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

Transcriptional programming mediated by the histone demethylase KDM5C regulates dendritic cell population heterogeneity and function

Functional and phenotypic heterogeneity of dendritic cells (DCs) play crucial roles in facilitating the development of diverse immune responses essential for host protection. Here, we report that KDM5C, a histone lysine demethylase, regulates conventional or classical DC (cDC) and plasmacytoid DC (pDC) population heterogeneity and function. Mice deficient in KDM5C in DCs have increased proportions of cDC2Bs and cDC1s, which is partly dependent on type I interferon (IFN) and pDCs. Loss of KDM5C results in an increase in Ly6C- pDCs, which, compared to Ly6C+ pDCs, have limited ability to produce type I IFN and more efficiently stimulate antigen-specific CD8 T cells. KDM5C-deficient DCs have increased expression of inflammatory genes, altered expression of lineage-specific genes, and decreased function. In response to Listeria infection, KDM5C-deficient mice mount reduced CD8 T cell responses due to decreased antigen presentation by cDC1s. Thus, KDM5C is a key regulator of DC heterogeneity and critical driver of the functional properties of DCs.

Microbiota-derived butyrate inhibits cDC development via HDAC inhibition, diminishing their ability to prime T cells

Conventional dendritic cells (cDC) are central to maintaining the balance between protective immune responses and tolerance to harmless antigens, especially in the intestine. Short chain fatty acids (SCFAs) such as butyrate play critical roles in regulating intestinal immunity, but the underlying mechanisms remain unclear. Here we demonstrate that microbiota-derived butyrate alters intestinal cDC populations in vivo resulting in decreased numbers of the cDC2 lineage. By establishing a novel in vitro culture model, we show that butyrate has a direct and selective ability to repress the development of cDC2 from cDC precursors, an effect that is independent of G-protein coupled receptors (GPCRs) and is due to inhibition of histone deacetylase 3. Finally, cDC derived from pre-cDC in the presence of butyrate in vitro express lower levels of costimulatory molecules and have a decreased ability to prime naïve T cells. Together, our data show that butyrate affects the developmental trajectory of cDC, selectively repressing the cDC2 lineage and reducing their ability to stimulate T cells. These properties may help explain the ability of butyrate to maintain homeostasis in the intestine.

Adrenal gland macrophages regulate glucocorticoid production through Trem2 and TGF-β

Glucocorticoid synthesis by adrenal glands (AGs) is regulated by the hypothalamic-pituitary-adrenal axis to facilitate stress responses when the host is exposed to stimuli. Recent studies implicate macrophages as potential steroidogenic regulators, but the molecular mechanisms by which AG macrophages exert such influence remain unclear. In this study, we investigated the role of AG macrophages in response to cold challenge or atherosclerotic inflammation as physiologic models of acute or chronic stress. Using single-cell RNA sequencing, we observed dynamic AG macrophage polarization toward classical activation and lipid-associated phenotypes following acute or chronic stimulation. Among transcriptional alterations induced in macrophages, triggering receptor expressed on myeloid cells 2 (Trem2) was highlighted because of its upregulation following stress. Conditional deletion of macrophage Trem2 revealed a protective role in stress responses. Mechanistically, Trem2 deletion led to increased AG macrophage death, abolished the TGF-β-producing capacity of AG macrophages, and resulted in enhanced glucocorticoid production. In addition, enhanced glucocorticoid production was replicated by blockade of TGF-β signaling. Together, these observations suggest that AG macrophages restrict steroidogenesis through Trem2 and TGF-β, which opens potential avenues for immunotherapeutic interventions to resolve stress-related disorders.

A lactate-SREBP2 signaling axis drives tolerogenic dendritic cell maturation and promotes cancer progression

Conventional dendritic cells (DCs) are essential mediators of antitumor immunity. As a result, cancers have developed poorly understood mechanisms to render DCs dysfunctional within the tumor microenvironment (TME). After identification of CD63 as a specific surface marker, we demonstrate that mature regulatory DCs (mregDCs) migrate to tumor-draining lymph node tissues and suppress DC antigen cross-presentation in trans while promoting T helper 2 and regulatory T cell differentiation. Transcriptional and metabolic studies showed that mregDC functionality is dependent on the mevalonate biosynthetic pathway and its master transcription factor, SREBP2. We found that melanoma-derived lactate activates SREBP2 in tumor DCs and drives conventional DC transformation into mregDCs via homeostatic or tolerogenic maturation. DC-specific genetic silencing and pharmacologic inhibition of SREBP2 promoted antitumor CD8+ T cell activation and suppressed melanoma progression. CD63+ mregDCs were found to reside within the lymph nodes of several preclinical tumor models and in the sentinel lymph nodes of patients with melanoma. Collectively, this work suggests that a tumor lactate-stimulated SREBP2-dependent program promotes CD63+ mregDC development and function while serving as a promising therapeutic target for overcoming immune tolerance in the TME.

Genome-wide screening identifies Trim33 as an essential regulator of dendritic cell differentiation

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.

An Organotypic Human Lymph Node Model Reveals the Importance of Fibroblastic Reticular Cells for Dendritic Cell Function

BACKGROUND

Human lymph node (HuLN) models have emerged with invaluable potential for immunological research and therapeutic application given their fundamental role in human health and disease. While fibroblastic reticular cells (FRCs) are instrumental to HuLN functioning, their inclusion and recognition of importance for organotypic in vitro lymphoid models remain limited.

METHODS

Here, we established an in vitro three-dimensional (3D) model in a collagen-fibrin hydrogel with primary FRCs and a dendritic cell (DC) cell line (MUTZ-3 DC). To study and characterise the cellular interactions seen in this 3D FRC-DC organotypic model compared to the native HuLN; flow cytometry, immunohistochemistry, immunofluorescence and cytokine/chemokine analysis were performed.

RESULTS

FRCs were pivotal for survival, proliferation and localisation of MUTZ-3 DCs. Additionally, we found that CD1a expression was absent on MUTZ-3 DCs that developed in the presence of FRCs during cytokine-induced MUTZ-3 DC differentiation, which was also seen with primary monocyte-derived DCs (moDCs). This phenotype resembled HuLN-resident DCs, which we detected in primary HuLNs, and these CD1a- MUTZ-3 DCs induced T cell proliferation within a mixed leukocyte reaction (MLR), indicating a functional DC status. FRCs expressed podoplanin (PDPN), CD90 (Thy-1), CD146 (MCAM) and Gremlin-1, thereby resembling the DC supporting stromal cell subset identified in HuLNs.

CONCLUSION

This 3D FRC-DC organotypic model highlights the influence and importance of FRCs for DC functioning in a more realistic HuLN microenvironment. As such, this work provides a starting point for the development of an in vitro HuLN.

