Spleen serves as a reservoir of osteoclast precursors through vitamin D-induced IL-34 expression in osteopetrotic op/op mice. Proc Natl Acad Sci U S A. 2012;109:10006-10011. [PMID: 22670054 DOI: 10.1073/pnas.1207361109]

Inflammatory and cardiac biomarkers in pulmonary arterial hypertension: The prognostic role of IL-34

BACKGROUND

Pulmonary arterial hypertension (PAH) is characterized by increased pulmonary artery pressure with significant morbidity and mortality. Inflammatory processes are crucial in PAH pathogenesis, with inflammatory cells and mediators present early in disease progression. IL-34 involvement in inflammatory pathways suggests that IL-34 could be an important player in the progression of PAH, influencing both pulmonary pressures and vascular changes.

OBJECTIVE

The purpose of this study was to investigate the correlation between IL-34 levels and pulmonary arterial hypertension (PAH), aiming to enhance the understanding of the molecular mechanisms underlying PAH and explore IL-34's potential as a biomarker.

METHODS

Consecutive PAH patients diagnosed via right-heart catheterization at Malatya Turgut Ozal Eğitim ve Araştırma Hastanesi (Dec 2022 - Apr 2024) were enrolled. Patients were classified into low-risk and high-risk groups based on comprehensive risk assessments that included clinical parameters, hemodynamic measurements and biomarkers, in-line with ESC/ERS guidelines. Serum IL-34, hs-CRP, and NT-proBNP levels were measured and compared with those of healthy controls. Echocardiographic assessments and statistical analyses, including ROC analysis, were conducted to evaluate biomarker significance and predictive capabilities.

RESULTS

The mean age of low-risk and high-risk PAH patients was 42 ± 7.2 years and 45 ± 5.5 years, respectively. The mean age of the control group was 40 ± 6.4 years. Males comprised 54.29 % of the low-risk group, 56 % of the high-risk group, and 53.3 % of the control group. IL-34 and hs-CRP levels were significantly elevated in PAH patients compared to controls. IL-34 correlated positively with systolic pulmonary artery pressure, RA area, and NT-proBNP levels. Multivariate analysis revealed that IL-34 and hs-CRP were independent predictors of PAH. IL-34 levels>29.8 pg/mL predicted PAH with 78 % sensitivity and 69 % specificity, while levels >44.4 pg/mL predicted high-risk PAH with 84 % sensitivity and 77 % specificity.

CONCLUSION

Elevated IL-34 and hs-CRP levels are associated with PAH severity and right ventricular dysfunction, suggesting IL-34's potential as a diagnostic and prognostic biomarker. Further research is needed to validate these findings and explore IL-34-targeted therapies in pH management.

Effects of IL-34 and anti-IL-34 neutralizing mAb on alveolar bone loss in a ligature-induced model of periodontitis

Macrophage colony-stimulating factor (M-CSF) and interleukin-34 (IL-34) are ligands for the colony-stimulating factor-1  receptor (CSF-1r) expressed on the surface of monocyte/macrophage lineage cells. The importance of coordinated signaling between M-CSF/receptor activator of the nuclear factor kappa-Β ligand (RANKL) in physiological and pathological bone remodeling and alveolar bone loss in response to oral bacterial colonization is well established. However, our knowledge about the IL-34/RANKL signaling in periodontal bone loss remains limited. Recently published cohort studies have demonstrated that the expression patterns of IL-34 are dramatically elevated in gingival crevicular fluid collected from patients with periodontitis. Therefore, the present study aims to evaluate the effects of IL-34 on osteoclastogenesis in vitro and in experimental ligature-mediated model of periodontitis using male mice. Our initial in vitro study demonstrated increased RANKL-induced osteoclastogenesis of IL-34-primed osteoclast precursors (OCPs) compared to M-CSF-primed OCPs. Using an experimental model of ligature-mediated periodontitis, we further demonstrated elevated expression of IL-34 in periodontal lesions. In contrast, M-CSF levels were dramatically reduced in these periodontal lesions. Furthermore, local injections of mouse recombinant IL-34 protein significantly elevated cathepsin K activity, increased the number of tartrate-resistant acid phosphatase (TRAP)-positive osteoclasts and promoted alveolar bone loss in periodontitis lesions. In contrast, anti-IL-34 neutralizing monoclonal antibody significantly reduced the level of alveolar bone loss and the number of TRAP-positive osteoclasts in periodontitis lesions. No beneficial effects of locally injected anti-M-CSF neutralizing antibody were observed in periodontal lesions. This study illustrates the role of IL-34 in promoting alveolar bone loss in periodontal lesions and proposes the potential of anti-IL34 monoclonal antibody (mAb)-based therapeutic regimens to suppress alveolar bone loss in periodontitis lesions.

Elevated circulating levels of IL-34 are strongly associated with osteoporosis

In this cross-sectional study, we observed a strong, age-independent association of circulating interleukin-34 (IL-34) levels with osteoporosis.

PURPOSE

The reported capacity of IL-34 to induce and enhance osteoclastogenesis suggests its potential involvement in the pathogenesis of osteoporosis. Our study aimed to evaluate whether there is an association between IL-34 expression and osteoporosis.

METHODS

We enrolled 30 women with osteoporosis and 230 age-matched non-osteoporotic women as a control group. Osteoporosis diagnosis was based on dual-energy X-ray absorptiometry (DXA) of the lumbar spine and femoral neck. Body composition parameters were assessed by the bioimpedance method. Plasma IL-34 levels were measured by ELISA.

RESULTS

In comparison with the control group, the mean plasma IL-34 levels were significantly higher in osteoporotic women (164.61 ± 36.40 pg/ml vs. 665.43 ± 253.67 pg/ml, p = 0.0002), whereas basal metabolic rate (BMR) was significantly lower (1422.03 ± 6.80 kcal vs. 1339.39 ± 17.52 kcal, p = 0.00007). Both variables remained statistically significant after adjustment for age (p < 0.001). We did not observe correlations between plasma IL-34 levels and body composition parameters in osteoporotic and control groups. Multiple logistic regression analysis with osteoporosis status as a dependent variable clearly showed that age, BMR and IL-34 levels were independently and significantly associated with osteoporosis. The calculated odds ratios (OR) were 1.66 (95% CI = 1.16-2.38) for IL-34 levels and 0.22 (95% CI = 0.07-0.65) for BMR.

CONCLUSION

The significant (fourfold) elevation of IL-34 plasma levels in osteoporosis patients suggests that circulating IL-34 could be used as a biomarker for osteoporosis.

TREM2 acts as a receptor for IL-34 to suppress acute myeloid leukemia in mice

The bone marrow microenvironment supports leukocyte mobilization and differentiation and controls the development of leukemias, including acute myeloid leukemia (AML). Here, we found that the development of AML xenotransplants was suppressed in mice with osteoclasts tuberous sclerosis 1 (Tsc1) deletion. Tsc1-deficient osteoclasts released a high level of interleukin-34 (IL-34), which efficiently induced AML cell differentiation and prevented AML progression in various preclinical models. Conversely, AML development was accelerated in mice deficient in IL-34. Interestingly, IL-34 inhibited AML independent of its known receptors but bound directly to triggering receptor expressed on myeloid cells 2 (TREM2), a key hub of immune signals. TREM2-deficient AML cells and normal myeloid cells were resistant to IL-34 treatment. Mechanistically, IL-34-TREM2 binding rapidly phosphorylated Ras protein activator like 3 and inactivated extracellular signal-regulated protein kinase 1/2 signaling to prevent AML cell proliferation and stimulate differentiation. Furthermore, TREM2 was downregulated in patients with AML and associated with a poor prognosis. This study identified TREM2 as a novel receptor for IL-34, indicating a promising strategy for overcoming AML differentiation blockade in patients with AML.

Significance of IL-34 and SDC-1 in the pathogenesis of RA cells and preclinical models

IL-34 shares a common receptor with M-CSF, while it can bind to other distinct receptors including protein-tyrosine phosphatase zeta (PTPζ), and syndecan1 (SDC-1). In physiological conditions, IL-34 has a critical role in the maintenance and development of Langerhans and microglial cells in part through PTPζ ligation. Conversely, in autoimmune diseases such as rheumatoid arthritis (RA), SDC-1-induced phosphorylation of M-CSFR was responsible for the pathological effect of IL-34 in patient cells and/or preclinical models. Intriguingly, enrichment of IL-34 is strongly linked to rheumatoid factor (RF), disease activity score (DAS)28, erythrocyte sedimentation rate (ESR), c-reactive protein (CRP), and radiographic progression. In parallel, IL-34-induced naïve cell reprogramming into glycolytic RA CD14+CD86+GLUT1+ macrophage was dysregulated via M-CSFR or SDC-1 antibody therapy. Moreover, the inflammatory and erosive imprints of IL-34 arthritic mice were mitigated by glucose uptake inhibition and SDC-1, or RAG deficiency through nullifying macrophage metabolic rewiring and their ability to advance Th1/Th17 cell polarization. Consistently, IL-34-/- and SDC-1-/- mice could effectively impair CIA joint inflammation, osteoclast formation, and neovascularization by restraining monocyte infiltration as well as suppressing the inflammatory macrophage and T effector cell reconfiguration via metabolic deactivation. In conclusion, targeting IL-34/SDC-1 signaling, or its interconnected metabolites can uniquely intercept the crosstalk between glycolytic RA myeloid and lymphoid cells and their ability to trigger arthritis.

