Excretory/secretory (ES) products are molecules including various proteins produced by parasitic nematodes including larval A. simplex which is occurring in numerous marine fish hosts. The function of these substances and their effect on host physiology has not been fully described. The present work elucidates the effect of ES substances on the fish immune system by measuring immune gene expression in spleen and liver of rainbow trout (Oncorhynchus mykiss) injected
intraperitoneally with ES products isolated from A. simplex third stage larvae. The overall gene expression profile of exposed fish showed a generalized down-regulation of the immune genes tested, suggesting a role of ES proteins in minimizing the immune reaction of rainbow trout against invading nematodes. We also tested the enzymatic activity of the ES proteins and found that lipase,
esterase lipase, valine and cysteine arylamidases, naphthol-AS-BI-phosphohydrolase and a-galactosidase activities were present in the ES solution. This type of hydrolytic enzyme activity may play a role in nematode penetration of host tissue. Based on the notion that A. simplex ES-proteins may have an immune-depressive effect, it could also be suggested that worm enzymes directly target host immune effector molecules which would add to the decreased host immune responses and increased worm survival.