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

In mammals, <i>CD22</i> 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 <i>CD22</i> homolog designated <i>On-CD22</i> was identified and cloned from Nile tilapia (<i>Oreochromis niloticus</i>). <i>On-CD22</i> transcripts were highly expressed in the head kidney and peripheral blood of healthy fish and showed significant expression changes following infection with <i>Streptococcus agalactiae</i>, <i>Aeromonas hydrophila</i>, or stimulation with poly(I:C). Subcellular localization analysis indicated that <i>On-CD22</i> is predominantly localized to the plasma membrane. Luciferase reporter assays performed in heterologous cell systems showed that overexpression of <i>On-CD22</i> 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 <i>On-CD22</i> expression was mainly confined to the B-cell population within head-kidney leukocytes. Collectively, these findings suggest that <i>On-CD22</i> may be involved in immune regulatory processes in Nile tilapia.