Bone marrow Adipoq-lineage progenitors are a major cellular source of M-CSF that dominates bone marrow macrophage development, osteoclastogenesis, and bone mass

M-CSF is a critical growth factor for myeloid lineage cells, including monocytes, macrophages, and osteoclasts. Tissue-resident macrophages in most organs rely on local M-CSF. However, it is unclear what specific cells in the bone marrow produce M-CSF to maintain myeloid homeostasis. Here, we found that Adipoq-lineage progenitors but not mature adipocytes in bone marrow or in peripheral adipose tissue, are a major cellular source of M-CSF, with these Adipoq-lineage progenitors producing M-CSF at levels much higher than those produced by osteoblast lineage cells. The Adipoq-lineage progenitors with high CSF1 expression also exist in human bone marrow. Deficiency of M-CSF in bone marrow Adipoq-lineage progenitors drastically reduces the generation of bone marrow macrophages and osteoclasts, leading to severe osteopetrosis in mice. Furthermore, the osteoporosis in ovariectomized mice can be significantly alleviated by the absence of M-CSF in bone marrow Adipoq-lineage progenitors. Our findings identify bone marrow Adipoq-lineage progenitors as a major cellular source of M-CSF in bone marrow and reveal their crucial contribution to bone marrow macrophage development, osteoclastogenesis, bone homeostasis, and pathological bone loss.

CSF1R as a Therapeutic Target in Bone Diseases: Obvious but Not so Simple

PURPOSE OF REVIEW

The purpose of the review is to summarize the expression and function of CSF1R and its ligands in bone homeostasis and constraints on therapeutic targeting of this axis.

RECENT FINDINGS

Bone development and homeostasis depends upon interactions between mesenchymal cells and cells of the mononuclear phagocyte lineage (MPS), macrophages, and osteoclasts (OCL). The homeostatic interaction is mediated in part by the systemic and local production of growth factors, macrophage colony-stimulating factor (CSF1), and interleukin 34 (IL34) that interact with a receptor (CSF1R) expressed exclusively by MPS cells and their progenitors. Loss-of-function mutations in CSF1 or CSF1R lead to loss of OCL and macrophages and dysregulation of postnatal bone development. MPS cells continuously degrade CSF1R ligands via receptor-mediated endocytosis. As a consequence, any local or systemic increase or decrease in macrophage or OCL abundance is rapidly reversible. In principle, both CSF1R agonists and antagonists have potential in bone regenerative medicine but their evaluation in disease models and therapeutic application needs to carefully consider the intrinsic feedback control of MPS biology.

OUP accepted manuscript

Osteopetrosis is a rare inherited disease characterized by impaired osteoclast activity causing defective bone resorption and bone marrow aplasia. It is fatal in early childhood unless hematopoietic stem cell transplantation is performed. But, the transplant course is complicated with engraftment failure. Recently, osteoclasts have been described as the potential regulators of hematopoietic stem cell (HSC) niche. Here we investigated the alterations in the HSC and mesenchymal stromal cell (MSC) components of osteopetrotic niche and their interactions to mimic the stem cell dynamics/trafficking in the BM niche after HSC transplantation. Induced pluripotent stem cells were generated from peripheral blood mononuclear cells of patients with osteopetrosis carrying TCIRG1 mutation. iPSC lines were differentiated into hematopoietic and myeloid progenitors, then into osteoclasts using a step-wise protocol. We first demonstrated a shift toward monocyte-macrophages lineage regarding hematopoietic differentiation potential of osteopetrotic iPSC-derived hematopoietic progenitors (HPCs) and phenotypically normal and functionally defective osteoclast formation. The expression of the genes involved in HSC homing and maintenance (Sdf-1, Jagged-1, Kit-L, and Opn) in osteopetrotic MSCs recovered significantly after coculture with healthy HPCs. Similarly, the restoration of phenotype, impaired differentiation, and migratory potential of osteopetrotic iHPCs were observed upon interaction with healthy MSCs. Our results establish significant alterations in both MSC and HPC compartments of the osteopetrotic niche, and support the impact of functionally impaired osteoclasts in defective niche formation.

Low-dose IL-34 has no effect on osteoclastogenesis but promotes osteogenesis of hBMSCs partly via activation of the PI3K/AKT and ERK signaling pathways

Background

Inflammatory microenvironment is significant to the differentiation and function of mesenchymal stem cells (MSCs). It evidentially influences the osteoblastogenesis of MSCs. IL-34, a newly discovered cytokine, playing a key role in metabolism. However, the research on its functional role in the osteogenesis of MSCs was rarely reported. Here, we described the regulatory effects of low-dose IL-34 on both osteoblastogenesis and osteoclastogenesis.

Methods

We performed the osteogenic effects of hBMSCs by exogenous and overexpressed IL-34 in vitro, so were the osteoclastogenesis effects of mBMMs by extracellular IL-34. CCK-8 was used to assess the effect of IL-34 on the viability of hBMSCs and mBMMs. ALP, ARS, and TRAP staining was used to evaluate ALP activity, mineral deposition, and osteoclastogenesis, respectively. qRT-PCR and Western blotting analysis were performed to detect the expression of target genes and proteins. ELISA was used to evaluate the concentrations of IL-34. In vivo, a rat tibial osteotomy model and an OVX model were established. Radiographic analysis and histological evaluation were performed to confirm the therapeutic effects of IL-34 in fracture healing and osteoporosis. Statistical differences were evaluated by two-tailed Student’s t test, one-way ANOVA with Bonferroni’s post hoc test, and two-way ANOVA with Bonferroni multiple comparisons post hoc test in the comparison of 2 groups, more than 2 groups, and different time points of treated groups, respectively.

Results

Promoted osteoblastogenesis of hBMSCs was observed after treated by exogenous or overexpressed IL-34 in vitro, confirmed by increased mineral deposits and ALP activity. Furthermore, exogenous or overexpressed IL-34 enhanced the expression of p-AKT and p-ERK. The specific AKT and ERK signaling pathway inhibitors suppressed the enhancement of osteoblastogenesis induced by IL-34. In a rat tibial osteotomy model, imaging and histological analyses testified the local injection of exogenous IL-34 improved bone healing. However, the additional IL-34 has no influence on both osteoclastogenesis of mBMMs in vitro and osteoporosis of OVX model of rat in vivo.

Conclusions

Collectively, our study demonstrate that low-dose IL-34 regulates osteogenesis of hBMSCs partly via the PIK/AKT and ERK signaling pathway and enhances fracture healing, with neither promoting nor preventing osteoclastogenesis in vitro and osteoporosis in vivo.

Human osteoclastogenesis in Epstein-Barr virus-induced erosive arthritis in humanized NOD/Shi-scid/IL-2Rγnull mice

Many human viruses, including Epstein-Barr virus (EBV), do not infect mice, which is challenging for biomedical research. We have previously reported that EBV infection induces erosive arthritis, which histologically resembles rheumatoid arthritis, in humanized NOD/Shi-scid/IL-2Rγ^null (hu-NOG) mice; however, the underlying mechanisms are not known. Osteoclast-like multinucleated cells were observed during bone erosion in this mouse model, and therefore, we aimed to determine whether the human or mouse immune system activated bone erosion and analyzed the characteristics and origin of the multinucleated cells in hu-NOG mice. Sections of the mice knee joint tissues were immunostained with anti-human antibodies against certain osteoclast markers, including cathepsin K and matrix metalloproteinase-9 (MMP-9). Multinucleated cells observed during bone erosion stained positively for human cathepsin K and MMP-9. These results indicate that human osteoclasts primarily induce erosive arthritis during EBV infections. Human osteoclast development from hematopoietic stem cells transplanted in hu-NOG mice remains unclear. To confirm their differentiation potential into human osteoclasts, we cultured bone marrow cells of EBV-infected hu-NOG mice and analyzed their characteristics. Multinucleated cells cultured from the bone marrow cells stained positive for human cathepsin K and human MMP-9, indicating that bone marrow cells of hu-NOG mice could differentiate from human osteoclast progenitor cells into human osteoclasts. These results indicate that the human immune response to EBV infection may induce human osteoclast activation and cause erosive arthritis in this mouse model. Moreover, this study is the first, to our knowledge, to demonstrate human osteoclastogenesis in humanized mice. We consider that this model is useful for studying associations of EBV infections with rheumatoid arthritis and human bone metabolism.

