Notch Signaling Facilitates In Vitro Generation of Cross-Presenting Classical Dendritic Cells

Synthetic Melanin Acts as Efficient Peptide Carrier in Cancer Vaccine Strategy

We previously reported that a novel peptide vaccine platform, based on synthetic melanin nanoaggregates, triggers strong cytotoxic immune responses and significantly suppresses tumor growth in mice. However, the mechanisms underlying such an efficacy remained poorly described. Herein, we investigated the role of dendritic cells (DCs) in presenting the antigen embedded in the vaccine formulation, as well as the potential stimulatory effect of melanin upon these cells, in vitro by coculture experiments and ELISA/flow cytometry analysis. The vaccine efficiency was evaluated in FLT3-L^-/- mice constitutively deficient in DC1, DC2, and pDCs, in Zbtb46^DTR chimera mice deficient in DC1 and DC2, and in Langerin^DTR mice deficient in dermal DC1 and Langerhans cells. We concluded that DCs, and especially migratory conventional type 1 dendritic cells, seem crucial for mounting the immune response after melanin-based vaccination. We also assessed the protective effect of L-DOPA melanin on peptides from enzymatic digestion, as well as the biodistribution of melanin-peptide nanoaggregates, after subcutaneous injection using [¹⁸F]MEL050 PET imaging in mice. L-DOPA melanin proved to act as an efficient carrier for peptides by fully protecting them from enzymatic degradation. L-DOPA melanin did not display any direct stimulatory effects on dendritic cells in vitro. Using PET imaging, we detected melanin-peptide nanoaggregates up to three weeks after subcutaneous injections within the secondary lymphoid tissues, which could explain the sustained immune response observed (up to 4 months) with this vaccine technology.

Hox-driven conditional immortalization of myeloid and lymphoid progenitors: Uses, advantages, and future potential

Those who study macrophage biology struggle with the decision whether to utilize primary macrophages derived directly from mice or opt for the convenience and genetic tractability of immortalized macrophage-like cell lines in in vitro studies. Particularly when it comes to studying phagocytosis and phagosomal maturation-a signature cellular process of the macrophage-many commonly used cell lines are not representative of what occurs in primary macrophages. A system developed by Mark Kamps' group, that utilizes conditionally constitutive activity of Hox transcription factors (Hoxb8 and Hoxa9) to immortalize differentiation-competent myeloid cell progenitors of mice, offers an alternative to the macrophage/macrophage-like dichotomy. In this resource, we will review the use of Hoxb8 and Hoxa9 as hematopoietic regulators to conditionally immortalize murine hematopoietic progenitor cells which retain their ability to differentiate into many functional immune cell types including macrophages, neutrophils, basophils, osteoclasts, eosinophils, dendritic cells, as well as limited potential for the generation of lymphocytes. We further demonstrate that the use of macrophages derived from Hoxb8/Hoxa9 immortalized progenitors and their similarities to bone marrow-derived macrophages. To supplement the existing data, mass spectrometry-based proteomics, flow cytometry, cytology, and in vitro phagosomal assays were conducted on macrophages derived from Hoxb8 immortalized progenitors and compared to bone marrow-derived macrophages and the macrophage-like cell line J774. We additionally propose the use of a standardized nomenclature to describe cells derived from the Hoxb8/Hoxa9 system in anticipation of their expanded use in the study of leukocyte cell biology.

Human dendritic cell subsets: An updated view of their ontogeny and functional specialization

Human DCs have been divided into several subsets based on their phenotype and ontogeny. Recent high throughput single-cell methods have revealed additional heterogeneity within human DC subsets, and new subpopulations have been proposed. In this review, we provide an updated view of the human DC subsets and of their ontogeny supported by recent clinical studies . We also summarize their main characteristics including their functional specialization.

NF kappa B regulator Bcl3 controls development and function of classical dendritic cells required for resistance to Toxoplasma gondii

The atypical IκB family member Bcl3 associates with p50/NF-κB1 or p52/NF-κB2 homodimers in the nucleus, and positively or negatively modulates transcription in a context-dependent manner. In mice lacking Bcl3 globally or specifically in CD11c+ cells, we previously reported that Toxoplasma gondii infection is uniformly fatal and is associated with an impaired Th1 immune response. Since Bcl3 expression in dendritic cells (DC) is pivotal for antigen presentation and since classical DCs (cDC) are major antigen presenting cells, we investigated the role of Bcl3 specifically in cDCs in vivo by crossing Zbtb46 cre mice with Bcl3flx/flx mice. Bcl3flx/flx Zbtb46 cre mice were as susceptible to lethal T. gondii infection as total Bcl3-/- mice and generated poor Th1 immune responses. Consistent with this, compared to wildtype controls, splenic Xcr1+ Bcl3-deficient cDC1 cells were defective in presenting Ova antigen to OT-I cells both for Ova257-264 peptide and after infection with Ovalbumin-expressing T. gondii. Moreover, splenic CD4+ and CD8+ T cells from infected Bcl3flx/flx Zbtb46 cre mice exhibited decreased T. gondii-specific priming as revealed by both reduced cytokine production and reduced T. gondii-specific tetramer staining. In vitro differentiation of cDCs from bone marrow progenitors also revealed Bcl3-dependent cDC-specific antigen-presentation activity. Consistent with this, splenocyte single cell RNA seq (scRNAseq) in infected mice revealed Bcl3-dependent expression of genes involved in antigen processing in cDCs. We also identified by scRNAseq, a unique Bcl3-dependent hybrid subpopulation of Zbtb46+ DCs co-expressing the monocyte/macrophage transcription factor Lysozyme M. This subpopulation exhibited Bcl3-dependent expansion after infection. Likewise, by flow cytometry we identified two T. gondii-induced hybrid subpopulations of Bcl3-dependent cDC1 and cDC2 cells both expressing monocyte/macrophage markers, designated as icDC1 and icDC2. Together, our results indicate that Bcl3 in classical DCs is a major determinant of protective T cell responses and survival in T. gondii-infection.

Application of dendritic cells in tumor immunotherapy and progress in the mechanism of anti-tumor effect of Astragalus polysaccharide (APS) modulating dendritic cells: a review

Dendritic cells (DCs) are potent antigen-presenting cells (APCs) that are essential in mediating the body's natural and adaptive immune responses. The body can regulate the function of DCs in various ways to enhance their antitumor effects. In the tumour microenvironment (TME), antigen-specific T cell responses are initiated through DC processing and delivery of tumour-associated antigens (TAAs); conversely, tumour cells inhibit DC recruitment by releasing metabolites, cytokines and other regulatory TME and function. Different subpopulations of DCs exist in tumour tissues, and their functions vary. Insight into DC subgroups in TME allows assessment of the effectiveness of tumour immunotherapy. Astragalus polysaccharide (APS) is the main component of the Chinese herb Astragalus membranaceus. The study found that the antitumor effects of APS are closely related to DCs. APS can promote the expression of surface molecules CD80 and CD86, promote the maturation of DCs, and activate CTL to exert antitumor effects. We reviewed the application of DCs in tumor immunotherapy and the mechanism of modulation of DCs by Astragalus polysaccharide to provide new directions and strategies for tumor therapy and new drug development.

