INCREASED GROUP 2 INNATE LYMPHOID CELLS IN PERIPHERAL BLOOD OF ADULTS WITH MASTOCYTOSIS.

ABSTRACT Background Systemic mastocytosis is a hematological disease in which aberrant mast cells accumulate due to gain-of-function mutations in the KIT receptor. Group 2 innate lymphoid cells (ILC2s) are effector cells of type 2 immune responses that also express KIT and co-localize with mast cells at barrier tissue sites. In mouse models, mast cell-ILC2 crosstalk can drive local inflammation. However, a possible role for ILC2s in the pathophysiology of mastocytosis remains unexplored. Objective To characterize circulating ILC2s in a clinically diverse cohort of mastocytosis patients. Methods We included 21 adults with systemic mastocytosis and 18 healthy controls. Peripheral blood ILC2 abundance and phenotype were analysed by flow cytometry and correlated to clinical characteristics, including the presence of the D816V KIT mutation. Results ILC2 levels were significantly higher in D816V+ mastocytosis patients compared to D816V- patients or healthy controls. We observed increased proportions of KIT+ ILC2s among patients with mastocytosis, regardless of D816V status. Patients with skin involvement and itch showed the highest levels of ILC2s, which was independent from atopy or serum tryptase levels. Allele-specific quantitative PCR showed that the vast majority of ILC2s did not carry the D816V mutation. Conclusion Our findings suggest a role for ILC2s and pathogenic ILC2-mast cell crosstalk in mastocytosis. We hypothesize that a high cutaneous D816V+ mast cell burden alters the skin microenvironment to induce chronic local ILC2 activation and their dissemination into the circulation. Activated ILC2s could contribute to skin symptoms by producing inflammatory mediators and by further augmenting mast cell mediator release.