Hepcidin, Cathelicidin-1 and IL-8 as immunological markers of responsiveness in early developmental stages of rainbow trout

Chronic hypoxia has differential effects on constitutive and antigen-stimulated immune function in Atlantic salmon (Salmo salar)

Chronic hypoxia events are a common occurrence in Atlantic salmon (Salmo salar) sea-cages, especially during the summer, and their frequency and severity are predicted to increase with climate change. Although hypoxia is considered a very important fish health and welfare issue by the aquaculture industry, few studies have investigated the impact of chronic hypoxia on the fish immune system and its response to pathogen exposure. We exposed post-smolt Atlantic salmon to hypoxia (40% air sat.) for 6 weeks. Thereafter, we sampled fish prior to (i.e., at Time 0, to assess constitutive immune function), and after they were intraperitoneally injected with PBS (phosphate buffered saline) or formalin-killed Aeromonas salmonicida. We measured several innate immune parameters including: hematological immune responses [respiratory burst (RB), hemolytic activity of alternate complement system and plasma lysozyme concentration], and the relative percentage of circulating blood cells (erythrocytes/immature erythrocytes, monocytes, and granulocytes and lymphocytes) at Time 0 and at 24 hours post-injection (hpi); and the transcript expression levels of 8 anti-bacterial biomarkers in the head kidney [interleukin-1 beta (il1b), interleukin-8a (il8a), cyclooxygenase-2 (cox2), toll-like receptor 5, secreted (strl5), CC chemokine-like 19b (ccl19b), serum amyloid A5 (saa5), hepcidin anti-microbial peptide a (hampa) and cathelicidin anti-microbial peptide b (campb)] at Time 0 and at 6 and 24 hpi. In addition, we measured serum immunoglobulin (IgM) levels at Time 0 and at 8 weeks post-injection (4 weeks after a 'boost' injection). Fish exposed to chronic hypoxia had greater numbers of monocytes, which was consistent with the increase in RB, plasma lysozyme concentration and upregulated head kidney anti-bacterial gene expression (i.e., campb, ccl19b, hampa, il8a, stlr5). In contrast, chronic hypoxia: reduced RB and leukocyte numbers at 24 hpi in Asal compared to PBS-injected fish, and the transcript levels of campb, il1b, saa5, il8a and stlr5 at 6- and/or 24- hpi; but had no effect on constitutive or post-stimulation serum IgM titers. Overall, our results indicate that chronic hypoxia has differential effects on salmon constitutive innate immune function vs. following antigen exposure, and thus, it is still unclear how chronic hypoxia will impact the capacity of fish to defend against pathogens.

Detection and Localization of IL-8 and CXCR1 in Rainbow Trout Larvae in Response to Pseudomonas aeruginosa Lipopolysaccharide

The salmonid industry faces challenges due to the susceptibility of fish to opportunistic pathogens, particularly in early developmental stages. Understanding the immunological capacity during these stages is crucial for developing effective disease control strategies. IL-8R, a member of the G-protein-coupled receptor family, acts as a receptor for Interleukin 8 (IL-8). The binding of IL-8 to IL-8R plays a major role in the pathophysiology of a wide spectrum of inflammatory conditions. This study focused on the immune response capacity of rainbow trout (Oncorhynchus mykiss) larvae by analyzing IL-8/CXCR1 response to lipopolysaccharide (LPS) from Pseudomonas aeruginosa. Previous research demonstrated that LPS from P. aeruginosa acts as a potent immunostimulant in teleost, enhancing pro-inflammatory cytokines. The methodology included in silico analysis and the synthesis and characterization of an omCXCR1-derived epitope peptide, which was used to produce omCXCR1-specific anti98 serum in mice. The research revealed that rainbow trout larvae 19 days post-hatching (dph) exhibited pronounced immune responses post-stimulation with 1 µg/mL of LPS. This was evidenced by the upregulated protein expression of IL-8 and omCXCR1 in trout larvae 2 and 8 h after LPS challenge, as analyzed by ELISA and immunohistochemistry. Furthermore, fluorescence microscopy successfully revealed the colocalization of IL-8 and its receptor in cells from mucosal tissues after LPS challenge in larvae 19 dph. These findings underscore the efficacy of LPS immersion as a method to activate the innate immune system in trout larvae. Furthermore, we propose IL-8 and its receptor as molecular markers for evaluating immunostimulation in the early developmental stages of salmonids.

