Acidophilic granulocytes in the gills of gilthead seabream Sparus aurata: evidence of their responses to a natural infection by a copepod ectoparasite

Immunohistochemical and ultrastructural studieswere conducted on the gills of gilthead seabream, Sparusaurata L., naturally infected with the copepod ectoparasiteErgasilus lizae (Krøyer, 1863) in order to assess pathologyand the host immune cell response. Gills of 56 giltheadseabream were screened for ectoparasites; 36 specimens(64.3%) harbored E. lizae. Intensity of infection was 32.7±8.7 (mean ± SE). Pathological alterations to the gills of thehost were more pronounced in close proximity to the copepodsite of attachment. The parasite attached to the gills bymeans of its modified second antennae, occluded the arteries,provoked epithelial hyperplasia and hemorrhages andmost often caused lamellar disruption. Numerous granularcells were encountered near the site of E. lizae attachment.In both infected and uninfected gills, the granular cells laywithin the filaments and frequently occurred within theconnective tissue inside and outside the blood vessels ofthe filaments. The type of granular cell was identified byimmunohistochemical staining by using the monoclonalantibody G7 (mAb G7), which specifically recognizesacidophilic granulocytes (AGs) of S. aurata and with ananti-histamine antibody (as a marker for mast cells,MCs) on sections from 13 uninfected gills and 21parasitized gills. The use of mAb G7 revealed that, ingills harboring copepods, the number of G7-positivecells (i.e., AGs; 32.9±3.9, mean number of cells per45,000 μm2 ± SE) was significantly higher than thedensity of the same cells in uninfected gills (15.3±3.8;ANOVA, P<0.05). Few histamine-positive granular cells(i.e., MCs) were found in the uninfected and parasitizedgills. Here, we show, for the first time in S. auratainfected gills, that AGs rather than MCs are recruitedand involved in the response to E. lizae infection inseabream.[...]