An optimized cocktail of small molecule inhibitors promotes the maturation of dendritic cells in GM-CSF mouse bone marrow culture.

Dendritic cells (DCs) are the most potent antigen-presenting cells, playing an essential role in the pathogen and tumor recognition, and anti-tumor immunity, and linking both the innate and adaptive immunity. The monocyte-derived DCs generated by ex vivo culture, have been used for cancer immunotherapy to eliminate tumor; however, the clinical efficacies are not sufficient, and further improvement is essential. In this study, we established a method to generate DCs using small molecule compounds for cancer immunotherapy. We observed an increase in the percentage of CD11c+I-A/I-Ehigh cells, representing DCs, by adding four small molecular inhibitors: Y27632, PD0325901, PD173074, and PD98059 (abbreviated as YPPP), in mouse bone marrow (BM) culture with granulocyte-macrophage colony stimulating factor (GM-CSF). BM-derived DCs cultured with YPPP (YPPP-DCs) showed high responsiveness to lipopolysaccharide stimulation, resulting in increased interleukin (IL) -12 production and enhanced proliferation activity when co-cultured with naïve T cells compared with the vehicle control. RNA-seq analysis revealed an upregulation of peroxisome proliferator - activated receptor (PPAR) γ associated genes increased in YPPP-DCs. In tumor models treated with anti-programmed death (PD) -1 therapies, mice injected intratumorally with YPPP-DCs as a DCs vaccine exhibited reduced tumor growth and increased survival. These findings suggested that our method would be useful for the induction of DCs that efficiently activate effector T cells for cancer immunotherapy.

PCBP1 acts as a regulator of CCL2 expression in macrophages to induce recruitment of monocyte-derived macrophages into the inflamed colon.

Intestinal macrophages with functional plasticity play essential roles in gut immune responses by increasing chemokines and cytokines, thereby contributing to the pathogenesis of inflammatory bowel disease (IBD). Poly(rC)-binding protein 1 (PCBP1), which is widely expressed in immune cells, binds to nucleic acids in mRNA processing, stabilization, translation and transcription. However, little is known about the influence of PCBP1 on macrophages and its specific mechanism in inflamed intestines. In this study, conditional depletion of Pcbp1 in macrophages protected mice from progression of dextran sulfate sodium induced colitis and resulted in significant alleviation of colitis. Pcbp1 deficiency markedly decreased C-C motif chemokine ligand 2 (CCL2) production by colonic CX3C motif chemokine receptor 1+ (CX3CR1+) macrophages and reduced accumulation of pro-inflammatory macrophages and production of pro-inflammatory cytokines, such as IL-6 and TNF-α, in the inflamed colon. RNA-immunoprecipitation analysis indicated that PCBP1 might interact with Ccl2 mRNA and regulate its expression in macrophages. PCBP1 expression in inflamed intestines also correlated significantly with IBD severity in patients, suggesting a critical involvement of PCBP1 in intestinal inflammation. We anticipate that our findings will facilitate the development of novel therapeutic approaches for IBD by targeting the specific function of immune cells in the local microenvironment, thereby helping to reduce adverse effects.

A Novel CD135+ Subset of Mouse Monocytes with a Distinct Differentiation Pathway and Antigen-Presenting Properties.

The mononuclear phagocyte system (MPS), composed of monocytes/macrophages and dendritic cells (DCs), plays a critical role at the interface of the innate and adaptive immune systems. However, the simplicity of MPS has been challenged recently by discoveries of novel cellular components. In the current study, we identified the CD135+ subset of monocytes as a novel class of APCs in mice. CD135+ monocytes were readily found in the bone marrow, spleen, and peripheral blood at steady state, and they expressed markers specific to DCs, including MHC class II and CD209a, along with markers for monocytes/macrophages. In addition, this subset phagocytosed bacteria and activated naive T lymphocytes, fulfilling the criteria for APCs. CD135+ monocytes were derived directly from macrophage DC progenitors, not from common monocyte progenitors or other monocytes, suggesting that these are distinct from conventional monocytes. These findings facilitate our understanding of the MPS network that regulates immune responses for host defense.

Commitment to dendritic cells and monocytes.

