The transcription factors Batf3 and IRF8 are required for development of CD8α+ conventional dendritic cells (cDCs), but the basis for their actions was unclear. Here, we identify two novel Zbtb46+ progenitors that separately generate CD8α+ and CD4+ cDCs and arise directly from the common DC progenitor (CDP). Irf8 expression in the CDP depends on prior PU.1-dependent autoactivation, and specification of pre-CD8 DC progenitors requires IRF8 but not Batf3. However, upon pre-CD8 DC specification, Irf8 autoactivation becomes Batf3-dependent at a CD8α+ cDC-specific enhancer containing multiple AP1-IRF composite elements (AICEs) within the Irf8 superenhancer. CDPs from Batf3−/− mice that specify toward pre-CD8 DCs fail to complete CD8α+ cDC development due to decay of Irf8 autoactivation, and divert to the CD4+ cDC lineage.

Summary The two major lineages of classical dendritic cells (cDCs) express and require either IRF8 or IRF4 transcription factors for their development and function. IRF8-dependent cDCs promote anti-viral and T-helper 1 (Th1) cell responses, whereas IRF4-expressing cDCs have been implicated in controlling both Th2 and Th17 cell responses. Here, we have provided evidence that Kruppel-like factor 4 (Klf4) is required in IRF4-expressing cDCs to promote Th2 but not Th17 cell responses in vivo. Conditional Klf4 deletion within cDCs impaired Th2 cell responses during Schistosoma mansoni infection, Schistosoma egg antigen (SEA) immunization, and house dust mite challenge (HDM), without affecting cytotoxic T lymphocyte (CTL), Th1 and Th17 cell responses to herpes simplex virus, Toxoplasma gondii and Citrobacter rodentium infections. Further, Klf4 deletion reduced IRF4 expression in pre-cDCs and resulted in selective loss of IRF4-expressing cDCs subsets in several tissues. These results indicate that Klf4 guides a transcriptional program promoting IRF4-expressing cDCs heterogeneity.

The dendritic cells (DCs) of the immune system function in innate and adaptive responses by directing activity of various effector cells rather than serving as effectors themselves. DCs and closely related myeloid lineages share expression of many surface receptors, presenting a challenge in distinguishing their unique in vivo functions. Recent work has taken advantage of unique transcriptional programs to identify and manipulate murine DCs in vivo. This work has assigned several nonredundant in vivo functions to distinct DC lineages, consisting of plasmacytoid DCs and several subsets of classical DCs that promote different immune effector modules in response to pathogens. In parallel, a correspondence between human and murine DC subsets has emerged, underlying structural similarities for the DC lineages between these species. Recent work has begun to unravel the transcriptional circuitry that controls the development and diversification of DCs from common progenitors in the bone marrow.

Induction of the transcription factor Irf8 in the common dendritic cell progenitor (CDP) is required for classical type 1 dendritic cell (cDC1) fate specification, but the mechanisms controlling this induction are unclear. Here we identified Irf8 enhancers via chromatin profiling of DCs and used CRISPR/Cas9 genome editing to assess their roles in Irf8 regulation. An enhancer 32 kilobases downstream of the Irf8 transcriptional start site (+32 kb Irf8 ) that was active in mature cDC1s was required for the development of this lineage, but not for its specification. Instead, a +41 kb Irf8 enhancer previously thought to be active only in plasmacytoid DCs was found to also be transiently accessible in cDC1 progenitors, and deleting this enhancer prevented the induction of Irf8 in CDPs and abolished cDC1 specification. Thus, cryptic activation of the +41 kb Irf8 enhancer in DC progenitors is responsible for cDC1 fate specification.

Summary Both classical DCs (cDCs) and monocyte-derived DCs (Mo-DCs) are capable of cross-priming CD8+ T cells in response to cell-associated antigens. We found that Ly-6ChiTREML4− monocytes can differentiate into Zbtb46+ Mo-DCs in response to GM-CSF and IL-4, but that Ly-6ChiTREML4+ monocytes were committed to differentiate into Ly-6CloTREML4+ monocytes. Differentiation of Zbtb46+ Mo-DCs capable of efficient cross-priming required both GM-CSF and IL-4, and was accompanied by induction of Batf3 and Irf4. However, monocytes require IRF4, but not BATF3, to differentiate into Zbtb46+ Mo-DCs capable of cross-priming CD8+ T cells. Instead, Irf4−/− monocytes differentiate into macrophages in response to GM-CSF and IL-4. Thus, cDCs and Mo-DCs require distinct transcriptional programs of differentiation in acquiring the capacity to prime CD8+ T cells. These differences may be of consideration in the use of therapeutic DC vaccines based on Mo-DCs.