Autocrine activation of canonical BMP signaling regulates PD-L1 and PD-L2 expression in human dendritic cells

Regulation of Tumor Dendritic Cells by Programmed Cell Death 1 Pathways

The advent of immune checkpoint blockade therapy has revolutionized cancer treatments and is partly responsible for the significant decline in cancer-related mortality observed during the last decade. Immune checkpoint inhibitors, such as anti-programmed cell death 1 (PD-1)/programmed cell death ligand 1 (PD-L1), have demonstrated remarkable clinical successes in a subset of cancer patients. However, a considerable proportion of patients remain refractory to immune checkpoint blockade, prompting the exploration of mechanisms of treatment resistance. Whereas much emphasis has been placed on the role of PD-L1 and PD-1 in regulating the activity of tumor-infiltrating T cells, recent studies have now shown that this immunoregulatory axis also directly regulates myeloid cell activity in the tumor microenvironment including tumor-infiltrating dendritic cells. In this review, I discuss the most recent advances in the understanding of how PD-1, PD-L1, and programmed cell death ligand 2 regulate the function of tumor-infiltrating dendritic cells, emphasizing the need for further mechanistic studies that could facilitate the development of novel combination immunotherapies for improved cancer patient benefit.

Unique macrophage phenotypes activated by BMP signaling in breast cancer bone metastases

Metastatic breast cancer (mBC) tissue in bone was systematically profiled to define the composition of the tumor microenvironment. Gene expression identified a high myeloid signature of patients with improved survival outcomes. Bone metastases were profiled by spatial proteomics to examine myeloid populations within the stroma that correlated with macrophage functions. Single-cell spatial analysis uncovered macrophage activation in the stroma of mBC bone lesions. Matched BC patient samples of primary breast tumor and bone metastasis tissues were compared for gene expression in the bone, where bone morphogenetic protein 2 (BMP2) was most significantly upregulated. Immune cell changes from breast to bone demonstrated a loss of lymphoid cells but a consistent population of macrophages. BMP-activated macrophages were increased uniquely in bone. Bone marrow-derived macrophage activation coupled with BMP inhibition increased inflammatory responses. Using experimental mouse models of mBC bone metastasis and trained immunity, we found that BMP inhibition restricts progression of metastases early in the macrophage activation state but not after tumors were established in the bone. This study revealed unique myeloid BMP activation states that are distinctly integrated with bone metastases.

Microenvironmental and cell intrinsic factors governing human cDC2 differentiation and monocyte reprogramming

cDC2s occur abundantly in peripheral tissues and arise from circulating blood cDC2s. However, the factors governing cDC2 differentiation in tissues, especially under inflammatory conditions, remained poorly defined. We here found that psoriatic cDC2s express the efferocytosis receptor Axl and exhibit a bone morphogenetic protein (BMP) and p38MAPK signaling signature. BMP7, strongly expressed within the lesional psoriatic epidermis, cooperates with canonical TGF-β1 signaling for inducing Axl+cDC2s from blood cDC2s in vitro. Moreover, downstream induced p38MAPK promotes Axl+cDC2s at the expense of Axl+CD207+ Langerhans cell differentiation from blood cDC2s. BMP7 supplementation allowed to model cDC2 generation and their further differentiation into LCs from CD34+ hematopoietic progenitor cells in defined serum-free medium. Additionally, p38MAPK promoted the generation of another cDC2 subset lacking Axl but expressing the non-classical NFkB transcription factor RelB in vitro. Such RelB+cDC2s occurred predominantly at dermal sites in the inflamed skin. Finally, we found that cDC2s can be induced to acquire high levels of the monocyte lineage identity factor kruppel-like-factor-4 (KLF4) along with monocyte-derived DC and macrophage phenotypic characteristics in vitro. In conclusion, inflammatory and psoriatic epidermal signals instruct blood cDC2s to acquire phenotypic characteristics of several tissue-resident cell subsets.