New Evidence for the Role of Pituitary Adenylate Cyclase-Activating Polypeptide as an Antimicrobial Peptide in Teleost Fish

Pituitary Adenylate Cyclase-Activating Polypeptide (PACAP) is a multifunctional neuropeptide that is widely distributed and conserved across species. We have previously shown that in teleost fish, PACAP not only possesses direct antimicrobial properties but also immunomodulatory effects against the bacterial pathogens Flavobacterium psychrophilum and Pseudomonas aeruginosa using in vitro and in vivo experiments. These previous results suggest PACAP can be used as an alternative to antibiotics to prevent and/or treat bacterial infections in the aquaculture industry. To accomplish this goal, more studies are needed to better understand the effect of PACAP on pathogens affecting fish in live infections. In the present study, the transcripts PACAP, PRP/PACAP, and VPAC2 receptor were examined in rainbow trout (Oncorhynchus mykiss) naturally infected with Yersinia ruckeri, which exhibited an increase in their expression in the spleen when compared to healthy fish. Synthetic Clarias gariepinus PACAP-38 has direct antimicrobial activity on Y. ruckeri and inhibits up to 60% of the bacterial growth when the peptide is at concentrations between 50 and 100 µM in TSB. The growth inhibition increased up to 90% in the presence of 12.5 µM of PACAP-38 when salt-free LB broth was used instead of TSB. It was also found to inhibit Y. ruckeri growth in a dose-dependent manner when the rainbow trout monocyte/macrophage-like cell line (RTS11) was pre-treated with lower concentrations of the peptide (0.02 and 0.1 µM) before going through infection. Differential gene expression was analyzed in this in vitro model. Overall, the results revealed new evidence to support the role of PACAP as an antimicrobial and immunomodulatory peptide treatment in teleosts.

Priming European Sea Bass Female Broodstock Improves the Antimicrobial Immunity of Their Offspring

Acquiring immunocompetence is essential in the development of fish embryos, as they are exposed to environmental pathogens even before they are fertilized. Despite the importance of the antimicrobial function as the first line of defense against foreign microorganisms, little knowledge is available about its role in larval development. In vertebrates, transgenerational immune priming influences the acquisition of immunocompetence of specimens, regulating the selective allocation of nongenetic resources to their progeny and modulating their development. In this work, we primed teleost European sea bass broodstock females with a viral protein expression vector in order to evaluate the innate immunity development of their offspring. Several antimicrobial functions, the pattern of expression of gene coding for different antimicrobial peptides (AMPs), and their protein levels, were evaluated in eggs and larvae during development. Our data determined the presence of antimicrobial proteins of maternal origin in eggs, and that female vaccination increases antimicrobial activities and the transcription and synthesis of AMPs during larval development.

Immunodetection of rainbow trout IL-8 cleaved-peptide: Tissue bioavailability and potential antibacterial activity in a bacterial infection context