RANKL/OPG ratio regulates odontoclastogenesis in damaged dental pulp

Bone-resorbing osteoclasts are regulated by the relative ratio of the differentiation factor, receptor activator NF-kappa B ligand (RANKL) and its decoy receptor, osteoprotegerin (OPG). Dental tissue-localized-resorbing cells called odontoclasts have regulatory factors considered as identical to those of osteoclasts; however, it is still unclear whether the RANKL/OPG ratio is a key factor for odontoclast regulation in dental pulp. Here, we showed that odontoclast regulators, macrophage colony-stimulating factor-1, RANKL, and OPG were detectable in mouse pulp of molars, but OPG was dominantly expressed. High OPG expression was expected to have a negative regulatory effect on odontoclastogenesis; however, odontoclasts were not detected in the dental pulp of OPG-deficient (KO) mice. In contrast, damage induced odontoclast-like cells were seen in wild-type pulp tissues, with their number significantly increased in OPG-KO mice. Relative ratio of RANKL/OPG in the damaged pulp was significantly higher than in undamaged control pulp. Pulp damages enhanced hypoxia inducible factor-1α and -2α, reported to increase RANKL or decrease OPG. These results reveal that the relative ratio of RANKL/OPG is significant to pulpal odontoclastogenesis, and that OPG expression is not required for maintenance of pulp homeostasis, but protects pulp from odontoclastogenesis caused by damages.

Monocyte/Macrophage Lineage Cells From Fetal Erythromyeloid Progenitors Orchestrate Bone Remodeling and Repair

A third of the population sustains a bone fracture, and the pace of fracture healing slows with age. The slower pace of repair is responsible for the increased morbidity in older individuals who sustain a fracture. Bone healing progresses through overlapping phases, initiated by cells of the monocyte/macrophage lineage. The repair process ends with remodeling. This last phase is controlled by osteoclasts, which are bone-specific multinucleated cells also of the monocyte/macrophage lineage. The slower rate of healing in aging can be rejuvenated by macrophages from young animals, and secreted proteins from macrophage regulate undifferentiated mesenchymal cells to become bone-forming osteoblasts. Macrophages can derive from fetal erythromyeloid progenitors or from adult hematopoietic progenitors. Recent studies show that fetal erythromyeloid progenitors are responsible for the osteoclasts that form the space in bone for hematopoiesis and the fetal osteoclast precursors reside in the spleen postnatally, traveling through the blood to participate in fracture repair. Differences in secreted proteins between macrophages from old and young animals regulate the efficiency of osteoblast differentiation from undifferentiated mesenchymal precursor cells. Interestingly, during the remodeling phase osteoclasts can form from the fusion between monocyte/macrophage lineage cells from the fetal and postnatal precursor populations. Data from single cell RNA sequencing identifies specific markers for populations derived from the different precursor populations, a finding that can be used in future studies. Here, we review the diversity of macrophages and osteoclasts, and discuss recent finding about their developmental origin and functions, which provides novel insights into their roles in bone homeostasis and repair.

Development of a method for the identification of receptor activator of nuclear factor-κB+ populations in vivo

OBJECTIVES

Osteoclasts are induced by macrophage colony-stimulating factor-1 (CSF-1) and receptor activator of nuclear factor-κB (RANK) ligand (RANKL). Monocyte/macrophage lineages are thought to be osteoclast precursors; however, such cells have not been fully characterized owing to a lack of tools for their identification. Osteoclast precursors express colony-stimulating factor-1 receptor (CSF-1R) and RANK. However, the capacity of conventional methods using anti-RANK antibodies to detect RANK⁺ cells by flow cytometry is insufficient. Here, we developed a high-sensitivity method for detecting RANK⁺ cells using biotinylated recombinant glutathione S-transferase-RANKL (GST-RANKL-biotin).

METHODS

We sorted sub-populations of mouse bone marrow (BM) or peripheral blood (PB) cells using GST-RANKL-biotin, anti-CSF1R, and anti-B220 antibodies and induced osteoclastogenesis in vitro.

RESULTS

The frequency of the RANK⁺ population in BM detected by GST-RANKL-biotin was significantly higher than that detected by anti-RANK antibodies. Although RANK⁺ cells were detected in both the B220⁺ and B220^- populations, the macrophage lineage was present only in B220^-. Unexpectedly, a significantly higher number of osteoclasts was induced in RANK^-CSF-1R⁺ cells than in RANK⁺CSF-1R⁺ cells contained in the B220^- population. In contrast, the PB-derived B220^-RANK⁺CSF-1R⁺ population contained a significantly higher frequency of osteoclast precursors than the B220^-RANK^-CSF-1R⁺ population.

CONCLUSIONS

These results suggest that GST-RANKL-biotin is useful for the detection of RANK⁺ cells and that RANK and CSF-1R may be helpful indicators of osteoclast precursors in PB.

The twin cytokines interleukin-34 and CSF-1: masterful conductors of macrophage homeostasis

Macrophages are specialized cells that control tissue homeostasis. They include non-resident and tissue-resident macrophage populations which are characterized by the expression of particular cell surface markers and the secretion of molecules with a wide range of biological functions. The differentiation and polarization of macrophages relies on specific growth factors and their receptors. Macrophage-colony stimulating factor (CSF-1) and interleukine-34 (IL-34), also known as "twin" cytokines, are part of this regluatory landscape. CSF-1 and IL-34 share a common receptor, the macrophage-colony stimulating factor receptor (CSF-1R), which is activated in a similar way by both factors and turns on identical signaling pathways. However, there is some discrete differential activation leading to specific activities. In this review, we disscuss recent progress in understanding of the role of the twin cytokines in macrophage differentiation, from their interaction with CSF-1R and the activation of signaling pathways, to their implication in macrophage polarization of non-resident and tissue-resident macrophages. A special focus on IL-34, its involvement in pathophsyiological contexts, and its potential as a theranostic target for macrophage therapy will be proposed.

RANKL as the master regulator of osteoclast differentiation

RANKL, the essential cue for osteoclast differentiation, is the membrane-bound factor expressed by osteoclastogenesis-supporting cells such as osteoblasts and osteocytes. In vivo evidence indicates that RANKL functions as the indispensable and irreplaceable in the program of osteoclast differentiation. The reason why RANKL plays a critical role in osteoclastogenesis is discussed from the viewpoint of the distinct signaling pathways mediated by co-stimulatory receptors and the key transcription factor NFATc1.

Emerging roles of IL-34 in health and disease

Macrophages are part of the innate immune system and are present in every organ of the body. They fulfill critical roles in tissue homeostasis and development and are involved in various pathologies. An essential factor for the development, homeostasis, and function of mononuclear phagocytes is the colony stimulating factor-1 receptor (CSF-1R), which has two known ligands: CSF-1 and interleukin-34 (IL-34). While CSF-1 has been extensively studied, the biology and functions of IL-34 are only now beginning to be uncovered. In this review, we discuss recent advances of IL-34 biology in health and disease with a specific focus on mononuclear phagocytes.

IL-34 regulates the inflammatory response and anti-bacterial immune defense of Japanese flounder Paralichthys olivaceus

Interleukin (IL)-34 is a relatively recently discovered cytokine with pleiotropic effects on various cellular activities, including immune response. In fish, the knowledge on the function of IL-34 is limited. In the present work, we investigated the function of Japanese flounder Paralichthys olivaceus IL-34 (PoIL-34) in association with inflammation and immune defense. PoIL-34 possesses the conserved structure of IL-34 superfamily and shares 21.52% sequence identity with murine IL-34. PoIL-34 expression was detected in a wide range of tissues of flounder, in particular intestine, and was regulated to a significant extent by bacterial infection in a time-dependent fashion. In vitro studies showed that recombinant PoIL-34 (rPoIL-34) bound peripheral blood leukocytes (PBLs) and promoted ROS production, acid phosphatase activity, and cellular resistance against bacterial infection. At the molecular level, rPoIL-34 enhanced the expressions of inflammatory cytokines and specific JAK and STAT genes. Similar stimulatory effects of rPoIL-34 were observed in vivo. When PoIL-34 was overexpressed in flounder, the expressions of pro- and anti-inflammatory mediators were significantly affected in a tissue-dependent manner, which correlated with an augmented ability of the fish to eliminate invading pathogens from tissues. Together, these results indicated that PoIL-34 regulated inflammatory response probably via specific JAK/STAT pathways and had a significant influence on the immune defense of flounder against bacterial infection.