The performance and perspectives of dendritic cell vaccines modified by immune checkpoint inhibitors or stimulants

Therapeutic dendritic cell (DC) vaccines stimulate the elimination of tumor cells by the immune system. However, while antigen-specific T cell responses induced by DC vaccines are commonly observed, the clinical response rate is relatively poor, necessitating vaccine optimization. There is evidence that the suppression of DC function by immune checkpoints hinders the anti-tumor immune responses mediated by DC vaccines, ultimately leading to the immune escape of the tumor cells. The use of immune checkpoint inhibitors (ICIs) and immune checkpoint activators (ICAs) has extended the immunotherapeutic range. It is known that both inhibitory and stimulatory checkpoint molecules are expressed by most DC subsets and can thus be used to manipulate the effectiveness of DC vaccines. Such manipulation has been investigated using strategies such as chemotherapy, agonistic or antagonistic antibodies, siRNA, shRNA, CRISPR-Cas9, soluble antibodies, lentiviruses, and adenoviruses to maximize the efficacy of DC vaccines. Thus, a deeper understanding of immune checkpoints may assist in the development of improved DC vaccines. Here, we review the actions of various ICIs or ICAs shown by preclinical studies, as well as their potential application in DC vaccines. New therapeutic interventional strategies for blocking and stimulating immune checkpoint molecules in DCs are also described in detail.

Regulation of the Immune System in Health and Disease by Members of the Bone Morphogenetic Protein Family

Bone morphogenetic proteins (BMPs) are potent signaling molecules initially described as osteopromoting proteins. BMPs represent one of the members of the larger TGFβ family and today are recognized for their important role in numerous processes. Among the wide array of functions recently attributed to them, BMPs were also described to be involved in the regulation of components of the innate and adaptive immune response. This review focuses on the signaling pathway of BMPs and highlights the effects of BMP signaling on the differentiation, activation, and function of the main cell types of the immune system.

Characterization and Manipulation of the Crosstalk Between Dendritic and Natural Killer Cells Within the Tumor Microenvironment

Cellular therapy has entered the daily clinical life with the approval of CAR T cell therapeutics and dendritic cell (DCs) vaccines in the US and the EU. In addition, numerous other adoptive cellular products, including natural killer (NK) cells, are currently evaluated in early phase I/ II clinical trials for the treatment of cancer patients. Despite these promising accomplishments, various challenges remain to be mastered in order to ensure sustained therapeutic success. These include the identification of strategies by which tumor cells escape the immune system or establish an immunosuppressive tumor microenvironment (TME). As part of the innate immune system, DCs and NK cells are both present within the TME of various tumor entities. While NK cells are well known for their intrinsic anti-tumor activity by their cytotoxicity capacities and the secretion of pro-inflammatory cytokines, the role of DCs within the TME is a double-edged sword as different DC subsets have been described with either tumor-promoting or -inhibiting characteristics. In this review, we will discuss recent findings on the interaction of DCs and NK cells under physiological conditions and within the TME. One focus is the crosstalk of various DC subsets with NK cells and their impact on the progression or inhibition of tumor growth. In addition, we will provide suggestions to overcome the immunosuppressive outcome of the interaction of DCs and NK cells within the TME.

Nuclear IL-33/SMAD signaling axis promotes cancer development in chronic inflammation

