The production of a lymphokine (macrophage activating factor) by rainbow trout, Oncorhynchus mykiss (Walbaum), leucocytes stimulated with the extracellular products of Mycobacterium sp.

The production of macrophage activation factor (MAF) by rainbow trout, Onco rhynchus mykiss (Walbaum), head kidney leucocytes was examined after cultur ing in vitro with extracellular products (ECP) collected from Mycobacterium sp. Cultures of leucocytes were prepared from naive fish, or fish previous ly vaccinated with either the ECP or with formalin killed whole cell prepar ations (WC) of the bacterium. The cells were then incubated with the ECP in vitro and the ability of their supernatants to activate macrophages assess ed. Macrophages from control fish were incubated with the supernatants, and their ability to reduce nitroblue tetrazolium (NBT); measured as an indica tor of macrophage activation. Incubation of head kidney macrophages from;na ive fish directly with 1, 10 or 100 mug mL-(1) of ECP for 48 h significantl y enhanced macrophage activation compared with control macrophages. Vaccina tion of fish with either ECP or WC had no significant effect on the respira tory burst of control macrophages 4 weeks post-vaccination. By the eighth w eek, however, absorbance levels of respiratory burst reflecting both the pr imary (cells from vaccinated fish cultured in vitro with PBS) and the secon dary (cells cultured in vitro with ECP) MAF responses of fish vaccinated wi th ECP and MC, had peaked and these were significantly different from the n on-vaccinated controls. This activity had fallen to levels similar to contr ol fish by week 12 for fish vaccinated with WC.