Ageing-related bone and immunity changes: insights into the complex interplay between the skeleton and the immune system.

Ageing as a natural irreversible process inherently results in the functional deterioration of numerous organ systems and tissues, including the skeletal and immune systems. Recent studies have elucidated the intricate bidirectional interactions between these two systems. In this review, we provide a comprehensive synthesis of molecular mechanisms of cell ageing. We further discuss how age-related skeletal changes influence the immune system and the consequent impact of immune system alterations on the skeletal system. Finally, we highlight the clinical implications of these findings and propose potential strategies to promote healthy ageing and reduce pathologic deterioration of both the skeletal and immune systems.

Obesity induces osteoimmunology imbalance: Molecular mechanisms and clinical implications.

The notion that obesity can be a protective factor for bone health is a topic of ongoing debate. Increased body weight may have a positive impact on bone health due to its mechanical effects and the production of estrogen by adipose tissue. However, recent studies have found a higher risk of bone fracture and delayed bone healing in elderly obese patients, which may be attributed to the heightened risk of bone immune regulation disruption associated with obesity. The balanced functions of bone cells such as osteoclasts, osteoblasts, and osteocytes, would be subverted by aberrant and prolonged immune responses under obese conditions. This review aims to explore the intricate relationship between obesity and bone health from the perspective of osteoimmunology, elucidate the impact of disturbances in bone immune regulation on the functioning of bone cells, including osteoclasts, osteoblasts, and osteocytes, highlighting the deleterious effects of obesity on various diseases development such as rheumatoid arthritis (RA), osteoarthritis (AS), bone fracture, periodontitis. On the one hand, weight loss may achieve significant therapeutic effects on the aforementioned diseases. On the other hand, for patients who have difficulty in losing weight, the osteoimmunological therapies could potentially serve as a viable approach in halting the progression of these disease. Additional research in the field of osteoimmunology is necessary to ascertain the optimal equilibrium between body weight and bone health.

The complex association between the immune system and the skeletal system in osteoporosis: A study of single-cell RNA sequencing.

OBJECTIVE: Osteoporosis (OP) is a disease characterized by decreased bone mass, deteriorated microstructure, and increased fragility and fracture risk. The diagnosis and prevention of OP and its complications have become major public health challenges. Therefore, exploring the complex ecological connections between the immune and skeletal systems may provide new insights for clinical prevention and treatment strategies. METHODS: First, we performed single-cell RNA sequencing on human lumbar lamina tissue and conducted clustering and subgroup analysis of quality-controlled single-cell transcriptome data to identify target subgroups. Subsequently, enrichment analysis and pseudotime analysis were performed. In addition, we conducted in-depth studies on the gene regulatory network between different cell subgroups and the communication between bone immune cells. RESULTS: In this study, we identified several cell subgroups that may be involved in the progression of OP. For example, the CCL4+ NKT and CXCL8+ neutrophils subgroups promote OP progression by mediating an inflammatory environment that disrupts bone homeostasis, and the MNDA+ Mac subgroup promotes osteoclast differentiation to promote OP. Moreover, the TNFAIP6+ Obl, NR4A2+ B and HMGN2+ erythrocyte subgroups promoted the balance of bone metabolism and suppressed OP. In the cell communication network, Obl closely interacts with immune cell subgroups through the CXCR4-CXCL12, CTGF-ITGB2, and TNFSF14-TNFRSF14 axes. CONCLUSION: Our research revealed specific subgroups and intercellular interactions that play crucial roles in the pathogenesis of OP, providing potential new insights for more precise therapeutic interventions for OP.

Three-gene signature revealing the dynamics of lymphocyte infiltration in subchondral bone during osteoarthritis progression.

Osteoarthritis (OA), a degenerative joint disorder, has an unclear immune infiltration mechanism in subchondral bone (SCB). Thus, this study aims to discern immune infiltration variations in SCB between early- and late-stages of OA and identify pertinent biomarkers. Utilizing the GSE515188 bulk-seq profile from the Gene Expression Omnibus database, we performed single-sample gene-set enrichment analysis alongside weighted gene co-expression network analysis to identify key cells and immune-related genes (IRGs) involved in SCB at both stages. At the meanwhile, differentially expressed genes (DEGs) were identified in the same dataset and intersected with IRGs to find IR-DEGs. Protein-protein interaction network and enrichment analyses and further gene filtering using LASSO regression led to the discovery of potential biomarkers, which were then validated by ROC curve analysis, single-cell RNA sequencing, qRT-PCR, western blot and immunofluorescence. ScRNA-seq analysis using GSE196678, qRT-PCR, western blot and immunofluorescence results confirmed the upregulation of their expression levels in early-stage OA SCB samples. Our comprehensive analysis revealed lymphocytes infiltration as a major feature in early OA SCB. A total of 13 IR-DEGs were identified, showing significant enrichment in T- or B-cell activation pathways. Three of them (CD247, POU2AF1, and TNFRSF13B) were selected via the LASSO regression analysis, and results from the ROC curve analyses indicated the diagnostic efficacy of these 3 genes as biomarkers. These findings may aid in investigating the mechanisms of SCB immune infiltration in OA, stratifying OA progression, and identifying relevant therapeutic targets.