Inhibition of Notch Signaling Stimulates Osteoclastogenesis From the Common Trilineage Progenitor Under Inflammatory Conditions

Osteoclasts, macrophages and dendritic cells (DCs) can be derived from a common trilineage myeloid progenitor of hematopoietic origin. Progenitor commitment is susceptible to regulation through Notch signaling. Our aim was to determine the effects of Notch modulation on trilineage progenitor commitment and functional properties of differentiated cells under inflammatory conditions. We used the conditional inducible CX3CR1CreERT2 mouse strain to achieve overexpression of the Notch 1 intracellular domain (NICD1) or to inhibit Notch signaling via deletion of the transcription factor RBP-J in a bone marrow population, used as a source of the trilineage progenitor (CD45+Ly6G−CD3−B220−NK1.1−CD11b–/loCD115+). Cre-recombinase, under the control of the CX3CR1 promoter, expressed in the monocyte/macrophage lineage, was induced in vitro by 4-hydroxytamoxifen. Differentiation of osteoclasts was induced by M-CSF/RANKL; macrophages by M-CSF; DCs by IL-4/GM-CSF, and inflammation by LPS. Functionally, DCs were tested for the ability to process and present antigen, macrophages to phagocytose E. coli particles, and osteoclasts to resorb bone and express tartrate-resistant acid phosphatase (TRAP). We found that Notch 1 signal activation suppressed osteoclast formation, whereas disruption of the Notch canonical pathway enhanced osteoclastogenesis, resulting in a higher number and size of osteoclasts. RANK protein and Ctsk gene expression were upregulated in osteoclastogenic cultures from RBP-J+ mice, with the opposing results in NICD1+ mice. Notch modulation did not affect the number of in vitro differentiated macrophages and DCs. However, RBP-J deletion stimulated Il12b and Cd86 expression in macrophages and DCs, respectively. Functional assays under inflammatory conditions confirmed that Notch silencing amplifies TRAP expression by osteoclasts, whereas the enhanced phagocytosis by macrophages was observed in both NICD1+ and RBP-J+ strains. Finally, antigen presentation by LPS-stimulated DCs was significantly downregulated with NICD1 overexpression. This experimental setting allowed us to define a cell-autonomous response to Notch signaling at the trilineage progenitor stage. Although Notch signaling modulation affected the activity of all three lineages, the major effect was observed in osteoclasts, resulting in enhanced differentiation and function with inhibition of canonical Notch signaling. Our results indicate that Notch signaling participates as the negative regulator of osteoclast activity during inflammation, which may be relevant in immune and bone diseases.

Pulmonary Alveolar Proteinosis and Multiple Infectious Diseases in a Child with Autosomal Recessive Complete IRF8 Deficiency

BACKGROUND

Autosomal recessive (AR) complete IRF8 deficiency is a rare severe inborn error of immunity underlying an absence of blood myeloid mononuclear cells, intracerebral calcifications, and multiple infections. Only three unrelated patients have been reported.

MATERIALS AND METHODS

We studied an Argentinian child with multiple infectious diseases and severe pulmonary alveolar proteinosis (PAP). We performed whole-exome sequencing (WES) and characterized his condition by genetic, immunological, and clinical means.

RESULTS

The patient was born and lived in Argentina. He had a history of viral pulmonary diseases, disseminated disease due to bacillus Calmette-Guérin (BCG), PAP, and cerebral calcifications. He died at the age of 10 months from refractory PAP. WES identified two compound heterozygous variants in IRF8: c.55del and p.R111*. In an overexpression system, the p.R111* cDNA was loss-of-expression, whereas the c.55del cDNA yielded a protein with a slightly lower molecular weight than the wild-type protein. The mutagenesis of methionine residues downstream from c.55del revealed a re-initiation of translation. However, both variants were loss-of-function in a luciferase assay, suggesting that the patient had AR complete IRF8 deficiency. The patient had no blood monocytes or dendritic cells, associated with neutrophilia, and normal counts of NK and other lymphoid cell subsets.

CONCLUSION

We describe the fourth patient with AR complete IRF8 deficiency. This diagnosis should be considered in children with PAP, which is probably due to the defective development or function of alveolar macrophages.

Dendritic Cells and Their Immunotherapeutic Potential for Treating Type 1 Diabetes

Type 1 diabetes (T1D) results from the destruction of pancreatic beta cells through a process that is primarily mediated by T cells. Emerging evidence suggests that dendritic cells (DCs) play a crucial role in initiating and developing this debilitating disease. DCs are professional antigen-presenting cells with the ability to integrate signals arising from tissue infection or injury that present processed antigens from these sites to naïve T cells in secondary lymphoid organs, thereby triggering naïve T cells to differentiate and modulate adaptive immune responses. Recent advancements in our knowledge of the various subsets of DCs and their cellular structures and methods of orchestration over time have resulted in a better understanding of how the T cell response is shaped. DCs employ various arsenal to maintain their tolerance, including the induction of effector T cell deletion or unresponsiveness and the generation and expansion of regulatory T cell populations. Therapies that suppress the immunogenic effects of dendritic cells by blocking T cell costimulatory pathways and proinflammatory cytokine production are currently being sought. Moreover, new strategies are being developed that can regulate DC differentiation and development and harness the tolerogenic capacity of these cells. Here, in this report, we focus on recent advances in the field of DC immunology and evaluate the prospects of DC-based therapeutic strategies to treat T1D.

Human cDC1 display constitutive activation of the UPR Sensor IRE1

The intracellular mechanisms safeguarding DC function are of biomedical interest in several immune‐related diseases. Type 1 conventional DCs (cDC1s) are prominent targets of immunotherapy typified by constitutive activation of the unfolded protein response (UPR) sensor IRE1. Through its RNase domain, IRE1 regulates key processes in cDC1s including survival, ER architecture and function. However, most evidence linking IRE1 RNase with cDC1 biology emerges from mouse studies and it is currently unknown whether human cDC1s also activate the enzyme to preserve cellular homeostasis.

In this work, we report that human cDC1s constitutively activate IRE1 RNase in steady state, which is evidenced by marked expression of IRE1, XBP1s, and target genes, and low levels of mRNA substrates of the IRE1 RNase domain. On a functional level, pharmacological inhibition of the IRE1 RNase domain curtailed IL‐12 and TNF production by cDC1s upon stimulation with TLR agonists. Altogether, this work demonstrates that activation of the IRE1/XBP1s axis is a conserved feature of cDC1s across species and suggests that the UPR sensor may also play a relevant role in the biology of the human lineage.

The In Vitro Differentiation of Human CD141+CLEC9A+ Dendritic Cells from Mobilized Peripheral Blood CD34+ Hematopoietic Stem Cells

As shown in various preclinical studies, conventional type-1 dendritic cells, or cDC1s, play a critical role in the immunological rejection of tumors and in the defense against pathogens. This indispensability stems from their potent capacity to activate cytotoxic T cells, especially via the cross-presentation of exogenous antigens. For this reason, cDC1s have become an attractive target for immunotherapy. Here we report a simplified method for generating large numbers of cDC1-like cells in vitro from mobilized human peripheral blood CD34+ hematopoietic stem cells using FMS-like tyrosine kinase 3 ligand (FLT3L) and granulocyte-macrophage colony-stimulating factor (GM-CSF). An important aspect of this Protocol is the growth of cells on a non-tissue culture-treated surface rather than on a tissue culture-treated surface since the latter suppresses cDC1-marker expression. The resulting CD11c+ DCs express high levels of cDC1-specific markers such as CD141, CLEC9A, TLR3, and several DC maturation markers. Compared to alternative differentiation methods, this method generates large numbers of cDC1-like cells without the need for immortalized feeder cells and should prove useful for studying cDC1 immunobiology and clinical applications of this DC subset. © 2022 Wiley Periodicals LLC. Basic Protocol: Generation of human CD141+CLEC9A+ dendritic cells from mobilized peripheral blood CD34+ hematopoietic stem cells Support Protocol: Flow cytometric immunophenotyping of CD141+ dendritic cells.

