Subcutaneous immunotherapy-induced IgG1 suppresses allergic airway inflammation through FcγRIIb-mediated inhibition of group 2 innate lymphoid cell proliferation

<h2>Abstract</h2><h3>Background</h3> Subcutaneous immunotherapy (SCIT), a form of allergy immunotherapy, can alter the natural course of allergic diseases and induce tolerance to causative allergens by modulating type 2 immune responses. While the regulatory effects of SCIT on Th2 cells have been extensively studied, its effects on group 2 innate lymphoid cells (ILC2s) remain poorly understood. In a murine model of asthma, we demonstrated that SCIT increased the production of allergen-specific IgG1, which is analogous to human IgG4. To clarify the role of IgG1 in the mechanisms underlying SCIT, we evaluated its effects on the development of allergic asthma and the proliferation of ILC2s. <h3>Methods</h3> BALB/c mice were sensitized with ovalbumin (OVA) and Al(OH)₃, followed by SCIT consisting of 3 subcutaneous OVA injections at a dose of 1 mg/animal. After SCIT, sera were collected, and total IgG1 was purified using the pH-gradient elution method. ILC2s were isolated from the lungs of OVA-challenged mice and stimulated with IL-33 in the presence of OVA and the purified IgG1 for 72 h, after which cell proliferation was assessed using an ATP assay. For in vivo experiments, the purified IgG1 was intratracheally administered to OVA-challenged asthmatic mice, and airway remodeling and lung leukocyte populations were subsequently analyzed. Animal experiments were approved by the Experimental Animal Research Committee of Setsunan University (approval Nos. K21-1, K22-1, K23-1, K24-1, and K25-1) and conducted in accordance with the ARRIVE 2.0 guidelines. <h3>Results</h3> (1) IgG1⁺ B cells were markedly increased in the lungs of SCIT-treated asthmatic mice (p = 0.0409) and were the most abundant among the 4 IgG⁺ B-cell subsets. (2) Intratracheal administration of IgG1 derived from SCIT mice (SCIT-IgG1) significantly suppressed epithelial thickening and mucus accumulation, and reduced the numbers of lung ILC2s and eosinophils in the asthma model (p = 0.0034, 0.0341, 0.0246, and 0.0041, respectively). (3) FcγRIIb expression on lung ILC2s derived from asthmatic mice was significantly increased (p = 0.0004). (4) SCIT-IgG1 significantly suppressed OVA and IL-33-induced in vitro proliferation of ILC2s (p = 0.0018), and this inhibitory effect disappeared in the presence of an <i>anti</i>-FcγRIIb antibody. <h3>Conclusions</h3> SCIT-IgG1 attenuates airway remodeling and limits ILC2 expansion in allergic airway inflammation. Mechanistically, SCIT-IgG1 restrains ILC2 proliferation via FcγRIIb engagement, revealing an antibody–ILC2 inhibitory axis that likely contributes to the efficacy of allergy immunotherapy and suggests therapeutic strategies that enhance inhibitory FcγR signaling to control type 2 inflammation.