Figure 1.

Schematic diagram of macrophage sources, distribution and their activation. Macrophages are derived from bone marrow and embryonic stem cells, and some bone marrow-derived monocytes subsequently flow into tissues to develop into tissue-resident macrophages together with embryonic hepatocytes and yolk sac cells, and specifically differentiate into different functional macrophages at different sites. They have in common that both can polarize into pro-inflammatory (M1) and anti-inflammatory (M2) phenotypes. M1-type macrophages are activated by TLRs ligands and involve several transcription factors, such as NF-κB, STAT1, STAT5, IRF3, IRF5 and HIF-1α, while releasing pro-inflammatory and chemokines including TNF-α, IL-1α, IL-1β M2 macrophages activate transcription factors including STAT3, STAT6, IRF4, KLF4, JMJD3, PPARδ, PPARγ and release anti-inflammatory substances, cytokines and chemokines including IL-10, TGF-β, CCL17, CCL18 and CCL22, which promote tissue repair and regeneration, immunosuppression.