In line with an important anti-inflammatory role of the Trem2+ lining macrophage subset, selective depletion of murine synovial lining macrophages (1) as well as global deletion of resident synovial macrophages (46) resulted in early onset and exacerbation of arthritis in mice, emphasizing their crucial role in the maintenance of joint homeostasis and the prevention of synovial inflammation. The gene discussed is TREM2; the disease is Arthritis.