Characterization of two novel MCSFR paralogues in rainbow trout Oncorhynchus mykiss: New insights into the molecular mechanism underlying macrophage differentiation and modulation in fish.

Colony-stimulating factor-1 (CSF-1) receptor, also known as macrophage colony-stimulating factor (MCSF) receptor, belongs to the type III protein tyrosine kinase receptor family. MCSF and IL-34 play essential roles in both innate and adaptive immune systems in vertebrates through their shared receptor MCSFR. While the functional study of MCSFR in mammals has been well-demonstrated, its role in fish remains limited. Therefore, this report aims to identify and study the expression of the MCSFR genes in rainbow trout Oncorhynchus mykiss, where four paralogues were found present at different genomic loci, two identified for the first time in this study. The deduced protein structure of these MCSFRs reveals five immunoglobulin (Ig)-like domains, a transmembrane domain and a conserved intracellular domain containing a glycine-rich motif (Gly-x-Gly-x-x-Gly), similar to other species. Phylogenetic and synteny analyses demonstrate that MCSFR are present throughout vertebrates, with two forms present in teleost fish more generally (type I and type II MCSFR), existing as pairs of genes (MCSFR1a/MCSFR1b, MCSFR2a/MCSFR2b) in trout. The MCSFR genes are widely expressed, with higher transcript levels observed in immune tissues such as the spleen, blood and head kidney. The paralogues showed marked differences in expression modulation. Following Yersinia ruckeri infection, MCSFR2a was highly induced but after stimulation of RTS-11 cells, a trout monocyte/macrophage-like cell line, with Y. ruckeri flagellin both MCSFR1b and MCSFR2a were induced. However, none of the different paralogues of MCSFR were induced by proinflammatory cytokines (trout rTNF-α, rIL-6 and rIFN-γ). This study adds to our knowledge of the molecules/pathways present in fish that drive macrophage regulation and activation, and emphasizes the complexity present with multiple ligands and receptors involved.