M2 macrophages and their role in rheumatic diseases

As a component of the innate immune system, macrophages play a crucial role in host defense against a variety of microbes. Conventionally, macrophages have been classified as M1 and M2 depending on their phenotype and role in immune regulation. M1 macrophages are generally pro-inflammatory, while M2 (also known as alternatively activated macrophages) are anti-inflammatory. M1 macrophages release pro-inflammatory cytokines, reactive nitrogen, and oxygen intermediates, and kill pathogens, whereas their M2 counterparts participate in the resolution of inflammation, remodeling of tissue, angiogenesis, and tissue repair. Macrophages are also crucial in the pathogenesis of immune-inflammatory disorders, such as, arthritis. In this review, we discuss the markers of human M2 macrophages, the role played by them in inflammation or progression of rheumatic diseases, their potential to act as biomarkers, and, finally, therapeutic strategies aiming at altering/enhancing the macrophage phenotype.