GM-CSF increases cross-presentation and CD103 expression by mouse CD8⁺ spleen dendritic cells

IL-33 and IL-33-derived DC-based tumor immunotherapy

Interleukin-33 (IL-33), a member of the IL-1 family, is a cytokine released in response to tissue damage and is recognized as an alarmin. The multifaceted roles of IL-33 in tumor progression have sparked controversy within the scientific community. However, most findings generally indicate that endogenous IL-33 has a protumor effect, while exogenous IL-33 often has an antitumor effect in most cases. This review covers the general characteristics of IL-33 and its effects on tumor growth, with detailed information on the immunological mechanisms associated with dendritic cells (DCs). Notably, DCs possess the capability to uptake, process, and present antigens to CD8+ T cells, positioning them as professional antigen-presenting cells. Recent findings from our research highlight the direct association between the tumor-suppressive effects of exogenous IL-33 and a novel subset of highly immunogenic cDC1s. Exogenous IL-33 induces the development of these highly immunogenic cDC1s through the activation of other ST2+ immune cells both in vivo and in vitro. Recognizing the pivotal role of the immunogenicity of DC vaccines in DC-based tumor immunotherapy, we propose compelling methods to enhance this immunogenicity through the addition of IL-33 and the promotion of highly immunogenic DC generation.

Systemic inflammatory response syndrome triggered by blood-borne pathogens induces prolonged dendritic cell paralysis and immunosuppression

Blood-borne pathogens can cause systemic inflammatory response syndrome (SIRS) followed by protracted, potentially lethal immunosuppression. The mechanisms responsible for impaired immunity post-SIRS remain unclear. We show that SIRS triggered by pathogen mimics or malaria infection leads to functional paralysis of conventional dendritic cells (cDCs). Paralysis affects several generations of cDCs and impairs immunity for 3-4 weeks. Paralyzed cDCs display distinct transcriptomic and phenotypic signatures and show impaired capacity to capture and present antigens in vivo. They also display altered cytokine production patterns upon stimulation. The paralysis program is not initiated in the bone marrow but during final cDC differentiation in peripheral tissues under the influence of local secondary signals that persist after resolution of SIRS. Vaccination with monoclonal antibodies that target cDC receptors or blockade of transforming growth factor β partially overcomes paralysis and immunosuppression. This work provides insights into the mechanisms of paralysis and describes strategies to restore immunocompetence post-SIRS.

Emerging aspects of cytokine storm in COVID-19: The role of proinflammatory cytokines and therapeutic prospects

COVID-19 has claimed millions of lives during the last 3 years since initial cases were reported in Wuhan, China, in 2019. Patients with COVID-19 suffer from severe pneumonia, high fever, acute respiratory distress syndrome (ARDS), and multiple-organ dysfunction, which may also result in fatality in extreme cases. Cytokine storm (CS) is hyperactivation of the immune system, wherein the dysregulated production of proinflammatory cytokines could result in excessive immune cell infiltrations in the pulmonary tissues, resulting in tissue damage. The immune cell infiltration could also occur in other tissues and organs and result in multiple organs' dysfunction. The key cytokines implicated in the onset of disease severity include TNF-α, IFN-γ, IL-6, IL-1β, GM-CSF, and G-CSF. Controlling the CS is critical in treating COVID-19 disease. Therefore, different strategies are employed to mitigate the effects of CS. These include using monoclonal antibodies directed against soluble cytokines or the cytokine receptors, combination therapies, mesenchymal stem cell therapy, therapeutic plasma exchange, and some non-conventional treatment methods to improve patient immunity. The current review describes the role/s of critical cytokines in COVID-19-mediated CS and the respective treatment modalities.

Discovery of highly immunogenic spleen-resident FCGR3+CD103+ cDC1s differentiated by IL-33-primed ST2+ basophils

Recombinant interleukin-33 (IL-33) inhibits tumor growth, but the detailed immunological mechanism is still unknown. IL-33-mediated tumor suppression did not occur in Batf3^-/- mice, indicating that conventional type 1 dendritic cells (cDC1s) play a key role in IL-33-mediated antitumor immunity. A population of CD103⁺ cDC1s, which were barely detectable in the spleens of normal mice, increased significantly in the spleens of IL-33-treated mice. The newly emerged splenic CD103⁺ cDC1s were distinct from conventional splenic cDC1s based on their spleen residency, robust effector T-cell priming ability, and surface expression of FCGR3. DCs and DC precursors did not express Suppressor of Tumorigenicity 2 (ST2). However, recombinant IL-33 induced spleen-resident FCGR3⁺CD103⁺ cDC1s, which were found to be differentiated from DC precursors by bystander ST2⁺ immune cells. Through immune cell fractionation and depletion assays, we found that IL-33-primed ST2⁺ basophils play a crucial role in the development of FCGR3⁺CD103⁺ cDC1s by secreting IL-33-driven extrinsic factors. Recombinant GM-CSF also induced the population of CD103⁺ cDC1s, but the population neither expressed FCGR3 nor induced any discernable antitumor immunity. The population of FCGR3⁺CD103⁺ cDC1s was also generated in vitro culture of Flt3L-mediated bone marrow-derived DCs (FL-BMDCs) when IL-33 was added in a pre-DC stage of culture. FL-BMDCs generated in the presence of IL-33 (FL-33-DCs) offered more potent tumor immunotherapy than control Flt3L-BMDCs (FL-DCs). Human monocyte-derived DCs were also more immunogenic when exposed to IL-33-induced factors. Our findings suggest that recombinant IL-33 or an IL-33-mediated DC vaccine could be an attractive protocol for better tumor immunotherapy.

Immune Tolerance in the Oral Mucosa

The oral mucosa is a site of intense immune activity, where a large variety of immune cells meet to provide a first line of defense against pathogenic organisms. Interestingly, the oral mucosa is exposed to a plethora of antigens from food and commensal bacteria that must be tolerated. The mechanisms that enable this tolerance are not yet fully defined. Many works have focused on active immune mechanisms involving dendritic and regulatory T cells. However, epithelial cells also make a major contribution to tolerance by influencing both innate and adaptive immunity. Therefore, the tolerogenic mechanisms concurring in the oral mucosa are intertwined. Here, we review them systematically, paying special attention to the role of oral epithelial cells.