Sex-Based Differences in Monocytic Lineage Cells Contribute to More Severe Collagen-Induced Arthritis in Female Rats Compared with Male Rats

Monocytes' plasticity has an important role in the development of rheumatoid arthritis (RA), an autoimmune disease exhibiting greater prevalence in women. Contribution of this phenomenon to sex bias in RA severity was investigated in rat collagen-induced arthritis (CIA) model of RA. The greater severity of CIA in females (exhibiting signs of bone resorption) was accompanied by the higher blood level of advanced oxidation protein products and a more pro-oxidant profile. Consistently, in females, the greater density of giant multinuclear cells (monocytes/macrophages and osteoclasts) in inflamed joint tissue was found. This correlated with the higher frequencies of CCR2- and CX3CR1- expressing cells (precursors of inflammatory monocytes/macrophages and osteoclasts) among CD11b+ splenocytes. This in conjunction with the enhanced migratory capacity of CD11b+ monocytic cells in females compared with males could be linked with the higher frequencies of CCR2+CX3CR1-CD43^lowCD11b+ and CCR2-CX3CR1+CD43^hiCD11b+ cells (corresponding to "classical" and "non-classical" monocytes, respectively) and the greater density of CD68+ cells (monocytes/macrophages and osteoclast precursors/osteoclasts) in blood and inflamed paws from female rats, respectively. Consistently, the higher levels of GM-CSF, TNF-α and IL-6, IL-1β (driving Th17 cell differentiation), and IL-17 followed by the lower level of IL-10 were measured in inflamed paw cultures from female compared with male rats. To the greater IL-17 production (associated with enhanced monocyte immigration and differentiation into osteoclasts) most likely contributed augmented Th17 cell generation in the lymph nodes draining arthritic joints from female compared with male rats. Overall, the study suggests the sex-specific contribution of monocytic lineage cells to CIA, and possibly RA development.

GDF11 promotes osteogenesis as opposed to MSTN, and follistatin, a MSTN/GDF11 inhibitor, increases muscle mass but weakens bone

MSTN, a member of the TGF-β family, has been widely shown to suppress muscle growth, leading to an intense effort being directed at targeting MSTN to treat patients with muscle loss. GDF11 is another TGF-β family member closely related to MSTN, but its postnatal function is less clear. Using conditional knockout techniques, we show that GDF11 enhances bone mass in contrast to MSTN, emphasizing that MSTN/GDF11 inhibitors, such as FST, can induce adverse effects on bone through GDF11 inhibition. Because most MSTN inhibitors also inhibit GDF11 due to the high sequence similarity between MSTN and GDF11, our findings suggest that their opposing roles must be carefully considered when developing MSTN inhibitors for clinical applications.

Growth and differentiation factor 11 (GDF11) and myostatin (MSTN) are closely related transforming growth factor β (TGF-β) family members, but their biological functions are quite distinct. While MSTN has been widely shown to inhibit muscle growth, GDF11 regulates skeletal patterning and organ development during embryogenesis. Postnatal functions of GDF11, however, remain less clear and controversial. Due to the perinatal lethality of Gdf11 null mice, previous studies used recombinant GDF11 protein to prove its postnatal function. However, recombinant GDF11 and MSTN proteins share nearly identical biochemical properties, and most GDF11-binding molecules have also been shown to bind MSTN, generating the possibility that the effects mediated by recombinant GDF11 protein actually reproduce the endogenous functions of MSTN. To clarify the endogenous functions of GDF11, here, we focus on genetic studies and show that Gdf11 null mice, despite significantly down-regulating Mstn expression, exhibit reduced bone mass through impaired osteoblast (OB) and chondrocyte (CH) maturations and increased osteoclastogenesis, while the opposite is observed in Mstn null mice that display enhanced bone mass. Mechanistically, Mstn deletion up-regulates Gdf11 expression, which activates bone morphogenetic protein (BMP) signaling pathway to enhance osteogenesis. Also, mice overexpressing follistatin (FST), a MSTN/GDF11 inhibitor, exhibit increased muscle mass accompanied by bone fractures, unlike Mstn null mice that display increased muscle mass without fractures, indicating that inhibition of GDF11 impairs bone strength. Together, our findings suggest that GDF11 promotes osteogenesis in contrast to MSTN, and these opposing roles of GDF11 and MSTN must be considered to avoid the detrimental effect of GDF11 inhibition when developing MSTN/GDF11 inhibitors for therapeutic purposes.

Neuron-Specific Vitamin D Signaling Attenuates Microglia Activation and CNS Autoimmunity

Low vitamin D during childhood is associated with an increased risk of developing multiple sclerosis (MS) as an adult. Given that vitamin D has anti-inflammatory properties, it has been postulated that the relationship between MS and low vitamin D is due to immune dysregulation. Since the vitamin D receptor (VDR) is expressed in many cell types, this study investigated an alternative hypothesis-neuron-specific VDR signaling induces anti-inflammatory molecules that protect the central nervous system from autoimmunity. Using media from neurons treated with calcitriol, the active form of vitamin D3, LPS-activated microglia had a reduction in pro-inflammatory molecules, and a reciprocal induction of anti-inflammatory molecules. Since IL-34 is critical to the homeostasis of microglia, and was previously shown to be induced in endothelial cells by vitamin D, we investigated IL-34 as the potential anti-inflammatory molecule induced in neurons by vitamin D. Treatment of LPS-activated microglia with IL-34 reduced pro-inflammatory cytokine production and enhanced the expression of anti-inflammatory transcripts. However, neutralizing IL-34 in vitamin D neuronal conditioned media only impacted IL-6 and not the broader anti-inflammatory phenotype of microglia. To mimic low vitamin D in children, we used a neuron-specific inducible mouse model in which VDR was partially deleted in juvenile mice. Partial deletion of VDR in neurons during early life resulted in exacerbated CNS autoimmunity in adult mice. Overall, the study illustrated that vitamin D signaling in neurons promotes an anti-inflammatory state in microglia, and low vitamin D in early life may enhance CNS autoimmunity.

Cytokines and Bone: Osteoimmunology

Cytokines and hematopoietic growth factors have traditionally been thought of as regulators of the development and function of immune and blood cells. However, an ever-expanding number of these factors have been discovered to have major effects on bone cells and the development of the skeleton in health and disease (Table 1). In addition, several cytokines have been directly linked to the development of osteoporosis in both animal models and in patients. In order to understand the mechanisms regulating bone cells and how this may be dysregulated in disease states, it is necessary to appreciate the diverse effects that cytokines and inflammation have on osteoblasts, osteoclasts, and bone mass. This chapter provides a broad overview of this topic with extensive references so that, if desired, readers can access specific references to delve into individual topics in greater detail.

Erythromyeloid progenitors give rise to a population of osteoclasts that contribute to bone homeostasis and repair

Osteoclasts are multinucleated cells of the monocyte/macrophage lineage that degrade bone. Here, we used lineage tracing studies-labelling cells expressing Cx3cr1, Csf1r or Flt3-to identify the precursors of osteoclasts in mice. We identified an erythromyeloid progenitor (EMP)-derived osteoclast precursor population. Yolk-sac macrophages of EMP origin produced neonatal osteoclasts that can create a space for postnatal bone marrow haematopoiesis. Furthermore, EMPs gave rise to long-lasting osteoclast precursors that contributed to postnatal bone remodelling in both physiological and pathological settings. Our single-cell RNA-sequencing data showed that EMP-derived osteoclast precursors arose independently of the haematopoietic stem cell (HSC) lineage and the data from fate tracking of EMP and HSC lineages indicated the possibility of cell-cell fusion between these two lineages. Cx3cr1+ yolk-sac macrophage descendants resided in the adult spleen, and parabiosis experiments showed that these cells migrated through the bloodstream to the remodelled bone after injury.

Elevated plasma interleukin 34 levels correlate with disease severity-reflecting parameters of patients with haemorrhagic fever with renal syndrome

Background: Haemorrhagic fever with renal syndrome (HFRS) is characterized by an uncontrolled cytokine storm that causes vascular leakage and kidney injury. The cytokine interleukin 34 (IL-34) enhances proliferation and differentiation of myeloid cells and secretion of pro-inflammatory cytokines, which is involved in the pathogenesis of some inflammatory and infectious diseases, including acute kidney injury. To date, however, the role of IL-34 in patients with HFRS is unclear. This study aims to determine the plasma IL-34 levels of HFRS patients and discuss the possible effects of IL-34 in the pathogenesis of HFRS. Methods: Plasma levels of IL-34 in 52 HFRS patients and 20 healthy controls were quantified using enzyme-linked immunosorbent assay. Results: Compared with healthy controls, the plasma IL-34 levels in HFRS patients were significantly elevated in acute phase [37.92 (0-215.45) pg/ml vs. 7.13 (0-19.44) pg/ml, p < .0001], and then decreased to the normal levels in convalescent phase. Importantly, IL-34 levels correlated positively with white blood cell counts and mononuclear cell counts (r = 0.592, p < .0001 and r = 0.458, p < .0001, respectively), and correlated negatively with platelet counts and serum albumin levels (r = -0.430, p < .0001 and r = -0.479, p = .0001, respectively). Conclusions: Plasma levels of IL-34 in HFRS patients were significantly elevated in acute phase and correlated with disease severity-reflecting parameters, which suggests a potential role of IL-34 in HFRS and should be future explored.

