Antibacterial activity of erythrocyte from grass carp (Ctenopharyngodon idella) is associated with phagocytosis and reactive oxygen species generation

Exploration of the immune response of grass carp (Ctenopharyngodon idellus) erythrocytes during bacterial infection

In teleost blood, red blood cells (RBCs) are the most common type of cell, and they differ from mammalian RBCs in having a nucleus and other organelles. As nucleated cells, teleost RBCs contribute to the immune response against pathogens, but their antibacterial mechanism remains unclear. Here, we utilized RNA-Seq to analyze gene expression patterns of grass carp (Ctenopharyngodon idellus) RBCs (GcRBCs) stimulated by Aeromonas hydrophila, Escherichia coli, and Staphylococcus aureus. Our transcriptomic data showed that bacterial stimulation generated many differentially expressed genes (DEGs). Furthermore, several inflammatory pathways responded to bacterial activation, and the TLR, IL-17, and tumor necrosis factor (TNF) signaling pathways were significantly activated based on Kyoto Encyclopedia of Genes and Genomes (KEGG) enrichment analysis. Furthermore, the findings of qRT-PCR showed markedly elevated expression of various cytokines, including IL-1β, IL4, IL6, IL8, IL12, and TNFα, in GcRBCs after incubation with bacteria. Reactive oxygen species (ROS) production in GcRBCs was markedly increased after the cells were stimulated with the three bacteria, and the expression of superoxide dismutase, glutathione peroxidase, and antioxidant enzymes, including catalase, was altered. Flow cytometry analysis showed that the apoptosis rate of GcRBCs was enhanced after stimulation with the three bacteria for different times. In summary, our findings reveal that bacterial stimulation activates the immune response of GcRBCs by regulating ROS release, cytokine expression, and the antioxidant system, leading to apoptosis of GcRBCs.

The infection of Aeromonas hydrophila activated Multiple programmed cell death pathways in red blood cells of Clarias fuscus

In contrast to mammalian red blood cells (RBCs), Osteichthyes RBCs contain a nucleus and organelles, suggesting the involvement of more intricate mechanisms, particularly in the context of ferroptosis. In this study, we utilized RBCs from Clarias fuscus (referred to as Cf-RBCs) as a model system. We conducted RNA-seq analysis to quantify gene expression levels in Cf-RBCs after exposure to both Aeromonas hydrophila and lipopolysaccharides. Our analysis unveiled 1326 differentially expressed genes (DEGs) in Cf-RBCs following 4 h of incubation with A. hydrophila, comprising 715 and 611 genes with upregulated and downregulated expression, respectively. These DEGs were further categorized into functional clusters: 292 related to cellular processes, 241 involved in environmental information processing, 272 associated with genetic information processing, and 399 linked to organismal systems. Additionally, notable changes were observed in genes associated with the autophagy pathway at 4 h, and alterations in the ferroptosis pathway were observed at 8 h following A. hydrophila incubation. To validate these findings, we assessed the expression of cytokines (DMT1, TFR1, LC3, and GSS). All selected genes were significantly upregulated after exposure to A. hydrophila. Using flow cytometry, we evaluated the extent of ferroptosis, and the group incubated with A. hydrophila for 8 h exhibited higher levels of lipid peroxidation compared with the 4-h incubation group, even under baseline conditions. An evaluation of the glutathione redox system through GSSG/GSH ratios indicated an increased ratio in Cf-RBCs after exposure to A. hydrophila. In summary, our data suggest that A. hydrophila may induce ferroptosis in Cf-RBCs, potentially by triggering the cystine/glutamate antiporter system (system XC-), while Cf-RBCs counteract ferroptosis through the regulation of the glutathione redox system. These findings contribute to our understanding of the iron overload mechanism in Osteichthyes RBCs, provide insights into the management of bacterial diseases in Clarias fuscus, and offer potential strategies to mitigate economic losses in aquaculture.