Leukemic mutation FLT3-ITD is retained in dendritic cells and disrupts their homeostasis leading to expanded Th17 frequency

Dendritic cells (DC) are mediators between innate and adaptive immune responses to pathogens and tumors. DC development is determined by signaling through the receptor tyrosine kinase Fms-like tyrosine kinase 3 (FLT3) in bone marrow myeloid progenitors. Recently the naming conventions for DC phenotypes have been updated to distinguish between "Conventional" DCs (cDCs) and plasmacytoid DCs (pDCs). Activating mutations of FLT3, including Internal Tandem Duplication (FLT3-ITD), are associated with poor prognosis for acute myeloid leukemia (AML) patients. Having a shared myeloid lineage it can be difficult to distinguish bone fide DCs from AML tumor cells. To date, there is little information on the effects of FLT3-ITD in DC biology. To further elucidate this relationship we utilized CITE-seq technology in combination with flow cytometry and multiplex immunoassays to measure changes to DCs in human and mouse tissues. We examined the cDC phenotype and frequency in bone marrow aspirates from patients with AML to understand the changes to cDCs associated with FLT3-ITD. When compared to healthy donor (HD) we found that a subset of FLT3-ITD+ AML patient samples have overrepresented populations of cDCs and disrupted phenotypes. Using a mouse model of FLT3-ITD+ AML, we found that cDCs were increased in percentage and number compared to control wild-type (WT) mice. Single cell RNA-seq identified FLT3-ITD+ cDCs as skewed towards a cDC2 T-bet- phenotype, previously shown to promote Th17 T cells. We assessed the phenotypes of CD4+ T cells in the AML mice and found significant enrichment of both Treg and Th17 CD4+ T cells in the bone marrow and spleen compartments. Ex vivo stimulation of CD4+ T cells also showed increased Th17 phenotype in AML mice. Moreover, co-culture of AML mouse-derived DCs and naïve OT-II cells preferentially skewed T cells into a Th17 phenotype. Together, our data suggests that FLT3-ITD+ leukemia-associated cDCs polarize CD4+ T cells into Th17 subsets, a population that has been shown to be negatively associated with survival in solid tumor contexts. This illustrates the complex tumor microenvironment of AML and highlights the need for further investigation into the effects of FLT3-ITD mutations on DC phenotypes and their downstream effects on Th polarization.

Distinct ontogenetic lineages dictate cDC2 heterogeneity

Conventional dendritic cells (cDCs) include functionally and phenotypically diverse populations, such as cDC1s and cDC2s. The latter population has been variously subdivided into Notch-dependent cDC2s, KLF4-dependent cDC2s, T-bet+ cDC2As and T-bet- cDC2Bs, but it is unclear how all these subtypes are interrelated and to what degree they represent cell states or cell subsets. All cDCs are derived from bone marrow progenitors called pre-cDCs, which circulate through the blood to colonize peripheral tissues. Here, we identified distinct mouse pre-cDC2 subsets biased to give rise to cDC2As or cDC2Bs. We showed that a Siglec-H+ pre-cDC2A population in the bone marrow preferentially gave rise to Siglec-H- CD8α+ pre-cDC2As in tissues, which differentiated into T-bet+ cDC2As. In contrast, a Siglec-H- fraction of pre-cDCs in the bone marrow and periphery mostly generated T-bet- cDC2Bs, a lineage marked by the expression of LysM. Our results showed that cDC2A versus cDC2B fate specification starts in the bone marrow and suggest that cDC2 subsets are ontogenetically determined lineages, rather than cell states imposed by the peripheral tissue environment.

A Simple and Rapid Protocol for the Isolation of Murine Bone Marrow Suitable for the Differentiation of Dendritic Cells

The generation of bone-marrow-derived dendritic cells is a widely used approach in immunological research to study antigen processing and presentation, as well as T-cell activation responses. However, the initial step of isolating the bone marrow can be time-consuming, especially when larger numbers of precursor cells are required. Here, we assessed whether an accelerated bone marrow isolation method using centrifugation is suitable for the differentiation of FMS-like tyrosine kinase 3 ligand-driven dendritic cells. Compared to the conventional flushing method, the centrifugation-based isolation method resulted in a similar bone marrow cell yield on Day 0, increased cell numbers by Day 8, similar proportions of dendritic cell subsets, and consequently a higher number of type 1 conventional dendritic cells (cDC1) from the culture. Although the primary purpose of this method of optimization was to improve experimental efficiency and increase the output of cDC1s, the protocol is also compatible with the differentiation of other dendritic cell subsets such as cDC2 and plasmacytoid dendritic cells, with an improved output cell count and a consistent phenotype.

Cancer vaccines in the clinic

Vaccines are an important tool in the rapidly evolving repertoire of immunotherapies in oncology. Although cancer vaccines have been investigated for over 30 years, very few have achieved meaningful clinical success. However, recent advances in areas such antigen identification, formulation development and manufacturing, combination therapy regimens, and indication and patient selection hold promise to reinvigorate the field. Here, we provide a timely update on the clinical status of cancer vaccines. We identify and critically analyze 360 active trials of cancer vaccines according to delivery vehicle, antigen type, indication, and other metrics, as well as highlight eight globally approved products. Finally, we discuss current limitations and future applications for clinical translation of cancer vaccines.

Decoding immunogenic cell death from a dendritic cell perspective

Dendritic cells (DCs) are myeloid cells bridging the innate and adaptive immune system. By cross-presenting tumor-associated antigens (TAAs) liberated upon spontaneous or therapy-induced tumor cell death to T cells, DCs occupy a pivotal position in the cancer immunity cycle. Over the last decades, the mechanisms linking cancer cell death to DC maturation, have been the focus of intense research. Growing evidence supports the concept that the mere transfer of TAAs during the process of cell death is insufficient to drive immunogenic DC maturation unless this process is coupled with the release of immunomodulatory signals by dying cancer cells. Malignant cells succumbing to a regulated cell death variant called immunogenic cell death (ICD), foster a proficient interface with DCs, enabling their immunogenic maturation and engagement of adaptive immunity against cancer. This property relies on the ability of ICD to exhibit pathogen-mimicry hallmarks and orchestrate the emission of a spectrum of constitutively present or de novo-induced danger signals, collectively known as damage-associated molecular patterns (DAMPs). In this review, we discuss how DCs perceive and decode danger signals emanating from malignant cells undergoing ICD and provide an outlook of the major signaling and functional consequences of this interaction for DCs and antitumor immunity.

mRNAs encoding self-DNA reactive cGAS enhance the immunogenicity of lipid nanoparticle vaccines

IMPORTANCE

Nucleic acid-based vaccines hold promise in preventing infections and treating cancer. The most common use of this technology is to encode antigenic proteins on mRNAs that are delivered to cells via lipid nanoparticle (LNP) formulations. In this study, we discovered that immunostimulatory proteins can also be encoded on mRNAs in LNPs. We found that an active mutant of the enzyme cGAS, referred to as cGAS∆N, acts as a catalytic adjuvant in LNP-encapsulated mRNA vaccines. The delivery of cGAS∆N mRNA via LNPs in combination with antigen mRNA-LNPs led to durable antigen-specific IFNγ-producing T cells that exceeded the efficiency of antigen-LNPs similar to those currently used in the clinic. This strategy did not compromise B cell responses; rather it induced Th1-biased antibody isotypes. This work unveils new vaccine design strategies using mRNA-encoded catalytic adjuvants that could be ideal for generating CD8+ T cell and B cell responses for immunotherapies.

