The crosstalk of monocyte-neutrophil in hair follicles regulates neutrophil transepidermal migration in contact dermatitis

The excessive accumulation of neutrophils within the epidermis is a significant hallmark of cutaneous diseases; however, the mechanisms governing neutrophil transepidermal migration (NTEM) remain inadequately understood. In this study, we develop trichromatic-fluorescence-labeled chimeric mice by utilizing Cx3cr1^GFP/+Lyz2^RFP/+ mice as bone marrow donors and Krt14^YFP/+ mice as recipients. This approach enables us to visualize the process of NTEM and the crosstalk between neutrophils and monocytes in a murine model of irritant contact dermatitis (ICD). Intravital imaging reveals a preferential transmigration of neutrophils through hair follicle (HF), where dermal neutrophils exhibit limited mobility and interact with dermal monocytes. Notably, 18 h following hapten exposure, dermal neutrophils continuously migrate toward HF regions and form clusters within 3 h. Importantly, MMP-9 is identified as essential for the NTEM process; the depletion of dermal monocytes results in a significant reduction of MMP-9 expression in the skin and inhibits the NTEM process in ICD. Mechanistically, dermal monocytes are found to be a crucial source of the cytokines TNF-α and CXCL2, which promote the upregulation of MMP-9 in neutrophils. Therefore, our results highlight HF regions as crucial gateways for dermal monocyte-modulated NTEM and provide visual insights into the crosstalk between neutrophils and monocytes in inflammatory skin disorders.