Spatial transcriptomic profiles of mouse uterine microenvironments at pregnancy day 7.5†

Uterine dysfunctions lead to fertility disorders and pregnancy complications. Normal uterine functions at pregnancy depend on crosstalk among multiple cell types in uterine microenvironments. Here, we performed the spatial transcriptomics and single-cell RNA-seq assays to determine local gene expression profiles at the embryo implantation site of the mouse uterus on pregnancy day 7.5 (D7.5). The spatial transcriptomic annotation identified 11 domains of distinct gene signatures, including a mesometrial myometrium, an anti-mesometrial myometrium, a mesometrial decidua enriched with natural killer cells, a vascular sinus zone for maternal vessel remodeling, a fetal-maternal interface, a primary decidual zone, a transition decidual zone, a secondary decidual zone, undifferentiated stroma, uterine glands, and the embryo. The scRNA-Seq identified 12 types of cells in the D7.5 uterus including three types of stromal fibroblasts with differentiated and undifferentiated markers, one cluster of epithelium including luminal and glandular epithelium, mesothelium, endothelia, pericytes, myelomonocytic cell, natural killer cells, and lymphocyte B. These single-cell RNA signatures were then utilized to deconvolute the cell-type compositions of each individual uterine microenvironment. Functional annotation assays on spatial transcriptomic data revealed uterine microenvironments with distinguished metabolic preferences, immune responses, and various cellular behaviors that are regulated by region-specific endocrine and paracrine signals. Global interactome among regions is also projected based on the spatial transcriptomic data. This study provides high-resolution transcriptome profiles with locality information at the embryo implantation site to facilitate further investigations on molecular mechanisms for normal pregnancy progression.

Notch signaling pathway: architecture, disease, and therapeutics

The NOTCH gene was identified approximately 110 years ago. Classical studies have revealed that NOTCH signaling is an evolutionarily conserved pathway. NOTCH receptors undergo three cleavages and translocate into the nucleus to regulate the transcription of target genes. NOTCH signaling deeply participates in the development and homeostasis of multiple tissues and organs, the aberration of which results in cancerous and noncancerous diseases. However, recent studies indicate that the outcomes of NOTCH signaling are changeable and highly dependent on context. In terms of cancers, NOTCH signaling can both promote and inhibit tumor development in various types of cancer. The overall performance of NOTCH-targeted therapies in clinical trials has failed to meet expectations. Additionally, NOTCH mutation has been proposed as a predictive biomarker for immune checkpoint blockade therapy in many cancers. Collectively, the NOTCH pathway needs to be integrally assessed with new perspectives to inspire discoveries and applications. In this review, we focus on both classical and the latest findings related to NOTCH signaling to illustrate the history, architecture, regulatory mechanisms, contributions to physiological development, related diseases, and therapeutic applications of the NOTCH pathway. The contributions of NOTCH signaling to the tumor immune microenvironment and cancer immunotherapy are also highlighted. We hope this review will help not only beginners but also experts to systematically and thoroughly understand the NOTCH signaling pathway.

Tim-3 adapter protein Bat3 acts as an endogenous regulator of tolerogenic dendritic cell function

Dendritic cells (DCs) sense environmental cues and adopt either an immune-stimulatory or regulatory phenotype, thereby fine-tuning immune responses. Identifying endogenous regulators that determine DC function can thus inform the development of therapeutic strategies for modulating the immune response in different disease contexts. Tim-3 plays an important role in regulating immune responses by inhibiting the activation status and the T cell priming ability of DC in the setting of cancer. Bat3 is an adaptor protein that binds to the tail of Tim-3; therefore, we studied its role in regulating the functional status of DCs. In murine models of autoimmunity (experimental autoimmune encephalomyelitis) and cancer (MC38-OVA-implanted tumor), lack of Bat3 expression in DCs alters the T cell compartment-it decreases TH1, TH17 and cytotoxic effector cells, increases regulatory T cells, and exhausted CD8+ tumor-infiltrating lymphocytes, resulting in the attenuation of autoimmunity and acceleration of tumor growth. We found that Bat3 expression levels were differentially regulated by activating versus inhibitory stimuli in DCs, indicating a role for Bat3 in the functional calibration of DC phenotypes. Mechanistically, loss of Bat3 in DCs led to hyperactive unfolded protein response and redirected acetyl-coenzyme A to increase cell intrinsic steroidogenesis. The enhanced steroidogenesis in Bat3-deficient DC suppressed T cell response in a paracrine manner. Our findings identified Bat3 as an endogenous regulator of DC function, which has implications for DC-based immunotherapies.

Single-cell transcriptional profiling informs efficient reprogramming of human somatic cells to cross-presenting dendritic cells

Type 1 conventional dendritic cells (cDC1s) are rare immune cells critical for the induction of antigen-specific cytotoxic CD8⁺ T cells, although the genetic program driving human cDC1 specification remains largely unexplored. We previously identified PU.1, IRF8, and BATF3 transcription factors as sufficient to induce cDC1 fate in mouse fibroblasts, but reprogramming of human somatic cells was limited by low efficiency. Here, we investigated single-cell transcriptional dynamics during human cDC1 reprogramming. Human induced cDC1s (hiDC1s) generated from embryonic fibroblasts gradually acquired a global cDC1 transcriptional profile and expressed antigen presentation signatures, whereas other DC subsets were not induced at the single-cell level during the reprogramming process. We extracted gene modules associated with successful reprogramming and identified inflammatory signaling and the cDC1-inducing transcription factor network as key drivers of the process. Combining IFN-γ, IFN-β, and TNF-α with constitutive expression of cDC1-inducing transcription factors led to improvement of reprogramming efficiency by 190-fold. hiDC1s engulfed dead cells, secreted inflammatory cytokines, and performed antigen cross-presentation, key cDC1 functions. This approach allowed efficient hiDC1 generation from adult fibroblasts and mesenchymal stromal cells. Mechanistically, PU.1 showed dominant and independent chromatin targeting at early phases of reprogramming, recruiting IRF8 and BATF3 to shared binding sites. The cooperative binding at open enhancers and promoters led to silencing of fibroblast genes and activation of a cDC1 program. These findings provide mechanistic insights into human cDC1 specification and reprogramming and represent a platform for generating patient-tailored cDC1s, a long-sought DC subset for vaccination strategies in cancer immunotherapy.

Epigenetics of Dendritic Cells in Tumor Immunology

Dendritic cells (DCs) are professional antigen-presenting cells with the distinctive property of inducing the priming and differentiation of naïve CD4+ and CD8+ T cells into helper and cytotoxic effector T cells to develop efficient tumor-immune responses. DCs display pathogenic and tumorigenic antigens on their surface through major histocompatibility complexes to directly influence the differentiation of T cells. Cells in the tumor microenvironment (TME), including cancer cells and other immune-infiltrated cells, can lead DCs to acquire an immune-tolerogenic phenotype that facilitates tumor progression. Epigenetic alterations contribute to cancer development, not only by directly affecting cancer cells, but also by their fundamental role in the differentiation of DCs that acquire a tolerogenic phenotype that, in turn, suppresses T cell-mediated responses. In this review, we focus on the epigenetic regulation of DCs that have infiltrated the TME and discuss how knowledge of the epigenetic control of DCs can be used to improve DC-based vaccines for cancer immunotherapy.

Generation of cDC-like cells from human induced pluripotent stem cells via Notch signaling

Background

Dendritic cells (DCs) play critical roles in regulating the innate and adaptive immune responses, and have long been a major focus of cancer immunotherapy. Accumulating evidence suggests that conventional type 1 DCs (cDC1s) excel in cross-presentation of exogenous antigens on MHC-I molecules and induction of antitumor CD8⁺ T cell immunity; however, obtaining large numbers of cDC1s is difficult. The use of reprogramming and differentiation technology is advantageous for obtaining unlimited numbers of autologous cDC1s especially for therapeutic interventions where repeated vaccinations are required. However, generation of cDC1s from human induced pluripotent stem cells (iPSCs) remains elusive.

Methods

Human iPSCs established from peripheral blood T cells and monocytes were differentiated to myeloid cells under on-feeder or feeder-free culture conditions in vitro. Phenotype, genomic and transcriptomic signature, and function of human iPSC-derived DCs were analyzed. The role of Notch signaling for the generation of HLA-DR⁺ cells from human iPSCs was interrogated by a loss- and gain-of-function approach.

