Purinergic signaling creates an anti-inflammatory profile in spleens of grass carp Ctenopharyngodon idella naturally infected by Saprolegnia parasitica: An attempt to prevent ATP pro-inflammatory effects.

Extracellular adenosine triphosphate (ATP) is as key mediator of immune and inflammatory responses. ATP is normally sequestered in the intracellular milieu and released by apoptotic and necrotic cells, where it acts as a pro-inflammatory mediator in the extracellular milieu. A limited number of studies have explored the involvement of purinergic signaling in oomycete infections, including Saprolegnia parasitica; this is a most destructive oomycete pathogen, associated with high mortality and severe economic losses for fish producers. The aim of this study was to determine whether purinergic signaling exerts anti- or pro-inflammatory effects in spleens of grass carp (Ctenopharyngodon idella) naturally infected by S. parasitica. Animals naturally infected with S. parasitica showed typical gross lesions characterized by cotton-wool tufts on the tail and fins, as well as severe histopathological lesions such as necrosis. Spleen ATP and metabolites of nitric oxide (NOx) levels were higher in fish naturally infected by S. parasitica compared to control on day 7 post-infection (PI). Spleen nucleoside triphosphate diphosphohydrolase (NTPDase) activity (ATP as substrate) was greater in fish naturally infected by S. parasitica than in uninfected on day 7 PI, while no significant differences were observed between groups with respect to NTPDase (adenosine diphosphate as substrate) and 5′-nucleotidase activities. Finally, adenosine deaminase (ADA) activity was lower in fish naturally infected by S. parasitica than in uninfected fish on day 7 PI. In summary, spleen tissue necrosis in the context of saprolegniosis provokes an intense release of ATP into the extracellular milieu, where it interacts with the P2X7 purine receptor and leads to a self-sustained pro-inflammatory deleterious cycle, contributing to an intense inflammatory process. In response to excessive ATP levels in the extracellular milieu, ATP and adenosine hydrolysis were modulated in an attempt to restrict the inflammatory process via upregulation of NTPDase and downregulation of ADA activities. We conclude that the purinergic signaling pathway modulates immune and inflammatory responses during natural infection with S. parasitica.