Macrophages in Bone and Synovial Inflammation in Rheumatoid Arthritis

Abstract

Macrophages are members of the innate immune system; that originate from monocyte cells from the myeloid stem cells. In response to the tissue environment, monocytes differentiate into two subtypes of macrophages, M1, or M2. The M1 or classically activated macrophages (CAM) aggravate immune responses by releasing reactive oxygen species (ROS), and pro-inflammatory cytokines. The alternatively activated or M2 macrophages (AAM), are involved in the process of tissue repair by suppressing immune responses. The other type of macrophages are called tissue-resident macrophages and they originate from the primitive myeloid precursor cells of the yolk sac. Different types of macrophages affect the immune system through various pathways. Rheumatic diseases are inflammatory; disorders with an auto-immune basis that usually involve the musculoskeletal system, lungs, kidneys, and eyes. In rheumatoid arthritis (RA), the main affected organs are joints, bones, and cartilage; which cause joint swelling, tenderness, redness, decreased range of motion, and morning stiffness. The roles of macrophages in RA include the production of cytokines, activation of autoreactive B- and T-cells, and impairment of tissue repair in joints. The main purpose of this review article is to further discuss the underlying role of macrophages in inflammatory articular changes in RA patients.