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

Effects of Aeromonas infection on the immune system, physical barriers and microflora structure in the intestine of juvenile grass carp (Ctenopharyngodon idella)

Grass carp (Ctenopharyngodon idella) is an intensively cultured and economically important herbivorous fish species in China, but its culture is often impacted by Aeromonas pathogens such as Aeromonas hydrophila and Aeromonas veronii. In this study, healthy grass carp were separately infected with A. hydrophila or A. veronii for 12, 24, 48 or 72 h. The results showed that the mRNA expression levels of intestinal inflammatory factors (tnf-α, il-1β and il-8), complement factors (c3 and c4), antimicrobial peptides (hepcidin, nk-lysin and β-defensin-1), immunoglobulins (igm and igt), and immune pathway-related signaling molecules (tlr1, tlr2, tlr4, myd88, irak4, irak1, traf6, nf-κb p65 and ap-1) were differentially upregulated in response to A. hydrophila and A. veronii challenge. Additionally, the expression levels of the intestinal pro-apoptotic genes tnfr1, tnfr2, tradd, caspase-8, caspase-3 and bax were significantly increased, whereas the expression of the inhibitory factor bcl-2 was significantly downregulated, indicating that Aeromonas infection significantly induced apoptosis in the intestine of grass carp. Moreover, the expression of intestinal tight junction proteins (occludin, zo-1, claudin b and claudin c) was significantly decreased after infection with Aeromonas. Histopathological analysis indicated the Aeromonas challenge caused severe damage to the intestinal villi with adhesions and detachment of intestinal villi accompanied by severe inflammatory cell infiltration at 12 h and 72 h. The 16S rRNA sequencing results showed that Aeromonas infection significantly altered the structure of the intestinal microflora of the grass carp at the phylum (Proteobacteria, Fusobacteria, Bacteroidetes and Firmicutes) and genus (Proteus, Cetobacterium, Bacteroides, and Aeromonas) levels. Take together, the findings of this study revealed that Aeromonas infection induces an intestinal immune response, triggers cell apoptosis, destroys physical barriers and alters microflora structure in the intestine of juvenile grass carp; the results will help to reveal the pathogenesis of intestinal bacterial diseases in grass carp.

Characterization of IgD and IgT with their expressional analysis following subtype II megalocytivirus vaccination and infection in rock bream (Oplegnathus fasciatus)

In this study, heavy chain genes of IgD and IgT were sequenced and characterized their gene expression in rock bream (Oplegnathus fasciatus). Rock bream (RB)-IgD cDNA is 3319 bp in length and encodes a leader region, variable domains, a μ1 domain, and seven constant domains (CH1-CH7). A membrane-bound (mIgT) and secretory form (sIgT) of RB-IgT cDNAs are 1902 bp and 1689 bp in length, respectively, and encode a leader region, variable domains, four constant domains (CH1-CH4) and C-terminus. Their predicted 3D-structure and phylogenetic relation were similar to those of other teleost. In healthy fish, RB-IgD and mIgT gene expressions were higher in major lymphoid organs and blood, while RB-sIgT gene was more highly expressed in midgut. IgT expressing cells were detected in melano-macrophage centers (MMC) of head kidney in immunohistochemistry analysis. Under immune stimulation in vitro, RB-IgD and IgT gene expressions were upregulated in head kidney and spleen cells by bovine serum albumin or a rock bream iridovirus (RBIV) vaccine. In vivo, their expressions were significantly upregulated in head kidney, blood, and gill upon vaccination. Especially, RB-mIgT gene expression in head kidney and blood was upregulated at day 3 after vaccination while upregulated at earlier time point of day 1 by challenge with RBIV. This may suggest that memory cells might be produced during the primary response by vaccination and rapidly proliferated by secondary immune response by viral infection. RB-sIgT gene expression was highly upregulated in peripheral blood in vaccinated fish after viral infection, indicating that IgT plays an important role in systemic immune response as well as mucosal immune system. Our findings provide information on the role of RB-IgT in adaptive immunity during vaccination and viral infection in the vaccinated fish.

