The study shows that the antimicrobial peptide LL-37 makes human mast cells release their stored chemicals and produce new inflammatory signals mainly by binding to a receptor called MrgX2. Turning off MrgX2 stops this reaction, while other receptors play only a minor role.

Human LL-37 is an important class of cationic antimicrobial peptide (CAP) that is known to stimulate mast cell activation. While many studies have been conducted on LL-37, to date little is known about the functional receptors for LL-37-induced human mast cell activation, in particular in terms of the release of de novo synthesized mediators. Thus, the aim of the present study is to identify the functional receptors for LL-37-induced human mast cell activation in terms of the degranulation and release of de novo synthesized mediators and investigate the downstream signalling pathways involved in mast cell activation. Overall, our study importantly demonstrates that LL-37-induced human mast cell degranulation and release of de novo synthesized mediators function primarily through the activation of MrgX2. We furthermore show that LL-37-induced human mast cell line LAD2 cells are involved in the degranulation and release of IL-8, and that FPRL1 and P2X7 have only a partial effect on IL-8 release, and no effect on mast cell degranulation triggered by LL-37. Instead, we find that silencing the expression of MrgX2 in human mast cell significantly inhibits the LL-37-induced degranulation and release of IL-8. Overall, this effect is associated with the activation of the G_i protein, PLC/PKC/Calcium/NFAT, PI3K/Akt and MAPKs signalling pathways.