Organization of the mouse and human DC network

Human dendritic cell interactions with the zoonotic parasite Cryptosporidium parvum result in activation and maturation

Cryptosporidiosis in humans is caused by infection of the zoonotic apicomplexan parasite Cryptosporidium parvum. In 2006, it was included by the World Health Organization (WHO) in the group of the most neglected poverty-related diseases. It is characterized by enteritis accompanied by profuse catarrhalic diarrhea with high morbidity and mortality, especially in children of developing countries under the age of 5 years and in HIV patients. The vulnerability of HIV patients indicates that a robust adaptive immune response is required to successfully fight this parasite. Little is known, however, about the adaptive immune response against C. parvum. To have an insight into the early events of the adaptive immune response, we generated primary human dendritic cells (DCs) from monocytes of healthy blood donors and exposed them to C. parvum oocysts and sporozoites in vitro. DCs are equipped with numerous receptors that detect microbial molecules and alarm signals. If stimulation is strong enough, an essential maturation process turns DCs into unique activators of naïve T cells, a prerequisite of any adaptive immune response. Parasite exposure highly induced the production of the pro-inflammatory cytokines/chemokines interleukin (IL)-6 and IL-8 in DCs. Moreover, antigen-presenting molecules (HLA-DR and CD1a), maturation markers, and costimulatory molecules required for T-cell stimulation (CD83, CD40, and CD86) and adhesion molecules (CD11b and CD58) were all upregulated. In addition, parasite-exposed human DCs showed enhanced cell adherence, increased mobility, and a boosted but time-limited phagocytosis of C. parvum oocysts and sporozoites, representing other prerequisites for antigen presentation. Unlike several other microbial stimuli, C. parvum exposure rather led to increased oxidative consumption rates (OCRs) than extracellular acidification rates (ECARs) in DCs, indicating that different metabolic pathways were used to provide energy for DC activation. Taken together, C. parvum-exposed human DCs showed all hallmarks of successful maturation, enabling them to mount an effective adaptive immune response.

Differential Activation of Splenic cDC1 and cDC2 Cell Subsets following Poxvirus Infection of BALB/c and C57BL/6 Mice

Conventional dendritic cells (cDCs) are innate immune cells that play a pivotal role in inducing antiviral adaptive immune responses due to their extraordinary ability to prime and polarize naïve T cells into different effector T helper (Th) subsets. The two major subpopulations of cDCs, cDC1 (CD8α+ in mice and CD141+ in human) and cDC2 (CD11b+ in mice and CD1c+ in human), can preferentially polarize T cells toward a Th1 and Th2 phenotype, respectively. During infection with ectromelia virus (ECTV), an orthopoxvirus from the Poxviridae family, the timing and activation of an appropriate Th immune response contributes to the resistance (Th1) or susceptibility (Th2) of inbred mouse strains to the lethal form of mousepox. Due to the high plasticity and diverse properties of cDC subpopulations in regulating the quality of a specific immune response, in the present study we compared the ability of splenic cDC1 and cDC2 originating from different ECTV-infected mouse strains to mature, activate, and polarize the Th immune response during mousepox. Our results demonstrated that during early stages of mousepox, both cDC subsets from resistant C57BL/6 and susceptible BALB/c mice were activated upon in vivo ECTV infection. These cells exhibited elevated levels of surface MHC class I and II, and co-stimulatory molecules and showed enhanced potential to produce cytokines. However, both cDC subsets from BALB/c mice displayed a higher maturation status than that of their counterparts from C57BL/6 mice. Despite their higher activation status, cDC1 and cDC2 from susceptible mice produced low amounts of Th1-polarizing cytokines, including IL-12 and IFN-γ, and the ability of these cells to stimulate the proliferation and Th1 polarization of allogeneic CD4+ T cells was severely compromised. In contrast, both cDC subsets from resistant mice produced significant amounts of Th1-polarizing cytokines and demonstrated greater capability in differentiating allogeneic T cells into Th1 cells compared to cDCs from BALB/c mice. Collectively, our results indicate that in the early stages of mousepox, splenic cDC subpopulations from the resistant mouse strain can better elicit a Th1 cell-mediated response than the susceptible strain can, probably contributing to the induction of the protective immune responses necessary for the control of virus dissemination and for survival from ECTV challenge.

Virus-Like Nanotherapeutic for Spatiotemporally Enhancing Antigen Presentation and Cross-Presentation toward Potential Personalized Immunotherapy

One of the major causes of immunotherapy resistance is the loss of major histocompatibility complex class I (MHC-I) molecules in tumor cells or the downregulation of the class I antigen presentation pathway. In this study, a novel virus-like nanotherapeutic (siRNA@HCM) is developed via encapsulating nanosized siRNA nanoparticles in a hybrid membrane comprising a personalized tumor cell membrane and a universal 293T membrane expressing the mutant vesicular stomatitis virus glycoprotein (mVSV-G). Upon intravenous administration, siRNA@HCM accumulates at the tumor site and provides two potent driving forces for antitumor immunity. First, mVSV-G induces the fusion of siRNA@HCM with tumor cell membranes and directly injects siRNAs into the cytoplasm, significantly improving tumor intrinsic MHC-I antigen presentation. Moreover, mVSV-G can promote the maturation of dendritic cells, thereby achieving highly efficient antigen cross-presentation. The results demonstrate that spatiotemporally enhancing tumor intrinsic antigen presentation and cross-presentation via siRNA@HCM can achieve satisfactory antitumor efficacy and excellent biocompatibility. Immune infiltration analysis shows that siRNA@HCM treatment turns cold tumors into hot tumors. In addition, it significantly promotes the therapeutic effect of programmed death-1 inhibitor. In summary, virus-like nanotherapeutics present a promising approach to enhance the antitumor immune response, with distinct advantages for potential personalized therapy and clinical applications.

Dendritic cells in inborn errors of immunity

Dendritic cells (DCs) are crucial cells for initiating and maintaining immune response. They play critical role in homeostasis, inflammation, and autoimmunity. A number of molecules regulate their functions including synapse formation, migration, immunity, and induction of tolerance. A number of IEI are characterized by mutations in genes encoding several of these molecules resulting in immunodeficiency, inflammation, and autoimmunity in IEI. Currently, there are 465 Inborn errors of immunity (IEI) that have been grouped in 10 different categories. However, comprehensive studies of DCs have been reported in only few IEI. Here we have reviewed biology of DCs in IEI classified according to recently published IUIS classification. We have reviewed DCs in selected IEI in each group category and discussed in depth changes in DCs where significant data are available regarding role of DCs in clinical and immunological manifestations. These include severe immunodeficiency diseases, antibody deficiencies, combined immunodeficiency with associated and syndromic features, especially disorders of synapse formation, and disorders of immune regulation.

How cell migration helps immune sentinels

The immune system relies on the migratory capacity of its cellular components, which must be mobile in order to defend the host from invading micro-organisms or malignant cells. This applies in particular to immune sentinels from the myeloid lineage, i.e. macrophages and dendritic cells. Cell migration is already at work during mammalian early development, when myeloid cell precursors migrate from the yolk sac, an extra embryonic structure, to colonize tissues and form the pool of tissue-resident macrophages. Later, this is accompanied by a migration wave of precursors and monocytes from the bone marrow to secondary lymphoid organs and the peripheral tissues. They differentiate into DCs and monocyte-derived macrophages. During adult life, cell migration endows immune cells with the ability to patrol their environment as well as to circulate between peripheral tissues and lymphoid organs. Hence migration of immune cells is key to building an efficient defense system for an organism. In this review, we will describe how cell migratory capacity regulates the various stages in the life of myeloid cells from development to tissue patrolling, and migration to lymph nodes. We will focus on the role of the actin cytoskeletal machinery and its regulators, and how it contributes to the establishment and function of the immune system.

