Mutations in the filaggrin gene, an epidermal structural protein, are the strongest risk factor identified for development of atopic dermatitis. Up to 50% of patients with moderate-to-severe atopic dermatitis in European populations have filaggrin-null alleles compared with a general population frequency of 7-10%.
This study aimed to investigate the relationship between filaggrin-null mutations and epidermal antigen-presenting cell maturation in individuals with and without atopic dermatitis. Additionally, we investigated whether the cis isomer of urocanic acid, a filaggrin breakdown product, exerts immunomodulatory effects on dendritic cells.
Epidermal antigen presenting cells from non-lesional skin were assessed by flow cytometry (n=27) and confocal microscopy (n=16). Monocyte-derived dendritic cells from healthy volunteers were used to assess effects of cis- and trans-urocanic acid on dendritic cell phenotype by flow cytometry (n=11).
Epidermal antigen presenting cells from filaggrin-null individuals had increased CD11c expression. Confocal microscopy confirmed this, and additionally revealed an increased number of epidermal CD83+ Langerhans cells in filaggrin-null individuals. In vitro, differentiation in the presence of cis-urocanic acid significantly reduced co-stimulatory molecule expression on monocyte-derived dendritic cells from healthy volunteers and increased their ability to induce a regulatory T cell phenotype in mixed lymphocyte reactions.
We show that individuals with filaggrin-null mutations have more mature Langerhans cells in non-lesional skin whether or not they have atopic dermatitis. We also demonstrate that cis-urocanic acid reduces maturation of dendritic cells and increases their capacity to induce regulatory T cells, suggesting a novel link between filaggrin deficiency and immune dysregulation.