Immunomodulation of Interleukin-34 and its Potential Significance as a Disease Biomarker and Therapeutic Target

Interleukin (IL)-34 is a cytokine discovered a few years ago and identified as the second colony-stimulating factor (CSF)-1 receptor (CSF-1R) ligand. Although CSF-1 and IL-34 share the same receptor through which they trigger similar effects, IL-34 also binds to receptors protein-tyrosine phosphatase (PTP)-ζ and syndecan-1. Thus, IL-34 is involved in several signaling pathways and participates in a wide array of biological actions. This review analyzes current studies on the role of IL-34 under physiological and pathological conditions, and explores its potential significance as a disease biomarker and therapeutic target. In physiological conditions, IL-34 expression is restricted to the microglia and Langerhans cells, with a fundamental role in cellular differentiation, adhesion and migration, proliferation, metabolism, and survival. It is released in response to inflammatory stimuli, such as pathogen-associated molecular patterns or pro-inflammatory cytokines, with effects over various immune cells, including monocytes, macrophages, and regulatory T cells that shape the immune microenvironment. Over the past decade, accumulating evidence has suggested a potent immune regulation of IL-34 in pathological states such as autoimmune diseases, cancer, transplant rejection, neurologic diseases, infections, and inflammatory diseases. Importantly, IL-34 may hold great promise for acting as a biomarker for monitoring disease severity and progression, and may serve as a new therapeutic target for the treatment of several diseases in clinical settings.

Biological functions and clinical implications of interleukin-34 in inflammatory diseases

Interleukin (IL)-34 is a recently discovered cytokine and ligand of the colony-stimulating factor (CSF)-1 receptor. Although CSF-1 and IL-34 share similar biological properties, their expression patterns and downstream signaling pathways are distinct. IL-34 can influence differentiation and has functions in multiple cell types (e.g., dendritic cells, monocytes, macrophages). In the pathological conditions, IL-34 is induced by pro-inflammatory stimuli (e.g., cytokines, pathogen-associated molecular patterns, and infection). Current evidence shows that IL-34 is a critical player in inflammatory response and is involved in the pathogenesis of inflammatory autoimmune dysfunction. Therefore, IL-34 may be a promising clinical biomarker and therapeutic target for treating inflammatory related disorders. In this article, we review the advances in biological functions of IL-34 and our understanding of its role in the development of inflammatory diseases as well as therapeutic applications.

Histopathological, immunohistochemical, and biomechanical effects of splenectomy on Achilles tendon healing in rats

PURPOSE

This study aimed to assess Achilles tendon repair in rats following splenectomy to simulate patients with musculoskeletal system injury who had splenectomy after spleen injury, a situation often seen in orthopedics and traumatology practice.

MATERIALS AND METHODS

The study included 32 male Sprague-Dawley rats (10 months old; average weight, 394.5 ± 28.3 g). The rats were fed with standard rodent food ad libitum at 22°C in a dark environment for 12 h. They were divided into two groups, namely the splenectomy (total splenectomy and Achilles tendon repair) and control groups (only Achilles tendon repair; n = 16). Four weeks after the surgery, the rats were euthanized, and their Achilles tendons were examined histopathologically, immunohistochemically, and biomechanically.

RESULTS

In the splenectomy group, proinflammatory cytokines, such as interleukin-1β, tumor necrosis factor-α, and interferon-γ, showed significantly lower values than those in the control group (p ˂0.01); moreover, the levels of anti-inflammatory cytokines like vascular endothelial growth factor, transforming growth factor-β1, interleukin-2, interleukin-10, and hepatocyte growth factor were significantly higher than in the control group (p ˂ 0.001). The average ultimate tensile strengths were 2.58 ± 0.5 in the splenectomy and 2.78 ± 0.3 in the control group (p = 0.043). The average εUTS values were 0.33 ± 0.5 in the splenectomy and 0.44 ± 0.1 in the control group (p = 0.021).

CONCLUSION

Splenectomy may positively influence Achilles tendon healing through modification of the proinflammatory/anti-inflammatory ratio in favor of anti-inflammatory cytokines by causing a decrease in spleen-originated inflammatory cells.

Interleukin-34, a comprehensive review

IL-34 is a novel cytokine that was identified in 2008 in a comprehensive proteomic analysis as a tissue-specific ligand of CSF-1 receptor (CSF-1R). IL-34 exists in all vertebrates including fish, amphibians, birds, and mammals, showing high conservation among species. Structurally, IL-34 belongs to the short-chain helical hematopoietic cytokine family but shows no apparent consensus structural domains, motifs, or sequence homology with other cytokines. IL-34 is synthesized as a secreted homodimeric glycoprotein that binds to the extracellular domains of CSF-1R and receptor-type protein-tyrosine phosphatase-zeta (PTP-ζ) in addition to the chondroitin sulfate chains of syndecan-1. These interactions result in activating several signaling pathways that regulate major cellular functions, including proliferation, differentiation, survival, metabolism, and cytokine/chemokine expression in addition to cellular adhesion and migration. In the steady state, IL-34 contributes to the development and maintenance of specific myeloid cell subsets in a tissue-specific manner: Langerhans cells in the skin and microglia in the brain. In pathological conditions, changes in IL-34 expression-increased or decreased-are involved in disease pathogenesis and correlate with progression, severity, and chronicity. One decade after its discovery, IL-34 has been introduced as a newcomer to the big family of interleukins with specific physiological functions, critical pathological roles, and promising clinical applications in disease diagnosis and treatment. In this review, we celebrate the 10th anniversary of IL-34 discovery, introducing its biological characteristics, and discussing the importance of IL-34 signaling network in health and disease.

Mechanisms involved in bone resorption regulated by vitamin D

Active forms of vitamin D enhance osteoclastogenesis in vitro and in vivo through the vitamin D receptor (VDR) in osteoblast-lineage cells consisting of osteoblasts and osteocytes. This pro-resorptive activity was evident basically with higher concentrations of active vitamin D than those expected in physiological conditions. Nevertheless, vitamin D compounds have been used in Japan for treating osteoporosis to increase bone mineral density (BMD). Of note, the increase in BMD by long-term treatment with pharmacological (=near-physiological) doses of vitamin D compounds was caused by the suppression of bone resorption. Therefore, whether vitamin D expresses pro-resorptive or anti-resorptive properties seems to be dependent on the treatment protocols. We established osteoblast lineage-specific and osteoclast-specific VDR conditional knockout (cKO) mice using Osterix-Cre transgenic mice and Cathepsin K-Cre knock-in mice, respectively. According to our observation using these cKO mouse lines, neither VDR in osteoblast-lineage cells nor that in osteoclasts played important roles for osteoclastogenesis and bone resorption at homeostasis. However, using our cKO lines, we observed that VDR in osteoblast-lineage cells, but not osteoclasts, was involved in the anti-resorptive properties of pharmacological doses of vitamin D compounds in vivo. Two different osteoblast-lineage VDR cKO mouse lines were reported. One is a VDR cKO mouse line using alpha 1, type I collagen (Col1a1)-Cre transgenic mice (here we call Col1a1-VDR-cKO mice) and the other is that using dentin matrix protein 1 (Dmp1)-Cre transgenic mice (Dmp1-VDR-cKO mice). Col1a1-VDR-cKO mice exhibited slightly increased bone mass due to lowered bone resorption. In contrast, Dmp1-VDR-cKO mice exhibited no difference in BMD in agreement with our results regarding Ob-VDR-cKO mice. Here we discuss contradictory results and multiple modes of actions of vitamin D in bone resorption in detail. (279 words).

Vitamin D3 Modulates Impaired Crosstalk Between RANK and Glucocorticoid Receptor Signaling in Bone Marrow Cells After Chronic Prednisolone Administration

The effectiveness of vitamin D₃ (cholecalciferol) in counteracting the side effects of glucocorticoid (GC) therapy has been demonstrated previously. Abnormalities in systemic hormonal and local (cytokine) regulation of bone marrow (BM) cells may underlie GC-induced imbalance between osteosynthesis and bone resorption. The cytokine system receptor activator of nuclear factor kappa-B (RANK)/RANK ligand (RANKL)/osteoprotegerin (OPG) is considered as an integrating link in the NF-κB-mediated interaction of various cells involved in maintaining osteoblastic-osteoclastic balance, which makes it a pharmacological target for regulation and correction of the bone remodeling process. We studied GC-induced impairments of the RANKL/RANK/OPG axis in BM cells depending on vitamin D bioavailability and whether these changes were mediated by glucocorticoid (GR) and/or vitamin D (VDR) receptors. Female Wistar rats administered with prednisolone (5 mg/kg b.w., 30 days) showed a decrease in the GR protein level and the number of GR-positive BM cells. GC caused a marked elevation of RANKL and RANK levels in BM, while OPG decreased. Flow cytometry data indicated GC-elicited increase in the number of circulating RANK-positive osteoclast precursors (OCPs) in BM, peripheral blood, and spleen. In full accordance with the data that the interaction of RANKL-RANK leads to transcriptional activation of NF-κB and subsequent differentiation of osteoclasts, we found an increase in the level of phosphorylated p65 subunit of NF-κB with a simultaneous decrease in the NF-κB inhibitor (IκB) level. These changes were accompanied by vitamin D insufficiency and downregulated expression of CYP27B1 and VDR, which are responsible for synthesis and hormonal signaling of 1,25(OH)₂D. Notably, we observed VDR and RANK co-localization in OCPs. Cholecalciferol co-administration (1,000 IU/kg b.w., 30 days) with prednisolone resulted in elevated GR synthesis in BM. Cholecalciferol prevented prednisolone-elicited disturbances of the RANKL/RANK/OPG, which correlated with improved bioavailability and vitamin D signaling through VDR. This caused the lowering of phosphoNF-κB p65 level and inhibiting NF-κB translocation to the nucleus that could reduce the circulating OCPs pool in BM, peripheral blood, and spleen. Our findings suggest that prednisolone-induced abnormalities in GR and RANKL/RANK/OPG signaling pathways are associated with the impairments of vitamin D auto/paracrine system in BM cells and can be ameliorated by cholecalciferol supplementation.

