Host immune responses elicited by invading pathogens depend on recognition of the pathogen by
specific receptors present on phagocytic cells. However, the response to viral, bacterial, parasitic
and fungal pathogens vary according to the pathogen-associated molecular patterns (PAMPs) on the
surface of the invader. Phagocytic cells are known to initiate a respiratory burst following an
exposure to the pathogen, but the underlying and associated specific elements are poorly elucidated
in fish. The present study describes the differential response of head kidney leukocytes from
rainbow trout (Oncorhynchus mykiss) to different PAMPs mimicking bacterial (flagellin and LPS),
viral (poly I:C) and fungal infections (zymosan and ß-glucan). Transcript of cytokines related to
inflammation (IL-1ß, IL-6, IL-10 and TNF-a) were highly up-regulated following LPS exposure
whereas flagellin or poly I:C induced merely moderate reactions. In contrast, IFN-¿ expression was
significantly higher in the poly I:C stimulated group compared to LPS group. When head kidney
cells were exposed to zymosan or ß-glucan, genes encoding IL-1ß, TNF-a, IL-6 and IL-10 became
up-regulated. Their level of up-regulation was comparable to LPS but the kinetics differed. In
particular, TNF-a induction was considerably slower when stimulated with zymosan or ß-glucan.
The gene encoding COX-2 enzyme, which is a central element in initiation of inflammatory
reactions, was significantly higher in stimulated cells but a depressing effect of high concentrations
of LPS and zymosan became evident after 4 h exposure. This study suggests that rainbow trout
leukocytes respond differently to viral, bacterial and fungal PAMPs, which may reflect activation of
specific signaling cascades eventually leading to activation of different immune effector molecules.