Natural killer cells and dendritic epidermal γδ T cells orchestrate type 1 conventional DC spatiotemporal repositioning toward CD8+ T cells

Successful immune responses rely on a regulated delivery of the right signals to the right cells at the right time. Here we show that natural killer (NK) and dendritic epidermal γδ T cells (DETCs) use similar mechanisms to spatiotemporally orchestrate conventional type 1 dendritic cell (cDC1) functions in the spleen, skin, and its draining lymph nodes (dLNs). Upon MCMV infection in the spleen, cDC1 clusterize with activated NK cells in marginal zones. This XCR1-dependent repositioning of cDC1 toward NK cells allows contact delivery of IL-12 and IL-15/IL-15Rα by cDC1, which is critical for NK cell responses. NK cells deliver granulocyte-macrophage colony-stimulating factor (GM-CSF) to cDC1, guiding their CCR7-dependent relocalization into the T cell zone. In MCMV-infected skin, XCL1-secreting DETCs promote cDC1 migration from the skin to the dLNs. This XCR1-dependent licensing of cDC1 both in the spleen and skin accelerates antiviral CD8⁺ T cell responses, revealing an additional mechanism through which cDC1 bridge innate and adaptive immunity.

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Upon viral infection in the spleen, NK cells clusterize with cDC1 in the marginal zone

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This XCL1/XCR1-dependent interaction allows mutual delivery of activating signals

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NK cell GM-CSF directs cDC1 migration to T cell zone boosting CD8+ T cell priming

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In the skin, DETCs contact cDC1 via XCL1/XCR1 to promote antiviral CD8+ T cell priming

Current strategies in engaging oncolytic viruses with antitumor immunity

Oncolytic virotherapy has produced promising yet limited results in preclinical and clinical studies. Besides direct oncolytic activity, a significant therapeutic mechanism of oncolytic virotherapy is the induction of tumor-specific immunity. Consequently, the efficacy of oncolytic viruses can be improved by the insertion of immune stimulator genes and rational combinatorial therapy with other immunotherapies. This article reviews recent efforts on arming oncolytic viruses with a variety of immune stimulator molecules, immune cell engagers, and other immune potentiating molecules. We outline what is known about the mechanisms of action and the corresponding results. The review also discusses recent preclinical and clinical studies of combining oncolytic virotherapy with immune-checkpoint inhibitors and the role of oncolytic virotherapy in changing the tumor microenvironment.

Blockade of the co-inhibitory molecule PD-1 unleashes ILC2-dependent antitumor immunity in melanoma

Group 2 innate lymphoid cells (ILC2s) are essential to maintain tissue homeostasis. In cancer, ILC2s can harbor both pro-tumorigenic and anti-tumorigenic functions, but we know little about their underlying mechanisms or whether they could be clinically relevant or targeted to improve patient outcomes. Here, we found that high ILC2 infiltration in human melanoma was associated with a good clinical prognosis. ILC2s are critical producers of the cytokine granulocyte-macrophage colony-stimulating factor, which coordinates the recruitment and activation of eosinophils to enhance antitumor responses. Tumor-infiltrating ILC2s expressed programmed cell death protein-1, which limited their intratumoral accumulation, proliferation and antitumor effector functions. This inhibition could be overcome in vivo by combining interleukin-33-driven ILC2 activation with programmed cell death protein-1 blockade to significantly increase antitumor responses. Together, our results identified ILC2s as a critical immune cell type involved in melanoma immunity and revealed a potential synergistic approach to harness ILC2 function for antitumor immunotherapies.

Decipher the Glioblastoma Microenvironment: The First Milestone for New Groundbreaking Therapeutic Strategies

Glioblastoma (GBM) is the most common primary malignant brain tumour in adults. Despite the combination of novel therapeutical approaches, it remains a deadly malignancy with an abysmal prognosis. GBM is a polymorphic tumour from both molecular and histological points of view. It consists of different malignant cells and various stromal cells, contributing to tumour initiation, progression, and treatment response. GBM’s microenvironment is multifaceted and is made up of soluble factors, extracellular matrix components, tissue-resident cell types (e.g., neurons, astrocytes, endothelial cells, pericytes, and fibroblasts) together with resident (e.g., microglia) or recruited (e.g., bone marrow-derived macrophages) immune cells. These latter constitute the so-called immune microenvironment, accounting for a substantial GBM’s tumour volume. Despite the abundance of immune cells, an intense state of tumour immunosuppression is promoted and developed; this represents the significant challenge for cancer cells’ immune-mediated destruction. Though literature data suggest that distinct GBM’s subtypes harbour differences in their microenvironment, its role in treatment response remains obscure. However, an in-depth investigation of GBM’s microenvironment may lead to novel therapeutic opportunities to improve patients’ outcomes. This review will elucidate the GBM’s microenvironment composition, highlighting the current state of the art in immunotherapy approaches. We will focus on novel strategies of active and passive immunotherapies, including vaccination, gene therapy, checkpoint blockade, and adoptive T-cell therapies.

Oncolytic virotherapy-mediated anti-tumor response: a single-cell perspective

Talimogene laherparepvec (T-VEC) is a genetically modified herpes simplex 1 virus (HSV-1) approved for cancer therapy. We investigate its effect on the clinical, histological, single-cell transcriptomic, and immune repertoire level using repeated fine-needle aspirates (FNAs) of injected and noninjected lesions in primary cutaneous B cell lymphoma (pCBCL). Thirteen patients received intralesional T-VEC, 11 of which demonstrate tumor response in the injected lesions. Using single-cell sequencing of the FNAs, we identify the malignant population and separate three pCBCL subtypes. Twenty-four hours after the injection, we detect HSV-1^T-VEC transcripts in malignant and nonmalignant cells of the injected lesion but not of the noninjected lesion. Oncolytic virotherapy results in a rapid eradication of malignant cells. It also leads to interferon pathway activation and early influx of natural killer cells, monocytes, and dendritic cells. These events are followed by enrichment in cytotoxic T cells and a decrease of regulatory T cells in injected and noninjected lesions.

Functional Human CD141+ Dendritic Cells in Human Immune System Mice

BACKGROUND

For the purpose of studying functional human dendritic cells (DCs) in a humanized mouse model that mimics the human immune system (HIS), a model referred to as HIS mice was established.

METHODS

Human immune system mice were made by engrafting NOD/SCID/IL2Rgammanull (NSG) mice with human hematopoietic stem cells (HSCs) following the transduction of genes encoding human cytokines and human leukocyte antigen (HLA)-A2.1 by adeno-associated virus serotype 9 (AAV9) vectors.

