Understanding the pathogenesis of Flavobacterium psychrophilum using the rainbow trout monocyte/macrophage-like cell line, RTS11, as an infection model

Innate and adaptive immune response of Rainbow trout (Oncorhynchus mykiss) naturally infected with Yersinia ruckeri

Rainbow trout is an important fish species for Peruvian artisanal aquaculture, comprising over 60 % of the total aquaculture production. However, their industry has been highly affected by several bacterial agents such as Yersinia ruckeri. This pathogen is the causative agent of Enteric Redmouth Disease, and causes high mortality in fingerlings and chronic infection in adult rainbow trout. To date, the immune response of rainbow trout against Y. ruckeri has been well studied in laboratory-controlled infection studies (i.e. intraperitoneal infection, bath immersion), however, the immune response during natural infection has not been explored. To address this, in this study, 35 clinically healthy O. mykiss without evidence of lesions or changes in behavior and 32 rainbow trout naturally infected by Y. ruckeri, were collected from semi-intensive fish farms located in the Central Highlands of Peru. To evaluate the effect on the immune response, RT-qPCR, western blotting, and ELISA were conducted using head kidney, spleen, and skin tissues to evaluate the relative gene expression and protein levels. Our results show a significant increase in the expression of the pro-inflammatory cytokines il1b, tnfa, and il6, as well as ifng in all three tissues, as well as increases in IL-1β and IFN-γ protein levels. The endogenous pathway of antigen presentation showed to play a key role in defense against Y. ruckeri, due to the upregulation of mhc-I, tapasin, and b2m transcripts, and the significant increase of Tapasin protein levels in infected rainbow trout. None of the genes associated with the exogenous pathway of antigen presentation showed a significant increase in infected fish, suggesting that this pathway is not involved in the response against this intracellular pathogen. Finally, the transcripts of immunoglobulins IgM and IgT did not show a modulation, nor were the protein levels evaluated in this study.

Characterisation of sRNAs enriched in outer membrane vesicles of pathogenic Flavobacterium psychrophilum causing Bacterial Cold Water Disease in rainbow trout

Flavobacterium psychrophilum (Fp) causes Bacterial Cold Water Disease in salmonids. During host-pathogen interactions, gram-negative bacteria, such as Fp, release external membrane vesicles (OMVs) harbouring cargos, such as DNA, RNA and virulence factors. This study aimed to characterise the potential role of the OMVs' small RNAs (sRNAs) in the Fp-rainbow trout host-pathogen interactions. sRNAs carried within OMVs were isolated from Fp. RNA-Seq datasets from whole-cell Fp and their isolated OMVs indicated substantial enrichment of specific sRNAs in the OMVs compared to the parent cell. Many of the OMV-packaged sRNAs were located in the pathogenicity islands of Fp. Conservation of sRNAs in 65 strains with variable degrees of virulence was reported. Dual RNA-Seq of host and pathogen transcriptomes on day 5 post-infection of Fp -resistant and -susceptible rainbow trout genetic lines revealed correlated expression of OMV-packaged sRNAs and their predicted host's immune gene targets. In vitro, treatment of the rainbow trout epithelial cell line RTgill-W1 with OMVs showed signs of cytotoxicity accompanied by dynamic changes in the expression of host genes when profiled 24 h following treatment. The OMV-treated cells, similar to the Fp -resistant fish, showed downregulated expression of the suppressor of cytokine signalling 1 (SOCS1) gene, suggesting induction of phagosomal maturation. Other signs of modulating the host gene expression following OMV-treatment include favouring elements from the phagocytic, endocytic and antigen presentation pathways in addition to HSP70, HSP90 and cochaperone proteins, which provide evidence for a potential role of OMVs in boosting the host immune response. In conclusion, the study identified novel microbial targets and inherent characteristics of OMVs that could open up new avenues of treatment and prevention of fish infections.

Systemic immune response of rainbow trout exposed to Flavobacterium psychrophilum infection

Bacterial cold-water disease (BCWD) caused by Flavobacterium psychrophilum is one of the most serious bacterial diseases leading to significant economic loss for rainbow trout (Oncorhynchus mykiss) aquaculture. However, little is known about the systemic immune response of rainbow trout against F. psychrophilum infection. This study investigated the immune response of rainbow trout to F. psychrophilum infection using multiple experiments, including bacterial load detection, phagocyte activity assessment, enzyme activity evaluation, and gene expression profiling. Results showed that the spleen index and intestinal pathogen load reached a peak at 3 days post-infection, with strong pro-inflammatory gene expression observed in rainbow trout. Leukocytes RBA and PKA were significantly elevated in the spleen, blood and intestine at 7 days post-infection. Heat map analysis demonstrated that the spleen had a more substantial pro-inflammatory response compared to the intestine post-infection and exhibited higher expression levels of immune-related genes, including IgM, il1β, il6, cd4, cd8a, cd8b, c1q, chathelicidin, inos, and lysozyme. Both Th1 and Th2 polarized responses in the spleen were activated, with Th2 (il4/13a, gata3) (FC > 4) being more intense than Th1 (tnfα, t-bet) (FC > 2). Tight junction proteins exhibited down-regulation followed by up-regulation post-infection. Collectively, the results of this study expand our current understanding of the immune response of rainbow trout post F. psychrophilum infection but also provide new avenues for investigation in salmonid aquaculture.

