Role of mannose-binding lectin in regulating monocytes/macrophages functions during Aeromonas hydrophila infection in grass carp, Ctenopharyngodon idella

Role of HIF in fish inflammation

The hypoxia-inducing factor (HIF) is a central transcription factor in cellular oxygen sensing and regulation. It is common that the inflammation always appears in many diseases, like infectious diseases in fishes, and the inflammation is often accompanied by hypoxia, as a hallmark of inflammation. Besides coordinating cellular responses to low oxygen, HIF-mediated hypoxia signaling pathway is also crucial for immune responses such as the regulations of innate immune cell phenotype and function, as well as metabolic reprogramming under the inflammation. However, the understanding of the molecular mechanisms by which HIFs regulate the inflammatory response in fish is still very limited. Here, we review the characteristics of HIF as well as its roles in innate immune cells and the infections caused by bacteria and viruses. The regulatory effects of HIF on the metabolic reprogramming of innate immune cells are also discussed and the future research directions are outlooked. This paper will serve as a reference for elucidating the molecular mechanism of HIF regulating inflammation and identifying treatment strategies to target HIF for fish disease.

The intestinal histopathology, innate immune response and antioxidant capacity of blunt snout bream (Megalobrama amblycephala) in response to Aeromonas hydrophila

The present study was performed to determine the effects of Aeromonas hydrophila infection on intestinal -histopathology, innate immune response and changes in antioxidant capacity of blunt snout bream (Megalobrama amblycephala). A series of histopathological changes, innate immune enzyme activities, antioxidant enzyme activities, and the corresponding mRNA relative genes expressions in intestines were measured at 0, 1, 2, and 3 weeks post-treatment of Aeromonas hydrophila (1✕10⁷ CFU mL^-1) infection. The results showed that Aeromonas hydrophila induced changes in intestinal morphology, including the decreased muscularis thickness, the proliferated goblet cells, and the atrophied intestine villi height. Moreover, the innate immune enzymes activities in serum such as acid phosphatase, alkaline phosphatase, lysozyme activities and immunoglobulin M were significantly reduced after infection at 1week, 2week and 3week. The contents of complement 3 and complement 4 were significantly decreased after infection as well. In addition, the antioxidant enzymes activities, including superoxide dismutase, catalase and glutathione peroxidase in the experimental groups were significantly decreased compared with the control group, whereas the content of malondialdehyde was significantly increased after infection at 1week, 2week and 3week. Furthermore, the mRNA relative expressions of the inflammatory cytokines such as tumor necrosis factor-α, interleukins-1β, interferon-γ, and interleukins-6 were significantly increased after infection with Aeromonas hydrophila. The TJ-related gene expressions in the intestine of zonula occluden-1, occludin, occludin-1, occludin-2 were significantly reduced throughout the infection period. The mRNA relative expressions of signal transducers and activators of transcription 4 and janus kinase-3 in the intestine were significantly ascended compared with the non-infected group. Overall, the results elucidated that the intestine tissue injury and innate immune response reduction, as well as antioxidant capacity attenuation were occurred against Aeromonas hydrophila infection of the blunt snout bream.

Functional miR-142a-3p Induces Apoptosis and Macrophage Polarization by Targeting tnfaip2 and glut3 in Grass Carp (Ctenopharyngodon idella)

In the process of microbial invasion, the inflammation reaction is induced to eliminate the pathogen. However, un-controlled or un-resolved inflammation can lead to tissue damage and death of the host. MicroRNAs (miRNAs) are the signaling regulators that prevent the uncontrolled progress of an inflammatory response. Our previous work strongly indicated that miR-142a-3p is related to the immune regulation in grass carp. In the present study, we found that the expression of miR-142a-3p was down-regulated after infection by Aeromonas hydrophila. tnfaip2 and glut3 were confirmed as be the target genes of miR-142a-3p, which were confirmed by expression correlation analysis, gene overexpression, and dual luciferase reporter assay. The miR-142a-3p can reduce cell viability and stimulate cell apoptosis by targeting tnfaip2 and glut3. In addition, miR-142a-3p also regulates macrophage polarization induced by A. hydrophila. Our results suggest that miR-142a-3p has multiple functions in host antibacterial immune response. Our research provides further understanding of the molecular mechanisms between miRNAs and their target genes, and provides a new insights for the development of pro-resolution strategies for the treatment of complex inflammatory diseases in fish.

