Modulation of the in vitro activity of European sea bass (Dicentrarchus labrax L.) phagocytes by the myxosporean parasite Sphaerospora dicentrarchi (Myxosporea: Bivalvulida).

Several in vitro studies were performed to study the cellular reaction of European sea bass (Dicentrarchus labrax L.) against Sphaerospora dicentrarchi. Head kidney phagocytes were obtained from parasitised (P) and non-parasitised (NP) fish. The production of superoxide anion (O2-), tested by the NBT method, was higher in P than in NP fish. The addition of increasing amounts of sea bass serum (SBS) produced a gradual increment of the respiratory burst with SBS from parasitised animals (P-SBS), whereas this increment reached a plateau at lower concentration with SBS from nonparasitised ones (NP-SBS). O2- production was higher when adding NP-SBS than with P-SBS or fetal bovine serum. Heat inactivation of NP-SBS and FBS reduced the respiratory burst significantly, whereas it did not change the effect of P-SBS. The number of NBT-positive cells after particulate stimulation was significantly higher using S. dicentrarchi spores than using SRBC, but lower than with phorbol myristate acetate. Phagocytes primed overnight with spore extracts produced higher amounts of O2- than those LPS-primed or non-primed ones. A similar percentage of phagocytosis was detected using glutaraldehyde fixed spores and SRBC. Most of the phagocytes engulfed three or more SRBC, whereas most of the phagocytes engulfed only one spore. Complement mediated opsonisation by NP-SBS may occur, as the phagocytic index was reduced when the serum was heat inactivated.