Transferrin and the innate immune response of fish: identification of a novel mechanism of macrophage activation.

We have previously demonstrated that a non-cytokine serum protein called transferrin was a primary activating molecule of the goldfish (Carassius auratus) macrophage antimicrobial response. The ability of the enzymatically cleaved forms of this protein to modulate fish macrophage function is novel and may represent a primitive and evolutionary conserved mechanism for the induction of NO response of macrophages. In the present study we confirm our earlier findings using immunoaffinity purified goldfish transferrin from mitogen-stimulated leukocyte supernatants. In addition we demonstrate that: (1). products released by necrotic/damaged cells contain transferrin-cleaving activity; (2). the cleavage site is located within the bridge peptide connecting the two lobes of the transferrin molecule; (3). transferrin is expressed by activated goldfish macrophages but not mitogen-stimulated kidney leukocytes; and (4). addition of transferrin significantly enhanced the killing response of goldfish macrophages exposed to different pathogens or pathogen products (e.g. lipopolysaccharide, Mycobacterium chelonei, Trypanosoma danilewskyi, Aeromonas salmonicida, and Leishmania major). We propose a model of fish macrophage activation that is mediated by a non-cytokine host protein (i.e. transferrin) in combination with highly conserved innate immunity recognition receptors that are almost certain to exist in teleost.