Mast Cell Interaction with Foxp3+ Regulatory T Cells Occur in the Dermis after Initiation of IgE-Mediated Cutaneous Anaphylaxis

Mast cells (MCs) are well-known for their role in IgE-mediated cutaneous anaphylactic responses, but their regulatory functions in the skin are still under intense scrutiny. Using a Red MC and Basophil reporter (RMB) mouse allowing red fluorescent detection and diphtheria toxin mediated depletion of MCs, we investigated the interaction of MCs, Foxp3⁺ regulatory T lymphocytes (Tregs) and Langerhans cells (LCs) during passive cutaneous anaphylaxis (PCA) responses. Using intravital imaging we show that MCs are sessile at homeostasis and during PCA. Breeding RMB mice with Langerin-eGFP mice revealed that dermal MCs do not interact with epidermal-localized LCs, the latter showing constant sprouting of their dendrites at homeostasis and during PCA. When bred with Foxp3-eGFP mice, we found that, although a few Foxp3⁺ Tregs are present at homeostasis, many Tregs transiently infiltrated the skin during PCA. While their velocity during PCA was not altered, Tregs increased the duration of their contact time with MCs compared to PCA-control mice. Antibody-mediated depletion of Tregs had no effect on the intensity of PCA. Hence, the observed increase in Treg numbers and contact time with MCs, regardless of an effect on the intensity of PCA responses, suggests an anti-inflammatory role dedicated to prevent further MC activation.

β2 Integrins on Dendritic Cells Modulate Cytokine Signaling and Inflammation-Associated Gene Expression, and Are Required for Induction of Autoimmune Encephalomyelitis

Heterodimeric β2 integrin surface receptors (CD11a-d/CD18) are specifically expressed by leukocytes that contribute to pathogen uptake, cell migration, immunological synapse formation and cell signaling. In humans, the loss of CD18 expression results in leukocyte adhesion deficiency syndrome (LAD-)1, largely characterized by recurrent severe infections. All available mouse models display the constitutive and ubiquitous knockout of either α or the common β2 (CD18) subunit, which hampers the analysis of the cell type-specific role of β2 integrins in vivo. To overcome this limitation, we generated a CD18 gene floxed mouse strain. Offspring generated from crossing with CD11c-Cre mice displayed the efficient knockdown of β2 integrins, specifically in dendritic cells (DCs). Stimulated β2-integrin-deficient splenic DCs showed enhanced cytokine production and the concomitantly elevated activity of signal transducers and activators of transcription (STAT) 1, 3 and 5, as well as the impaired expression of suppressor of cytokine signaling (SOCS) 2–6 as assessed in bone marrow-derived (BM) DCs. Paradoxically, these BMDCs also showed the attenuated expression of genes involved in inflammatory signaling. In line, in experimental autoimmune encephalomyelitis mice with a conditional DC-specific β2 integrin knockdown presented with a delayed onset and milder course of disease, associated with lower frequencies of T helper cell populations (Th)1/Th17 in the inflamed spinal cord. Altogether, our mouse model may prove to be a valuable tool to study the leukocyte-specific functions of β2 integrins in vivo.

Antigen Targeting of Porcine Skin DEC205+ Dendritic Cells

Dendritic cell (DC) targeting by DEC205+ cells effectively promotes the internalization of antigens that may trigger a specific immune response. In this study, we evaluated the ability of a recombinant antibody, anti-DEC205 (rAb ZH9F7), to trigger cellular endocytosis in subpopulations of DCs and targeted cells after intradermal injection and subsequent migration toward lymph nodes. Furthermore, the cellular immune response was evaluated in pigs after intradermal application of the antigenized rAb ZH9F7 combined with porcine circovirus type 2 cap antigen (rAb ZH9F7-Cap). We demonstrated that rAb ZH9F7 recognized conventional type 1 and 2 DCs from the blood and skin and monocytes. It promoted receptor-mediated endocytosis and migration of cDCs and moDCs toward regional lymph nodes. Intradermal application of rAb ZH9F7-Cap induced a higher frequency of IFN-γ-secreting CD4+CD8+ T lymphocytes and antibodies against Cap protein than that in the control group. In conclusion, the rAb ZH9F7-Cap system promoted the target of skin cDC1 and cDC2, provoking migration to the regional lymph nodes and inducing a Th1 response, as evidenced by the proliferation of double-positive CD4+CD8+ T cells, which correlates with an enhanced ability to target the cDC1 subset both in vitro and in vivo.

Dendritic cells in cancer immunology

The clinical success of immune checkpoint therapy (ICT) has produced explosive growth in tumor immunology research because ICT was discovered through basic studies of immune regulation. Much of the current translational efforts are aimed at enhancing ICT by identifying therapeutic targets that synergize with CTLA4 or PD1/PD-L1 blockade and are solidly developed on the basis of currently accepted principles. Expanding these principles through continuous basic research may help broaden translational efforts. With this mindset, we focused this review on three threads of basic research directly relating to mechanisms underlying ICT. Specifically, this review covers three aspects of dendritic cell (DC) biology connected with antitumor immune responses but are not specifically oriented toward therapeutic use. First, we review recent advances in the development of the cDC1 subset of DCs, identifying important features distinguishing these cells from other types of DCs. Second, we review the antigen-processing pathway called cross-presentation, which was discovered in the mid-1970s and remains an enigma. This pathway serves an essential in vivo function unique to cDC1s and may be both a physiologic bottleneck and therapeutic target. Finally, we review the longstanding field of helper cells and the related area of DC licensing, in which CD4 T cells influence the strength or quality of CD8 T cell responses. Each topic is connected with ICT in some manner but is also a fundamental aspect of cell-mediated immunity directed toward intracellular pathogens.

Integrin αE(CD103)β7 in Epithelial Cancer

Interactions of both the innate and the adaptive immune system with tumors are complex and often influence courses and therapeutic treatments in unanticipated ways. Based on the concept that CD8+T cells can mediate important antitumor effects, several therapies now aim to amplify their specific activity. A subpopulation of CD8+ tissue-resident T lymphocytes that express the αE(CD103)β7 integrin has raised particular interest. This receptor presumably contributes to the recruitment and retention of tumor-infiltrating immune cells through interaction with its ligand, E-cadherin. It appears to have regulatory functions and is thought to be a component of some immunological synapses. In TGF-rich environments, the αE(CD103)β7/E-cadherin-interaction enhances the binding strength between tumor cells and infiltrating T lymphocytes. This activity facilitates the release of lytic granule contents and cytokines as well as further immune responses and the killing of target cells. Expression of αE(CD103)β7 in some tumors is associated with a rather favorable prognosis, perhaps with the notable exception of squamous cell carcinoma of the skin. Although epithelial skin tumors are by far the most common tumors of fair-skinned people, there have been very few studies on the distribution of αE(CD103)β7 expressing cells in these neoplasms. Given this background, we describe here that αE(CD103)β7 is scarcely present in basal cell carcinomas, but much more abundant in squamous cell carcinomas with heterogeneous distribution. Notwithstanding a substantial number of studies, the role of αE(CD103)β7 in the tumor context is still far from clear. Here, we summarize the essential current knowledge on αE(CD103)β7 and outline that it is worthwhile to further explore this intriguing receptor with regard to the pathophysiology, therapy, and prognosis of solid tumors.

Macrophage-targeted therapy unlocks antitumoral crosstalk between IFN𝛾-secreting lymphocytes and IL12-producing dendritic cells

Macrophages often abound within tumors, express colony-stimulating factor 1 receptor (CSF1R), and are linked to adverse patient survival. Drugs blocking CSF1R signaling have been used to suppress tumor-promoting macrophage responses; however, their mechanisms of action remain incompletely understood. Here, we assessed the lung tumor immune microenvironment in mice treated with BLZ945, a prototypical small molecule CSF1R inhibitor, using single-cell RNA sequencing and mechanistic validation approaches. We showed that tumor control was not caused by CSF1R+ cell depletion; instead, CSF1R targeting reshaped the CSF1R+ cell landscape, which unlocked crosstalk between antitumoral CSF1R- cells. These cells included IFNγ-producing NK and T cells, and an IL12-producing dendritic cell subset, denoted as DC3, which were all necessary for CSF1R inhibitor-mediated lung tumor control. These data indicate that CSF1R targeting can activate a cardinal crosstalk between cells that are not macrophages and that are essential to mediate the effects of T cell-targeted immunotherapies and promote antitumor immunity.

