Bcl-2 controls dendritic cell longevity in vivo

Advancing the understanding of the role of apoptosis in lung cancer immunotherapy: Global research trends, key themes, and emerging frontiers

Apoptosis is vital for improving the efficacy of lung cancer (LC) immunotherapy by targeting cancer cell elimination. Despite its importance, there is a lack of comprehensive bibliometric studies analyzing global research on apoptosis in LC immunotherapy. This analysis aims to address this gap by highlighting key trends, contributors, and future directions. A total of 969 publications from 1996 to 2024 were extracted from the Web of Science Core Collection. Analysis was conducted using VOSviewer, CiteSpace, and the R package 'bibliometrix.' The study included contributions from 6,894 researchers across 1,469 institutions in 61 countries, with research published in 356 journals. The volume of publications has steadily increased, led by China and the United States, with Sichuan University as the top contributor. The journal Cancers published the most articles, while Cancer Research had the highest co-citations. Yu-Quan Wei was the leading author, and Jemal, A. was the most frequently co-cited. Key research themes include "cell death mechanisms," "immune regulation," "combination therapies," "gene and nanomedicine applications," and "traditional Chinese medicine (TCM)." Future research is likely to focus on "coordinated regulation of multiple cell death pathways," "modulation of the tumor immune microenvironment," "optimization of combination therapies," "novel strategies in gene regulation," and the "integration of TCM" for personalized treatment. This is the first bibliometric analysis on the role of apoptosis in LC immunotherapy, providing an landscape of global research patterns and emerging therapeutic strategies. The findings offer insights to guide future research and optimize treatment approaches.

Antigen self-presenting dendrosomes swallowing nanovaccines boost antigens and STING agonists codelivery for cancer immunotherapy

Cancer vaccines show promise by eliciting tumor-specific cytotoxic T lymphocytes (CTL) responses. Efficient cytosolic co-delivery of antigens and adjuvants to dendritic cells (DCs) is crucial for vaccines to induce anti-tumor immunity. However, peptide- or nucleic acid-based biomolecules like tumor antigens and STING agonist cyclic-di-GMP (cdGMP) are prone to endosomal degradation, resulting in low cytosolic delivery and CTL response rates. Cationic nanocarriers can improve cytosolic delivery, but their positive charges induce off-target effects. Here, we develop cationic poly(ester amide) based nanoparticles co-loaded with antigens and adjuvant cdGMP (NP(cG, OVA)) for efficient cytosolic delivery and swallow them within antigen self-presenting DCs-derived dendrosomes (ODs) for lymph nodes (LNs) homing. The constructed dendrosomes swallowing nanovaccines ODs/NP(cG, OVA) demonstrated significantly reduced liver accumulation and enhanced LNs and DCs targeting compared to NP(cG, OVA). ODs/NP(cG, OVA) effectively cross-dressed the antigen epitopes on the shell to DCs and facilitated internalization of NP(cG, OVA), realizing DCs cytosolic co-delivery of antigens and adjuvants, thereby promoting antigen presentation, maturation and inflammatory cytokines secretion of DCs. Consequently, DCs stimulated by ODs/NP(cG, OVA) effectively induced activation, proliferation, and differentiation of antigen-specific CTLs that provided robust immune protection against tumor invasion. This work presents a powerful vaccine strategy for cancer immunotherapy.

Combined KRAS Inhibition and Immune Therapy Generates Durable Complete Responses in an Autochthonous PDAC Model

SIGNIFICANCE

Clinically available KRAS* inhibitors and IO agents alleviated the immunosuppressive tumor microenvironment in PDAC. Profound tumor regression and prolonged survival in an autochthonous PDAC model provide a compelling rationale for combining KRAS* inhibition with IO agents targeting multiple arms of the immunity cycle to combat PDAC.

Quantitative trait loci mapping provides insights into the genetic regulation of dendritic cell numbers in mouse tissues

To explore the influence of genetics on homeostatic regulation of dendritic cell (DC) numbers, we present a screen of DCs and their progenitors in lymphoid and non-lymphoid tissues in Collaborative Cross (CC) and Diversity Outbred (DO) mice. We report 30 and 71 loci with logarithm of the odds (LOD) scores >8.18 and ranging from 6.67 to 8.19, respectively. The analysis reveals the highly polygenic and pleiotropic architecture of this complex trait, including many of the previously identified genetic regulators of DC development and maturation. Two SNPs in genes potentially underlying variation in DC homeostasis, a splice variant in Gramd4 (rs235532740) and a missense variant in Orai3 (rs216659754), are confirmed by gene editing using CRISPR-Cas9. Gramd4 is a central regulator of DC homeostasis that impacts the entire DC lineage, and Orai3 regulates cDC2 numbers in tissues. Overall, the data reveal a large number of candidate genes regulating DC homeostasis in vivo.

Immune Modulation with RANKL Blockade through Denosumab Treatment in Patients with Cancer

Denosumab is a fully human mAb that binds receptor activator of NFκB ligand (RANKL). It is routinely administered to patients with cancer to reduce the incidence of new bone metastasis. RANK-RANKL interactions regulate bone turnover by controlling osteoclast recruitment, development, and activity. However, these interactions also can regulate immune cells including dendritic cells and medullary thymic epithelial cells. Inhibition of the latter results in reduced thymic negative selection of T cells and could enhance the generation of tumor-specific T cells. We examined whether administering denosumab could modify modulate circulating immune cells in patients with cancer. Blood was collected from 23 patients with prostate cancer and 3 patients with renal cell carcinoma, all of whom had advanced disease and were receiving denosumab, prior to and during denosumab treatment. Using high-dimensional mass cytometry, we found that denosumab treatment by itself induced modest effects on circulating immune cell frequency and activation. We also found minimal changes in the circulating T-cell repertoire and the frequency of new thymic emigrants with denosumab treatment. However, when we stratified patients by whether they were receiving chemotherapy and/or steroids, patients receiving these concomitant treatments showed significantly greater immune modulation, including an increase in the frequency of natural killer cells early and classical monocytes later. We also saw broad induction of CTLA-4 and TIM3 expression in circulating lymphocytes and some monocyte populations. These findings suggest that denosumab treatment by itself has modest immunomodulatory effects, but when combined with conventional cancer treatments, can lead to the induction of immunologic checkpoints. See related Spotlight by Nasrollahi and Davar, p. 383.

BCL2 Inhibition Reveals a Dendritic Cell-Specific Immune Checkpoint That Controls Tumor Immunosurveillance

We developed a phenotypic screening platform for the functional exploration of dendritic cells (DC). Here, we report a genome-wide CRISPR screen that revealed BCL2 as an endogenous inhibitor of DC function. Knockout of BCL2 enhanced DC antigen presentation and activation as well as the capacity of DCs to control tumors and to synergize with PD-1 blockade. The pharmacologic BCL2 inhibitors venetoclax and navitoclax phenocopied these effects and caused a cDC1-dependent regression of orthotopic lung cancers and fibrosarcomas. Thus, solid tumors failed to respond to BCL2 inhibition in mice constitutively devoid of cDC1, and this was reversed by the infusion of DCs. Moreover, cDC1 depletion reduced the therapeutic efficacy of BCL2 inhibitors alone or in combination with PD-1 blockade and treatment with venetoclax caused cDC1 activation, both in mice and in patients. In conclusion, genetic and pharmacologic BCL2 inhibition unveils a DC-specific immune checkpoint that restrains tumor immunosurveillance.