Cytokines in the pathogenesis of rheumatoid arthritis: new players and therapeutic targets

In recent years, the landscape of pro- and anti-inflammatory cytokines has rapidly expanded with the identification of new members proven to be involved at different extent in the pathogenesis of chronic immune mediated inflammatory diseases including rheumatoid arthritis (RA). The advance of our understanding of mediators involved in the pathogenesis of RA and in consequence, the development of novel targeted therapies is necessary to provide patients not responding to currently available strategies with novel compounds. The aim of this review article is to provide an overview on recently identified cytokines, emphasizing their pathogenic role and therapeutic potential in RA. A systematic literature review was performed to retrieve articles related to every cytokine discussed in the review. In some cases, evidence from animal models and RA patients is already consistent to move forward into drug development. In others, conflicting observation and the paucity of data require further investigations.Forty years after the discovery of IL-1, the landscape of cytokines is continuously expanding with increasing possibilities to develop novel therapeutic strategies in RA.

Interleukin-34 Promotes Fibrocyte Proliferation

Rheumatoid arthritis (RA) is a systemic autoimmune disease affecting multiple joints. It remains unclear which factors in the circulation are associated with the systemic spread of the disease. Fibrocytes are pluripotent mesenchymal stem cells present in the circulation of RA patients. Our earlier findings implicated activated fibrocytes in the etiology of onset and pathogenesis of RA. Elevated levels of interleukin-34 (IL-34) in the serum and synovial fluid of RA patients are associated with rheumatoid factor and anticyclic citrullinated peptide antibodies, indicators of RA. Moreover, IL-34 levels are independent predictors of radiographic progression in RA patients. We provide evidence of simultaneous elevated levels of IL-34 and increased numbers of activated fibrocytes in the circulation of mice induced to develop arthritis. In vitro, IL-34 treatment induced the proliferation of fibrocytes, mediated by activation of cognate CSF-R1s on fibrocytes. Taken together, we infer that IL-34 has a role in stimulating fibrocyte proliferation and activation during arthritis, thereby contributing to both onset of RA and systemic spread of disease.

Sirtuin-3 Promotes Adipogenesis, Osteoclastogenesis, and Bone Loss in Aging Male Mice

Sirtuin-3 (Sirt3) is an essential metabolic regulatory enzyme that plays an important role in mitochondrial metabolism, but its role in bone marrow and skeletal homeostasis remains largely unknown. In this study, we hypothesize that increased expression of Sirt3 plays a role in skeletal aging. Using mice that overexpress Sirt3 [i.e., Sirt3 transgenic (Sirt3Tg)], we show that Sirt3 is a positive regulator of adipogenesis and osteoclastogenesis and a negative regulator of skeletal homeostasis. Sirt3Tg mice exhibited more adipocytes in the tibia compared with control mice. Bone marrow stromal cells (BMSCs) from Sirt3Tg mice displayed an enhanced ability to differentiate into adipocytes compared with control BMSCs. We found a 2.5-fold increase in the number of osteoclasts on the bone surface in Sirt3Tg mice compared with control mice (P < 0.03), and increased osteoclastogenesis in vitro. Importantly, Sirt3 activates the mechanistic target of rapamycin (mTOR) pathway to regulate osteoclastogenesis. Sirt3Tg male mice exhibited a significant reduction in cortical thickness at the tibiofibular junction (P < 0.05). In summary, Sirt3 activity in bone marrow cells is associated with increased adipogenesis, increased osteoclastogenesis through activation of mTOR signaling, and reduced bone mass. Interestingly, Sirt3 expression in bone marrow cells increases during aging, suggesting that Sirt3 promotes age-related adipogenesis and osteoclastogenesis associated with bone loss. These findings identify Sirt3 as an important regulator of adipogenesis and skeletal homeostasis in vivo and identify Sirt3 as a potential target for the treatment of osteoporosis.

1α,25-Dihydroxyvitamin D3 up-regulates IL-34 expression in SH-SY5Y neural cells

Vitamin D supplementation is regarded as a novel approach to treat Alzheimer’s disease, but the underlying mechanism remains elusive. The cytokine IL-34 provides strong neuroprotective and survival signals in brain injury and neurodegeneration and could be an immunological mediator for the vitamin D-induced protection. The aim of this study was to investigate whether human IL-34 is up-regulated in neuronal cells by the hormonally active form of vitamin D, 1α,25-dihydroxyvitamin D3 [1α,25(OH)2D3]. We found that IL-34 was detectable in a variety of cell lines and its expression was strongly induced in SH-SY5Y neural cells in a dose- and time-dependent manner by 1α,25(OH)2D3 through the vitamin D receptor (VDR). Furthermore, we identified the core promoter of IL-34 gene and a VDR binding site (CGCCCT) that was required for 1α,25(OH)2D3-induced IL-34 expression. These findings suggest that the induction of IL-34 expression by 1α,25(OH)2D3 may constitute a mechanism that explains the protective function of vitamin D in Alzheimer’s disease.

VDR in Osteoblast-Lineage Cells Primarily Mediates Vitamin D Treatment-Induced Increase in Bone Mass by Suppressing Bone Resorption

Long-term treatment with active vitamin D [1α,25(OH)₂ D₃ ] and its derivatives is effective for increasing bone mass in patients with primary and secondary osteoporosis. Derivatives of 1α,25(OH)₂ D₃ , including eldecalcitol (ELD), exert their actions through the vitamin D receptor (VDR). ELD is more resistant to metabolic degradation than 1α,25(OH)₂ D₃ . It is reported that ELD treatment causes a net increase in bone mass by suppressing bone resorption rather than by increasing bone formation in animals and humans. VDR in bone and extraskeletal tissues regulates bone mass and secretion of osteotropic hormones. Therefore, it is unclear what types of cells expressing VDR preferentially regulate the vitamin D-induced increase in bone mass. Here, we examined the effects of 4-week treatment with ELD (50 ng/kg/day) on bone using osteoblast lineage-specific VDR conditional knockout (Ob-VDR-cKO) and osteoclast-specific VDR cKO (Ocl-VDR-cKO) male mice aged 10 weeks. Immunohistochemically, VDR in bone was detected preferentially in osteoblasts and osteocytes. Ob-VDR-cKO mice showed normal bone phenotypes, despite no appreciable immunostaining of VDR in bone. Ob-VDR-cKO mice failed to increase bone mass in response to ELD treatment. Ocl-VDR-cKO mice also exhibited normal bone phenotypes, but normally responded to ELD. ELD-induced FGF23 production in bone was regulated by VDR in osteoblast-lineage cells. These findings suggest that the vitamin D treatment-induced increase in bone mass is mediated by suppressing bone resorption through VDR in osteoblast-lineage cells. © 2017 The Authors. Journal of Bone and Mineral Research Published by Wiley Periodicals Inc.

LPS administration increases CD11b+ c-Fms+ CD14+ cell population that possesses osteoclast differentiation potential in mice

Osteoclasts are multinucleated giant cells that originate from a monocyte/macrophage lineage, and are involved in the inflammatory bone destruction accompanied by periodontitis. Recent studies have shown that osteoclast precursors reside not only in the bone marrow, but also in the peripheral blood and spleen, though the precise characteristics of each precursor have not been analyzed. We hypothesized that the number of osteoclast precursors in those tissues may increase under pathological conditions and contribute to osteoclast formation in vivo in a mouse model. To test this hypothesis, we attempted to identify cell populations that possess osteoclast differentiation potential in the bone marrow, spleen, and blood by analyzing macrophage/monocyte-related cell surface markers such as CD11b, CD14, and colony-stimulating factor-1 receptor (c-Fms). In the bone marrow, the CD11b^- cell population, but not the CD11b⁺ cell population, differentiated into osteoclasts in the presence of receptor activator of nuclear factor-κB ligand and macrophage colony-stimulating factor. On the other hand, in the spleen and blood, CD11b⁺ cells differentiated into osteoclasts. Interestingly, lipopolysaccharide (LPS) administration to the mice dramatically increased the proportion of CD11b⁺ c-Fms⁺ CD14⁺ cells, which differentiated into osteoclasts, in the bone marrow and spleen. These results suggest that LPS administration increases the proportion of a distinct cell population expressing CD11b⁺, c-Fms⁺, and CD14⁺ in the bone marrow and spleen. Thus, these cell populations are considered to contribute to the increase in osteoclast number during inflammatory bone destruction such as periodontitis.