Identifying the function of the PI3K-AKT pathway during the pathogenic infection of Macrobrachium rosenbergii

The phosphoinositide-3-kinase/protein kinase b (PI3K-Akt) pathway is a signalling pathway based on protein phosphorylation and can be activated by a wide range of factors. To investigate the function of the PI3K-AKT signalling pathway in antibacterial immunity, we analysed the gene expression level of three key factors (PI3K, AKT and FoxO) and innate immune factors in immune tissues at different time points after Vibrio parahaemolyticus and Staphylococcus aureus infection. Tissues analysis showed that PI3K, AKT, and FoxO were expressed at high levels in the intestinal, hemocytes and hepatopancreas. Moreover, the expression levels of PI3K, AKT and FoxO can be regulated postinfection by different pathogens. In hemocytes and the intestine, V. parahaemolyticus infection was found to regulate the levels of PI3K, AKT, and FoxO more rapidly; however, an S. aureus infection regulated the levels of these factors more rapidly in the hepatopancreas and gills. Analysis showed that V. parahaemolyticus and S. aureus infection caused changes in the gene expression level of crustin, caspase 3 and NF-κB. Therefore, PI3K-AKT regulates the downstream immune pathway differentially in different immune tissues and participates in the regulation of cell apoptosis and the inflammatory response by activating caspase and NF-κB, respectively, following infection with V. parahaemolyticus and S. aureus.

Characterization and Expression Analysis of Genes from Megalobrama amblycephala Encoding Hemoglobins with Extracellular Microbicidal Activity

Hemoglobin (Hb) usually comprises two α and two β subunits, forming a tetramer responsible for oxygen transportation and storage. Few studies have elucidated fish hemoglobin immune functions. Megalobrama amblycephala is a freshwater-cultured fish prevalent in China. We identified two M. amblycephala hemoglobin subunits and analyzed their expression patterns and antibacterial activities. The respective full-length cDNA sequences of the M. amblycephala Hb α (MaHbα) and β (MaHbβ) subunits were 588 and 603 bp, encoding 143 and 148 amino acids. MaHbα and MaHbβ were highly homologous to hemoglobins from other fish, displaying typical globin-like domains, most heme-binding sites, and tetramer interface regions highly conserved in teleosts. In phylogenetic analyses, the hemoglobin genes from M. amblycephala and other cypriniformes clustered into one branch, and those from other fishes and mammals clustered into other branches, revealing fish hemoglobin conservation. These M. amblycephala Hb subunits exhibit different expression patterns in various tissues and during development. MaHbα is mainly expressed in the blood and brain, while MaHbβ gene expression is highest in the muscle. MaHbα expression was detectable and abundant post-fertilization, with levels fluctuating during the developmental stages. MaHbβ expression began at 3 dph and gradually increased. Expression of both M. amblycephala Hb subunits was down-regulated in most examined tissues and time points post-Aeromonas hydrophila infection, which might be due to red blood cell (RBC) and hematopoietic organ damage. Synthetic MaHbα and MaHbβ peptides showed excellent antimicrobial activities, which could inhibit survival and growth in five aquatic pathogens. Two M. amblycephala hemoglobin subunits were identified, and their expression patterns and antibacterial activities were analyzed, thereby providing a basis for the understanding of evolution and functions of fish hemoglobins.

Teleost innate immunity, an intricate game between immune cells and parasites of fish organs: who wins, who loses

Fish, comprising over 27,000 species, represent the oldest vertebrate group and possess both innate and adaptive immune systems. The susceptibility of most wild fish to parasitic infections and related diseases is well-established. Among all vertebrates, the digestive tract creates a remarkably favorable and nutrient-rich environment, which, in turn, renders it susceptible to microparasites and macroparasites. Consequently, metazoan parasites emerge as important disease agents, impacting both wild and farmed fish and resulting in substantial economic losses. Given their status as pathogenic organisms, these parasites warrant considerable attention. Helminths, a general term encompassing worms, constitute one of the most important groups of metazoan parasites in fish. This group includes various species of platyhelminthes (digeneans, cestodes), nematodes, and acanthocephalans. In addition, myxozoans, microscopic metazoan endoparasites, are found in water-dwelling invertebrates and vertebrate hosts. It is worth noting that several innate immune cells within the fish alimentary canal and certain visceral organs (e.g., liver, spleen, and gonads) play active roles in the immune response against parasites. These immune cells include macrophages, neutrophils, rodlet cells, and mast cells also known as eosinophilic granular cells. At the site of intestinal infection, helminths often impact mucous cells number and alter mucus composition. This paper presents an overview of the state of the art on the occurrence and characteristics of innate immune cells in the digestive tract and other visceral organs in different fish-parasite systems. The data, coming especially from studies employed immunohistochemical, histopathological, and ultrastructural analyses, provide evidence supporting the involvement of teleost innate immune cells in modulating inflammatory responses to metazoan and protozoan parasitic infections.