Activation of Dendritic Cells in Tonsils Is Associated with CD8 T Cell Responses following Vaccination with Live Attenuated Classical Swine Fever Virus

Classical swine fever (CSF) is a highly contagious disease caused by the classical swine fever virus (CSFV). The live attenuated C-strain vaccine is highly efficacious, initiating protection within several days of delivery. The vaccine strain is detected in the tonsil early after inoculation, yet little is known of the role that tonsillar immune cells might play in initiating protection. Comparing the C-strain vaccine with the pathogenic CSFV Alfort-187 strain, changes in the myeloid cell compartment of the tonsil were observed. CSFV infection led to the emergence of an additional CD163⁺CD14⁺ cell population, which showed the highest levels of Alfort-187 and C-strain infection. There was also an increase in both the frequency and activation status (as shown by increased MHC-II expression) of the tonsillar conventional dendritic cells 1 (cDC1) in pigs inoculated with the C-strain. Notably, the activation of cDC1 cells coincided in time with the induction of a local CSFV-specific IFN-γ⁺ CD8 T cell response in C-strain vaccinated pigs, but not in pigs that received Alfort-187. Moreover, the frequency of CSFV-specific IFN-γ⁺ CD8 T cells was inversely correlated to the viral load in the tonsils of individual animals. Accordingly, we hypothesise that the activation of cDC1 is key in initiating local CSFV-specific CD8 T cell responses which curtail early virus replication and dissemination.

In Vivo CD4+ T Cell Differentiation and Function: Revisiting the Th1/Th2 Paradigm

The discovery of CD4⁺ T cell subset-defining master transcription factors and framing of the Th1/Th2 paradigm ignited the CD4⁺ T cell field. Advances in in vivo experimental systems, however, have revealed that more complex lineage-defining transcriptional networks direct CD4⁺ T cell differentiation in the lymphoid organs and tissues. This review focuses on the layers of fate decisions that inform CD4⁺ T cell differentiation in vivo. Cytokine production by antigen-presenting cells and other innate cells influences the CD4⁺ T cell effector program [e.g., T helper type 1 (Th1), Th2, Th17]. Signals downstream of the T cell receptor influence whether individual clones bearing hallmarks of this effector program become T follicular helper cells, supporting development of B cells expressing specific antibody isotypes, or T effector cells, which activate microbicidal innate cells in tissues. These bifurcated, parallel axes allow CD4⁺ T cells to augment their particular effector program and prevent disease.

The AP-1 transcription factors c-Jun and JunB are essential for CD8α conventional dendritic cell identity

Dendritic cell (DC) development is orchestrated by lineage-determining transcription factors (TFs). Although, members of the activator-protein-1 (AP-1) family, including Batf3, have been implicated in conventional (c)DC specification, the role of Jun proteins is poorly understood. Here, we identified c-Jun and JunB as essential for cDC1 fate specification and function. In mice, Jun proteins regulate extrinsic and intrinsic pathways, which control CD8α cDC1 diversification, whereas CD103 cDC1 development is unaffected. The loss of c-Jun and JunB in DC progenitors diminishes the CD8α cDC1 pool and thus confers resistance to Listeria monocytogenes infection. Their absence in CD8α cDC1 results in impaired TLR triggering and antigen cross-presentation. Both TFs are required for the maintenance of the CD8α cDC1 subset and suppression of cDC2 identity on a transcriptional and phenotypic level. Taken together, these results demonstrate the essential role of c-Jun and JunB in CD8α cDC1 diversification, function, and maintenance of their identity.

Swine Dendritic Cell Response to Porcine Reproductive and Respiratory Syndrome Virus: An Update

Dendritic cells (DCs) are the most potent antigen-presenting cells, unique to initiate and coordinate the adaptive immune response. In pigs, conventional DCs (cDCs), plasmacytoid DCs (pDCs), and monocyte-derived DCs (moDCs) have been described in blood and tissues. Different pathogens, such as viruses, could infect these cells, and in some cases, compromise their response. The understanding of the interaction between DCs and viruses is critical to comprehend viral immunopathological responses. Porcine reproductive and respiratory syndrome virus (PRRSV) is the most important respiratory pathogen in the global pig population. Different reports support the notion that PRRSV modulates pig immune response in addition to their genetic and antigenic variability. The interaction of PRRSV with DCs is a mostly unexplored area with conflicting results and lots of uncertainties. Among the scarce certainties, cDCs and pDCs are refractory to PRRSV infection in contrast to moDCs. Additionally, response of DCs to PRRSV can be different depending on the type of DCs and maybe is related to the virulence of the viral isolate. The precise impact of this virus-DC interaction upon the development of the specific immune response is not fully elucidated. The present review briefly summarizes and discusses the previous studies on the interaction of in vitro derived bone marrow (bm)- and moDCs, and in vivo isolated cDCs, pDCs, and moDCs with PRRSV1 and 2.

Assessment of medullary and extramedullary myelopoiesis in cardiovascular diseases

Recruitment of innate immune cells and their accumulation in the arterial wall and infarcted myocardium has been recognized as a central feature of atherosclerosis and cardiac ischemic injury, respectively. In both, steady state and under pathological conditions, majority of these cells have a finite life span and are continuously replenished from haematopoietic stem/progenitor cell pool residing in the bone marrow and extramedullary sites. While having a crucial role in the cardiovascular disease development, proliferation and differentiation of innate immune cells within haematopoietic compartments is greatly affected by the ongoing cardiovascular pathology. In the current review, we summarize key cells, processes and tissue compartments that are involved in myelopoiesis under the steady state, during atherosclerosis development and in myocardial infarction.

The pig as a medical model for acquired respiratory diseases and dysfunctions: An immunological perspective

By definition no model is perfect, and this also holds for biology and health sciences. In medicine, murine models are, and will be indispensable for long, thanks to their reasonable cost and huge choice of transgenic strains and molecular tools. On the other side, non-human primates remain the best animal models although their use is limited because of financial and obvious ethical reasons. In the field of respiratory diseases, specific clinical models such as sheep and cotton rat for bronchiolitis, or ferret and Syrian hamster for influenza and Covid-19, have been successfully developed, however, in these species, the toolbox for biological analysis remains scarce. In this view the porcine medical model is appearing as the third, intermediate, choice, between murine and primate. Herein we would like to present the pros and cons of pig as a model for acquired respiratory conditions, through an immunological point of view. Indeed, important progresses have been made in pig immunology during the last decade that allowed the precise description of immune molecules and cell phenotypes and functions. These progresses might allow the use of pig as clinical model of human respiratory diseases but also as a species of interest to perform basic research explorations.

Select hyperactivating NLRP3 ligands enhance the TH1- and TH17-inducing potential of human type 2 conventional dendritic cells

The detection of microorganisms and danger signals by pattern recognition receptors on dendritic cells (DCs) and the consequent formation of inflammasomes are pivotal for initiating protective immune responses. Although the activation of inflammasomes leading to secretion of the cytokine IL-1β is typically accompanied by pyroptosis (an inflammatory form of lytic programmed cell death), some cells can survive and exist in a state of hyperactivation. Here, we found that the conventional type 2 DC (cDC2) subset is the major human DC subset that is transcriptionally and functionally poised for inflammasome formation and response without pyroptosis. When cDC2 were stimulated with ligands that relatively weakly activated the inflammasome, the cells did not enter pyroptosis but instead secreted IL-12 family cytokines and IL-1β. These cytokines induced prominent T helper type 1 (T_H1) and T_H17 responses that were superior to those seen in response to Toll-like receptor (TLR) stimulation alone or to stronger, classical inflammasome ligands. These findings not only define the human cDC2 subpopulation as a prime target for the treatment of inflammasome-dependent inflammatory diseases but may also inform new approaches for adjuvant and vaccine development.