SIGNIFICANCE

BCL2 inhibition improves the capacity of DCs to stimulate anticancer immunity and restrain cancer growth in an immunocompetent context but not in mice lacking cDC1 or mature T cells. This study indicates that BCL2 blockade can be used to sensitize solid cancers to PD-1/PD-L1-targeting immunotherapy. This article is featured in Selected Articles from This Issue, p. 2293.

Dendritic Cell Vaccines: A Shift from Conventional Approach to New Generations

In the emerging era of cancer immunotherapy, immune checkpoint blockades (ICBs) and adoptive cell transfer therapies (ACTs) have gained significant attention. However, their therapeutic efficacies are limited due to the presence of cold type tumors, immunosuppressive tumor microenvironment, and immune-related side effects. On the other hand, dendritic cell (DC)-based vaccines have been suggested as a new cancer immunotherapy regimen that can address the limitations encountered by ICBs and ACTs. Despite the success of the first generation of DC-based vaccines, represented by the first FDA-approved DC-based therapeutic cancer vaccine Provenge, several challenges remain unsolved. Therefore, new DC vaccine strategies have been actively investigated. This review addresses the limitations of the currently most adopted classical DC vaccine and evaluates new generations of DC vaccines in detail, including biomaterial-based, immunogenic cell death-inducing, mRNA-pulsed, DC small extracellular vesicle (sEV)-based, and tumor sEV-based DC vaccines. These innovative DC vaccines are envisioned to provide a significant breakthrough in cancer immunotherapy landscape and are expected to be supported by further preclinical and clinical studies.

Glucose metabolism and its role in the maturation and migration of human CD1c+ dendritic cells following exposure to BCG

Introduction

Tuberculosis (TB) still kills over 1 million people annually. The only approved vaccine, BCG, prevents disseminated disease in children but shows low efficacy at preventing pulmonary TB. Myeloid dendritic cells (mDCs) are promising targets for vaccines and immunotherapies to combat infectious diseases due to their essential role in linking innate and adaptive immune responses. DCs undergo metabolic reprogramming following exposure to TLR agonists, which is thought to be a prerequisite for a successful host response to infection. We hypothesized that metabolic rewiring also plays a vital role in the maturation and migration of DCs stimulated with BCG. Consequently, we investigated the role of glycolysis in the activation of primary human myeloid CD1c⁺ DCs in response to BCG.

Methods/results

We show that CD1c⁺ mDC mature and acquire a more energetic phenotype upon challenge with BCG. Pharmacological inhibition of glycolysis with 2-deoxy-D-glucose (2-DG) decreased cytokine secretion and altered cell surface expression of both CD40 and CCR7 on BCG-challenged, compared to untreated, mDCs. Furthermore, inhibition of glycolysis had differential effects on infected and uninfected bystander mDCs in BCG-challenged cultures. For example, CCR7 expression was increased by 2-DG treatment following challenge with BCG and this increase in expression was seen only in BCG-infected mDCs. Moreover, although 2-DG treatment inhibited CCR7-mediated migration of bystander CD1C⁺ DCs in a transwell assay, migration of BCG-infected cells proceeded independently of glycolysis.

Discussion

Our results provide the first evidence that glycolysis plays divergent roles in the maturation and migration of human CD1c⁺ mDC exposed to BCG, segregating with infection status. Further investigation of cellular metabolism in DC subsets will be required to determine whether glycolysis can be targeted to elicit better protective immunity against Mtb.

A nanovaccine for antigen self-presentation and immunosuppression reversal as a personalized cancer immunotherapy strategy

The strategy of combining a vaccine with immune checkpoint inhibitors has been widely investigated in cancer management, but the complete response rate for this strategy is still unresolved. We describe a genetically engineered cell membrane nanovesicle that integrates antigen self-presentation and immunosuppression reversal (ASPIRE) for cancer immunotherapy. The ASPIRE nanovaccine is derived from recombinant adenovirus-infected dendritic cells in which specific peptide-major histocompatibility complex class I (pMHC-I), anti-PD1 antibody and B7 co-stimulatory molecules are simultaneously anchored by a programmed process. ASPIRE can markedly improve antigen delivery to lymphoid organs and generate broad-spectrum T-cell responses that eliminate established tumours. This work presents a powerful vaccine formula that can directly activate both native T cells and exhausted T cells, and suggests a general strategy for personalized cancer immunotherapy.

Ablation of Cbl-b and c-Cbl in dendritic cells causes spontaneous liver cirrhosis via altering multiple properties of CD103+ cDC1s

The Casitas B-lineage lymphoma (Cbl) family proteins are E3 ubiquitin ligases implicated in the regulation of various immune cells. However, their function in dendritic cells (DCs) remains unclear. To investigate the role of Cbl family members in DCs, we created dendritic cell double-deficient Casitas B lymphoma-b (Cbl-b) and Casitas B lineage lymphoma (c-Cbl) mice by crossing Cbl-b^−/− mice with c-Cbl^flox/flox CD11c-Cre⁺ mice. We found that specific deletion of Cbl-b and c-Cbl in CD11c⁺ cells, predominantly in DCs, led to liver fibrosis, cirrhosis, and accumulation of systemic conventional Type I DCs (cDC1s) due to enhanced cell proliferation and decreased cell apoptosis. In addition to a change in DC number, double knockout (dKO) cDC1s exhibited a partially activated status as indicated by high basal expression levels of certain cytokines and possessed an enhanced capacity to prime T cells. After adoptive transfer, dKO cDC1s could drive liver fibrosis too. In further experiments, we demonstrated that Cbl-b and c-Cbl could target signal transducer and activator of transcription 5 (STAT5), a transcriptional repressor for the pro-apoptotic protein Bim, to promote ubiquitination-mediated degradation and cell apoptosis in cDC1s. Further extensive experiments revealed that Cbl-b mediated K27-linked ubiquitination of lysine 164 of STAT5a while c-Cbl induced K29-linked ubiquitination of lysine 696 of STAT5a and K27-linked ubiquitination of lysine 140 and 694 of STAT5b. Thus, our findings indicate a functional redundancy of Cbl-b and c-Cbl in cDC homeostasis and maturation.

