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Cytokine-like factor 1 (CYTL1) is a small cytokine and has diverse biological functions in mammals. However, whether CYTL1 exists in lower vertebrates is not clear. In this study, we identified cytl homologs in fish and characterized the immune functions in a teleost species, grass carp (Ctenopharyngodon idella). Fish CYTL1 homologs share conserved molecular features with their mammalian counterparts, including 6 cysteine residues in the mature peptide, genomic organization and synteny. Gene expression analysis revealed that cytl1 was constitutively expressed in tissues of grass carp, with the highest expression detected in the heart. Upon infection with Aeromonas hydrophila (A. hydrophila) , cytl1 was downregulated in the hindgut, head kidney, skin, and spleen. In the primary head kidney leukocytes (HKLs), stimulation with inactivated A. hydrophila , LPS , poly(I:C), IL-22, IFN-a or IFN-γrel resulted in downregulation of cytl1 expression. Recombinant grass carp CYTL1 protein produced in the HEK293-F cells was potent to induce il-10 expression, but had little effect on the expression of il-1β and il-6. In vivo experiments revealed that CYTL1 was effective to recruit macrophages to the muscle injected with cytl expression plasmids. Taken together, our results indicate that CYTL1 is a potent chemokine for recruitment of macrophages in fish. • Fish CYTL1s share conserved molecular features with their mammalian counterparts. • Grass carp cytl1 is downregulated upon infection with Aeromonas hydrophila. • Grass carp cytl1 is downregulated by LPS , poly(I:C), IL-22, IFN-a and IFN-γrel. • Grass carp CYTL1 is a chemoattractant for macrophages. [ABSTRACT FROM AUTHOR]