Cellular requirements for lymphokine secretion by rainbow trout Salmo gairdneri leucocytes

The ability of different populations of rainbow trout blood leucocytes to produce MAF following stimulation with Con A/PMA was assessed by the amount of NBT reduction in target macrophages. The effect of varying lymphocyte or macrophage number on MAF production in the presence of a constant number of macrophages or lymphocytes respectively, showed that in both cases MAF activity initially increased with increasing cell number and then plateaued. Macrophages alone did not produce MAF whereas some MAF activity was produced by macrophage-depleted lymphocytes, although significantly lower than in the presence of macrophages. Separation of leucocytes into sIg- and sIg+ cells by panning showed that only sIg- lymphocytes could produce MAF and that macrophages were necessary as accessory cells. These results support the contention that fish lymphocytes can be divided into sIg- T cells and sIg+ B cells.