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.

Origin of monocytes and macrophages in a committed progenitor.

Monocytes, macrophages and dendritic cells (DCs) are developmentally related regulators of the immune system that share the monocyte-macrophage DC progenitor (MDP) as a common precursor. Unlike differentiation into DCs, the distal pathways for differentiation into monocytes and monocyte-derived macrophages are not fully elucidated. We have now demonstrated the existence of a clonogenic, monocyte- and macrophage-restricted progenitor cell derived from the MDP. This progenitor was a Ly6C(+) proliferating cell present in the bone marrow and spleen that generated the major monocyte subsets and macrophages, but not DCs or neutrophils. By in-depth quantitative proteomics, we characterized changes in the proteome during monocyte differentiation, which provided insight into the molecular principles of developing monocytes, such as their functional maturation. Thus, we found that monocytes and macrophages were renewed independently of DCs from a committed progenitor.

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.

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

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

Inflammatory macrophage derived TNFα downregulates estrogen receptor α via FOXO3a inactivation in human breast cancer cells.

Patients with estrogen receptor α positive (ERα+) breast cancer can respond to endocrine therapy, but treatment resistance is common and associated with downregulation of ERα expression in the dormant residual cells. Here we show, using long-term NSG xenograft models of human breast cancer and primary human monocytes, in vitro primary cell cultures and tumors from breast cancer patients, that macrophage derived tumor necrosis factor alpha (TNFα) downregulates ERα in breast cancer cells via inactivation of the transcription factor Forkhead box O transcription factor 3a (FOXO3a). Moreover, presence of tumor associated macrophages in the primary tumor of breast cancer patients, was associated with ERα negativity, and with worse prognosis in patients with ERα+ tumors. We propose that pro-inflammatory macrophages, despite being tumoricidal, may have direct effects on tumor progression and endocrine resistance in breast cancer patients. Our findings suggest that TNFα antagonists should be evaluated for treatment of ERα+ breast cancer.

Different monocyte phenotypes result in proresolving macrophages in conjugated linoleic acid-induced attenuated progression and regression of atherosclerosis.

Monocytes/macrophages drive progression and regression of atherosclerosis. Conjugated linoleic acid (CLA), an anti-inflammatory lipid, mediates atheroprotective effects. We investigated how CLA alters monocyte/macrophage phenotype during attenuated progression and regression of atherosclerosis. Apolipoprotein E knockout (ApoE-/-) mice were fed a high-fat (60%) high-cholesterol (1%) diet (HFHCD) for 2 wk, followed by 6-wk 1% CLA 80:20 supplementation to investigate disease progression. Simultaneously, ApoE-/- mice were fed a 12-wk HFHCD with/without CLA for the final 4 wk to investigate regression. Aortic lesions were quantified by en face staining. Proteomic analysis, real-time quantitative PCR and flow cytometry were used to interrogate monocyte/macrophage phenotypes. CLA supplementation inhibited atherosclerosis progression coincident with decreased proinflammatory and increased anti-inflammatory macrophages. However, CLA-induced regression was associated with increased proinflammatory monocytes resulting in increased proresolving M2 bone marrow-derived macrophages, splenic macrophages, and dendritic cells in lesion-draining lymph nodes. Proteomic analysis confirmed regulation of a proinflammatory bone marrow response, which was abolished upon macrophage differentiation. Thus, in attenuation and regression of atherosclerosis, regardless of the monocyte signature, during monocyte to macrophage differentiation, proresolving macrophages prevail, mediating vascular repair. This study provides novel mechanistic insight into the monocyte/macrophage phenotypes in halted atherosclerosis progression and regression of atherosclerosis.-Bruen, R., Curley, S., Kajani, S., Lynch, G., O'Reilly, M. E., Dillon, E. T., Fitzsimons, S., Mthunzi, L., McGillicuddy, F. C., Belton, O. Different monocyte phenotypes result in proresolving macrophages in conjugated linoleic acid-induced attenuated progression and regression of atherosclerosis.

JNK1 regulates RANKL-induced osteoclastogenesis via activation of a novel Bcl-2-Beclin1-autophagy pathway.

