Molecular characterization and functional analysis of On-Siglec2 in the antibacterial response of Nile tilapia to Streptococcus agalactiae

Sialic acid-binding immunoglobulin-type lectin 2 (Siglec2, also known as CD22) is a key immunoregulatory molecule in mammals; however, its role in antibacterial immunity remains poorly understood in teleost fish. In this study, we identified and characterized a Siglec2 homolog from Nile tilapia (Oreochromis niloticus), designated On-Siglec2. The open reading frame of On-Siglec2 is 1767 bp and encodes a type I transmembrane protein of 588 amino acids. On-Siglec2 was broadly expressed in all examined tissues, with the highest transcript levels detected in the head kidney, and its expression was significantly upregulated following Streptococcus agalactiae infection. A recombinant extracellular fragment of On-Siglec2 (rOn-Siglec2) exhibited binding activity toward both Gram-positive and Gram-negative bacteria but did not significantly alter monocyte/macrophage phagocytic activity under the conditions tested. In vivo administration of rOn-Siglec2 was associated with a modest improvement in survival and reduced bacterial burdens. rOn-Siglec2 administration was also associated with altered transcription of immune-related genes. Collectively, these findings suggest a potential involvement of On-Siglec2 in the antibacterial immune response of Nile tilapia, while rOn-Siglec2 administration may confer partial protection against S. agalactiae infection. The physiological role of endogenous On-Siglec2 and the underlying molecular mechanisms remain to be elucidated.