Molecular cloning and expression analysis of immunoglobulin M heavy chain gene of blunt snout bream (Megalobrama amblycephala)

Immunogenicity and protective efficacy of OmpA subunit vaccine against Aeromonas hydrophila infection in Megalobrama amblycephala: An effective alternative to the inactivated vaccine

Aeromonas hydrophila is a kind of zoonotic pathogen, which can cause bacterial septicemia in fish and bring huge economic losses to global aquaculture. Outer membrane proteins (Omps) are conserved antigens of Aeromonas hydrophila, which can be developed as subunit vaccines. To evaluate the protective efficacy of inactivated vaccine and recombinant outer membrane protein A (OmpA) subunit vaccine against A. hydrophila in juvenile Megalobrama amblycephala, the present study investigated the immunogenicity and protective effects of both vaccines, as well as the non-specific and specific immune response of M. amblycephala. Compared with the non-vaccinated group, both inactivated and OmpA subunit vaccines improved the survival rate of M. amblycephala upon infection. The protective effects of OmpA vaccine groups were better than that of the inactivated vaccine groups, which should be attributed to the reduced bacterial load and enhanced host immunity in the vaccinated fish. ELISA assay showed that the titer of serum immunoglobulin M (IgM) specific to A. hydrophila up-regulated significantly in the OmpA subunit vaccine groups at 14 d post infection (dpi), which should contribute to better immune protective effects. In addition, vaccination enhanced host bactericidal abilities might also attribute to the regulation of the activities of hepatic and serum antimicrobial enzymes. Moreover, the expression of immune-related genes (SAA, iNOS, IL-1 β, IL-6, IL-10, TNF α, C3, MHC I, MHC II, CD4, CD8, TCR α, IgM, IgD and IgZ) increased in all groups post infection, which was more significant in the vaccinated groups. Furthermore, the number of immunopositive cells exhibiting different epitopes (CD8, IgM, IgD and IgZ) that were detected by immunohistochemical assay had increased in the vaccinated groups post infection. These results show that vaccination effectively stimulated host immune response (especially OmpA vaccine groups). In conclusion, these results indicated that both the inactivated vaccine and OmpA subunit vaccine could protect juvenile M. amblycephala against A. hydrophila infection, of which OmpA subunit vaccine provided more effective immune protection and can be used as an ideal candidate for the A. hydrophila vaccine.

Evaluation of Ammonia Nitrogen Exposure in Immune Defenses Present on Spleen and Head-Kidney of Wuchang Bream (Megalobrama amblycephala)

Ammonia is one of the most important environmental factors in aquatic ecosystems. However, there are limited studies on the effects of chronic or long-term ammonia stress and its potential molecular mechanism in fish. This study aimed to investigate the immune response and molecular mechanisms in the spleen and head-kidney of fish following chronic ammonia exposure. Megalobrama amblycephala (9.98 ± 0.48 g) were exposed to different concentrations of total ammonia nitrogen (0-30 mg/L) for 30 days. Ammonia exposure caused significant increases in cortisol levels and decreases in lysozyme and complement 3/4 concentrations in the serum, indicating inhibitory effects of ammonia stress on innate immune responses. Ammonia exposure also induced concentration-dependent increases in ammonia concentrations in tissue, pathological damage and indexes of spleen and head-kidney. Additionally, the contents of immunoglobulin M (IgM), interleukin 1β (IL-1β) and tumor necrosis factor α (TNF-α) as well as mRNA levels of toll-like receptors (TLRs)/Myeloid differentiation factor 88 (MyD88)-independent signaling molecules in the spleen and head-kidney were significantly downregulated after ammonia exposure. Our findings suggested that chronic ammonia exposure caused the suppression of innate and adaptive immune responses through downregulating TLR/MyD88-independent signaling. Adverse influences of chronic ammonia stress were more severe in the spleen than in the head-kidney.