Identification, Virulence, and Molecular Characterization of a Recombinant Isolate of Grass Carp Reovirus Genotype I

The hemorrhagic disease of grass carp (HDGC) caused by grass carp reovirus (GCRV) still poses a great threat to the grass carp industry. Isolation and identification of the GCRV genotype I (GCRV-I) has been rarely reported in the past decade. In this study, a new GCRV was isolated from diseased fish with severe symptoms of enteritis and mild hemorrhages on the body surface. The isolate was further identified by cell culture, transmission electron, indirect immunofluorescence, and SDS-PAGE electrophoretic pattern analysis of genomic RNA. The results were consistent with the new isolate as a GCRV-I member and tentatively named GCRV-GZ1208. Both grass carp and rare minnow infected by the GCRV-GZ1208 have no obvious hemorrhagic symptoms, and the final mortality rate was ≤10%, indicating that it may be a low virulent isolate. GZ1208 possessed highest genomic homology to 873/GCHV (GCRV-I) and golden shiner reovirus (GSRV). Additionally, it was found a 90.7-98.3% nucleotide identity, a 96.4-100% amino acid identity, and <50% identity with GCRV-II and III genotypes. Interestingly, the sequences of some segments of GZ1208 were similar to GCRV-8733/GCHV, whereas the remaining segments were more closely related to GSRV, suggesting that a recombination event had occurred. Bootscan analysis of the complete genomic sequence confirmed this hypothesis, and recombination events between 873/GCHV and other GSRV-like viruses were also accompanied by gene mutations.

Hypoxia-inducible factor-1α involved in macrophage regulation in ayu (Plecoglossus altivelis) under hypoxia

Hypoxia-inducible factor-1α (HIF-1α) plays a critical role in immune and inflammatory responses and is important in controlling a variety of processes in monocytes and macrophages. However, the role of HIF-1α in the teleost immune system remains less known. In this study, we cloned the cDNA sequence of HIF-1α from the ayu (Plecoglossus altivelis, PaHIF-1α). Sequence and phylogenetic tree analysis showed that PaHIF-1α clustered within the fish HIF-1α tree and was closely related to that of Northern pike (Esox lucius). PaHIF-1α was expressed in all tested tissues and expression increased in liver, head kidney, and body kidney upon Vibrio anguillarum infection. PaHIF-1α was found to regulate the expression of cytokines in ayu monocytes/macrophages (MO/MФ). PaHIF-1α mediated hypoxia-induced enhancement of MO/MФ phagocytic and bactericidal activities to enhance host defenses. Compared with the control, intermittent hypoxia further increased the expression of PaHIF-1α mRNA, improved the survival rate, and reduced the bacterial load of V. anguillarum-infected ayu. Therefore, PaHIF-1α may play a predominant role in the modulation of ayu MO/MФ function.

Mannose-Binding Lectin Possesses Agglutination Activity and Promotes Opsonophagocytosis of Macrophages with Calreticulin Interaction in an Early Vertebrate

The innate immune system is an ancient defense system in the process of biological evolution, which can quickly and efficiently resist pathogen infection. In mammals, mannose-binding lectin (MBL) is a key molecule in the innate immune and plays an essential role in the first line of host defense against pathogenic bacteria. However, the evolutionary origins and ancient roles of immune defense of MBL and its mechanism in clearance of microbial pathogens are still unclear, especially in early vertebrates. In this study, Oreochromis niloticus MBL (OnMBL) was successfully isolated and purified from the serum of Nile tilapia (O. niloticus). The OnMBL was able to bind and agglutinate with two important pathogens of tilapia, Streptococcus agalactiae and Aeromonas hydrophila Interestingly, the OnMBL was able to significantly inhibit the proliferation of pathogenic bacteria and reduce the inflammatory response. Upon bacterial challenge, the downregulation of OnMBL expression by RNA interference could lead to rapid proliferation of the pathogenic bacteria, ultimately resulting in tilapia death. However, the phenotype was rescued by reinjection of the OnMBL, which restored the healthy status of the knockdown tilapia. Moreover, a mechanistic analysis revealed that the OnMBL could clear pathogenic bacteria by collaborating with cell-surface calreticulin to facilitate phagocytosis in a complement activation-independent manner. To our knowledge, these results provide the first evidence on the antibacterial response mechanism of MBL performing evolutionary conserved function to promote opsonophagocytosis of macrophages in early vertebrates and reveals new insights into the understanding of the evolutionary origins and ancient roles basis of the C-type lectins in the innate immune defense.