Non-specific immune response of Nile tilapia,Oreochromis nilotica,to the extracellular products of Mycobacterium spp. and to various adjuvants

Nile tilapia were immunized by injecting extracellular products (ECP) of Mycobacterium spp. (strain TB40, TB267 or the type strain Mycobacterium marinum) into their swim bladders. A variety of adjuvants – Freund's complete adjuvant (FCA), Freund's incomplete adjuvant (FIA) and Titremax – were similarly injected into additional groups of tilapia. Phosphate-buffered saline (PBS) was used as a control. The number of nitroblue tetrazolium (NBT)-positive cells observed in the swim bladder of the immunized fish had significantly increased by the fourth day post-immunization. By day 8, the number of NBT-positive cells in fish immunized with ECP from mycobacteria strains TB40 or TB267 were fewer than in fish immunized with ECP from M. marinum or fish injected with FCA or FIA. The level of lysozyme activity detected in the serum of fish 4 days after being immunized with ECP from various Mycobacterium spp. was also significantly higher than that found in the serum of the control fish. Head kidney macrophages showed an enhanced reduction of NBT when cultured in vitro with 1 μg ml^–1 of ECP. Concentrations greater than this (10 or 100 μg ml^–1) were found to suppress the reduction of NBT by the macrophages.