Essential role of 4E-BP1 for lymphocyte activation and proliferation in the adaptive immune response of Nile tilapia

Eukaryotic translation initiation factor 4E-binding protein 1 (4E-BP1) is a translation repressor downstream of mTORC1 pathway, and plays a pivotal role in the adaptive immune response. However, whether 4E-BP1 participates in the regulation of adaptive immunity of early vertebrates is still unclear. In present study, using Nile tilapia Oreochromis niloticus as a model, we investigated the regulatory roles of 4E-BP1 (On-4E-BP1) on lymphocyte-mediated adaptive immune response of fish species. On-4E-BP1 is highly conserved compared with the homologues from other vertebrates, and it is widely distributed in the immune-related tissues of Nile tilapia with the highest expression level in the gill. Once the animals were infected by Aeromonas hydrophila, transcription level of On-4E-BP1 in spleen lymphocytes was significantly induced on day 5 after infection. Meanwhile, phosphorylation of On-4E-BP1 in lymphocytes was also enhanced during the primary adaptive immune response, suggesting that 4E-BP1 is involved in the adaptive immunity of Nile tilapia. Furthermore, when lymphocytes were activated by agonist PMA or T cell specific mitogen PHA in vitro, phosphorylation of On-4E-BP1 was obviously up-regulated. More importantly, once mTORC1/4E-BP1 activity was blocked by specific inhibitor, the inducible expression of T cell activation markers IFN-γ and CD122 was severely impaired during PHA-induced T cell activation, suggesting this signaling regulates T lymphocyte activation of Nile tilapia. In addition, blockade of mTORC1/4E-BP1 axis also resulted in a crippled proliferation of lymphocyte during the primary response of anti-bacterial adaptive immunity. Collectively, we found that mTORC1/4E-BP1 signaling participates in the adaptive immune response by regulating lymphocyte activation and proliferation in Nile tilapia. This study enriches our current knowledge on teleost adaptive immunity, and provides novel perspective to understand the evolution of adaptive immune system.