A DLL1-culture system to propagate murine patrolling monocytes from bone marrow progenitors

Abstract Patrolling monocytes (pMos) scavenge debris from vessel walls and mediate antibody-dependent cellular phagocytosis, making them attractive cell therapy candidates for cancer, atherosclerosis, and Alzheimer’s disease. However, methods to generate enhanced numbers of pMos are not available. To address this, we developed a two-step procedure to produce pMos in vitro from murine bone-marrow (BM). First, myeloid progenitors were expanded and enriched from BM using cytokines for four days. Second, expanded progenitors were differentiated into pMos on DLL1-expressing monolayers for eight days. We confirmed that in vitro grown pMos expressed the transcription factor Nr4a1 (Nur77) and other canonical pMos surface proteins, and depended on Notch signalling for their development. RNA sequencing revealed that in vitro pMos expressed hallmark pMos genes, including Cx3cr1, Itgax (CD11c), CD43, Fcγr4, and Cd274 (PD-L1), and their gene signatures clustered closely with in vivo blood and BM pMos. Transcriptomic and phenotypic analyses further demonstrated that in vitro pMos were distinct from classical BM macrophages. Phagocytosis assays demonstrated the function of in vitro pMos in cancer cell uptake. Adoptive transfer studies demonstrated that in vitro pMos persisted within the circulation and lung vasculature during the early post-transfer period compared with BM-derived macrophages, consistent with the vascular-patrolling properties of pMos. Adoptive transfer of pMos reduced lung tumour burden in a metastatic model, supporting an anti-tumoural role for pMos and their ability to mediate immune surveillance in vivo. These findings demonstrate that the DLL1 culture system allows for propagation of functional pMos, enabling studies of pMos biology and their therapeutic potential.