Intervention of Inflammatory Monocyte Activity Limits Dermal Fibrosis

Monocytes and monocyte-derived cells are important players in the initiation, progression, and resolution of inflammatory skin reactions. As inflammation is a prerequisite for fibrosis development, we focused on the role of monocytes in cutaneous fibrosis, the clinical hallmark of patients suffering from systemic sclerosis. Investigating the function of monocytes in reactive oxygen species-induced dermal fibrosis, we observed that early monocyte depletion partially reduced disease severity. Low numbers of inflammatory Ly6C^high monocytes, as well as inhibition of CCR2 and CCL2 in wild type animals by a specific L-RNA aptamer, mitigated disease parameters, indicating a pivotal role for CCR2⁺ inflammatory monocytes and the CCR2/CCL2 axis in fibrosis development. Of note, mice lacking splenic reservoirs failed to recruit monocytes to the skin and developed less fibrosis. Furthermore, enforced monocyte conversion into noninflammatory, patrolling Ly6C^low monocytes by a nuclear receptor Nur77-agonist also resulted in significantly impaired cutaneous inflammation and dermal fibrosis. Most evident, pronounced monocyte conversion in interferon stimulated gene 12-deficient mice with pronounced nuclear Nur77 signaling completely protected from dermal fibrosis. Our study shows that inflammatory monocytes that are recruited from splenic reservoirs play a key role in the development of skin fibrosis and can be therapeutically challenged by forced conversion via the Nur77/interferon stimulated gene 12 axis.