Characterization of Conventional Dendritic Cells and Macrophages in the Spleen Using the CSF1R-Reporter Transgenic Chickens

The spleen is a major site for the immunological responses to blood-borne antigens that is coordinated by cells of the mononuclear phagocyte system (MPS). The chicken spleen is populated with a number of different macrophages while the presence of conventional dendritic cells (cDC) has been described. However, a detailed characterization of the phenotype and function of different macrophage subsets and cDC in the chicken spleen is limited. Using the CSF1R-reporter transgenic chickens (CSF1R-tg), in which cells of the MPS express a transgene under the control elements of the chicken CSF1R, we carried out an in-depth characterization of these cells in the spleen. Immunohistological analysis demonstrated differential expression of MRC1L-B by periarteriolar lymphoid sheaths (PALS)-associated CSF1R-tg+ cells. In the chicken's equivalent of the mammalian marginal zone, the peri-ellipsoid white-pulp (PWP), we identified high expression of putative CD11c by ellipsoid-associated cells compared to ellipsoid-associated macrophages. In addition, we identified a novel ellipsoid macrophage subset that expressed MHCII, CD11c, MRC1L-B, and CSF1R but not the CSF1R-tg. In flow cytometric analysis, diverse expression of the CSF1R-tg and MHCII was observed leading to the categorization of CSF1R-tg cells into CSF1R-tgdim MHCIIinter−hi, CSF1R-tghi MHCIIhi, and CSF1R-tghi MHCIIinter subpopulations. Low levels of CD80, CD40, MHCI, CD44, and Ch74.2 were expressed by the CSF1R-tghi MHCIIinter cells. Functionally, in vivo fluorescent bead uptake was significantly higher in the CSF1R-tghi MHCIIhi MRC1L-B+ cells compared to the CSF1R-tgdim and CSF1R-tghi MHCIIinter MRC1L-B+ subpopulations while LPS enhanced phagocytosis by the CSF1R-tghi MHCIIinter subpopulation. The analysis of bead localization in the spleen suggests the presence of ellipsoid-associated macrophage subsets. In addition, we demonstrated the functionality of ex vivo derived CSF1R-tg+ MRC1L-Bneg cDC. Finally, RNA-seq analysis of the CSF1R-tg subpopulations demonstrated that separating the CSF1R-tghi subpopulation into CD11chi and CD11cdim cells enriched for cDC and macrophage lineages, respectively, while the CSF1R-tghi MHCIIinter subpopulation was enriched for red pulp macrophages. However, our analysis could not define the cell lineage of the heterogeneous CSF1R-tgdim subpopulation. This detailed overview of the MPS in the chicken spleen will contribute to future research on their role in antigen uptake and presentation.

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

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

Immunization with alloantibodies-covered melanoma cells induces regional antitumor effects that become systemic when combined with 5-FU treatment

Alloantibodies, in particular immunoglobulin G (allo-IgG), confer a rejection advantage to tumors sharing the same major histocompatibility complex (MHC) in mice. However, when administrated intratumorally, this effect can only be achieved in combination with dendritic cells (DCs) activation. Here, we developed high titer allo-IgG by multiple rounds of immunization with allogenic B16 melanoma cells, which allows for the strong binding with B16 cells. We demonstrate that B16 cells incubated with these allo-IgG (referred to as allo-IgG-B16) become highly immunogenic, which release tumor antigens that are efficiently presented by classic DCs in lymph nodes (LNs). Injection of allo-IgG-B16 turns the tumor into an immune hot one and even elicits a systemic antitumor response when used together with 5-fluorouracil (5-FU). This systemic response is tumor-specific and relies on the critical site - LNs. Our findings provide a rationale for the use of allo-IgG in cancer treatment.

Th17 Cell-Mediated Colitis Is Positively Regulated by Interferon Regulatory Factor 4 in a T Cell-Extrinsic Manner

Inflammatory bowel diseases (IBDs) are characterized by chronic, inflammatory gastrointestinal lesions and often require life-long treatment with immunosuppressants and repetitive surgical interventions. Despite progress in respect to the characterization of molecular mechanisms e.g. exerted by TNF-alpha, currently clinically approved therapeutics fail to provide long-term disease control for most patients. The transcription factor interferon regulatory factor 4 (IRF4) has been shown to play important developmental as well as functional roles within multiple immune cells. In the context of colitis, a T cell-intrinsic role of IRF4 in driving immune-mediated gut pathology is established. Here, we conversely addressed the impact of IRF4 inactivation in non-T cells on T cell driven colitis in vivo. Employing the CD4+CD25- naïve T cell transfer model, we found that T cells fail to elicit colitis in IRF4-deficient compared to IRF4-proficient Rag1-/- mice. Reduced colitis activity in the absence of IRF4 was accompanied by hampered T cell expansion both within the mesenteric lymph node (MLN) and colonic lamina propria (cLP). Furthermore, the influx of various myeloids, presumably inflammation-promoting cells was abrogated overall leading to a less disrupted intestinal barrier. Mechanistically, gene profiling experiments revealed a Th17 response dominated molecular expression signature in colon tissues of IRF4-proficient, colitic Rag1-/- but not in colitis-protected Rag1-/-Irf4-/- mice. Colitis mitigation in Rag1-/-Irf4-/- T cell recipients resulted in reduced frequencies and absolute numbers of IL-17a-producing T cell subsets in MLN and cLP possibly due to a regulation of conventional dendritic cell subset 2 (cDC2) known to impact Th17 differentiation. Together, extending the T cell-intrinsic role for IRF4 in the context of Th17 cell driven colitis, the provided data demonstrate a Th17-inducing and thereby colitis-promoting role of IRF4 through a T cell-extrinsic mechanism highlighting IRF4 as a putative molecular master switch among transcriptional regulators driving immune-mediated intestinal inflammation through both T cell-intrinsic and T cell-extrinsic mechanisms. Future studies need to further dissect IRF4 controlled pathways within distinct IRF4-expressing myeloid cell types, especially cDC2s, to elucidate the precise mechanisms accounting for hampered Th17 formation and, according to our data, the predominant mechanism of colitis protection in Rag1-/-Irf4-/- T cell receiving mice.

Augmenting E Protein Activity Impairs cDC2 Differentiation at the Pre-cDC Stage

Dendritic cell (DC) specification and differentiation are controlled by a circuit of transcription factors, which regulate the expression of DC effector genes as well as the transcription factors themselves. E proteins are a widely expressed basic helix-loop-helix family of transcription factors whose activity is suppressed by their inhibitors, ID proteins. Loss-of-function studies have demonstrated the essential role of both E and ID proteins in different aspects of DC development. In this study, we employed a gain-of-function approach to illustrate the importance of the temporal control of E protein function in maintaining balanced differentiation of conventional DC (cDC) subsets, cDC1 and cDC2. We expressed an E protein mutant, ET2, which dimerizes with endogenous E proteins to overcome inhibition by ID proteins and activate the transcription of E protein targets. Induction of ET2 expression at the hematopoietic progenitor stage led to a dramatic reduction in cDC2 precursors (pre-cDC2s) with little impact on pre-cDC1s. Consequently, we observed decreased numbers of cDC2s in the spleen and lung, as well as in FLT3L-driven bone marrow-derived DC cultures. Furthermore, in mice bearing ET2, we detected increased expression of the IRF8 transcription factor in cDC2s, in which IRF8 is normally down-regulated and IRF4 up-regulated. This aberrant expression of IRF8 induced by ET2 may contribute to the impairment of cDC2 differentiation. In addition, analyses of the transcriptomes of splenic cDC1s and cDC2s revealed that ET2 expression led to a shift, at least in part, of the transcriptional profile characteristic of cDC2s to that of cDC1. Together, these results suggest that a precise control of E protein activity is crucial for balanced DC differentiation.