Chemokines such as IL-8 are part of an important group of proinflammatory response molecules, as well as cell recruitment. However, it has been described in both higher vertebrates and fish that IL-8 has an additional functional role by acting as an antimicrobial effector, either directly or by cleavage of a peptide derived from its C-terminal end. Nevertheless, it is still unknown whether this fragment is released in the context of infection by bacterial pathogens and if it could be immunodetected in tissues of infected salmonids. Therefore, the objective of this research was to demonstrate that the C-terminal end of IL-8 from Oncorhynchus mykiss is cleaved, retaining its antibacterial properties, and that is detectable in tissues of infected rainbow trout. SDS-PAGE and mass spectrometry demonstrated the cleavage of a fragment of about 2 kDa when the recombinant IL-8 was subjected to acidic conditions. By chemical synthesis, it was possible to synthesize this fragment called omIL-8α_80-97 peptide, which has antibacterial activity against Gram-negative and Gram-positive bacteria at concentrations over 10 μM. Besides, by fluorescence microscopy, it was possible to locate the omIL-8α_80-97 peptide both on the cell surface and in the cytoplasm of the bacteria, as well as inside the monocyte/macrophage-like cell. Finally, by indirect ELISA, Western blot, and mass spectrometry, the presence of the fragment derived from the C-terminal end of IL-8 was detected in the spleen of trout infected with Piscirickettsia salmonis. The results reported in this work present the first evidence about the immunodetection of an antibacterial, and probably cell-penetrating peptide cleaved from the C-terminal end of IL-8 in monocyte/macrophage-like cell and tissue of infected rainbow trout.

New insight to the rol of α-enolase (Eno-1) as immunological marker in rainbow trout fry

α-Enolase is an enzyme of the glycolytic pathway that has also been involved in vertebrate inflammatory processes through its interaction with plasminogen. However, its participation in the immune response of lower vertebrates during early life development is unknown. Opportunistic pathogens in salmon farming are the principal cause of mortality in the fry stage. For that reason, molecular indicators of their immunological status are required to ensure the success of the large-scale cultivation. Thus, the objective of this work was to analyze if ENO-1 is involved in the immune response of rainbow trout fry. For this purpose, the coding sequence of trout ENO-1 was characterized, identifying the plasminogen-binding domain that has been described for homologs of this enzyme in higher vertebrates. A peptide-epitope of α-enolase was used for producing mice antiserum. The specificity of polyclonal antibodies was confirmed by dot blot, ELISA and Western blot. Then, the antiserum was used to evaluate α-enolase expression in fry between 152 and 264 degree-days post-hatching after 2, 8, and 12 h of challenge with lipopolysaccharide from Pseudomona auroginosa. The expression of α-enolase at both transcriptional (RT-qPCR) and protein (ELISA) levels was significantly increased after 8 h post-challenge with lipopolysaccharide. These results were confirmed by proteomic analysis by 2D-difference gel electrophoresis (DIGE). This work provides the first evidence of the involvement of α-enolase in the early immune response of salmonids. Future research will be required to understand the possible interaction of α-enolase with plasminogen in cells and tissues of the salmonid immune system.

Antimicrobial Peptides of Salmonid Fish: From Form to Function

Antimicrobial peptides (AMPs) are small, usually cationic, and amphiphilic molecules that play a crucial role in molecular and cellular host defense against pathogens, tissue damage, and infection. AMPs are present in all metazoans and several have been discovered in teleosts. Some teleosts, such as salmonids, have undergone whole genome duplication events and retained a diverse AMP repertoire. Salmonid AMPs have also been shown to possess diverse and potent antibacterial, antiviral, and antiparasitic activity and are induced by a variety of factors, including dietary components and specific molecules also known as pathogen-associated molecular patterns (PAMPs), which may activate downstream signals to initiate transcription of AMP genes. Moreover, a multitude of cell lines have been established from various salmonid species, making it possible to study host-pathogen interactions in vitro, and several of these cell lines have been shown to express various AMPs. In this review, the structure, function, transcriptional regulation, and immunomodulatory role of salmonid AMPs are highlighted in health and disease. It is important to characterize and understand how salmonid AMPs function as this may lead to a better understanding of host-pathogen interactions with implications for aquaculture and medicine.

