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Interleukin-21 (IL-21) is produced mainly by activated CD4+ T cells and promotes the proliferation, survival and differentiation of immune cells. This study characterised a high affinity binding receptor of IL-21 (IL-21 Rα) from grass carp (Ctenopharyngodon idella, Ci) and analyzed the expression of CiIL-21 and CiIL-21 Rα. It has been shown that they were modulated by PAMPs and during infection with bacterial and viral pathogens. Recombinant CiIL-21 protein produced in bacteria was active in inducing the expression of genes involved in Th responses. A monoclonal CiIL-21 antibody was generated against the rCiIL-21 and showed good reactive specificity with the rCiIL-21 proteins expressed in E. coli cells, and HEK293 T cells and the native protein from PHA-stimulated kidney leukocytes. In addition, the IL-21-producing cells detected by the CiIL-21 monoclonal antibody were found to be increased to in the kidney, spleen and hindgut after infection with Flavobacterium columnare. Our data indicate that IL-21 is involved in the regulation of immune response to bacterial and viral infection.

CC chemokine ligand 19 (CCL19) plays a key role in the regulation of immune responses including homeostasis, inflammation, and immune tolerance. In this study, two variants of CCL19 homologues (CCL19a2 and CCL19b) and CCR7 were investigated in grass carp Ctenopharyngodon idella. The three genes were widely expressed in immune tissues and could be modulated by stimulation with LPS, PHA and poly(I:C), and infection with Flavobacterium columnare and grass carp reovirus. In an in vitro chemotaxis assay, the recombinant CCL19a2 and CCL19b were active to promote the migration of HEK293 T cells expressing CCR7 and leucocytes isolated from the gills, head kidney and spleen. Moreover, their chemotactive effects were validated in vivo. We found that the cells recruited by CCL19a2 and CCl19b are mainly monocytes/macrophages expressing high levels of IL-1β, IFN-γ, colony stimulating factor 1 receptor (CSF1R) and MHC II. Our work suggests that CCL19a2 and CCl19b are involved in recruitment of antigen presenting cells in fish.