Osteoporosis: interferon-gamma-mediated bone remodeling in osteoimmunology.

As the world population ages, osteoporosis, the most common disease of bone metabolism, affects more than 200 million people worldwide. The etiology is an imbalance in bone remodeling process resulting in more significant bone resorption than bone remodeling. With the advent of the osteoimmunology field, the immune system's role in skeletal pathologies is gradually being discovered. The cytokine interferon-gamma (IFN-γ), a member of the interferon family, is an important factor in the etiology and treatment of osteoporosis because it mediates bone remodeling. This review starts with bone remodeling process and includes the cellular and key signaling pathways of bone remodeling. The effects of IFN-γ on osteoblasts, osteoclasts, and bone mass are discussed separately, while the overall effects of IFN-γ on primary and secondary osteoporosis are summarized. The net effect of IFN-γ on bone appears to be highly dependent on the environment, dose, concentration, and stage of cellular differentiation. This review focuses on the mechanisms of bone remodeling and bone immunology, with a comprehensive discussion of the relationship between IFN-γ and osteoporosis. Finding the paradoxical balance of IFN-γ in bone immunology and exploring the potential of its clinical application provide new ideas for the clinical treatment of osteoporosis and drug development.

Interplay Between Skeletal and Hematopoietic Cells in the Bone Marrow Microenvironment in Homeostasis and Aging.

PURPOSE OF THE REVIEW: In this review, we discuss the most recent scientific advances on the reciprocal regulatory interactions between the skeletal and hematopoietic stem cell niche, focusing on immunomodulation and its interplay with the cell's mitochondrial function, and how this impacts osteoimmune health during aging and disease. RECENT FINDINGS: Osteoimmunology investigates interactions between cells that make up the skeletal stem cell niche and immune system. Much work has investigated the complexity of the bone marrow microenvironment with respect to the skeletal and hematopoietic stem cells that regulate skeletal formation and immune health respectively. It has now become clear that these cellular components cooperate to maintain homeostasis and that dysfunction in their interaction can lead to aging and disease. Having a deeper, mechanistic appreciation for osteoimmune regulation will lead to better research perspective and therapeutics with the potential to improve the aging process, skeletal and hematologic regeneration, and disease targeting.

The immune cells in modulating osteoclast formation and bone metabolism.

Osteoclasts are pivotal in regulating bone metabolism, with immune cells significantly influencing both physiological and pathological processes by modulating osteoclast functions. This is particularly evident in conditions of inflammatory bone resorption, such as rheumatoid arthritis and periodontitis. This review summarizes and comprehensively analyzes the research progress on the regulation of osteoclast formation by immune cells, aiming to unveil the underlying mechanisms and pathways through which diseases, such as rheumatoid arthritis and periodontitis, impact bone metabolism.

Excess glucocorticoids inhibit murine bone turnover via modulating the immunometabolism of the skeletal microenvironment.

Elevated bone resorption and diminished bone formation have been recognized as the primary features of glucocorticoid-associated skeletal disorders. However, the direct effects of excess glucocorticoids on bone turnover remain unclear. Here, we explored the outcomes of exogenous glucocorticoid treatment on bone loss and delayed fracture healing in mice and found that reduced bone turnover was a dominant feature, resulting in a net loss of bone mass. The primary effect of glucocorticoids on osteogenic differentiation was not inhibitory; instead, they cooperated with macrophages to facilitate osteogenesis. Impaired local nutrient status - notably, obstructed fatty acid transportation - was a key factor contributing to glucocorticoid-induced impairment of bone turnover in vivo. Furthermore, fatty acid oxidation in macrophages fueled the ability of glucocorticoid-liganded receptors to enter the nucleus and then promoted the expression of BMP2, a key cytokine that facilitates osteogenesis. Metabolic reprogramming by localized fatty acid delivery partly rescued glucocorticoid-induced pathology by restoring a healthier immune-metabolic milieu. These data provide insights into the multifactorial metabolic mechanisms by which glucocorticoids generate skeletal disorders, thus suggesting possible therapeutic avenues.