JNK1 plays an important role in osteoclastogenesis in response to the osteoclastogenic cytokine receptor activator for nuclear factor-κB ligand (RANKL). JNK1 is widely accepted as an autophagy regulator under stress conditions. However, the role of JNK1-mediated autophagy in osteoclastogenesis remains largely unknown. In the current study, our data showed that JNK1 inhibition by a pharmacological inhibitor or RNA interference significantly reduced the autophagic response induced by RANKL in osteoclast precursors (OCPs) derived from bone marrow-derived macrophages. Overexpression of the key autophagy protein Beclin1 rescued autophagy deficiency and osteoclastogenesis in the presence of a JNK inhibitor (SP600125). In contrast, JNK activator (anisomycin)-induced autophagy was blocked by Beclin1 knockdown in OCPs. In addition, JNK1 inhibition increased apoptosis and blocked autophagy, whereas overexpression of Beclin1 reversed the enhanced apoptosis induced by JNK1 inhibition in OCPs. Furthermore, RANKL could induce the phosphorylation of Bcl-2, subsequently dissociating Beclin1 from the Bcl-2-Beclin1 complex, which could be blocked by JNK1 inhibition. Collectively, this study revealed that JNK1 regulated osteoclastogenesis by activating Bcl-2-Beclin1-autophagy signaling in addition to the classic c-Jun/activator protein 1 pathway, which provided the first evidence for the contribution of JNK1 signaling to OCP autophagy and the autophagic mechanism underlying JNK1-regulated osteoclastogenesis. An important osteoclastogenesis-regulating signaling pathway (JNK1-Bcl-2-Beclin1-autophagy activation) was identified, which provides novel potential targets for the clinical therapy of metabolic bone diseases.-Ke, D., Ji, L., Wang, Y., Fu, X., Chen, J., Wang, F., Zhao, D., Xue, Y., Lan, X., Hou, J. JNK1 regulates RANKL-induced osteoclastogenesis via activation of a novel Bcl-2-Beclin1-autophagy pathway.

Epitranscriptomic profiling across cell types reveals associations between APOBEC1-mediated RNA editing, gene expression outcomes, and cellular function.

Epitranscriptomics refers to posttranscriptional alterations on an mRNA sequence that are dynamic and reproducible, and affect gene expression in a similar way to epigenetic modifications. However, the functional relevance of those modifications for the transcript, the cell, and the organism remain poorly understood. Here, we focus on RNA editing and show that Apolipoprotein B mRNA-editing enzyme, catalytic polypeptide-1 (APOBEC1), together with its cofactor RBM47, mediates robust editing in different tissues. The majority of editing events alter the sequence of the 3'UTR of targeted transcripts, and we focus on one cell type (monocytes) and on a small set of highly edited transcripts within it to show that editing alters gene expression by modulating translation (but not RNA stability or localization). We further show that specific cellular processes (phagocytosis and transendothelial migration) are enriched for transcripts that are targets of editing and that editing alters their function. Finally, we survey bone marrow progenitors and demonstrate that common monocyte progenitor cells express high levels of APOBEC1 and are susceptible to loss of the editing enzyme. Overall, APOBEC1-mediated transcriptome diversification is required for the fine-tuning of protein expression in monocytes, suggesting an epitranscriptomic mechanism for the proper maintenance of homeostasis in innate immune cells.

Induction of functional human macrophages from bone marrow promonocytes by M-CSF in humanized mice.

Engraftment of human CD34⁺ hematopoietic stem/progenitor cells into immunodeficient mice leads to robust reconstitution of human T and B cells but not monocytes and macrophages. To identify the cause underlying the poor monocyte and macrophage reconstitution, we analyzed human myeloid cell development in humanized mice and found that it was blocked at the promonocyte stage in the bone marrow. Expression of human M-CSF or GM-CSF by hydrodynamic injection of cytokine-encoding plasmid completely abolished the accumulation of promonocytes in the bone marrow. M-CSF promoted the development of mature monocytes and tissue-resident macrophages whereas GM-CSF did not. Moreover, correlating with an increased human macrophages at the sites of infection, M-CSF-treated humanized mice exhibited an enhanced protection against influenza virus and Mycobacterium infection. Our study identifies the precise stage at which human monocyte/macrophage development is blocked in humanized mice and reveals overlapping and distinct functions of M-CSF and GM-CSF in human monocyte and macrophage development. The improved reconstitution and functionality of monocytes/macrophages in the humanized mice following M-CSF expression provide a superior in vivo system to investigate the role of macrophages in physiological and pathological processes.

Body fluid exosomes promote secretion of inflammatory cytokines in monocytic cells via Toll-like receptor signaling.

