LA RISPOSTA DEL SISTEMA IMMUNITARIO DEI PESCI ALLE INFESTAZIONI PARASSITARIE: IL RUOLO DELLE MAST CELLS NELLA DIFESA DELL’OSPITE

The ability to avoid infection depends on animal mechanisms of innate immunity. The innate immune system is responsible for the primary response and one type of granular cells, mast cells (MCs), has been reported to play a critical role as part of the defence function against pathogens and evidence for its involvement in the immune system of fish is growing. Antimicrobial peptides (AMPs) are widely distributed in nature and are considered to be ancestral components in the evolution on innate immunity. Several types of AMPs have been isolated from fish and they are present in mucosal surfaces and in immune cells such as MCs. One type of AMPs expressed by fish MCs is piscidins, which are 22-44 residue, α-helical AMPs that were originally isolated from MCs of hybrid striped bass. In this study on gills of 45 Dicentrarchus labrax, the MCs positive to piscidin 3 and 4 synthesized greater amounts of these peptides in the presence of metazoans, irrespective of the number of parasites or of the proximity to them. In the 46 specimens of Sparus aurata heavily parasitized the number of positive cells to piscidin 3 was higher than both those lightly parasitized and healthy ones in all the investigated tissue districts (central venous sinus, capillaries of secondary lamellae and extravascular tissue), irrespective of the vicinity of the parasite. In the extravascular tissue, furthermore, the intensity of the immunostaining was stronger in heavily parasitized fish compared to both lightly parasitized and healthy. The study on 24 Perca fluviatilis showed that the intensity of immunostaining for both piscidin 3 and 4 was stronger in healthy fish than those parasitized and that there were no differences in abundance of MCs expressing piscidins in the presence or absence of helminths. Our report documents the response of gills of D. Labrax and S. Aurata, and intestine of P. Fluviatilis and provides further evidence that MCs and their AMPs may play a role in response to branchial ectoparasite and intestine endoparasite infection.