Results

Flow cytometric analyses and single-cell profiling of HLA-DR⁺ cells revealed that human iPSCs gave rise to CD141⁺XCR1⁺CLEC9A⁺ cells (cDC1s), CLEC4A^hiCLEC10A^–CD1c⁺ cells (cDC2As), CLEC4A^loCLEC10A⁺CD1c⁺ cells (cDC2Bs), CD163^–CD5⁺CD1c⁺ cells (CD5⁺cDC2s), and AXL⁺SIGLEC6⁺ cells (AS-DCs) on OP9 feeder cells expressing the Notch ligand delta-like 1 (OP9-DL1) while the majority of iPSC-derived cells differentiated on OP9 cells were CD163⁺CD5^–CD1c⁺ cells (DC3s) and monocytes. Plasmacytoid DCs were not differentiated from iPSCs on either OP9 or OP9-DL1 cells. Inhibition of Notch signaling during co-culture of iPSC-derived CD34⁺ hematopoietic progenitor cells with OP9-DL1 cells abrogated generation of cDC1s, cDC2As, cDC2Bs, CD5⁺cDC2s, and AS-DCs but increased frequency of DC3s. Notch-activated human iPSC-derived XCR1⁺CLEC9A⁺HLA-DR⁺CD11c⁺ cells exhibited similar gene expression profile with peripheral blood cDC1s. Human iPSC-derived DCs have phagocytic, T-cell proliferative, and cytokine-producing functions.

Conclusions

Our study demonstrates a critical role of Notch signaling in regulating developmental pathway of human cDCs. These findings provide insights into the future development of personalized treatment with unlimited numbers of autologous cDCs from human iPSCs.

CRISPR/Cas9 editing in conditionally immortalized HoxB8 cells for studying gene regulation in mouse dendritic cells

HoxB8 multipotent progenitors (MPP) are obtained by expression of the estrogen receptor hormone binding domain (ERHBD) HoxB8 fusion gene in mouse BM cells. HoxB8 MPP generate (i) the full complement of DC subsets (cDC1, cDC2, and pDC) in vitro and in vivo and (ii) allow CRISPR/Cas9-mediated gene editing, for example, generating homozygous deletions in cis-acting DNA elements at high precision, and (iii) efficient gene repression by dCas9-KRAB for studying gene regulation in DC differentiation.

Strategy and clinical application of up-regulating cross presentation by DCs in anti-tumor therapy

The cross presentation of exogenous antigen (Ag) by dendritic cells (DCs) facilitates a diversified mode of T-cell activation, orchestrates specific humoral and cellular immunity, and contributes to an efficient anti-tumor immune response. DCs-mediated cross presentation is subject to both intrinsic and extrinsic factors, including the homing and phenotype of DCs, the spatiotemporal trafficking and degradation kinetics of Ag, and multiple microenvironmental clues, with many details largely unexplored. Here, we systemically review the current mechanistic understanding and regulation strategies of cross presentation by heterogeneous DC populations. We also provide insights into the future exploitation of DCs cross presentation for a better clinical efficacy in anti-tumor therapy.

Recruitment of dendritic cell progenitors to foci of influenza A virus infection sustains immunity

Protection from infection with respiratory viruses such as influenza A virus (IAV) requires T cell–mediated immune responses initiated by conventional dendritic cells (cDCs) that reside in the respiratory tract. Here, we show that effective induction of T cell responses against IAV in mice requires reinforcement of the resident lung cDC network by cDC progenitors. We found that CCR2-binding chemokines produced during IAV infection recruit pre-cDCs from blood and direct them to foci of infection, increasing the number of progeny cDCs next to sites of viral replication. Ablation of CCR2 in the cDC lineage prevented this increase and resulted in a deficit in IAV-specific T cell responses and diminished resistance to reinfection. These data suggest that the homeostatic network of cDCs in tissues is insufficient for immunity and reveal a chemokine-driven mechanism of expansion of lung cDC numbers that amplifies T cell responses against respiratory viruses.

Type 1 conventional dendritic cells: ontogeny, function, and emerging roles in cancer immunotherapy

Dendritic cells (DCs) are key immune sentinels that orchestrate protective immune responses against pathogens or cancers. DCs have evolved into multiple phenotypically, anatomically, and functionally distinct cell types. One of these DC types, Type 1 conventional DCs (cDC1s), are uniquely equipped to promote cytotoxic CD8⁺ T cell differentiation and, therefore, represent a promising target for harnessing antitumor immunity. Indeed, recent studies have highlighted the importance of cDC1s in tumor immunotherapy using immune checkpoint inhibitors. Here, we review the progress in defining the key developmental and functional attributes of cDC1s and the approaches to optimizing the potency of cDC1s for anticancer immunity.

Conventional type 1 dendritic cells (cDC1) as cancer therapeutics: challenges and opportunities

INTRODUCTION

The use of dendritic cell (DC)-based cancer vaccines over three decades has shown them to be a safe therapeutic approach against a range of hematological and solid malignancies. However, underwhelming and inconsistent results from clinical trials have seen them move in and out of favor. The limitations of ex vivo generated monocyte-derived DC (MoDC) in these therapies provide a pointed explanation for the varying and somewhat disappointing clinical outcomes. The identification of a specialized role for the rare conventional type 1 dendritic cell (cDC1) subset in orchestrating tumor immunity via the initiation of CD8⁺ T cell responses has led to a new concept of targeting cDC1 as a therapeutic option to address the unmet need of enhancing the immune response in cancer patients.

AREAS COVERED

This article reviews several current challenges and key opportunities associated with the development and use of next generation cDC1 cancer vaccines.

EXPERT OPINION

Manipulation of cDC1 quantity and quality holds enormous potential to improve tumor immunogenicity, as already demonstrated in preclinical models. New technologies are rapidly advancing the understanding of cDC1 in human cancer patients and facilitating the generation of these extremely rare cells in vitro, providing feasible new approaches toward clinical translation.

DL4-μbeads induce T cell lineage differentiation from stem cells in a stromal cell-free system

T cells are pivotal effectors of the immune system and can be harnessed as therapeutics for regenerative medicine and cancer immunotherapy. An unmet challenge in the field is the development of a clinically relevant system that is readily scalable to generate large numbers of T-lineage cells from hematopoietic stem/progenitor cells (HSPCs). Here, we report a stromal cell-free, microbead-based approach that supports the efficient in vitro development of both human progenitor T (proT) cells and T-lineage cells from CD34⁺cells sourced from cord blood, GCSF-mobilized peripheral blood, and pluripotent stem cells (PSCs). DL4-μbeads, along with lymphopoietic cytokines, induce an ordered sequence of differentiation from CD34⁺ cells to CD34⁺CD7⁺CD5⁺ proT cells to CD3⁺αβ T cells. Single-cell RNA sequencing of human PSC-derived proT cells reveals a transcriptional profile similar to the earliest thymocytes found in the embryonic and fetal thymus. Furthermore, the adoptive transfer of CD34⁺CD7⁺ proT cells into immunodeficient mice demonstrates efficient thymic engraftment and functional maturation of peripheral T cells. DL4-μbeads provide a simple and robust platform to both study human T cell development and facilitate the development of engineered T cell therapies from renewable sources.

T cells derived from stem cells can be harnessed for regenerative medicine and cancer immunotherapy, but current technologies limit production and translation. Here, the authors present a serum-free, stromal-cell free DLL4-coated microbead method for the scalable production of T-lineage cells from multiple sources of stem cells.