Interleukin (IL)-33 cytokine plays a critical role in allergic diseases and cancer. IL-33 also has a nuclear localization signal. However, the nuclear function of IL-33 and its impact on cancer is unknown. Here, we demonstrate that nuclear IL-33-mediated activation of SMAD signaling pathway in epithelial cells is essential for cancer development in chronic inflammation. Using RNA and ChIP sequencing, we found that nuclear IL-33 repressed the expression of an inhibitory SMAD, Smad6, by interacting with its transcription factor, RUNX2. IL-33 was highly expressed in the skin and pancreatic epithelial cells in chronic inflammation, leading to a markedly repressed Smad6 expression as well as dramatically upregulated p-SMAD2/3 and p-SMAD1/5 in the epithelial cells. Blocking TGF-β/SMAD signaling attenuated the IL-33-induced cell proliferation in vitro and inhibited IL-33-dependent epidermal hyperplasia and skin cancer development in vivo. IL-33 and SMAD signaling were upregulated in human skin cancer, pancreatitis, and pancreatitis-associated pancreatic cancer. Collectively, our findings reveal that nuclear IL-33/SMAD signaling is a cell-autonomous tumor-promoting axis in chronic inflammation, which can be targeted by small-molecule inhibitors for cancer treatment and prevention.

Emerging concepts in PD-1 checkpoint biology

The PD-1 pathway is a cornerstone in immune regulation. While the PD-1 pathway has received considerable attention for its role in contributing to the maintenance of T cell exhaustion in chronic infection and cancer, the PD-1 pathway plays diverse roles in regulating host immunity beyond T cell exhaustion. Here, we discuss emerging concepts in the PD-1 pathway, including (1) the impact of PD-1 inhibitors on diverse T cell differentiation states including effector and memory T cell development during acute infection, as well as T cell exhaustion during chronic infection and cancer, (2) the role of PD-1 in regulating Treg cells, NK cells, and ILCs, and (3) the functions of PD-L1/B7-1 and PD-L2/RGMb/neogenin interactions. We then discuss the emerging use of neoadjuvant PD-1 blockade in the treatment of early-stage cancers and how the timing of PD-1 blockade may improve clinical outcomes. The diverse binding partners of PD-1 and its associated ligands, broad expression patterns of the receptors and ligands, differential impact of PD-1 modulation on cells depending on location and state of differentiation, and timing of PD-1 blockade add additional layers of complexity to the PD-1 pathway, and are important considerations for improving the efficacy and safety of PD-1 pathway therapeutics.

Comprehensive assessment of PD-L1 and PD-L2 dysregulation in gastrointestinal cancers

Background: PD-L1 and PD-L2 are ligands of PD-1. Their overexpression has been reported in different cancers. However, the underlying mechanism of PD-L1 and PD-L2 dysregulation and their related signaling pathways are still unclear in gastrointestinal cancers. Materials & methods: The expression of PD-L1 and PD-L2 were studied in The Cancer Genome Atlas and Genotype-Tissue Expression databases. The gene and protein alteration of PD-L1 and PD-L2 were analyzed in cBioportal. The direct transcription factor regulating PD-L1/PD-L2 was determined with ChIP-seq data. The association of PD-L1/PD-L2 expression with clinicopathological parameters, survival, immune infiltration and tumor mutation burden were investigated with data from The Cancer Genome Atlas. Potential targets and pathways of PD-L1 and PD-L2 were determined by protein enrichment, WebGestalt and gene ontology. Results: Comprehensive analysis revealed that PD-L1 and PD-L2 were significantly upregulated in most types of gastrointestinal cancers and their expressions were positively correlated. SP1 was a key transcription factor regulating the expression of PD-L1. Conclusion: Higher PD-L1 or PD-L2 expression was significantly associated with poor overall survival, higher tumor mutation burden and more immune and stromal cell populations. Finally, HIF-1, ERBB and mTOR signaling pathways were most significantly affected by PD-L1 and PD-L2 dysregulation. Altogether, this study provided comprehensive analysis of the dysregulation of PD-L1 and PD-L2, its underlying mechanism and downstream pathways, which add to the knowledge of manipulating PD-L1/PD-L2 for cancer immunotherapy.

