Faculty Bibliography

Dendritic cells (DCs) facilitate the maintenance of immunological tolerance in the steady state. We report that transcription factor Etv3 is preferentially expressed in mature DCs, including tissue-derived migratory DCs (migDCs), and facilitates their homeostatic maturation and CCR7-dependent migration. Mice with global or DC-specific deletion of Etv3 manifested the expansion of CD25^low regulatory T (T_reg) cells, spontaneous activation of conventional T cells, and multiorgan T cell infiltration. Etv3 deficiency exacerbated TLR7-driven systemic lupus erythematosus (SLE)-like disease, supporting the reported genetic association of human ETV3 with SLE. Etv3-deficient migDCs up-regulated multiple costimulatory molecules, including OX40 ligand (OX40L/TNFSF4), whose blockade partially rescued the T_reg cell abnormalities. These results identify Etv3 as an essential regulator of the tolerogenic function of DCs and implicate it in the regulation of human autoimmunity.

Type I interferons (IFN-I), including IFN-β and multiple IFN-α subtypes, are key antiviral proteins encoded within a single large locus. Here, we studied how the chromatin organization of this locus controls cell-type-specific IFN-I responses. The professional IFN-I-producing plasmacytoid dendritic cells (pDCs) simultaneously induced nearly all IFN-I subtypes across the locus. During pDC differentiation, the IFN-I locus translocated into the active intranuclear chromosomal compartment. It also underwent cohesin-dependent reorganization of its three-dimensional chromatin structure; accordingly, IFN-I production by pDCs was cohesin dependent. The promoters of most IFN-I genes harbored open chromatin peaks specifically in pDCs. The preemptive intranuclear translocation and promoter opening of IFN-I genes in pDCs were mediated by the pDC-enriched transcription factor interferon regulatory factor (IRF)8. Several IRF8- and/or cohesin-binding regulatory regions within the IFN-I locus facilitated IFN-I gene induction in pDCs, as confirmed by single-cell multiome analysis. Thus, the unique IFN-I-producing capacity of pDCs is facilitated by anticipatory chromatin organization imparted by IRF8 and cohesin.

The cohesin complex extrudes chromatin loops, stopping at sites bound by CCCTC-binding factor (CTCF) and organizing chromosomes into topologically associated domains, yet biological implications of this process remain obscure. We show that cohesin controls the in vivo differentiation and function of murine antigen-presenting dendritic cells (DCs), particularly antigen cross-presentation and interleukin-12 (IL-12) secretion by type 1 conventional DCs (cDC1s). The chromatin organization of DCs was shaped by cohesin and the transcription factor IRF8, which facilitated chromatin looping and chromosome compartmentalization, respectively. Optimal expression of IRF8 itself required CTCF/cohesin binding sites demarcating the Irf8 gene. During DC activation, cohesin enabled the induction of a subset of genes that were preferentially located in Polycomb-repressed regions and enriched in more distal enhancers. Accordingly, deletion of CTCF sites flanking the Il12b gene in mice reduced IL-12 production by cDC1s. Our data reveal an essential role of cohesin-mediated chromatin folding in cell differentiation and function in vivo and its bidirectional cross-talk with lineage-specifying transcription factors.

Priming rare subdominant precursor B cells in germinal centers (GCs) is a central goal of vaccination to generate broadly neutralizing antibodies (bnAbs) against HIV. Multivalent immunogen display on protein nanoparticle scaffolds can promote such responses, but it also generates scaffold-specific B cells that could theoretically limit bnAb precursor expansion in GCs. We rationally designed DNA origami-based virus-like particles (DNA-VLPs) displaying a germline-targeting HIV envelope protein immunogen, which elicited no scaffold-specific antibody responses. Compared with a state-of-the-art clinical protein nanoparticle, these DNA-VLPs increased the expansion of epitope-specific GC B cells relative to off-target B cells and enhanced expansion of bnAb-lineage B cells in a humanized mouse model of CD4 binding site priming. Thus, minimizing off-target responses enhances bnAb priming and indicates that DNA-VLPs are a promising vaccine platform.

Plasmacytoid dendritic cells (pDCs) mount powerful antiviral type I interferon (IFN-I) responses, yet only a fraction of pDCs produces high levels of IFN-I. Here we report that peripheral pDCs in naive mice comprise three subsets (termed A, B and C) that represent progressive differentiation stages. This heterogeneity was generated by tonic IFN-I signaling elicited in part by the cGAS/STING and TLR9 DNA-sensing pathways. A small 'IFN-I-naive' subset (pDC-A) could give rise to other subsets; it was expanded in STING deficiency or after the IFN-I receptor blockade, but was abolished by exogenous IFN-I. In response to RNA viruses, pDC-A showed increased Bcl2-dependent survival and superior IFN-I responses, but was susceptible to virus infection. Conversely, the majority of pDCs comprised the 'IFN-I-primed' subsets (pDC-B/C) that showed lower IFN-I responses and poor survival, but did not support virus replication. Thus, tonic IFN-I signaling decreases the cytokine-producing capacity and survival of pDCs but increases their virus resistance, facilitating optimal antiviral responses.

