D-dopachrome tautomerase (DDT),a member of the macrophage migration inhibitory factor (MIF) protein superfamily,is a newly described cytokine with chemokine-like characteristics.However,research on fish DDT remains limited.In this study,we identified a DDT homolog (LjDDT) from the Japanese sea bass,Lateolabrax japonicus.Sequence analysis showed that LjDDT had typical sequence features of known DDT and MIF homologs and was most closely related to DDT of rock bream (Oplegnathus fasciatus).LjDDT transcripts were detected in all tested tissues of healthy Japanese sea bass,with the highest expression found in the liver.Upon infection with Vibrio harveyi,LjDDT transcripts were significantly down-regulated in the three tested tissues,including the liver,spleen,and head kidney.Recombinant LjDDT (rLjDDT) and the corresponding antibody (anti-rLjDDT) were subsequently prepared.The administration of 100 μg/g anti-rLjDDT had a statistically significant protective effect on the survival of V.harveyi-infected fish.Moreover,rLjDDT was able to induce the migration of monocytes/macrophages (MO/MΦ) and lymphocytes both in vitro and in vivo,but without significant influence on the migration of neutrophils.rLjDDT exhibited chemotactic activity for lipopolysaccharide (LPS)-stimulated M1-type MO/MΦ in vitro,but not for cAMP-stimulated M2-type MO/MΦ.Furthermore,the knockdown of LjCD74,but not LjCXCR4,significantly down-regulated the rLjDDT-enhanced migration of MO/MΦ and relieved the rLjMIF-inhibited migration of MO/MΦ.These results indicate that LjCD74 may be the major chemotactic receptor of LjDDT and LjMIF in Japanese sea bass MO/MΦ.Combined rLjDDT+ rLjMIF treatment had no significant effect on the migration of MsiRNA,LjCD74si-,or LjCXCR4-sitreated MO/MΦ compared to the control group,suggesting that the roles of LjDDT and LjMIF may be antagonistic.In conclusion,our study demonstrates for the first time that DDT may play a role in the immune responses of fish against bacterial infection through chemotactic recruitment of MO/MΦ via mediation of CD74 as an antagonist of MIF.