Grouper (Epinephelus coioides) Mpeg1s: Molecular identification, expression analysis, and antimicrobial activity

Accumulation and Phagocytosis of Fluorescently Visualized Macrophages Against Edwardsiella piscicida Infection in Established mpeg1.1-Transgenic Japanese Medaka Oryzias latipes

Intracellular bacteria such as those belonging to the genus Edwardsiella can survive and proliferate within macrophages. However, the detailed mechanisms underlying the host macrophage immune response and pathogen evasion strategies remain unknown. To advance the field of host macrophage research, we successfully established transgenic (Tg) Japanese medaka Oryzias latipes that possesses fluorescently visualized macrophages. As a macrophage marker, the macrophage-expressed gene 1.1 (mpeg1.1) was selected because of its predominant expression across various tissues in medaka. To validate the macrophage characteristics of the fluorescently labeled cells, May-Grünwald Giemsa staining and peroxidase staining were conducted. The labeled cells exhibited morphological features consistent with those of monocyte/macrophage-like cells and tested negative for peroxidase activity. Through co-localization studies, the fluorescently labeled cells co-localized with E. piscicida in the intestines and kidneys of infected medaka larvae, confirming the ingestion of bacteria through phagocytosis. In addition, the labeled cells expressed macrophage markers but lacked a neutrophil marker. These results suggested that the fluorescently labeled cells of Tg[mpeg1.1:mCherry/mAG] medaka were monocytes/macrophages, which will be useful for future studies aimed at understanding the mechanisms of macrophage-mediated bacterial infections.

Single-cell atlas of rainbow trout peripheral blood leukocytes and profiling of their early response to infectious pancreatic necrosis virus

The recent development of single cell sequencing technologies has revolutionized the state-of-art of cell biology, allowing the simultaneous measurement of thousands of genes in single cells. This technology has been applied to study the transcriptome of single cells in homeostasis and also in response to pathogenic exposure, greatly increasing our knowledge of the immune response to infectious agents. Yet the number of these studies performed in aquacultured fish species is still very limited. Thus, in the current study, we have used the 10x Genomics single cell RNA sequencing technology to study the response of rainbow trout (Oncorhynchus mykiss) peripheral blood leukocytes (PBLs) to infectious pancreatic necrosis virus (IPNV), an important trout pathogen. The study allowed us to obtain a transcriptomic profile of 12 transcriptionally distinct leukocyte cell subpopulations that included four different subsets of B cells, T cells, monocytes, two populations of dendritic-like cells (DCs), hematopoietic progenitor cells, non-specific cytotoxic cells (NCC), neutrophils and thrombocytes. The transcriptional pattern of these leukocyte subpopulations was compared in PBL cultures that had been exposed in vitro to IPNV for 24 h and mock-infected cultures. Our results revealed that monocytes and neutrophils showed the highest number of upregulated protein-coding genes in response to IPNV. Interestingly, IgM+IgD+ and IgT+ B cells also upregulated an important number of genes to the virus, but a much fainter response was observed in ccl4 + or plasma-like cells (irf4 + cells). A substantial number of protein-coding genes and genes coding for ribosomal proteins were also transcriptionally upregulated in response to IPNV in T cells and thrombocytes. Interestingly, although genes coding for ribosomal proteins were regulated in all affected PBL subpopulations, the number of such genes transcriptionally regulated was higher in IgM+IgD+ and IgT+ B cells. A further analysis dissected which of the regulated genes were common and which were specific to the different cell clusters, identifying eight genes that were transcriptionally upregulated in all the affected groups. The data provided constitutes a comprehensive transcriptional perspective of how the different leukocyte populations present in blood respond to an early viral encounter in fish.