Identification of dendritic cell precursor from the CD11c+ cells expressing high levels of MHC class II molecules in the culture of bone marrow with FLT3 ligand

Dendritic cells (DCs) are readily generated from the culture of mouse bone marrow (BM) treated with either granulocyte macrophage-colony stimulating factor (GM-CSF) or FMS-like tyrosine kinase 3 ligand (FLT3L). CD11c+MHCII+ or CD11c+MHCIIhi cells are routinely isolated from those BM cultures and generally used as in vitro-generated DCs for a variety of experiments and therapies. Here, we examined CD11c+ cells in the BM culture with GM-CSF or FLT3L by staining with a monoclonal antibody 2A1 that is known to recognize mature or activated DCs. Most of the cells within the CD11c+MHCIIhi DC gate were 2A1+ in the BM culture with GM-CSF (GM-BM culture). In the BM culture with FLT3L (FL-BM culture), almost of all the CD11c+MHCIIhi cells were within the classical DC2 (cDC2) gate. The analysis of FL-BM culture revealed that a majority of cDC2-gated CD11c+MHCIIhi cells exhibited a 2A1-CD83-CD115+CX3CR1+ phenotype, and the others consisted of 2A1+CD83+CD115-CX3CR1- and 2A1-CD83-CD115-CX3CR1- cells. According to the antigen uptake and presentation, morphologies, and gene expression profiles, 2A1-CD83-CD115-CX3CR1- cells were immature cDC2s and 2A1+CD83+CD115-CX3CR1- cells were mature cDC2s. Unexpectedly, however, 2A1-CD83-CD115+CX3CR1+ cells, the most abundant cDC2-gated MHCIIhi cell subset in FL-BM culture, were non-DCs. Adoptive cell transfer experiments in the FL-BM culture confirmed that the cDC2-gated MHCIIhi non-DCs were precursors to cDC2s, i.e., MHCIIhi pre-cDC2s. MHCIIhi pre-cDC2s also expressed the higher level of DC-specific transcription factor Zbtb46 as similarly as immature cDC2s. Besides, MHCIIhi pre-cDC2s were generated only from pre-cDCs and common DC progenitor (CDP) cells but not from monocytes and common monocyte progenitor (cMoP) cells, verifying that MHCIIhi pre-cDC2s are close lineage to cDCs. All in all, our study identified and characterized a new cDC precursor, exhibiting a CD11c+MHCIIhiCD115+CX3CR1+ phenotype, in FL-BM culture.

Guidelines for mouse and human DC generation

This article is part of the Dendritic Cell Guidelines article series, which provides a collection of state-of-the-art protocols for the preparation, phenotype analysis by flow cytometry, generation, fluorescence microscopy, and functional characterization of mouse and human dendritic cells (DC) from lymphoid organs and various non-lymphoid tissues. This article provides protocols with top ticks and pitfalls for preparation and successful generation of mouse and human DC from different cellular sources, such as murine BM and HoxB8 cells, as well as human CD34+ cells from cord blood, BM, and peripheral blood or peripheral blood monocytes. We describe murine cDC1, cDC2, and pDC generation with Flt3L and the generation of BM-derived DC with GM-CSF. Protocols for human DC generation focus on CD34+ cell culture on OP9 cell layers for cDC1, cDC2, cDC3, and pDC subset generation and DC generation from peripheral blood monocytes (MoDC). Additional protocols include enrichment of murine DC subsets, CRISPR/Cas9 editing, and clinical grade human DC generation. While all protocols were written by experienced scientists who routinely use them in their work, this article was also peer-reviewed by leading experts and approved by all co-authors, making it an essential resource for basic and clinical DC immunologists.

Dendritic Cell Vaccination in Non-Small Cell Lung Cancer: Remodeling the Tumor Immune Microenvironment

Non-small-cell lung cancer (NSCLC) remains one of the leading causes of death worldwide. While NSCLCs possess antigens that can potentially elicit T cell responses, defective tumor antigen presentation and T cell activation hinder host anti-tumor immune responses. The NSCLC tumor microenvironment (TME) is composed of cellular and soluble mediators that can promote or combat tumor growth. The composition of the TME plays a critical role in promoting tumorigenesis and dictating anti-tumor immune responses to immunotherapy. Dendritic cells (DCs) are critical immune cells that activate anti-tumor T cell responses and sustain effector responses. DC vaccination is a promising cellular immunotherapy that has the potential to facilitate anti-tumor immune responses and transform the composition of the NSCLC TME via tumor antigen presentation and cell-cell communication. Here, we will review the features of the NSCLC TME with an emphasis on the immune cell phenotypes that directly interact with DCs. Additionally, we will summarize the major preclinical and clinical approaches for DC vaccine generation and examine how effective DC vaccination can transform the NSCLC TME toward a state of sustained anti-tumor immune signaling.

The Potential of Dendritic Cell Subsets in the Development of Personalized Immunotherapy for Cancer Treatment

Since the discovery of dendritic cells (DCs) in 1973 by Ralph Steinman, a tremendous amount of knowledge regarding these innate immunity cells has been accumulating. Their role in regulating both innate and adaptive immune processes is gradually being uncovered. DCs are proficient antigen-presenting cells capable of activating naive T-lymphocytes to initiate and generate effective anti-tumor responses. Although DC-based immunotherapy has not yielded significant results, the substantial number of ongoing clinical trials underscores the relevance of DC vaccines, particularly as adjunctive therapy or in combination with other treatment options. This review presents an overview of current knowledge regarding human DCs, their classification, and the functions of distinct DC populations. The stepwise process of developing therapeutic DC vaccines to treat oncological diseases is discussed, along with speculation on the potential of combined therapy approaches and the role of DC vaccines in modern immunotherapy.

Leukemic mutation FLT3-ITD is retained in dendritic cells and disrupts their homeostasis leading to expanded Th17 frequency

Dendritic cells (DC) are mediators of adaptive immune responses to pathogens and tumors. DC development is determined by signaling through the receptor tyrosine kinase Fms-like tyrosine kinase 3 (FLT3) in bone marrow myeloid progenitors. Recently the naming conventions for DC phenotypes have been updated to distinguish between "Conventional" DCs (cDCs) and plasmacytoid DCs (pDCs). Activating mutations of FLT3, including Internal Tandem Duplication (FLT3-ITD), are associated with poor prognosis for leukemia patients. To date, there is little information on the effects of FLT3-ITD in DC biology. We examined the cDC phenotype and frequency in bone marrow aspirates from patients with acute myeloid leukemia (AML) to understand the changes to cDCs associated with FLT3-ITD. When compared to healthy donor (HD) we found that a subset of FLT3-ITD+ AML patient samples have overrepresented populations of cDCs and disrupted phenotypes. Using a mouse model of FLT3-ITD+ AML, we found that cDCs were increased in percentage and number compared to control wild-type (WT) mice. Single cell RNA-seq identified FLT3-ITD+ cDCs as skewed towards a cDC2 T-bet - phenotype, previously shown to promote Th17 T cells. We assessed the phenotypes of CD4+ T cells in the AML mice and found significant enrichment of both Treg and Th17 CD4+ T cells. Furthermore, co-culture of AML mouse- derived DCs and naïve OT-II cells preferentially skewed T cells into a Th17 phenotype. Together, our data suggests that FLT3-ITD+ leukemia-associated cDCs polarize CD4+ T cells into Th17 subsets, a population that has been shown to be negatively associated with survival in solid tumor contexts. This illustrates the complex tumor microenvironment of AML and highlights the need for further investigation into the effects of FLT3-ITD mutations on DC phenotypes.