Expression of Interleukin-1β protein in vitro,exvivo and in vivo salmonid models

Interleukin-1β (IL-1β) is one of the first cytokines expressed during immune responses, and its levels are affected by many factors, including stress. To date, it has only been possible to measure IL-1β transcript (mRNA) expression quantitatively in fish using qPCR. This is because previous studies that measured IL-1β protein concentrations in these taxa used western blotting, which only provides qualitative data. To advance our knowledge of fish IL-1β biology, and because post-translational processing plays a critical role in the activation of this molecule, we developed a quantitative enzyme-linked immunosorbent assay (ELISA) to accurately measure the concentration of IL-1β protein in several cell cultures and in vivo in salmonids. We compared changes in IL-1β protein levels to the expression of its mRNA. The developed ELISA was quite sensitive and has a detection limit of 12.5 pg/mL. The tools developed, and information generated through this research, will allow for a more accurate and complete understanding of IL-1β's role in the immune response of salmonids.The assay described here has the potential to significantly advance our ability to assess fish health and immune status.

Biofilm formation of pathogenic bacteria isolated from aquatic animals

Bacterial biofilm formation is one of the dynamic processes, which facilitates bacteria cells to attach to a surface and accumulate as a colony. With the help of biofilm formation, pathogenic bacteria can survive by adapting to their external environment. These bacterial colonies have several resistance properties with a higher survival rate in the environment. Especially, pathogenic bacteria can grow as biofilms and can be protected from antimicrobial compounds and other substances. In aquaculture, biofilm formation by pathogenic bacteria has emerged with an increased infection rate in aquatic animals. Studies show that Vibrio anguillarum, V. parahaemolyticus, V. alginolyticus, V. harveyi, V. campbellii, V. fischeri, Aeromonas hydrophila, A. salmonicida, Yersinia ruckeri, Flavobacterium columnare, F. psychrophilum, Piscirickettsia salmonis, Edwardsiella tarda, E. ictaluri, E. piscicida, Streptococcus parauberis, and S. iniae can survive in the environment by transforming their planktonic form to biofilm form. Therefore, the present review was intended to highlight the principles behind biofilm formation, major biofilm-forming pathogenic bacteria found in aquaculture systems, gene expression of those bacterial biofilms and possible controlling methods. In addition, the possibility of these pathogenic bacteria can be a serious threat to aquaculture systems.

DOK3 maintains intestinal homeostasis by suppressing JAK2/STAT3 signaling and S100a8/9 production in neutrophils

How pathogenesis of inflammatory bowel disease (IBD) depends on the complex interplay of host genetics, microbiome and the immune system is not fully understood. Here, we showed that Downstream of Kinase 3 (DOK3), an adapter protein involved in immune signaling, confers protection of mice from dextran sodium sulfate (DSS)-induced colitis. DOK3-deficiency promotes gut microbial dysbiosis and enhanced colitis susceptibility, which can be reversed by the transfer of normal microbiota from wild-type mice. Mechanistically, DOK3 exerts its protective effect by suppressing JAK2/STAT3 signaling in colonic neutrophils to limit their S100a8/9 production, thereby maintaining gut microbial ecology and colon homeostasis. Hence, our findings reveal that the immune system and microbiome function in a feed-forward manner, whereby DOK3 maintains colonic neutrophils in a quiescent state to establish a gut microbiome essential for intestinal homeostasis and protection from IBD.

Breeding Strategy Shapes the Composition of Bacterial Communities in Female Nile Tilapia Reared in a Recirculating Aquaculture System

In industrial animal production, breeding strategies are essential to produce offspring of better quality and vitality. It is also known that host microbiome has a bearing on its health. Here, we report for the first time the influence of crossbreeding strategy, inbreeding or outbreeding, on the buccal and intestinal bacterial communities in female Nile tilapia (Oreochromis niloticus). Crossbreeding was performed within a family and between different fish families to obtain the inbred and outbred study groups, respectively. The genetic relationship and structure analysis revealed significant genetic differentiation between the inbred and outbred groups. We also employed a 16S rRNA gene sequencing technique to understand the significant differences between the diversities of the bacterial communities of the inbred and outbred groups. The core microbiota composition in the mouth and the intestine was not affected by the crossbreeding strategy but their abundance varied between the two groups. Furthermore, opportunistic bacteria were abundant in the buccal cavity and intestine of the outbred group, whereas beneficial bacteria were abundant in the intestine of the inbred group. The present study indicates that crossbreeding can influence the abundance of beneficial bacteria, core microbiome and the inter-individual variation in the microbiome.

Salmonid Antibacterial Immunity: An Aquaculture Perspective

The aquaculture industry is continuously threatened by infectious diseases, including those of bacterial origin. Regardless of the disease burden, aquaculture is already the main method for producing fish protein, having displaced capture fisheries. One attractive sector within this industry is the culture of salmonids, which are (a) uniquely under pressure due to overfishing and (b) the most valuable finfish per unit of weight. There are still knowledge gaps in the understanding of fish immunity, leading to vaccines that are not as effective as in terrestrial species, thus a common method to combat bacterial disease outbreaks is the use of antibiotics. Though effective, this method increases both the prevalence and risk of generating antibiotic-resistant bacteria. To facilitate vaccine design and/or alternative treatment efforts, a deeper understanding of the teleost immune system is essential. This review highlights the current state of teleost antibacterial immunity in the context of salmonid aquaculture. Additionally, the success of current techniques/methods used to combat bacterial diseases in salmonid aquaculture will be addressed. Filling the immunology knowledge gaps highlighted here will assist in reducing aquaculture losses in the future.