Upregulation of Toll-like Receptor (TLR) Expression and Release of Cytokines from Mast Cells by IL-12

Background: It has been reported that peptidoglycan (PGN) and lipopolysaccharide (LPS) can provoke mast cells to release an array of cytokines via TLR2 and TLR4, respectively. However, little is known of the regulatory mechanism of TLR2 and TLR4 mediated cytokine production in mast cells. Methods: Since IL-12 plays important roles in protection of the body from microorganism infection and mast cell is a crucial source of IL-12, we investigated effects of IL-12 on expression of TLR2 and TLR4, and cytokine production in mast cells by using quantitative real time PCR, flow cytometry analysis and cellular activation of signaling ELISA techniques. Results: The results showed that IL-12 induced significant increase in expression of TLR2 and TLR4 mRNAs and proteins, respectively. It can also synergistically enhance LPS-induced TLR4 expression in P815 cells. IL-12 not only by itself, but also synergistically enhanced LPS-induced IL-13 release from P815 cells. It appears that IL-12 induced IL-13 release and TLR4 expression is through activation of MAPK and PI3K/Akt signaling pathways, whereas IL-12 induced upregulation of TLR2 is via activation of PI3K/Akt signaling pathway, but not MAPK pathway. Conclusion: The ability of IL-12 in modulation of expression of TLR2 and TLR4 in mast cells, and in stimulation of IL-13 release from mast cells provides further evidence that this cytokine may play a role in the protective immunity against bacteria infection.