Enhanced in vitro type 1 conventional dendritic cell generation via the recruitment of hematopoietic stem cells and early progenitors by Kit ligand

In vitro culture of bone marrow (BM) with Fms-like tyrosine kinase 3 ligand (Flt3L) is widely used to study development and function of type 1 conventional dendritic cells (cDC1). Hematopoietic stem cells (HSCs) and many progenitor populations that possess cDC1 potential in vivo do not express Flt3 and thus may not contribute to Flt3L-mediated cDC1 production in vitro. Here, we present a KitL/Flt3L protocol that recruits such HSCs and progenitors into the production of cDC1. Kit ligand (KitL) is used to expand HSCs and early progenitors lacking Flt3 expression into later stage where Flt3 is expressed. Following this initial KitL phase, a second Flt3L phase is used to support the final production of DCs. With this two-stage culture, we achieved approximately tenfold increased production of both cDC1 and cDC2 compared to Flt3L culture. cDC1 derived from this culture are similar to in vivo cDC1 in their dependence on IRF8, ability to produce IL-12, and induction of tumor regression in cDC1-deficient tumor-bearing mice. This KitL/Flt3L system for cDC1 production will be useful in further analysis of cDC1 that rely on in vitro generation from BM.

Circulating IRF8-expressing CD123+CD127+ lymphoid progenitors: key players in human hematopoiesis

Lymphopoiesis is the process in which B and T cells, and innate lymphoid cells (ILCs) develop from hematopoietic progenitors that exhibit early lymphoid priming. The branching points where lymphoid-primed human progenitors are further specified to B/T/ILC differentiation trajectories remain unclear. Here, we discuss the emerging role of interferon regulatory factor (IRF)8 as a key factor to bridge human lymphoid and dendritic cell (DC) differentiation, and the current evidence for the existence of circulating and tissue-resident CD123+CD127+ lymphoid progenitors. We propose a model whereby DC/B/T/ILC lineage programs in circulating CD123+CD127+ lymphoid progenitors are expressed in balance. Upon tissue seeding, the tissue microenvironment tilts this molecular balance towards a specific lineage, thereby determining in vivo lineage fates. Finally, we discuss the translational implication of these lymphoid precursors.

Bcl6, Irf2, and Notch2 promote nonclassical monocyte development

Ly6Clo monocytes are a myeloid subset that specializes in the surveillance of vascular endothelium. Ly6Clo monocytes have been shown to derive from Ly6Chi monocytes. NOTCH2 signaling has been implicated as a trigger for Ly6Clo monocyte development, but the basis for this effect is unclear. Here, we examined the impact of NOTCH2 signaling of myeloid progenitors on the development of Ly6Clo monocytes in vitro. NOTCH2 signaling induced by delta-like ligand 1 (DLL1) efficiently induced the transition of Ly6Chi TREML4- monocytes into Ly6Clo TREML4+ monocytes. We further identified two additional transcriptional requirements for development of Ly6Clo monocytes. Deletion of BCL6 from myeloid progenitors abrogated development of Ly6Clo monocytes. IRF2 was also required for Ly6Clo monocyte development in a cell-intrinsic manner. DLL1-induced in vitro transition into Ly6Clo TREML4+ monocytes required IRF2 but unexpectedly could occur in the absence of NUR77 or BCL6. These results imply a transcriptional hierarchy for these factors in controlling Ly6Clo monocyte development.

Transitional dendritic cells are distinct from conventional DC2 precursors and mediate proinflammatory antiviral responses

High-dimensional approaches have revealed heterogeneity amongst dendritic cells (DCs), including a population of transitional DCs (tDCs) in mice and humans. However, the origin and relationship of tDCs to other DC subsets has been unclear. Here we show that tDCs are distinct from other well-characterized DCs and conventional DC precursors (pre-cDCs). We demonstrate that tDCs originate from bone marrow progenitors shared with plasmacytoid DCs (pDCs). In the periphery, tDCs contribute to the pool of ESAM+ type 2 DCs (DC2s), and these DC2s have pDC-related developmental features. Different from pre-cDCs, tDCs have less turnover, capture antigen, respond to stimuli and activate antigen-specific naïve T cells, all characteristics of differentiated DCs. Different from pDCs, viral sensing by tDCs results in IL-1β secretion and fatal immune pathology in a murine coronavirus model. Our findings suggest that tDCs are a distinct pDC-related subset with a DC2 differentiation potential and unique proinflammatory function during viral infections.

The Unfolded Protein Response Sensor IRE1 Regulates Activation of In Vitro Differentiated Type 1 Conventional DCs with Viral Stimuli

Type 1 conventional dendritic cells (cDC1s) are leukocytes competent to coordinate antiviral immunity, and thus, the intracellular mechanisms controlling cDC1 function are a matter of intense research. The unfolded protein response (UPR) sensor IRE1 and its associated transcription factor XBP1s control relevant functional aspects in cDC1s including antigen cross-presentation and survival. However, most studies connecting IRE1 and cDC1 function are undertaken in vivo. Thus, the aim of this work is to elucidate whether IRE1 RNase activity can also be modeled in cDC1s differentiated in vitro and reveal the functional consequences of such activation in cells stimulated with viral components. Our data show that cultures of optimally differentiated cDC1s recapitulate several features of IRE1 activation noticed in in vivo counterparts and identify the viral analog Poly(I:C) as a potent UPR inducer in the lineage. In vitro differentiated cDC1s display constitutive IRE1 RNase activity and hyperactivate IRE1 RNase upon genetic deletion of XBP1s, which regulates production of the proinflammatory cytokines IL-12p40, TNF-α and IL-6, Ifna and Ifnb upon Poly(I:C) stimulation. Our results show that a strict regulation of the IRE1/XBP1s axis regulates cDC1 activation to viral agonists, expanding the scope of this UPR branch in potential DC-based therapies.

Nuanced role for dendritic cell intrinsic IRE1 RNase in the regulation of antitumor adaptive immunity

In cancer, activation of the IRE1/XBP1s axis of the unfolded protein response (UPR) promotes immunosuppression and tumor growth, by acting in cancer cells and tumor infiltrating immune cells. However, the role of IRE1/XBP1s in dendritic cells (DCs) in tumors, particularly in conventional type 1 DCs (cDC1s) which are cellular targets in immunotherapy, has not been fully elucidated. Here, we studied the role of IRE1/XBP1s in subcutaneous B16/B78 melanoma and MC38 tumors by generating loss-of-function models of IRE1 and/or XBP1s in DCs or in cDC1s. Data show that concomitant deletion of the RNase domain of IRE1 and XBP1s in DCs and cDC1s does not influence the kinetics of B16/B78 and MC38 tumor growth or the effector profile of tumor infiltrating T cells. A modest effect is observed in mice bearing single deletion of XBP1s in DCs, which showed slight acceleration of melanoma tumor growth and dysfunctional T cell responses, however, this effect was not recapitulated in animals lacking XBP1 only in cDC1s. Thus, evidence presented here argues against a general pro-tumorigenic role of the IRE1/XBP1s pathway in tumor associated DC subsets.

