Title: Lin-CD117+CD34+FceRI+ progenitor cells are elevated in atopic dermatitisTo the Editor, We recently demonstrated that Lin-CD117+CD34+FcεRI+progenitor cells predict treatment response to omalizumab in chronic spontaneous urticaria (CSU) (1). These heterogenous myeloid progenitors acquired FcεRI expression with advancing maturity and a phenotype previously described as mast cell progenitors (2). A growing body of research suggests that inflammation drives the egress of myeloid and mast cell progenitors from the bone marrow (1, 3). Atopic dermatitis (AD) is a common inflammatory skin disorder characterised by recurrent eczematous lesions with the dominant symptom of intense itch (4). While the role of T-cell mediated type 2 inflammation is integral to the pathogenesis of AD, mast cells and the pre-formed pruritogens they release have an important role. We sought to measure Lin-CD117+CD34+FcεRI+progenitor cells in the peripheral blood of 10 individuals with a clinical diagnosis of AD and 10 healthy controls. Ethical approval was granted by the Joint Research Ethics Committee in Tallaght University Hospital and St. James’s Hospital in Ireland. Informed consent was obtained. Healthy control subjects had no diagnosis of atopic disease which we defined as the absence of self-reported AD, allergic rhinitis, asthma, food allergy, eosinophilic oesophagitis or CSU. We excluded patients with AD who were in receipt of any systemic treatment. Peripheral blood mononuclear cells (PBMC) were incubated in PBS, pH 7.4 with 2% heat‐inactivated foetal calf serum with the following fluorescent‐labelled antibodies; V500 CD4 (RPA‐T4), V500 CD8 (RPA‐T8), PerCP-Cy5.5 CD14 (M5E2), V500 CD19 (HIB19), PE CD34 (581), APC CD117 (104D2) and BV421 FcεRI (AER‐37). Absolute numbers of cells per ml were calculated using BD Biosciences Liquid Counting Beads. Sample acquisition was performed on a FACSCanto II flow cytometer (BD Biosciences). Data analysis was performed using FlowJo (v10, BD Biosciences). Fluorescence minus one (FMO) controls were used to confirm CD34, CD117 and FcεRI positivity. We categorised progenitor cells according to whether they were Lin-CD34+CD117+FcεRI-(hereafter, FcεRI- progenitors), Lin-CD34+CD117+Fc εRI+ (hereafter, FcεRI+ progenitors), and Lin-CD34+CD117+Fc εRIhi( hereafter, FcεRIhi progenitors) (See Supplemental Figure 1). FcεRIhi progenitors represent an extremely rare cell type with the phenotype of true mast cell precursors (2).Baseline characteristics are illustrated in Table 1.CD34+CD117+Fc εRI+cells were elevated in individuals with AD when compared with healthy controls (n=10; p = 0.0067) (Figure 1). There was no difference in FcεRI- or FcεRIhiprogenitors between individuals with AD and healthy controls (p = 0.356; p = 0.276 respectively). In keeping with findings observed in CSU, mean total serum IgE was not correlated with the number of myeloid progenitors in peripheral blood in individuals with AD (r = 0.486, p = 0.223) or in healthy controls (r = -0.267, p = 0.522). The mean number of circulating FcεRI+ progenitors in males with AD (n=6) was 413.3 per mL of peripheral blood compared to 212.8 per mL in females but this was not statistically significant (p = 0.102). Although myeloid progenitors have been found to predict treatment response in CSU, CSU differs from AD in primarily being a mast cell driven disease. In contrast, mast cell activation in AD represents an adjacent process to the more pivotal role of Th2 polarisation. Our findings therefore propose an intriguing difference in FcεRI+ myeloid progenitors in AD and suggest that the bone marrow may be engaged in myeloid cell replenishment. This phenomenon may provide supportive evidence for the potential efficacy of Bruton Tyrosine Kinase inhibitors or MRGPRX2 targeted therapies in AD (5, 6). Paired analyses of individuals off and on systemic treatment modalities in AD may provide additional insights into the triggers of progenitor egress. 1. Ridge K, Moran B, Alvarado-Vazquez PA, Hallgren J, Little MA, Irvine AD, et al. Lin. Allergy. 2024.2. Dahlin JS, Malinovschi A, Öhrvik H, Sandelin M, Janson C, Alving K, et al. Lin- CD34hi CD117int/hi FcεRI+ cells in human blood constitute a rare population of mast cell progenitors. Blood. 2016;127(4):383-91.3. Lei Y, Guo X, Luo Y, Niu X, Xi Y, Xiao L, et al. Synovial microenvironment-influenced mast cells promote the progression of rheumatoid arthritis. Nat Commun. 2024;15(1):113.4. Langan SM, Irvine AD, Weidinger S. Atopic dermatitis. Lancet. 2020;396(10247):345-60.5. Robak E, Robak T. Bruton’s Kinase Inhibitors for the Treatment of Immunological Diseases: Current Status and Perspectives. J Clin Med. 2022;11(10).6. Wollam J, Solomon M, Villescaz C, Lanier M, Evans S, Bacon C, et al. Inhibition of Mast Cell Degranulation by Novel Small Molecule MRGPRX2 Antagonists. J Allergy Clin Immunol. 2024.