Recent Advances in Good Manufacturing Practice-Grade Generation of Dendritic Cells

Dendritic cells (DCs) are pivotal regulators of immune responses, specialized in antigen presentation and bridging the gap between the innate and adaptive immune system. Due to these key features, DCs have become a pillar of the continuously growing field of cellular therapies. Here we review recent advances in good manufacturing practice strategies and their individual specificities in relation to DC production for clinical applications. These take into account both small-scale experimental approaches as well as automated systems for patient care.

Helminth-derived peptide GK-1 induces Myd88-dependent pro-inflammatory signaling events in bone marrow-derived antigen-presenting cells

GK-1 is an immunomodulatory, 18-aa-long peptide that has been proved to promote the activation of mouse peritoneal macrophages and LPS-pulsed mouse bone marrow-derived dendritic cells (BM-DCs). This study is aimed to explore the mechanisms underlying the activation of these antigen-presenting cells (APCs) by GK-1. In our study, GK-1 up-regulated in vitro the expression of CD86 and CD40, and it increased the secretion of NO in bone marrow-derived macrophages (BMDMs). In BM-DCs, GK-1 upregulated the expression of MHC class II and CD86. Additionally, GK-1 was found to be involved in the phosphorylation of MAPK p38, JNK and ERK 1/2 and in Myd88-dependent activation of NF-κB in both antigen-presenting cell types. In vivo, GK-1 increased the secretion of IL-15, CCL2, and IL-6 through a Myd88-dependent mechanism. This study demonstrated that GK-1 promotes the activation and effector activity of APCs through a mechanism dependent on Myd88, probably involving a Toll-like receptor as a target.

Development of Pig Conventional Dendritic Cells From Bone Marrow Hematopoietic Cells in vitro

In recent years, porcine dendritic cells (DCs) have been identified from pig tissues. However, studying the interaction of porcine DCs with pathogens is still difficult due to the scarcity of DCs in tissues. In the present work, the Flt3-ligand (Flt3L)-based in vitro derivation system was further characterized and compared with other cytokine derivation models using a combination of factors: stem cell factor (SCF), GM-CSF, and IL-4. The method using Flt3L alone or combined with SCF supported the development of pig bone marrow hematopoietic cells into in vivo equivalent conventional DCs (cDCs). The equivalent cDC1 (the minor population in the cultures) were characterized as CADM1⁺CD14^–MHC-II⁺CD172a^–/loCD1^–CD163^– DEC205⁺CD11R3^loCD11R1⁺CD33⁺CD80/86⁺. They expressed high levels of FLT3, ZBTB46, XCR1, and IRF8 mRNA, were efficient in endocytosing dextran and in proliferating allogenic CD4⁺CD8⁺ T cells, but were deficient in phagocyting inactivated Staphylococcus aureus (S. aureus). Also, after poly I:C stimulation, they predominantly produced IL-12p40a and matured as indicated by the increase of MHC-I, MHC-II, and CD80/86. The equivalent cDC2 (the main population) were CADM1⁺CD14^–MHC-II⁺C D172a⁺CD1⁺CD163^–/loDEC205^loCD11R3⁺CD11R1⁺CD33⁺CD80/86⁺; meanwhile, they overexpressed FcεR1α and IRF4 mRNA. They showed high efficiency in the endocytosis of dextran, but weak in phagocytosing bacteria. They supported allogenic CD4⁺CD8^–/CD4⁺CD8⁺ T cell proliferation and were high producers of IL-12p40 (upon TLR7 stimulation) and IL-10 (upon TLR7 stimulation). TLR ligand stimulation also induced their maturation. In addition, a CD14⁺ population was identified with the phenotype CADM1⁺CD14⁺MHC-II⁺CD172a⁺ CD1⁺CD163⁺DEC205^–CD11R3⁺CD11R1⁺CD33^–/loCD80/86⁺. They shared some functional similarities with cDC2 and were distinguishable from macrophages. This CD14⁺ population was efficient in phagocyting S. aureus but showed less maturation upon TLR ligand stimulation than cDC1 or cDC2. The alternative methods of DC derivation including GM-CSF and/or IL-4 produced mostly CADM1^– cells that did not fulfill the canonical phenotype of bona fide porcine DCs. Our study provides an exhaustive characterization of Flt3L-derived DCs with different methods that can help the in vitro study of the interaction of DCs with porcine-relevant pathogens.

Prostaglandin D2 signaling in dendritic cells is critical for the development of EAE

Priming of autoreactive T cells in lymph nodes by dendritic cells (DCs) is critical for the pathogenesis of experimental autoimmune encephalitis (EAE). DC activation reflects a balance of pro- and anti-inflammatory signals. One anti-inflammatory factor is prostaglandin D2 signaling through its cognate receptor, D-prostanoid receptor 1 (PTGDR), on myeloid cells. Loss of PTGDR signaling might be expected to enhance DC activation and EAE but here we show that PTGDR^−/− mice developed only mild signs of MOG_35-55 peptide immunization-induced EAE. Compared to wild type mice, PTGDR^−/− mice exhibited less demyelination, decreased leukocyte infiltration and diminished microglia activation. These effects resulted from increased pro-inflammatory responses in the lymph nodes, most notably in IL-1β production, with the unexpected consequence of increased activation-induced apoptosis of MOG_35-55 peptide-specific T cells. Conditional deletion of PTGDR on DCs, and not other myeloid cells ameliorated EAE. Together, these results demonstrate the indispensable role that PGD₂/PTGDR signaling on DCs has in development of pathogenic T cells in autoimmune demyelination.

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Increased T cell activation occurred in PTGDR^−/- mice resulting in T cell apoptosis.

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AICD decreased T cell infiltration into, and demyelination in CNS during EAE.

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Decreased PGD₂/PTGDR signaling in DCs resulted in increased IL-1β expression.

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Anakinra treatment in PTGDR^−/- mice increased EAE severity.

Regulatory T cells in inflammatory skin disease: from mice to humans

The skin is the largest organ in the body and one of the primary barriers to the environment. In order to optimally protect the host, the skin is home to numerous immune cell subsets that interact with each other and other non-immune cells to maintain organ integrity and function. Regulatory T cells (Tregs) are one of the largest immune cell subsets in skin. They play a critical role in regulating inflammation and facilitating organ repair. In doing so, they adopt unique and specialized tissue-specific functions. In this review, we compare and contrast the role of Tregs in cutaneous immune disorders from mice and humans, with a specific focus on scleroderma, alopecia areata, atopic dermatitis, cutaneous lupus erythematosus and psoriasis.

Involvement of the myeloid cell compartment in fibrogenesis and systemic sclerosis

Systemic sclerosis (SSc) is an autoimmune fibrotic disease of unknown aetiology that is characterized by vascular changes in the skin and visceral organs. Autologous haematopoietic stem cell transplantation can improve skin and organ fibrosis in patients with progressive disease and a high risk of organ failure, indicating that cells originating in the bone marrow are important contributors to the pathogenesis of SSc. Animal studies also indicate a pivotal function of myeloid cells in the development of fibrosis leading to changes in the tissue architecture and dysfunction in multiple organs such as the heart, lungs, liver and kidney. In this Review, we summarize current knowledge about the function of myeloid cells in fibrogenesis that occurs in patients with SSc. Targeted therapies currently in clinical studies for SSc might affect myeloid cell-related pathways. Therefore, myeloid cells might be used as cellular biomarkers of disease through the application of high-dimensional techniques such as mass cytometry and single-cell RNA sequencing.

