Polarizing Macrophages In Vitro

Macrophages are a heterogeneous population of innate myeloid cells involved in health and disease, which are the most functionally diverse cells of the hematopoietic system. The main functions of macrophages include responding to pathogens and modulating the adaptive immune response, induction and resolution of inflammation, tissue repair, and homeostasis. Macrophages exhibit remarkable plasticity, in which the different populations of macrophages with distinct physiological and pathological roles can be developed in response to different stimuli. Depending on the types of stimuli that macrophages are exposed to, these cells will be able to polarize to M1 (pro-inflammatory) or M2 (anti-inflammatory) macrophages. M1 macrophages are induced by stimulation with IFNγ and LPS. Based on the stimuli and the achieved transcriptional changes, the M2 macrophages can be classified into four subdivisions: alternative activated macrophages (M2a, activated by IL-4 or IL-13), type 2 macrophages (M2b, activated by immune complexes and LPS), deactivated macrophages (M2c, activated by glucocorticoids or IL-10), and M2-like macrophages (M2d, activated by adenosines or IL-6). In this chapter, we describe the methods to generate different phenotypes of activated macrophages by adding certain stimuli and we also summarize the distinct markers and/or cytokines used for verification of each activated macrophage.