The development of the gut microbiota in rainbow trout (<em>Oncorhynchus mykiss</em>) is affected by first feeding and diet type

An influence of the intestinal microbiota in connection to first-feeding of rainbow trout (Oncorhynchus mykiss) fry was demonstrated using Illumina HiSeq sequencing. The period from the end of yolk-sac feeding and until seven weeks post first-feeding was examined after administration of either a marine- or plant based diet with or without the probiont Pediococcus acidilactici. Before first feeding the main part of the sequence reads grouped to the genus Sediminibacterium probably originating from the surrounding water. The microbial abundance and diversity increased after first-feeding and the microbiota then changed towards Firmicutes phylum dominance for plant based fed fish and towards dominance of phylum Proteobacteria for the marine fed fish. After first-feeding, there were significantly higher abundances of Streptococcus,Leuconostoc and Weissella in fish fed the plant-based diet. The microbiota clustered separately according to the diet type, but only minor effects were seen from the probiont when using PCAanalysis. The constitutive transcription level of most examined immune genes increased during the ontogenic shift, but the results could not explain the differences in the composition of the microbiota dependent on diet treatment after first-feeding. The results suggest that the intestine of rainbow trout is colonised at an early state, but is guided in new and different directions dependent on the diet type

Early Immune Responses in Rainbow Trout Liver upon Viral Hemorrhagic Septicemia Virus (VHSV) Infection

<div><p>Among the essential metabolic functions of the liver, in mammals, a role as mediator of systemic and local innate immunity has also been reported. Although the presence of an important leukocyte population in mammalian liver is well documented, the characterization of leukocyte populations in the teleost liver has been only scarcely addressed. In the current work, we have confirmed the presence of IgM⁺, IgD⁺, IgT⁺, CD8α⁺, CD3⁺ cells, and cells expressing major histocompatibility complex (MHC-II) in rainbow trout (<i>Oncorhynchus mykiss</i>) liver by flow cytometry and/or immunohistochemistry analysis. Additionally, the effect of viral hemorrhagic septicemia virus (VHSV) on the liver immune response was assessed. First, we studied the effect of viral intraperitoneal injection on the transcription of a wide selection of immune genes at days 1, 2 and 5 post-infection. These included a group of leukocyte markers genes, pattern recognition receptors (PRRs), chemokines, chemokine receptor genes, and other genes involved in the early immune response and in acute phase reaction. Our results indicate that T lymphocytes play a key role in the initial response to VHSV in the liver, since CD3, CD8, CD4, perforin, Mx and interferon (IFN) transcription levels were up-regulated in response to VHSV. Consequently, flow cytometry analysis of CD8α⁺ cells in liver and spleen at day 5 post-infection revealed a decrease in the number of CD8α⁺ cells in the spleen and an increased population in the liver. No differences were found however in the percentages of B lymphocyte (IgM⁺ or IgD⁺) populations. In addition, a strong up-regulation in the transcription levels of several PRRs and chemokines was observed from the second day of infection, indicating an important role of these factors in the response of the liver to viral infections.</p></div

Transcriptional Heterogeneity of IgM⁺ Cells in Rainbow Trout (<i>Oncorhynchus mykiss</i>) Tissues

<div><p>Two major classes of B lymphocytes have been described to date in rainbow trout: IgM⁺ and IgT⁺ cells. IgM⁺ cells are mainly localized in the spleen, peripheral blood and kidney but are also found in other tissues. However, differences among IgM⁺ cell populations attending to its location are poorly defined in fish. Thus, the aim of this work was to characterize the expression of different immune molecules such as chemokine receptors, Toll-like receptors (TLRs) and transcription factors on sorted IgM⁺ lymphocytes from different rainbow trout tissues. IgM⁺ populations from blood, spleen, kidney, gills, intestine and liver were isolated by cell sorting and the constitutive levels of transcription of these genes evaluated by real-time PCR. To further characterize B cells, we identified an MS4A sequence. In humans, the MS4A family includes several genes with immune functions, such as the B cell marker CD20 or FcRβ. Subsequently, we have also evaluated the mRNA levels of this MS4A gene in the different IgM⁺ populations. The relevant differences in transcriptional patterns observed for each of these IgM⁺ populations analyzed, point to the presence of functionally different tissue-specific B cell populations in rainbow trout. The data shown provides a pattern of genes transcribed in IgM⁺ B cells not previously revealed in teleost fish. Furthermore, the constitutive expression of all the TLR genes analyzed in IgM⁺ cells suggests an important role for these cells in innate immunity.</p> </div