In vitro opsonin-independent interactions between cells of smooth and rough phenotypes of Flavobacterium psychrophilum and rainbow trout (Oncorhynchus mykiss) head kidney macrophages

The cytotoxic activity of smooth and rough phenotypic cells of the fish pathogenic Gram-negative bacterium Flavobacterium psychrophilum to rainbow trout (Oncorhynchus mykiss) head kidney macrophages was investigated in vitro. The cytotoxicity to macrophages was significantly higher for rough cells compared with the smooth cells. The cytotoxic activity increased for both cell types with increasing temperature and the cells retained their cytotoxic nature after metabolical inactivation by heat, suggesting a cell-bound cytotoxic mechanism. The cytotoxicity was significantly reduced in both cell types after treatment with sodium (meta)periodate, indicating that the major bacterial structure involved in the cytotoxicity is of carbohydrate nature. Trypsin treatment further reduced the cytotoxicity in smooth cells, while sialic acid treatment reduced the cytotoxicity in rough cells, suggesting different lysing mechanisms for the two phenotypic variants. The results from the present study therefore suggest that the cytotoxic activity of F. psychrophilum to rainbow trout macrophages in vitro is stronger expressed in the rough phenotype and that it is opsonin-independent and initiated by binding of bacterial surface carbohydrates to lectins on the surface of the macrophages. How the lysis of the macrophages is executed is still unclear but it is suggested to function by different mechanisms in the smooth and the rough cells. The migration of rainbow trout macrophages toward smooth and rough cells of F. psychrophilum was further investigated. The results show that the macrophages were able to recognize both cell types, but the migration rate did not differ between the two phenotypes.