RESULTS

Our results indicate that human DC subsets, such as CD141+CD11c+ and CD1c+CD11c+ myeloid DCs, distribute throughout several organs in HIS mice including blood, bone marrow, spleen, and draining lymph nodes. The CD141+CD11c+ and CD1c+CD11c+ human DCs isolated from HIS mice immunized with adenoviruses expressing malaria/human immunodeficiency virus (HIV) epitopes were able to induce the proliferation of malaria/HIV epitopes-specific human CD8+ T cells in vitro. Upregulation of CD1c was also observed in human CD141+ DCs 1 day after immunization with the adenovirus-based vaccines.

CONCLUSIONS

Establishment of such a humanized mouse model that mounts functional human DCs enables preclinical assessment of the immunogenicity of human vaccines in vivo.

Selective inhibition of mTORC1 in tumor vessels increases antitumor immunity

A tumor blood vessel is a key regulator of tissue perfusion, immune cell trafficking, cancer metastasis, and therapeutic responsiveness. mTORC1 is a signaling node downstream of multiple angiogenic factors in the endothelium. However, mTORC1 inhibitors have limited efficacy in most solid tumors, in part due to inhibition of immune function at high doses used in oncology patients and compensatory PI3K signaling triggered by mTORC1 inhibition in tumor cells. Here we show that low-dose RAD001/everolimus, an mTORC1 inhibitor, selectively targets mTORC1 signaling in endothelial cells (ECs) without affecting tumor cells or immune cells, resulting in tumor vessel normalization and increased antitumor immunity. Notably, this phenotype was recapitulated upon targeted inducible gene ablation of the mTORC1 component Raptor in tumor ECs (RaptorECKO). Tumors grown in RaptorECKO mice displayed a robust increase in tumor-infiltrating lymphocytes due to GM-CSF-mediated activation of CD103+ dendritic cells and displayed decreased tumor growth and metastasis. GM-CSF neutralization restored tumor growth and metastasis, as did T cell depletion. Importantly, analyses of human tumor data sets support our animal studies. Collectively, these findings demonstrate that endothelial mTORC1 is an actionable target for tumor vessel normalization, which could be leveraged to enhance antitumor immune therapies.

NOD2 modulates immune tolerance via the GM-CSF-dependent generation of CD103+ dendritic cells

Four decades ago, it was identified that muramyl dipeptide (MDP), a peptidoglycan-derived bacterial cell wall component, could display immunosuppressive functions in animals through mechanisms that remain unexplored. We sought to revisit these pioneering observations because mutations in NOD2, the gene encoding the host sensor of MDP, are associated with increased risk of developing the inflammatory bowel disease Crohn's disease, thus suggesting that the loss of the immunomodulatory functions of NOD2 could contribute to the development of inflammatory disease. Here, we demonstrate that intraperitoneal (i.p.) administration of MDP triggered regulatory T cells and the accumulation of a population of tolerogenic CD103⁺ dendritic cells (DCs) in the spleen. This was found to occur not through direct sensing of MDP by DCs themselves, but rather via the production of the cytokine GM-CSF, another factor with an established regulatory role in Crohn's disease pathogenesis. Moreover, we demonstrate that populations of CD103-expressing DCs in the gut lamina propria are enhanced by the activation of NOD2, indicating that MDP sensing plays a critical role in shaping the immune response to intestinal antigens by promoting a tolerogenic environment via manipulation of DC populations.

Naringenin Enhances the Antitumor Effect of Therapeutic Vaccines by Promoting Antigen Cross-Presentation

Dendritic cells (DCs) can internalize and cross-present exogenous Ags to CD8⁺ T cells for pathogen or tumor cell elimination. Recently, growing evidences suggest the possible immunoregulatory role of flavonoids through modulating the Ag presentation of DCs. In this study, we report that naringenin, a grapefruit-derived flavonoid, possesses the ability to increase the Ag cross-presentation in both murine DC line DC2.4 as well as bone marrow-derived DCs, and naringenin-induced moderate intracellular oxidative stress that contributed to the disruption of lysosomal membrane enhanced Ag leakage to cytosol and cross-presentation. Moreover, in a murine colon adenocarcinoma model, naringenin induced more CD103⁺ DCs infiltration into tumor and facilitated the activation of CD8⁺ T cells and strengthened the performance of therapeutic E7 vaccine against TC-1 murine lung cancer. Our investigations may inspire novel thoughts for vaccine design and open a new field of potential applications of flavonoids as immunomodulators to improve host protection against infection and tumor.

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

The IRF8-dependent subset of classical dendritic cells (cDCs), termed cDC1, is important for cross-priming cytotoxic T cell responses against pathogens and tumors. Culture of hematopoietic progenitors with DC growth factor FLT3 ligand (FLT3L) yields very few cDC1s (in humans) or only immature “cDC1-like” cells (in the mouse). We report that OP9 stromal cells expressing the Notch ligand Delta-like 1 (OP9-DL1) optimize FLT3L-driven development of cDC1s from murine immortalized progenitors and primary bone marrow cells. Co-culture with OP9-DL1 induced IRF8-dependent cDC1s with a phenotype (CD103⁺ Dec205⁺ CD8α⁺) and expression profile resembling primary splenic cDC1s. OP9-DL1-induced cDC1s showed preferential migration toward CCR7 ligands in vitro and superior T cell cross-priming and antitumor vaccination in vivo. Co-culture with OP9-DL1 also greatly increased the yield of IRF8-dependent CD141⁺ cDC1s from human bone marrow progenitors cultured with FLT3L. Thus, Notch signaling optimizes cDC generation in vitro and yields authentic cDC1s for functional studies and translational applications.

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DL1-Notch2 signaling induces differentiation of murine CD8α⁺ CD103⁺ cDC1s in vitro

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Notch-induced cDC1s show improved expression profile and CCR7-dependent migration

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Notch-induced cDC1s mediate superior T cell cross-priming and antitumor vaccination

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DL1 signaling facilitates in vitro generation of human IRF8-dependent CD141⁺ cDC1s

The Pleiotropic Effects of the GM-CSF Rheostat on Myeloid Cell Differentiation and Function: More Than a Numbers Game

