A Siglec3-like lectin from Nile tilapia (Oreochromis niloticus): Transcriptional profiling upon Streptococcus agalactiae infection and its potential role in immune escape

Siglec-3, also known as CD33, is a type of sialic acid binding immunoglobulin-like lectin that plays a role in regulating cellular activation in the innate immune system and subsequent inflammatory responses. In this study, we cloned the cDNA encoding OnSiglec3-like ORF sequence (1170 bp) in Nile tilapia, contained two Ig-like domains in the extracellular region and one inhibitory ITIM-like motif in the intracellular region. Through qRT-PCR analysis, we found that the mRNA of OnSiglec3-like lectin was highly expressed in the brain and head kidney. When challenged with Streptococcus agalactiae both in vivo and in vitro, we observed a significant up-regulation in the expression of OnSiglec3-like lectin in the head kidney tissues and leukocytes, respectively. Additionally, we investigated the interaction between a recombinant extracellular region protein (referred to as (r)exSiglec3-like protein) and three different phenotypes of S. agalactiae: WT (wild type), ∆cps (cps-deficient mutant), and ∆neuA (sia-deficient mutant) using ELISA. We found that the exOnSiglec3-like protein, when combined with RNA interference and bacterial infection, significantly enhanced the phagocytic ability against S. agalactiae. Furthermore, the expression of four common inflammatory factors (OnIL-6, OnIL-10, OnTNF-α, and OnTGF-β) showed a significant up-regulation. Overall, this study suggests that OnSigelc3-like lectin may play a role in host defence against bacterial infection and mediate immune escape in Nile tilapia.