Chracterisation of the Epithelial barrier in early life and in atopic dermatitis

Atopic dermatitis (AD) is the most common inflammatory disease of childhood. It is characterised clinically by chronic skin inflammation and pruritus. Atopic dermatitis is associated with several co-morbidities, including food allergies and asthma. It is a disease of early life, with a typical onset in infancy and the clinical manifestations varying with age. The cheek skin is the site of disease initiation and the focus of inflammation. Atopic dermatitis is associated with a significant negative impact on quality of life for the patient and their family. Pathologically AD is characterised by an epidermal barrier abnormality, immune dysregulation, and microbiome alteration. The discovery of loss-of-function (LOF) mutations in filaggrin (FLG), a barrier protein expressed in the outer layers of the epidermis, as the strongest known genetic risk factor for the development of AD was fundamental in highlighting that skin barrier dysfunction is a central pathomechanism in the disease. A defective epidermal permeability barrier is consistently found in AD skin, at both clinically affected and unaffected sites. Patients with AD with FLG LOF mutations have more persistent disease, more severe disease, and a greater risk of food allergies and eczema herpeticum. The focus of this PhD is the investigation of early disease mechanisms in AD. This work included characterisation of the epithelial barrier in early life, both in the setting of health and disease, as well as biomarker discovery in infantile AD. I aimed to investigate regional and age dependent variations in stratum corneum (SC) structure and function, with a focus on filaggrin metabolism and corneocyte maturity, to determine if these could explain the observed predilection sites for AD in early life. I sought to explore the relationship between FLG genotype, filaggrin breakdown products (natural moisturisingfactor [NMF]), and corneocyte morphology in patients with AD. I also aimed to explore biomarkers in infantile AD, both from the skin and systemically, and compare these to structural and functional measures of the skin barrier. This thesis presents several important original findings that represent progress in this research field. I have established that filaggrin processing, corneocyte maturity, and protease activities show regional and temporal differences in infant skin. These findings help explain disease patterns in early life AD, particularly why infantile AD initially affects the cheeks. The results are supportive of the cheek site being highly relevant for allergen priming in early childhood. I have demonstrated that filaggrin deficiency is associated with physical abnormalities in corneocytes in AD that persist despite clinical improvement in disease. These structural differences help explain clinical variances between AD endophenotypes, and have the potential to facilitate assessment of therapeutic interventions. Finally, I have identified clinically relevant AD biomarkers, including novel markers, easily sampled and typed in infants with AD. These markers provide insights into pathogenesis of infant AD, and may be potentially useful for disease stratification. Further research opportunities arise from this work. How the findings of this thesis can be utilised to guide directed therapeutics and preventative strategies, and the optimal timing of such interventions, are the next challenges to address