EFFECT OF TEMPERATURE ON MACROPHAGE ACTIVATION AND THE PRODUCTION OF MACROPHAGE ACTIVATING FACTOR BY RAINBOW-TROUT (ONCORHYNCHUS-MYKISS) LEUKOCYTES

Production of macrophage activating factor (MAF) by rainbow trout leuc ocytes has been shown to be temperature dependent in vivo and in vitro . Cells from fish held at 14 degrees C and stimulated to produce MAF i mmediately after isolation were capable of secreting MAF down to 6 deg rees C (the lowest temperature tested). However, after 48 h at 6 degre es C, these leucocytes show impaired MAF secretion. Acclimation of fis h to low temperatures (7 degrees C) did not recover the inhibitory eff ects of low in vitro temperatures on MAF production, but if these leuc ocytes were preincubated at 10 or 18 degrees C for 48 h, MAF was produ ced from these cells. Interestingly, macrophages isolated from fish ke pt at 7 or 14 degrees C and cultured at low temperatures (6 degrees C) were responsive to MAF-containing supernatants, and showed a higher r elative increase in respiratory burst activity compared with their cou nterparts cultured at 10 and 18 degrees C. Such observations clearly d emonstrate that a major impairment of bactericidal activity at low tem peratures resides within the specific immune compartment of fish. The implications for fish health are discussed.