Discovery and expression of 3 siglecs-like in Oreochromis niloticus neutrophil, and their interaction with group B streptococcal sialylated capsular polysaccharides.

Sialic acid - binding immunoglobulin - like lectins (Siglecs) are members of the largest superfamily of immune receptors; they recognize sialic acid and are mainly expressed in immune cells. Studies on mammals indicate that Streptococcus agalactiae (GBS) evade immune reactions by interacting with the host immune cells via the sialic acid of sialylated capsular polysaccharides. However, it is currently unknown if fish-derived GBS can interact with Siglecs to evade host immunity. In this study, we examined the binding of FITC-GBS with neutrophils to determine the presence of receptors that binds with GBS. Furthermore, 3 Siglec-like genes, (OnSiglec-1-like/-4b-like/-14-like) from the neutrophils cDNA were screened by PCR. All the genes had specific domains (immunostimulation and immunosuppression domains), conserved amino acid residues, and sialic acid polysaccharide binding sites that are found in mammalian Siglecs. Flow cytometry of Siglecs-like/COS-7 cells and ELISA of Siglecs/Ex-Fc fusion proteins confirmed that 3 Siglecs-like have high binding activity with GBS. Erythrocytes adhesion assays and sialylated glycans binding assay confirmed that 3 Siglecs-like bind to sialic acid polysaccharides. Siglecs-like had high expression levels in the spleen, gill, and kidney in Oreochromis niloticus by qPCR. After experimental infection, Siglec-1-like/-14-like showed a significant upregulated initially and later downregulated in liver, spleen, kidney, and gill. However, Siglec-4b-like was downregulated in most tissues, except that in liver. The results indicate that 3 OnSiglecs-like may recognize GBS sialylated capsular polysaccharides. GBS infections led to significant changes in Siglecs-like expression in immune-related tissues. However, immunostimulation or immunosuppression via the recognition of GBS by different Siglecs-like molecules requires additional studies.