Bim Expression Promotes the Clearance of Mononuclear Phagocytes during Choroidal Neovascularization, Mitigating Scar Formation in Mice

Inflammation is increasingly recognized as an important modulator in the pathogenesis of neovascular age-related macular degeneration (nAMD). Although significant progress has been made in delineating the pathways that contribute to the recruitment of inflammatory cells and their contribution to nAMD, we know little about what drives the resolution of these inflammatory responses. Gaining a better understanding of how immune cells are cleared in the choroid will give a novel insight into how sustained inflammation could influence the pathogenesis of nAMD. The pro-apoptotic Bcl-2 family member Bim is a master regulator of immune cell homeostasis. In its absence, immune cell lifespan and numbers increase. Most therapeutic regimes that squelch inflammation do so by enhancing immune cell apoptosis through enhanced Bim expression and activity. To test the hypothesis that Bim expression tempers inflammation during the pathogenesis of nAMD, we used the mouse laser-induced choroidal neovascularization (CNV) model in which inflammation acts as a facilitator of CNV. Here, we showed minimal to no change in the recruitment of F4/80-, CD80-, CD11b-, and Iba1-positive myeloid-derived mononuclear phagocytes to the site of laser photocoagulation in the absence of Bim expression. However, the resolution of these cells from the choroid of Bim-deficient (Bim -/-) mice was significantly diminished following laser photocoagulation. With time, we noted increased scar formation, demonstrated by collagen I staining, in Bim -/- mice with no change in the resolution of neovascularization compared to wild-type littermates. We also noted that mice lacking Bim expression in mononuclear phagocytes (BimFlox/Flox; Lyz2-Cre (BimMP) mice) had delayed resolution of F4/80-, CD80-, CD11b-, and Iba1-positive cells, while those lacking Bim expression in endothelial cells (BimFlox/Flox; Cad5-Cre (BimEC) mice) had delayed resolution of only CD11b- and Iba1-positive cells. Both BimMP and BimEC mice demonstrated increased scar formation, albeit to differing degrees. Thus, our studies show that resolving inflammation plays an important role in moderating scar formation in nAMD, and it is impacted by Bim expression in both the endothelium and mononuclear phagocyte lineages.

Tolerogenic Dendritic Cells Shape a Transmissible Gut Microbiota That Protects From Metabolic Diseases

Excess chronic contact between microbial motifs and intestinal immune cells is known to trigger a low-grade inflammation involved in many pathologies such as obesity and diabetes. The important skewing of intestinal adaptive immunity in the context of diet-induced obesity (DIO) is well described, but how dendritic cells (DCs) participate in these changes is still poorly documented. To address this question, we challenged transgenic mice with enhanced DC life span and immunogenicity (DC^hBcl-2 mice) with a high-fat diet. Those mice display resistance to DIO and metabolic alterations. The DIO-resistant phenotype is associated with healthier parameters of intestinal barrier function and lower intestinal inflammation. DC^hBcl-2 DIO-resistant mice demonstrate a particular increase in tolerogenic DC numbers and function, which is associated with strong intestinal IgA, T helper 17, and regulatory T-cell immune responses. Microbiota composition and function analyses reveal that the DC^hBcl-2 mice microbiota is characterized by lower immunogenicity and an enhanced butyrate production. Cohousing experiments and fecal microbial transplantations are sufficient to transfer the DIO resistance status to wild-type mice, demonstrating that maintenance of DCs' tolerogenic ability sustains a microbiota able to drive DIO resistance. The tolerogenic function of DCs is revealed as a new potent target in metabolic disease management.

Aging and Interferons: Impacts on Inflammation and Viral Disease Outcomes

As highlighted by the COVID-19 global pandemic, elderly individuals comprise the majority of cases of severe viral infection outcomes and death. A combined inability to control viral replication and exacerbated inflammatory immune activation in elderly patients causes irreparable immune-mediated tissue pathology in response to infection. Key to these responses are type I, II, and III interferons (IFNs), which are involved in inducing an antiviral response, as well as controlling and suppressing inflammation and immunopathology. IFNs support monocyte/macrophage-stimulated immune responses that clear infection and promote their immunosuppressive functions that prevent excess inflammation and immune-mediated pathology. The timing and magnitude of IFN responses to infection are critical towards their immunoregulatory functions and ability to prevent immunopathology. Aging is associated with multiple defects in the ability of macrophages and dendritic cells to produce IFNs in response to viral infection, leading to a dysregulation of inflammatory immune responses. Understanding the implications of aging on IFN-regulated inflammation will give critical insights on how to treat and prevent severe infection in vulnerable individuals. In this review, we describe the causes of impaired IFN production in aging, and the evidence to suggest that these impairments impact the regulation of the innate and adaptive immune response to infection, thereby causing disease pathology.

Enhanced Human T Lymphocyte Antigen Priming by Cytokine-Matured Dendritic Cells Overexpressing Bcl-2 and IL-12

Dendritic cell (DC)-based vaccination is a promising immunotherapeutic strategy for cancer. However, clinical trials have shown only limited efficacy, suggesting the need to optimize protocols for human DC vaccine preparation. In this study, we systemically compared five different human DC vaccine maturation protocols used in clinical trials: (1) a four-cytokine cocktail (TNF-α, IL-6, IL-1β, and PGE2); (2) an α-DC-cytokine cocktail (TNF-α, IL-1β, IFN-α, IFN-γ, and poly I:C); (3) lipopolysaccharide (LPS)/IFN-γ; (4) TNF-α and PGE2; and (5) TriMix (mRNAs encoding CD40L, CD70, and constitutively active Toll-like receptor 4 electroporated into immature DCs). We found that the four-cytokine cocktail induced high levels of costimulatory and HLA molecules, as well as CCR7, in DCs. Mature DCs (mDCs) matured with the four-cytokine cocktail had higher viability than those obtained with the other protocols. Based on these features, we chose the four-cytokine cocktail protocol to further improve the immunizing capability of DCs by introducing exogenous genes. We showed that introducing exogenous Bcl-2 increased DC survival. Furthermore, introducing IL-12p70 rescued the inhibition of IL-12 secretion by PGE2 without impairing the DC phenotype. Introducing both Bcl-2 and IL-12p70 mRNAs into DCs induced enhanced cytomegalovirus pp65-specific CD8⁺ T cells secreting IFN-γ and TNF-α. Taken together, our data suggest that DC matured by the four-cytokine cocktail combined with exogenous Bcl-2 and IL-12p70 gene expression represents a promising approach for clinical applications in cancer immunotherapy.

Expression of the Phosphatase Ppef2 Controls Survival and Function of CD8+ Dendritic Cells

Apoptotic cell death of Dendritic cells (DCs) is critical for immune homeostasis. Although intrinsic mechanisms controlling DC death have not been fully characterized up to now, experimentally enforced inhibition of DC-death causes various autoimmune diseases in model systems. We have generated mice deficient for Protein Phosphatase with EF-Hands 2 (Ppef2), which is selectively expressed in CD8⁺ DCs, but not in other related DC subtypes such as tissue CD103⁺ DCs. Ppef2 is down-regulated rapidly upon maturation of DCs by toll-like receptor stimuli, but not upon triggering of CD40. Ppef2-deficient CD8⁺ DCs accumulate the pro-apoptotic Bcl-2-like protein 11 (Bim) and show increased apoptosis and reduced competitve repopulation capacities. Furthermore, Ppef2^−/− CD8⁺ DCs have strongly diminished antigen presentation capacities in vivo, as CD8⁺ T cells primed by Ppef2^−/− CD8⁺ DCs undergo reduced expansion. In conclusion, our data suggests that Ppef2 is crucial to support survival of immature CD8⁺ DCs, while Ppef2 down-regulation during DC-maturation limits T cell responses.

