Modulating dendritic cell function in allergic asthma with <i>Toxoplasma gondii</i> serine protease inhibitor 1

Tolerogenic adjuvants can enhance allergy vaccine efficacy. We previously showed that intranasal administration of recombinant Toxoplasma gondii serine protease inhibitor 1 (rTgPI-1) with ovalbumin alleviates asthma symptoms in mice. This study investigates the immunomodulatory mechanisms of rTgPI-1, focusing on its effect on dendritic cells (DCs). Bone marrow-derived DCs (BMDCs) were generated in the presence of rTgPI-1 and analyzed for their phenotype. rTgPI-1 exposed BMDCs showed reduced CD80, CD86, and MHCII, increased PDL-1 and CD45Rb, and a higher IL-10/IL-12 ratio. These BMDCs induced fewer CD69⁺CD4⁺ T cells, decreased proliferation and secretion of IL-17 and IL-4, and increased CD4⁺FoxP3⁺ T cells. In vivo, intranasal co-administration of rTgPI-1 with allergen in asthmatic mice reduced CD80high/CD86high DCs, expanded lung CD4⁺FoxP3⁺ regulatory T cells, and decreased the cDC2 subset, correlating with reduced IL-4 levels. Importantly, nafamostat mesylate-a synthetic serine protease inhibitor previously shown to alleviate asthma symptoms-failed to induce FoxP3⁺ T cells both in vitro and in vivo, underscoring the unique tolerogenic activity of rTgPI-1. Finally, stimulation of peripheral blood mononuclear cells from mite-allergic patients with house dust mite extract in the presence of rTgPI-1 led to reduced allergen-specific IL-4 and IL-5 secretion. These findings demonstrate that rTgPI-1 promotes a semimature, tolerogenic DC phenotype, suppresses T cell activation, and fosters regulatory T cell differentiation. Moreover, rTgPI-1 selectively modulates DC subsets in vivo. The observed effects on patient-derived PBMCs support its potential for further preclinical development as an adjuvant for allergen-specific immunotherapy.