Efficient expansion of mouse hematopoietic stem cells ex vivo by membrane anchored Angptl2

Hematopoietic stem cell (HSC) transplantation represents an important curative therapy for numerous hematological and immune diseases. Many efforts have been applied to achieve attainable ex vivo HSC expansion. We previously showed that angiopoietin-like proteins 2 (Angptl2) binds and activates the immune inhibitory receptor human leukocyte immunoglobulin (Ig)-like receptor B2 (LILRB2) to support the expansion of HSC. However, soluble Angptl2 is unstable and the downstream signaling would be attenuated by ligand-binding triggered receptor endocytosis, compromising the potential of Angptl2 to expand HSCs. We proposed that membrane anchored Angptl2 will overcome these limitations. In this study, we constructed the C-terminal and N-terminal anchored membrane Angptl2 (Cm-Angptl2 and Nm-Angptl2) by adding a transmembrane domain at the C-terminal or an anchor sequence at the N-terminal respectively. Both forms of Angptl2 showed efficient expression on the surface of feeder cells. Nm-Angptl2, but not Cm-Angptl2, induces a potent activation of LILRB2 reporter, indicating the fibronectin (FBN) domain at the C-terminus of Angptl2 is essential to stimulate LILRB2 signaling. Compared to soluble Angptl2, Nm-Angptl2 displays higher activities to activate LILRB2 reporter, and to promote the expansion of mouse HSCs as determined by transplantation and limiting dilution assay. Our study revealed the importance of FBN domain for Angptl2 to activate LILRB2 and demonstrated that Nm-Angptl2 have enhanced activities than the soluble protein in LILRB2 activation and HSC expansion, providing a strategy to explore the mode of ligand induced receptor signaling, and an optimized approach to expand HSCs ex vivo.