**8. Tie2/Angiopoietin signaling in immune cells**

The Tie2/Angiopoietin signaling axis was first identified for its key role in the regulation of angiogenic pathways, but this receptor complex is also gaining increasing recognition for its importance in peripheral immune cells. The receptor tyrosine kinase Tie2 (also known as Tek) interacts with its ligands, the angiopoietin family of proteins, to influence vascular development [57]. Studies in endothelial cells have shown that Tie2 is differentially regulated by its ligands Angiopoietin 1 (Angpt1) and Angiopoietin 2 (Angpt2). Angpt1 typically acts as an agonist for Tie2, while Angpt2 serves as an antagonist with several exceptions [58, 59]. Although Tie2/Angiopoietin signaling has been most studied for its role in regulation of vascular function, Tie2 is also expressed in a subpopulation of monocyte/macrophages called Tie2-expressing monocytes (TEMs) implicated in tumor formation and inflammation [60]. This review will discuss the mechanisms involved in the Tie2/Angiopoietin signaling axis and investigate the function of TEMs in various cellular contexts.
