Staphylococcus aureus exploits epidermal barrier defects in atopic dermatitis to trigger cytokine expression

Patients with atopic dermatitis (AD) have an abnormal skin barrier and are frequently colonized by S. aureus. In this study we investigated if S. aureus penetrates the epidermal barrier of subjects with AD and sought to understand the mechanism and functional significance of this entry. S. aureus was observed to be more abundant in the dermis of lesional skin from AD patients. Bacterial entry past the epidermis was observed in cultured human skin equivalents and in mice, but found to be increased in the skin of cathelicidin antimicrobial peptide knockout (Camp-/-) and ovalbumin-sensitized filaggrin mutant (FLGft/ft ) mice. S. aureus penetration through the epidermis was dependent on bacterial viability and protease activity as killed bacteria or a protease-null mutant strain of S. aureus was unable to penetrate. Entry of S. aureus directly correlated with increased expression of IL4, IL13, IL22, TSLP and other cytokines associated with AD, and with decreased expression of cathelicidin. These data illustrate how abnormalities of the epidermal barrier in AD can alter the balance of S. aureus entry into the dermis and provides an explanation for how such dermal dysbiosis results in increased inflammatory cytokines and exacerbation of disease.