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, Oncorhynchus mykiss (Walbaum) , head kidney leucocytes was examined after culturing in vitro with extracellular products (ECP) collected from Mycobacterium sp. Cultures of leucocytes were prepared from naive fish, or fish previously vaccinated with either the ECP or with formalin killed whole cell preparations (WC) of the bacterium. The cells were then incubated with the ECP in vitro and the ability of their supernatants to activate macrophages assessed. Macrophages from control fish were incubated with the supernatants, and their ability to reduce nitroblue tetrazolium (NBT) measured as an indicator of macrophage activation. Incubation of head kidney macrophages from naive fish directly with 1, 10 or 100 μg mL –1 of ECP for 48 h significantly enhanced macrophage activation compared with control macrophages. Vaccination of fish with either ECP or WC had no significant effect on the respiratory burst of control macrophages 4 weeks post‐vaccination. By the eighth week, however, absorbance levels of respiratory burst reflecting both the primary (cells from vaccinated fish cultured in vitro with PBS) and the secondary (cells cultured in vitro with ECP) MAF responses of fish vaccinated with ECP and WC, had peaked and these were significantly different from the non‐vaccinated controls. This activity had fallen to levels similar to control fish by week 12 for fish vaccinated with WC.