Granulocyte Macrophage-Colony Stimulating Factor (GM-CSF) is a myelopoietic growth factor that has pleiotropic effects not only in promoting the differentiation of immature precursors into polymorphonuclear neutrophils (PMNs), monocytes/macrophages (MØs) and dendritic cells (DCs), but also in controlling the function of fully mature myeloid cells. This broad spectrum of GM-CSF action may elicit paradoxical outcomes-both immunostimulation and immunosuppression-in infection, inflammation, and cancer. The complexity of GM-CSF action remains to be fully unraveled. Several aspects of GM-CSF action could contribute to its diverse biological consequences. Firstly, GM-CSF as a single cytokine affects development of most myeloid cells from progenitors to mature immune cells. Secondly, GM-CSF activates JAK2/STAT5 and also activate multiple signaling modules and transcriptional factors that direct different biological processes. Thirdly, GM-CSF can be produced by different cell types including tumor cells in response to different environmental cues; thus, GM-CSF quantity can vary greatly under different pathophysiological settings. Finally, GM-CSF signaling is also fine-tuned by other less defined feedback mechanisms. In this review, we will discuss the role of GM-CSF in orchestrating the differentiation, survival, and proliferation during the generation of multiple lineages of myeloid cells (PMNs, MØs, and DCs). We will also discuss the role of GM-CSF in regulating the function of DCs and the functional polarization of MØs. We highlight how the dose of GM-CSF and corresponding signal strength acts as a rheostat to fine-tune cell fate, and thus the way GM-CSF may best be targeted for immuno-intervention in infection, inflammation and cancer.

Molecular regulation of dendritic cell development and function in homeostasis, inflammation, and cancer

Dendritic cells (DCs) are the principal antigen-presenting cells of the immune system and play key roles in controlling immune tolerance and activation. As such, DCs are chief mediators of tumor immunity. DCs can regulate tolerogenic immune responses that facilitate unchecked tumor growth. Importantly, however, DCs also mediate immune-stimulatory activity that restrains tumor progression. For instance, emerging evidence indicates the cDC1 subset has important functions in delivering tumor antigens to lymph nodes and inducing antigen-specific lymphocyte responses to tumors. Moreover, DCs control specific therapeutic responses in cancer including those resulting from immune checkpoint blockade. DC generation and function is influenced profoundly by cytokines, as well as their intracellular signaling proteins including STAT transcription factors. Regardless, our understanding of DC regulation in the cytokine-rich tumor microenvironment is still developing and must be better defined to advance cancer treatment. Here, we review literature focused on the molecular control of DCs, with a particular emphasis on cytokine- and STAT-mediated DC regulation. In addition, we highlight recent studies that delineate the importance of DCs in anti-tumor immunity and immune therapy, with the overall goal of improving knowledge of tumor-associated factors and intrinsic DC signaling cascades that influence DC function in cancer.

Langerin+CD8+ Dendritic Cells in the Splenic Marginal Zone: Not So Marginal After All

Dendritic cells (DC) fulfill an essential sentinel function within the immune system, acting at the interface of innate and adaptive immunity. The DC family, both in mouse and man, shows high functional heterogeneity in order to orchestrate immune responses toward the immense variety of pathogens and other immunological threats. In this review, we focus on the Langerin⁺CD8⁺ DC subpopulation in the spleen. Langerin⁺CD8⁺ DC exhibit a high ability to take up apoptotic/dying cells, and therefore they are essential to prime and shape CD8⁺ T cell responses. Next to the induction of immunity toward blood-borne pathogens, i.e., viruses, these DC are important for the regulation of tolerance toward cell-associated self-antigens. The ontogeny and differentiation pathways of CD8⁺CD103⁺ DC should be further explored to better understand the immunological role of these cells as a prerequisite of their therapeutic application.

Arming oncolytic reovirus with GM-CSF gene to enhance immunity

Oncolytic reovirus administration has been well tolerated by cancer patients in clinical trials. However, its anti-cancer efficacy as a monotherapy remains to be augmented. We and others have previously demonstrated the feasibility of producing replication-competent reoviruses expressing a heterologous transgene. Here, we describe the production of recombinant reoviruses expressing murine (mm) or human (hs) GM-CSF (rS1-mmGMCSF and rS1-hsGMCSF, respectively). The viruses could be propagated up to 10 passages while deletion mutants occurred only occasionally. In infected cell cultures, the secretion of GM-CSF protein (up to 481 ng/10⁶ cells per day) was demonstrated by ELISA. The secreted mmGM-CSF protein was functional in cell culture, as demonstrated by the capacity to stimulate the survival and proliferation of the GM-CSF-dependent dendritic cell (DC) line D1, and by its ability to generate DCs from murine bone marrow cells. Importantly, in a murine model of pancreatic cancer we found a systemic increase in DC and T-cell activation upon intratumoral administration of rS1-mmGMCSF. These data demonstrate that reoviruses expressing functional GM-CSF can be generated and have the potential to enhance anti-tumor immune responses. The GM-CSF reoviruses represent a promising new agent for use in oncolytic virotherapy strategies.

Neonatal mice possess two phenotypically and functionally distinct lung-migratory CD103+ dendritic cell populations following respiratory infection

The CD103⁺ subset of lung-migratory dendritic cells (DCs) plays an important role in the generation of CD8⁺ T cell responses following respiratory infection. Here, we demonstrate that the dependence on CD103⁺ DCs for stimulation of RSV-specific T cells is both epitope and age-dependent. CD103⁺ DCs in neonatal mice develop two phenotypically and functionally distinct populations following respiratory infection. Neonatal CD103⁺ DCs expressing low levels of CD103 (CD103lo DCs) and other lineage and maturation markers including costimulatory molecules are phenotypically immature and functionally limited. CD103lo DCs sorted from infected neonates were unable to stimulate cells of the K^dM2_82-90 specificity, which are potently stimulated by CD103hi DCs sorted from the same animals. These data suggest that the delayed maturation of CD103⁺ DCs in the neonate limits the K^dM2_82-90-specific response and explain the distinct CD8⁺ T cell response hierarchy displayed in neonatal mice that differs from the hierarchy seen in adult mice. These findings have implications for the development of early-life vaccines, where the promotion of responses with less age bias may prove advantageous. Alternately, specific approaches may be used to enhance the maturation and function of the CD103lo DC population in neonates to promote more adult-like T cell responses.

Dendritic cells in cancer: the role revisited

Dendritic cells (DCs) with their potent antigen presenting ability are long considered as critical factor in antitumor immunity. Despite high potential in promoting antitumor responses, tumor-associated DCs are largely defective in their functional activity and can contribute to immune suppression in cancer. In recent years existence of immune suppressive regulatory DCs in tumor microenvironment was described. Monocytic myeloid derived suppressor cells (M-MDSCs) can contribute to the pool of tumor associated DCs by differentiating to inflammatory DCs (inf-DCs), which appear to have specific phenotype and is critical component of antitumor response. Here we examine the role of inf-DCs along with other DC subsets in the regulation of immune responses in cancer. These novel data expand our view on the role of DCs in cancer and may provide new targets for immunotherapy.

