Vitellogenin mediates phagocytosis through interaction with FcγR

Vitellogenin (Vg), once reported to be a female-specific protein, has been identified in both male and juvenile fishes. However, the biological significance of the production of Vg in the male and juvenile fishes is elusive. Our previous studies showed that Vg is an opsonin capable of enhancing phagocytosis, but the mechanism by which Vg mediates phagocytosis is unknown. In this study we demonstrated that Vg-opsonized phagocytosis was characterized by pseudopod extension and depended upon tyrosine kinase. In contrast, inhibition of Rho family proteins and microtubule depolymerization had little effects on Vg-opsonized phagocytosis. Besides, Vg-opsonized phagocytosis was substantially blocked by monoclonal antibodies against FcγRs but not by CR3 antibody. Moreover, theoretical prediction analysis further revealed that Vg had the potency to interact with Fcγ receptors. Finally, the expression of proinflammatory cytokine genes tnf-α and il-1β was significantly up-regulated by Vg, and this up-regulation was inhibited by selective inhibitors of FcR signaling pathways, wortmannin and piceatannol. Taken together, these results suggest that Vg plays an IgG-like role in that it activates FcγR-mediated phagocytosis, thus establishing an antibody-like function for Vg for the first time.