Molecular characterization and functional analysis of EBF1 reveals its critical role in B-cell immunity in Nile tilapia.

The transcription factor EBF1 is essential for B-cell development and adaptive immunity, yet its function in teleost (Nile tilapia) remains unclear. In this study, we identified and characterized the On-EBF1 gene in Nile tilapia (Oreochromis niloticus). It features an open reading frame of approximately 1.8 kb that encodes a 599-amino acid nuclear protein with conserved IPT and HLH domains, and the encoded protein shares over 80% homology with other teleosts and over 70% homology with mammals. On-EBF1 is highly expressed in the spleen and is specifically expressed in the B cell subset of the head kidney leukocytes. Upon Streptococcus agalactiae (S. agalactiae) infection, temporal upregulation of On-EBF1 was observed across multiple tissues but was significantly downregulated in B cells at 24 h post-stimulation with either S. agalactiae or Aeromonas hydrophila (A. hydrophila). Following S. agalactiae challenge, in vivo silencing of On-EBF1 markedly reduced fish survival, which was accompanied by dysregulated inflammation, aberrant apoptosis, complement system disruption, and impaired tissue-specific expression of key B cell differentiation factors (Pax5, CD19a/b, IgM, Blimp1). Collectively, our research results demonstrate that On-EBF1 is the central regulatory factor for bacterial immunity in Nile tilapia, suggesting that its strictly regulated expression is crucial for adaptive immunity by regulating B-cell differentiation. This work provides new insights into the immune network of lower vertebrates.