Immunocytochemical localization of pleurocidin to the cytoplasmic granules of eosinophilic granular cells from the winter flounder gill

The teleost gill is considered to be of significant immunological importance, as it is one of the first tissues exposed to environmental or pathogenic challenge and thus should be well equipped to mount an effective immune response. This study characterizes ultrastructurally and immunocytochemically a tissue granulocyte (eosinophilic granular cell) from the winter flounder gill that was previously determined to be involved in the gene expression and synthesis of a known antimicrobial peptide (pleurocidin). The cell is irregular in shape with a cytoplasm characterized by numerous large, electron‐dense, membrane‐bounded granules. The nucleus is euchromatic and closely associated with a prominent rough endoplasmic reticulum. The cytoplasm typically contains two to three mitochondria and a centralized Golgi apparatus surrounded by numerous electron‐lucent vesicles. Immunogold staining of the cells with an anti‐pleurocidin antibody shows large number of gold particles in direct association with the electron‐dense granules. These data provide the first evidence definitively showing storage of an antimicrobial peptide in the cytoplasmic granules of an eosinophilic granule cell resident in gill tissue.