Protective macrophages New insights into the role of synovial macrophages in inflammatory joint diseases

Macrophages are among the phylogenetically oldest cells of the immune system and are found in all tissues and organs. In addition to playing an important role in immune response against pathogenic microorganisms, these cells were previously described to play a vital role in chronic inflammatory diseases such as rheumatoid arthritis. Using novel techniques such as single-cell sequencing and advanced microscopy techniques it has now been shown that macrophages are far more versatile. Thus, these cells contribute considerably to tissue homeostasis and tissue regeneration. As each tissue has to fulfill special requirements, macrophages vary in their phenotype and function between organs. New data have now identified a specialised population of epithelioid macrophages that exert a protective and anti-inflammatory function in synovial tissue and prevent the initial onset as well as episodes of joint inflammation in rheumatoid arthritis.