Bone morphogenetic protein signaling regulates skin inflammation via modulating dendritic cell function

BACKGROUND

Bone morphogenetic proteins (BMPs) are members of the TGF-β family that signal via the BMP receptor (BMPR) signaling cascade, distinct from canonical TGF-β signaling. BMP downstream signaling is strongly induced within epidermal keratinocytes in cutaneous psoriatic lesions, and BMP7 instructs monocytic cells to acquire characteristics of psoriasis-associated Langerhans dendritic cells (DCs). Regulatory T (Treg)-cell numbers strongly increase during psoriatic skin inflammation and were recently shown to limit psoriatic skin inflammation. However, the factors mediating Treg-cell accumulation in psoriatic skin currently remain unknown.

OBJECTIVE

We sought to investigate the role of BMP signaling in Treg-cell accumulation in psoriasis.

METHODS

The following methods were used: immunohistology of patients and healthy controls; ex vivo models of Treg-cell generation in the presence or absence of Langerhans cells; analysis of BMP versus canonical TGF-β signaling in DCs and Treg cells; and modeling of psoriatic skin inflammation in mice lacking the BMPR type 1a in CD11c⁺ cells.

RESULTS

We here demonstrated a positive correlation between Treg-cell numbers and epidermal BMP7 expression in cutaneous psoriatic lesions and show that unlike Treg cells from healthy skin, a portion of inflammation-associated Treg cells exhibit constitutive-active BMP signaling. We further found that BMPR signaling licenses inflammation-associated Langerhans cell/DC to gain an enhanced capacity to promote Treg cells via BMPR-mediated CD25 induction and that this effect is associated with reduced skin inflammation.

CONCLUSIONS

Psoriatic lesions are marked by constitutive high BMP7/BMPR signaling in keratinocytes, which instructs inflammatory DCs to gain enhanced Treg-cell-stimulatory activity. Locally secreted BMP7 can directly promote Treg-cell generation through the BMP signaling cascade.

Nivolumab increases pulmonary artery pressure in patients treated for non-small cell lung cancer

PURPOSE

The widespread use of Nivolumab results in an increasing number of side effects and adverse events. Herein, we evaluated the impact of Nivolumab on crude and normalized pulmonary artery diameter (PAD).

METHODS

We analyzed clinical, morphometric, pathological and radiological data of lung cancer patients treated by Nivolumab in an 18-month period. Blinded radiological evaluation was performed, by three observers measuring axial PAD and Aorta diameter (AoD) in secondarily matched pre- and post-Nivolumab CT-scans. Correlation between ΔPAD and clinicopathological data was investigated.

RESULTS

59 patients receiving Nivolumab for treatment of advanced lung carcinoma were identified. Pre-and post-Nivolumab comparison of CT-scan measures revealed that mean PAD was 26.3 ± 2.8 mm versus 28.0 ± 3.0 mm (p < 0.001), and mean PAD/AoD ratio was 0.82 ± 0.09 versus 0.87 ± 0.11 (p <  0.001), respectively. Median ΔPAD was 0.05 [0.01-0.122] was significantly higher in hypometabolic patients exhibiting low Rest Energy Expenditure (p = 0.03). Patients exhibiting ΔPAD > 1% had significantly lower serum albumin level (p = 0.03), and higher nutritional risk (p = 0.02), compared to others. Unlike Nivolumab therapy, there was no increase of PAD after chemotherapy in the same cohort of patients with available scans (n = 45, 25.9 ± 2.9 mm pre-chemotherapy versus 25.7 ± 2.4 mm post-chemotherapy, p = 0.51). Anti-PD-1 treatment was associated with immune-related adverse events in 11 (18.6%) cases including 2 cases of life-threatening acute pulmonary hypertension, both exhibiting post-treatment PAD/AoD ratio > 1.

CONCLUSION

Nivolumab is associated to PAD enlargement, a potential marker of pulmonary hypertension, sometimes leading to lethal adverse events. Careful CT-scan and echocardiographic evaluation of PAD should be part of the therapeutic work-up of patients receiving Nivolumab, especially those suffering cancer-associated malnutrition.

