Tongue Sole CD209: A Pattern-Recognition Receptor that Binds a Broad Range of Microbes and Promotes Phagocytosis

CD209 is an immune receptor that plays an important role in the initiation of innate immunity and activation of adaptive immunity in mammals. However, much less is known about the immunological function of CD209 in lower vertebrates. In the present study, we examined the immune effect of a CD209 homologue (CsCD209) from the teleost fish tongue sole Cynoglossus semilaevis. CsCD209 possesses a lectin domain that shares high levels of similarity with the lectin domains of human and mouse CD209. CsCD209 expression was most abundant in kidney and blood and was significantly upregulated during bacterial infection. CsCD209 exhibited a subcellular localization mainly on the cell surface of myelomonocytes. Recombinant CsCD209 displayed apparent binding capacities to a broad range of bacteria and fungi, and significantly promoted the phagocytosis of the bound bacteria by C. semilaevis leukocytes. Collectively, the results indicate that teleost CD209 serves as a pattern recognition receptor that exerts an influence on the phagocytosis process during pathogen infections.