Immune-like response and autophagy modulation in gill epithelial cells of rainbow trout challenged with Piscirickettsia salmonis

Chile is the largest producer of the rainbow trout worldwide, where salmonid farming represents the second most important economic activity generating returns of over 5000 million dollars. However, during the productive cycle the fish is affected by a diverse range of pathogenic outbreaks, where the intracellular bacteria Piscirickettsia salmonis is the biggest threat of the industry. One of the main entryways for P. salmonis, are the gills. This tissue can deploy an immune response against pathogens using different mechanisms like infiltrated immune cells and mucosal immune molecules. Among the cellular mechanisms involved in the gill health and immune activity we can find the autophagy. In mammals, autophagy can participate in the clearance of intracellular pathogens and its activation can be mediated by pattern recognition receptors (PRRs). In fish, P. salmonis can trigger a PRRs response, however, the induction or modulation of the autophagy process in fish gill cells challenged against intracellular pathogens of commercial interests like P. salmonis are not described. In this context, the aim of the present work was to evaluate the modulation of autophagy and the immune-like response of RTgill-W1 cell line challenged with Piscirickettsia salmonis for 2, 4, 6, 8, 24, 48, 72 and 96 h. Results showed that P. salmonis triggered an immune response in RTgill-W1 cells through the up-regulation of pro-inflammatory cytokines (IL-1β, IL-8 and TNFα) and PRRs (TLRs and NLRs) gene expression. Moreover, genes involved in the autophagy were up-regulated after the challenge with the bacteria. Finally, Western blot and immunofluorescence microscopy showed that the protein content of autophagy molecular marker LC3-II/LC3-I was increased in the challenged cells. These results suggest that P. salmonis is able of triggering autophagy during a developing immune response through the increase in the expression of genes associated with this process, as well as through the increase in the protein content of LC3-II. Overall, the results suggest a tight relation between the epithelial immune response and autophagy process in rainbow trout. These findings are relevant, as they provide key insights for expanding the development of strategies targeting the control of intracellular pathogens in the salmon farming industry.