Role of splenic reservoir monocytes in pulmonary vascular monocyte accumulation in experimental hepatopulmonary syndrome

BACKGROUND AND AIM

Pulmonary monocyte infiltration plays a significant role in the development of angiogenesis in experimental hepatopulmonary syndrome (HPS) after common bile duct ligation (CBDL). Hepatic monocytes are also increased after CBDL, but the origins remain unclear. Splenic reservoir monocytes have been identified as a major source of monocytes that accumulate in injured tissues. Whether splenic monocytes contribute to monocyte alterations after CBDL is unknown. This study evaluates monocyte distributions and assesses effects of splenectomy on monocyte levels and pulmonary vascular and hepatic abnormalities in experimental HPS.

METHODS

Splenectomy was performed in CBDL animals. Monocyte levels in different tissues and circulation were assessed with CD68. Pulmonary alterations of HPS were evaluated with vascular endothelial growth factor-A (VEGF-A) levels, angiogenesis, and alveolar-arterial oxygen gradient (AaPO₂ ). Liver abnormalities were evaluated with fibrosis (Sirius red), bile duct proliferation (CK-19), and enzymatic changes.

RESULTS

Monocyte levels increased in the lung and liver after CBDL and were accompanied by elevated circulating monocyte numbers. Splenectomy significantly decreased monocyte accumulation, VEGF-A levels, and angiogenesis in CBDL animal lung and improved AaPO₂ levels. In contrast, hepatic monocyte levels, fibrosis, and functional abnormalities were further exacerbated by spleen removal.

CONCLUSIONS

Splenic reservoir monocytes are a major source for lung monocyte accumulation after CBDL, and spleen removal attenuates the development of experimental HPS. Liver monocytes may have different origins, and accumulation is exacerbated after depletion of splenic reservoir monocytes. Tissue specific monocyte alterations, influenced by the spleen reservoir, have a significant impact on pulmonary complications of liver disease.

Functions of interleukin-34 and its emerging association with rheumatoid arthritis

Rheumatoid arthritis (RA) is a systemic autoimmune disease characterized by chronic, synovial inflammation affecting multiple joints, finally leading to extra-articular lesions for which limited effective treatment options are currently available. Interleukin-34 (IL-34), recently discovered as the second colony-stimulating factor-1 receptor (CSF-1R) ligand, is a newly discovered cytokine. Accumulating evidence has disclosed crucial roles of IL-34 in the proliferation and differentiation of mononuclear phagocyte lineage cells, osteoclastogenesis and inflammation. Recently, IL-34 was detected at high levels in patients with active RA and in experimental models of inflammatory arthritis. Blockade of functional IL-34 with a specific monoclonal antibody can reduce the severity of inflammatory arthritis, suggesting that targeting IL-34 or its receptors may constitute a novel therapeutic strategy for autoimmune diseases such as RA. Here, we have comprehensively discussed the structure and biological functions of IL-34, and reviewed recent advances in our understanding of the emerging role of IL-34 in the development of RA as well as its potential utility as a therapeutic target.

Of skin and bone: did Langerhans cells and osteoclasts evolve from a common ancestor?

Skin Langerhans cells are antigen-presenting cells of the interfollicular epidermis and the upper part of the hair follicle, whereas osteoclasts are specialized bone-resorbing macrophages. Although at first view these two cell types appear to have little in common, a closer analysis reveals shared features, and when taking into account their surrounding environment, a hypothesis can be developed that Langerhans cells and osteoclasts have evolved from a common ancestral cell type. In this mini-review, we have compared the ontogenetic features of Langerhans cells and osteoclasts from a genetic and a functional point of view, an issue that so far has been overlooked. The gene programs that control cell differentiation, and the body parts where they reside, present surprising similarities. Whereas the function of osteoclasts in bone degradation has been established since the first vertebrates, Langerhans cells may have undergone a stepwise adaptation from aquatic to terrestrial life. Their cell function co-evolved with the imperatives of the skin to protect against physical impact, heat, water loss and pathogens, which implied the capacity of Langerhans cells to associate with skin appendages and to develop immunostimulatory functions. For the highly versatile and efficient immune system of modern vertebrates, Langerhans cells may be a memory of the past.

Differential Responses to Vitamin D2 and Vitamin D3 Are Associated With Variations in Free 25-Hydroxyvitamin D

25-Hydroxyvitamin D (25D) circulates bound primarily to serum vitamin D binding protein (DBP), with DBP showing higher binding affinity for 25D3 than 25D2. We therefore hypothesized that vitamin D2 (D2) promotes higher serum levels of unbound 25D (free 25D), with different functional responses, relative to vitamin D3 (D3). Week 3 C56BL/6 mice were placed on diets containing either D2 or D3 alone (both 1000 IU/kg). At week 8 and week 16, D2 mice had only 25D2 in circulation (26.6 ± 1.9 and 33.3 ± 4.4 ng/mL), and D3 mice had only 25D3 (28.3 ± 2.0 and 31.7 ± 2.1 ng/mL). At week 8 (44.5 ± 6.4 vs 62.4 ± 11.6 pg/mL, P < .05) and week 16 (78.4 ± 12.6 vs 95.5 ± 11.6), D2 mice had lower serum 1,25-dihydroxyvitamin D relative to D3 mice. By contrast, measured free 25D was significantly higher in D2 mice at week 8 (16.8 ± 0.65 vs 8.4 ± 0.63 pg/mL, P < .001) and week 16 (17.4 ± 0.43 vs 8.4 ± 0.44, P < .001). A two-way ANOVA of bone histomorphometry showed that week 8 D2 mice had significantly higher osteoclast surface/bone surface, eroded surface/bone surface, and mineral apposition rate compared with D3 mice. Osteoblast surface/bone surface was higher in week 8 D2 females but not week 8 D2 males. At week 16, D2 mice had significantly higher bone volume/total volume and trabecular number compared with D3 mice. Differences in bone phenotype were observed despite D2 mice reaching similar serum 25D levels and lower 1,25D levels compared with D3 mice. These data indicate that 25D2 binds less well to DBP than 25D3, with resulting higher levels of free 25D promoting differential effects on bone in mice exposed to D2 alone.

Regulation and Biological Significance of Formation of Osteoclasts and Foreign Body Giant Cells in an Extraskeletal Implantation Model

The implantation of biomaterials induces a granulomatous reaction accompanied by foreign body giant cells (FBGCs). The characterization of multinucleated giant cells (MNGCs) around bone substitutes implanted in bone defects is more complicated because of healing with bone admixed with residual bone substitutes and their hybrid, and the appearance of two kinds of MNGCs, osteoclasts and FBGCs. Furthermore, the clinical significance of osteoclasts and FBGCs in the healing of implanted regions remains unclear. The aim of the present study was to characterize MNGCs around bone substitutes using an extraskeletal implantation model and evaluate the clinical significance of osteoclasts and FBGCs. Beta-tricalcium phosphate (β-TCP) granules were implanted into rat subcutaneous tissue with or without bone marrow mesenchymal cells (BMMCs), which include osteogenic progenitor cells. We also compared the biological significance of plasma and purified fibrin, which were used as binders for implants. Twelve weeks after implantation, osteogenesis was only detected in specimens implanted with BMMCs. The expression of two typical osteoclast markers, tartrate-resistant acid phosphatase (TRAP) and cathepsin-K (CTSK), was analyzed, and TRAP-positive and CTSK-positive osteoclasts were only detected beside bone. In contrast, most of the MNGCs in specimens without the implantation of BMMCs were FBGCs that were negative for TRAP, whereas the degradation of β-TCP was detected. In the region implanted with β-TCP granules with plasma, FBGCs tested positive for CTSK, and when β-TCP granules were implanted with purified fibrin, FBGCs tested negative for CTSK. These results showed that osteogenesis was essential to osteoclastogenesis, two kinds of FBGCs, CTSK-positive and CTSK-negative, were induced, and the expression of CTSK was plasma-dependent. In addition, the implantation of BMMCs was suggested to contribute to osteogenesis and the replacement of implanted β-TCP granules to bone.

