Molecular Characterization and Expression Analysis of CD22 in Nile Tilapia (Oreochromis niloticus) and Its Potential Role in Immune Responses

In mammals, CD22 is a member of the Siglec family and plays essential roles in B-cell activation, signal transduction, and immune regulation. However, its functions in teleost fish remain largely unclear. In this study, a CD22 homolog designated On-CD22 was identified and cloned from Nile tilapia (Oreochromis niloticus). On-CD22 transcripts were highly expressed in the head kidney and peripheral blood of healthy fish and showed significant expression changes following infection with Streptococcus agalactiae, Aeromonas hydrophila, or stimulation with poly(I:C). Subcellular localization analysis indicated that On-CD22 is predominantly localized to the plasma membrane. Luciferase reporter assays performed in heterologous cell systems showed that overexpression of On-CD22 was associated with changes in the basal transcriptional activities of NF-κB, IFN1, IFN3, and STAT1 responsive promoters under unstimulated conditions. Furthermore, single-cell transcriptomic analysis revealed that On-CD22 expression was mainly confined to the B-cell population within head-kidney leukocytes. Collectively, these findings suggest that On-CD22 may be involved in immune regulatory processes in Nile tilapia.