Airway fibroblasts direct mast cell progenitor proliferation and polarization

Mast cells (MCs) are central regulators of type 2 inflammatory diseases, including asthma and nasal polyposis, producing factors that elicit bronchoconstriction and leukocyte infiltration. MCs develop from circulating progenitors that mature in peripheral tissues under the regulation of microenvironmental factors. In nasal polyposis, MC expansion is partially driven by proliferation of transcriptionally immature MCs within the subepithelium that correlates with disease severity. Thus, we hypothesized that structural cells within the subepithelium direct the proliferation of immature MCs. Using single-cell RNA sequencing, we identified a population of CD34+ cells expressing MC-associated transcripts (TPSAB1, HDC, FCER1A) present in peripheral blood and characterized by a discrete cell surface protein expression profile (CD71, CD131, CD33, CD203c). Following flow cytometric isolation, these MC progenitors had limited proliferation capacity in monoculture, but robustly proliferated when co-cultured with airway fibroblasts. Fibroblast co-culture further enhanced intracellular chymase levels; a marker of subepithelial MCs; and cell surface expression of the innate activating receptor MRGPRX2. Thus, our findings suggest a central role for fibroblasts in both driving MC expansion and shaping the subepithelial MC phenotype.