Response of the rainbow trout monocyte/macrophage cell line, RTS11 to the water molds Achlya and Saprolegnia.

The Saprolegniales are responsible for various fish mycoses worldwide and considered the most important fungi afflicting fresh water fish. Saprolegniosis leads to massive epidermal destruction and macrophage recruitment, yet little is known regarding the cytological response of their piscine hosts. The objective of this study was to explore the response of fish macrophage to members of the Saprolegniales using the rainbow trout monocyte/macrophage cell line, RTS11. After 48 h in co-culture, RTS11 demonstrated chemotaxis, adherence and homotypic aggregation to both live and heat-killed fungal spores and mycelia. This aggregation was enhanced when using conditioned media from co-cultured RTS11 and Achlya, suggesting the presence of synergistic effectors of aggregation. Although fungal toxins were not evident, as cells remained viable throughout fungal overgrowth, phagocytosis was inhibited due to large fungal spore size, allowing these molds to evade macrophage defenses. Although class I MH and other viral response genes showed no significant change in expression, calreticulin and interleukin-8 were moderately up-regulated implicating calcium modulation and chemotactic response, respectively. Cyclooxygenase (COX-2) and the cytokines IL-1beta and TNFalpha were strongly up-regulated in the presence of Achlya, while gene expression of the class II major histocompatibility (MH II) receptor and associated molecules appeared down-regulated, suggesting fungal interference of immune function. Previous studies have shown an increased dependence of macrophage in immune function at low temperatures; based upon data presented here, this reduction of macrophage MH II receptor expression and inability to phagocytose spores may limit host response thereby providing increased susceptibility to these opportunistic pathogens.