Serpinb9 is a marker of antigen cross-presenting dendritic cells

Serpinb9 (Sb9, also called Spi6) is an intracellular inhibitor of granzyme B (grB) that protects cytotoxic lymphocytes from grB-mediated death. In addition, Sb9 is also expressed in accessory immune cells, including dendritic cells (DCs), although its role is debated. Recently, we have demonstrated that Sb9 plays a grB-independent role in cross-presentation of antigens by CD8⁺ DCs. Here, using a mouse line expressing green fluorescent protein knocked in under the control of the Sb9 promoter, we demonstrate that Sb9 expression is highest in those tissue-resident and migratory DC subsets capable of cross-presentation. Further, we show that CD8⁺ DCs can be divided into two subsets based on Sb9 expression, and that only the subset expressing higher levels of Sb9 is capable of cross-presentation. These findings add support for role for Sb9 cross-presentation, and indicate that high Sb9 expression is a novel marker of cross-presentation capable DCs.

Targeting dendritic cells to accelerate T-cell activation overcomes a bottleneck in tuberculosis vaccine efficacy

The development of a tuberculosis (TB) vaccine that induces sterilizing immunity to Mycobacterium tuberculosis infection has been elusive. Absence of sterilizing immunity induced by TB vaccines may be due to delayed activation of mucosal dendritic cells (DCs), and subsequent delay in antigen presentation and activation of vaccine-induced CD4⁺ T-cell responses. Here we show that pulmonary delivery of activated M. tuberculosis antigen-primed DCs into vaccinated mice, at the time of M. tuberculosis exposure, can overcome the delay in accumulation of vaccine-induced CD4⁺ T-cell responses. In addition, activating endogenous host CD103⁺ DCs and the CD40–CD40L pathway can similarly induce rapid accumulation of vaccine-induced lung CD4⁺ T-cell responses and limit early M. tuberculosis growth. Thus, our study provides proof of concept that targeting mucosal DCs can accelerate vaccine-induced T-cell responses on M. tuberculosis infection, and provide insights to overcome bottlenecks in TB vaccine efficacy.

A delay in T cell responses is postulated as a possible explanation for the limited efficacy of vaccines against tuberculosis. Here the authors demonstrate this T-cell block and remove it by activating endogenous dendritic cells or delivering activated dendritic cells to the lungs, enhancing immunity of mice to Mycobacterium tuberculosis.

MyD88/CD40 Genetic Adjuvant Function in Cutaneous Atypical Antigen-Presenting Cells Contributes to DNA Vaccine Immunogenicity

Therapeutic DNA-based vaccines aim to prime an adaptive host immune response against tumor-associated antigens, eliminating cancer cells primarily through CD8+ cytotoxic T cell-mediated destruction. To be optimally effective, immunological adjuvants are required for the activation of tumor-specific CD8+ T cells responses by DNA vaccination. Here, we describe enhanced anti-tumor efficacy of an in vivo electroporation-delivered DNA vaccine by inclusion of a genetically encoded chimeric MyD88/CD40 (MC) adjuvant, which integrates both innate and adaptive immune signaling pathways. When incorporated into a DNA vaccine, signaling by the MC adjuvant increased antigen-specific CD8+ T cells and promoted elimination of pre-established tumors. Interestingly, MC-enhanced vaccine efficacy did not require direct-expression of either antigen or adjuvant by local antigen-presenting cells, but rather our data supports a key role for MC function in "atypical" antigen-presenting cells of skin. In particular, MC adjuvant-modified keratinocytes increased inflammatory cytokine secretion, upregulated surface MHC class I, and were able to increase in vitro and in vivo priming of antigen-specific CD8+ T cells. Furthermore, in the absence of critical CD8α+/CD103+ cross-priming dendritic cells, MC was still able to promote immune priming in vivo, albeit at a reduced level. Altogether, our data support a mechanism by which MC signaling activates an inflammatory phenotype in atypical antigen-presenting cells within the cutaneous vaccination site, leading to an enhanced CD8+ T cell response against DNA vaccine-encoded antigens, through both CD8α+/CD103+ dendritic cell-dependent and independent pathways.

Bypassing STAT3-mediated inhibition of the transcriptional regulator ID2 improves the antitumor efficacy of dendritic cells

Despite the potent ability of dendritic cells (DCs) to stimulate lymphocyte responses and host immunity, granulocyte-macrophage colony-stimulating factor-derived DCs (GM-DCs) used as antitumor vaccines have demonstrated relatively modest success in cancer immunotherapy. We found that injecting GM-DCs into melanoma tumors in mice, or culturing GM-DCs with melanoma-secreted cytokines or melanoma-conditioned medium, rapidly suppressed DC-intrinsic expression of the gene encoding inhibitor of differentiation 2 (ID2), a transcriptional regulator. Melanoma-associated cytokines repressed Id2 transcription in murine DCs through the activation of signal transducer and activator of transcription 3 (STAT3). Enforced expression of ID2 in GM-DCs (ID2-GM-DCs) suppressed their production of the proinflammatory cytokine tumor necrosis factor-α (TNF-α). Vaccination with ID2-GM-DCs slowed the progression of melanoma tumors and enhanced animal survival, which was associated with an increased abundance of tumor-infiltrating interferon-γ-positive CD4(+) effector and CD8(+) cytotoxic T cells and a decreased number of tumor-infiltrating regulatory CD4(+) T cells. The efficacy of the ID2-GM-DC vaccine was improved by combinatorial treatment with a blocking antibody to programmed cell death protein-1 (PD-1), a current immunotherapy that overcomes suppressive immune checkpoint signaling. Collectively, our data reveal a previously unrecognized STAT3-mediated immunosuppressive mechanism in DCs and indicate that DC-intrinsic ID2 promotes tumor immunity by modulating tumor-associated CD4(+) T cell responses. Thus, inhibiting STAT3 or overexpressing ID2 selectively in DCs may improve the efficiency of DC vaccines in cancer therapy.