Inflammatory compound lipopolysaccharide promotes the survival of GM-CSF cultured dendritic cell via PI3 kinase-dependent upregulation of Bcl-x

As professional antigen-presenting cells, dendritic cells (DCs) initiate and regulate immune responses against inflammation. The invasion of pathogens into the body, however, can in turn cause the change of DCs in both activity and viability, which ultimately affect immune homeostasis. The exact mechanisms that the bacteria utilize to alter the lifespan of DCs, however, are far from clear. In this study, we found that the bacterial wall compound lipopolysaccharide (LPS) can promote the survival of GM-CSF-differentiated DCs (GM-DCs). At molecular levels, we demonstrated that GM-DCs had distinct pattern of mRNA expression for anti-apoptotic BCL-2 family members, of which, Bcl-x increased significantly following LPS stimulation. Interestingly, specific inhibition of BCL-XL protein alone was sufficient to remove the anti-apoptotic effects of LPS on BM-DCs. Further study of the signaling mechanisms revealed that although LPS can activate both Erk MAP kinase and PI3 kinase pathways, only blocking of PI3K abolished both Bcl-x upregulation and the enhanced survival phenotype, suggesting that the PI3K signaling mediated the upregulation of Bcl-x for the LPS-induced pro-survival in GM-DCs. Collectively, this study unveils a molecular mechanism that DCs adapt to maintain innate immunity against the invasion of pathogens.

Bim suppresses the development of SLE by limiting myeloid inflammatory responses

Tsai et al. demonstrate that loss of Bim (BCL2L11) in myeloid cells in mice (LysM^CreBim^fl/fl) is sufficient to induce systemic autoimmunity. Kidney macrophages in LysM^CreBim^fl/fl mice possess a proinflammatory transcriptional signature and signal through TRIF to cause end-stage glomerulonephritis.

The Bcl-2 family is considered the guardian of the mitochondrial apoptotic pathway. We demonstrate that Bim acts as a molecular rheostat by controlling macrophage function not only in lymphoid organs but also in end organs, thereby preventing the break in tolerance. Mice lacking Bim in myeloid cells (LysM^CreBim^fl/fl) develop a systemic lupus erythematosus (SLE)–like disease that mirrors aged Bim^−/− mice, including loss of marginal zone macrophages, splenomegaly, lymphadenopathy, autoantibodies (including anti-DNA IgG), and a type I interferon signature. LysM^CreBim^fl/fl mice exhibit increased mortality attributed to glomerulonephritis (GN). Moreover, the toll-like receptor signaling adaptor protein TRIF (TIR-domain–containing adapter-inducing interferon-β) is essential for GN, but not systemic autoimmunity in LysM^CreBim^fl/fl mice. Bim-deleted kidney macrophages exhibit a novel transcriptional lupus signature that is conserved within the gene expression profiles from whole kidney biopsies of patients with SLE. Collectively, these data suggest that the Bim may be a novel therapeutic target in the treatment of SLE.

The inflammatory role of phagocyte apoptotic pathways in rheumatic diseases

Rheumatoid arthritis affects nearly 1% of the world's population and is a debilitating autoimmune condition that can result in joint destruction. During the past decade, inflammatory functions have been described for signalling molecules classically involved in apoptotic and non-apoptotic death pathways, including, but not limited to, Toll-like receptor signalling, inflammasome activation, cytokine production, macrophage polarization and antigen citrullination. In light of these remarkable advances in the understanding of inflammatory mechanisms of the death machinery, this Review provides a snapshot of the available evidence implicating death pathways, especially within the phagocyte populations of the innate immune system, in the perpetuation of rheumatoid arthritis and other rheumatic diseases. Elevated levels of signalling mediators of both extrinsic and intrinsic apoptosis, as well as the autophagy, are observed in the joints of patients with rheumatoid arthritis. Furthermore, risk polymorphisms are present in signalling molecules of the extrinsic apoptotic and autophagy death pathways. Although research into the mechanisms underlying these pathways has made considerable progress, this Review highlights areas where further investigation is particularly needed. This exploration is critical, as new discoveries in this field could lead to the development of novel therapies for rheumatoid arthritis and other rheumatic diseases.

The contribution of the programmed cell death machinery in innate immune cells to lupus nephritis

Systemic lupus erythematosus (SLE) is a chronic multi-factorial autoimmune disease initiated by genetic and environmental factors, which in combination trigger disease onset in susceptible individuals. Damage to the kidney as a consequence of lupus nephritis (LN) is one of the most prevalent and severe outcomes, as LN affects up to 60% of SLE patients and accounts for much of SLE-associated morbidity and mortality. As remarkable strides have been made in unlocking new inflammatory mechanisms associated with signaling molecules of programmed cell death pathways, this review explores the available evidence implicating the action of these pathways specifically within dendritic cells and macrophages in the control of kidney disease. Although advancements into the underlying mechanisms responsible for inducing cell death inflammatory pathways have been made, there still exist areas of unmet need. By understanding the molecular mechanisms by which dendritic cells and macrophages contribute to LN pathogenesis, we can improve their viability as potential therapeutic targets to promote remission.

Inhibition of SIRPα in dendritic cells potentiates potent antitumor immunity

Despite their central function in tumor immunity, dendritic cells (DCs) can respond to inhibitory signals and become tolerogenic, curtailing T cell responses in vivo. Here, we provide the evidence for an inhibitory function of signal regulatory protein (SIRP) α in DC survival and activation. In tumors from human liver cancer patients, infiltrative DCs expressed elevated levels of SIRPα, which is correlated with the induction of immune tolerance within the tumors. Silencing of SIRPα resulted in a significant increase in the longevity of antigen-pulsed DCs in the draining lymph nodes. In addition, SIRPα controls the activation and output of DCs. Silencing of DC-expressed SIRPα induced spontaneous and enhanced production of IL12 and costimulatory molecules, resulting in more potent cytotoxic T lymphocyte responses, including the eradication of previously established solid tumors. SIRPα exerted such effects, at least in part, via the association and sequestration of p85 subunit of PI3K. Thus, SIRPα is a critical regulator of DC lifespan and activity, and its inhibition might improve the clinical efficacy of DC-based tumor vaccines.

A virtual lymph node model to dissect the requirements for T-cell activation by synapses and kinapses

The initiation of T-cell responses in lymph nodes requires T cells to integrate signals delivered by dendritic cells (DCs) during long-lasting contacts (synapses) or more transient interactions (kinapses). However, it remains extremely challenging to understand how a specific sequence of contacts established by T cells ultimately dictates T-cell fate. Here, we have coupled a computational model of T-cell migration and interactions with DCs with a real-time, flow cytometry-like representation of T-cell activation. In this model, low-affinity peptides trigger T-cell proliferation through kinapses but we show that this process is only effective under conditions of high DC densities and prolonged antigen availability. By contrast, high-affinity peptides favor synapse formation and a vigorous proliferation under a wide range of antigen presentation conditions. In line with the predictions, decreasing the DC density in vivo selectively abolished proliferation induced by the low-affinity peptide. Finally, our results suggest that T cells possess a biochemical memory of previous stimulations of at least 1–2 days. We propose that the stability of T-cell–DC interactions, apart from their signaling potency, profoundly influences the robustness of T-cell activation. By offering the ability to control parameters that are difficult to manipulate experimentally, the virtual lymph node model provides new possibilities to tackle the fundamental mechanisms that regulate T-cell responses elicited by infections or vaccines.

