Two birds with one stone: human SIRPα nanobodies for functional modulation and in vivo imaging of myeloid cells

Signal-regulatory protein (SIRP) expressed by myeloid cells is of particular interest for therapeutic strategies targeting the interaction between SIRP and the "dont eat me" ligand CD47 and as a marker to monitor macrophage infiltration into tumor lesions. To address both approaches, we developed a set of novel human SIRP (hSIRP)-specific nanobodies (Nbs). We identified three high-affinity Nbs targeting the hSIRP/hCD47 interface, thereby enhancing antibody-dependent cellular phagocytosis (ADCP). For non-invasive in vivo imaging, we chose S36 Nb as a non-modulating binder. By quantitative positron emission tomography (PET) in novel hSIRP/hCD47 knock-in (KI) mice, we demonstrated the applicability of 64Cu-hSIRP-S36 Nb to visualize tumor infiltration of myeloid cells. We envision that the hSIRP-Nbs presented in this study have potential as versatile probes, including novel myeloid-specific checkpoint inhibitors for combinatorial treatment approaches and for in vivo stratification and monitoring of individual responses during cancer immunotherapies.