Dietary phytic acid weakened the antimicrobial activity and aggravated the inflammatory status of head kidney, spleen and skin in on-growing grass carp (Ctenopharyngodon idella)

The present study aimed to explore the effects of phytic acid (PA) on the antimicrobial activity and inflammatory response in three immune organs (head kidney, spleen and skin) of on-growing grass carp (Ctenopharyngodon idella). To achieve this goal, we first conducted a 60-day growth trial by feeding fish with graded levels of PA (0, 0.8, 1.6, 2.4, 3.2 and 4.0%). Then, the fish were challenged with Aeromonas hydrophila for 6 days. Compared with the control group, the following results were obtained regarding supplementation with certain levels of PA in the diet. (1) There was an increase in skin haemorrhage and lesion morbidity in fish. (2) There was a decrease in activities or contents of immune factors, including lysozyme (LZ), complement 3 (C3), C4 and immunoglobulin M (IgM), and there was downregulation of gene expression levels of hepcidin, liver-expressed antimicrobial peptide 2A (LEAP-2A), LEAP-2B, and β-defensin-1 in immune organs. (3) There was upregulation in the gene expression of the following pro-inflammatory cytokines: tumour necrosis factor α (TNF-α), interleukin 1β (IL-1β) (except in the spleen), interferon γ2 (IFN-γ2), IL-6 (except in the spleen), IL-8, IL-12p40, IL-15 and IL-17D. These changes were partly related to the nuclear factor kappa B (NF-κB) signalling pathway, but downregulation of mRNA levels of anti-inflammatory cytokines (transforming growth factor β1 (TGF-β1), TGF-β2, IL-413/A, IL-413/B, IL-10 (except in the skin) and IL-11) occurred in a manner partially related to the target of rapamycin (TOR) signalling pathway. Finally, based on the broken-line analysis of skin haemorrhage and lesion morbidity and IgM content in the head kidney, the maximum tolerance levels of PA for on-growing grass carp (120.56-452.00 g) were estimated to be 1.79 and 1.31% of the diet, respectively.

Phytic acid disrupted intestinal immune status and suppressed growth performance in on-growing grass carp (Ctenopharyngodon idella)

Phytic acid (PA) is one of the most common anti-nutritional factors in plant-derived protein feeds, and it poses considerable threats to aquaculture production. However, little is known about the effects of PA on fish intestinal health. This study aimed to investigate the impacts of PA on intestinal immune function in on-growing grass carp. To achieve this goal, a growth trial was conducted for 60 days by feeding 540 fish (120.56 ± 0.51 g) with six semi-purified diets containing graded levels of PA (0, 0.8, 1.6, 2.4, 3.2 and 4.0%). Then fish were challenged with Aeromonas hydrophila for 6 days. The results indicated that, compared with the control group (0% PA), PA did the following: (1) suppressed fish growth performance (percentage weight gain and feed efficiency) and reduced their ability to resist enteritis; (2) decreased fish intestinal antimicrobial ability by reducing intestinal lysozyme (LZ) activities, the contents of complement 3 (C3), C4 and immunoglobulin M (IgM), and downregulating the mRNA levels of hepcidin, liver-expressed antimicrobial peptide 2A (LEAP-2A), LEAP-2B, and β-defensin-1; and (3) aggravated fish intestinal inflammation responses by upregulating the mRNA levels of pro-inflammatory cytokines including tumour necrosis factor α (TNF-α), interleukin 1β (IL-1β) (except in the DI), interferon γ2 (IFN-γ2), IL-8, IL-12p40, IL-15 (except in the DI) and IL-17D, which is partly related to the nuclear factor kappa B (NF-κB) signalling pathway, whereas downregulating the mRNA levels of anti-inflammatory cytokines including transforming growth factor β1 (TGF-β1), IL-4/13A, IL-4/13B, IL-10 and IL-11, which is partially associated with the target of rapamycin (TOR) signalling pathway. The possible reasons for some distinctive gene expression patterns in fish three intestinal segments were discussed. Finally, based on the percent weight gain, enteritis morbidity, IgM content and LZ activity in the PI, the maximum tolerance levels of PA for on-growing grass carp were estimated to be 2.17, 1.68, 1.47 and 1.18% of the diet, respectively.