Tumor-derived exosomes have been shown to induce various immunomodulatory effects. However, the underlying signaling pathways are poorly understood. Here, we analyzed the effects of ex vivo-derived exosomes on monocytic cell differentiation/activation using THP-1 cells as model. We isolated exosomes from various body fluids such as amniotic fluid, liver cirrhosis ascites, and malignant ascites of ovarian cancer patients. We observed that exosomes were internalized by THP-1 cells and induced the production of IL-1ß, TNF-α, and IL-6. Analysis of the signaling pathways revealed a fast triggering of NFκB and a delayed activation of STAT3. Pharmacologic and antibody-blocking experiments showed that the initial production of IL-6 was instrumental for subsequent activation of STAT3. Importantly, triggering of cell signaling was not a unique property of tumor exosomes but was also observed with exosomes of noncancerous origin. Exosomal signaling was TLR-dependent as the knockdown of Toll-like receptor 2 (TLR2) and TLR4 blocked NFκB and STAT3 activation. Similar results were obtained with TLR-neutralizing antibodies. Exosomes also triggered the release of cytokines from mouse bone marrow-derived dendritic cells or macrophages. This process was MyD88-dependent, further supporting a role of TLR signaling. Our results suggest that exosomes trigger TLR-dependent signaling pathways in monocytic precursor cells but possibly also in other immune cells. This process could be important for the induction of immunosuppressive mechanisms during cancer progression and inflammatory diseases.

BMP-7 attenuates adverse cardiac remodeling mediated through M2 macrophages in prediabetic cardiomyopathy.

The main objective of this study was to determine whether or not monocyte infiltration occurs in the prediabetic (PD) heart and its role in PD cardiomyopathy. We hypothesized that the PD heart is significantly populated with monocytes and that bone morphogenetic protein (BMP)-7, a novel mediator of monocyte polarization, activates infiltrated monocytes into anti-inflammatory M2 macrophages, thereby inhibiting apoptosis and fibrosis and improving cardiac function. C57Bl6 mice were assigned to control, PD, or PD + BMP-7 groups. PD and PD + BMP-7 groups were administered streptozotocin (50 mg/kg), whereas control animals received sodium citrate buffer. Afterward, the PD + BMP-7 group was administered BMP-7 (200 µg/kg) for 3 days. Our data showed significantly increased infiltrated monocytes and associated pro-inflammatory cytokines, adverse cardiac remodeling, and heart dysfunction in the PD group (P < 0.05). Interestingly, M2 macrophage differentiation and associated anti-inflammatory cytokines were enhanced and there were reduced adverse cardiac remodeling and improved cardiac function in the PD + BMP-7 group (P < 0.05). In conclusion, our data suggest that PD cardiomyopathy is associated with increased monocyte infiltration and released proinflammatory cytokines, which contributes to adverse cardiac remodeling and cardiac dysfunction. Moreover, we report that BMP-7 possesses novel therapeutic potential in its ability to differentiate monocytes into M2 macrophages and confer cardiac protection in the PD heart.

Hoxb8 conditionally immortalised macrophage lines model inflammatory monocytic cells with important similarity to dendritic cells.

We have examined the potential to generate bona fide macrophages (MØ) from conditionally immortalised murine bone marrow precursors. MØ can be derived from Hoxb8 conditionally immortalised macrophage precursor cell lines (MØP) using either M-CSF or GM-CSF. When differentiated in GM-CSF (GM-MØP) the resultant cells resemble GM-CSF bone marrow-derived dendritic cells (BMDC) in morphological phenotype, antigen phenotype and functional responses to microbial stimuli. In spite of this high similarity between the two cell types and the ability of GM-MØP to effectively present antigen to a T-cell hybridoma, these cells are comparatively poor at priming the expansion of IFN-γ responses from naïve CD4(+) T cells. The generation of MØP from transgenic or genetically aberrant mice provides an excellent opportunity to study the inflammatory role of GM-MØP, and reduces the need for mouse colonies in many studies. Hence differentiation of conditionally immortalised MØPs in GM-CSF represents a unique in vitro model of inflammatory monocyte-like cells, with important differences from bone marrow-derived dendritic cells, which will facilitate functional studies relating to the many 'sub-phenotypes' of inflammatory monocytes.

In vitro effects of pulmonary surfactant on macrophage morphology and function.

The effects of pulmonary surfactant on the morphology and functioning of young macrophages were studied on the model of monocyte/macrophage differentiation in vitro and on macrophages of the bronchial alveolar lavage fluid. Surfactant is not a differentiation inductor, but it stimulated the maturation and phagocytic activity of young macrophages. The stimulatory effect of surfactant on phagocytic activity of macrophages persisted even after its removal from the culture medium.

M-CSF elevates c-Fos and phospho-C/EBPalpha(S21) via ERK whereas G-CSF stimulates SHP2 phosphorylation in marrow progenitors to contribute to myeloid lineage specification.