First report of L1 cell adhesion molecule (L1CAM) in flat fish (Starry flounder): Molecular features and expression analysis

In this study, the starry flounder L1 cell adhesion molecule (L1CAM) sequence was obtained using next-generation sequencing, and the integrity of the sequence was verified by cloning and sequencing. First, the amino acid sequence was predicted using the cDNA sequence, and the gene was then identified through multiple sequence alignment analysis with related sequences and phylogenetic analysis. Thus, homogeneity was confirmed. The expression level of PsL1CAM (Platichthys stellatus L1CAM) mRNA in healthy starry flounder was detected in all tissues used in the experiment, and tissue- and gene-specific expression levels were confirmed. In addition, as a result of mRNA expression analysis after artificial infection with viral hemorrhagic septicemia virus (VHSV) and Streptococcus parauberis PH0710, significant expression changes and characteristics were confirmed following infection with VHSV and S. parauberis PH0710. After artificial infection with VHSV, the expression level of PsL1CAM mRNA was significantly upregulated in almost all major tissues of the starry flounder, whereas it was significantly downregulated in mucosal-associated lymphoid tissues, such as the gills and intestine. Infection with S. parauberis PH0710 significantly upregulated the expression of PsL1CAM mRNA in almost all major tissues of the starry flounder, whereas it was significantly downregulated in the heart after infection. Our results indicate that PsL1CAM may be involved in the host immune response to starry flounders.

The evolutionary diversification and antimicrobial potential of MPEG1 in Metazoa

Macrophage-expressed gene 1 (MPEG1) is an ancient immune effector known to exist in Cnidaria, Mollusca, Actinopterygii, and Mammalia. In this study, we examined the evolution and antibacterial potential of MPEG1 across Metazoa. By unbiased data-mining, MPEG1 orthologs were found in 11 of 34 screened phyla. In invertebrates, MPEG1 is present in the major phyla and exhibits intensive duplication. In vertebrates, class-based clades were formed by the major, generic MPEG1 (gMPEG1) in each class. However, there is a minority of unique MPEG1 (uMPEG1) from 71 species of 4 classes that clustered into a separate clade detached from all major class-based clades. gMPEG1 and uMPEG1 exhibit strong genomic collinearity and are surrounded by high-density transposons. gMPEG1 and uMPEG1 transcript expressions were most abundant in immune organs, but differed markedly in tissue specificity. Systematic analysis identified an antimicrobial peptide (AMP)-like segment in the C-terminal (CT) tail of MPEG1. Peptides based on the AMP-like regions of 35 representative MPEG1 were synthesized. Bactericidal activities were displayed by all peptides. Together these results suggest transposon-propelled evolutionary diversification of MPEG1 in Metazoa that has likely led to functional specialisation. This study also reveals a possible antimicrobial mechanism mediated directly and solely by the CT tail of MPEG1.

Vibrio harveyi co-infected with Cryptocaryon irritans to orange-spotted groupers Epinephelus coioides

The orange-spotted grouper (Epinephelus coioides) is a high economic value aquacultural fish in China, however, it often suffers from the outbreak of parasitic ciliate Cryptocaryon irritans as well as bacterium Vibrio harveyi which bring great loss in grouper farming. In the present study, we established a high dose C. irritans local-infected model which caused the mortality of groupers which showed low vitality and histopathological analysis demonstrated inflammatory response and degeneration in infected skin, gill and liver. In addition, gene expression of inflammatory cytokines was detected to assist the estimate of inflammatory response. Furthermore, we also found that the activity of Na⁺/K⁺ ATPase in gill was decreased in groupers infected C. irritans and the concentration of Na⁺/Cl^- in blood were varied. Base on the morbidity symptom occurring in noninfected organs, we hypothesized that the result of morbidity and mortality were due to secondary bacterial infection post parasitism of C. irritans. Moreover, four strains of bacteria were isolated from the infected site skin and liver of local-infected groupers which were identified as V. harveyi in accordance of phenotypic traits, biochemical characterization and molecular analysis of 16S rDNA genes, housekeeping genes (gyrB and cpn60) and species-specific gene Vhhp2. Regression tests of injecting the isolated strain V. harveyi has showed high pathogenicity to groupers. In conclusion, these findings provide the evidence of coinfections with C. irritans and V. harveyi in orange-spotted grouper.

Host responses against the fish parasitizing ciliate Cryptocaryon irritans

The parasitic ciliate Cryptocaryon irritans, which infects almost all marine fish species occurring in both tropical and subtropical regions throughout the world. The disease, cryptocaryonosis, accounts for significant economic losses to the aquaculture industry. This review attempts to provide a comprehensive overview of the biology of the parasite, host-parasite interactions and both specific and non-specific host defense mechanisms are responsible for the protection of fish against challenge infections with this ciliate. Also, this article reflects the current interest in this subject area and the quest to develop an available vaccine against the disease. Due to the high frequency of clinical fish cryptocaryonosis, the study of fish immune responses to C. irritans provides an optimal experimental model for understanding immunity against extracellular protozoa.