Effect of dietary inclusion of microalgae (Nannochloropsis gaditana and Schizochytrium spp) on non-specific immunity and erythrocyte maturity in Atlantic salmon fingerlings

The parr-smolt transformation in salmonids involves a critical period characterized by systemic changes associated with the fish's immune response. In this context, as a dietary ingredient in functional diets, microalgae offer an alternative due to their nutritional and bioactive compounds that could strengthen the immune status. This study evaluated the effect of a diet supplemented with Schizochytrium spp and Nannochloropsis gaditana on the expression of genes associated with the antibacterial response. Additionally, the study assessed the effect on the leukocyte population and erythrocyte maturity in Salmo salar blood. Fish were fed for 30 days with a microalgal mixture (1:1) at a 10% inclusion. Each diet was randomly assigned to a tank using a completely randomized design (CRD) with four replications. Each tank was stocked with 70 Atlantic salmon fingerlings with an initial mean weight of 78.87 ± 0.84. Transcription levels were quantified and analyzed by qRT-PCR from cell isolates and mucus tissue. Furthermore, cell count and identification of leukocytes and classification of cellular maturity of erythrocytes using a neural network with a multilayer perceptron (MLP) were performed. Our results showed a significant (p < 0.05) increase in fold change expression of C3 (2.54 ± 0.65) and NK-Lysine (6.84 ± 0.94) in erythrocytes of microalgae-supplemented fish. Moreover, a significant increase of 1.59 and 2.35 times in monocytes and immature erythrocytes, respectively, was observed in the same group of fish (p < 0.05). This study's results indicate that dual microalgae (Schizochytrium spp and N. gaditana) supplementation can increase innate humoral antibacterial components, particularly in erythrocyte tissue, and increase phagocytic cells and immature erythrocytes in S. salar blood.

Characterization analysis of TLR5a and TLR5b immune response after different bacterial infection in grass carp (Ctenopharyngodon idella)

Toll-like receptor (TLR) is an important pattern recognition receptor, which specifically recognizes microbial components, and TLR5 recognizes bacterial flagellin in vertebrates and invertebrates. In this study, two forms of TLR5 (TLR5a and TLR5b) were identified in grass carp (Ctenopharyngodon idella). Aeromonas hydrophila and Staphylococcus aureus were used to investigate the role of grass carp TLR5a and TLR5b against bacteria (flagellate and non-flagellate) in innate immunity, and the expression of TLR5a and TLR5b genes and proteins were detected in immune-related tissues. Quantitative real-time polymerase chain reaction results showed that TLR5a and TLR5b genes of grass carp were highly expressed in the liver, spleen, and head kidney, and their expression patterns were similar in tissues. Meanwhile, the TLR5b gene expression was higher than TLR5a in most tissues. Following exposure to A. hydrophila and S. aureus, the expression levels of TLR5a and TLR5b genes in the liver, spleen, and head kidney were up-regulated significantly. Moreover, the downstream gene, NF-κB, was up-regulated significantly. After A. hydrophila infection, the expression of TLR5a gene was up-regulated in the liver and spleen at 24 h, while TLR5b was up-regulated at 6 h. In the head kidney, TLR5a was up-regulated at 6 h, while TLR5b was up-regulated at 6 h and 12 h. After S. aureus infection, TLR5a and TLR5b were up-regulated at 6 h in the liver and 12 h in the spleen. However, in the head kidney, TLR5a was down-regulated, while TLR5b was up-regulated. Compared with TLR5a, TLR5b had a higher expression level and stronger response to pathogen stimulation. The immunofluorescence results showed that TLR5a and TLR5b proteins in the liver of grass carp infected with A. hydrophila and S. aureus were similar but different in the spleen and head kidney. The results indicated that TLR5a and TLR5b play a critical role in resisting bacterial infection, and TLR5a and TLR5b had obvious tissue and pathogen specificity. TLR5b may play a major role in immune tissues, while TLR5a may play an auxiliary regulatory role in early infection. In addition, TLR5a and TLR5b have an irreplaceable regulatory role in response to flagellate and non-flagellate bacteria. This lays a foundation to explore further the role of TLR5 in resisting flagellate and non-flagellate infections in fish and provides a reference for the innate immunity research of grass carp.