Dendritic cells (DCs) and monocytes are widely conserved immune cells in vertebrates that arise from hematopoietic stem cells via intermediate progenitors. The progenitors that strictly give rise to DCs or monocytes have been recently identified both in humans and in mice, thereby revealing their differentiation pathways. Advances in analysis technologies have further deepened our understanding of the development of DCs and monocytes from progenitor population-based to individual progenitor cell-based commitment. Since DC-committed progenitors, common DC progenitors (CDPs) and precursor conventional DCs (pre-cDCs) do not differentiate into monocytes, DCs are a distinct lineage from monocytes, although monocytes can acquire DC functions upon activation at tissues where they arrive.

Detailed structure of mouse interferon α2 and its interaction with Sortilin.

Interferon α (IFNα) is a type I interferon, an essential cytokine employed by the immune system to fight viruses. Although a number of the structures of type I interferons have been reported, most of the known structures of IFNα are in complex with its receptors. There are only two examples of structures of free IFNα: one is a dimeric X-ray structure without side-chain information; and another is an NMR structure of human IFNα. Although we have shown that Sortilin is involved in the secretion of IFNα, the details of the molecular interaction and the secretion mechanism remain unclear. Recently, we solved the X-ray structure of mouse Sortilin, but the structure of mouse IFNα remained unknown. In this study, we determined the crystal structure of mouse IFNα2 at 2.1 Å resolution and investigated its interaction with Sortilin. Docking simulations suggested that Arg22 of mouse IFNα2 is important for the interaction with mouse Sortilin. Mutation of Arg22 to alanine facilitated IFNα2 secretion, as determined by flow cytometry, highlighting the contribution of this residue to the interaction with Sortilin. These results suggest an important role for Arg22 in mouse IFNα for Sortilin-mediated IFNα trafficking.

PCBP2 post-transcriptionally regulates sortilin expression by binding to a C-rich element in its 3' UTR.

Post-transcriptional regulation of cytokine production is crucial to ensure appropriate immune responses. We previously demonstrated that poly-rC-binding protein-1 (PCBP1) can act as a trans-acting factor to stabilize transcripts encoding sortilin, which mediates cytokine trafficking. Here, we report that PCBP2, which strongly resembles PCBP1, can stabilize sortilin transcripts in macrophages using the same mechanism employed by PCBP1. PCBP2 recognized the C-rich element in the 3' UTR of sortilin mRNA, and PCBP2 knockdown decreased sortilin transcripts, indicating that PCBP2 stabilizes sortilin mRNA by binding to its 3' UTR. Zn2+ reversibly inhibited the nucleotide binding ability of PCBP2 in vitro. These findings suggest that both PCBP2 and PCBP1 may control the stability of sortilin transcripts by sensing intracellular Zn2+ levels in immune cells.

Calcium Pyrophosphate Dihydrate Crystals Increase the Granulocyte/Monocyte Progenitor (GMP) and Enhance Granulocyte and Monocyte Differentiation In Vivo.

Calcium pyrophosphate dihydrate (CPPD) crystals are formed locally within the joints, leading to pseudogout. Although the mobilization of local granulocytes can be observed in joints where pseudogout has manifested, the mechanism of this activity remains poorly understood. In this study, CPPD crystals were administered to mice, and the dynamics of splenic and peripheral blood myeloid cells were analyzed. As a result, levels of both granulocytes and monocytes were found to increase following CPPD crystal administration in a concentration-dependent manner, with a concomitant decrease in lymphocytes in the peripheral blood. In contrast, the levels of other cells, such as dendritic cell subsets, T-cells, and B-cells, remained unchanged in the spleen, following CPPD crystal administration. Furthermore, an increase in granulocytes/monocyte progenitors (GMPs) and a decrease in megakaryocyte/erythrocyte progenitors (MEPs) were also observed in the bone marrow. In addition, CPPD administration induced production of IL-1ß, which acts on hematopoietic stem cells and hematopoietic progenitors and promotes myeloid cell differentiation and expansion. These results suggest that CPPD crystals act as a "danger signal" to induce IL-1ß production, resulting in changes in course of hematopoietic progenitor cell differentiation and in increased granulocyte/monocyte levels, and contributing to the development of gout.

Crystal structure of the ligand-free form of the Vps10 ectodomain of dimerized Sortilin at acidic pH.

