Proliferating Langerhans cells dampen inflammation in established mouse psoriatic lesions. Mus musculus

Psoriasis is a chronic inflammatory skin disease of unknown etiology. Although macrophages and dendritic cells (DCs) have been proposed to drive the psoriatic cascade, their largely overlapping phenotype hampered studying their respective role. Topical application of Imiquimod, a Toll-like receptor 7 agonist, induces psoriasis in patients and psoriasiform inflammation in mice. We showed that daily application of Imiquimod for 14 days recapitulated both the initiation and the maintenance phase of psoriasis. Based on our ability to discriminate Langerhans cells (LCs), conventional DCs, monocytes, monocyte-derived DCs and macrophages in the skin, we characterized their dynamics during both phases of psoriasis. During the initiation phase, neutrophils infiltrated the epidermis whereas monocytes and monocyte-derived DCs were predominant in the dermis. During the maintenance phase, LCs and macrophage numbers increased in the epidermis and dermis, respectively. LC expansion resulted from local proliferation, a conclusion supported by transcriptional analysis. Continuous depletion of LCs during the course of Imiquimod treatment aggravated chronic psoriatic symptoms as documented by an increased influx of neutrophils and a stronger inflammation. Therefore, by developing a mouse model that mimics the human disease more accurately, we established that LCs play a negative regulatory role during the maintenance phase of psoriasis. Overall design: DCs and other myeloid cell types were isolated from lymphoid organs as previously described (Henri et al. 2010). Briefly, LNs were cut into small pieces and digested for 20 min at room temperature with a mixture of type II collagenase (Worthington Biochemical) and DNase I (Sigma-Aldrich). The resulting cell suspension was treated with 5 mM EDTA to disrupt DC-T cell conjugates. After eliminating undigested material, light-density cells were enriched by centrifugation on an Optiprep solution (d = 1.32 g/mL, Abcys). To extract skin mononuclear phagocytic cells, ears were splitted into dorsal and ventral parts and incubated with a solution of PBS containing 1 mg/mL dispase (Roche) for 2 h at 37° C or overnight at 4° C, as specified. The dorsal and ventral parts were then cut into small pieces and incubated for 90 min at 37° C with RPMI containing 1 mg/mL DNase and 1 mg/mL Collagenase IV (Worthington Biochemical). The resulting single cell suspension was subjected to centrifugation on a Percoll gradient (Amersham-Pharmacia).