Insights Into Dendritic Cells in Cancer Immunotherapy: From Bench to Clinical Applications

Dendritic cells (DCs) are efficient antigen-presenting cells (APCs) and potent activators of naïve T cells. Therefore, they act as a connective ring between innate and adaptive immunity. DC subsets are heterogeneous in their ontogeny and functions. They have proven to potentially take up and process tumor-associated antigens (TAAs). In this regard, researchers have developed strategies such as genetically engineered or TAA-pulsed DC vaccines; these manipulated DCs have shown significant outcomes in clinical and preclinical models. Here, we review DC classification and address how DCs are skewed into an immunosuppressive phenotype in cancer patients. Additionally, we present the advancements in DCs as a platform for cancer immunotherapy, emphasizing the technologies used for in vivo targeting of endogenous DCs, ex vivo generated vaccines from peripheral blood monocytes, and induced pluripotent stem cell-derived DCs (iPSC-DCs) to boost antitumoral immunity.

In situ delivery of iPSC-derived dendritic cells with local radiotherapy generates systemic antitumor immunity and potentiates PD-L1 blockade in preclinical poorly immunogenic tumor models

BACKGROUND

Dendritic cells (DCs) are a promising therapeutic target in cancer immunotherapy given their ability to prime antigen-specific T cells, and initiate antitumor immune response. A major obstacle for DC-based immunotherapy is the difficulty to obtain a sufficient number of functional DCs. Theoretically, this limitation can be overcome by using induced pluripotent stem cells (iPSCs); however, therapeutic strategies to engage iPSC-derived DCs (iPSC-DCs) into cancer immunotherapy remain to be elucidated. Accumulating evidence showing that induction of tumor-residing DCs enhances immunomodulatory effect of radiotherapy (RT) prompted us to investigate antitumor efficacy of combining intratumoral administration of iPSC-DCs with local RT.

METHODS

Mouse iPSCs were differentiated to iPSC-DCs on OP9 stromal cells expressing the notch ligand delta-like 1 in the presence of granulocyte macrophage colony-stimulating factor. Phenotype and the capacities of iPSC-DCs to traffic tumor-draining lymph nodes (TdLNs) and prime antigen-specific T cells were evaluated by flow cytometry and imaging flow cytometry. Antitumor efficacy of intratumoral injection of iPSC-DCs and RT was tested in syngeneic orthotopic mouse tumor models resistant to anti-PD-1 ligand 1 (PD-L1) therapy.

RESULTS

Mouse iPSC-DCs phenotypically resembled conventional type 2 DCs, and had a capacity to promote activation, proliferation and effector differentiation of antigen-specific CD8+ T cells in the presence of the cognate antigen in vitro. Combination of in situ administration of iPSC-DCs and RT facilitated the priming of tumor-specific CD8+ T cells, and synergistically delayed the growth of not only the treated tumor but also the distant non-irradiated tumors. Mechanistically, RT enhanced trafficking of intratumorally injected iPSC-DCs to the TdLN, upregulated CD40 expression, and increased the frequency of DC/CD8+ T cell aggregates. Phenotypic analysis of tumor-infiltrating CD8+ T cells and myeloid cells revealed an increase of stem-like Slamf6+ TIM3- CD8+ T cells and PD-L1 expression in tumor-associated macrophages and DCs. Consequently, combined therapy rendered poorly immunogenic tumors responsive to anti-PD-L1 therapy along with the development of tumor-specific immunological memory.

CONCLUSIONS

Our findings illustrate the translational potential of iPSC-DCs, and identify the therapeutic efficacy of a combinatorial platform to engage them for overcoming resistance to anti-PD-L1 therapy in poorly immunogenic tumors.

Notch signaling in cancer: Complexity and challenges on the path to clinical translation

Notch receptors participate in a conserved pathway in which ligands expressed on neighboring cells trigger a series of proteolytic cleavages that allow the intracellular portion of the receptor to travel to the nucleus and form a short-lived transcription complex that turns on target gene expression. The directness and seeming simplicity of this signaling mechanism belies the complexity of the outcomes of Notch signaling in normal cells, which are highly context and dosage dependent. This complexity is reflected in the diverse roles of Notch in cancers of various types, in which Notch may be oncogenic or tumor suppressive and may have a wide spectrum of effects on tumor cells and stromal elements. This review provides an overview of the roles of Notch in cancer and discusses challenges to clinical translation of Notch targeting agents as well as approaches that may overcome these hurdles.

Prenatal Development and Function of Human Mononuclear Phagocytes

The human mononuclear phagocyte (MP) system, which includes dendritic cells, monocytes, and macrophages, is a critical regulator of innate and adaptive immune responses. During embryonic development, MPs derive sequentially in yolk sac progenitors, fetal liver, and bone marrow haematopoietic stem cells. MPs maintain tissue homeostasis and confer protective immunity in post-natal life. Recent evidence - primarily in animal models - highlight their critical role in coordinating the remodeling, maturation, and repair of target organs during embryonic and fetal development. However, the molecular regulation governing chemotaxis, homeostasis, and functional diversification of resident MP cells in their respective organ systems during development remains elusive. In this review, we summarize the current understanding of the development and functional contribution of tissue MPs during human organ development and morphogenesis and its relevance to regenerative medicine. We outline how single-cell multi-omic approaches and next-generation ex-vivo organ-on-chip models provide new experimental platforms to study the role of human MPs during development and disease.

Clinically applicable CD34+-derived blood dendritic cell subsets exhibit key subset-specific features and potently boost anti-tumor T and NK cell responses

Allogeneic stem cell transplantation (alloSCT), following induction chemotherapy, can be curative for hemato-oncology patients due to powerful graft-versus-tumor immunity. However, disease recurrence remains the major cause of treatment failure, emphasizing the need for potent adjuvant immunotherapy. In this regard, dendritic cell (DC) vaccination is highly attractive, as DCs are the key orchestrators of innate and adaptive immunity. Natural DC subsets are postulated to be more powerful compared with monocyte-derived DCs, due to their unique functional properties and cross-talk capacity. Yet, obtaining sufficient numbers of natural DCs, particularly type 1 conventional DCs (cDC1s), is challenging due to low frequencies in human blood. We developed a clinically applicable culture protocol using donor-derived G-CSF mobilized CD34⁺ hematopoietic progenitor cells (HPCs) for simultaneous generation of high numbers of cDC1s, cDC2s and plasmacytoid DCs (pDCs). Transcriptomic analyses demonstrated that these ex vivo-generated DCs highly resemble their in vivo blood counterparts. In more detail, we demonstrated that the CD141⁺CLEG9A⁺ cDC1 subset exhibited key features of in vivo cDC1s, reflected by high expression of co-stimulatory molecules and release of IL-12p70 and TNF-α. Furthermore, cDC1s efficiently primed alloreactive T cells, potently cross-presented long-peptides and boosted expansion of minor histocompatibility antigen-experienced T cells. Moreover, they strongly enhanced NK cell activation, degranulation and anti-leukemic reactivity. Together, we developed a robust culture protocol to generate highly functional blood DC subsets for in vivo application as tailored adjuvant immunotherapy to boost innate and adaptive anti-tumor immunity in alloSCT patients.

The Flt3L/Flt3 Axis in Dendritic Cell Biology and Cancer Immunotherapy

Dendritic cells (DCs) prime anti-tumor T cell responses in tumor-draining lymph nodes and can restimulate T effector responses in the tumor site. Thus, in addition to unleashing T cell effector activity, current immunotherapies should be directed to boost DC function. Herein, we review the potential function of Flt3L as a tool for cancer immunotherapy. Flt3L is a growth factor that acts in Flt3-expressing multipotent progenitors and common lymphoid progenitors. Despite the broad expression of Flt3 in the hematopoietic progenitors, the main effect of the Flt3/Flt3L axis, revealed by the characterization of mice deficient in these genes, is the generation of conventional DCs (cDCs) and plasmacytoid DCs (pDCs). However, Flt3 signaling through PI3K and mTOR may also affect the function of mature DCs. We recapitulate the use of Flt3L in preclinical studies either as a single agent or in combination with other cancer therapies. We also analyze the use of Flt3L in clinical trials. The strong correlation between type 1 cDC (cDC1) infiltration of human cancers with overall survival in many cancer types suggests the potential use of Flt3L to boost expansion of this DC subset. However, this may need the combination of Flt3L with other immunomodulatory agents to boost cancer immunotherapy.

