Immune Functions of the M2 Macrophages: Host Defense, Self-tolerance, and Autoimmunity

Abstract

This chapter summarizes the immune functions of M2 macrophages. A bona fide M2 macrophage activation is associated with Th2 immune response against helminth infections. Some intracellular pathogens can hijack macrophage activation, induce Th2/M2 immune response, and survive within M2 macrophages. This macrophage activation also shows some elements of an incomplete M1 activation, skewed toward an M2 state by the intracellular pathogen. The term tolerogenic macrophage is sometimes used to tag M2 macrophages. This term reflects one important trait of M2 macrophages, the maintenance of immunological self-tolerance, that is, to avoid the development of immunity against self-antigens, such as nucleic acids, nuclear fragments, cell debris, apoptotic bodies, and apoptotic cells. This function has been linked to the phagocytosing ability of macrophages, which clears potential self-antigens from the tissues. This function avoids the activation of other immune cells, and eventually acts against the development of inflammation and autoimmunity. Tolerogenic immune behavior of M2 macrophages is also present in the tumor environment, where M2 traits impede the recognition and elimination of tumor cells. This chapter discusses these immune functions of M2 macrophages.