GM-CSF as a therapeutic target in autoimmune diseases

Granulocyte-macrophage colony-stimulating factor (GM-CSF) has been known as a hematopoietic growth factor and immune modulator. Recent studies revealed that GM-CSF also had pro-inflammatory functions and contributed to the pathogenicity of Th17 cells in the development of Th17-mediated autoimmune diseases. GM-CSF inhibition in some animal models of autoimmune diseases showed significant beneficial effects. Therefore, several agents targeting GM-CSF are being developed and are expected to be a useful strategy for the treatment of autoimmune diseases. Particularly, in clinical trials for rheumatoid arthritis (RA) patients, GM-CSF inhibition showed rapid and significant efficacy with no serious side effects. This article summarizes recent findings of GM-CSF and information of clinical trials targeting GM-CSF in autoimmune diseases.

Origin, Localization, and Immunoregulatory Properties of Pulmonary Phagocytes in Allergic Asthma

Allergic asthma is a chronic inflammatory disease of the airways that is driven by maladaptive T helper 2 (Th2) and Th17 immune responses against harmless, airborne substances. Pulmonary phagocytes represent the first line of defense in the lung where they constantly sense the local environment for potential threats. They comprise two distinct cell types, i.e., macrophages and dendritic cells (DC) that differ in their origins and functions. Alveolar macrophages quickly take up most of the inhaled allergens, yet do not deliver their cargo to naive T cells sampling in draining lymph nodes. In contrast, pulmonary DCs instruct CD4(+) T cells develop into Th2 and Th17 effectors, initiating the maladaptive immune responses toward harmless environmental substances observed in allergic individuals. Unraveling the mechanisms underlying this mistaken identity of harmless, airborne substances by innate immune cells is one of the great challenges in asthma research. The identification of different pulmonary DC subsets, their role in antigen uptake, migration to the draining lymph nodes, and their potential to instruct distinct T cell responses has set the stage to unravel this mystery. However, at this point, a detailed understanding of the spatiotemporal resolution of DC subset localization, allergen uptake, processing, autocrine and paracrine cellular crosstalk, and the humoral factors that define the activation status of DCs is still lacking. In addition to DCs, at least two distinct macrophage populations have been identified in the lung that are either located in the airway/alveolar lumen or in the interstitium. Recent data suggest that such populations can exert either pro- or anti-inflammatory functions. Similar to the DC subsets, detailed insights into the individual roles of alveolar and interstitial macrophages during the different phases of asthma development are still missing. Here, we will provide an update on the current understanding of the origin, localization, and function of the diverse pulmonary antigen-presenting cell subsets, in particular with regard to the development and regulation of allergic asthma. While most data are from mouse models of experimental asthma, we have also included available human data to judge the translational value of the findings obtained in experimental asthma models.

Generation of mouse and human dendritic cells in vitro

Dendritic cells (DC) that can orchestrate immune responses and maintain host homeostasis, are indispensable components of the immune system. Although distributed widely in many lymphoid and non-lymphoid tissues, their rarity in number has become a limiting factor for DC related research and therapies. Therefore, methods for efficiently generating large numbers of DC resembling their in vivo counterparts are urgently needed for DC related research and therapies. Herein we summarize the current methods for generating mouse and human DC in vitro and hope that these will facilitate both studies of DC biology and their clinical applications.

Design and characterization of the tumor vaccine MGN1601, allogeneic fourfold gene-modified vaccine cells combined with a TLR-9 agonist

The tumor vaccine MGN1601 was designed and developed for treatment of metastatic renal cell carcinoma (mRCC). MGN1601 consists of a combination of fourfold gene-modified cells with the toll-like receptor 9 agonist dSLIM, a powerful connector of innate and adaptive immunity. Vaccine cells originate from a renal cell carcinoma cell line (grown from renal cell carcinoma tissue), express a variety of known tumor-associated antigens (TAA), and are gene modified to transiently express two co-stimulatory molecules, CD80 and CD154, and two cytokines, GM-CSF and IL-7, aimed to support immune response. Proof of concept of the designed vaccine was shown in mice: The murine homologue of the vaccine efficiently (100%) prevented tumor growth when used as prophylactic vaccine in a syngeneic setting. Use of the vaccine in a therapeutic setting showed complete response in 92% of mice as well as synergistic action and necessity of the components. In addition, specific cellular and humoral immune responses in mice were found when used in an allogeneic setting. Immune response to the vaccine was also shown in mRCC patients treated with MGN1601: Peptide array analysis revealed humoral CD4-based immune response to TAA expressed on vaccine cells, including survivin, cyclin D1, and stromelysin.

The Isolation and Enrichment of Large Numbers of Highly Purified Mouse Spleen Dendritic Cell Populations and Their In Vitro Equivalents

Dendritic cells (DCs) form a complex network of cells that initiate and orchestrate immune responses against a vast array of pathogenic challenges. Developmentally and functionally distinct DC subtypes differentially regulate T-cell function. Importantly it is the ability of DC to capture and process antigen, whether from pathogens, vaccines, or self-components, and present it to naive T cells that is the key to their ability to initiate an immune response. Our typical isolation procedure for DC from murine spleen was designed to efficiently extract all DC subtypes, without bias and without alteration to their in vivo phenotype, and involves a short collagenase digestion of the tissue, followed by selection for cells of light density and finally negative selection for DC. The isolation procedure can accommodate DC numbers that have been artificially increased via administration of fms-like tyrosine kinase 3 ligand (Flt3L), either directly through a series of subcutaneous injections or by seeding with an Flt3L secreting murine melanoma. Flt3L may also be added to bone marrow cultures to produce large numbers of in vitro equivalents of the spleen DC subsets. Total DC, or their subsets, may be further purified using immunofluorescent labeling and flow cytometric cell sorting. Cell sorting may be completely bypassed by separating DC subsets using a combination of fluorescent antibody labeling and anti-fluorochrome magnetic beads. Our procedure enables efficient separation of the distinct DC subsets, even in cases where mouse numbers or flow cytometric cell sorting time is limiting.

Transcriptional Control of Dendritic Cell Development

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

Monocyte-Derived Dendritic Cells Promote Th Polarization, whereas Conventional Dendritic Cells Promote Th Proliferation

Monocyte-derived dendritic cells (moDCs) dramatically increase in numbers upon infection and inflammation; accordingly, we found that this also occurs during allogeneic responses. Despite their prominence, how emergent moDCs and resident conventional DCs (cDCs) divide their labor as APCs remain undefined. Hence, we compared both direct and indirect presentation by murine moDCs versus cDCs. We found that, despite having equivalent MHC class II expression and in vitro survival, moDCs were 20-fold less efficient than cDCs at inducing CD4(+) T cell proliferation through both direct and indirect Ag presentation. Despite this, moDCs were more potent at inducing Th1 and Th17 differentiation (e.g., 8-fold higher IFN-γ and 2-fold higher IL-17A in T cell cocultures), whereas cDCs induced 10-fold higher IL-2 production. Intriguingly, moDCs potently reduced the ability of cDCs to stimulate T cell proliferation in vitro and in vivo, partially through NO production. We surmise that such division of labor between moDCs and cDCs has implications for their respective roles in the immune response.