Allergen-Induced C5a/C5aR1 Axis Activation in Pulmonary CD11b+ cDCs Promotes Pulmonary Tolerance through Downregulation of CD40

Activation of the C5/C5a/C5a receptor 1 (C5aR1) axis during allergen sensitization protects from maladaptive T cell activation. To explore the underlying regulatory mechanisms, we analyzed the impact of C5aR1 activation on pulmonary CD11b⁺ conventional dendritic cells (cDCs) in the context of house-dust-mite (HDM) exposure. BALB/c mice were intratracheally immunized with an HDM/ovalbumin (OVA) mixture. After 24 h, we detected two CD11b⁺ cDC populations that could be distinguished on the basis of C5aR1 expression. C5aR1⁻ but not C5aR1⁺ cDCs strongly induced T cell proliferation of OVA-reactive transgenic CD4⁺ T cells after re-exposure to antigen in vitro. C5aR1⁻ cDCs expressed higher levels of MHC-II and CD40 than their C5aR1⁺ counterparts, which correlated directly with a higher frequency of interactions with cognate CD4⁺ T cells. Priming of OVA-specific T cells by C5aR1⁺ cDCs could be markedly increased by in vitro blockade of C5aR1 and this was associated with increased CD40 expression. Simultaneous blockade of C5aR1 and CD40L on C5aR1⁺ cDCs decreased T cell proliferation. Finally, pulsing with OVA-induced C5 production and its cleavage into C5a by both populations of CD11b⁺ cDCs. Thus, we propose a model in which allergen-induced autocrine C5a generation and subsequent C5aR1 activation in pulmonary CD11b⁺ cDCs promotes tolerance towards aeroallergens through downregulation of CD40.

Understanding the Role of the Unfolded Protein Response Sensor IRE1 in the Biology of Antigen Presenting Cells

The unfolded protein response (UPR) is an adaptive response that maintains the fidelity of the cellular proteome in conditions that subvert the folding capacity of the cell, such as those noticed in infection and inflammatory contexts. In immunity, the UPR sensor IRE1 (Inositol-requiring enzyme 1-alpha) has emerged as a critical regulator of the homeostasis of antigen presenting cells (APCs). In the past few years, it has become clear that IRE1 plays canonical and non-canonical roles in APCs, many of which intersect with key features of these cells, including the initiation of inflammation, antibody production, and antigen presentation. The aims of the present review are to provide recent insights on the mechanisms by which IRE1 regulates the diversity of APC functions and to highlight its relevance in the coordination of innate and adaptive immunity.

Transcriptional control of dendritic cell development and functions

Dendritic cells (DCs) are major regulators of adaptive immunity, as they are not only capable to induce efficient immune responses, but are also crucial to maintain peripheral tolerance and thereby inhibit autoimmune reactions. DCs bridge the innate and the adaptive immune system by presenting peptides of self and foreign antigens as peptide MHC complexes to T cells. These properties render DCs as interesting target cells for immunomodulatory therapies in cancer, but also autoimmune diseases. Several subsets of DCs with special properties and functions have been described. Recent achievements in understanding transcriptional programs on single cell level, together with the generation of new murine models targeting specific DC subsets, advanced our current understanding of DC development and function. Thus, DCs arise from precursor cells in the bone marrow with distinct progenitor cell populations splitting the monocyte populations and macrophage populations from the DC lineage, which upon lineage commitment can be separated into conventional cDC1, cDC2, and plasmacytoid DCs (pDCs). The DC populations harbor intrinsic programs enabling them to react for specific pathogens in dependency on the DC subset, and thereby orchestrate T cell immune responses. Similarities, but also varieties, between human and murine DC subpopulations are challenging, and will require further investigation of human specimens under consideration of the influence of the tissue micromilieu and DC subset localization in the future.

Analysis of Swine Conventional Dendritic Cells, DEC205+CD172a+/-CADM1+, from Blood and Spleen in Response to PRRSV and PEDV

Conventional dendritic cells (cDCs) cannot be infected by porcine reproductive and respiratory syndrome virus (PRRSV) but respond to infection via cytokine production, indicating a possible role in initiation/regulation of the immune response against PRRSV. In this work, we evaluated the responses of splenic and blood cDCs, with DEC205⁺CADM1⁺CD172a^+/− phenotype, as well as those of CD163⁺ cells against PRRSV and porcine epidemic diarrhea virus (PEDV). Both populations were incubated in the presence of PRRSV or PEDV with and without naïve CD3⁺ T cells, and cytokine responses were evaluated by qPCR and ELISA. Our results showed that cDCs, but not CD163⁺ cells, produced IL-12 in response to PRRSV. PEDV did not induce IL-12 production. Cocultures of cDCs and autologous naïve CD3⁺ cells resulted in decreased IL-12 production and low expression of IFN-γ transcripts in response to PRRSV. Interestingly, cDCs increased the proliferation of naïve T cells in the presence of PRRSV compared with that achieved with monocytes and peripheral blood mononuclear cells (PBMCs). Cocultures of CD163⁺ cells induced IL-10 and IL-4 expression in the presence of PRRSV and PEDV, respectively. In conclusion, cDCs can selectively produce IL-12 in response to PRRSV but poorly participate in the activation of naïve T cells.

Myeloid and Plasmacytoid Dendritic Cells and Cancer - New Insights

Dendritic cells (DCs) use effective mechanisms to combat antigens and to bring about adaptive immune responses through their ability to stimulate näive T cells. At present, four major cell types are categorised as DCs: Classical or conventional (cDCs), Plasmacytoid (pDCs), Langerhans cells (LCs), and monocyte-derived DCs (Mo-DCs). It was suggested that pDCs, CD1c+ DCs and CD141+ DCs in humans are equivalent to mouse pDCs, CD11b+ DCs and CD8α+ DCs, respectively. Human CD141+ DCs compared to mouse CD8α+ DCs have remarkable functional and transcriptomic similarities. Characteristic markers, transcription factors, toll-like receptors, T helpers (Th) polarisation, cytokines, etc. of DCs are discussed in this review. Major histocompatibility complex (MHC) I and II antigen presentation, cross-presentation and Th polarisation are defined, and the dual role of DCs in the tumour is discussed. Human DCs are the main immune cells that orchestrate the immune response in the tumour microenvironment.

Mind the Map: Technology Shapes the Myeloid Cell Space

The myeloid cell system shows very high plasticity, which is crucial to quickly adapt to changes during an immune response. From the beginning, this high plasticity has made cell type classification within the myeloid cell system difficult. Not surprising, naming schemes have been frequently changed. Recent advancements in multidimensional technologies, including mass cytometry and single-cell RNA sequencing, are challenging our current understanding of cell types, cell subsets, and functional states of cells. Despite the power of these technologies to create new reference maps for the myeloid cell system, it is essential to put these new results into context with previous knowledge that was established over decades. Here we report on earlier attempts of cell type classification in the myeloid cell system, discuss current approaches and their pros and cons, and propose future strategies for cell type classification within the myeloid cell system that can be easily extended to other cell types.

Emerging Principles in Myelopoiesis at Homeostasis and during Infection and Inflammation

Myelopoiesis ensures the steady state of the myeloid cell compartment. Technological advances in fate mapping and genetic engineering, as well as the advent of single cell RNA-sequencing, have highlighted the heterogeneity of the hematopoietic system and revealed new concepts in myeloid cell ontogeny. These technologies are also shedding light on mechanisms of myelopoiesis at homeostasis and at different phases of infection and inflammation, illustrating important feedback loops between affected tissues and the bone marrow. We review these findings here and revisit principles in myelopoiesis in light of the evolving understanding of myeloid cell ontogeny and heterogeneity. We argue for the importance of system-wide evaluation of changes in myelopoiesis and discuss how even after the resolution of inflammation, long-lasting alterations in myelopoiesis may play a role in innate immune memory or trained immunity.

Single cell RNA-Seq reveals pre-cDCs fate determined by transcription factor combinatorial dose

Background

Classic dendritic cells (cDCs) play a central role in the immune system by processing and presenting antigens to activate T cells, and consist of two major subsets: CD141⁺ cDC (cDC1) and CD1c⁺ cDC (cDC2). A population of migratory precursor cells, the pre-cDCs, is the immediate precursors to both cDC subsets. Previous studies showed that there were two pre-committed pre-cDC subpopulations. However, the key molecular drivers of pre-commitment in human pre-cDCs were not investigated.

