis, indicating the presence of an NF-κB-dependent IL-17A signaling pathway for coordinating adaptive and innate immunity in fish. Our results suggest that fish NF-κB couples TCR and IL-17 signals to modulate ancestral T-cell immunity against bacterial infection, and the regulation of T-cell immunity by NF-κB and IL-17 is a strategy that existed prior to the divergence of the tetrapod lineage from teleost fish. This study, therefore, provides a new perspective on the evolution of adaptive immunity.