On the surface: Skin microbial exposure contributes to allergic disease

Objective: To discuss the skin microbiome modulates immunity by interactions between skin immunology with keratinocytes to combat pathogens. Allergic disorders are classified by immunoglobulin E sensitivity and aberrant T_H2 cell responses, and an increasing number of studies have described the associations with skin microbiome fluctuations. In this review, we discuss commensal-epidermal homeostasis and its influence on allergic disease.

Data sources: All included references were obtained from the PubMed database.

Study selections: Studies addressing relevant aspects of commensal-epidermal homeostasis, skin microbiome dysbiosis, microbiome-targeted therapeutics, and prevention in allergy were included.

Results: Homeostasis between the commensal microbiome and the epidermis is important in protecting against allergic disease. Commensals promote antiallergic T_H1 and T_H17 immunophenotypes within the skin and induce keratinocytes to secrete antimicrobial peptides and alarmins that enhance barrier function and antagonize proallergic organisms. Perturbations in this homeostasis, however, is associated with allergic disease development. Atopic dermatitis is associated with decreases in skin commensals and increases in the pathogen, Staphylococcus aureus. Fluctuations in the skin microbiome contributes to decreased barrier dysfunction, allergic sensitization, and T_H2 cytokine secretion. Little is known about how the skin microbiome affects food allergy, allergic rhinitis, and asthma, and it is poorly understood how cutaneous inflammation influences systemic allergic responses. Therapies are targeted toward maintenance of the skin barrier, replacement of healthy commensals, and anti-T_H2 biologic therapy.

Conclusion: Although the effects of commensal-epidermal homeostasis on allergy within the skin are becoming increasingly clear, future studies are necessary to assess its effects on extracutaneous allergic disorders and explore potential therapeutics targeting the skin microbiome.