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.

Aim: The 4th Davos Declaration, convened during the Global Allergy Forum (GAF) in Davos, aimed to elevate patient care for patients with atopic dermatitis (AD) by uniting experts and stakeholders. The forum addressed the high prevalence of AD, with a strategic focus on advancing research, treatment, and management to meet the evolving challenges in the field. Methods: This multidisciplinary forum brought together top leaders from research, clinical practice, policy, and patient advocacy to discuss the critical aspects of AD, including neuroimmunology, environmental factors, comorbidities, and breakthroughs in prevention, diagnosis, and treatment. The discussions were geared towards fostering a collaborative approach to integrate these advancements into practical, patient-centric care. Results The forum underlined the mounting burden of AD, attributing it to significant environmental and lifestyle changes. It acknowledged the progress in understanding AD and in developing targeted therapies but recognized a gap in translating these innovations into clinical practice. Emphasis was placed on the need for enhanced awareness, education, and stakeholder engagement to address this gap effectively and to consider environmental and lifestyle factors in a comprehensive disease management strategy. Conclusion: The 4th Davos Declaration marks a significant milestone in the journey to improve care for people with AD. By promoting a holistic approach that combines research, education, and clinical application, the Forum sets a roadmap for stakeholders to work together to improve patient outcomes in AD, reflecting a commitment to adapt and respond to the dynamic challenges of AD in a changing world.

Background: Skin barrier dysfunction is a key component of the pathogenesis of atopic dermatitis (AD). Recent research on barrier optimization to prevent AD has shown mixed results. The aim of this study was to assess the relationship between emollient bathing at two months and the trajectory of AD in the first two years of life in a large unselected observational birth cohort study. Methods: The Babies After SCOPE: Evaluating the Longitudinal Impact Using Neurological and Nutritional Endpoints Birth Cohort study enrolled 2183 infants. Variables extracted from the database related to early skincare, skin barrier function, parental history of atopy, and AD outcomes. Statistical analysis was performed to adjust for potential confounding variables. Results: 1,505 children had data on AD status available at six, 12, and 24 months. Prevalence of AD was 18.6% at six months, 15.2% at 12 months, and 16.5% at 24 months. Adjusted for potential confounding variables, the odds of AD at any point were higher among infants who had emollient baths at two months (OR (95% CI): 2.41 (1.56 to 3.72), p<0.001). Following multivariable analysis, the odds of AD were higher among infants who had both emollient baths and frequent emollient application at two months, compared to infants who had neither (OR (95% CI) at six months 1.74 (1.18-2.58), p=0.038), (OR (95% CI) at 12 months 2.59 (1.69-3.94), p<0.001), (OR (95% CI) at 24 months 1.87 (1.21-2.90), p=0.009). Conclusion: Early emollient bathing was associated with greater development of AD by two years of age in this unselected birth cohort study.

Background Protecting the skin barrier in early infancy may prevent atopic dermatitis (AD). We investigated if daily emollient use from birth to 2 months reduced AD incidence in high risk infants at 12 months. Methods This was a single-center, two-armed, investigator-blinded, randomized controlled clinical trial (NCT03871998). Term infants identified as high risk for AD (parental history of AD, asthma or allergic rhinitis) were recruited within 4 days of birth and randomised 1:1 to either twice-daily emollient application for the first 8 weeks of life (intervention group), using an emollient specifically formulated for very dry, AD-prone skin, or to standard routine skin care (control group). The primary outcome was cumulative AD incidence at 12 months. AD <6 months was diagnosed based on clinical presence of AD. The UK Working Party Diagnostic Criteria were applied when diagnosing AD between 6 and 12 months. Results 321 infants were randomised (161 intervention and 160 control), with 61 withdrawals (41 intervention, 20 control). The cumulative incidence of AD at 12 months was 32.8% in the intervention group vs. 46.4% in the control group, p = 0.036 [Relative risk (95%CI): 0.707 (0.516, 0.965)]. One infant in the intervention group was withdrawn from the study following development of a rash that had a potential relationship with the emollient. There was no significant difference in the incidence of skin infections between the intervention and control groups during the intervention period (5.0% vs. 5.7%, P>0.05). Conclusions This study has demonstrated that early initiation of daily specialized emollient use until 2 months reduces the incidence of AD in the first year of life in high-risk infants.