Activation-Induced Cell Death of Dendritic Cells Is Dependent on Sphingosine Kinase 1

Sphingosine 1-phosphate (S1P) is an immune modulatory lipid mediator and has been implicated in numerous pathophysiological processes. S1P is produced by sphingosine kinase 1 (Sphk1) and Sphk2. Dendritic cells (DCs) are central for the direction of immune responses and crucially involved in autoimmunity and cancerogenesis. In this study we examined the function and survival of bone marrow-derived DCs under long-term inflammatory stimulation. We observed that differentiated cells undergo activation-induced cell death (AICD) upon LPS stimulation with an increased metabolic activity shortly after stimulation, followed by a rapid activation of caspase 3 and subsequent augmented apoptosis. Importantly, we highlight a profound role of Sphk1 in secretion of inflammatory cytokines and survival of dendritic cells that might be mediated by a change in sphingolipid levels as well as by a change in STAT3 expression. Cell growth during differentiation of Sphk1-deficient cells treated with the functional S1P receptor antagonist FTYP was reduced. Importantly, in dendritic cells we did not observe a compensatory regulation of Sphk2 mRNA in Sphk1-deficient cells. Instead, we discovered a massive increase in Sphk1 mRNA concentration upon long-term stimulation with LPS in wild type cells that might function as an attempt to rescue from inflammation-caused cell death. Taken together, in this investigation we describe details of a crucial involvement of sphingolipids and Sphk1 in AICD during long-term immunogenic activity of DCs that might play an important role in autoimmunity and might explain the differences in immune response observed in in vivo studies of Sphk1 modulation.

Antitumor cytotoxicity induced by bone-marrow-derived antigen-presenting cells is facilitated by the tumor suppressor protein p53 via regulation of IL-12

Activated antigen-presenting cells (APC) deliver the three signals cytotoxic T cells require to differentiate into effector cells that destroy the tumor. These comprise antigen, co-stimulatory signals and cytokines. Once these cells have carried out their function, they apoptose. We hypothesized that the tumor suppressor protein, p53, played an important role in generating the antitumor response facilitated by APC. CD11c⁺ APC derived from p53 wild-type (wt) mouse (wt p53) GM-CSF bone marrow cultures (BMAPC) and activated had reduced survival compared to BMAPC from p53 null consistent with p53-mediated apoptosis following activation. There was a lower percentage of antigenic peptide/MHC I complexes on antigen-pulsed p53 null cells suggesting p53 played a role in antigen processing but there was no difference in antigen-specific T cell proliferative responses to these cells in vivo. In contrast, antigen-specific cytotoxicity in vivo was markedly reduced in response to p53 null BMAPC. When these cells were pulsed with a model tumor antigen and delivered as a prophylactic vaccination, they provided no protection against melanoma cell growth whereas wt BMAPC were very effective. This suggested that p53 might regulate the requisite third signal and, indeed, we found that p53 null BMAPC produced less IL-12 than wt p53 BMAPC and that p53 bound to the promoter region of IL-12. This work suggests that p53 in activated BMAPC is associated with the generation of IL-12 required for the differentiation of cytotoxic immune responses and an effective antitumor response. This is a completely new role for this protein that has implications for BMAPC-mediated immunotherapy.

Reduction of conventional dendritic cells during Plasmodium infection is dependent on activation induced cell death by type I and II interferons

Dendritic cells (DCs) play critical roles in innate and adaptive immunity and in pathogenesis during the blood stage of malaria infection. The mechanisms underlying DC homeostasis during malaria infection are not well understood. In this study, the numbers of conventional DCs (cDCs) and plasmacytoid DCs (pDCs) in the spleens after lethal rodent malaria infection were examined, and were found to be significantly reduced. Concomitant with up-regulation of maturation-associated molecules, activation of caspase-3 was significantly increased, suggesting induction of cell death. Studies using neutralizing antibody and gene-deficient mice showed that type I and II interferons were critically involved in activation induced cell death of cDCs during malaria infection. These results demonstrate that DCs rapidly disappeared following IFN-mediated DC activation, and that homeostasis of DCs was significantly impaired during malaria infection.

Control of Homeostasis and Dendritic Cell Survival by the GTPase RhoA

Tissues accommodate defined numbers of dendritic cells (DCs) in highly specific niches where different intrinsic and environmental stimuli control DC life span and numbers. DC homeostasis in tissues is important, because experimental changes in DC numbers influence immunity and tolerance toward various immune catastrophes and inflammation. However, the precise molecular mechanisms regulating DC life span and homeostasis are unclear. We report that the GTPase RhoA controls homeostatic proliferation, cytokinesis, survival, and turnover of cDCs. Deletion of RhoA strongly decreased the numbers of CD11b(-)CD8(+) and CD11b(+)Esam(hi) DC subsets, whereas CD11b(+)Esam(lo) DCs were not affected in conditional RhoA-deficient mice. Proteome analyses revealed a defective prosurvival pathway via PI3K/protein kinase B (Akt1)/Bcl-2-associated death promoter in the absence of RhoA. Taken together, our findings identify RhoA as a central regulator of DC homeostasis, and its deletion decreases DC numbers below critical thresholds for immune protection and homeostasis, causing aberrant compensatory DC proliferation.

Prosurvival Bcl-2 family members reveal a distinct apoptotic identity between conventional and plasmacytoid dendritic cells

Dendritic cells (DCs) are heterogeneous, comprising subsets with functional specializations that play distinct roles in immunity as well as immunopathology. We investigated the molecular control of cell survival of two main DC subsets: plasmacytoid DCs (pDCs) and conventional DCs (cDCs) and their dependence on individual antiapoptotic BCL-2 family members. Compared with cDCs, pDCs had higher expression of BCL-2, lower A1, and similar levels of MCL-1 and BCL-XL. Transgenic overexpression of BCL-2 increased the pDC pool size in vivo with only minor impact on cDCs. With a view to immune intervention, we tested BCL-2 inhibitors and found that ABT-199 (the BCL-2 specific inhibitor) selectively killed pDCs but not cDCs. Conversely, genetic knockdown of A1 profoundly reduced the proportion of cDCs but not pDCs. We also found that conditional ablation of MCL-1 significantly reduced the size of both DC populations in mice and impeded DC-mediated immune responses. Thus, we revealed that the two DC types have different cell survival requirements. The molecular basis of survival of different DC subsets thus advocates the antagonism of selective BCL-2 family members for treating diseases pertaining to distinct DC subsets.

In vivo 4-1BB deficiency in myeloid cells enhances peripheral T cell proliferation by increasing IL-15

4-1BB signals are considered positive regulators of T cell responses against viruses and tumors, but recent studies suggest that they have more complex roles in modulating T cell responses. Although dual roles of 4-1BB signaling in T cell responses have been suggested, the underlying mechanisms are still not fully understood. In this study, we tested whether 4-1BB expression affected T cell responses differently when expressed in myeloid versus lymphoid cells in vivo. By assessing the proliferation of 4-1BB(+/+) and 4-1BB(-/-) T cells in lymphocyte-deficient RAG2(-/-) and RAG2(-/-)4-1BB(-/-) mice, we were able to compare the effects on T cell responses of 4-1BB expression on myeloid versus T cells. Surprisingly, adoptively transferred T cells were more responsive in tumor-bearing RAG2(-/-)4-1BB(-/-) mice than in RAG2(-/-) mice, and this enhanced T cell proliferation was further enhanced if the T cells were 4-1BB deficient. Dendritic cells (DCs) rather than NK or tissue cells were the myeloid lineage cells primarily responsible for the enhanced T cell proliferation. However, individual 4-1BB(-/-) DCs were less effective in T cell priming in vivo than 4-1BB(+/+) DCs; instead, more DCs in the secondary lymphoid organs of RAG2(-/-)4-1BB(-/-) mice appeared to induce the enhanced T cell proliferation by producing and transpresenting more IL-15. Therefore, we conclude that in vivo 4-1BB signaling of myeloid cells negatively regulates peripheral T cell responses by limiting the differentiation of DCs and their accumulation in secondary lymphoid organs.

