Synovial macrophages in cartilage destruction and regeneration-lessons learnt from osteoarthritis and synovial chondromatosis

Inflammation is a critical process in disease pathogenesis and the restoration of tissue structure and function, for example, in joints such as the knee and temporomandibular. Within the innate immunity process, the body's first defense response in joints when physical and chemical barriers are breached is the synovial macrophages, the main innate immune effector cells, which are responsible for triggering the initial inflammatory reaction. Macrophage is broadly divided into three phenotypes of resting M0, pro-inflammatory M1-like (referred to below as M1), and anti-inflammatory M2-like (referred to below as M2). The synovial macrophage M1-to-M2 transition can affect the chondrogenic differentiation of mesenchymal stem cells (MSCs) in joints. On the other hand, MSCs can also influence the transition between M1 and M2. Failure of the chondrogenic differentiation of MSCs can result in persistent cartilage destruction leading to osteoarthritis. However, excessive chondrogenic differentiation of MSCs may cause distorted cartilage formation in the synovium, which is evidenced in the case of synovial chondromatosis. This review summarizes the role of macrophage polarization in the process of both cartilage destruction and regeneration, and postulates that the transition of macrophage phenotype in an inflammatory joint environment may play a key role in determining the fate of joint cartilage.