Molecular characterization and expression analysis of septin gene family and phagocytic function of recombinant septin 2, 3 and 8 of starry flounder (Platichthys stellatus)

Septin is an evolutionarily conserved family of GTP-binding proteins. Septins are known to be involved in a variety of cellular processes, including cell division, chromosome separation, cell polarity, motility, membrane dynamics, exocytosis, apoptosis, phagocytosis, DNA damage responses, and other immune responses. In this study, the sequences of the septin gene family of starry flounder were obtained using NGS sequencing, and the integrity of the sequences was verified through cloning and sequencing. At first, the amino acid sequence was annotated using the cDNA sequence, and then, the gene sequence was verified through multiple sequence alignment and phylogenetic analyses using the related conserved sequences. The septin gene family was classified into three subgroups based on the phylogenetic analysis. High conservation within the domain and homology between the genes reported in different species were confirmed. The expression level of septin gene family mRNA in each tissue of healthy starry flounder was evaluated to confirm the tissue- and gene-specific expression levels. Additionally, as a result of the analysis of mRNA expression after simulated pathogen infection, significant expression changes and characteristics were confirmed upon infection with bacteria (Streptococcus parauberis PH0710) and virus (VHSV). Based on the current results and that of previous studies, to confirm the immunological function, Septin 2, 3, and 8 were produced as recombinant proteins based on the amino acid sequences, and their role in phagocytosis was further investigated. The results of this study indicate that septin gene family plays a complex and crucial role in the host immune response to pathogens of starry flounder.

Production of monoclonal antibody against grouper (Epinephelus coioides) CD4-1 and the distribution of CD4-1+ cells

CD4-a transmembrane glycoprotein molecule expressed on the surface of helper T (Th) cells-plays a central role in adaptive immune protection. In the current study, we developed a monoclonal antibody (mAb) against the grouper CD4-1. Western blotting and immunohistochemistry results revealed that the CD4-1 mAb could recognize the recombinant and natural protein of grouper CD4-1 as well as the CD4-1⁺ cells in the various tissues from grouper. Tissue distribution analyses revealed that the grouper CD4-1⁺ cells were expressed in all tissues tested in the healthy grouper, with greater localization in the thymus, head kidney, and spleen tissues. In addition, we tested the changes in the proportion of CD4-1⁺ cells in the thymus, head kidney, and the gills of grouper post the infection by C. irritans. Our data suggest that the CD4-1 mAb produced against grouper in the current study can be used as a tool to characterize CD4-1⁺ cells and to investigate the functions of the grouper CD4-1⁺ cells in the host response against pathogens infection.

The dynamic immune responses of Mandarin fish (Siniperca chuatsi) to ISKNV in early infection based on full-length transcriptome analysis and weighted gene co-expression network analysis

Mandarin fish (Siniperca chuatsi) been seriously harmed by infectious spleen and kidney necrosis virus (ISKNV) in recent years, but the early immune response mechanism of infection is still unknown. Here, we performed RNA sequencing on the spleens of mandarin fish infected with ISKNV at 0, 12, 24, 48, and 72 h post-infection (hpi) using short-read Illumina RNA sequencing and long-read Pacific Biosciences isoform sequencing to generate a full-length transcriptome. The immune responses of mandarin fish infected with ISKNV at the molecular level were characterized by RNA-seq analysis and weighted gene co-expression network analysis (WGCNA). A total of 26,528 full-length transcript sequences were obtained. There were 2,729 (1,680 up-regulated and 1,112 down-regulated), 1,874 (1,136 up-regulated and 738 down-regulated), 2,032 (1,158 up-regulated and 847 down-regulated), and 4,176 (2,233 up-regulated and 1,943 down-regulated) differentially expressed genes (DEGs) in mandarin fish at 12, 24, 48, and 72 hpi, compared with uninfected fish, respectively. A total of four modules of co-expressed DEGs identified by WGCNA were significantly positively correlated to the four time points after infection, respectively. Kyoto Encyclopedia of Genes and Genomes (KEGG) analysis revealed that the immune-related DEGs in all these modules were mainly enriched in Phagosome, Endocytosis, Herpes simplex infection, and Cytokine-cytokine receptor interaction pathways. Further analysis showed that oher signaling pathways, including CAMs, NOD-like receptor and ER protein processing, Intestinal immune network for IgA production, TLR pathway, and Apoptosis significantly enriched in four modules corresponding to 12, 24, 48, and 72 hpi respectively, had specifically participated in the immune response. Hub genes identified based on the high-degree nodes in the WGCN, including CAM3, IL-8, CCL21, STING, SNX1, PFR and TBK1, and some DEGs such as MHCI, MHCII, TfR, STING, TNF α, TBK1, IRF1, and NF-kB, BCR, IgA and Bcl-XL had involved in dynamic molecular response of mandarin fish to ISKNV infection. In sum, this study provides a set of full-length transcriptome of the spleen tissue of mandarin fish for the first time and revealed a group of immune genes and pathways involved in different temporal responses to ISKNV infection, which has implications for resource conservation and aiding the development of strategies to prevent virus early infection for mandarin fish.