A novel MEK-ERK-AMPK signaling axis controls chemokine receptor CCR7-dependent survival in human mature dendritic cells

Chemokine receptor CCR7 directs mature dendritic cells (mDCs) to secondary lymph nodes where these cells regulate the activation of T cells. CCR7 also promotes survival in mDCs, which is believed to take place largely through Akt-dependent signaling mechanisms. We have analyzed the involvement of the AMP-dependent kinase (AMPK) in the control of CCR7-dependent survival. A pro-apoptotic role for AMPK is suggested by the finding that pharmacological activators induce apoptosis, whereas knocking down of AMPK with siRNA extends mDC survival. Pharmacological activation of AMPK also induces apoptosis of mDCs in the lymph nodes. Stimulation of CCR7 leads to inhibition of AMPK, through phosphorylation of Ser-485, which was mediated by G(i)/Gβγ, but not by Akt or S6K, two kinases that control the phosphorylation of AMPK on Ser-485 in other settings. Using selective pharmacological inhibitors, we show that CCR7-induced phosphorylation of AMPK on Ser-485 is mediated by MEK and ERK. Coimmunoprecipitation analysis and proximity ligation assays indicate that AMPK associates with ERK, but not with MEK. These results suggest that in addition to Akt-dependent signaling mechanisms, CCR7 can also promote survival of mDCs through a novel MEK1/2-ERK1/2-AMPK signaling axis. The data also suggest that AMPK may be a potential target to modulate mDC lifespan and the immune response.

IFN-γ regulates survival and function of tumor-induced CD11b+ Gr-1high myeloid derived suppressor cells by modulating the anti-apoptotic molecule Bcl2a1

Myeloid derived suppressor cells (MDSCs) play a critical role in suppression of immune responses in cancer and inflammation. Here, we describe how regulation of Bcl2a1 by cytokines controls the suppressor function of CD11b(+) Gr-1(high) granulocytic MDSCs. Coculture of CD11b(+) Gr-1(high) granulocytic MDSCs with antigen-stimulated T cells and simultaneous blockade of IFN-γ by the use of anti-IFN-γ blocking antibody, IFN-γ(-/-) effector T cells, IFN-γR(-/-) MDSCs or STAT1(-/-) MDSCs led to upregulation of Bcl2a1 in CD11b(+) Gr-1(high) cells, improved survival, and enhanced their suppressor function. Molecular studies revealed that GM-CSF released by antigen-stimulated CD8(+) T cells induced Bcl2a1 upregulation, which was repressed in the presence of IFN-γ by a direct interaction of phosphorylated STAT-1 with the Bcl2a1 promotor. Bcl2a1 overexpressing granulocytic MDSCs demonstrated prolonged survival and enhanced suppressor function in vitro. Our data suggest that IFN-γ/ STAT1-dependent regulation of Bcl2a1 regulates survival and thereby suppressor function of granulocytic MDSCs.

Loss of the death receptor CD95 (Fas) expression by dendritic cells protects from a chronic viral infection

Chronic viral infections incapacitate adaptive immune responses by "exhausting" virus-specific T cells, inducing their deletion and reducing productive T-cell memory. Viral infection rapidly induces death receptor CD95 (Fas) expression by dendritic cells (DCs), making them susceptible to elimination by the immune response. Lymphocytic choriomeningitis virus (LCMV) clone 13, which normally establishes a chronic infection, is rapidly cleared in C57Black6/J mice with conditional deletion of Fas in DCs. The immune response to LCMV is characterized by an extended survival of virus-specific effector T cells. Moreover, transfer of Fas-negative DCs from noninfected mice to preinfected animals results in either complete clearance of the virus or a significant reduction of viral titers. Thus, DC-specific Fas expression plays a role in regulation of antiviral responses and suggests a strategy for stimulation of T cells in chronically infected animals and humans to achieve the clearance of persistent viruses.

Detecting apoptosis of leukocytes in mouse lymph nodes

Although there are multiple methods for analyzing apoptosis in cultured cells, methodologies for analyzing apoptosis in vivo are sparse. In this protocol, we describe how to detect apoptosis of leukocytes in mouse lymph nodes (LNs) via the detection of apoptotic caspases. We have previously used this protocol to study factors that modulate dendritic cell (DC) survival in LNs; however, it can also be used to analyze other leukocytes that migrate to the LNs. DCs labeled with a fluorescent cell tracker are subcutaneously injected in the posterior footpads of mice. Once the labeled DCs reach the popliteal LN (PLN), the animals are intravenously injected with FLIVO, a permeant fluorescent reagent that selectively marks active caspases and consequently apoptotic cells. Explanted PLNs are then examined under a two-photon microscope to look for the presence of apoptotic cells among the DCs injected. The protocol requires 6-6.5 h for preparation and analysis plus an additional 34-40 h to allow apoptosis of the injected DCs in the PLN.

Bcl-2 family member Bcl-G is not a proapoptotic protein

The three major subgroups of the Bcl-2 family, including the prosurvival Bcl-2-like proteins, the proapoptotic Bcl-2 homology (BH)3-only proteins and Bax/Bak proteins, regulate the mitochondrial apoptotic pathway. In addition, some outliers within the Bcl-2 family do not fit into these subgroups. One of them, Bcl-G, has a BH2 and a BH3 region, and was proposed to trigger apoptosis. To investigate the physiological role of Bcl-G, we have inactivated the gene in the mouse and generated monoclonal antibodies to determine its expression. Although two isoforms of Bcl-G exist in human, only one is found in mice. mBcl-G is expressed in a range of epithelial as well as in dendritic cells. Loss of Bcl-G did not appear to affect any of these cell types. mBcl-G only binds weakly to prosurvival members of the Bcl-2 family, and in a manner that is independent of its BH3 domain. To understand what the physiological role of Bcl-G might be, we searched for Bcl-G-binding partners through immunoprecipitation/mass spectroscopy and yeast-two-hybrid screening. Although we did not uncover any Bcl-2 family member in these screens, we found that Bcl-G interacts specifically with proteins of the transport particle protein complex. We conclude that Bcl-G most probably does not function in the classical stress-induced apoptosis pathway, but rather has a role in protein trafficking inside the cell.

