Until quite recently, the pathogenesis of atopic dermatitis (AD) has been attributed to primary abnormalities of the immune system. Intensive study revealed the key roles played by T H 1/T H 2 cell dysregulation, IgE production, mast cell hyperactivity, and dendritic cell signaling in the evolution of the chronic, pruritic, inflammatory dermatosis that characterizes AD. Accordingly, current therapy has been largely directed toward ameliorating T H 2-mediated inflammation and pruritus. In this review we will assess emerging evidence that inflammation in AD results from inherited and acquired insults to the barrier and the therapeutic implications of this paradigm.
KeywordsAntimicrobial peptides; atopic dermatitis; barrier function; barrier repair; cytokines; filaggrin; pH; psychologic stress; Staphylococcus aureus; serine proteases; T H 2 cells Until recently, atopic dermatitis (AD) has been viewed largely as a disease of immunologic etiology. 1-5 Yet, the epidermis generates a set of protective/defensive functions (Table I) mediated by its unique differentiation end product, the stratum corneum (SC). 6,7 These functions include the permeability barrier, which retards transcutaneous evaporative water loss, allowing survival in a potentially desiccating external environment, and an antimicrobial barrier, which simultaneously encourages colonization by nonpathogenic ''normal'' flora while resisting growth of microbial pathogens. 8 Although both a defective epidermal permeability 9-13 and a propensity to secondary infection 14,15 are well-recognized features of AD, these abnormalities have been widely assumed to reflect downstream consequences of a primary immunologic abnormality (the historical inside-outside view of AD pathogenesis). We and others have long proposed that the permeability barrier abnormality inADis not merely an epiphenomenon but rather the ''driver'' of disease activity (ie, the reverse outside-inside view of disease pathogenesis) 16-19 for the following reasons: (1) the extent of the permeability barrier abnormality parallels the severity of the disease phenotype in AD 9,10,12 ; (2) both clinically uninvolved skin sites and skin cleared of inflammation for as long as 5 years continue to display significant barrier abnormalities 10,13 ; (3) emollient therapy comprises effective ancillary therapy 20 ; and most importantly, (4) specific replacement therapy, which targets the
NIH-PA Author ManuscriptNIH-PA Author Manuscript NIH-PA Author Manuscript prominent lipid abnormalities that account for the barrier abnormality in AD (see below), corrects both the permeability barrier abnormality and comprises effective anti-inflammatory therapy for AD (see the Therapeutic implications section below).
BROAD BARRIER FAILURE IN ADLike permeability barrier dysfunction, the antimicrobial barrier is also compromised in patients with AD. Colonization by Staphylococcus aureus is a common feature of AD, 21 and although colonization is highest on lesional skin, colony counts often are high on the clinically no...