Non-lysosomal Activation in Macrophages of Atlantic Salmon (Salmo salar) After Infection With Piscirickettsia salmonis

Abstract

Piscirickettsia salmonis is a facultative intracellular pathogen and etiological agent of the systemic disease salmonid rickettsial septicemia. It has been suggested that P salmonis is able to survive in host macrophages, localized within a vacuole like-compartment which prevents lysosomal degradation. However, the relevant aspects of the pathogenesis of P salmonis as the host modulation that allow its intracellular survival have been poorly characterized. In this study, we evaluated the role of lysosomes in the response to P salmonis infection in macrophage-enriched cell cultures established from Atlantic salmon head kidneys. Bacterial infection was confirmed using confocal microscopy. A gentamicin protection assay was performed to recover intracellular bacteria and the 16S rDNA copy number was quantified through quantitative polymerase chain reaction in order to determine the replication of P salmonis within macrophages. Lysosomal activity in Atlantic salmon macrophage-enriched cell cultures infected with P salmonis was evaluated by analyzing the lysosomal pH and proteolytic ability through confocal microscopy. The results showed that P salmonis can survive >= 120 h in Atlantic salmon macrophage-enriched cell cultures, accompanied by an increase in the detection of the 16S rDNA copy number/cell. The latter finding suggests that P salmonis also replicates in Atlantic salmon macrophage-enriched cell cultures. Moreover, this bacterial survival and replication appears to be favored by a perturbation of the lysosomal degradation system. We observed a modulation in the total number of lysosomes and lysosomal acidification following infection with P salmonis. Collectively, the results of this study showed that infection of Atlantic salmon macrophages with P salmonis induced limited lysosomal response which may be associated with host immune evasion mechanisms of P salmonis that have not been previously reported.