Phagocytic clearance of targeted cells with a synthetic ligand.

During the process of engulfment, phosphatidylserine is exposed on the surface of dead cells as an 'eat-me' signal and is recognized by Protein S (ProS), a secreted factor that also binds to the Mer tyrosine kinase (MerTK) on phagocytes. Despite its robust activity, this engulfment mechanism has not been exploited for therapeutic purposes. Here we develop a synthetic protein modality called Crunch (connector for removal of unwanted cell habitat) by modifying ProS, inspired by the high engulfment capability of the ProS-MerTK pathway. In Crunch, the phosphatidylserine-binding motif of ProS is replaced with a nanobody or single-chain variable fragment that recognizes the surface proteins of targeted cells. Green fluorescent protein nanobody-conjugated Crunch eliminates green fluorescent protein-expressing melanoma cells in transplantation mouse models. In addition, CD19B cells are eliminated by anti-CD19 single-chain variable fragment-conjugated Crunch, resulting in a therapeutic effect on systemic lupus erythematosus. Both mouse and human versions of Crunch are effective, establishing this synthetic ligand as a promising tool for the elimination of targeted cells.