Azithromycin impairs TLR7 signaling in dendritic cells and improves the severity of imiquimod-induced psoriasis-like skin inflammation in mice.

BACKGROUND: The activation of Toll-like receptor 7 (TLR7) in dendritic cells (DCs) plays a crucial role in the pathogenesis of psoriasis. The macrolide antibiotic azithromycin (AZM) had been demonstrated to inhibit the TLR4 agonist-induced maturation and activation of murine bone marrow-derived DCs (BMDCs).

OBJECTIVE: To investigate the effects of AZM on the induction of DC maturation and activation by imiquimod (IMQ), a synthetic TLR7 agonist, as well as its potential as a therapeutic agent for psoriasis.

METHODS: The effects of AZM on IMQ-induced DC activation were investigated based on the expression of cell surface markers and cytokine secretion. The lysosomal pH, post-translational processing of TLR7, and TLR7 signaling were also examined in DCs. The therapeutic effects of AZM on psoriasis were evaluated in a murine model of IMQ-induced psoriasis-like skin inflammation.

RESULTS: AZM significantly inhibited the expression of co-stimulatory molecules (CD40 and CD80) and reduced TNF-α, IL-10, IL-12p40, IL-12p70, IL-23p19 in BMDCs and IFN-α production in plasmacytoid DCs. AZM treatment impaired lysosomal acidification, interrupted TLR7 maturation in the lysosome, and ultimately blocked the IMQ-induced NF-κB and IRF-7 nuclear translocation in DCs. AZM treatment decreased signs of IMQ-induced skin inflammation in BALB/c mice. In addition to decreasing keratinocyte hyper-proliferation and restoring their terminal differentiation, AZM treatment decreased the accumulation of DCs as well as CD4, CD8 T cells and IL-17 producing cells in psoriatic skin lesions. AZM treatment improved splenomegaly, decreased the populations of Th17 and γδ T cells, and reduced the expression of cytokines known to be involved in the pathogenesis of psoriasis, such as IL-17A, IL-17F, IL-22 and IL-23, in the skin and spleen.

CONCLUSION: AZM impaired IMQ-induced DC activation by decreasing lysosomal acidification and disrupting TLR7 maturation and signaling. AZM significantly improved the IMQ-induced psoriasis-like inflammation in mice. AZM may be a potential therapeutic candidate for psoriasis treatment.