Sortilin is a multifunctional sorting receptor involved in cytokine production in immune cells. To understand the mechanism of Sortilin-mediated cytokine trafficking, we determined the 2.45-Å structure of the dimerized Sortilin ectodomain (sSortilin or the Vps10-domain) crystallized at acidic pH. Substantial conformational changes upon dimerization lead to the intermolecular hydrophobic interaction between the conserved E455 and F137. Analysis of the electrostatic surface and size-exclusion chromatography revealed that sSortilin dimerization occurs due to an increase in hydrophobic interactions at the neutral dimer interface at acidic pH. The N682-attached N-glycan in the vicinity of the dimer interface implies its involvement in the dimerization. The disruption of Sortilin dimerization by mutations impairs efficient interferon-alpha secretion from cells. These results suggest the functional importance of Sortilin dimerization in cytokine trafficking.

Flexible fate commitment of E2-2high common DC progenitors implies tuning in tissue microenvironments.

The basic helix-loop-helix transcription factor E2-2 is essential for the development of plasmacytoid dendritic cells (pDCs) but not conventional DCs (cDCs). Here, we generated E2-2 reporter mice and demonstrated that an E2-2high fraction among common DC progenitors, which are a major source of pDCs and cDCs in the steady state, strictly gave rise to pDCs in the presence of Flt3 (Fms-like tyrosine kinase receptor-3) ligand ex vivo or in the secondary lymphoid organs when transferred in vivo. However, in the small intestine, some of these E2-2high progenitors differentiated into cDCs that produced retinoic acid. This transdifferentiation was driven by signaling via the common ß receptor, a receptor for the cytokines IL-3, IL-5 and GM-CSF, which are abundant in the gut. In the presence of GM-CSF and Flt3 ligand, E2-2high-progenitor-derived cDCs consistently induced Foxp3+ Treg cells ex vivo. Our findings reveal the commitment and flexibility of E2-2high progenitor differentiation and imply that pertinent tuning machinery is present in the gut microenvironment.

Mast cell hyperactivity underpins the development of oxygen-induced retinopathy.

Mast cells are classically thought to play an important role in protection against helminth infections and in the induction of allergic diseases; however, recent studies indicate that these cells also contribute to neovascularization, which is critical for tissue remodeling, chronic inflammation, and carcinogenesis. Here, we demonstrate that mast cells are essential for sprouting angiogenesis in a murine model of oxygen-induced retinopathy (OIR). Although mouse strains lacking mast cells did not exhibit retinal neovascularization following hypoxia, these mice developed OIR following infusion of mast cells or after injection of mast cell tryptase (MCT). Relative hypoxia stimulated mast cell degranulation via transient receptor potential ankyrin 1. Subsequent surges in MCT stimulated retinal endothelial cells to produce monocyte chemotactic protein-1 (MCP1) and angiogenic factors, leading to sprouting angiogenesis. Mast cell stabilizers as well as specific tryptase and MCP1 inhibitors prevented the development of OIR in WT mice. Preterm infants with early retinopathy of prematurity had markedly higher plasma MCT levels than age-matched infants without disease, suggesting mast cells contribute to human disease. Together, these results suggest therapies that suppress mast cell activity should be further explored as a potential option for preventing eye diseases and subsequent blindness induced by neovascularization.

Intrinsic Autophagy Is Required for the Maintenance of Intestinal Stem Cells and for Irradiation-Induced Intestinal Regeneration.

Autophagy is a lysosomal degradation pathway with important roles in physiological homeostasis and disease. However, the role of autophagy in intestinal stem cells (ISCs) is unclear. Here, we show that intrinsic autophagy in ISCs is important for ISC homeostasis. Mice lacking autophagy protein 5 (ATG5) in intestinal epithelial cells (iECs) (Villin-Cre: Atg5fl/fl, hereafter Atg5ΔIEC mice) or in all iECs except Paneth cells (Ah-Cre: Atg5fl/fl mice) had significantly fewer ISCs than did control mice and showed impaired ISC-dependent intestinal recovery after irradiation. Crypt ISCs from Atg5ΔIEC mice had significantly higher reactive oxygen species (ROS) levels than did those from control mice. A ROS-inducing reagent decreased the ISC number and impaired ISC regenerative capacity ex vivo, and treating Atg5ΔIEC mice with an antioxidant rescued their defects. Our results show that intrinsic autophagy supports ISC maintenance by reducing excessive ROS. Optimizing autophagy may lead to autophagy-based therapies for intestinal injuries.

Identification of a Human Clonogenic Progenitor with Strict Monocyte Differentiation Potential: A Counterpart of Mouse cMoPs.