Results

To identify the key molecular drivers for pre-commitment in human pre-cDCs, we performed single cell RNA sequencing (RNA-Seq) of two cDC subsets and pre-cDCs, and bulk RNA-Seq of pre-cDCs and cDCs from human peripheral blood. We found that pre-DC subpopulations cannot be separated by either variable genes within pre-cDCs or differentially expressed genes between cDC1 and cDC2. In contrast, they were separated by 16 transcription factors that are themselves differentially expressed or have regulated targets enriched in the differentially expressed genes between bulk cDC1 and cDC2, with one subpopulation close to cDC1 and the other close to cDC2. More importantly, these two pre-cDC sub-populations are correlated with ratio of IRF8 to IRF4 expression level more than their individual expression level. We also verified these findings using three recently published datasets.

Conclusions

In this study, we demonstrate that single cell transcriptome profiling can reveal pre-cDCs differentiation map, and our results suggest the concept that combinatorial dose of transcription factors determines cell differentiation fate.

Electronic supplementary material

The online version of this article (10.1186/s12860-019-0199-y) contains supplementary material, which is available to authorized users.

Unleashing Tumour-Dendritic Cells to Fight Cancer by Tackling Their Three A's: Abundance, Activation and Antigen-Delivery

Recent advances in cancer immunotherapy have mainly focused on re-activating T-cell responses against cancer cells. However, both priming and activation of effector T-cell responses against cancer-specific antigens require cross-talk with dendritic cells (DCs), which are responsible for the capturing, processing and presentation of tumour-(neo)antigens to T cells. DCs consequently constitute an essential target in efforts to generate therapeutic immunity against cancer. This review will discuss recent research that is unlocking the cancer-fighting potential of tumour-infiltrating DCs. First, the complexity of DCs in the tumour microenvironment regarding the different subsets and the difficulty of translating mouse data into equivalent human data will be briefly touched upon. Mainly, possible solutions to problems currently faced in DC-based cancer treatments will be discussed, including their infiltration into tumours, activation strategies, and antigen delivery methods. In this way, we hope to put together a broad picture of potential synergistic therapies that could be implemented to harness the full capacity of tumour-infiltrating DCs to stimulate anti-tumour immune responses in patients.

Immunological aspects of allogeneic pancreatic islet transplantation: a comparison between mouse and human

Pancreatic islet allotransplantation is a treatment for patients with severe forms of type 1 diabetes. As long-term graft function and survival are not yet optimal, additional studies are warranted in order to continue improving transplant outcomes. The mechanisms of islet graft loss and tolerance induction are often studied in murine diabetes models. Despite numerous islet transplantation studies successfully performed over recent years, translation from experimental mouse models to human clinical application remains elusive. This review aims at critically discussing the strengths and limitations of current mouse models of diabetes and experimental islet transplantation. In particular, we will analyze the causes leading to diabetes and compare the immunological mechanisms responsible for rejection between mouse and human. A better understanding of the experimental mouse models should facilitate translation to human clinical application.

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.

Hepatocellular carcinoma-associated antigen 59 of Haemonchus contortus modulates the functions of PBMCs and the differentiation and maturation of monocyte-derived dendritic cells of goats in vitro

BACKGROUND

Hepatocellular carcinoma-associated antigen 59 (HCA59), which is one of the most important excretory/secretory products of Haemonchus contortus (HcESPs), is known to have antigenic functions. However, its immunomodulatory effects on host cells are poorly understood.

METHODS

Here, we cloned the HCA59 gene and expressed the recombinant protein of HCA59 (rHCA59). Binding activities of rHCA59 to goat peripheral blood mononuclear cells (PBMCs) and dendritic cells (DCs) were checked by immunofluorescence assay (IFA) and the immunoregulatory effects of rHCA59 on cytokine secretions, cell migration, cell proliferation, nitric oxide production, and changes in expression of genes in related pathways were observed by co-incubation of rHCA59 with goat PBMCs and DCs. Monocyte phagocytosis and characterization of goat blood DC subsets were detected by flow cytometry.

RESULTS

The IFA results revealed that rHCA59 could bind to PBMCs and DCs. Treatment of PBMCs with rHCA59 significantly increased cellular proliferation and NO production in a dose-dependent manner, while cell migration was vigorously blocked. Treatment with rHCA59 significantly suppressed monocytes phagocytosis. The quantity of surface marker CD80 on DCs increased significantly after rHCA59 treatment. In addition, the expression of genes included in the WNT pathway was related to the differentiation and maturation of DCs, and the production of IL-10 and IL-17 produced by PBMCs was altered.

CONCLUSIONS

Our findings illustrated that rHCA59 could enhance host immune responses by regulating the functions of goat PBMCs and DCs, which would benefit our understanding of HCA59 from parasitic nematodes contributing to the mechanism of parasitic immune evasion.

Metabolic Regulation of Dendritic Cell Differentiation

Dendritic cells (DCs) are important antigen-presenting cells (APCs) that play essential roles in bridging innate and adaptive immune responses. Differentiation stages of DC subsets from bone marrow progenitor cells have been well-defined during the past decades. Features that distinguish DC progenitor cells from each differentiation stages, related signaling pathways and transcription factors that are crucial for DC lineage commitment have been well-elucidated in numerous studies. Recently, growing evidence are showing that cellular metabolism, as one of the most fundamental process of cells, has essential role in the modulation of immune system. There have been multiple reports and reviews that focus on the metabolic modulations on DC functions, however little attention had been paid to the metabolic regulation of DC development and differentiation. In recent years, increasing evidence suggests that metabolic regulations also exert significant impact on DC differentiation, as well as on the homeostasis of tissue resident DCs. The focus of this review is to summarize the findings from recent studies on the metabolic regulation of DC differentiation and to discuss the impacts of the three major aspects of metabolism on the processes of DC development and differentiation, namely the changes in metabolic pathways, the molecular signaling pathways that modulate cell metabolism, and the effects of metabolites and nutrients. The aim of this review is to draw attentions to this important and exciting research field where the effects of metabolic process and their regulation in DC differentiation need to be further explored.

The Myeloid Cell Compartment-Cell by Cell

Myeloid cells are a major cellular compartment of the immune system comprising monocytes, dendritic cells, tissue macrophages, and granulocytes. Models of cellular ontogeny, activation, differentiation, and tissue-specific functions of myeloid cells have been revisited during the last years with surprising results; for example, most tissue macrophages are yolk sac derived, monocytes and macrophages follow a multidimensional model of activation, and tissue signals have a significant impact on the functionality of all these cells. While these exciting results have brought these cells back to center stage, their enormous plasticity and heterogeneity, during both homeostasis and disease, are far from understood. At the same time, the ongoing revolution in single-cell genomics, with single-cell RNA sequencing (scRNA-seq) leading the way, promises to change this. Prevailing models of hematopoiesis with distinct intermediates are challenged by scRNA-seq data suggesting more continuous developmental trajectories in the myeloid cell compartment. Cell subset structures previously defined by protein marker expression need to be revised based on unbiased analyses of scRNA-seq data. Particularly in inflammatory conditions, myeloid cells exhibit substantially vaster heterogeneity than previously anticipated, and work performed within large international projects, such as the Human Cell Atlas, has already revealed novel tissue macrophage subsets. Based on these exciting developments, we propose the next steps to a full understanding of the myeloid cell compartment in health and diseases.