Breast tumors harbor dynamic microenvironments, with multiple immune cell types playing opposing roles during tumor progression and/or response to therapy. Tumor-associated macrophages promote mammary tumorigenesis, whereas the role of mammary tissue macrophages (MTMs) remains incompletely understood. High-dimensional immunostaining of murine mammary tumor progression revealed that MTMs were localized in the peritumoral stroma and associated with eosinophils, which were previously shown to facilitate antitumor T cell responses. The depletion of MTMs accelerated tumorigenesis in both spontaneous and orthotopically transplanted mammary tumor models. Upon induction of a productive antitumor response via the depletion of regulatory T cells, MTMs assumed an alternatively activated state and expressed eotaxins, thereby attracting eosinophils to peritumoral regions. MTMs expressed the receptor for the alarmin IL-33, which induced both MTM activation and eosinophil recruitment. These results suggest that MTMs can sense IL-33 and recruit eosinophils to facilitate antitumor immunity, a mechanism that may operate during tumor progression and be further enhanced during productive antitumor responses.

Leukemia-driving mutations are thought to arise in hematopoietic stem cells (HSC), yet the natural history of their spread is poorly understood. We genetically induced mutations within endogenous murine HSC and traced them in unmanipulated animals. In contrast to mutations associated with clonal hematopoiesis (such as Tet2 deletion), the leukemogenic KrasG12D mutation dramatically accelerated HSC contribution to all hematopoietic lineages. The acceleration was mediated by KrasG12D-expressing multipotent progenitors (MPP) that lacked self-renewal but showed increased proliferation and aberrant transcriptome. The deletion of osteopontin, a secreted negative regulator of stem/progenitor cells, delayed the early expansion of mutant progenitors. KrasG12D-carrying cells showed increased CXCR4-driven motility in the bone marrow, and the blockade of CXCR4 reduced the expansion of MPP in vivo. Finally, therapeutic blockade of KRASG12D spared mutant HSC but reduced the expansion of mutant MPP and their mature progeny. Thus, transforming mutations facilitate their own spread from stem cells by reprogramming MPP, creating a preleukemic state via a two-component stem/progenitor circuit.

Plasmacytoid dendritic cells (pDCs) represent a unique cell type within the innate immune system. Their defining property is the recognition of pathogen-derived nucleic acids through endosomal Toll-like receptors and the ensuing production of type I interferon and other soluble mediators, which orchestrate innate and adaptive responses. We review several aspects of pDC biology that have recently come to the fore. We discuss emerging questions regarding the lineage affiliation and origin of pDCs and argue that these cells constitute an integral part of the dendritic cell lineage. We emphasize the specific function of pDCs as innate sentinels of virus infection, particularly their recognition of and distinct response to virus-infected cells. This essential evolutionary role of pDCs has been particularly important for the control of coronaviruses, as demonstrated by the recent COVID-19 pandemic. Finally, we highlight the key contribution of pDCs to systemic lupus erythematosus, in which therapeutic targeting of pDCs is currently underway.

The development of dendritic cells (DCs), including antigen-presenting conventional DCs (cDCs) and cytokine-producing plasmacytoid DCs (pDCs), is controlled by the growth factor Flt3 ligand (Flt3L) and its receptor Flt3. We genetically dissected Flt3L-driven DC differentiation using CRISPR-Cas9-based screening. Genome-wide screening identified multiple regulators of DC differentiation including subunits of TSC and GATOR1 complexes, which restricted progenitor growth but enabled DC differentiation by inhibiting mTOR signaling. An orthogonal screen identified the transcriptional repressor Trim33 (TIF-1γ) as a regulator of DC differentiation. Conditional targeting in vivo revealed an essential role of Trim33 in the development of all DCs, but not of monocytes or granulocytes. In particular, deletion of Trim33 caused rapid loss of DC progenitors, pDCs, and the cross-presenting cDC1 subset. Trim33-deficient Flt3⁺ progenitors up-regulated pro-inflammatory and macrophage-specific genes but failed to induce the DC differentiation program. Collectively, these data elucidate mechanisms that control Flt3L-driven differentiation of the entire DC lineage and identify Trim33 as its essential regulator.

The hierarchical model of hematopoiesis posits that self-renewing, multipotent hematopoietic stem cells (HSCs) give rise to all blood cell lineages. While this model accounts for hematopoiesis in transplant settings, its applicability to steady-state hematopoiesis remains to be clarified. Here, we used inducible clonal DNA barcoding of endogenous adult HSCs to trace their contribution to major hematopoietic cell lineages in unmanipulated animals. While the majority of barcodes were unique to a single lineage, we also observed frequent barcode sharing between multiple lineages, specifically between lymphocytes and myeloid cells. These results suggest that both single-lineage and multilineage contributions by HSCs collectively drive continuous hematopoiesis, and highlight a close relationship of myeloid and lymphoid development.