T Helper Cytokines (Th1, Th2 and Th17) Differentially Regulate the Epidermal Tight Junction Barrier

Atopic dermatitis (AD) is characterized by an epidermal tight junction (TJ) defect and a mixed but primarily Th2 immune response. We hypothesized that Th cytokines might alter the TJ barrier. To test this, we evaluated the effect of Th1, Th2 and Th17 cytokines on epidermal TJ protein function/expression. Epidermal mRNA expression of TJ proteins, systemic Th2 biomarkers were measured in AD (nonlesional [ADNL]; lesional [ADL]) and nonatopic (NA) subjects. TJ function was evaluated by fluorescein permeability in healthy human epidermis ± Th cytokines. Expression of TJ proteins from primary human keratinocytes ± Th cytokine stimulation was quantified by qPCR and Western blotting. AD epidermis (n=8) expressed less TJ proteins than NA epidermis (n=10) (claudin-4 [CLDN4], NA vs ADNL: P=0.02; NA vs ADL: P=0.01; CLDN23, NA vs ADNL: P=0.05; NA vs ADL: P=0.003). CLDN4 expression inversely correlated with systemic Th2 biomarkers (serum IgE, r=−0.53, P=0.02; eosinophil counts, r=−0.44, P=0.04). Th2 cytokines (IL4+IL13) enhanced TJ permeability (n=7, P<0.01) and dose-dependently inhibited CLDN4 expression (n=5, P<0.001). In contrast, Th1 (IFNγ) and Th17 (IL17A) cytokines decreased TJ permeability (n= 6, P<0.01; n= 5, P<0.05, respectively) and dose-dependently enhanced TJ protein expression (Th1: CLDN23, n=6, P<0.05; Th17: CLDN4, n=4, P<0.05; CLDN23, n=4, P<0.01) Several claudin family members (CLDN4 and CLDN23) are reduced in AD skin and this correlates with systemic Th2 biomarkers. In contrast with the TJ enhancing effects of Th1 and Th17 cytokines, Th2 cytokines inhibit epidermal TJ protein expression/function. This suggests that Th2 cytokines may cause the TJ barrier defects observed in AD.