Cellular immune response of gilthead sea bream (Sparus aurata L.) to Enteromyxum leei (Myxozoa)

Abstract

Enteromyxosis in gilthead sea bream (GSB) (Sparus aurata) consists of severe chronic catharral enteritis causing a cachectic syndrome and death. The ethiologic agent is the intestinal parasite Enteromyxum leei, which penetrates and proliferates in the paracellular space between enterocytes. The parasite disrupts the epithelial organization and provokes epithelial desquamation triggering an intense inflammatory response locally. By means of light microscopy and immunohistochemistry, the distribution pattern of some leukocytic populations was studied at intestinal and haematopoietic levels in GSB anally intubated with E. leei (R, recipient). Tissue sections of Bouin fixed and paraffin embedded portions of anterior intestine (Ai), posterior intestine (Pi), head kidney (Hk), spleen (Sp) and thymus (Th) were taken at 15 and 40 days post inoculum (d.p.i.). For immunohistochemistry, the G7 monoclonal antibody (Mab) against GSB acidophilic granulocytes (AGs), a polyclonal antibody (Pab) against histamine, which is stored in mast cell (MC) granules, and a Pab against GSB IgM labeling plasma cells and B cells (PCs/BCs) were applied. In splenic sections, melanomacrophage centres (MMCs) and their sizes were quantified. PCs/BCs and MCs increased significantly, whereas AGs decreased in the inflammatory infiltrates of parasitized (PAR) intestinal sections, compared to intestines of non-parasitized (NON-PAR) and unexposed (CTRL) fish. These differences were stronger at the Pi section, the main target of the parasite, and at 40 d.p.i. In Hk and Sp of R fish at 40 d.p.i., PCs/BCs and MCs also increased, whereas AGs decreased. No differences were found in the Th. In NON-PAR GSB (vs. PAR and CTRL groups), the number of MMCs and the percentage of splenic surface occupied by MMCs increased, though only significantly for the latter. To conclude, during enteromyxosis in GSB, PCs/BCs, MCs and MMCs seem to proliferate in haematopoietic tissues and PCs/BCs and MCs are recruited to the site of infection, while AGs show an overall depletion.