Expression and functional analysis of properdin in zebrafish Danio rerio

Properdin, an upregulator of the alternative complement pathway, has been thoroughly studied in the mammalian species, but its research in the lower vertebrates such as fish is rather limited. Additionally, information regarding the structure-activity relationship of properdin remains rather fragmentary. In this report, we showed that zebrafish properdin gene zfp was abundantly expressed in the liver of adult fish, while it was primarily expressed in the brain, neural plate, developing lens, and neutrophil in the early embryos/larvae. Recombinant TSR modules of zfP were demonstrated to be able to bind to C3b, LPS, LTA and both gram-negative and positive bacteria. Moreover, TSR5 of zfP was able to enhance the phagocytosis of microbes by macrophages. These results together support the notion that properdin is a pattern recognition molecule capable of identifying non-self antigens/structures, and indicate that TSR5 plays a central role in the capacity of properdin to promote phagocytosis. It is also suggested that properdin is associated with the pattern formation and immune defense of early developing embryos/larvae.