Human CD141+ dendritic cells (cDC1) are impaired in patients with advanced melanoma but can be targeted to enhance anti-PD-1 in a humanized mouse model

BACKGROUND

The conventional type 1 dendritic cell subset (cDC1) is indispensable for tumor immune responses and the efficacy of immune checkpoint inhibitor (ICI) therapies in animal models but little is known about the role of the human CD141+ DC cDC1 equivalent in patients with melanoma.

METHODS

We developed a flow cytometry assay to quantify and characterize human blood DC subsets in healthy donors and patients with stage 3 and stage 4 metastatic melanoma. To examine whether harnessing CD141+ DCs could improve responses to ICIs in human melanoma, we developed a humanized mouse model by engrafting immunodeficient NSG-SGM3 mice with human CD34+ hematopoietic stem cells (HSCs) from umbilical cord blood followed by transplantation of a human melanoma cell line and treatment with anti-programmed cell death protein-1 (anti-PD-1).

RESULTS

Blood CD141+ DC numbers were significantly reduced in patients with stage 4 melanoma compared with healthy controls. Moreover, CD141+ DCs in patients with melanoma were selectively impaired in their ability to upregulate CD83 expression after stimulation with toll-like receptor 3 (TLR3) and TLR7/8 agonists ex vivo. Although DC numbers did not correlate with responses to anti-PD-1 and/or anti-cytotoxic T-lymphocyte-associated protein 4 (CTLA-4) ICIs, their numbers and capacity to upregulate CD83 declined further during treatment in non-responding patients. Treatment with anti-PD-1 was ineffective at controlling tumor growth in humanized mice but efficacy was enhanced by indirectly expanding and activating DCs in vivo with fms-like tyrosine kinase-3 ligand (Flt3L) and a TLR3 agonist. Moreover, intratumoral injections of CD141+ DCs resulted in reduced tumor growth when combined with anti-PD-1 treatment.

CONCLUSIONS

These data illustrate quantitative and qualitative impairments in circulating CD141+ DCs in patients with advanced melanoma and that increasing CD141+ DC number and function is an attractive strategy to enhance immunogenicity and response rates to ICIs.

Enhancing the immunogenicity of cancer vaccines by harnessing CLEC9A

Dendritic cell (DC) vaccines are a safe and effective means of inducing tumor immune responses, however, a better understanding of DC biology is required in order to realize their full potential. Recent advances in DC biology have identified a crucial role for cDC1 in tumor immune responses, making this DC subset an attractive vaccine target. Human cDC1 exclusively express the C-type-lectin-like receptor, CLEC9A (DNGR-1) that plays an important role in cross-presentation, the process by which effective CD8⁺ T cell responses are generated. CLEC9A antibodies deliver antigen specifically to cDC1 for the induction of humoral, CD4⁺ and CD8⁺ T cell responses and are therefore promising candidates to develop as vaccines for infectious diseases and cancer. The development of human CLEC9A antibodies now facilitates their application as vaccines for cancer immunotherapy. Here we discuss the recent advances in CLEC9A targeting antibodies as vaccines for cancer and their translation to the clinic.

Harnessing the cDC1-NK Cross-Talk in the Tumor Microenvironment to Battle Cancer

Immunotherapeutic approaches have revolutionized the treatment of several diseases such as cancer. The main goal of immunotherapy for cancer is to modulate the anti-tumor immune responses by favoring the recognition and destruction of tumor cells. Recently, a better understanding of the suppressive effect of the tumor microenvironment (TME) on immune cells, indicates that restoring the suppressive effect of the TME is crucial for an efficient immunotherapy. Natural killer (NK) cells and dendritic cells (DCs) are cell types that are currently administered to cancer patients. NK cells are used because of their ability to kill tumor cells directly via cytotoxic granzymes. DCs are employed to enhance anti-tumor T cell responses based on their ability to present antigens and induce tumor-antigen specific CD8⁺ T cell responses. In preclinical models, a particular DC subset, conventional type 1 DCs (cDC1s) is shown to be specialized in cross-presenting extracellular antigens to CD8⁺ T cells. This feature makes them a promising DC subset for cancer treatment. Within the TME, cDC1s show a bidirectional cross-talk with NK cells, resulting in a higher cDC1 recruitment, differentiation, and maturation as well as activation and stimulation of NK cells. Consequently, the presence of cDC1s and NK cells within the TME might be of utmost importance for the success of immunotherapy. In this review, we discuss the function of cDC1s and NK cells, their bidirectional cross-talk and potential strategies that could improve cancer immunotherapy.

The UPR sensor IRE1α promotes dendritic cell responses to control Toxoplasma gondii infection

The unfolded protein response (UPR) has emerged as a central regulator of immune cell responses in several pathologic contexts including infections. However, how intracellular residing pathogens modulate the UPR in dendritic cells (DCs) and thereby affect T cell-mediated immunity remains uncharacterized. Here, we demonstrate that infection of DCs with Toxoplasma gondii (T. gondii) triggers a unique UPR signature hallmarked by the MyD88-dependent activation of the IRE1α pathway and the inhibition of the ATF6 pathway. Induction of XBP1s controls pro-inflammatory cytokine secretion in infected DCs, while IRE1α promotes MHCI antigen presentation of secreted parasite antigens. In mice, infection leads to a specific activation of the IRE1α pathway, which is restricted to the cDC1 subset. Mice deficient for IRE1α and XBP1 in DCs display a severe susceptibility to T. gondii and succumb during the acute phase of the infection. This early mortality is correlated with increased parasite burden and a defect in splenic T-cell responses. Thus, we identify the IRE1α/XBP1s branch of the UPR as a key regulator of host defense upon T. gondii infection.

Dendritic cell metabolism: moving beyond in vitro-culture-generated paradigms

Dendritic cells (DCs) are key orchestrators of immunity and tolerance. It has become evident that DC function can be influenced by cellular metabolic programs. However, conclusions from early metabolic studies using in vitro GM-CSF DC cultures fail to correlate with bona fide DC populations. Here, we discuss the existing paradigms in the DC metabolism field, focusing on the limitations of the models utilized. Furthermore, we introduce alternative models to generate DCs in vitro that better emulate DCs found in vivo. Finally, we highlight new techniques to evaluate DC metabolism at the single-cell level. The combination of these two strategies could help advance the DC metabolism field towards a more physiological understanding, which is crucial for the development of effective DC-based therapies.

Deficiency of Rbpj Leads to Defective Stress-Induced Hematopoietic Stem Cell Functions and Hif Mediated Activation of Non-canonical Notch Signaling Pathways

Deregulated notch signaling has been associated with human pathobiology. However, functions of notch pathways in hematopoiesis remain incompletely understood. Here, we ablated canonical notch pathways, through genetic deletion of Rbpj, in hematopoietic stem cells (HSCs). Our data identified that loss of canonical notch results in normal adult HSC pool, at steady state conditions. However, HSC maintenance and functions in response to radiation-, chemotherapy-, and cytokine- induced stress were compromised in the absence of canonical notch. Rbpj deficient HSCs exhibit decreased proliferation rates and elevated expression of p57^Kip2. Surprisingly, loss of Rbpj resulted in upregulation of key notch target genes and augmented binding of Hes1 to p57 and Gata2 promoters. Further molecular analyses identified an increase in notch activity, elevated expression and nuclear translocation of Hif proteins, and augmented binding of Hif1α to Hes1 promoter in the absence of Rbpj. These studies, for the first time, identify a previously unknown role for non-canonical notch signaling and establish a functional link between Hif and Notch pathways in hematopoiesis.