Stromal cell-derived factor-1 mediates changes of bone marrow stem cells during the bone repair process

Osteoblasts, osteoclasts, chondrocytes, and macrophages that participate in the bone repair process are derived from hematopoietic stem cells (HSCs) and mesenchymal stem cells (MSCs). However, the roles of these stem cells during the repair of injured bone tissue are still unclear. In the present study, we examined the effects of bone defect on HSCs and MSCs in bone marrow and spleen in 75 mice and its mechanism. We analyzed the HSC and MSC populations in these tissues of a mouse with femoral bone damage by using flow cytometry. The number of HSCs in the bone marrow of mice with damaged femurs was significantly lower than the number of these cells in the bone marrow of the contralateral intact femurs on day 2 after injury. Meanwhile, the number of MSCs in the bone marrow of mice with damaged femurs was significantly higher than that of the contralateral femurs. Both intraperitoneal administration of AMD3100, a C-X-C chemokine receptor 4 (CXCR4) antagonist, and local treatment with an anti-stromal cell-derived factor-1 (SDF-1) antibody blunted the observed decrease in HSC and increase in MSC populations within the bone marrow of injured femurs. In conclusion, the present study revealed that there is a concurrent decrease and increase in the numbers of HSCs and MSCs, respectively, in the bone marrow during repair of mouse femoral bone damage. Furthermore, the SDF-1/CXCR4 system was implicated as contributing to the changes in these stem cell populations upon bone injury.

Enhancement of osteogenesis post-splenectomy does not attenuate bone loss in ovariectomized rats

The roles of different immune cell populations and cytokines in bone metabolism have been extensively investigated. However, the influence of whole immune organ removal on osteopathology remains unknown. In the current study, we investigated the effects of splenectomy on bone metabolism and microarchitecture in rats with or without concurrent ovariectomy. Ovariectomized (OVX) rats were used as osteoporosis model. Sixty 12-week-old female rats were randomized into 4 groups (n = 15): sham, splenectomized (SP), ovariectomized, as well as ovariectomized and splenectomized (OVX + SP). Bone microarchitecture was assessed by micro CT analysis at 4 week and 12 week post-operation, respectively. Bone pathology and metabolism were evaluated via immunohistochemical staining. The serum levels of alkaline phosphatase (ALP), tumor necrosis factor-alpha (TNF-α), tartrate-resistant acid phosphatase 5b (Tracp5b), and C-terminal telopeptide (CTx) were analyzed at 4 and 12 weeks post-operation. Removal of the spleen led to alterations in the homeostasis of bone metabolism and increased bone formation in rats. In this study, our findings indicate that the spleen is involved in skeletal metabolism.

Interleukin-34 restores blood-brain barrier integrity by upregulating tight junction proteins in endothelial cells

Interleukin-34 (IL-34) is a newly discovered cytokine as an additional ligand for colony stimulating factor-1 receptor (CSF1R), and its functions are expected to overlap with colony stimulating factor-1/macrophage-colony stimulating factor. We have previously shown that the IL-34 is primarily produced by neurons in the central nervous system (CNS) and induces proliferation and neuroprotective properties of microglia which express CSF1R. However, the functions of IL-34 in the CNS are still elucidative. Here we show that CNS capillary endothelial cells also express CSF1R. IL-34 protected blood–brain barrier integrity by restored expression levels of tight junction proteins, which were downregulated by pro-inflammatory cytokines. The novel function of IL-34 on the blood–brain barrier may give us a clue for new therapeutic strategies in neuroinflammatory and neurodegenerative diseases such as multiple sclerosis and Alzheimer's disease.

Roles of osteoclasts in the control of medullary hematopoietic niches

Bone marrow is the major site of hematopoiesis in mammals. The bone marrow environment plays an essential role in the regulation of hematopoietic stem and progenitor cells by providing specialized niches in which these cells are maintained. Many cell types participate to the composition and regulation of hematopoietic stem cell (HSC) niches, integrating complex signals from the bone, immune and nervous systems. Among these cells, the bone-resorbing osteoclasts (OCLs) have been described as main regulators of HSC niches. They are not limited to carving space for HSCs, but they also provide signals that affect the molecular and cellular niche components. However, their exact role in HSC niches remains unclear because of the variety of models, signals and conditions used to address the question. The present review will discuss the importance of the implication of OCLs focusing on the formation of HSC niches, the maintenance of HSCs in these niches and the mobilization of HSCs from the bone marrow. It will underline the importance of OCLs in HSC niches.

Targeting IL-34 in chronic inflammation

A second ligand for colony-stimulating factor-1 receptor (CSF-1R) with distinct biologic activities had long been implicated but not appreciated until the recent discovery of interleukin (IL)-34. IL-34 and CSF-1 signal through this common receptor to mediate the biology of mononuclear phagocytic cells. Aberrant macrophage activation by CSF-1 and/or IL-34 is associated with numerous diseases, and clinical therapies targeting this pathway are being tested. Although IL-34 and CSF-1 have distinct activities under physiologic conditions, they appear functionally redundant in various disease states. Thus, blocking the activity of both might be necessary for maximal efficacy.

Interkeukin-34, a cytokine crucial for the differentiation and maintenance of tissue resident macrophages and Langerhans cells

IL-34 is a recently discovered cytokine that acts on tissue resident macrophages and Langerhans cells upon binding the receptor for CSF-1, CSF-1R. The existence of two ligands for CSF-1R, IL-34, and CSF-1, raises several intriguing questions. Are IL-34 and CSF-1 redundant or does each perform temporally and spatially distinct functions? Is IL-34 involved in human pathology? Would therapeutic strategies based on selective inhibition or administration of either IL-34 or CSF-1 be advantageous for preventing human pathology? Recent in vivo studies indicate that IL-34 promotes the development, survival, and function of microglia and Langerhans cells; therefore, this cytokine may predominately function in brain and skin biology. Here, we review the evidence for IL-34 as a key cytokine in the development and function of these two diverse cell types and discuss its potential role in pathological conditions.

Vitamin D endocrine system and osteoclasts

Vitamin D was discovered as an anti-rachitic agent preventing a failure in bone mineralization, but it is now established that the active form of vitamin D3 (1α,25(OH)2D3) induces bone resorption. Discovery of the receptor activator of nuclear factor -κB ligand (RANKL) uncovered the molecular mechanism by which 1α,25(OH)2D3 stimulates bone resorption. Treating osteoblastic cells with 1α,25(OH)2D3 stimulates RANKL expression, which in turn induces osteoclastogenesis. Nevertheless, active vitamin D compounds such as calcitriol (1α,25(OH)2D3), alfacalcidol (1α(OH)D3) and eldecalcitol (1α,25-dihydroxy-2β-(3-hydroxypropoxy) vitamin D3) have been used as therapeutic drugs for osteoporosis, as they increase bone mineral density (BMD) in osteoporotic patients. Paradoxically, the increase in BMD is caused by the suppression of bone resorption. Several studies have been performed to elucidate the mechanism by which active vitamin D compounds suppress bone resorption in vivo. Our study showed that daily administration of eldecalcitol to mice suppressed neither the number of osteoclast precursors in the bone marrow nor the number of osteoclasts formed in ex vivo cultures. Eldecalcitol administration suppressed RANKL expression in osteoblasts. This review discusses how the difference between in vitro and in vivo effects of active vitamin D compounds on bone resorption is induced.

CSF-1 receptor-mediated differentiation of a new type of monocytic cell with B cell-stimulating activity: its selective dependence on IL-34

With the use of a mouse FDC line, FL-Y, we have been analyzing roles for FDCs in controlling B cell fate in GCs. Beside these regulatory functions, we fortuitously found that FL-Y cells induced a new type of CD11b⁺ monocytic cells (F4/80⁺, Gr-1⁻, Ly6C⁻, I-A/E(-/lo), CD11c⁻, CD115⁺, CXCR4⁺, CCR2⁺, CX₃CR1⁻) when cultured with a Lin⁻c-kit⁺ population from mouse spleen cells. The developed CD11b⁺ cells shared a similar gene-expression profile to mononuclear phagocytes and were designated as FDMCs. Here, we describe characteristic immunological functions and the induction mechanism of FDMCs. Proliferation of anti-CD40 antibody-stimulated B cells was markedly accelerated in the presence of FDMCs. In addition, the FDMC-activated B cells efficiently acquired GC B cell-associated markers (Fas and GL-7). We observed an increase of FDMC-like cells in mice after immunization. On the other hand, FL-Y cells were found to produce CSF-1 as well as IL-34, both of which are known to induce development of macrophages and monocytes by binding to the common receptor, CSF-1R, expressed on the progenitors. However, we show that FL-Y-derived IL-34, but not CSF-1, was selectively responsible for FDMC generation using neutralizing antibodies and RNAi. We also confirmed that FDMC generation was strictly dependent on CSF-1R. To our knowledge, a CSF-1R-mediated differentiation process that is intrinsically specific for IL-34 has not been reported. Our results provide new insights into understanding the diversity of IL-34 and CSF-1 signaling pathways through CSF-1R.