Monocytes give rise to macrophages and dendritic cells (DCs) under steady-state and inflammatory conditions, thereby contributing to host defense and tissue pathology. A common monocyte progenitor (cMoP) that is strictly committed to the monocyte lineage has been recently identified in mice. Here, we identified human cMoPs as a CLEC12AhiCD64hi subpopulation of conventional granulocyte-monocyte progenitors (cGMPs) in umbilical cord blood and in bone marrow. Human cMoPs gave rise to monocyte subsets without showing any potential for differentiating into myeloid or lymphoid cells. Within the cGMP population, we also identified revised GMPs that completely lacked DC and lymphoid potential. Collectively, our findings expand and revise the current understanding of human myeloid cell differentiation pathways.

Isolation of Dendritic Cell Progenitor and Bone Marrow Progenitor Cells from Mouse.

Dendritic cells (DCs) comprise two major subsets, conventional DC (cDC) and plasmacytoid DC (pDC) in the steady-state lymphoid organ. These cells have a short half-life and therefore, require continuous generation from hematopoietic stem cells and progenitor cells. Recently, we identified DC-restricted progenitors called common DC progenitors (CDPs) in the bone marrow of mouse. The CDPs can be isolated from mouse bone marrow based on the hematopoietic cytokine receptors, such as Flt3 (Fms-related tyrosine kinase 3) (CD135), c-kit (CD117), M-CSF (macrophage colony-stimulating factor) receptor (CD115), and IL-7 (interleukin-7) receptor-α (CD127). The CDPs comprise of two progenitors, CD115(+) CDPs and CD115(-) CDPs, and give rise to only DC subsets in both in vitro and in vivo. The former CDPs are the main source of cDC, while the later CDPs are the main source of pDC in vivo. Here, we provide a protocol for the isolation of dendritic cell progenitor and bone marrow progenitor cells from mouse.

Cytosine-Phosphorothionate-Guanine Oligodeoxynucleotides Exacerbates Hemophagocytosis by Inducing Tumor Necrosis Factor-Alpha Production in Mice after Bone Marrow Transplantation.

Hemophagocytic syndrome (HPS) is frequently associated with hematopoietic stem cell transplantation and is treated with some benefit derived from TNF-α inhibitors. However, the mechanisms of how HPS occurs and how a TNF-α inhibitor exerts some benefit to HPS management have remained unclear. We evaluated the effect of toll-like receptor (TLR) ligands, especially focusing on cytosine-phosphorothionate-guanine oligodeoxynucleotide (CpG), a TLR9 ligand, on HPS in mice that underwent transplantation with syngeneic or allogeneic bone marrow (BM) cells (Syn-BMT, Allo-BMT), or with allogeneic BM cells plus splenocytes to promote graft-versus-host disease (GVHD mice). Hemophagocytosis was a common feature early after all BMT, but it subsided in Syn-BMT and Allo-BMT mice. In GVHD mice, however, hemophagocytosis persisted and was accompanied by upregulated production of IFN-γ but not TNF-α, and it was suppressed by blockade of IFN-γ but not TNF-α. A single injection of the TLR9 ligand CpG promoted HPS in all BMT mice and was lethal in GVHD mice, accompanied by greatly upregulated production of TNF-α, IL-6, and IFN-γ. Blocking of TNF-α, but not IL-6 or IFN-γ, suppressed CpG-induced HPS in all BMT mice and rescued GVHD mice from CpG-induced mortality. Thus, TLR9 signaling mediates TNF-α-driven HPS in BMT mice and is effectively treated through TNF-α inhibition.

Bipotent or Oligopotent? A macrophage and DC progenitor revisited.

Macrophage and dendritic cell (DC) progenitors (MDPs) produce macrophages and DCs but not other hematopoietic lineages. In this issue of Immunity, Sathe et al. (2014) show that isolated MDP populations hardly contain such bipotent progenitors at clonal levels, arguing against the existence of MDPs.

Dendritic cells, monocytes and macrophages: a unified nomenclature based on ontogeny.

The mononuclear phagocyte system (MPS) has historically been categorized into monocytes, dendritic cells and macrophages on the basis of functional and phenotypical characteristics. However, considering that these characteristics are often overlapping, the distinction between and classification of these cell types has been challenging. In this Opinion article, we propose a unified nomenclature for the MPS. We suggest that these cells can be classified primarily by their ontogeny and secondarily by their location, function and phenotype. We believe that this system permits a more robust classification during both steady-state and inflammatory conditions, with the benefit of spanning different tissues and across species.

Monocyte-derived dendritic cells perform hemophagocytosis to fine-tune excessive immune responses.