Notch-Regulated Dendritic Cells Restrain Inflammation-Associated Colorectal Carcinogenesis

Conventional dendritic cells (cDC) play a central role in T-cell antitumor responses. We studied the significance of Notch-regulated DC immune responses in a mouse model of colitis-associated colorectal cancer in which there is epithelial downregulation of Notch/Hes1 signaling. This defect phenocopies that caused by GMDS (GDP-mannose 4,6-dehydratase) mutation in human colorectal cancers. We found that, although wild-type immune cells restrained dysplasia progression and decreased the incidence of adenocarcinoma in chimeric mice, the immune system with Notch2 deleted in all blood lineages or in only DCs promoted inflammation-associated transformation. Notch2 signaling deficiency not only impaired cDC terminal differentiation, but also downregulated CCR7 expression, reduced DC migration, and suppressed antigen cross-presentation to CD8⁺ T cells. Transfer of Notch-primed DCs restrained inflammation-associated dysplasia progression. Consistent with the mouse data, we observed a correlation between infiltrating cDC1 and Notch2 signaling in human colorectal cancers and found that GMDS-mutant colorectal cancers showed decreased CCR7 expression and suppressed cDC1 signature gene expression. Suppressed cDC1 gene signature expression in human colorectal cancer was associated with a poor prognosis. In summary, our study supports an important role for Notch2 signaling in cDC1-mediated antitumor immunity and indicates that Notch2-controlled DCs restrain inflammation-associated colon cancer development in mice.

TAO-kinase 3 governs the terminal differentiation of NOTCH2-dependent splenic conventional dendritic cells

Antigen-presenting conventional dendritic cells (cDCs) are broadly divided into type 1 and type 2 subsets that further adapt their phenotype and function to perform specialized tasks in the immune system. The precise signals controlling tissue-specific adaptation and differentiation of cDCs are currently poorly understood. We found that mice deficient in the Ste20 kinase Thousand and One Kinase 3 (TAOK3) lacked terminally differentiated ESAM+ CD4+ cDC2s in the spleen and failed to prime CD4+ T cells in response to allogeneic red-blood-cell transfusion. These NOTCH2- and ADAM10-dependent cDC2s were absent selectively in the spleen, but not in the intestine of Taok3-/- and CD11c-cre Taok3fl/fl mice. The loss of splenic ESAM+ cDC2s was cell-intrinsic and could be rescued by conditional overexpression of the constitutively active NOTCH intracellular domain in CD11c-expressing cells. Therefore, TAOK3 controls the terminal differentiation of NOTCH2-dependent splenic cDC2s.

Comparison of the PU.1 transcriptional regulome and interactome in human and mouse inflammatory dendritic cells

Dendritic cells (DCs) are key immune modulators and are able to mount immune responses or tolerance. DC differentiation and activation imply a plethora of molecular and cellular responses, including transcriptional changes. PU.1 is a highly expressed transcription factor in DCs and coordinates relevant aspects of DC biology. Due to their role as immune regulators, DCs pose as a promising immunotherapy tool. However, some of their functional features, such as survival, activation, or migration, are compromised due to the limitations to simulate in vitro the physiologic DC differentiation process. A better knowledge of transcriptional programs would allow the identification of potential targets for manipulation with the aim of obtaining "qualified" DCs for immunotherapy purposes. Most of the current knowledge regarding DC biology derives from studies using mouse models, which not always find a parallel in human. In the present study, we dissect the PU.1 transcriptional regulome and interactome in mouse and human DCs, in the steady state or LPS activated. The PU.1 transcriptional regulome was identified by performing PU.1 chromatin immunoprecipitation followed by high-throughput sequencing and pairing these data with RNAsequencing data. The PU.1 interactome was identified by performing PU.1 immunoprecipitation followed by mass spectrometry analysis. Our results portray PU.1 as a pivotal factor that plays an important role in the regulation of genes required for proper DC activation and function, and assures the repression of nonlineage genes. The interspecies differences between human and mouse DCs are surprisingly substantial, highlighting the need to study the biology of human DCs.

A Comprehensive Experimental Guide to Studying Cross-Presentation in Dendritic Cells In Vitro

Cross-presentation was first observed serendipitously in the 1970s. The importance of it was quickly realized and subsequently attracted great attention from immunologists. Since then, our knowledge of the ability of certain antigen presenting cells to internalize, process, and load exogenous antigens onto MHC-I molecules to cross-prime CD8+ T cells has increased significantly. Dendritic cells (DCs) are exceptional cross-presenters, thus making them a great tool to study cross-presentation but the relative rarity of DCs in circulation and in tissues makes it challenging to isolate sufficient numbers of cells to study this process in vitro. In this paper, we describe in detail two methods to culture DCs from bone-marrow progenitors and a method to expand the numbers of DCs present in vivo as a source of endogenous bona-fide cross-presenting DCs. We also describe methods to assess cross-presentation by DCs using the activation of primary CD8+ T cells as a readout. © 2020 Wiley Periodicals LLC. Basic Protocol 1: Isolation of bone marrow progenitor cells Basic Protocol 2: In vitro differentiation of dendritic cells with GM-CSF Support Protocol 1: Preparation of conditioned medium from GM-CSF producing J558L cells Basic Protocol 3: In vitro differentiation of dendritic cells with Flt3L Support Protocol 2: Preparation of Flt3L containing medium from B16-Flt3L cells Basic Protocol 4: Expansion of cDC1s in vivo for use in ex vivo experiments Basic Protocol 5: Characterizing resting and activated dendritic cells Basic Protocol 6: Dendritic cell stimulation, antigenic cargo, and fixation Support Protocol 3: Preparation of model antigen coated microbeads Support Protocol 4: Preparation of apoptotic cells Support Protocol 5: Preparation of recombinant bacteria Basic Protocol 7: Immunocytochemistry immunofluorescence (ICC/IF) Support Protocol 6: Preparation of Alcian blue-coated coverslips Basic Protocol 8: CD8+ T cell activation to assess cross-presentation Support Protocol 7: Isolation and labeling of CD8+ T cells with CFSE.

Dexosomes as a cell-free vaccine for cancer immunotherapy

Dendritic cells (DCs) secrete vast quantities of exosomes termed as dexosomes. Dexosomes are symmetric nanoscale heat-stable vesicles that consist of a lipid bilayer displaying a characteristic series of lipid and protein molecules. They include tetraspanins and all established proteins for presenting antigenic material such as the major histocompatibility complex class I/II (MHC I/II) and CD1a, b, c, d proteins and CD86 costimulatory molecule. Dexosomes contribute to antigen-specific cellular immune responses by incorporating the MHC proteins with antigen molecules and transferring the antigen-MHC complexes and other associated molecules to naïve DCs. A variety of ex vivo and in vivo studies demonstrated that antigen-loaded dexosomes were able to initiate potent antitumor immunity. Human dexosomes can be easily prepared using monocyte-derived DCs isolated by leukapheresis of peripheral blood and treated ex vivo by cytokines and other factors. The feasibility of implementing dexosomes as therapeutic antitumor vaccines has been verified in two phase I and one phase II clinical trials in malignant melanoma and non small cell lung carcinoma patients. These studies proved the safety of dexosome administration and showed that dexosome vaccines have the capacity to trigger both the adaptive (T lymphocytes) and the innate (natural killer cells) immune cell recalls. In the current review, we will focus on the perspective of utilizing dexosome vaccines in the context of cancer immunotherapy.

