Spatial Mapping of Myelopoiesis Reveals the Bone Marrow Niche for Monocyte Dendritic Cell Progenitors

Monocytes, macrophages and dendritic cells are indispensable for innate immunity. Myelopoiesis takes place in the bone marrow (BM) through differentiation of a common myeloid progenitor into monocyte dendritic cell progenitors (MDP) or granulocyte monocyte progenitors (GMP). MDP differentiate into myeloid progenitors (MoP) or common dendritic progenitors (cDP) that give rise to monocytes or dendritic cells (DC) repectively. GMP can also generate neutrophil-like monocytes via an intermediate monocyte progenitor [MP; Immunity. 2017 47(5):890] and neutrophils via a granulocyte progenitor (GP). CSF1 (M-CSF) is the key cytokine that regulates monopoiesis, CSF1 loss causes profound defects in monocyte, dendritic cell, macrophage and osteoclast generation. Classical studies using Csf1-/- chimeric mice demonstrated that CSF1 is produced by BM stromal cells. However the identity of these CSF1-producing cells is unknown. A major limitation is the lack of immunofluorescence protocols to map progenitor interaction with candidate niches. We have found that CD11b-Ly6C-CD117+CD115+ cells are MDP; Lin-Ly6C-CD117+CD16/32+CD115- cells are GMP; CD11b-Ly6C+CD117+CD115+ are MP/MoP; CD11b-Ly6C+CD117+CD115- are GP; CD11b+CD115+Ly6Chi and CD11b+CD115+Ly6Clo cells are classical and non-classical monocytes; and MHCIIhi reticular cells are DC. We used the markers above to map the 3D position of every myeloid cell in the sternum and assessed the relationships between myeloid progenitors, their offspring and candidate niches in situ with single cell resolution. To test whether the interactions observed were specific we obtained the X, Y and Z coordinates for every hematopoietic cell in the sternum (detected using αCD45 and αTer119). We then used these coordinates to randomly place each type of myeloid cell, at the same frequencies found in vivo, through the BM to generate a random distribution for each myeloid cell type. HSC localize to sinusoidal, arteriolar and endosteal niches. However, myeloid progenitors are exclusively perisinusoidal (mean MDP distance to sinusoids, arterioles, and endosteum observed 5, 134, and 105μm vs 9, 86, and 69µm in the random simulation). Myeloid progenitors rarely localize with HSC indicating that progenitors abandon the HSC niche upon differentiation. Strikingly, we found that granulopoiesis, monopoiesis and DCpoiesis occur in distinct sinusoidal locations and that MDP are tightly associated with sinusoids, dendritic cells and Ly6Clo monocytes (2.0 DC