Clinical symptoms
Lethargy, anorexia and other clinical disease signs were detected in infected
E. awoara (Fig 1). Skin lining, hemorrhagic spots were also found. Congestion with ulceration was the common features.
Expression of MIF in fish tissues in response to V. alginolyticus infection
MIF has been detected in all healthy and diseased fish organs (Fig 2). The sampled tissues consisted of head kidney, spleen, liver, brain, intestine, gill, heart, stomach and muscle. PBS and
V. alginolyticus were injected into fishes and served as control groups and treatment groups. MIF mRNA levels were low in the tissues sampled in control groups. The MIF mRNA levels of the head kidney, liver, brain and spleen in diseased
E. awoara increased compared with those in control groups (
P<0.05) (Fig 2).
MIF, TNF-α and IL-1tissue levels
ELISA was used for detecting MIF, TNF-α and IL-1 levels
. The tissues consisted of spleen, head kidney, muscle, intestine and liver. Tissue levels of MIF (Fig 3A), TNF-α (Fig 3B) and IL-1 (Fig 3C) were found higher in infected
E. awoara compared to the levels in the untreated infected fish tissues. These average values were from all 5 fishes. The MIF levels of liver were 6.280±0.110 ng/mL in diseased
E. awoara, while 5.309±0.031 ng/mL in that of control. Tissue MIF levels of spleen, head kidney, intestine and muscle infected fishes were 6.095±0.070 ng/mL, 6.027±0.035 ng/mL, 6.115±0.046 ng/mL and 6.075±0.116 ng/mL higher than the MIF levels in untreated infected fishes (5.737±0.039 ng/mL, 5.478±0.047 ng/mL, 5.478±0.047 ng/mL and 4.988±0.037 ng/mL). There was a significant difference between uninfected, untreated control groups and infected (
P<0.05, Fig 3A).
Likewise, TNF-α levels in all the tissues were higher in treatment groups than those in controls. With regard to the tissues of
E. awoara, TNF-α levels of liver, spleen, head kidney, intestine and muscle in the infected groups were 0.690±0.005 ng/mL, 0.326±0.003 ng/mL, 0.358±0.005 ng/mL and 0.316±0.002 ng/mL respectively and 0.476±0.007 ng/mL, 0.291±0.004 ng/mL, 0.294±0.008 ng/mL, 0.279 ±0.005 ng/mL and 0.281±0.007 ng/mL in controls. TNF-α levels in the tissues were increased in treatment groups compared to controls (
P<0.05, Fig 3B).
The IL-1 levels in spleen, head kidney, intestine, muscle and liver of diseased fishes were 0.155±0.001 ng/mL, 0.158±0.001 ng/mL, 0.158±0.002 ng/mL, 0.157±0.004 ng/mL and 0.159±0.004 ng/mL, respectively. Infected fishes displayed higher tissue levels of IL-1 detected by ELISA than those found in the non-infected groups. The IL-1 levels of spleen, head kidney, intestine, muscle and liver of untreated infected fishes were significantly lower (0.139±0.001 ng/mL, 0.147±0.002 ng/mL, 0.134±0.002 ng/mL, 0.149±0.003 and 0.145±0.002 ng/mL, respectively). The two groups had statistically significant differences (
P<0.05, Fig 3C). These results show MIF, TNF-α and IL-1 play a role in diseased fishes and the levels in the spleen, head kidney, intestine and muscles were much higher than those in controls (Fig 3A, B, C). MIF, TNF-α and IL-1 levels in
V. alginolyticus -affected fishes were positively correlated with bacteria in the fishes. The MIF, TNF-α and IL-1 could be detected in controls.
V. alginolyticus had been isolated from some diseased marine invertebrates, including
E. awoara (
Mohamad et al., 2019), shrimp
(Yin et al., 2022), Epinephelus coioides (Wang
et al., 2021) and
Charybdis japonica (Zhang et al., 2015).
E. awoara fishes infected with
V. alginolyticus had anorexia, local hemorrhagic ulcers in head and body and other clinical signs of
V.
alginolyticus caused mortality in
E. awoara.
MIF plays an important role in the immunity system. MIF is ancient in its evolutionary basis and is a highly conserved cytokine with multiple functions. MIF promotes colorectal cancer and tumour invasion. MIF deficiency reduced chronic inflammation in the tissue.
We reported that MIF levels were higher in tissues of
E. awoara infected with
V.
alginolyticus than those in controls. MIFmRNA levels of the examined tissues have been detected by qPCR analysis in other teleo steans
(Xu et al., 2016; Xu et al., 2019) and mammals
(Poulsen et al., 2021; Wang et al., 2021). MIF levels of the tissues in infected with
Edwardsiella tarda groups were significantly lower than those from fishes in the untreated infected groups. MIF could be determined in various cells and tissues of the endocrine system.The system includes lung, skin and gastro-intestinal tissues in mammals. In vertebrate species (
Howait et al., 2019) and in sea bass
(Xu et al., 2019), the thymus had the highest MIF mRNA levels. However, the MIF mRNA levels in the gut and spleen were lower than those in gills and head kidney in
Tetraodon nigroviridis. MIF mRNA levels of the tissues in
V. alginolyticus-affected fishes were detected higher than those of untreated healthy fish groups. Brain, head kidney, spleen and liver have higher levels of MIF mRNA than those in other tissues, including muscle, gill, heart, stomach and intestines in diseased fishes. MIF was relevant to
E. awoara immune defence against invading
V. alginolyticus. MIF played a role in fish susceptibility to in flammatory diseases. High levels of MIF expressions were associated with autoimmune diseases (
Baños-Hernández et al., 2019). As a cytokine, MIF also played a critical role in inflammatory and infectious diseases.
In this study, the qRT-PCR analysis results were consistent with the results of ELISA analyses. In infected
E. awoaras, MIF levels in head kidney and liver were higher than in other tissues. Statistically significant differences in TNF-α and IL-1 expressions were observed in treated organs compared to control organs. This result was similar to previous reports that an overreaction in host response can occur that results in IL-1 and TNF-α and leading to the loss of periodontal attachment
(Werber et al., 2021). MIF could induce IL-1 and TNF-α (
Howait et al., 2019;
Jiao et al., 2020). MIF increased tumour cell proliferation by releasing cytokines including TNF-α, IL-1b and IL-6
(Kolostova et al., 2022). In our study, tissue levels of MIF, TNF-α and IL-1 were up-regulated in infected
E. awoara; these results were similar to previously reported results
(Sam et al., 2021).These results may offer a novel therapeutic strategy for treating bacterial disease.