Serum level of hemokinin-1 is significantly lower in patients with chronic spontaneous urticaria than in healthy subjects

Background: We previously reported upregulation of expression of Mas–related G protein–coupled receptor X2 (MRGPRX2) on mast cells (MCs) in the skin of patients with severe chronic spontaneous urticaria (CSU). Serum levels of substance P (SP) were reportedly significantly elevated, in correlation with the severity of CSU. Hemokinin-1 (HK-1) reportedly induced histamine release from LAD2 cells via MRGPRX2. We aimed to investigate HK-1's role in CSU. Methods: The concentrations of HK-1 and SP were measured using ELISAs. Skin– and synovium–derived cultured MCs were generated by culturing dispersed skin and synovial cells, respectively, with stem cell factor. MRGPRX2 expression in the MCs was reduced using a lentiviral shRNA silencing technique. Results: Anti–SP Ab used in the SP ELISA showed 100% cross–reactivity to HK-1, but anti–HK-1 Ab showed 0% cross–reactivity to SP. The serum level of HK-1 was significantly lower in patients with CSU (n = 151) than in non–atopic healthy control (NC) subjects (n = 114). The EC₅₀ of histamine release from MCs induced by HK-1 (5056 nM) was 12–fold higher than by SP (426 nM). Brief pretreatment of MCs with HK-1 at concentrations of 3.0–10 μM significantly reduced histamine release by 0.1 μM SP. However, brief incubation of MCs with HK-1 did not elicit rapid MRGPRX2 internalization. Conclusions: In NC subjects, high HK-1 concentrations may desensitize MGRPRX2–mediated MC activation, thereby preventing MC degranulation by SP.