Control of Macrophage Function by the Microenvironment

Macrophages are versatile cells of the immune system that play an important role in both advancing and resolving inflammation through both signaling and phagocytosis. To perform their diverse functional roles, these cells respond dynamically to cues in their microenvironment. Although macrophage activation has been described as a binary and exclusive polarization to an inflammatory M1 function or a wound-healing M2 function, macrophages expressing markers associated with both M1 and M2 activity are observed in vivo. Using flow cytometry, the macrophage population’s response to combined M1 and M2 activation signals, presented either simultaneously or sequentially, was assessed. Macrophages exposed to a combination of LPS, IFN‑γ, IL‑4, and IL‑13 acquired a mixed activation state, with individual cells expressing both M1 marker CD86 and M2 marker CD206 instead of polarizing to discrete phenotypes. These results corroborate a multidimensional model of macrophage activation and demonstrate that phenotypic markers evolve with time and with exposure to complex signals. Further, the migration of macrophages along extracellular matrix materials characteristic of the wound environment was explored. Migration speed is sensitive to the composition and concentration of the matrix along which macrophages migrate. Activation with cocktails of LPS and IFN‑γ slowed macrophage migration, while treatment with IL‑4 and IL‑13 directed cells along linear tracks. The effect of macrophage activation on extracellular matrix (ECM) concentration-dependent macrophage migration was characterized and the effect of interrupting integrin-mediated cell-matrix interactions was assessed. Together, the studies help to elucidate the roles of the cytokine and ECM environment in macrophage polarization, plasticity, and migration