Differential IRF8 Transcription Factor Requirement Defines Two Pathways of Dendritic Cell Development in Humans

Abstract

Summary The formation of mammalian dendritic cells (DCs) is controlled by multiple hematopoietic transcription factors, including IRF8. Loss of IRF8 exerts a differential effect on DC subsets, including plasmacytoid DCs (pDCs) and the classical DC lineages cDC1 and cDC2. In humans, cDC2-related subsets have been described including AXL + SIGLEC6 + pre-DC, DC2 and DC3. The origin of this heterogeneity is unknown. Using high-dimensional analysis, in vitro differentiation, and an allelic series of human IRF8 deficiency, we demonstrated that cDC2 (CD1c + DC) heterogeneity originates from two distinct pathways of development. The lymphoid-primed IRF8 hi pathway, marked by CD123 and BTLA, carried pDC, cDC1, and DC2 trajectories, while the common myeloid IRF8 lo pathway, expressing SIRPA, formed DC3s and monocytes. We traced distinct trajectories through the granulocyte-macrophage progenitor (GMP) compartment showing that AXL + SIGLEC6 + pre-DCs mapped exclusively to the DC2 pathway. In keeping with their lower requirement for IRF8, DC3s expand to replace DC2s in human partial IRF8 deficiency.

Subsets of CD1c+ DCs: Dendritic Cell Versus Monocyte Lineage

“…Here, it was noted that the CD1c + CD163 + cells, when stimulated with a mixture of TLR ligands followed by intracellular staining and flow cytometry, showed IL-1b production as high as monocytes, while the CD1c + CD163cells showed a low level production of this cytokine (see summary in Figure 2). Similar results were obtained for IL-10, while the two DC subsets produced comparable amounts of IL-12 (63).…”

“…lin − = lineage negative. The human data are a summary of a series of studies compiled in Figure 2 differential transcription factor requirements for the CD14 + and the CD5 + subset of DC2s were apparent, and the CD1c + CD14 + DC2s showed co-segregation of with monocytes (63). While the issue of lineage assignment of the CD1c + CD14 + cells in human blood still needs to be resolved, it has become clear that there may be more than two subsets of DC2.…”

“…This went along with depletion of cDC1 and pDC but increased blood monocytes and normal numbers of CD1c + DC2s. Analysis of subsets, however, revealed an almost complete absence of the CD1c + CD5 + cells, while the CD1c + CD5 − subset was expanded (63). Whereas the moderate level activity of IRF8 is apparently sufficient to allow for development of the CD14 + monocyte subsets and of the CD1c + CD14 + DC subset, it is not sufficient to allow for generation of CD1c + CD14 − CD5 + DCs.…”

“…Moreover, DC2s can be generated in vitro from CD34 + hematopoietic stem cells (64) and more specifically from cells with the phenotype of MLPs, CMPs, and GMPs (65). The generation of subsets of DC2 was only studied recently (63). Here, CMPs and CD33 + GMPs were found to have CD1c + CD14 + DC2 potential and the CD14 + subset was shown to segregate with monocytes.…”

“…Cytlak et al looked at CD1c + DCs in a kindred with a heterozygous dominant-negative mutation of IRF8, which led to a moderate deficiency of this transcription factor (63). This went along with depletion of cDC1 and pDC but increased blood monocytes and normal numbers of CD1c + DC2s.…”