PACAP/PAC1R activation promotes group 2 innate lymphoid cells-dependent allergic rhinitis via ERK pathway

Allergic rhinitis (AR) is a T helper type 2 (Th2)-mediated inflammatory disease. It has been reported that Group 2 innate lymphoid cells (ILC2s) may contribute to the pathogenesis of AR. While Pituitary Adenylate Cyclase-Activating Polypeptide (PACAP) has demonstrated anti-inflammatory properties in allergic contact dermatitis, its regulatory effects on ILC2s remain unclear. This study aimed to investigate the regulatory role of PACAP in ICL2 proliferation under allergic inflammation. In an ovalbumin (OVA)-induced AR mouse model, we observed significant elevations in both PACAP levels and ILC2 populations. Both in vivo and in vitro experiments confirmed that PACAP effectively promoted the expansion of IL-5⁺ and IL-13⁺ ILC2s subsets and enhances ILC2 proliferation. PACAP receptor PAC1R knockdown or PAC1R receptor antagonist PA-8 markedly suppressed ILC2s proliferation and cytokine production. Furthermore, in vivo experiments demonstrated that PACAP inhibition reduced ILC2 proliferation, thereby alleviating nasal mucosal inflammatory responses, confirming that PACAP exacerbates allergic inflammation through PAC1R-dependent activation of ILC2s. Mechanistic studies revealed that PACAP/PAC1R signaling activated the ERK pathway, as evidenced by upregulated p-ERK expression and increased IL-5/IL-13 secretion in ILC2s. These effects were effectively reversed by ERK inhibitor PD98059. Importantly, both PAC1R knockdown and ERK inhibition significantly decreased p-ERK expression and ILC2s proliferation, while ameliorating AR pathological features. Our findings revealed that PACAP/PAC1R activation promoted ILC2s proliferation and allergic inflammation through ERK pathway, which provides novel insights into the regulation of ILC2s and potential therapeutic targets in allergic rhinitis.