Identification of sea bass pIgR shows its interaction with vitellogenin inducing antibody-like activities in HEK 293T cells

Fish vitellogenin (Vg) has been shown to mediate the phagocytosis via interaction with a Fcγ-like phagocytic receptor on macrophages, but identification of such a receptor and its functional characterization remains lacking. In this study, we isolated a cDNA of polymeric immunoglobulin receptor (pIgR) from sea bass, which encoded a single-spanning transmembrane protein of 326 amino acids including a 21-amino acid signal peptide, an extracellular region, a transmembrane region and a 36-amino acid intracellular region included two Ig-like domains (ILDs), and was expressed in multiple lymphoid organs. We then showed that recombinant extracellular domain of sea bass pIgR was capable of binding to Vg as well as IgG and IgM. We also showed that Vg as well as IgG and IgM interacted with pIgR-expressing HEK 293T cells. Importantly, we demonstrated that Vg as well as IgG and IgM were all capable of enhancing phagocytosis by HEK 293T cells and inducing expression of tnf-α and il-1β, via interacting with pIgR. Collectively, these results suggest that fish Vg, analogous to IgG and IgM, can interact with pIgR and result in similar down-stream immune responses, providing an additional evidence that Vg plays an antibody-like role.