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.

Low and high IgE is linked to improvement and worsening of chronic urticaria during pregnancy, respectively Kocatürk E, Thomsen SF, Al-Ahmad M, Gimenez-Arnau A, Conlon N, Savk E, Criado RF, Danilycheva I, Fomina D, Khoshkhui M, Gelincik A, Degirmentepe EN, Demir S, Ensina LF, Kasperska-Zajac A, Rudenko M, Bauer A, Medina I, Maurer M.1 Urticaria Center of Reference and Excellence (UCARE), Dept. of Dermatology, Koç University School of Medicine, Istanbul, Turkey [email protected] Urticaria Center of Reference and Excellence (UCARE), Center of Reference and Excellence (UCARE), Department of Dermatology, Bispebjerg Hospital, Copenhagen, Denmark [email protected] Urticaria Center of Reference and Excellence (UCARE), Microbiology Department, Faculty of Medicine, Kuwait University, Safat, Kuwait [email protected] Urticaria Center of Reference and Excellence (UCARE), Department of Dermatology, Hospital del Mar, IMIM, Universitat Autònoma, Barcelona, Spain [email protected] Urticaria Center of Reference and Excellence (UCARE), Dermatology, and Immunology, St James’s Hospital, Dublin, Ireland [email protected] Aydın Adnan Menderes University, Aydın, Turkey [email protected] Urticaria Center of Reference and Excellence (UCARE), Faculdade de Medicina do ABC (FMABC), Santo André, Brazil [email protected] Urticaria Center of Reference and Excellence (UCARE), NRC Institute of Immunology FMBA of Russia, Moscow, Russia [email protected] Urticaria Center of Reference and Excellence (UCARE), First Moscow State Medical University, Moscow Center of Allergy and Immunology , Clinical Hospital 52 , Ministry of Moscow Healthcare, Moscow, Russia [email protected] Urticaria Center of Reference and Excellence (UCARE), Allergy Research Center, Mashhad University of Medical Sciences, Mashhad, Iran [email protected] Urticaria Center of Reference and Excellence (UCARE), Istanbul Faculty of Medicine Istanbul University, Istanbul, Turkey [email protected] Urticaria Center of Reference and Excellence (UCARE), Okmeydani Training and Research Hospital, Istanbul, Turkey [email protected] Urticaria Center of Reference and Excellence (UCARE), Istanbul Faculty of Medicine Istanbul University, Istanbul, Turkey [email protected] Urticaria Center of Reference and Excellence (UCARE), Federal University of São Paulo, Sao Paulo, Brazil [email protected] Urticaria Center of Reference and Excellence (UCARE), European Center for Diagnosis and Treatment of Urticaria (GA2LEN UCARE Network) Medical University of Silesia in Katowice, Poland [email protected] Urticaria Center of Reference and Excellence (UCARE), The London Allergy & Immunology Centre, London, United Kingdom [email protected] Urticaria Center of Reference and Excellence (UCARE), Department of Dermatology, University Allergy Center, University Hospital Carl Gustav Carus, Technical University Dresden, Germany. [email protected] Urticaria Center of Reference and Excellence (UCARE), the Centro Médico Vitae, Buenos Aires, Argentina [email protected] Urticaria Center of Reference and Excellence (UCARE), Dermatological Allergology, Allergie-Centrum-Charité, Dept. of Dermatology and Allergy, Charité – Universitätsmedizin Berlin, Germany [email protected] Editor,PREG-CU, the recent study on pregnancy and chronic urticaria (CU) by the Urticaria Centers of Reference and Excellence (UCAREs), showed that CU improves in half (51.1%) of patients during pregnancy, whereas 28.9% and 20% of patients, respectively, experienced worsening and no change. Low disease activity, no angioedema, and no treatment before pregnancy were risk factors for worsening during pregnancy (1).We hypothesized that patients with chronic spontaneous urticaria (CSU) that worsens during pregnancy are more likely to have type I autoimmune CSU, also called autoallergic CSU. We also hypothesized that patients who improve during pregnancy are more likely to have type IIb autoimmune CSU (2). This hypothesis is supported by the immunological changes observed during pregnancy, i.e., decreased Th1 and Th17 immunity and a switch to a Th2-type cytokine profile (3).To test this hypothesis, we retrieved total IgE levels of CSU patients who gave consent to be included in the PREG-CU study (1). Elevated IgE levels have been reported to be linked to autoallergic CSU, whereas low IgE is a marker of type IIb autoimmune CSU (4).Total IgE blood levels were available for 115 of the 218 CSU patients not treated with omalizumab enrolled in PREG-CU. The median IgE level was 106 (range: 3-1664 IU/mL), more than half of patients (51.3%) had elevated IgE (≥100 IU/mL), and 17.4% had low IgE (<40 IU/mL). Most patients with mild disease (51%) or moderate disease (61%), but only one in four patients with severe disease (26%) had elevated IgE levels (≥100 IU/mL). IgE levels were lower in patients with severe disease (68 IU/mL) vs mild (112 IU/mL; p=0.009) or moderate disease (128 IU/mL; p=0.018), and low IgE levels (<40 IU/mL) were more frequent in patients with severe than mild disease (36.8 vs 11.6%; p=0.034).CSU patients who got worse during pregnancy had higher IgE levels (154 vs. 82.2 IU/mL; p=0.033) and numerically higher rates of elevated IgE (57.5 vs. 46%) compared to patients who got better during pregnancy. In contrast, patients who improved during pregnancy more often had low IgE levels than patients who deteriorated (22 vs. 12.5%), but this was not statistically significant. One in three of our patients (34.9%) had elevated anti-TPO, another marker of type IIb autoimmune CSU, but this was not linked to improvement during pregnancy.Worsening of CSU during pregnancy in patients with high IgE levels may be explained, in part, by the role that IgE and Th2 immunity play in the pathogenesis of their CSU. High IgE, in CSU, has been linked, in some studies, to autoallergy, characterized by the presence of IgE autoantibodies (5). Pregnancy skews immunity towards Th2 responses and patients with Th2-driven diseases, including allergies, often experience worsening of their disease during pregnancy (3). Improvement of CSU during pregnancy in patients with low IgE may point to a role of Th1 and Th17 cytokines in the pathogenesis of their disease. Low IgE is a type IIb autoimmune CSU marker, which is linked to Th1 and Th17 autoimmunity (6). Pregnancy decreases Th1/Th17 immunity, and patients with TH1/TH17-driven autoimmune diseases often experience improvement during pregnancy (3). Our finding that elevated IgG-anti-TPO, another marker of type IIb autoimmune CSU, is not linked to CSU improvement during pregnancy remains unexplained. Many CSU patients with IgG-anti-TPO also have IgE-anti-TPO and vice versa, which could point to both autoallergic and autoimmune drivers of their CSU. Better biomarkers are needed to identify which patients have autoallergic CSU, autoimmune CSU, both or none of these.Our findings support the notion that CSU is a heterogeneous disease, with at least two endotypes, i.e., autoallergic and autoimmune. Further studies are needed to better characterize the course of disease during and after pregnancy, in patients with autoallergic CSU and with autoimmune CSU. IgE levels may help to predict which CSU patients get worse and which improve when they get pregnant.