Abstract
Psoriasis is a chronic inflammatory skin disease of unknown etiology. Previous studies showed that short-term, 5-7 d-long application of imiquimod (IMQ), a TLR7 agonist, to the skin of mice triggers a psoriasis-like inflammation. In the current study, by applying IMQ for 14 consecutive d, we established an improved mouse psoriasis-like model in that it recapitulated many of the clinical and cellular hallmarks observed in human patients during both the early-onset and the late-stable phase of psoriasis. Although macrophages and dendritic cells (DCs) have been proposed to drive the psoriatic cascade, their largely overlapping phenotype hampered studying their respective role. Based on our ability to discriminate Langerhans cells (LCs), conventional DCs, monocytes, monocyte-derived DCs, macrophages, and plasmacytoid DCs in the skin, we addressed their dynamics during both phases of our biphasic psoriasis-like model. Plasmacytoid DCs were not detectable during the whole course of IMQ treatment. During the early phase, neutrophils infiltrated the epidermis, whereas monocytes and monocyte-derived DCs were predominant in the dermis. During the late phase, LCs and macrophage numbers transiently increased in the epidermis and dermis, respectively. LC expansion resulted from local proliferation, a conclusion supported by global transcriptional analysis. Genetic depletion of LCs permitted to evaluate their function during both phases of the biphasic psoriasis-like model and demonstrated that their absence resulted in a late phase that is associated with enhanced neutrophil infiltration. Therefore, our data support an anti-inflammatory role of LCs during the course of psoriasis-like inflammation.