MCSFR+ monocytes/macrophages are activated to produce IL-4/13 in the gill lamellae of grass carp after infection with bacterial pathogens

Macrophage colony-stimulating factor receptor (MCSFR) is a cell lineage marker for monocytes/macrophages and has been applied for cell typing in mammals. However, lack of high-quality monoclonal antibodies hampers isolation and functional characterization of monocytes/macrophages in fish. A monoclonal antibody (mAb) against the extracellular region of grass carp (Ctenopharyngodon idella, Ci) MCSFR using the recombinant protein produced in the E. coli cells was generated in this study. Western blotting showed the mAb could recognize the native CiMCSFR protein derived from tissues and the recombinant CiMCSFR protein expressed in the E. coli cells and HEK293 cells. The antibody did not cross-react with the MCSFR homolog in zebrafish, indicating that it has good species specificity. Immunofluorescence analysis showed MCSFR+ monocytes/macrophages distributed in the gills, hindgut, head kidney and spleen of healthy grass carp. Furthermore, Aeromonas hydrophila infection resulted in increased MCSFR+ monocytes/macrophages in the lamina propria and epithelium of hindgut mucosa. In addition, Flavobacterium columnare infection led to the infiltration of MCSFR+ monocytes/macrophages in the gills, and interestingly, the MCSFR+ monocytes/macrophages in the gill lamellae but not filaments are activated to produce IL-4/13A and IL-4/13B, while MCSFR+ monocytes/macrophages in the hindgut did not produce IL-4/13A and IL-4/13B. Our results indicate that monocytes/macrophages are participated in mediating mucosal immune responses to bacterial infections and will help understand host immune defense to bacterial diseases.