Transforming growth factor beta-activated kinase 1 (TAK1)-dependent checkpoint in the survival of dendritic cells promotes immune homeostasis and function

Homeostatic control of dendritic cell (DC) survival is crucial for adaptive immunity, but the molecular mechanism is not well defined. Moreover, how DCs influence immune homeostasis under steady state remains unclear. Combining DC-specific and -inducible deletion systems, we report that transforming growth factor beta-activated kinase 1 (TAK1) is an essential regulator of DC survival and immune system homeostasis and function. Deficiency of TAK1 in CD11c(+) cells induced markedly elevated apoptosis, leading to the depletion of DC populations, especially the CD8(+) and CD103(+) DC subsets in lymphoid and nonlymphoid tissues, respectively. TAK1 also contributed to DC development by promoting the generation of DC precursors. Prosurvival signals from Toll-like receptors, CD40 and receptor activator of nuclear factor-κB (RANK) are integrated by TAK1 in DCs, which in turn mediated activation of downstream NF-κB and AKT-Foxo pathways and established a gene-expression program. TAK1 deficiency in DCs caused a myeloid proliferative disorder characterized by expansion of neutrophils and inflammatory monocytes, disrupted T-cell homeostasis, and prevented effective T-cell priming and generation of regulatory T cells. Moreover, TAK1 signaling in DCs was required to prevent myeloid proliferation even in the absence of lymphocytes, indicating a previously unappreciated regulatory mechanism of DC-mediated control of myeloid cell-dependent inflammation. Therefore, TAK1 orchestrates a prosurvival checkpoint in DCs that affects the homeostasis and function of the immune system.

Regulation of Immune Responses by Spontaneous and T cell-mediated Dendritic Cell Death

In response to antigen stimulations, cells in the immune system undergo dynamic activation, differentiation, expansion and turnover. Programmed cell death is important for maintaining homeostasis of different cell types in the immune system. Dendritic cells (DCs) are a heterogeneous population of antigen presenting cells that capture, process and present antigens to stimulate lymphocytes. DCs have also emerged as major regulators of both innate and adaptive immune responses. Conventional myeloid DCs are relatively short-lived compared to lymphocytes in lymphoid organs. Mitochondrion-dependent apoptosis governed by Bcl-2 family members plays a major role in regulating spontaneous DC turnover. Killing of DCs by antigen-specific T cells also provides a negative feedback mechanism to restrict the duration and the scope of immune responses. Defects in cell death in DCs lead to DC accumulation, resulting in overactivation of lymphocytes and the development of autoimmunity in mice. Programmed cell death in DCs may play essential roles in the regulation of the duration and magnitude of immune responses, and in the protection against autoimmunity and uncontrolled inflammation.

Effects of 4-1BB signaling on the biological function of murine dendritic cells

4-1BB signaling has profound effects on the T cell-induced cell immune response, but its biological function in dendritic cells (DCs) has remained largely uncharacterized. In this study, we investigated the function of 4-1BB in murine DCs with an agonistic mAb to 4-1BB. Interleukin (IL)-6 and IL-12 production was assessed by an enzyme-linked immunosorbent assay (ELISA). Co-stimulatory molecules (CD80 and CD86) in DCs were analyzed by flow cytometry. The results showed that 4-1BB was strongly expressed in DCs during the maturation process. Triggering 4-1BB increased the secretion of IL-6 and IL-12 and the upregulation of co-stimulatory molecules (CD80 and CD86) from DCs, indicating that agonistic mAb to 4-1BB directly improves the activation of DCs. Moreover, triggering 4-1BB induced a higher survival rate of DCs compared to that of hamster IgG isotype control, due to the upregulated expression of Bcl-2 and Bcl-xL. To further assess the role of 4-1BB on DCs stimulating T-cell proliferation, allogeneic mixed lymphocyte reactions were analyzed. The agonistic anti-4-1BB mAb induced a higher T-cell proliferation. These results suggest that 4-1BB affects the duration, DC-T interaction and immunogenicity of DCs.

Type I interferon negatively controls plasmacytoid dendritic cell numbers in vivo

Production of type I interferon in response to virus infection reduces plasmacytoid dendritic cell numbers, thus creating a negative feedback loop.

Plasmacytoid dendritic cells (pDCs) specialize in the secretion of type I interferons (IFN-I) and thus are considered critical mediators of antiviral responses. We recently reported that pDCs have a very early but limited and transient capacity to curtail viral infections. Additionally, pDC numbers are not sustained in human infections caused by Hepatitis B or C viruses (HBV and HCV) and HIV. Thus, the numbers and/or function of pDCs appear to be regulated during the course of viral infection. In this study, we show that splenic pDCs are reduced in vivo during several systemic viral infections and after administration of synthetic toll-like receptor ligands. We demonstrate that IFN-I, regardless of the source, contributes to this decline and mediates pDC death via the intrinsic apoptosis pathway. These findings demonstrate a feedback control mechanism by which IFN-I modulates pDC numbers, thus fine-tuning systemic IFN-I response to viruses. IFN-I–mediated control of pDCs may explain the loss of pDCs during human infections caused by HBV, HCV, or HIV and has important therapeutic implications for settings in which IFN-I is used to treat infections and autoimmune diseases.

Immune regulation through mitochondrion-dependent dendritic cell death induced by T regulatory cells

Dendritic cells (DCs) harbor an active mitochondrion-dependent cell death pathway regulated by Bcl-2 family members and undergo rapid turnover in vivo. However, the functions for mitochondrion-dependent cell death of DCs in immune regulation remain to be elucidated. In this article, we show that DC-specific knockout of proapoptotic Bcl-2 family members, Bax and Bak, induced spontaneous T cell activation and autoimmunity in mice. In addition to a defect in spontaneous cell death, Bax(-/-)Bak(-/-) DCs were resistant to killing by CD4(+)Foxp3(+) T regulatory cells (Tregs) compared with wild-type DCs. Tregs inhibited the activation of T effector cells by wild-type, but not Bax(-/-)Bak(-/-), DCs. Bax(-/-)Bak(-/-) DCs showed increased propensity for inducing autoantibodies. Moreover, the autoimmune potential of Bax(-/-)Bak(-/-) DCs was resistant to suppression by Tregs. Our data suggested that Bax and Bak mediate intrinsic spontaneous cell death in DCs, as well as regulate DC killing triggered by Tregs. Bax- and Bak-dependent cell death mechanisms help to maintain DC homeostasis and contribute to the regulation of T cell activation and the suppression of autoimmunity.

Applications of myeloid-specific promoters in transgenic mice support in vivo imaging and functional genomics but do not support the concept of distinct macrophage and dendritic cell lineages or roles in immunity

Myeloid lineage cells contribute to innate and acquired immunity, homeostasis, wound repair, and inflammation. There is considerable interest in manipulation of their function in transgenic mice using myeloid-specific promoters. This review considers the applications and specificity of some of the most widely studied transgenes, driven by promoter elements of the lysM, csf1r, CD11c, CD68, macrophage SRA, and CD11b genes, as well as several others. Transgenes have been used in mice to generate myeloid lineage-specific cell ablation, expression of genes of interest, including fluorescent reporters, or deletion via recombination. In general, the specificity of such transgenes has been overinterpreted, and none of them provide well-documented, reliable, differential expression in any specific myeloid cell subset, macrophages, granulocytes, or myeloid DCs. Nevertheless, they have proved valuable in cell isolation, functional genomics, and live imaging of myeloid cell behavior in many different pathologies.

