Skin macrophage subsets in health and disease.

Macrophages are heterogeneous tissue-resident immune cells distributed throughout the body. The majority of tissue-resident macrophages are seeded in the peripheral tissues during embryogenesis, and these embryonic precursors form tissue-specific macrophages dependent on their specific microenvironment. The skin is the first line of immune defence and contains various uniquely differentiated tissue-resident macrophage populations, including Langerhans cells in the epidermis, perivascular and sensory nerve-associated macrophages in the dermis, and LYVE1 + and CD169 + macrophages in the hypodermis. Cutaneous melanoma is the most aggressive skin cancer, and macrophages are abundant immune cells found in these tumours. Due to the diversity of macrophage subsets in the skin, macrophages have highly heterogeneous roles in melanoma lesions, including exerting both tumour-suppressing and promoting functions. Additionally, macrophage subsets in the tumour-draining lymph node play important roles in tumour immunity and progression. A comprehensive understanding of the different macrophage subsets in both tumour and tumour-draining lymph nodes may lead to improved therapeutic outcomes for cutaneous melanoma.