Single-Cell RNA Sequencing Reveals Microevolution of the Stickleback Immune System

The risk and severity of pathogen infections in humans, livestock, or wild organisms depend on host immune function, which can vary between closely related host populations or even among individuals. This immune variation can entail between-population differences in immune gene coding sequences, copy number, or expression. In recent years, many studies have focused on population divergence in immunity using whole-tissue transcriptomics. But, whole-tissue transcriptomics cannot distinguish between evolved differences in gene regulation within cells, versus changes in cell composition within the focal tissue. Here, we leverage single-cell transcriptomic approaches to document signatures of microevolution of immune system structure in a natural system, the three-spined stickleback (Gasterosteus aculeatus). We sampled nine adult fish from three populations with variability in resistance to a cestode parasite, Schistocephalus solidus, to create the first comprehensive immune cell atlas for G. aculeatus. Eight broad immune cell types, corresponding to major vertebrate immune cells, were identified. We were also able to document significant variation in both abundance and expression profiles of the individual immune cell types among the three populations of fish. Furthermore, we demonstrate that identified cell type markers can be used to reinterpret traditional transcriptomic data: we reevaluate previously published whole-tissue transcriptome data from a quantitative genetic experimental infection study to gain better resolution relating infection outcomes to inferred cell type variation. Our combined study demonstrates the power of single-cell sequencing to not only document evolutionary phenomena (i.e., microevolution of immune cells) but also increase the power of traditional transcriptomic data sets.

Effects of metacercariae of Prosorhynchoides sp. (Trematoda: Bucephalidae) on the swimming ability and blood parameters of the intertidal fish Girella laevifrons (Osteichthyes: Kyphosidae)

Parasites may have various effects on their host's health. It is important to determine the type of the effects but also to decipher the physiological mechanisms underlying a parasitic infection. In this study, swimming ability (by a fixed swim speed test) and haematological analyses (by blood parameters) were compared between juvenile fish Girella laevifrons non-parasitised and parasitised with metacercariae of Prosorhynchoides sp. Fish were infested in the laboratory using natural parasitised mussels. Aerobic swimming time, total erythrocyte and leukocyte counts were compared among four groups of fish: non-parasitised (NP), non-parasitised but manipulated (NPM), parasitised 3 days (PT1), and 10 days (PT2) post-infection. Differentiated haematological components were also compared among NP, PT1 and PT2 fish groups. Parasitised fish (PT1 and PT2) swam between 32% and 55% less time than unparasitised fish. Erythrocyte counts were lower in PT1, indicating anemia, and total leukocyte counts were higher in PT1, showing immune action. Macrophages and basophils were observed only in parasitised fish, independent of the post-infection time. There was a negative correlation between swimming time and leukocyte counts. Increased leukocytes in the blood indicate activation of the innate cellular immune response, which may be metabolically costly for the host. Moreover, the anemia in parasitised fish would reduce the oxygen transport necessary to carry out aerobic exercise, reducing the swimming capacity. In fish several days post-infection (PT2), the erythrocytes count reached normal values, and some kinds of leukocytes decreased in comparison to fish PT1. This suggests that parasitised fish with metacercariae were recovering.

The Antibacterial Activity of Erythrocytes From Goose (Anser domesticus) Can Be Associated With Phagocytosis and Respiratory Burst Generation

In the lumen of blood vessels, there are large numbers of erythrocytes, which are approximately 95% of the total blood cells. Although the function of erythrocytes is to transport oxygen in the organism, recent studies have shown that mammalian and teleost erythrocytes are involved in the immune response against bacterial infections. However, the immune mechanisms used by avian erythrocytes are not yet clear. Here, we demonstrated that erythrocytes from goose have the ability to phagocytose as well as conduct antimicrobial activity. Firstly, we revealed the phagocytosis or adhesion activity of goose erythrocytes for latex beads 0.1-1.0 μm in diameter by fluorescence microscopy, and scanning and transmission electron microscopy. The low cytometry results also proved that goose erythrocytes had a wide range of phagocytic or adhesion activity for different bacteria. Followed, the low cytometry analysis data further explored that the goose erythrocytes contain the ability to produce reactive oxygen species (ROS) and inducible nitric oxide synthase (iNOS) in response to bacterial stimulation, and also up-regulated the expression of NOX family includes NOX1 and NOX5. Finally, we also found that goose erythrocytes showed a powerful antibacterial activity against all the three bacteria, meanwhile the stimulation of three kinds of bacteria up-regulated the expression of inflammatory factors, and increased the production of antioxidant enzymes to protect the cells from oxidative damage. Herein, our results demonstrate that goose Erythrocytes possess a certain phagocytic capacity and antioxidant system, and that the antimicrobial activity of erythrocytes can occurred through the production of unique respiratory burst against foreign pathogenic bacteria, which provides new clues to the interaction between bacteria and avian erythrocytes.

