Abstract

The CC motif chemokine ligand 25 (CCL25), which specifically binds to the CC chemokine receptor 9 (CCR9), plays a critical role in immune responses during bacterial infections. Although its importance is well-established in mammals, comprehensive studies on the CCL25/CCR9 axis in fish remain limited. In this study, we identified and characterized two alternatively spliced isoforms of CCL25 ( On CCL25a and On CCL25b) and two isoforms of CCR9 ( On CCR9a and On CCR9b) in Nile tilapia ( Oreochromis niloticus ). These genes were expressed in various tissues, with OnCCR9 showing significantly higher expression in IgM + B cells compared to IgM − leukocytes. Following infection with Streptococcus agalactiae or Aeromonas hydrophila , the expression levels of the CCL25/CCR9 axis components were significantly up-regulated in leukocytes from peripheral blood (PBL), spleen (SPL), and head kidney (HK). To investigate functional roles, recombinant On CCL25a and On CCL25b proteins were utilized. Our finding revealed that the CCL25/CCR9 axis promotes B cells differentiation without inducing their differentiation into plasma cells. Additionally, the axis did not influence the migration of PBL, SPL, HK leukocytes, or IgM + B cells, nor did it affect the behavior of HK monocytes/macrophages. Most importantly, the CCL25/CCR9 axis exhibited strong antibacterial activity, including both direct antibacterial effects of On CCL25 and indirect in vivo effects mediated through immune regulation. Specifically, the axis reduced bacterial load and improved survival in Streptococcus agalactiae -infected Nile tilapia. This study provides the first comprehensive characterization of the CCL25/CCR9 axis in Nile tilapia, elucidating its regulatory mechanism on immune cells and highlight s its critical role in host defense against bacterial pathogens.