Genome-wide investigation and expression analysis of MACPF gene family reveals its immune role in response to bacterial challenge of Manila clam

In this study, 18 MACPF genes (RpMACPF) were identified and classed into three types (Macrophage-expressed gene 1, Apextrin, and MACPF domain contain protein) based on gene structure and phylogenetic relationship in R. philippinarum. The length of RpMACPF proteins varied from 287 to 785 amino acids. The molecular weights and Theoretical PI values ranged from 3.2 kDa to 8.7 kDa and 4.7 to 8.6, respectively. RNA-seq data analysis revealed that 14 of 18 RpMACPF genes were highly expressed at the pediveliger larvae stage indicate RpMACPF might contribute to the early development and metamorphosis processes of the R. philippinarum. Besides, we found RpMACPF genes were significantly regulated by pathogen-associated molecular patterns (PAMPs) and Vibrio parahemolyticus, which indicates RpMACPF genes may play significant roles in response to invading pathogens. The results obtained in this work will provide valuable insight into the immune function of MACPF gene in R. philippinarum.

Breaching the Bacterial Envelope: The Pivotal Role of Perforin-2 (MPEG1) Within Phagocytes

The membrane attack complex (MAC) of the complement system and Perforin-1 are well characterized innate immune effectors. MAC is composed of C9 and other complement proteins that target the envelope of gram-negative bacteria. Perforin-1 is deployed when killer lymphocytes degranulate to destroy virally infected or cancerous cells. These molecules polymerize with MAC-perforin/cholesterol-dependent cytolysin (MACPF/CDC) domains of each monomer deploying amphipathic β-strands to form pores through target lipid bilayers. In this review we discuss one of the most recently discovered members of this family; Perforin-2, the product of the Mpeg1 gene. Since their initial description more than 100 years ago, innumerable studies have made macrophages and other phagocytes some of the best understood cells of the immune system. Yet remarkably it was only recently revealed that Perforin-2 underpins a pivotal function of phagocytes; the destruction of phagocytosed microbes. Several studies have established that phagocytosed bacteria persist and in some cases flourish within phagocytes that lack Perforin-2. When challenged with either gram-negative or gram-positive pathogens Mpeg1 knockout mice succumb to infectious doses that the majority of wild-type mice survive. As expected by their immunocompromised phenotype, bacterial pathogens replicate and disseminate to deeper tissues of Mpeg1 knockout mice. Thus, this evolutionarily ancient gene endows phagocytes with potent bactericidal capability across taxa spanning sponges to humans. The recently elucidated structures of mammalian Perforin-2 reveal it to be a homopolymer that depends upon low pH, such as within phagosomes, to transition to its membrane-spanning pore conformation. Clinical manifestations of Mpeg1 missense mutations further highlight the pivotal role of Perforin-2 within phagocytes. Controversies and gaps within the field of Perforin-2 research are also discussed as well as animal models that may be used to resolve the outstanding issues. Our review concludes with a discussion of bacterial counter measures against Perforin-2.