The Oxidative Injury of Extracellular Hemoglobin Is Associated With Reactive Oxygen Species Generation of Grass Carp (Ctenopharyngodon idella)

Intravascular hemolysis is a fundamental feature of hemorrhagic venereal infection or tissue and releases the endogenous damage-associated molecular pattern hemoglobin (Hb) into the plasma or tissues, which results in systemic inflammation, vasomotor dysfunction, thrombophilia, and proliferative vasculopathy. However, how the cytotoxic Hb affects the tissues of grass carp remains unclear. Here, we established a hemolysis model in grass carp by injecting phenylhydrazine (PHZ). The data revealed that the PHZ-induced hemolysis increased the content of Hb and activated the antioxidant system in plasma. The histopathology analysis data showed that the PHZ-induced hemolysis increased the accumulation of Hb and iron both in the head and middle kidney. The results of quantitative real-time PCR (qRT-PCR) detection suggested that the hemolysis upregulated the expressions of iron metabolism-related genes. In addition, the immunofluorescence and immunohistochemistry data revealed that the hemolysis caused an obvious deposition of collagen fiber, malondialdehyde (MDA), and 4-hydroxynonenal (4-HNE) accumulation and increased the content of oxidative-related enzymes such as β-galactosidase (β-GAL), lipid peroxide (LPO), and MDA in both the head and middle kidney. Furthermore, the PHZ-induced hemolysis significantly increased the production of reactive oxygen species (ROS), which resulted in apoptosis and modulated the expressions of cytokine-related genes. Taken together, excess of Hb released from hemolysis caused tissue oxidative damage, which may be associated with ROS and inflammation generation.

Hemeprotein amplifies the innate immune receptors of Ctenopharyngodon idellus kidney cells through NF-κB- and MAPK-dependent reactive oxygen species generation

Infectious bacterial and viral diseases that cause hemolysis are considered life-threatening to grass carp (Ctenopharyngodon idellus), which is a species used in aquaculture worldwide. After heme and hemeproteins (Hb) are released as a result of hemolysis, the effect of excess Hb and heme on tissues remains to be characterized. To decipher the mechanisms, after incubation with Hb, we showed that lipopolysaccharide (LPS), Hb, and heme increased the cytotoxicity and secretion of inflammatory cytokines such as interleukin (IL)-6, chemokine (C-C motif) ligand 1 (CCL1), tumor necrosis factor (TNF)-α, IL-6, and IL-1β in vitro, which was due to stimulation of the expression of innate immune receptors, such as nucleotide-binding oligomerization domain (NOD2), toll-like receptor 2 (TLR2), TLR 4, and TLR3. The formation of reactive oxygen species (ROS) and the activation of mitogen-activated protein kinases (MAPKs) and nuclear factor (NF)-κB were important for increasing the cytokine production to induce heme, Hb, and LPS. Moreover, we confirmed that after LPS, Hb, and heme challenge, superoxide dismutase (SOD) and glutathione (GSH) synthetase (GSS) also caused remarkable destruction. However, catalase (CAT) and heme oxygenase-1 (HO-1) were strongly activated. In summary, our research findings present a framework through which heme and Hb concentrations amplify the secretions of inflammatory cytokines, which are induced by pattern recognition receptor (PRR) activation and present possible paths for immune intervention during infection with viral diseases and hemolytic bacterial.