Molecular characterization and expression analysis of secretory immunoglobulin M (IgM) heavy chain gene in rohu, Labeo rohita

Immunoglobulin M (IgM) is the major isotype among teleost immunoglobulins. The present study was aimed to explore IgM heavy chain gene and its expression profile in rohu. Full-length IgM heavy chain cDNA of rohu consisted of 1994 bp encoding a polypeptide of 576 amino acid residues including a leader peptide, variable (VH) and constant (CH1-CH2-CH3-CH4) domains confirming the secretory form of IgM. The sequence carries conserved residues such as cysteine, tryptophan and amino acid motifs like 'YYCAR' and 'FDYWGKGT-VTV-S'. The predicted 3 D model confirmed various domains of rohu IgM heavy chain. Phylogenetic tree analysis revealed that IgM heavy chain gene of rohu shared the same cluster with that of other cyprinid fishes. Tissue distribution analysis showed the predominant level of IgM heavy chain gene expression in kidney, spleen and intestine. IgM heavy chain gene expression in rohu kidney was found to be up-regulated and reached a maximum at 7 days post-challenge with Aeromonas hydrophila. These findings demonstrate the first report of full-length secretory IgM heavy chain gene in rohu. Besides, IgM heavy chain gene was highly expressed in major lymphoid tissues and bacterial challenge influenced its expression which further confirmed its role in the adaptive humoral immune response.

Cloning and expressional analysis of secretory and membrane-bound IgM in rock bream (Oplegnathus fasciatus) under megalocytivirus infection and vaccination

In this study, for better understanding the humoral immunity of rock bream (Oplegnathus fasciatus), 2 transcripts of immunoglobulin M (IgM) heavy chain gene including membrane bound (m-IgM) and secretory (s-IgM) forms were sequenced and analyzed their tissue distribution and differential expression in rock bream under rock bream iridovirus (RBIV) infection and vaccination since RBIV has caused mass mortality in rock bream aquaculture in Korea. Consequently, s-IgM cDNA was 1902 bp in length encoding a leader region, a variable region, four constant regions (CH1, CH2, CH3, CH4) and a C-terminal region while m-IgM cDNA was 1689 bp in length encoding shorter three constant regions (CH1, CH2, CH3) and two transmembrane regions. The predicted s-IgM and m-IgM represent a high structural similarity to other species including human. In tissue distribution analysis in healthy fish, the highest expression of s-IgM was observed in head kidney followed by body kidney, spleen, and mid gut whereas m-IgM expression was the highest in blood followed by head kidney and spleen. In vitro, s-IgM expression was up-regulated by LPS in head kidney and spleen cells at 24 h with no change of m-IgM expression. In vivo upon vaccination, s-IgM expression was up-regulated in liver and blood but not in head kidney while m-IgM expression was only up-regulated in head kidney. After challenge with RBIV, s-IgM expression level was higher in vaccinated fish than in unvaccinated fish and m-IgM expression was up-regulated in head kidney of vaccinated group. In conclusion, differential expression of m-IgM and s-IgM may indicate their differential functions to produce the most effective IgM during adaptive immune response. Although it is not able to assess specific IgM at protein level due to a lack of antibody against rock bream IgM, the present study on s-IgM and m-IgM gene expressions upon infection and vaccination will be useful in developing efficient vaccines in the future.

Effects of dietary toxic cyanobacteria and ammonia exposure on immune function of blunt snout bream (Megalabrama amblycephala)

Cyanobacterial blooms caused by water eutrophication have become a worldwide problem. During the degradation of toxic cyanobacterial blooms, elevated ammonia and microcystins concentrations co-occur and exert toxicity on fish. Up to now, the combined effect of microcystins and ammonia on fish immunotoxicity has not been reported. The present study investigated immune responses of blunt snout bream (Megalabrama amblycephala) to dietary toxic cyanobacteria and ammonia exposure. Megalobrama amblycephala were exposed to solutions with different concentrations of NH₃-N (0, 0.06, 0.12 mg/L) and fed with diets containing 15% and 30% of toxic cyanobacteria lyophilized powder for 30 d. The microcystins concentration in different organs of Megalobrama amblycephala was in the following sequence: head kidney > liver > intestine > gonad > spleen > gill > trunk kidney > brain > muscle > heart. In both head kidney and spleen, the MC-LR and MC-RR concentration increased significantly with increasing NH₃-N concentration. It indicates that NH₃-N maybe promote the accumulation of microcystins in immune organs of Megalobrama amblycephala. Meanwhile, broadened peripheral interspace of lymphocytes, nucleus shrivel and edematous mitochondria were observed in head kidney lymphocyte of toxic treatment fish. Moreover, there were significant interactions between dietary toxic cyanobacteria and ammonia exposure on head kidney macrophage phagocytosis activity, respiratory burst activities, total number of white blood cells and the transcriptional levels of sIgM, mIgD and sIgZ genes. Our data clearly demonstrated that dietary toxic cyanobacteria combined with ammonia exposure showed a synergistic effect on Megalobrama amblycephala immunotoxicity.