Functional characterization and gene expression profile of perforin-2 in starry flounder (Platichthys stellatus)

The membrane attack complex/perforin (MACPF) superfamily consists of multifunctional proteins that form pores on the membrane surface of microorganisms to induce their death and have various immune-related functions. PFN2 is a perforin-like protein with an MACPF domain, and humans with deficient PFN2 levels have increased susceptibility to bacterial infection, which can lead to fatal consequences for some patients. Therefore, in this study, we confirmed the antimicrobial function of PFN2 in starry flounder (Platichthys stellatus). The molecular properties were confirmed based on the verified amino acid sequence of PsPFN2. In addition, the expression characteristics of tissue-specific and pathogen-specific PsPFN2 mRNA were also confirmed. The recombinant protein was produced using Escherichia coli, and the antimicrobial activity was then confirmed. The coding sequence of PFN2 (PsPFN2) in P. stellatus consists of 710 residues. The MACPF domain was conserved throughout evolution, as shown by multiple sequence alignment and phylogenetic analysis. PsPFN2 mRNA is abundantly distributed in immune-related organs such as the spleen and gills of healthy starry flounder, and significant expression changes were confirmed after artificial infection by bacteria or viruses. We cloned the MACPF domain region of PFN2 to produce a recombinant protein (rPFN2) and confirmed its antibacterial effect against a wide range of bacterial species and the parasite (Miamiensis avidus).

Ancient but Not Forgotten: New Insights Into MPEG1, a Macrophage Perforin-Like Immune Effector

Macrophage-expressed gene 1 [MPEG1/Perforin-2 (PRF2)] is an ancient metazoan protein belonging to the Membrane Attack Complex/Perforin (MACPF) branch of the MACPF/Cholesterol Dependent Cytolysin (CDC) superfamily of pore-forming proteins (PFPs). MACPF/CDC proteins are a large and extremely diverse superfamily that forms large transmembrane aqueous channels in target membranes. In humans, MACPFs have known roles in immunity and development. Like perforin (PRF) and the membrane attack complex (MAC), MPEG1 is also postulated to perform a role in immunity. Indeed, bioinformatic studies suggest that gene duplications of MPEG1 likely gave rise to PRF and MAC components. Studies reveal partial or complete loss of MPEG1 causes an increased susceptibility to microbial infection in both cells and animals. To this end, MPEG1 expression is upregulated in response to proinflammatory signals such as tumor necrosis factor α (TNFα) and lipopolysaccharides (LPS). Furthermore, germline mutations in MPEG1 have been identified in connection with recurrent pulmonary mycobacterial infections in humans. Structural studies on MPEG1 revealed that it can form oligomeric pre-pores and pores. Strikingly, the unusual domain arrangement within the MPEG1 architecture suggests a novel mechanism of pore formation that may have evolved to guard against unwanted lysis of the host cell. Collectively, the available data suggest that MPEG1 likely functions as an intracellular pore-forming immune effector. Herein, we review the current understanding of MPEG1 evolution, regulation, and function. Furthermore, recent structural studies of MPEG1 are discussed, including the proposed mechanisms of action for MPEG1 bactericidal activity. Lastly limitations, outstanding questions, and implications of MPEG1 models are explored in the context of the broader literature and in light of newly available structural data.

Distribution of Mpeg1+ cells in healthy grouper (Epinephelus coioides) and after Cryptocaryon irritans infection

Cryptocaryon irritans is an extremely harmful ciliated obligate parasite that is responsible for large economic losses in aquaculture. C. irritans infection can cause an insect-resistant immune response in fish, and many immune cells can be observed in the local infection site. However, it is unclear whether macrophages are involved in the host defense against C. irritans infection. The Mpeg1 protein can form pores and destroy the cell membrane of invading pathogens, and is also used as a macrophage-specific marker in mammals. Therefore, a polyclonal antibody against grouper recombinant Mpeg1a was produced to mark macrophages in this study, which could recognize both isoforms of Mpeg1 (Mpeg1a/b). Immunofluorescence revealed that EcMpeg1 positive cells were mostly distributed in the head kidney and spleen in healthy grouper. Immunofluorescence and immunohistochemistry showed that the number of EcMpeg1 positive cells increased in the gills after infection with C. irritans, implying that EcMpeg1 positive cells may be involved in the process of grouper resistance against C. irritans infection.