The antibacterial activity of erythrocytes from Clarias fuscus associated with phagocytosis and respiratory burst generation

It is widely known that red blood cells (RBCs) are responsible for respiration and the transport of gas. However, recent reports have also described the immune properties of RBCs, therefore creating new understanding for the functionality of RBCs. However, little is known about the immunological role of RBCs in bony fish. In this study, we used RBCs from Clarias fuscus as a model and demonstrate that these cells exhibited phagocytic ability with both latex beads and bacteria. Scanning electron microscopy and transmission electron microscopy provided visual confirmation of the phagocytotic process in RBCs. In addition, we used flow cytometry and fluorescence microscopy to analyse the rate of phagocytosis in RBCs. We found that RBCs exhibited stable phagocytotic ability with latex beads ranging from 0.5 to 1.0 μm in size. In response to bacterial stimulation, RBCs produced reactive oxygen species (ROS) and nitric oxide synthase (NOS), which are harmful to bacteria. RBCs also have an antioxidant system. Under conditions of oxidative stress, the expression levels of antioxidant enzymes, and particularly those of superoxide dismutase（SOD） increased significantly. Our results show that the erythrocytes of bony fish are phagocytic and also produce ROS which are toxic to bacteria. In addition, RBCs have an antioxidant system that removes excess ROS production to protect cells from oxidative damage.

Expression and functional analysis of the BCL2-Associated agonist of cell death (BAD) gene in grass carp (Ctenopharyngodon idella) during bacterial infection

The BCL2-associated agonist of cell death protein is a key participant in apoptosis dependent on mitochondria and in disease progression that involves the regulation of cell death, such as tumorigenesis, diabetes, sepsis shock, and epilepsy. Nevertheless, the mechanisms underlying the immune responses to teleost BAD bacterial infection and mitochondrial-dependent apoptosis remains unclear. In order to elucidate the mechanisms involved, in this study, a Ctenopharyngodon idella (grass carp) BAD gene named GcBAD1 was firstly cloned and characterized. The results indicated that the ORF (open reading frame) of GcBAD1 was 438 bp in length, encoding a 145-amino acid putative protein of 16.66 kDa. This deduced amino acid sequence has a better identity than another teleost species according to a phylogenetic analysis, and contains a Bcl2-BAD-1 domain. In healthy grass carp fish, the mRNA transcripts of GcBAD1 were widely present in the studied tissues, which could be ranked as follows; spleen > brain > middle-kidney > head-kidney > liver > gills > intestines > heart and muscle. In addition, during infection by Aeromonas hydrophila and Staphylococcus aureus, the mRNA transcription and protein levels expression of GcBAD1 in the head-kidney, spleen, and liver tissues of the fish were significantly up-regulated. Moreover, when the C. idellus kidney cell line (CIK) cells were incubated with Lipopolysaccharide (LPS) and lipoteichoic acid (LTA), the GcBAD1 expression transcripts were also significantly up-regulated. Additionally, overexpression of GcBAD1 in CIK cells was able to activate apoptosis-related genes, including those encoding p53, Cytochrome C (CytoC), caspase-3, and caspase-9. Besides, in the TUNEL assays, when pEGFP-BAD1 was over-expressed, the number of red signals associated with apoptosis were significantly increased in the CIK cells infected with LPS or LTA at 12 h. This study demonstrates that GcBAD1 has a significant role in the mitochondrial apoptosis pathway of grass carp's innate immunity. Our findings provide new insight into the potential mechanisms of teleost antibacterial immunity.

Oxycytosis and the role of triboelectricity and oxidation in bacteria clearing from the bloodstream

Until recently, little was known about the mechanism for killing and clearing bacteria from the bloodstream. Leukocyte phagocytosis could not be a mechanism for catching, killing and removing bacteria from the bloodstream because of many reasons. Recently accumulated data have led to the conclusion that in bacteremia, bacteria are quickly removed from the blood and erythrocytes are the main cells that capture, kill and remove bacteria. Data were also obtained that erythrocytes catch bacteria by triboelectric charge attraction and kill them by oxygen released from oxyhemoglobin. This phenomenon has been named oxycytosis by analogy with the term phagocytosis. Oxycytosis has been discussed in a number of published articles, but the specific mechanism of triboelectric charging and the mechanism of killing bacteria by oxidation, have not yet been detailed. The purpose of this review is to provide a more detailed explanation of the process of triboelectric charging and capture of bacteria by erythrocytes and destruction of bacteria by oxidation. For the first time, the review presents various variants of oxycytosis (two-stage, three-stage, multi-stage), depending on the resistance of the pathogen to oxidation. The review also discusses the biological significance of oxycytosis and its impact on the understanding of bacteremia and sepsis.