Tissue-resident, extravascular Ly6c- monocytes are critical for inflammation in the synovium

Monocytes are abundant immune cells that infiltrate inflamed organs. However, the majority of monocyte studies focus on circulating cells, rather than those in tissue. Here, we identify and characterize an intravascular synovial monocyte population resembling circulating non-classical monocytes and an extravascular tissue-resident monocyte-lineage cell (TR-MC) population distinct in surface marker and transcriptional profile from circulating monocytes, dendritic cells, and tissue macrophages that are conserved in rheumatoid arthritis (RA) patients. TR-MCs are independent of NR4A1 and CCR2, long lived, and embryonically derived. TR-MCs undergo increased proliferation and reverse diapedesis dependent on LFA1 in response to arthrogenic stimuli and are required for the development of RA-like disease. Moreover, pathways that are activated in TR-MCs at the peak of arthritis overlap with those that are downregulated in LFA1-/- TR-MCs. These findings show a facet of mononuclear cell biology that could be imperative to understanding tissue-resident myeloid cell function in RA.

Effect of the Lipoxin Receptor Agonist BML-111 on Cigarette Smoke Extract-Induced Macrophage Polarization and Inflammation in RAW264.7 Cells

Background

Macrophages are known to play a crucial role in the chronic inflammation associated with Chronic Obstructive Pulmonary Disease (COPD). BML-111, acting as a lipoxin A4 (LXA4) receptor agonist, has shown to be effective in protecting against COPD. However, the precise mechanism by which BML-111 exerts its protective effect remains unclear.

Methods

In order to establish a cell model of inflammation, cigarette smoke extract (CSE) was used on the RAW264.7 cell line. Afterwards, an Enzyme-linked immunosorbent assay (ELISA) kit was employed to measure concentrations of tumor necrosis factor-α (TNF-α), interleukin-1beta (IL-1β), interleukin-18 (IL-18), and interleukin-10 (IL-10) in the cell supernatants of the RAW264.7 cells.In this study, we examined the markers of macrophage polarization using two methods: quantitative real-time polymerase chain reaction (qRT-PCR) and Western blot analysis. Additionally, we detected the expression of Notch-1 and Hes-1 through Western blotting.

Results

BML-111 effectively suppressed the expression of pro-inflammatory cytokines TNF-α, IL-1β, and IL-18, as well as inflammasome factors NLRP3 and Caspase-1, while simultaneously up-regulating the expression of the anti-inflammatory cytokine IL-10 induced by CSE. Moreover, BML-111 reduced the expression of iNOS, which is associated with M1 macrophage polarization, and increased the expression of Arg-1, which is associated with M2 phenotype. Additionally, BML-111 downregulated the expression of Hes-1 and the ratio of activated Notch-1 to Notch-1 induced by CSE. The effect of BML-111 on inflammation and macrophage polarization was reversed upon administration of the Notch-1 signaling pathway agonist Jagged1.

Conclusion

BML-111 has the potential to suppress inflammation and modulate M1/M2 macrophage polarization in RAW264.7 cells. The underlying mechanism may involve the Notch-1 signaling pathway.

TIM-3 increases the abundance of type-2 dendritic cells during Leishmania donovani infection by enhancing IL-10 production via STAT3

The outcome of the disease visceral leishmaniasis (VL), caused by Leishmania donovani (LD), largely relies on the relative dominance of host-protective type-1 T helper (Th1) cell response versus disease-promoting type-2 T helper (Th2) cell response. The Th1 and Th2 responses, in turn, are believed to be elicited by type-1 conventional dendritic cells (cDC1) and type-2 conventional DCs (cDC2), respectively. However, it is still unknown which DC subtype (cDC1 or cDC2) predominates during chronic LD infection and the molecular mechanism governing such occurrence. Here we report that in chronically infected mice, the splenic cDC1-cDC2 balance shifted toward the cDC2 subtype and that the receptor T cell immunoglobulin and mucin protein-3 (TIM-3) expressed by DCs played a key role in mediating this effect. Transfer of TIM-3-silenced DCs in fact prevented the predominance of the cDC2 subtype in mice with chronic LD infection. We also found that LD actually upregulated TIM-3 expression on DCs by triggering a TIM-3-mediated signaling pathway STAT3 (signal transducer and activator of transcription 3)→interleukin (IL)-10→c-Src→transcription factors Ets1, Ets2, USF1, and USF2. Notably, TIM-3 promoted STAT3 activation via a non-receptor tyrosine kinase Btk. Adoptive transfer experiments further demonstrated a critical role for STAT3-driven TIM-3 upregulation on DCs in increasing cDC2 abundance in chronically infected mice, which ultimately aided disease pathogenesis by augmenting Th2 responses. These findings document a new immunoregulatory mechanism contributing to disease pathology during LD infection and define TIM-3 as a key mediator of this process.

Single cell RNA sequencing reveals distinct clusters of Irf8-expressing pulmonary conventional dendritic cells

A single population of interferon-regulatory factor 8 (Irf8)-dependent conventional dendritic cell (cDC type1) is considered to be responsible for both immunogenic and tolerogenic responses depending on the surrounding cytokine milieu. Here, we challenge this concept of an omnipotent single Irf8-dependent cDC1 cluster through analysis of pulmonary cDCs at single cell resolution. We report existence of a pulmonary cDC1 cluster lacking Xcr1 with an immunogenic signature that clearly differs from the Xcr1 positive cDC1 cluster. The Irf8⁺Batf3⁺Xcr1^- cluster expresses high levels of pro-inflammatory genes associated with antigen presentation, migration and co-stimulation such as Ccr7, Cd74, MHC-II, Ccl5, Il12b and Relb while, the Xcr1⁺ cDC1 cluster expresses genes corresponding to immune tolerance mechanisms like Clec9a, Pbx1, Cadm1, Btla and Clec12a. In concordance with their pro-inflammatory gene expression profile, the ratio of Xcr1^- cDC1s but not Xcr1⁺cDC1 is increased in the lungs of allergen-treated mice compared to the control group, in which both cDC1 clusters are present in comparable ratios. The existence of two distinct Xcr1⁺ and Xcr1^- cDC1 clusters is furthermore supported by velocity analysis showing markedly different temporal patterns of Xcr1^- and Xcr1⁺cDC1s. In summary, we present evidence for the existence of two different cDC1 clusters with distinct immunogenic profiles in vivo. Our findings have important implications for DC-targeting immunomodulatory therapies.