The effect of Aeromonas hydrophila infection on the non-specific immunity of blunt snout bream (Megalobrama amblycephala)

Aeromonas hydrophila is the main reason of epidemic septicaemia for freshwater fish. In the present study, the effect of Aeromonas hydrophila infection on the non-specific immunity of blunt snout bream (Megalobrama amblycephala) was studied. After Aeromonas hydrophila challenge, lysozyme activity was significantly increased at 4 h, 1 d, 3 d, 5 d, 14 d and 21 d. An increased level of lysozyme activity indicated a natural protective mechanism in fish. The significant increases of superoxide dismutase activity and catalase activity in treatment group were detected at 4 h, 1 d, 3 d, 5 d, 14 d and 21 d after Aeromonas hydrophila challenge. Increase in the superoxide anion and H2O2 is considered to be beneficial for self-protection from disease. Acid phosphatase activity increased significantly at 1 d, 3 d and 5 d after Aeromonas hydrophila challenge. Alkaline phosphatase activity in treatment group showed significant increase at 4 h, 1 d, 3 d, 5 d, 14 d and 21 d compared to control group. Increased phosphatase activity indicates higher breakdown of the energy reserve, which is utilized for the growth and survival of fish. These results revealed that the non-specific immunity of fish played an important role in self-protection after pathogens infection.

sIgZ exhibited maternal transmission in embryonic development and played a prominent role in mucosal immune response of Megalabrama amblycephala

IgZ is considered to be the last immunoglobulin discovered in vertebrate species. In this study, the structure of secreted form of blunt snout bream (Megalabrama amblycephala) IgZ (sIgZ) heavy chain gene is VH-Cζ1-Cζ2-Cζ3-Cζ4, in which Cζ4 provides the specificity to the IgZ isotype. The deduced amino acid sequence of sIgZ shows highest similarity with that of Ctenopharyngodon idella (79%). The ontogeny of sIgZ gene expression showed a V-shape change pattern: decreased initially from unfertilized egg stage to 16-cell embryos and increased significantly from blastula stage, which exhibited maternal transmission effects. Compared with the juvenile fish, sIgZ mRNA level was higher in head kidney, spleen, trunk kidney, liver, intestine and gill of adult fish. In both juvenile and adult fish, sIgZ mRNA was detected in intestine and gill. Aeromonas hydrophila challenge experiment showed that sIgZ transcription significantly increased in skin, gill and intestine, indicating a prominent mucosal immune response. The results of Western blot also verified the protein alterations of sIgZ in mucosal organs in M. amblycephal. Our studies indicate a prominent role of IgZ in mucosa-associated lymphoid tissue immunity and further support the specialized role of IgZ in teleost mucosal immune responses.

Spatio-temporal expression of blunt snout bream (Megalobrama amblycephala) mIgD and its immune response to Aeromonas hydrophila

The function of IgD in fish and mammals has not been fully understood since its discovery. In this study, we have isolated and characterized the cDNA that encodes membrane-bound form of the immunoglobulin D heavy chain gene (mIgD) of blunt snout bream (Megalobrama amblycephala) using RT-PCR and rapid amplification of cDNA ends (RACE). The full-length cDNA of mIgD consisted of 3313 bp, encoding a putative protein of 943 amino acids. The structure of blunt snout bream mIgD is VDJ-μ1-δ1-δ2-δ3-δ4-δ5-δ6-δ7-TM. Multiple alignment and phylogenetic analyses indicated that blunt snout bream mIgD clusters with the homologues of cyprinid fish and that its highest identity is with that of C. idella (82%). The mIgD expression in early different developmental stages showed that the level of mIgD mRNA decreased dramatically from the unfertilized egg stage to the 32-cell stage, suggesting that mIgD mRNA was maternally transferred. As cell differentiation initially took place in the blastula stage, the mIgD expression increased significantly from the blastula stage to prelarva, which might be attributed to embryonic stem cell differentiation processes. Compared with juvenile fish, the expression and tissue distribution patterns of mIgD in adult individuals exhibited considerable variation. After the injection of Aeromonas hydrophila, mIgD expression was up-regulated in various tissues, reaching the peak expression at 5 d, 14 d or 21 d (depending on the tissue type). The present study provides a theoretical basis for further research of the teleost immune system.