Because immune responses simultaneously defend and injure the host, the immune system must be finely regulated to ensure the host's survival. Here, we have shown that when injected with high Toll-like receptor ligand doses or infected with lymphocytic choriomeningitis virus (LCMV) clone 13, which has a high viral turnover, inflammatory monocyte-derived dendritic cells (Mo-DCs) engulfed apoptotic erythroid cells. In this process, called hemophagocytosis, phosphatidylserine (PS) served as an "eat-me" signal. Type I interferons were necessary for both PS exposure on erythroid cells and the expression of PS receptors in the Mo-DCs. Importantly, hemophagocytosis was required for interleukin-10 (IL-10) production from Mo-DCs. Blocking hemophagocytosis or Mo-DC-derived IL-10 significantly increased cytotoxic T cell lymphocyte activity, tissue damage, and mortality in virus-infected hosts, suggesting that hemophagocytosis moderates immune responses to ensure the host's survival in vivo. This sheds light on the physiological relevance of hemophagocytosis in severe inflammatory and infectious diseases.

A clonogenic progenitor with prominent plasmacytoid dendritic cell developmental potential.

Macrophage and dendritic cell (DC) progenitors (MDPs) and common DC progenitors (CDPs) are bone marrow (BM) progenitors with DC differentiation potential. However, both MDPs and CDPs give rise to large numbers of conventional DCs (cDCs) and few plasmacytoid DCs (pDCs), implying that more dedicated pDC progenitors remain to be identified. Here we have described DC progenitors with a prominent pDC differentiation potential. Although both MDPs and CDPs express the macrophage colony stimulating factor (M-CSF) receptor (M-CSFR), the progenitors were confined to a M-CSFR(-) fraction, identified as Lin(-)c-Kit(int/lo)Flt3(+)M-CSFR(-), and expressed high amounts of E2-2 (also known as Tcf4) an essential transcription factor for pDC development. Importantly, they appeared to be directly derived from either CDPs or lymphoid-primed multipotent progenitors (LMPPs). Collectively, our findings provide insight into DC differentiation pathways and may lead to progenitor-based therapeutic applications for infection and autoimmune disease.

CpG-ODN 2006 and human parvovirus B19 genome consensus sequences selectively inhibit growth and development of erythroid progenitor cells.

Recent studies have shown that anemia is commonly observed after exposure to pathogens or pathogen-derived products, which are recognized via Toll-like receptor 9 (TLR9). In the current study, we demonstrate that CpG oligodeoxynucleotide-2006, a TLR9 ligand with phosphodiester (PO; 2006-PO) but not with the phosphorothioate backbone, selectively inhibits the erythroid growth derived from human CD34(+) cells. The 2006-PO was internalized by the erythroid progenitors within 30 minutes; however, expression of TLR9 mRNA was not detected in these cells. The 2006-PO directly inhibited burst-forming unit-erythroid growth, resulted in the accumulation of cells in S and G(2)/M phases, and increased cell size and frequency of apoptotic cells. These features were similar to those observed in erythroid progenitors infected with human parvovirus B19 that causes pure red cell aplasia. The consensus sequence of 2006-PO was defined as 5'-GTTTTGT-3', which was located in the P6-promoter region of B19 and inhibited erythroid growth in a sequence-specific manner and down-regulated expression of erythropoietin receptor (EPOR) mRNA and EPOR. B19 genome extracted from serum also inhibited erythroid growth and down-regulated expression of EPOR on glycophorin A(+) cells. These results provide a possible insight into our understanding of the mechanisms of human parvovirus B19-mediated inhibition of erythropoiesis.

Isolation of common dendritic cell progenitors (CDP) from mouse bone marrow.

In the steady-state lymphoid organ, dendritic cells (DCs) are classified into two major subsets, plasmacytoid DC (pDC) and conventional DC (cDC). A standing question was whether a common progenitor for plasmacytoid and conventional dendritic cells exists during the sequential differentiation from hematopoietic stem cells to dendritic cells. We have recently identified such a common clonogenic plasmacytoid and dendritic cell progenitor (CDP) from mouse bone marrow using antibodies for c-kit, Flt3, and M-CSFR. CDPs generated almost exclusively pDC and cDC in vitro and upon transfer in irradiated and steady-state mice in vivo. Single-cell analysis revealed the existence of clonal progenitors giving rise to both pDC and cDC within the CDP population. Thus, these results prove the existence of a common developmental pathway for at least some pDCs and cDCs in lymphoid organs in vivo.