A BCWD-Resistant line of rainbow trout is less sensitive to cortisol implant-induced changes in IgM response as compared to a susceptible (control) line

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.

Microplastics exacerbate virus-mediated mortality in fish

Microplastics are a persistent and increasing environmental hazard. They have been reported to interact with a variety of biotic and abiotic environmental stressors, but the ramifications of such interactions are largely unknown. We investigated virus-induced mortalities in a commercially important salmonid following exposure to microplastics, plastic microfibers, and natural (non-plastic) microparticles. Microplastics or microparticles alone were not lethal. Mortality increased significantly when fish were co-exposed to virus and microplastics, particularly microfibers, compared to virus alone. This presents the unique finding that microplastics (not natural microparticulate matter) may have a significant impact on population health when presented with another stressor. Further, we found that mortality correlated with host viral load, mild gill inflammation, immune responses, and transmission potential. We hypothesize that microplastics can compromise host tissues, allowing pathogens to bypass defenses. Further research regarding this mechanism and the interplay between microplastics and infectious disease are paramount, considering microplastics increasing environmental burden.

The early function of cortisol in liver during Aeromonas hydrophila infection: Dynamics of the transcriptome and accessible chromatin landscapes

In China, channel catfish (Ictalurus punctatus) is an important aquaculture species; however, haemorrhagic disease (Aeromonas hydrophila induced disease) in these fish has caused tremendous economic loss due to high morbidity and mass mortality in the breeding industry. The role of cortisol in bacterial diseases, particularly in the acute phase, remains unclear. In this study, liver transcriptome (RNA-seq) and chromatin accessibility (ATAC-seq) analyses were employed to investigate the early functional role of cortisol in Aeromonas hydrophila-stimulated responses. Our experiments confirmed that A. hydrophila infection can initially significantly increase serum cortisol levels at 1 h after infection. At this time point, the increased serum cortisol levels can significantly regulate A. hydrophila-regulated genes by affecting both transcriptome and chromatin accessibility. Cross-analysis of RNA-seq and ATAC-seq revealed that a certain gene group (92 target_DEGs) was regulated at an early time point by cortisol. KEGG enrichment analysis revealed that the top three pathways according to target_DEGs were cancer, glutathione metabolism, and the Notch signalling pathway. The protein-protein interaction analysis of target_DEGs revealed that they may be primarily involved in cell proliferation, CD8⁺ T cell function, glutathione synthesis, and activation of the NF-κB signalling pathway. This suggests that after the emergence of immune stress, the early regulation of cortisol is positive against the immune response. It is possible that in this situation, the animal is attempting to avoid dangerous situations and risks and then cope with the imbalance produced by the stressor to ultimately restore homeostasis. Our results will contribute to future research on fish and provide valuable insight regarding the mechanism of immune regulation by cortisol and the study of bacterial haemorrhagic disease in channel catfish.

Polystyrene microplastics reduce abundance of developing B cells in rainbow trout (Oncorhynchus mykiss) primary cultures

Environmental microplastic pollution (including polystyrene, PS) may have detrimental effects on the health of aquatic organisms. Accumulation of PS microplastics has been reported to affect innate immune cells and inflammatory responses in fish. To date, knowledge on effects of microplastics on the antibody response is still very limited. Here, we investigated effects of small (0.8-20 μm) PS microplastics on the abundance of B lineage cells in primary cultures of developing immune cells from the anterior kidney of rainbow trout. Both purchased PS microbeads and PS microparticles generated from consumer products were used as microplastic sources. We first show that rainbow trout phagocytic B cells efficiently took up small (0.83-3.1 μm) PS microbeads within hours of exposure. In addition, our data revealed that PS microplastic exposure most significantly decreased the abundance of a population of non-phagocytic developing B cells, using both flow cytometry and RT-qPCR. PS microplastics-induced loss of developing B cells further correlated with reduced gene expression of RAG1 and the membrane form of immunoglobulin heavy chains mu and tau. Based on the induced loss of developing B cells observed in our in vitro studies, we speculate that in vivo, chronic PS microplastic-exposure may lead to suboptimal IgM/IgT levels in response to pathogens in teleost species. Considering the highly conserved nature of vertebrate B lymphopoiesis it is likely that PS microplastics will similarly reduce antibody responses in higher vertebrate species, including humans. Further, RAG1 provides an effective biomarker to determine effects of PS microplastics on B cell development in teleost species.