Targeting GM-CSF in inflammatory diseases

Granulocyte-macrophage colony stimulating factor (GM-CSF) is a growth factor first identified as an inducer of differentiation and proliferation of granulocytes and macrophages derived from haematopoietic progenitor cells. Later studies have shown that GM-CSF is involved in a wide range of biological processes in both innate and adaptive immunity, with its production being tightly linked to the response to danger signals. Given that the functions of GM-CSF span multiple tissues and biological processes, this cytokine has shown potential as a new and important therapeutic target in several autoimmune and inflammatory disorders - particularly in rheumatoid arthritis. Indeed, GM-CSF was one of the first cytokines detected in human synovial fluid from inflamed joints. Therapies that target GM-CSF or its receptor have been tested in preclinical studies with promising results, further supporting the potential of targeting the GM-CSF pathway. In this Review, we discuss our expanding view of the biology of GM-CSF, outline what has been learnt about GM-CSF from studies of animal models and human diseases, and summarize the results of early phase clinical trials evaluating GM-CSF antagonism in inflammatory disorders.

Fate mapping of dendritic cells

Dendritic cells (DCs) are a heterogeneous group of mononuclear phagocytes with versatile roles in immunity. They are classified predominantly based on phenotypic and functional properties, namely their stellate morphology, expression of the integrin CD11c, and major histocompatibility class II molecules, as well as their superior capacity to migrate to secondary lymphoid organs and stimulate naïve T cells. However, these attributes are not exclusive to DCs and often change within inflammatory or infectious environments. This led to debates over cell identification and questioned even the mere existence of DCs as distinct leukocyte lineage. Here, we review experimental approaches taken to fate map DCs and discuss how these have shaped our understanding of DC ontogeny and lineage affiliation. Considering the ontogenetic properties of DCs will help to overcome the inherent shortcomings of purely phenotypic- and function-based approaches to cell definition and will yield a more robust way of DC classification.

Modulation of tolerogenic dendritic cells and autoimmunity

A key function of dendritic cells (DCs) is to induce either immune tolerance or immune activation. Many new DC subsets are being recognized, and it is now clear that each DC subset has a specialized function. For example, different DC subsets may express different cell surface molecules and respond differently to activation by secretion of a unique cytokine profile. Apart from intrinsic differences among DC subsets, various immune modulators in the microenvironment may influence DC function; inappropriate DC function is closely related to the development of immune disorders. The most exciting recent advance in DC biology is appreciation of human DC subsets. In this review, we discuss functionally different mouse and human DC subsets both in lymphoid organs and non-lymphoid organs, the molecules that regulate DC function, and the emerging understanding of the contribution of DCs to autoimmune diseases.

Th17 cytokine deficiency in patients with Aspergillus skull base osteomyelitis

BACKGROUND

Fungal skull base osteomyelitis (SBO) is a severe complication of otitis externa or sinonasal infection, and is mainly caused by Aspergillus species. Here we investigate innate and adaptive immune responses in patients with Aspergillus SBO to identify defects in the immune response that could explain the susceptibility to this devastating disease.

METHODS

Peripheral blood mononuclear cells isolated from six patients with Aspergillus SBO and healthy volunteers were stimulated with various microbial stimuli, among which also the fungal pathogens Candida albicans and Aspergillus fumigatus. The proinflammatory cytokines IL-6, TNFα and IL-1β, and the T-helper cell-derived cytokines IFNγ, IL-17 and IL-22 were measured in cell culture supernatants by ELISA.

RESULTS

Proinflammatory cytokine responses did not differ between SBO patients and healthy volunteers. The Candida- and Aspergillus-specific Th17 response (production of IL-17 and IL-22) was significantly decreased in the SBO patients compared to healthy individuals, while Th1 cytokine response (IFNγ production) did not differ between the two groups.

CONCLUSIONS

We show that patients with Aspergillus skull base osteomyelitis infection have specific defects in Th17 responses. Since IL-17 and IL-22 are important for stimulating antifungal host defense, we hypothesize that strategies that have the ability to improve IL-17 and IL-22 production may be useful as adjuvant immunotherapy in patients with Aspergillus SBO.

Pivotal roles of GM-CSF in autoimmunity and inflammation

Granulocyte macrophage-colony stimulating factor (GM-CSF) is a hematopoietic growth factor, which stimulates the proliferation of granulocytes and macrophages from bone marrow precursor cells. In autoimmune and inflammatory diseases, Th17 cells have been considered as strong inducers of tissue inflammation. However, recent evidence indicates that GM-CSF has prominent proinflammatory functions and that this growth factor (not IL-17) is critical for the pathogenicity of CD4⁺ T cells. Therefore, the mechanism of GM-CSF-producing CD4⁺ T cell differentiation and the role of GM-CSF in the development of autoimmune and inflammatory diseases are gaining increasing attention. This review summarizes the latest knowledge of GM-CSF and its relationship with autoimmune and inflammatory diseases. The potential therapies targeting GM-CSF as well as their possible side effects have also been addressed in this review.

GM-CSF as a target in inflammatory/autoimmune disease: current evidence and future therapeutic potential

Granulocyte-macrophage colony-stimulating factor (GM-CSF) can be viewed as a pro-inflammatory cytokine rather than as a key regulator of steady state and systemic myelopoiesis. Key aspects of GM-CSF biology need to be clarified such as pro-survival vs activation/differentiation function, its cellular sources, its responsive cell populations, its downstream mediators/pathways, and when GM-CSF is relevant. Striking effects of GM-CSF depletion/deletion in some pre-clinical autoimmune/inflammation models have been reported. Systemic effects of administered GM-CSF are not necessarily informative about its local blockade in disease. Recent clinical RA trials, particularly Phase II trials with mavrilimumab (anti-GM-CSFRα Ab), show rapid and impressive efficacy with no significant adverse effects. Larger and longer trials targeting GM-CSF are needed and with careful monitoring of unwanted side effects. This review summarizes the most recent information on these topics.

