The opsonising activity of a pentraxin-like protein isolated from snapper (Pagrus auratus, Sparidae) serum

In mammals the pentraxins, C-reactive protein (CRP) and serum amyloid P component (SAP) are important components of the immune response. Although pentraxins have been isolated from a number of fish species few studies detail their functional immunological role. In this paper we report the establishment of a flow cytometry based assay to measure the phagocytic activity of isolated snapper head kidney leukocytes (HKLs). This assay was then used to examine the ability of a pentraxin-like protein isolated from the serum of snapper (P. auratus) (Sn-PLP) to act as an opsonin. Incubation of snapper head kidney leukocytes (HKL) with FITC-labelled beads resulted in uptake of these particles by approximately 35% of HKLs. Incubation of beads with Sn-PLP enhanced phagocytosis by snapper HKLs in a dose-dependant manner. Enhanced phagocytosis could be inhibited by addition of a rabbit anti-Sn-PLP antibody suggesting that Sn-PLP may act as a ligand for a HKL cell surface receptor. This study provides further evidence toward a functional role for pentraxins in the host defence repertoire of fish.