Subsets of CD1c+ DCs: Dendritic Cell Versus Monocyte Lineage

Currently three bona fide dendritic cell (DC) types are distinguished in human blood. Herein we focus on type 2 DCs (DC2s) and compare the three defining markers CD1c, CD172, and CD301. When using CD1c to define DC2s, a CD14⁺ and a CD14⁻ subset can be detected. The CD14⁺ subset shares features with monocytes, and this includes substantially higher expression levels for CD64, CD115, CD163, and S100A8/9. We review the current knowledge of these CD1c⁺CD14⁺ cells as compared to the CD1c⁺CD14⁻ cells with respect to phenotype, function, transcriptomics, and ontogeny. Here, we discuss informative mutations, which suggest that two populations have different developmental requirements. In addition, we cover subsets of CD11c⁺CD8⁻ DC2s in the mouse, where CLEC12A⁺ESAM^low cells, as compared to the CLEC12A⁻ESAM^high subset, also express higher levels of monocyte-associated markers CD14, CD3, and CD115. Finally, we summarize, for both man and mouse, the data on lower antigen presentation and higher cytokine production in the monocyte-marker expressing DC2 subset, which demonstrate that the DC2 subsets are also functionally distinct.

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

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 IRF8hi pathway, marked by CD123 and BTLA, carried pDC, cDC1, and DC2 trajectories, while the common myeloid IRF8lo 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.

Ex vivo pulsed dendritic cell vaccination against cancer

As the most powerful antigen-presenting cell type, dendritic cells (DCs) can induce potent antigen-specific immune responses in vivo, hence becoming optimal cell population for vaccination purposes. DCs can be derived ex vivo in quantity and manipulated extensively to be endowed with adequate immune-stimulating capacity. After pulsing with cancer antigens in various ways, the matured DCs are administrated back into the patient. DCs home to lymphoid organs to present antigens to and activate specific lymphocytes that react to a given cancer. Ex vivo pulsed DC vaccines have been vigorously investigated for decades, registering encouraging results in relevant immunotherapeutic clinical trials, while facing some solid challenges. With more details in DC biology understood, new theory proposed, and novel technology introduced (featuring recently emerged mRNA vaccine technology), it is becoming increasingly likely that ex vivo pulsed DC vaccine will fulfill its potential in cancer immunotherapy.

Dendritic cell development at a clonal level within a revised 'continuous' model of haematopoiesis

Understanding development of the dendritic cell (DC) subtypes continues to evolve. The origin and relationship of conventional DC type 1 (cDC1), cDC type 2 (cDC2) and plasmacytoid DCs (pDCs) to each other, and in relation to classic myeloid and lymphoid cells, has had a long and controversial history and is still not fully resolved. This review summarises the technological developments and findings that have been achieved at a clonal level, and how that has enhanced our knowledge of the process. It summarises the single cell lineage tracing technologies that have emerged, their application in in vitro and in vivo studies, in both mouse and human settings, and places the findings in a wider context of understanding haematopoiesis at a single cell or clonal level. In particular, it addresses the fate heterogeneity observed in many phenotypically defined progenitor subsets and how these findings have led to a departure from the classic ball-and-stick models of haematopoiesis to the emerging continuous model. Prior contradictions in DC development may be reconciled if they are framed within this revised model, where commitment to a lineage or cell type does not occur in an all-or-nothing process in defined progenitors but rather can occur at many stages of haematopoiesis in a dynamic process.

Breakthrough concepts in immune-oncology: Cancer vaccines at the bedside

Clinical approval of the immune checkpoint blockade (ICB) agents for multiple cancer types has reinvigorated the long-standing work on cancer vaccines. In the pre-ICB era, clinical efforts focused on the Ag, the adjuvants, the formulation, and the mode of delivery. These translational efforts on therapeutic vaccines range from cell-based (e.g., dendritic cells vaccine Sipuleucel-T) to DNA/RNA-based platforms with various formulations (liposome), vectors (Listeria monocytogenes), or modes of delivery (intratumoral, gene gun, etc.). Despite promising preclinical results, cancer vaccine trials without ICB have historically shown little clinical activity. With the anticipation and expansion of combinatorial immunotherapeutic trials with ICB, the cancer vaccine field has entered the personalized medicine arena with recent advances in immunogenic neoantigen-based vaccines. In this article, we review the literature to organize the different cancer vaccines in the clinical space, and we will discuss their advantages, limits, and recent progress to overcome their challenges. Furthermore, we will also discuss recent preclinical advances and clinical strategies to combine vaccines with checkpoint blockade to improve therapeutic outcome and present a translational perspective on future directions.

Notch signaling at the crossroads of innate and adaptive immunity

Notch signaling is an evolutionarily conserved cell-to-cell signaling pathway that regulates cellular differentiation and function across multiple tissue types and developmental stages. In this review, we discuss our current understanding of Notch signaling in mammalian innate and adaptive immunity. The importance of Notch signaling is pervasive throughout the immune system, as it elicits lineage and context-dependent effects in a wide repertoire of cells. Although regulation of binary cell fate decisions encompasses many of the functions first ascribed to Notch in the immune system, recent advances in the field have refined and expanded our view of the Notch pathway beyond this initial concept. From establishing T cell identity in the thymus to regulating mature T cell function in the periphery, the Notch pathway is an essential, recurring signal for the T cell lineage. Among B cells, Notch signaling is required for the development and maintenance of marginal zone B cells in the spleen. Emerging roles for Notch signaling in innate and innate-like lineages such as classical dendritic cells and innate lymphoid cells are likewise coming into view. Lastly, we speculate on the molecular underpinnings that shape the activity and versatility of the Notch pathway.

Free Fatty Acid Receptors 2 and 3 as Microbial Metabolite Sensors to Shape Host Health: Pharmacophysiological View

The role of the gut microbiome in human health is becoming apparent. The major functional impact of the gut microbiome is transmitted through the microbial metabolites that are produced in the gut and interact with host cells either in the local gut environment or are absorbed into circulation to impact distant cells/organs. Short-chain fatty acids (SCFAs) are the major microbial metabolites that are produced in the gut through the fermentation of non-digestible fibers. SCFAs are known to function through various mechanisms, however, their signaling through free fatty acid receptors 2 and 3 (FFAR2/3; type of G-coupled protein receptors) is a new therapeutic approach. FFAR2/3 are widely expressed in diverse cell types in human and mice, and function as sensors of SCFAs to change several physiological and cellular functions. FFAR2/3 modulate neurological signaling, energy metabolism, intestinal cellular homeostasis, immune response, and hormone synthesis. FFAR2/3 function through Gi and/or Gq signaling, that is mediated through specific structural features of SCFAs-FFAR2/3 bindings and modulating specific signaling pathway. In this review, we discuss the wide-spread expression and structural homologies between human and mice FFAR2/3, and their role in different human health conditions. This information can unlock opportunities to weigh the potential of FFAR2/3 as a drug target to prevent human diseases.

Functional Role of Dendritic Cell Subsets in Cancer Progression and Clinical Implications

Dendritic cells (DCs) constitute a complex network of cell subsets with common functions but also with many divergent aspects. All dendritic cell subsets share the ability to prime T cell response and to undergo a complex trafficking program related to their stage of maturation and function. For these reasons, dendritic cells are implicated in a large variety of both protective and detrimental immune responses, including a crucial role in promoting anti-tumor responses. Although cDC1s are the most potent subset in tumor antigen cross-presentation, they are not sufficient to induce full-strength anti-tumor cytotoxic T cell response and need close interaction and cooperativity with the other dendritic cell subsets, namely cDC2s and pDCs. This review will take into consideration different aspects of DC biology, including the functional role of dendritic cell subsets in both fostering and suppressing tumor growth, the mechanisms underlying their recruitment into the tumor microenvironment, as well as the prognostic value and the potentiality of dendritic cell therapeutic targeting. Understanding the specificity of dendritic cell subsets will allow to gain insights on role of these cells in pathological conditions and to design new selective promising therapeutic approaches.