IL-4/13A1 promotes the proliferation of IgT⁺ B cells while enhancing their apoptosis in tilapia

Interleukin-4/13 (IL-4/13) is a kind of bony fish cytokine, which is the homolog of Th2 cytokines IL-4 or IL-13 in mammals. Produced by activated T cells, these Th2 cytokines regulate B cell proliferation, survival, and antibody responses. Although IL-4/13 has been shown to participate in immune responses in bony fish, whether and how it regulates the teleost-specific IgT⁺ B cell subset, a lineage unique to bony fish remains unclear. In present study, we identified two conserved IL-4/13 from the Nile tilapia Oreochromis niloticus (OnIL-4/13A1 and OnIL-4/13A2), and suggested their wide expression in immunological tissues. OnIL-4/13A1 transcription was markedly upregulated in spleen and liver leukocytes upon bacterial infection, and can be dramatically induced by PHA- or CD3 mAb-induced T cell activation, suggesting activated T cells are a key cellular source of this cytokine. More importantly, we found that 2- or 4-days' in vivo administration of the recombinant OnIL-4/13A1 (rOnIL-4/13A1) caused an obvious expansion of IgT⁺ but not IgM⁺ B cells in tilapia. Further investigation demonstrated that this expansion was due to cellular proliferation, because the BrdU incorporation in IgT⁺ B cells was obviously increased, while the Erk1/2 and NF-κB phosphorylation that contributing to the proliferation was also enhanced in IgT⁺ B cells. Interestingly, as revealed by the Annexin V staining, we found that rOnIL-4/13A1 treatment elevated the apoptosis of IgT⁺ B cells, probably by initiating the Caspase-8/Caspase-3 pathway. Therefore, our results suggest that IL-4/13A1 involves in the anti-bacterial immune response of tilapia by promoting IgT⁺ B cell proliferation and maintaining IgT⁺ B cell hemostasis, thus shed novel light on the B cells response in bony fish.