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

## TL;DR

This study identifies and analyzes a CD22 homolog in Nile tilapia, suggesting it plays a role in immune regulation.

## Contribution

The study identifies and functionally characterizes a CD22 homolog in Nile tilapia for the first time.

## Key findings

- On-CD22 is highly expressed in immune-related tissues and changes dynamically after immune challenges.
- On-CD22 is localized to the plasma membrane and affects NF-κB, interferon, and STAT1 signaling.
- On-CD22 expression is mainly confined to B-cells in head-kidney leukocytes.

## Abstract

CD22 (Siglec-2) is an inhibitory co-receptor on mammalian B cells that plays an important role in immune regulation, whereas its functions in teleost fish are poorly understood. In this study, a CD22 homolog (On-CD22) was identified in Nile tilapia and examined under immune challenge conditions. On-CD22 was highly expressed in immune-related tissues and exhibited dynamic expression changes following bacterial infection or viral mimic stimulation. The protein was localized to the plasma membrane, and overexpression assays were associated with alterations in the basal activities of NF-κB, interferon, and STAT1 related signaling reporters. These findings suggest that On-CD22 may be involved in immune regulatory processes in Nile tilapia.

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.