CSF1R and rheumatoid arthritis: Experimentation in synovial biopsies cannot address the possibility that circulating pro-inflammatory monocytes might replace synovial macrophages following anti-CSF1R treatment, but studies of monocyte migration using scintigraphy in RA patients has indicated that macrophage turnover in synovial tissue is slow, and even during successful therapy, the rate of monocyte immigration into synovial tissue is unchanged [50, 51], raising the possibility that CSF-1 maintains distinct monocyte populations in mouse and man.