Toolbox of Clickable Benzylguanines for Labeling of HoxB8-Derived Macrophages via SNAP-Tag and Bioorthogonal Chemistry.

Inflammation is a dynamic process which importantly involves migration of immune cells. Understanding the onset, acute phase and resolution of inflammation is greatly facilitated by their imaging. However, immune cells are sensitive, difficult to genetically manipulate and prone to changes in response to contact, hindering the application of well-established cell labeling methods. We generated the first reported SNAP-tag expressing conditionally immortalized early hematopoietic progenitor cells (ER-HoxB8) and synthesized benzylguanine (BG) analogs with bioorthogonal groups for SNAP-tag mediated cell surface labeling. Comparative investigation of SNAP-tag positive HeLa cells, ER-HoxB8 progenitor cells and ER-HoxB8-derived macrophages as well as neutrophiles revealed remarkable differences in labeling depending on the bioorthogonal group and fluorescent reporter used. HeLa cells and ER-HoxB8 progenitor cells were efficiently labeled with BG analogs bearing an azide and a dioxolan-fused -cyclooctene (dTCO). When we differentiated ER-HoxB8 cells into macrophages, labeling was less bright due to lower SNAP-tag expression and only the tetrazine ligation of dTCO proved suitable for cell-type specific labeling. These results show that exploring different reactions and bioorthogonal groups is important to address the challenge of labeling differentiated immune cells. Combining the SNAP-tag with click chemistry provides a modular approach for cell-specific labeling and the combination of SNAP-tag and dTCO presents an improved system for labeling ER-HoxB8-derived macrophages.