Figure 2.

Signal pathways of macrophage polarization. The figure illustrates several mechanisms underlying macrophage polarization and shows the feedback regulation between M1 and M2 signal pathways. Those include the activation of STAT1 mediated by IFN-γ receptor, increase in IRF5, NF-κB, as well as AP1 expression mediated by Toll-like receptor 4 (TLR4), enhanced AP1 expression mediated by cytokine receptor, activation of STAT6 and increased IRF4 mediated by IL-4 receptor, increased level of PPARγ mediated by fatty acid receptor, and enhanced expression in CREB by TLR4. The feedback regulation between M1 and M2 are implemented by STAT1-STAT6, IRF5-IRF4, NF-κB-PPARγ, AP1-CREB, and AP1-PPARγ, and they play essential roles in the initiation, development, and cessation of inflammatory diseases.