Extrahepatic synthesis of complement components in the rainbow trout (Oncorhynchus mykiss)

The components of activated complement display a wide range of functions in inflammatory reactions, including chemotaxis, opsonisation and the destruction of microorganisms. In fish, the complement system is believed to be more complex than in mammals. Teleost fish possess several complement components encoded by multiple genes giving rise to complement subtypes, such as C3-1, C3-3 and C3-4. Complement synthesis is mainly localised in the liver and little attention has been paid to studying production at extrahepatic sites in fish. In mammalian species, the monocyte-macrophage lineage is known to be an important contributor to extrahepatic C3 levels. In vitro studies of piscine macrophages have, however, revealed absence of basal transcription of C3. The objective of this study was to quantify and localise the C3 and C7 subtypes, C4, C5 and factor B in rainbow trout using real-time RT-PCR and in situ hybridisation. Widespread extrahepatic synthesis of all complement components studied was detected, though with high degrees of individual variation between fish. Compared to the levels of complement mRNA in the liver, extrahepatic synthesis was low, but indeed high enough to imply biological significance. This study represents the first comprehensive mapping of the distribution and levels of complement components in any vertebrate species.