Figure 1.

The spectrum of macrophage activation. Macrophages can respond to a wide range of stimuli, resulting in the induction of a spectrum of macrophage activation states. These include M1 macrophages, involved in the protection against bacteria, and M2 macrophages, induced by Th2 cytokines, anti-inflammatory cytokines (IL-10, TGF-β), immune complexes and glucocorticoids, and participate in anti-parasite immune responses, tissue remodeling/wound healing and inhibition of immune responses. Furthermore, stimuli associated with chronic inflammation, including PGE₂, TNF-α and the TLR2-ligand PC3, induce a macrophage activation state distinct from the M1/M2 macrophages that have the potential to inhibit T cell proliferation. Defining molecules for murine and human M1 and M2 macrophages are indicated under each specific polarization state. GC, glucocorticoids; IC, immune complexes; IDO, indoleamine 2,3-dioxygenase; iNOS, inducible nitric oxide synthase.