Extracellular BMP Antagonists, Multifaceted Orchestrators in the Tumor and Its Microenvironment

The bone morphogenetic proteins (BMPs), a subgroup of the transforming growth factor-β (TGF-β) superfamily, are involved in multiple biological processes such as embryonic development and maintenance of adult tissue homeostasis. The importance of a functional BMP pathway is underlined by various diseases, including cancer, which can arise as a consequence of dysregulated BMP signaling. Mutations in crucial elements of this signaling pathway, such as receptors, have been reported to disrupt BMP signaling. Next to that, aberrant expression of BMP antagonists could also contribute to abrogated signaling. In this review we set out to highlight how BMP antagonists affect not only the cancer cells, but also the other cells present in the microenvironment to influence cancer progression.

Acute Lymphoblastic Leukaemia Cells Impair Dendritic Cell and Macrophage Differentiation: Role of BMP4

Dendritic cells and macrophages are common components of the tumour immune microenvironment and can contribute to immune suppression in both solid and haematological cancers. The Bone Morphogenetic Protein (BMP) pathway has been reported to be involved in cancer, and more recently in leukaemia development and progression. In the present study, we analyse whether acute lymphoblastic leukaemia (ALL) cells can affect the differentiation of dendritic cells and macrophages and the involvement of BMP pathway in the process. We show that ALL cells produce BMP4 and that conditioned media from ALL cells promote the generation of dendritic cells with immunosuppressive features and skew M1-like macrophage polarization towards a less pro-inflammatory phenotype. Likewise, BMP4 overexpression in ALL cells potentiates their ability to induce immunosuppressive dendritic cells and favours the generation of M2-like macrophages with pro-tumoral features. These results suggest that BMP4 is in part responsible for the alterations in dendritic cell and macrophage differentiation produced by ALL cells.

P2RY1/ALK3-Expressing Cells within the Adult Human Exocrine Pancreas Are BMP-7 Expandable and Exhibit Progenitor-like Characteristics

Treatment of human pancreatic non-endocrine tissue with Bone Morphogenetic Protein 7 (BMP-7) leads to the formation of glucose-responsive β-like cells. Here, we show that BMP-7 acts on extrainsular cells expressing PDX1 and the BMP receptor activin-like kinase 3 (ALK3/BMPR1A). In vitro lineage tracing indicates that ALK3⁺ cell populations are multipotent. PDX1⁺/ALK3⁺ cells are absent from islets but prominently represented in the major pancreatic ducts and pancreatic duct glands. We identified the purinergic receptor P2Y1 (P2RY1) as a surrogate surface marker for PDX1. Sorted P2RY1⁺/ALK3^bright+ cells form BMP-7-expandable colonies characterized by NKX6.1 and PDX1 expression. Unlike the negative fraction controls, these colonies can be differentiated into multiple pancreatic lineages upon BMP-7 withdrawal. RNA-seq further corroborates the progenitor-like nature of P2RY1⁺/ALK3^bright+ cells and their multilineage differentiation potential. Our studies confirm the existence of progenitor cells in the adult human pancreas and suggest a specific anatomical location within the ductal and glandular networks.

TGFβ/BMP immune signaling affects abundance and function of C. elegans gut commensals

The gut microbiota contributes to host health and fitness, and imbalances in its composition are associated with pathology. However, what shapes microbiota composition is not clear, in particular the role of genetic factors. Previous work in Caenorhabditis elegans defined a characteristic worm gut microbiota significantly influenced by host genetics. The current work explores the role of central regulators of host immunity and stress resistance, employing qPCR and CFU counts to measure abundance of core microbiota taxa in mutants raised on synthetic communities of previously-isolated worm gut commensals. This revealed a bloom, specifically of Enterobacter species, in immune-compromised TGFβ/BMP mutants. Imaging of fluorescently labeled Enterobacter showed that TGFβ/BMP-exerted control operated primarily in the anterior gut and depended on multi-tissue contributions. Enterobacter commensals are common in the worm gut, contributing to infection resistance. However, disruption of TGFβ/BMP signaling turned a normally beneficial Enterobacter commensal to pathogenic. These results demonstrate specificity in gene-microbe interactions underlying gut microbial homeostasis and highlight the pathogenic potential of their disruption.

