CD32b defines distinct dendritic cell lineages generated from the culture of bone marrow with GM-CSF

Culturing bone marrow (BM) with granulocyte macrophage-colony stimulating factor (GM-CSF) is the most commonly used standard method of generating mouse dendritic cells (DCs) in vitro, although the development of those MHC II^hi GM-CSF-induced DCs (GM-DCs) remains poorly elucidated. Here, we have characterized that in vitro-generated GM-DCs from the BM comprise two distinct subsets distinguished by the expression of CD32b, arising at a different time from a separate progenitor lineage. Monocyte-dendritic cell progenitors (MDPs) give rise to CD32b^- GM-DCs within the first week of the BM culture, while granulocyte-monocyte progenitors (GMPs) generate CD32b⁺ GM-DCs at a slower rate and become numerically dominant in the later stages. In addition, there exist separate populations of pre-GM-DCs in the BM culture; Ly6C^-CD32b^-MHC II^int pre-GM-DCs for CD32b^- GM-DCs and Ly6C^-CD32b⁺MHC II^int pre-GM-DCs for CD32b⁺ GM-DCs. These two GM-DC subsets also exhibit distinct functions; specifically, CD32b⁺ GM-DCs have enhanced capacity to stimulate CD4⁺ T cells. Notably, when adoptively transferred to mice treated with GM-CSF, GMPs selectively generate CD32b⁺ GM-DCs in vivo with delayed kinetics, compared to MDPs that give rise to heterogeneous GM-DC subsets in vivo rather quickly. Therefore, we have identified CD32b as a marker to differentiate two developmentally and functionally distinct GM-DCs, thus demonstrating the heterogeneity of GM-DCs, for the first time.