NOD mice are functionally deficient in the capacity of cross-presentation

Cross-presentation by CD8(+) conventional dendritic cells (cDCs) is involved in the maintenance of peripheral tolerance and this process is termed cross-tolerance. Previous reports showed that non-obese diabetic (NOD) mice have reduced number of splenic CD8(+) cDCs compared with non-diabetic strains, and that the administration of Flt3L to enhance DC development resulted in reduced diabetes incidence. As CD8(+) cDCs are the most efficient antigen cross-presenting cells, it was assumed that reduced cross-presentation by non-activated, tolerogenic CD8(+) cDC predisposes to autoimmune diabetogenesis. Here we show for the first time that indeed NOD mice have a defect in autoantigen cross-presentation capacity. First, we showed that NOD CD8(+) cDCs were less sensitive to iatrogenic cytochrome c, which had previously been shown to selectively deplete CD8(+) cDCs that functionally cross-present. Second, we found that proliferation of islet-specific glucose-6-phosphatase catalytic subunit-related protein (IGRP)-specific CD8(+) T cells was impaired in NOD compared with non-obese diabetes resistant mice after immunization with cell associated recombinant fusion protein containing the cognate IGRP peptide. This study, therefore, suggests that the reduced number of CD8(+) cDCs in NOD mice, coupled with the reduced capacity to cross-present self-antigens, reduces the overall capacity to maintain peripheral tolerance in the spontaneous autoimmune type 1 diabetes mice.

MIDGE Technology for the Production of a Fourfold Gene-Modified, Allogenic Cell-Based Vaccine for Cancer Therapy

Gene modification of eukaryotic cells by electroporation is a widely used method to express selected genes in a defined cell population for various purposes, like gene correction or production of therapeutics. Here, we describe the generation of a cell-based tumor vaccine via fourfold transient gene modification of a human renal cell carcinoma (RCC) cell line for high expression of CD80, CD154, GM-CSF, and IL-7 by use of MIDGE(®) vectors. The two co-stimulatory molecules CD80 and CD154 are expressed at the cell surface, whereas the two cytokines GM-CSF and IL-7 are secreted yielding cells with enhanced immunological properties. These fourfold gene-modified cells have been used as a cell-based tumor vaccine for the treatment of RCC.

Lymphoid tissue and plasmacytoid dendritic cells and macrophages do not share a common macrophage-dendritic cell-restricted progenitor

The relationship between dendritic cells (DCs) and macrophages is often debated. Here we ask whether steady-state, lymphoid-tissue-resident conventional DCs (cDCs), plasmacytoid DCs (pDCs), and macrophages share a common macrophage-DC-restricted precursor (MDP). Using new clonal culture assays combined with adoptive transfer, we found that MDP fractions isolated by previous strategies are dominated by precursors of macrophages and monocytes, include some multipotent precursors of other hematopoietic lineages, but contain few precursors of resident cDCs and pDCs and no detectable common precursors restricted to these DC types and macrophages. Overall we find no evidence for a common restricted MDP leading to both macrophages and FL-dependent, resident cDCs and pDCs.

Modulation of dendritic cell antigen presentation by pathogens, tissue damage and secondary inflammatory signals

Antigen presentation by dendritic cells (DC) is regulated directly by pathogen-associated or cell death-associated cues, or indirectly by immunomodulatory molecules produced during infection or tissue damage. DC modulation by direct encounter of pathogen-associated compounds has been thoroughly studied; the effects of molecules associated with cell death are less well characterized; modulation by secondary signals remain poorly understood. In this review we describe recent studies on the role of these three categories of immunomodulatory compounds on DC. We conclude that characterization of the role of secondary immunomodulators is an area in dare need of further study. The outcomes of this endeavor will be new opportunities for the development of better vaccines and compounds applicable to the therapeutic immunomodulation of DC function.

Selective and efficient generation of functional Batf3-dependent CD103+ dendritic cells from mouse bone marrow

Multiple subsets of FMS-like tyrosine kinase 3 ligand (FLT3L)-dependent dendritic cells (DCs) control T-cell tolerance and immunity. In mice, Batf3-dependent CD103(+) DCs efficiently enter lymph nodes and cross-present antigens, rendering this conserved DC subset a promising target for tolerance induction or vaccination. However, only limited numbers of CD103(+) DCs can be isolated with current methods. Established bone marrow culture protocols efficiently generate monocyte-derived DCs or produce a mixture of FLT3L-dependent DC subsets. We show that CD103(+) DC development requires prolonged culture time and continuous action of both FLT3L and granulocyte macrophage colony-stimulating factor (GM-CSF), explained by a dual effect of GM-CSF on DC precursors and differentiating CD103(+) DCs. Accordingly, we established a novel method to generate large numbers of CD103(+) DCs (iCD103-DCs) with limited presence of other DC subsets. iCD103-DCs develop in a Batf3- and Irf8-dependent fashion, express a CD8α/CD103 DC gene signature, cross-present cell-associated antigens, and respond to TLR3 stimulation. Thus, iCD103-DCs reflect key features of tissue CD103(+) DCs. Importantly, iCD103-DCs express high levels of CCR7 upon maturation and migrate to lymph nodes more efficiently than classical monocyte-derived DCs. Finally, iCD103-DCs induce T cell-mediated protective immunity in vivo. Our study provides insights into CD103(+) DC development and function.

Ontogenic, Phenotypic, and Functional Characterization of XCR1(+) Dendritic Cells Leads to a Consistent Classification of Intestinal Dendritic Cells Based on the Expression of XCR1 and SIRPα

In the past, lack of lineage markers confounded the classification of dendritic cells (DC) in the intestine and impeded a full understanding of their location and function. We have recently shown that the chemokine receptor XCR1 is a lineage marker for cross-presenting DC in the spleen. Now, we provide evidence that intestinal XCR1⁺ DC largely, but not fully, overlap with CD103⁺ CD11b⁻ DC, the hypothesized correlate of “cross-presenting DC” in the intestine, and are selectively dependent in their development on the transcription factor Batf3. XCR1⁺ DC are located in the villi of the lamina propria of the small intestine, the T cell zones of Peyer’s patches, and in the T cell zones and sinuses of the draining mesenteric lymph node. Functionally, we could demonstrate for the first time that XCR1⁺/CD103⁺ CD11b⁻ DC excel in the cross-presentation of orally applied antigen. Together, our data show that XCR1 is a lineage marker for cross-presenting DC also in the intestinal immune system. Further, extensive phenotypic analyses reveal that expression of the integrin SIRPα consistently demarcates the XCR1⁻ DC population. We propose a simplified and consistent classification system for intestinal DC based on the expression of XCR1 and SIRPα.