The ferroptosis inducer erastin promotes proliferation and differentiation in human peripheral blood mononuclear cells

Peripheral blood mononuclear cells (PBMCs) contain multipotent progenitor cell populations and possess the potential to differentiate into various types of immune cells under both physiological and pathological conditions. Ferroptosis is a type of oxidative stress-associated cell death that is mainly mediated by lipid peroxidation. However, the function of ferroptosis in cell differentiation remains unknown. Here, we showed that the ferroptosis inducer erastin did not cause cell death in human PBMCs. In contrast, erastin-induced lipid peroxidation promoted human PBMC proliferation and differentiation into B cells and natural killer cells through inhibition of bone morphogenetic protein family expression. These findings uncover a new immune modulation function of erastin in promoting PBMC proliferation and differentiation.

Novel bone morphogenetic protein receptor inhibitor JL5 suppresses tumor cell survival signaling and induces regression of human lung cancer

BMP receptor inhibitors induce death of cancer cells through the downregulation of antiapoptotic proteins XIAP, pTAK1, and Id1-Id3. However, the current most potent BMP receptor inhibitor, DMH2, does not downregulate BMP signaling in vivo because of metabolic instability and poor pharmacokinetics. Here we identified the site of metabolic instability of DMH2 and designed a novel BMP receptor inhibitor, JL5. We show that JL5 has a greater volume of distribution and suppresses the expression of Id1 and pTak1 in tumor xenografts. Moreover, we demonstrate JL5-induced tumor cell death and tumor regression in xenograft mouse models without immune cells and humanized with adoptively transferred human immune cells. In humanized mice, JL5 additionally induces the infiltration of immune cells within the tumor microenvironment. Our studies show that the BMP signaling pathway is targetable in vivo and BMP receptor inhibitors can be developed as a therapeutic to treat cancer patients.

Dendritic Cells and Programmed Death-1 Blockade: A Joint Venture to Combat Cancer

Two decades of clinical cancer research with dendritic cell (DC)-based vaccination have proved that this type of personalized medicine is safe and has the capacity to improve survival, but monotherapy is unlikely to cure the cancer. Designed to empower the patient’s antitumor immunity, huge research efforts are set to improve the efficacy of next-generation DC vaccines and to find synergistic combinations with existing cancer therapies. Immune checkpoint approaches, aiming to breach immune suppression and evasion to reinforce antitumor immunity, have been a revelation in the immunotherapy field. Early success of therapeutic antibodies blocking the programmed death-1 (PD-1) pathway has sparked the development of novel inhibitors and combination therapies. Hence, merging immunoregulatory tumor-specific DC strategies with PD-1-targeted approaches is a promising path to explore. In this review, we focus on the role of PD-1-signaling in DC-mediated antitumor immunity. In the quest of exploiting the full potential of DC therapy, different strategies to leverage DC immunopotency by impeding PD-1-mediated immune regulation are discussed, including the most advanced research on targeted therapeutic antibodies, lessons learned from chemotherapy-induced immune activation, and more recent developments with soluble molecules and gene-silencing techniques. An overview of DC/PD-1 immunotherapy combinations that are currently under preclinical and clinical investigation substantiates the clinical potential of such combination strategies.

TGF-β, Bone Morphogenetic Protein, and Activin Signaling and the Tumor Microenvironment

The cellular and noncellular components surrounding the tumor cells influence many aspects of tumor progression. Transforming growth factor β (TGF-β), bone morphogenetic proteins (BMPs), and activins have been shown to regulate the phenotype and functions of the microenvironment and are attractive targets to attenuate protumorigenic microenvironmental changes. Given the pleiotropic nature of the cytokines involved, a full understanding of their effects on numerous cell types in many contexts is necessary for proper clinical intervention. In this review, we will explore the various effects of TGF-β, BMP, and activin signaling on stromal phenotypes known to associate with cancer progression. We will summarize these findings in the context of their tumor suppressive or promoting effects, as well as the molecular changes that these cytokines induce to influence stromal phenotypes.