The role of hematopoietic cytokines in lineage commitment remains uncertain. To gain insight into the contribution of cytokine signaling to myeloid lineage specification, we compared granulocyte colony-stimulating factor (G-CSF) and macrophage colony-stimulating factor (M-CSF) signaling in Ba/F3 cells expressing both the G-CSF and M-CSF receptors and in lineage-negative murine marrow cells. G-CSF and M-CSF serve as prototypes for additional cytokines that also influence immature myeloid cells. G-CSF specifically activated signal transducer and activator of transcription 3 and induced Src homology region 2 domain-containing phosphatase 2 (SHP2) phosphorylation, whereas M-CSF preferentially activated phospholipase Cgamma2, and thereby extracellular signal-regulated kinase (ERK), to stabilize c-Fos and stimulate CCAAT/enhancer-binding protein (C/EBP)alpha(S21) phosphorylation. In contrast, activation of Jun kinase or c-Jun was similar in response to either cytokine. Inhibition of ERK prevented induction of c-Fos by M-CSF and reduced C/EBPalpha phosphorylation and formation of colony-forming unit-monocytes. SHP2 inhibition reduced ERK activation in G-CSF, but not M-CSF, and reduced colony-forming unit-granulocytes, underscoring divergent pathways to ERK activation. Phorbol ester mimicked the effect of M-CSF, activating ERK independent of SHP2. In summary, M-CSF activates ERK more potently than G-CSF, and thereby induces higher levels of c-Fos and phospho-C/EBPalpha(S21), which may directly interact to favor monopoiesis, whereas G-CSF activates signal transducer and activator of transcription 3 and SHP2, potentially shifting the balance to granulopoiesis via gene induction by C/EBPalpha homodimers and via effects of SHP2 on regulators besides ERK.

Selective expansion of the monocytic lineage directed by bacterial infection.

CCR2-mediated recruitment of Ly6C(high) monocytes is essential for defense against a range of microbial pathogens. Although our understanding of monocyte trafficking to inflammatory sites is increasing, how innate immune inflammation influences monocyte development and maturation during microbial infection remains undefined. Herein, we demonstrate that infection with the intracellular bacterial pathogen Listeria monocytogenes specifically and selectively promotes monopoiesis. Systemic infection with virulent L. monocytogenes induces marked proliferation of bone marrow monocyte precursors and results in depletion of myeloid progenitors. Proliferation of monocyte precursors correlates with the intensity of systemic infection and is unaffected by the density of monocytes in the bone marrow. Although MyD88/Trif-mediated signaling is not required for early emigration of the mature monocyte population from the bone marrow, replenishment of monocyte populations depends on MyD88/Trif. Our studies demonstrate that TLR-mediated signals play an essential role in the maintenance of monocyte homeostasis during systemic bacterial infection.

[RANKL signaling and bone diseases. Quiescent osteoclast precursors and RANKL signaling].

It is well known that bone marrow macrophages differentiate into osteoclasts in response to M-CSF and RANKL in culture. However, the characteristics and dynamics of osteoclast precursors in vivo are not clear. Cell cycle arrest in osteoclast precursors is a prerequisite step for their differentiation into osteoclasts. We named such precursors "QOPs (cell cycle-arrested quiescent osteoclast precursors) " . Injection of 2MD [a potent analogue of 1α, 25 (OH) (2)D(3)] , RANKL, or M-CSF to mice induced osteoclast differentiation from QOPs. Moreover, osteoclasts appearing in BMP (bone morphogenetic protein) -induced ectopic bone were also differentiated from QOPs without cell proliferation. These results indicate that (1)osteoclasts are formed from QOPs in response to bone resorbing stimuli in vivo, (2) QOPs circulate in the bloodstream and settle in the right place for osteoclastogenesis. Here I review recent advances in our understanding of osteoclast precursors in vivo.

Inhibitory immunoglobulin-like receptors LILRB and PIR-B negatively regulate osteoclast development.

Osteoclasts, multinucleated cells of myeloid-monocytic origin, are responsible for bone resorption, which is crucial for maintenance of bone homeostasis in concert with bone-forming osteoblasts of nonhematopoietic, mesenchymal origin. Receptor activator of NF-kappaB ligand (RANKL) and M-CSF, expressed on the surface of and secreted by osteoblasts, respectively, are essential factors that facilitate osteoclast formation. In contrast to the activation processes for osteoclast formation, inhibitory mechanisms for it are poorly understood. Herein we demonstrate that inhibitory Ig-like receptors recruiting Src homology 2 domain-containing tyrosine phosphatase 1 (SHP-1) are expressed on osteoclast precursor cells like other myeloid cells, and that they play a regulatory role in the development of osteoclasts. We detected cell-surface expression of paired Ig-like receptor (PIR)-B and four isoforms of leukocyte Ig-like receptor (LILR)B on cultured osteoclast precursor cells of mouse and human origin, respectively, and showed that all of these ITIM-harboring inhibitory receptors constitutively recruit SHP-1 in the presence of RANKL and M-CSF, and that some of them can suppress osteoclast development in vitro. Fluorescence energy transfer analyses have suggested that the constitutive binding of either murine PIR-B or its human ortholog LILRB1 to MHC class I molecules on the same cell surface comprises one of the mechanisms for developmental regulation. These results constitute the first evidence of the regulation of osteoclast formation by cell-surface, ITIM-harboring Ig-like receptors. Modulation of these regulatory receptors may be a novel way to control various skeletal system disorders and inflammatory arthritis.

