Abstract
Lactococcus garvieae is a significant pathogen in aquaculture with a potential zoonotic risk. To begin to characterize the late immune response of trout to lactococcosis, we selected infected individuals showing clinical signs of lactococcosis. At the time lactococcosis clinical signs appeared, infection by L. garvieae induced a robust inflammatory response in the spleen of rainbow trout, which correlated with abundant granulomatous lesions. The response in kidney goes in parallel with that of spleen, and most of the gene regulations are similar in both organs. A correlation existed between the early inflammatory granulomas in spleen (containing macrophages with internalized L. garvieae) and up-regulated gene sets, which defined the presence of macrophages and neutrophils. This is the first analysis of the immune transcriptome of rainbow trout following L. garvieae infection during the initiation of adaptive immune mechanisms and shows a transcriptome induction of antibody response by both IgM (+) and IgT (+) spleen B cells to respond to systemic infection. These results increase our understanding of lactococcosis and pave the way for future research to improve control measures of lactococcosis on fish farms.
Highlights
Fish lactococcosis is a haemorrhagic septicaemia caused by Lactococcus garvieae
Overview of spleen and head kidney immune‐related transcripts from L. garvieae infected trout To increase the detection of immune-related transcripts expressed at low concentrations after L. garvieae infection, an immune-targeted microarray designed with selected genes from GenBank mRNA published sequences was used
In this work, we have performed a transcriptome analysis to study the immune responses in the spleen and head kidney of trout with clinical signs of lactococcosis 72 h after L. garvieae inoculation, to analyze a fully developed response
Summary
Fish lactococcosis is a haemorrhagic septicaemia caused by Lactococcus garvieae. This pathogen has been isolated worldwide from numerous cultured and wild fish species, but lactococcosis is prevalent and economically relevant in farmed rainbow trout (Oncorhynchus mykiss), and usually occurs when water temperatures rise to 18 °C [1, 2]. Analysis of fish immune responses to pathogenic bacterial infections has benefited from the application of transcriptome profiling technologies. For L. garvieae, the early transcriptome response of immune organs following a lactococcosis-induced infection in grey mullet (Mugil cephalus) has been recently reported [11]. Similar transcriptome studies analysing the immune response to L. garvieae infection in rainbow trout are missing, despite that this fish species is the most affected by L. garvieae infections [1, 2]
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