The role of opsonization by antibody and complement in in vitro phagocytosis of microsporidian parasites by turbot spleen cells

We investigated the role played by opsonization by antibody and complement in in vitro phagocytosis of microsporidian spores by turbot adherent phagocytes. Most turbot adherent cells displaying phagocytic activity are probably macrophages. Phagocytosis of yeast cells and polystyrene beads was greatly enhanced in the presence of both the Ig and the non-Ig (i.e. complement-containing) fractions of normal turbot serum, but phagocytosis of Glugea caulleryi or Tetramicra brevifilum spores was not affected by either fraction. Neither anti-G. caulleryi immune serum, nor anti-T. brevifilum immune serum (which cross-reacted considerably with G. caulleryi antigens), enhanced phagocytosis of G. caulleryi spores. Finally, spores treated with sodium m-periodate (to modify the structure of surface-borne sugars) were less effectively ingested than untreated spores, suggesting that phagocytosis of microsporidian spores involves recognition of such sugars by the phagocytic cell. The results of this study support the hypothesis that microsporidian parasites of fish in some way modulate the host phagocytic responses.