Pathogenicity of Plesiomonas shigelloides causing mass mortalities of largemouth bass (Micropterus salmoides) and its induced host immune response

The outbreak of mass mortality of M. salmoides occurred in an aquaculture farm in Jiangsu province of China, showing signs of skin ulceration and haemorrhages. The bacteria were isolated from diseased largemouth bass, and identified as Plesiomonas shigelloides based on morphological, physiological and biochemical features, as well as 16S rRNA gene sequence analysis. The pathogenicity of P. shigelloides was determined by challenge experiments, and the median lethal dosage (LD₅₀) of the isolate NJS1 for M. salmoides was calculated as 1.6 × 10⁵ CFU/mL at 7 d post-infection. Histopathological analysis revealed that extensive necrosis, vacuolization and inflammation were presented in the kidney, liver and gill of the diseased fish. Detection of virulence-related genes showed that P. shigelloides NJS1 was positive for astA, astB, astD, astE, actP and 6 ahpA. Additionally, the host defensive response of M. salmoides infected by P. shigelloides was analyzed by quantitive real-time PCR (qRT-PCR), and the results showed that the expression levels of Cas3, Hep1, HIF, IgM, IL15 and TGF were significantly up-regulated in head kidney, liver and spleen in different hours post-infection, which revealed varying expression profiles and clear transcriptional activation of immune related genes. The results suggested that P. shigelloides was an etiological element in the mass mortalities of M. salmoides and this study provided deeper insights for the pathogenesis and host defensive system in P. shigelloides invasion.

Effects of Dietary Enterococcus faecalis YFI-G720 on the Growth, Immunity, Serum Biochemical, Intestinal Morphology, Intestinal Microbiota, and Disease Resistance of Crucian Carp (Carassius auratus)

Diseases of crucian carp (Carassius auratus) are closely related to intestinal parameters. Enterococcus faecalis has strong colonization ability in the intestinal tract, and produces natural antibiotics, bacteriocin, and other bacteriostatic substances, which can effectively inhibit some pathogenic bacteria and improve the intestinal microenvironment. This study aimed to assess the effects of E. faecalis YFI-G720 which was isolated from the intestinal of crucian carp on the growth, immunity, intestinal health, and disease resistance of crucian carp. Fish (48.16 ± 0.55 g) were fed four diets, commercial diet or diet containing E. faecalis at 105 CFU/g (EF1), 106 CFU/g (EF2), or 107 CFU/g (EF3) for 28 days. The results showed that supplementation of E. faecalis significantly improved the weight gain ratio (WGR) and the specific growth rate (SGR) compared with control group (p < 0.05). Intestinal mucosal epithelial cells in EF2 were intact and normal, but there was obvious vacuolation in CG. Compared with CG, serum C3 and IgM in EF2 were significantly increased at the end of the experiment (p < 0.05), and serum alkaline phosphatase was significantly higher in all experimental groups (p < 0.05). Among studied immune-related genes, expression was detected by qPCR, C3, IgM, and IL-1βwere upregulated in all experimental groups to varying degrees from 14 days, with highest expression in EF2 at 28 days. Intestinal microbiota structure analyzed through high-throughput sequencing, and the results showed that the relative abundance of Aeromonas and Acinetobacter decreased while Cetobacterium increased in all experimental groups, with the greatest changes in EF2. Challenge tests showed that fish fed E. faecalis were more resistant to Aeromonas veronii (p < 0.05). In conclusion, dietary E. faecalis YFI-G720 at 106 CFU/g can improve the health status, immune parameters, intestinal microbiota composition, and disease resistance of crucian carp.

