The effects of novel immunostimulants on immunity and disease resistance in the rainbow trout (Oncorhynchus mykiss)
Recently, substantial attention has focused on the role of immunostimulants as prophylactic disease control agents in aquaculture. The purpose of this study was to investigate the effects of a range of putative immunostimulants on immune parameters and disease resisting capabilities in the rainbow trout, Oncorhynchus mykiss. Two plant derived immunostimulants, Ergosan and Chevimmun, were investigated of the modulatory effects on localised and systemic innate immune responses following intraperitoneal administration. With respect to localised effects, injection of ³1mg ml-1 Ergosan or 75% Chevimmun resulted in enhanced leucocyte migration, phagocytosis and Nitroblue Tetrazolium (NBT) reduction. In addition, Ergosan stimulated cytokine (IL-1b, IL-8 and TNF-a(2)) gene expression in peritoneal leucocytes. Of the humoral parameters studied, complement activity was augment following 1mg ml-1 Ergosan administration, whilst 10mg ml -1 stimulated antiprotease activity. Interestingly, Ergosan, but not Chevimmun, enhanced the resistance of trout to Aeromonas salmonicida and Viral Haemorrhagic Septicaemia Virus (VHSV). A third immunostimulant, recombinant trout IL-1b (rIL-1b) enhanced leucocyte migration, phagocytosis and bactericidal activity in vitro at concentrations ³10ng ml-1. In vivo studies demonstrated that intraperitoneally administered rIL-1b (typically 1mg/fish) could attract peritoneal leucocytes, particularly neutrophils, as well as enhancing the phagocytic activity of elicited cells. Finally, the two isoforms of trout rTNF-a, referred to as rTNF-a(1) and rTNF-a(2), were shown to function as chemoattractants in vivo, in addition to modulating the phagocytic response of head kidney leucocytes. The second isoform was the more potent inducer of these two important parameters. Conversely, rTNF-a(1), but not rTNF-a(2), increased bactericidal activity of trout macrophages exposed to an avirulent strain of A. salmonicida.