The Notch signaling pathway: a potential target for cancer immunotherapy

Dysregulation of the Notch signaling pathway, which is highly conserved across species, can drive aberrant epigenetic modification, transcription, and translation. Defective gene regulation caused by dysregulated Notch signaling often affects networks controlling oncogenesis and tumor progression. Meanwhile, Notch signaling can modulate immune cells involved in anti- or pro-tumor responses and tumor immunogenicity. A comprehensive understanding of these processes can help with designing new drugs that target Notch signaling, thereby enhancing the effects of cancer immunotherapy. Here, we provide an up-to-date and comprehensive overview of how Notch signaling intrinsically regulates immune cells and how alterations in Notch signaling in tumor cells or stromal cells extrinsically regulate immune responses in the tumor microenvironment (TME). We also discuss the potential role of Notch signaling in tumor immunity mediated by gut microbiota. Finally, we propose strategies for targeting Notch signaling in cancer immunotherapy. These include oncolytic virotherapy combined with inhibition of Notch signaling, nanoparticles (NPs) loaded with Notch signaling regulators to specifically target tumor-associated macrophages (TAMs) to repolarize their functions and remodel the TME, combining specific and efficient inhibitors or activators of Notch signaling with immune checkpoint blockers (ICBs) for synergistic anti-tumor therapy, and implementing a customized and effective synNotch circuit system to enhance safety of chimeric antigen receptor (CAR) immune cells. Collectively, this review aims to summarize how Notch signaling intrinsically and extrinsically shapes immune responses to improve immunotherapy.

Strategy and application of manipulating DCs chemotaxis in disease treatment and vaccine design

As the most versatile antigen-presenting cells (APCs), dendritic cells (DCs) function as the cardinal commanders in orchestrating innate and adaptive immunity for either eliciting protective immune responses against canceration and microbial invasion or maintaining immune homeostasis/tolerance. In fact, in physiological or pathological conditions, the diversified migratory patterns and exquisite chemotaxis of DCs, prominently manipulate their biological activities in both secondary lymphoid organs (SLOs) as well as homeostatic/inflammatory peripheral tissues in vivo. Thus, the inherent mechanisms or regulation strategies to modulate the directional migration of DCs even could be regarded as the crucial cartographers of the immune system. Herein, we systemically reviewed the existing mechanistic understandings and regulation measures of trafficking both endogenous DC subtypes and reinfused DCs vaccines towards either SLOs or inflammatory foci (including neoplastic lesions, infections, acute/chronic tissue inflammations, autoimmune diseases and graft sites). Furthermore, we briefly introduced the DCs-participated prophylactic and therapeutic clinical application against disparate diseases, and also provided insights into the future clinical immunotherapies development as well as the vaccines design associated with modulating DCs mobilization modes.

Tumor-derived GCSF Alters Tumor and Systemic Immune System Cell Subset Composition and Signaling

While immunotherapies such as immune checkpoint blockade and adoptive T-cell therapy improve survival for a subset of human malignancies, many patients fail to respond. Phagocytes including dendritic cells (DC), monocytes, and macrophages (MF) orchestrate innate and adaptive immune responses against tumors. However, tumor-derived factors may limit immunotherapy effectiveness by altering phagocyte signal transduction, development, and activity. Using Cytometry by Time-of-Flight, we found that tumor-derived GCSF altered myeloid cell distribution both locally and systemically. We distinguished a large number of GCSF-induced immune cell subset and signal transduction pathway perturbations in tumor-bearing mice, including a prominent increase in immature neutrophil/myeloid-derived suppressor cell (Neut/MDSC) subsets and tumor-resident PD-L1⁺ Neut/MDSCs. GCSF expression was also linked to distinct tumor-associated MF populations, decreased conventional DCs, and splenomegaly characterized by increased splenic progenitors with diminished DC differentiation potential. GCSF-dependent dysregulation of DC development was recapitulated in bone marrow cultures in vitro, using medium derived from GCSF-expressing tumor cell cultures. Importantly, tumor-derived GCSF impaired T-cell adoptive cell therapy effectiveness and was associated with increased tumor volume and diminished survival of mice with mammary cancer. Treatment with neutralizing anti-GCSF antibodies reduced colonic and circulatory Neut/MDSCs, normalized colonic immune cell composition and diminished tumor burden in a spontaneous model of mouse colon cancer. Analysis of human colorectal cancer patient gene expression data revealed a significant correlation between survival and low GCSF and Neut/MDSC gene expression. Our data suggest that normalizing GCSF bioactivity may improve immunotherapy in cancers associated with GCSF overexpression.

Significance

Tumor-derived GCSF leads to systemic immune population changes. GCSF blockade restores immune populations, improves immunotherapy, and reduces tumor size, paralleling human colorectal cancer data. GCSF inhibition may synergize with current immunotherapies to treat GCSF-secreting tumors.

Intrathymic dendritic cell-biased precursors promote human T cell lineage specification through IRF8-driven transmembrane TNF

The cross-talk between thymocytes and thymic stromal cells is fundamental for T cell development. In humans, intrathymic development of dendritic cells (DCs) is evident but its physiological significance is unknown. Here we showed that DC-biased precursors depended on the expression of the transcription factor IRF8 to express the membrane-bound precursor form of the cytokine TNF (tmTNF) to promote differentiation of thymus seeding hematopoietic progenitors into T-lineage specified precursors through activation of the TNF receptor (TNFR)-2 instead of TNFR1. In vitro recapitulation of TNFR2 signaling by providing low-density tmTNF or a selective TNFR2 agonist enhanced the generation of human T cell precursors. Our study shows that, in addition to mediating thymocyte selection and maturation, DCs function as hematopoietic stromal support for the early stages of human T cell development and provide proof of concept that selective targeting of TNFR2 can enhance the in vitro generation of T cell precursors for clinical application.

A novel 3D cell culture model to study the human small intestinal immune landscape

Several subsets of mononuclear phagocytes and DCs (MDC) populate the small intestine (SI), and these cells reportedly exert specialized functions in anti-microbial immunity and tolerance. Given the specialized phenotype of these cells, differing from other MDC family members, including their putative circulating blood precursors, local intestinal factors play key instructive roles in their differentiation. We designed an SI cell culture model composed of three intestinal epithelial cell (IEC) types, including absorptive enterocytes (E cells), antigen delivering microfold (M) cells, and mucus-producing goblet (G) cells plus T lymphocytes and soluble B cell-derived factors. This model was used to study the differentiation fate of CD34⁺ hematopoietic progenitor cell-derived monocyte/DC precursors. Progeny cells can be analyzed after a 3-week co-culture period, mimicking the physiologic turn-over time of intestinal MDC. A dominant monocyte differentiation pathway was suppressed, in favor of partial differentiation along DC and macrophage pathways, with low percentages of cells acquired DC or macrophage markers. Moreover, E and G cells play opposing roles in CX3CR1⁺ vs CD103^dim cell differentiation, indicating that both together might counter-balance M/DC differentiation. Thus, SI epithelial cells suppress M/DC differentiation, supporting a key role for exogenous factors in M/DC differentiation.