Tonsil conventional dendritic cells are not infected by porcine reproductive and respiratory syndrome virus

Porcine reproductive and respiratory syndrome virus (PRRSV) infects monocyte-derived DCs, and previous reports have shown that PRRSV does not infect conventional DCs (cDCs) in vitro, but the effects on cDCs from lymphoid tissues are unknown. This study analyzed the response and susceptibility of tonsil DEC205⁺cDCs from infected pigs. We confirmed the phenotype and lineage of bona fide tonsil cDCs with the mRNA expression of FLT3⁺ and the phenotype MHCII⁺CADM1^highDEC205⁺ (DEC205⁺cDCs). These cells were not infected by PRRSV, whereas CD163⁺ tonsil cells were infected. The numbers of tonsil cDCs and CD163⁺ cells were not affected by PRRSV, in contrast to the reduction in alveolar macrophage numbers. DEC205⁺cDCs exhibited an increase in the expression of IL-12 at 5 days postinfection, suggesting a proinflammatory response by these cells to the virus. In summary, this study confirms that, in vitro and in vivo, cDCs are not susceptible to PRRSV but can respond against it.

Successful Anti-PD-1 Cancer Immunotherapy Requires T Cell-Dendritic Cell Crosstalk Involving the Cytokines IFN-γ and IL-12

Anti-PD-1 immune checkpoint blockers can induce sustained clinical responses in cancer but how they function in vivo remains incompletely understood. Here, we combined intravital real-time imaging with single-cell RNA sequencing analysis and mouse models to uncover anti-PD-1 pharmacodynamics directly within tumors. We showed that effective antitumor responses required a subset of tumor-infiltrating dendritic cells (DCs), which produced interleukin 12 (IL-12). These DCs did not bind anti-PD-1 but produced IL-12 upon sensing interferon γ (IFN-γ) that was released from neighboring T cells. In turn, DC-derived IL-12 stimulated antitumor T cell immunity. These findings suggest that full-fledged activation of antitumor T cells by anti-PD-1 is not direct, but rather involves T cell:DC crosstalk and is licensed by IFN-γ and IL-12. Furthermore, we found that activating the non-canonical NF-κB transcription factor pathway amplified IL-12-producing DCs and sensitized tumors to anti-PD-1 treatment, suggesting a therapeutic strategy to improve responses to checkpoint blockade.

Novel mechanism for estrogen receptor alpha modulation of murine lupus

Female sex is a risk factor for lupus. Sex hormones, sex chromosomes and hormone receptors are implicated in the pathogenic pathways in lupus. Estrogen receptor alpha (ERα) knockout (KO) mice are used for defining hormone receptor effects in lupus. Prior studies of ERα KO in lupus have conflicting results, likely due to sex hormone levels, different lupus strains and different ERα KO constructs. Our objective was to compare a complete KO of ERα vs. the original functional KO of ERα (expressing a short ERα) on disease expression and immune phenotype, while controlling sex hormone levels. We studied female lupus prone NZM2410 WT and ERα mutant mice. All mice (n = 44) were ovariectomized (OVX) for hormonal control. Groups of each genotype were estrogen (E2)-repleted after OVX. We found that OVXed NZM mice expressing the truncated ERα (ERα short) had significantly reduced nephritis and prolonged survival compared to both wildtype and the complete ERαKO (ERα null) mice, but surprisingly only if E2-repleted. ERα null mice were not protected regardless of E2 status. We observed significant differences in splenic B cells and dendritic cells and a decrease in cDC2 (CD11b+CD8^-) dendritic cells, without a concomitant decrease in cDC1 (CD11b-CD8a+) cells comparing ERα short to ERα null or WT mice. Our data support a protective role for the ERα short protein. ERα short is similar to an endogenously expressed ERα variant (ERα46). Modulating its expression/activity represents a potential approach for treating female-predominant autoimmune diseases.

Precise Delineation and Transcriptional Characterization of Bovine Blood Dendritic-Cell and Monocyte Subsets

A clear-cut delineation of bovine bona fide dendritic cells (DC) from monocytes has proved challenging, given the high phenotypic and functional plasticity of these innate immune cells and the marked phenotypic differences between species. Here, we demonstrate that, based on expression of Flt3, CD172a, CD13, and CD4, a precise identification of bovine blood conventional DC type 1 and 2 (cDC1, cDC2), plasmacytoid DC (pDC), and monocytes is possible with cDC1 being Flt3⁺CD172a^dimCD13⁺CD4⁻, cDC2 being Flt3⁺CD172a⁺CD13⁻CD4⁻, pDC being Flt3⁺CD172a^dimCD13⁻CD4⁺, and monocytes being Flt3⁻CD172a^highCD13⁻CD4⁻. The phenotype of these subsets was characterized in further detail, and a subset-specific differential expression of CD2, CD5, CD11b, CD11c, CD14, CD16, CD26, CD62L, CD71, CD163, and CD205 was found. Subset identity was confirmed by transcriptomic analysis and subset-specific transcription of conserved key genes. We also sorted monocyte subsets based on their differential expression of CD14 and CD16. Classical monocytes (CD14⁺CD16⁻) clustered clearly apart from the two CD16⁺ monocyte subsets probably representing intermediate and non-classical monocytes described in human. The transcriptomic data also revealed differential gene transcription for molecules involved in antigen presentation, pathogen sensing, and migration, and therefore gives insights into functional differences between bovine DC and monocyte subsets. The identification of cell-type- and subset-specific gene transcription will assist in the quest for “marker molecules” that—when targeted by flow cytometry—will greatly facilitate research on bovine DC and monocytes. Overall, species comparisons will elucidate basic principles of DC and monocyte biology and will help to translate experimental findings from one species to another.

Enhanced effect of checkpoint inhibitors when given after or together with IMM-101: significant responses in four advanced melanoma patients with no additional major toxicity

Background

The use of checkpoint inhibitors (ipilimumab, pembrolizumab, nivolumab) has revolutionised the treatment of metastatic melanoma. However still more than the half the patients do not respond to single-agent immunotherapy. This has led to the development of combining these agents in an attempt to enhance the anti-cancer activity. More than 300 different studies with 15 different drug doses are currently ongoing. Combining different checkpoint inhibitors (CPIs) does indeed lead to an increase in response rate, but this is associated with significant toxicity. IMM-101 is a heat killed Mycobacterium preparation which induces marked immune modulation and little systemic toxicity. It has been reported as having activity in melanoma as single agent and in pancreatic cancer in combination with gemcitabine, the latter in a randomised study.

Methods

Here we report the effect of adding CPIs to 3 patients who had previously been on IMM-101, either as a trial or a named patient programme and a patient who received the IMM-101 together with nivolumab.

Results

All 4 patients had rapid and very good responses, three of them maintained over 18 months with no significant additional toxicity.

Conclusions

The rapid and complete clinical responses seen in these patients may suggest that IMM-101 is activating a complementary pathway which is synergistic with CPI treatment.

Conventional Dendritic Cells Impair Recovery after Myocardial Infarction

Ischemic myocardial injury results in sterile cardiac inflammation that leads to tissue repair, two processes controlled by mononuclear phagocytes. Despite global burden of cardiovascular diseases, we do not understand the functional contribution to pathogenesis of specific cardiac mononuclear phagocyte lineages, in particular dendritic cells. To address this limitation, we used detailed lineage tracing and genetic studies to identify bona fide murine and human CD103⁺ conventional dendritic cell (cDC)1s, CD11b⁺ cDC2s, and plasmacytoid DCs (pDCs) in the heart of normal mice and immunocompromised NSG mice reconstituted with human CD34⁺ cells, respectively. After myocardial infarction (MI), the specific depletion of cDCs, but not pDCs, improved cardiac function and prevented adverse cardiac remodeling. Our results showed that fractional shortening measured after MI was not influenced by the absence of pDCs. Interestingly, however, depletion of cDCs significantly improved reduction in fractional shortening. Moreover, fibrosis and cell areas were reduced in infarcted zones. This correlated with reduced numbers of cardiac macrophages, neutrophils, and T cells, indicating a blunted inflammatory response. Accordingly, mRNA levels of proinflammatory cytokines IL-1β and IFN-γ were reduced. Collectively, our results demonstrate the unequivocal pathological role of cDCs following MI.