Escherichia coli induced ferroptosis in red blood cells of grass carp (Ctenopharyngodon idella)

The red blood cells (RBCs) of fish make up around 95% of the total peripheral blood cells, and the long-held paradigm is that RBCs are mainly responsible for transporting oxygen. Previous studies have showed that the RBCs can be involved in the immune response against bacterial infection; however, this mechanism remains enigmatic. Here, we explored the structure of grass carp RBCs (GcRBCs). The results showed that the GcRBCs released a pseudopodia-like structure when grown in a 24-well plate, and the transmission electron microscopy (TEM) result showed that GcRBCs contained some organelle-like structures. To further verify the organelle-like structures might be the mitochondria and lysosome which similar to other immune cells, a fluorescent labeling assay was used to verify it. To decipher the antibacterial immunity of GcRBCs, transcriptomic profiling of grass carp RBCs after the incubation with E. coli was analyzed. The results showed that there were 4099 differently expressed genes (DEGs) of GcRBCs upon E. coli incubation, including 2041 up-regulated and 2058 down-regulated genes. In addition, to validate our transcriptomic data, we checked the expression of several cytokines, such as CCL4, CCL20, IL4, IL12 and IFN-α, and the results showed that all the selected gens were significantly up-regulated after E. coli incubation. Furthermore, E. coli incubation induced hemoglobin oxidation and increased the heme in GcRBCs, which further activated the expression of heme oxygenase 1 (HO-1), autophagy related genes 5 (ATG5), and ferritin. In contrast, E. coli incubation inhibited the expression of Ferroportin-1 (FPN1), which increased intracellular iron levels, induced Fenton reaction to release reactive oxygen species (ROS), and activated the ferroptosis signaling pathway in GcRBCs. Herein, we demonstrate that E. coli can induce teleost RBCs cell death through an iron-mediated ferroptosis pathway, which sheds new light on the interaction between bacteria and teleost RBCs.

The immune function of heme oxygenase-1 from grass carp (Ctenopharyngodon idellus) in response to bacterial infection

Heme oxygenase (HO)-1, a rate-limiting enzyme in heme catabolism, results in the formation of equivalent amounts of biliverdin (BV), carbon monoxide (CO) and ferrous iron (Fe²⁺). Previous studies have revealed that HO-1 plays an important role in immune responses. However, the mechanism underlying the immune responses against bacterial infection of teleost HO-1 remains enigmatic. To decipher the mechanisms, we have cloned and characterized the HO-1 gene of grass carp (designated as GcHO-1) in this research. The results showed that the open reading frame (ORF) of GcHO-1 was 819 bp, which encoded a putative protein of 272 amino acids. The deduced amino acid sequence phylogenetically shared the highest identity with other teleosts, and contained two domains of heme-oxygenase and a single-pass transmembrane domain. The mRNA expressions of GcHO-1 in healthy grass carp have widely existed in examined tissues in the following order of spleen > head-kidney > middle head-kidney > intestines > liver > gills > heart > muscle > brain. Besides, the mRNA and protein transcription of GcHO-1 were both significantly up-regulated in the liver and head-kidney tissues after Staphylococcus aureus and Aeromonas hydrophila infection. In addition, overexpression of GcHO-1 in kidney cell line (CIK) cells of grass carp could reduce the expression of inflammatory cytokines (IL-1β, IL-8, TNFα, CCL1 and IL-6). Herein, we demonstrate that GcHO-1 plays an anti-inflammatory role in innate immunity. Our results shed new light on the mechanisms underlying the antibacterial immunity of teleost.

Immune Functions of Erythrocytes in Osteichthyes

Red blood cells (RBCs)—erythrocytes—of Osteichthyes are primarily known for their involvement in the process of gas exchange and respiration. Currently, physiological properties of RCBs in fish should also include their ability to participate in defense processes as part of the innate and adaptive immune mechanisms. In response to viruses, bacteria, and fungi or recombinant nanoparticles, they can modulate expression of genes responsible for immune reactions, influence activity of leukocytes, and produce cytokines, antimicrobial peptides, and paracrine intercellular signaling molecules. Via the complement system (CR1 receptor) and owing to their phagocytic properties (erythrophagocytosis), RBCs of Osteichthyes can eliminate pathogens. In addition, they are probably involved in the immune response as antigen-presenting cells via major histocompatibility complex class II antigens.