The canonical BMP signaling pathway is involved in human monocyte-derived dendritic cell maturation

Bone morphogenetic proteins (BMPs), members of the transforming growth factor-β superfamily, are multifunctional polypeptides regulating a broad spectrum of functions in embryonic and adult tissues. Recent reports have demonstrated that BMPs regulate the survival, proliferation and differentiation of several cell types in the immune system. In this study, we investigate the effects of BMP signaling activation on the capacity of human dendritic cells (DCs) to stimulate immune responses. Human DCs express type I and type II BMP receptors (BMPRIA, BMPRIB, type IA activin receptor, BMPRII) and BMP signal transduction molecules (Smad1, 5, and 8, as well as Smad4). On BMP stimulation, Id1-3 (inhibitor of differentiation 1-3/DNA binding) mRNA expression is upregulated and this effect can be blocked with the inhibitor dorsomorphin, showing that the canonical BMP signal transduction pathway is functionally active in DCs. BMP signaling activation promotes the phenotypic maturation of human DCs by increasing the expression of co-stimulatory molecules and also CD83, programmed cell death ligand 1 (PD-L1) and PD-L2, and stimulates cytokine secretion, mainly interleukin-8 and tumor necrosis factor-α. Accordingly, BMP-treated DCs exhibit an enhanced T-cell stimulatory capacity. BMP signaling also enhances the survival of human DCs increasing the Bcl-2/Bax ratio. Finally, the expression of Runx transcription factors is increased in mature DCs, and the mRNA levels of Runx1-3 are upregulated in response to BMP stimulation, indicating that Runx transcription factor family may mediate the effects of BMP signaling in human DC maturation.

A nonredundant role for canonical NF-κB in human myeloid dendritic cell development and function

The plastic role of dendritic cells (DCs) in the regulation of immune responses has made them interesting targets for immunotherapy, but also for pathogens or tumors to evade immunity. Functional alterations of DCs are often ascribed to manipulation of canonical NF-κB activity. However, though this pathway has been linked to murine myeloid DC biology, a detailed analysis of its importance in human myeloid DC differentiation, survival, maturation, and function is lacking. The myeloid DC subsets include interstitial DCs and Langerhans cells. In this study, we investigated the role of canonical NF-κB in human myeloid DCs generated from monocytes (monocyte-derived DCs [mo-DCs]) or CD34(+) progenitors (CD34-derived myeloid DCs [CD34-mDCs]). Inhibition of NF-κB activation during and after mo-DC, CD34-interstitial DC, or CD34-Langerhans cell differentiation resulted in apoptosis induction associated with caspase 3 activation and loss of mitochondrial transmembrane potential. Besides regulating survival, canonical NF-κB activity was required for the acquisition of a DC phenotype. Despite phenotypic differences, however, Ag uptake, costimulatory molecule and CCR7 expression, as well as T cell stimulatory capacity of cells generated under NF-κB inhibition were comparable to control DCs, indicating that canonical NF-κB activity during differentiation is redundant for the development of functional APCs. However, both mo-DC and CD34-mDC functionality were reduced by NF-κB inhibition during activation. In conclusion, canonical NF-κB activity is essential for the development and function of mo-DCs as well as CD34-mDCs. Insight into the role of this pathway may help in understanding how pathogens and tumors escape immunity and aid in developing novel treatment strategies aiming to interfere with human immune responses.

Contracting the 'mus cells'--does down-sizing suit us for diving into the memory pool?

Maintenance of T-cell homeostasis is critical for normal functioning of the immune system. After thymocyte selection, T cells enter the peripheral lymphoid organs, where they are maintained as naive cells. Transient disruption of homeostasis occurs when naive T cells undergo antigen-driven expansion and acquire effector functions. Effector T cells then either undergo apoptosis (i.e. contraction at the population level) or survive to become memory cells. This apoptotic process is crucial: it resets T-cell homeostasis, promotes protective immunity, and limits autoimmunity. Although initial studies using in vitro models supported a role for death receptor signaling, more recent in vivo studies have implicated Bcl-2 family members as being critical for the culling of T-cell responses. While several Bcl-2 family members likely contribute to T-cell contraction, the pro-apoptotic molecule Bim and its anti-apoptotic antagonist Bcl-2 are essential regulators of the process. This review discusses the progress made in our understanding of the mechanisms underlying contraction of T-cell responses and how some cells avoid this cell death and become memory T cells.

Programmed cell death of dendritic cells in immune regulation

Programmed cell death is essential for the maintenance of lymphocyte homeostasis and immune tolerance. Dendritic cells (DCs), the most efficient antigen-presenting cells, represent a small cell population in the immune system. However, DCs play major roles in the regulation of both innate and adaptive immune responses. Programmed cell death in DCs is essential for regulating DC homeostasis and consequently, the scope of immune responses. Interestingly, different DC subsets show varied turnover rates in vivo. The conventional DCs are relatively short-lived in most lymphoid organs, while plasmacytoid DCs are long-lived cells. Mitochondrion-dependent programmed cell death plays an important role in regulating spontaneous DC turnover. Antigen-specific T cells are also capable of killing DCs, thereby providing a mechanism for negative feedback regulation of immune responses. It has been shown that a surplus of DCs due to defects in programmed cell death leads to overactivation of lymphocytes and the onset of autoimmunity. Studying programmed cell death in DCs will shed light on the roles for DC turnover in the regulation of the duration and magnitude of immune responses in vivo and in the maintenance of immune tolerance.

Human CD34-derived myeloid dendritic cell development requires intact phosphatidylinositol 3-kinase-protein kinase B-mammalian target of rapamycin signaling

Dendritic cells (DCs) are composed of different subsets that exhibit distinct functionality in the induction and regulation of immune responses. The myeloid DC subsets, including interstitial DCs and Langerhans cells (LCs), develop from CD34+ hematopoietic progenitors via direct DC precursors or monocytes. The molecular mechanisms regulating DC development are still largely unknown and mostly studied in mice. Phosphatidylinositol 3-kinase (PI3K) regulates multiple processes in myeloid cells. This study investigated the role of PI3K signaling in the development of human CD34-derived myeloid DCs. Pharmacologic inhibition of PI3K or one of its downstream targets mTOR reduced interstitial DC and LC numbers in vitro. Increased activity of this signaling module by introduction of constitutively active protein kinase B (PKB/c-Akt) increased the yields of human DC precursors in vitro as well as in transplanted beta2-microglobulin-/- NOD/SCID mice in vivo. Signaling inhibition during differentiation did not affect the acquisition of a DC phenotype, whereas proliferation and survival strongly depended on intact PI3K-PKB-mTOR signaling. Interestingly, however, this pathway became redundant for survival regulation upon terminal differentiation, which was associated with an altered expression of apoptosis regulating genes. Although dispensable for costimulatory molecule expression, the PI3K-PKB-mTOR signaling module was required for other important processes associated with DC function, including Ag uptake, LPS-induced cytokine secretion, CCR7 expression, and T cell stimulation. Thus, PI3K-PKB-mTOR signaling plays a crucial role in the development of functional CD34-derived myeloid DCs. These findings could be used as a strategy to manipulate DC subset distribution and function to regulate immunity.

Dendritic cells in systemic lupus erythematosus

Systemic lupus erythematosus (SLE) persists as a chronic inflammatory autoimmune disease and is characterized by the production of autoantibodies and immune complexes that affect multiple organs. The underlying mechanism that triggers and sustains disease are complex and involve certain susceptibility genes and environmental factors. There have been several immune mediators linked to SLE including cytokines and chemokines that have been reviewed elsewhere [ 1-3 ]. A number of articles have reviewed the role of B cells and T cells in SLE [ 4-10 ]. Here, we focus on the role of dendritic cells (DC) and innate immune factors that may regulate autoreactive B cells.