Different routes of MHC-I delivery to phagosomes and their consequences to CD8 T cell immunity

Dendritic cells (DCs) present internalized antigens to CD8 T cells through cross-presentation by major histocompatibility complex class I (MHC-I) molecules. While conventional cDC1 excel at cross-presentation, cDC2 can be licensed to cross-present during infection by signals from inflammatory receptors, most prominently Toll-like receptors (TLRs). At the core of the regulation of cross-presentation by TLRs is the control of subcellular MHC-I traffic. Within DCs, MHC-I are enriched within endosomal recycling compartments (ERC) and traffic to microbe-carrying phagosomes under the control of phagosome-compartmentalized TLR signals to favor CD8 T cell cross-priming to microbial antigens. Viral blockade of the transporter associated with antigen processing (TAP), known to inhibit the classic MHC-I presentation of cytoplasmic protein-derived peptides, depletes the ERC stores of MHC-I to simultaneously also block TLR-regulated cross-presentation. DCs counter this impairment in the two major pathways of MHC-I presentation to CD8 T cells by mobilizing noncanonical cross-presentation, which delivers MHC-I to phagosomes from a new location in the ER-Golgi intermediate compartment (ERGIC) where MHC-I abnormally accumulate upon TAP blockade. Noncanonical cross-presentation thus rescues MHC-I presentation and cross-primes TAP-independent CD8 T cells best-matched against target cells infected with immune evasive viruses. Because noncanonical cross-presentation relies on a phagosome delivery route of MHC-I that is not under TLR control, it risks potential cross-presentation of self-antigens during infection. Here I review these findings to illustrate how the subcellular route of MHC-I to phagosomes critically impacts the regulation of cross-presentation and the nature of the CD8 T cell response to infection and cancer. I highlight important and novel implications to CD8 T cell vaccines and immunotherapy.

CRISPR/Cas9-based Engineering of Immunoglobulin Loci in Hybridoma Cells

Development of the hybridoma technology by Köhler and Milstein (1975) has revolutionized the immunological field by enabling routine use of monoclonal antibodies (mAbs) in research and development efforts, resulting in their successful application in the clinic today. While recombinant good manufacturing practices production technologies are required to produce clinical grade mAbs, academic laboratories and biotechnology companies still rely on the original hybridoma lines to stably and effortlessly produce high antibody yields at a modest price. In our own work, we were confronted with a major issue when using hybridoma-derived mAbs: there was no control over the antibody format that was produced, a flexibility that recombinant production does allow. We set out to remove this hurdle by genetically engineering antibodies directly in the immunoglobulin (Ig) locus of hybridoma cells. We used clustered regularly interspaced short palindromic repeats (CRISPR)/CRISPR-associated protein 9 (Cas9) and homology-directed repair (HDR) to modify antibody's format [mAb or antigen-binding fragment (Fab')] and isotype. This protocol describes a straightforward approach, with little hands-on time, leading to stable cell lines secreting high levels of engineered antibodies. Parental hybridoma cells are maintained in culture, transfected with a guide RNA (gRNA) targeting the site of interest in the Ig locus and an HDR template to knock in the desired insert and an antibiotic resistance gene. By applying antibiotic pressure, resistant clones are expanded and characterized at the genetic and protein level for their ability to produce modified mAbs instead of the parental protein. Finally, the modified antibody is characterized in functional assays. To demonstrate the versatility of our strategy, we illustrate this protocol with examples where we have (i) exchanged the constant heavy region of the antibody, creating chimeric mAb of a novel isotype, (ii) truncated the antibody to create an antigenic peptide-fused Fab' fragment to produce a dendritic cell-targeted vaccine, and (iii) modified both the constant heavy (CH)1 domain of the heavy chain (HC) and the constant kappa (Cκ) light chain (LC) to introduce site-selective modification tags for further derivatization of the purified protein. Only standard laboratory equipment is required, which facilitates its application across various labs. We hope that this protocol will further disseminate our technology and help other researchers. Graphical abstract.

Emerging Strategies of Engineering and Tracking Dendritic Cells for Cancer Immunotherapy

Dendritic cells (DCs), a kind of specialized immune cells, play key roles in antitumor immune response and promotion of innate and adaptive immune responses. Recently, many strategies have been developed to utilize DCs in cancer therapy, such as delivering antigens and adjuvants to DCs and using scaffold to recruit and activate DCs. Here we outline how different DC subsets influence antitumor immunity, summarize the FDA-approved vaccines and cancer vaccines under clinical trials, discuss the strategies for engineering DCs and noninvasive tracking of DCs to improve antitumor immunotherapy, and reveal the potential of artificial neural networks for the design of DC based vaccines.

In Vitro Generation of Human Cross-Presenting Type 1 Conventional Dendritic Cells (cDC1s) and Plasmacytoid Dendritic Cells (pDCs)

Dendritic cells (DCs) represent one of the most important immune cell subsets in preventing the host from pathogen invasion by promoting both innate and adaptive immunity. Most research on human dendritic cells has focused on the easy-to-obtain dendritic cells derived in vitro from monocytes (MoDCs). However, many questions remain unanswered regarding the role of different dendritic cell types. The investigation of their roles in human immunity is hampered by their rarity and fragility, which especially holds true for type 1 conventional dendritic cells (cDC1s) and for plasmacytoid dendritic cells (pDCs). In vitro differentiation from hematopoietic progenitors emerged as a common way to produce different DC types, but the efficiency and reproducibility of these protocols needed to be improved and the extent to which the DCs generated in vitro resembled their in vivo counterparts required a more rigorous and global assessment. Here, we describe a cost-effective and robust in vitro differentiation system for the production of cDC1s and pDCs equivalent to their blood counterparts, from cord blood CD34⁺ hematopoietic stem cells (HSCs) cultured on a stromal feeder layer with a combination of cytokines and growth factors.

Enrichment of Large Numbers of Splenic Mouse Dendritic Cells After Injection of Flt3L-Producing Tumor Cells

Dendritic cells (DCs) are antigen-presenting cells (APCs) that shape innate and adaptive immunity. There are multiple subsets of DCs distinguished according to their phenotype and functional specialization. DCs are present in lymphoid organs and across multiple tissues. However, their frequency and numbers at these sites are very low making their functional study difficult. Multiple protocols have been developed to generate DCs in vitro from bone marrow progenitors, but they do not fully recapitulate DC complexity found in vivo. Therefore, directly amplifying endogenous DCs in vivo appears as an option to overcome this specific caveat. In this chapter, we describe a protocol to amplify murine DCs in vivo by the injection of a B16 melanoma cell line expressing the trophic factor FMS-like tyrosine kinase 3 ligand (Flt3L). We have also compared two methods of magnetic sorting of amplified DCs, both giving high yields of total murine DCs, but different representation of the main DC subsets found in vivo.