Characterization of human fibroblastic reticular cells as potential immunotherapeutic tools

Fibroblastic reticular cells (FRCs) are essential players during adaptive immune responses not only as a structural support for the encounter of antigen-presenting cells and naive T lymphocytes but also as a source of modulatory signals. However, little is known about this cell population in humans. To address the phenotypical and functional analysis of human FRCs here we established splenic (SP) and mesenteric lymph node (LN) CD45^-CD31^-CD90⁺podoplanin⁺ myofibroblastic cell cultures. They shared the phenotypical characteristics distinctive of FRCs, including the expression of immunomodulatory factors and peripheral tissue antigens. Nevertheless, human FRCs also showed particular features, some differing from mouse FRCs, like the lack of nitric oxide synthase (NOS2) expression after interferon (IFN)γstimulation. Interestingly, SP-FRCs expressed higher levels of interleukin (IL)-6, BMP4, CCL2, CXCL12 and Notch molecules, and strongly adapted their functional profile to lipopolysaccharide (LPS), polyinosinic:polycytidylic acid (Poly I:C) and IFNγ stimulation. In contrast, we found higher expression of transforming growth factor (TGF)β and Activin A in LN-FRCs that barely responded via Toll-Like Receptor (TLR)3 and constitutively expressed retinaldehyde dehydrogenase 1 enzyme, absent in SP-FRCs. This study reveals human FRCs can be valuable models to increase our knowledge about the physiology of human secondary lymphoid organs in health and disease and to explore the therapeutic options of FRCs.

The BMP Pathway Participates in Human Naive CD4+ T Cell Activation and Homeostasis

Bone Morphogenetic Proteins (BMPs) form a group of secreted factors that belongs to the TGF-β superfamily. Among different roles in a number of immune cell types, BMPs are known to regulate T cell development within the thymus, although the role of BMP signaling in human mature T cells remains elusive. In this study, we demonstrate that canonical BMP signaling is necessary during two critical events that regulate the size and function of human naive CD4⁺ T cell population: activation and homeostasis. Upon stimulation via TCR, naive CD4⁺ T cells upregulate the expression of BMP ligands triggering canonical BMP signaling in CD25⁺ cells. Blockade of BMP signaling severely impairs CD4⁺ T cell proliferation after activation mainly through regulation of IL-2, since the addition of this cytokine recuperates normal T cell expansion after inhibition of BMP signaling. Similarly, activation of canonical BMP pathway is required for both the maintenance of cell survival and the homeostatic proliferation induced by IL-7, a key factor for T cell homeostasis. Moreover, upregulation of two critical receptors for T cell homeostasis, CXCR4 and CCR9, triggered by IL-7 is also abrogated in the absence of BMP signaling. Collectively, we describe important roles of the canonical BMP signaling in human naive CD4⁺ T cell activation and homeostasis that could be valuable for clinical application.

The TGF-β superfamily in dendritic cell biology

The TGF-β superfamily consists of a large group of pleiotropic cytokines that are involved in the regulation of many developmental, physiological and pathological processes. Dendritic cells are antigen-presenting cells that play a key role in innate and adaptive immune responses. Dendritic cells have a complex relationship with the TGF-β cytokine superfamily being both source and targets for many of these cytokines. Some TGF-β family members are expressed by dendritic cells and modulate immune responses, for instance through the induction of T cell polarization. Others play a crucial role in the development and function of the different dendritic cell subsets. This review summarizes the current knowledge on the role of TGF-β family cytokines in dendritic cell biology, focusing on TGF-β as well as on other, less characterized, members of these important immune mediators.