Thymic Macrophages (TM)

Thymic macrophages are immune cells that are crucial for clearing apoptotic thymocytes in the thymus. Remarkably, thymic macrophages only make up 0.1% of all cells in the thymus. Macrophages are a major population of thymic cells along with lymphocytes and epithelial cells. They are distributed in an apparently random manner throughout the cortex and medulla. Thymic macrophages express all of the various identifying characteristics associated with macrophages throughout the body including expression of a high level of class-I and II MHC products. They account for at least 99% of thymic Ia positivity. Thymic macrophages exhibit the property of binding thymic lymphocytes, and in some cases those bound lymphocytes are phagocytosed. This can result in the production of 'nurse cells'. Thymic macrophages can induce maturation of thymic lymphocytes and studies with non-thymic macrophages suggest that the macrophage-induced maturation is MHC restricted.