In Vitro Generation of Murine CD8α+ DEC205+ XCR1+ Cross-Presenting Dendritic Cells from Bone Marrow-Derived Hematopoietic Progenitors

Dendritic cells (DCs) comprise a heterogeneous population of antigen (Ag)-presenting cells that play a critical role in both innate and adaptive immunity. DCs orchestrate protective responses against pathogens and tumors while mediating tolerance to host tissues. Evolutionary conservation between species has allowed the successful use of murine models to identify and characterize DC types and functions relevant to human health. Among DCs, type 1 classical DCs (cDC1) are uniquely capable of inducing antitumor responses and therefore present a promising therapeutic target. However, the rarity of DCs, particularly cDC1, limits the number of cells that can be isolated for study. Despite significant effort, progress in the field has been hampered by inadequate methods to produce large quantities of functionally mature DCs in vitro. To overcome this challenge, we developed a culture system in which mouse primary bone marrow cells are cocultured with OP9 stromal cells expressing Notch ligand Delta-like 1 (OP9-DL1) to produce CD8α⁺ DEC205⁺ XCR1⁺ cDC1 (Notch cDC1). This novel method provides a valuable tool to facilitate the generation of unlimited cDC1 for functional studies and translational applications such as antitumor vaccination and immunotherapy.

In Vitro Generation of Murine Bone Marrow-Derived Dendritic Cells

Dendritic cells (DCs) are mononuclear phagocytes of hematopoietic origin residing in lymphoid and nonlymphoid tissues. DCs are often referred as the sentinels of the immune system as they can sense pathogens and danger signals. Upon activation, DCs migrate to the draining lymph nodes and present antigens to naïve T cells to trigger adaptive immunity. Hematopoietic progenitors for DCs reside in the adult bone marrow (BM). Therefore, BM cell culture systems have been developed to generate large amounts of primary DCs in vitro conveniently enabling to analyze their developmental and functional features. Here, we review various protocols enabling to generate DCs in vitro from murine BM cells and discuss the cellular heterogeneity of each culture system.

In Vitro Generation of Human Dendritic Cell Subsets from CD34+ Cord Blood Progenitors

Dendritic cells (DCs) are professional antigen-presenting cells controlling the activation of T cells and thus regulating adaptive immune response against pathogens or tumors. Modeling human DC differentiation and function is crucial for our understanding of immune response and the development of new therapies. Considering DC rarity in human blood, in vitro systems allowing their faithful generation are needed. This chapter will describe a DC differentiation method based on the co-culture of CD34⁺ cord blood progenitors together with mesenchymal stromal cells (eMSCs) engineered to deliver growth factors and chemokines.

In Vitro and In Vivo Assays to Evaluate Dendritic Cell Phagocytic Capacity

Phagocytosis is a process by which specific immune cells such as macrophages or dendritic cells engulf large particles. It is an important innate immune defense mechanism for removing a wide variety of pathogens and apoptotic cells. Following phagocytosis, nascent phagosomes are formed which, when fused to lysosome to become phagolysosome containing acidic proteases, will allow the degradation of ingested material. This chapter describes in vitro and in vivo assays to measure phagocytosis by murine dendritic cells using amine beads coupled with streptavidin Alexa 488. This protocol can also be applied to monitor phagocytosis in human dendritic cells.

Metabolic regulation of dendritic cell activation and immune function during inflammation

Dendritic cells (DCs) are antigen-presenting cells that bridge innate and adaptive immune responses. Multiple cell types, including DCs, rely on cellular metabolism to determine their fate. DCs substantially alter cellular metabolic pathways during activation, such as oxidative phosphorylation, glycolysis, fatty acid and amino acid metabolism, which have crucial implications for their functionality. In this review, we summarize and discuss recent progress in DC metabolic studies, focusing on how metabolic reprogramming influences DC activation and functionality and the potential metabolic differences among DC subsets. Improving the understanding of the relationship between DC biology and metabolic regulation may provide promising therapeutic targets for immune-mediated inflammatory diseases.

The Human Dendritic Cell Atlas: An Integrated Transcriptional Tool to Study Human Dendritic Cell Biology

Dendritic cells (DCs) are functionally diverse and are present in most adult tissues, but deep understanding of human DC biology is hampered by relatively small numbers of these in circulation and their short lifespan in human tissues. We built a transcriptional atlas of human DCs by combining samples from 14 expression profiling studies derived from 10 laboratories. We identified significant gene expression variation of DC subset-defining markers across tissue type and upon viral or bacterial stimulation. We further highlight critical gaps between in vitro-derived DC subsets and their in vivo counterparts and provide evidence that monocytes or cord blood progenitor in vitro-differentiated DCs fail to capture the repertoire of primary DC subsets or behaviors. In constructing a reference DC atlas, we provide an important resource for the community wishing to identify and annotate tissue-specific DC subsets from single-cell datasets, or benchmark new in vitro models of DC biology.

Dendritic cells generated from induced pluripotent stem cells and by direct reprogramming of somatic cells

Novel and exciting avenues allow generating dendritic cells (DC) by reprogramming of somatic cells. DC are obtained from induced pluripotent stem cells (iPS cells), referred to as ipDC, and by direct reprogramming of cells toward DC, referred to as induced DC (iDC). iPS cells represent pluripotent stem cells generated by reprogramming of somatic cells and can differentiate into all cell types of the body, including DC. This makes iPS cells and ipDC derived thereof useful for studying various DC subsets, acquiring high cell numbers for research and clinical use, or applying genome editing to generate DC with wanted properties. Thereby, ipDC overcome limitations in specific DC subsets, which are only found in low abundance in blood or lymphoid organs. iDC are generated by direct reprogramming of somatic cells with a specific set of transcription factors and offer an avenue to obtain DC without a pluripotent cell intermediate. ipDC and iDC retain patient and disease-specific mutations and this opens new perspectives for studying DC in disease. This review summarizes the current techniques used to generate ipDC and iDC, and the types and functionality of the DC generated.

Flt3L, LIF, and IL-10 combination promotes the selective in vitro development of ESAMlow cDC2B from murine bone marrow

The development of two conventional dendritic cells (DC) subsets (cDC1 and cDC2) and the plasmacytoid DC (pDC) in vivo and in cultures of bone marrow (BM) cells is mediated by the growth factor Flt3L. However, little is known about the factors that direct the development of the individual DC subsets. Here, we describe the selective in vitro generation of murine ESAM^low CD103^- XCR1^- CD172a⁺ CD11b⁺ cDC2 from BM by treatment with a combination of Flt3L, LIF, and IL-10 (collectively named as FL10). FL10 promotes common dendritic cell progenitors (CDP) proliferation in the cultures, similar to Flt3L and CDP sorted and cultured in FL10 generate exclusively cDC2. These cDC2 express the transcription factors Irf4, Klf4, and Notch2, and their growth is reduced using BM from Irf4^-/- mice, but the expression of Batf3 and Tcf4 is low. Functionally they respond to TLR3, TLR4, and TLR9 signals by upregulation of the surface maturation markers MHC II, CD80, CD86, and CD40, while they poorly secrete proinflammatory cytokines. Peptide presentation to TCR transgenic OT-II cells induced proliferation and IFN-γ production that was similar to GM-CSF-generated BM-DC and higher than Flt3L-generated DC. Together, our data support that FL10 culture of BM cells selectively promotes CDP-derived ESAM^low cDC2 (cDC2B) development and survival in vitro.

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.