Immune response and protective efficacy of two new adjuvants, Montanide™ ISA 763B VG and Montanide™ GEL02, administered with a Streptococcus agalactiae ghost vaccine in Nile tilapia (Oreochromis niloticus)

Streptococcus agalactiae is one of the most important pathogens infecting tilapia worldwide and causes meningoencephalitis, septicemia and high mortalities with considerable losses. Various types of vaccines have been developed against S. agalactiae infection, such as inactivated vaccines, live attenuated vaccines and subunit vaccines. Bacterial ghosts (BGs) are nonliving, empty cell envelopes and have been reported as novel vaccine candidates. Therefore, the main aims of this study were to develop an S. agalactiae ghost vaccine (SAGV) and to evaluate the immune response and protective effect of SAGV against S. agalactiae with two novel adjuvants, Montanide™ ISA 763B VG and Montanide™ GEL02. Nile tilapia, mean weight 50 g, were divided into four groups as follows; 1) fish injected with PBS as control, 2) fish injected with the SAGV alone; 3) fish injected with the SAGV+Montanide™ ISA 763B VG; and 4) fish injected with SAGV+Montanide™ GEL02. Following vaccination, innate immunity parameters including serum lysozyme, myeloperoxidase, catalase, and bactericidal activity were all significantly enhanced. Moreover, specific serum IgM antibodies were induced and reached their highest level 2-8 weeks post vaccination. Importantly, the relative percent survival of tilapia vaccinated against the SAGV formulated with both adjuvants was 80-93%. Furthermore, the transcription of immune-related genes (IgM, TCRβ, IL-1β, IL-8 and TNFα) were up-regulated in tilapia after vaccination, indicating that both cellular and humoral immune responses were induced by these adjuvanted vaccines. In summary, Montanide™ ISA 763B VG and Montanide™ GEL02 can enhance immunoprotection induced by the SAGV vaccine against streptococcosis, demonstrating that both have value as potential adjuvants of fish vaccines.

Cloning and characterization of tumor necrosis factor superfamily 15 in rock bream, Oplegnathus fasciatus; phylogenetic, in silico, and expressional analysis

Tumor necrosis factor superfamily (TNFSF)15 is a member of TNFSF which shares a high homology with other TNFSFs, especially lymphotoxin (LT)-α in teleost. In this study, we have cloned a putative TNFSF15 gene in rock bream which was highly homologous with other fish TNFSF15 and performed bioinformatic analysis to confirm the membership. The RB-TNFSF15 cDNA consists of 3192 bp (193 bp of 5'-untranslated region (UTR), 732 bp of ORF, and 2267 bp of 3'-UTR) and encodes a polypeptide of 243 amino acids containing a predicted TNF superfamily signature with 43-61% identities with fish TNFSF15. The predicted 3D structure was similar to human TNFSF15 with β barrel structure containing 10 β strands and 1 α helix while human LT-α and β contain 10 β strands and 2 α helices. Consequently, the synteny and phylogenetic analysis of fish TNFSF15 genes and structural similarity of the predicted protein to mammalian TNFSF15 implicate that they can be identified as TNFSF15. In healthy rock bream, RB-TNFSF15 gene expression level was the highest in fin and the lowest in blood. In vitro, TNFSF15 gene expression was up-regulated by lipopolysaccharide, polyinosinic:polycytidylic acid (poly I:C) and rock bream iridovirus (RBIV) in head kidney, while up-regulated by poly I:C and RBIV at later time in spleen. In vivo, RB-TNFSF15 gene expression was up-regulated in head kidney, liver and blood after vaccination with a formalin inactivated RBIV. After challenging with RBIV, RB-TNFSF15 gene expression was up-regulated in unvaccinated group at day 3 post-infection in head kidney. In gill, it was significantly up-regulated in vaccinated group at day 1 post-challenge and all groups at day 7, indicating that RB-TNFSF may play a key role in mucosal immunity during viral infection. Since the regulation mechanism of TNFSF15 gene expression in fish has not yet been elucidated, the present study will help to understand the roles of TNFSF15 in fish immune system.