Estrogens decrease osteoclast number by attenuating mitochondria oxidative phosphorylation and ATP production in early osteoclast precursors.

Loss of estrogens at menopause is a major cause of osteoporosis and increased fracture risk. Estrogens protect against bone loss by decreasing osteoclast number through direct actions on cells of the myeloid lineage. Here, we investigated the molecular mechanism of this effect. We report that 17ß-estradiol (E2) decreased osteoclast number by promoting the apoptosis of early osteoclast progenitors, but not mature osteoclasts. This effect was abrogated in cells lacking Bak/Bax-two pro-apoptotic members of the Bcl-2 family of proteins required for mitochondrial apoptotic death. FasL has been previously implicated in the pro-apoptotic actions of E2. However, we show herein that FasL-deficient mice lose bone mass following ovariectomy indistinguishably from FasL-intact controls, indicating that FasL is not a major contributor to the anti-osteoclastogenic actions of estrogens. Instead, using microarray analysis we have elucidated that ERα-mediated estrogen signaling in osteoclast progenitors decreases "oxidative phosphorylation" and the expression of mitochondria complex I genes. Additionally, E2 decreased the activity of complex I and oxygen consumption rate. Similar to E2, the complex I inhibitor Rotenone decreased osteoclastogenesis by promoting osteoclast progenitor apoptosis via Bak/Bax. These findings demonstrate that estrogens decrease osteoclast number by attenuating respiration, and thereby, promoting mitochondrial apoptotic death of early osteoclast progenitors.

Hyaluronan preconditioning of monocytes/macrophages affects their angiogenic behavior and regulation of TSG-6 expression in a tumor type-specific manner.

Hyaluronan is a glycosaminoglycan normally present in the extracellular matrix in most tissues. Hyaluronan is a crucial player in many processes associated with cancer, such as angiogenesis, invasion, and metastasis. However, little has been reported regarding the action of hyaluronan on monocytes/macrophages (Mo/MØ) in tumor angiogenesis and its consequences on tumor development. In the present study, we investigated the effects of hyaluronan of different sizes on human Mo/MØ angiogenic behavior in colorectal and breast carcinoma. In vitro, the treatment of Mo/MØ with lysates and conditioned media from a breast but not from colorectal carcinoma cell line plus high-molecular weight hyaluronan induced: (a) an increased expression of angiogenic factors VEGF, IL-8, FGF-2, and MMP-2, (b) an increased endothelial cell migration, and (c) a differential expression of hyaluronan-binding protein TSG-6. Similar results were observed in Mo/MØ derived from breast cancer patients treated with tumor lysates. Besides, macrophages primed with high-molecular weight hyaluronan and inoculated in human breast cancer xenograft tumor increased blood vessel formation and diminished TSG-6 levels. In contrast, the effects triggered by high-molecular weight hyaluronan on Mo/MØ in breast cancer context were not observed in the context of colorectal carcinoma. Taken together, these results indicate that the effect of high-molecular weight hyaluronan as an inductor of the angiogenic behavior of macrophages in breast tumor context is in part consequence of the presence of TSG-6.

Developmental control of macrophage function.

The combination between novel fate-mapping tools and single-cell RNA-sequencing technology has revealed the presence of multiple macrophage progenitors. This raises the fascinating possibility that what was once perceived as immense functional plasticity of macrophages could in fact come down to separate macrophage subsets performing distinct functions because of their differential cellular origin. The question of macrophage plasticity versus macrophage heterogeneity is broader than the difference between macrophages of embryonic or adult hematopoietic origin and is particularly relevant in the context of inflammation. In this manuscript, we review the potential impact of cellular origin on the function of macrophages. We also highlight the need for novel 'functional fate-mapping' tools that would reveal the history of the functional state of macrophages, rather than their cellular origin, in order to finally study their true plasticity in vivo.