Integration of Transcriptome, Gross Morphology and Histopathology in the Gill of Sea Farmed Atlantic Salmon (Salmo salar): Lessons From Multi-Site Sampling

The gill of teleost fish is a multifunctional organ involved in many physiological processes such as gas exchange, osmotic and ionic regulation, acid-base balance and excretion of nitrogenous waste. Due to its extensive interface with the environment, the gill plays a key role as a primary mucosal defense tissue against pathogens, as manifested by the presence of the gill-associated lymphoid tissue (GIALT). In recent years, the prevalence of multifactorial gill pathologies has increased significantly, causing substantial losses in Atlantic salmon aquaculture. The transition from healthy to unhealthy gill phenotypes and the progression of multifactorial gill pathologies, such as proliferative gill disease (PGD), proliferative gill inflammation (PGI) and complex gill disorder (CGD), are commonly characterized by epithelial hyperplasia, lamellar fusion and inflammation. Routine monitoring for PGD relies on visual inspection and non-invasive scoring of the gill tissue (gross morphology), coupled with histopathological examination of gill sections. To explore the underlying molecular events that are associated with the progression of PGD, we sampled Atlantic salmon from three different marine production sites in Scotland and examined the gill tissue at three different levels of organization: gross morphology with the use of PGD scores (macroscopic examination), whole transcriptome (gene expression by RNA-seq) and histopathology (microscopic examination). Our results strongly suggested that the changes in PGD scores of the gill tissue were not associated with the changes in gene expression or histopathology. In contrast, integration of the gill RNA-seq data with the gill histopathology enabled us to identify common gene expression patterns associated with multifactorial gill disease, independently from the origin of samples. We demonstrated that the gene expression patterns associated with multifactorial gill disease were dominated by two processes: a range of immune responses driven by pro-inflammatory cytokines and the events associated with tissue damage and repair, driven by caspases and angiogenin.

NK-lysin, dicentracin and hepcidin antimicrobial peptides in European sea bass. Ontogenetic development and modulation in juveniles by nodavirus

Antimicrobial peptides (AMPs) are considered to be amongst the most powerful tools for the fight against pathogens in fish, since they form part of the innate immune response, which is especially vital in eggs and early larval stages, when the immune system is developing. The fish responsible for a large part of the profits in Mediterranean aquaculture is European sea bass (Dicentrarchus labrax), a species greatly susceptible to nodavirus (NNV), especially in the larval and juvenile stages. In this work, polyclonal antibodies were developed and used to detect and quantify NK-lysin, dicentracin and hepcidin AMPs in European sea bass eggs and during larval development, as well as to evaluate their regulation in juvenile specimens upon NNV infection. Basal and detectable levels of all the AMPs studied were present in eggs, confirming the maternal transfer of peptides, which increased in one or two waves during larval development up to 69 days post-fertilization. After NNV infection, the mRNA of all the AMPs analysed was up-regulated five days after infection in most of the tissues, whilst peptide quantification of all three AMPs decreased in the brain, the target tissue for NNV, but increased in the head-kidney 5 days after infection. Further research should be carried out to ascertain the role of AMPs in fish innate immunity and to understand how NNV evades the immune response to be disseminated.

Fish skeletal muscle tissue is an important focus of immune reactions during pathogen infection

Skeletal muscle in mammals can express and secrete immune-related molecules during pathogen infection. Despite in fish is known that classical immune tissues participate in innate immunity, the role of skeletal muscle in this function is poorly understood. To determine the immunocompetence of fish skeletal muscle, juvenile fine flounder (Paralichthys adpersus) were challenged with Vibrio ordalii. Different Toll-like receptors, pro-inflammatory cytokines (TNFα, Il-1β, and IL-8), and immune-effector molecules (NKEF and the antimicrobial peptides hepcidin and LEAP-2) were analyzed. Infection initially triggered IL-1β upregulation and P38-MAPK/AP-1 pathway activation. Next, the NFĸB pathway was activated, together with an upregulation of intracellular Toll-like receptor expressions (tlr3, tlr8a tlr9, and tlr21), TNFα production, and leap-2 expression. Finally, transcriptions of il-1β, il-8, tnfα, nkef-a, and hepcidin were also upregulated. These results suggest that fish skeletal muscle is an immunologically active organ that could play an important role against pathogens.