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/MФ) 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.