The bone-liver interaction modulates immune and hematopoietic function through Pinch-Cxcl12-Mbl2 pathway.

Mesenchymal stromal cells (MSCs) are used to treat infectious and immune diseases and disorders; however, its mechanism(s) remain incompletely defined. Here we find that bone marrow stromal cells (BMSCs) lacking Pinch1/2 proteins display dramatically reduced ability to suppress lipopolysaccharide (LPS)-induced acute lung injury and dextran sulfate sodium (DSS)-induced inflammatory bowel disease in mice. Prx1-Cre; Pinch1f/f; Pinch2-/- transgenic mice have severe defects in both immune and hematopoietic functions, resulting in premature death, which can be restored by intravenous injection of wild-type BMSCs. Single cell sequencing analyses reveal dramatic alterations in subpopulations of the BMSCs in Pinch mutant mice. Pinch loss in Prx1+ cells blocks differentiation and maturation of hematopoietic cells in the bone marrow and increases production of pro-inflammatory cytokines TNF-α and IL-1ß in monocytes. We find that Pinch is critical for expression of Cxcl12 in BMSCs; reduced production of Cxcl12 protein from Pinch-deficient BMSCs reduces expression of the Mbl2 complement in hepatocytes, thus impairing the innate immunity and thereby contributing to infection and death. Administration of recombinant Mbl2 protein restores the lethality induced by Pinch loss in mice. Collectively, we demonstrate that the novel Pinch-Cxcl12-Mbl2 signaling pathway promotes the interactions between bone and liver to modulate immunity and hematopoiesis and may provide a useful therapeutic target for immune and infectious diseases.

How does Hashimoto's thyroiditis affect bone metabolism?

Bone marrow contains resident cellular components that are not only involved in bone maintenance but also regulate hematopoiesis and immune responses. The immune system and bone interact with each other, coined osteoimmunology. Hashimoto's thyroiditis (HT) is one of the most common chronic autoimmune diseases which is accompanied by lymphocytic infiltration. It shows elevating thyroid autoantibody levels at an early stage and progresses to thyroid dysfunction ultimately. Different effects exert on bone metabolism during different phases of HT. In this review, we summarized the mechanisms of the long-term effects of HT on bone and the relationship between thyroid autoimmunity and osteoimmunology. For patients with HT, the bone is affected not only by thyroid function and the value of TSH, but also by the setting of the autoimmune background. The autoimmune background implies a breakdown of the mechanisms that control self-reactive system, featuring abnormal immune activation and presence of autoantibodies. The etiology of thyroid autoimmunity and osteoimmunology is complex and involves a number of immune cells, cytokines and chemokines, which regulate the pathogenesis of HT and osteoporosis at the same time, and have potential to affect each other. In addition, vitamin D works as a potent immunomodulator to influence both thyroid immunity and osteoimmunology. We conclude that HT affects bone metabolism at least through endocrine and immune pathways.

Inflammatory Arthritis and Bone Metabolism Regulated by Type 2 Innate and Adaptive Immunity.

While type 2 immunity has traditionally been associated with the control of parasitic infections and allergic reactions, increasing evidence suggests that type 2 immunity exerts regulatory functions on inflammatory diseases such as arthritis, and also on bone homeostasis. This review summarizes the current evidence of the regulatory role of type 2 immunity in arthritis and bone. Key type 2 cytokines, like interleukin (IL)-4 and IL-13, but also others such as IL-5, IL-9, IL-25, and IL-33, exert regulatory properties on arthritis, dampening inflammation and inducing resolution of joint swelling. Furthermore, these cytokines share anti-osteoclastogenic properties and thereby reduce bone resorption and protect bone. Cellular effectors of this action are both T cells (i.e., Th2 and Th9 cells), but also non-T cells, like type 2 innate lymphoid cells (ILC2). Key regulatory actions mediated by type 2 cytokines and immune cells on both inflammation as well as bone homeostasis are discussed.

Supercritical carbon dioxide dried double layer laponite XLS and alginate/polyacrylamide construct and immune response.

Fabrication of immunocompatible tissue constructs for bone-cartilage defect regeneration is of prime importance. In this study, a double layer hydrogel was successfully synthesized, where alginate/polyacrylamide were formulated to represent cartilage layer (5-10 % (w/w) total polymer ratio) and laponite XLS (2-5-8% (w/w))/alginate/polyacrylamide formed bone layer. Hydrogels were dried by supercritical CO2 at 100 and 200 bar, 45 °C, 5 g/min CO2 flow rate for 2 h. Constructs were treated with collagen, then cellularized and embedded in cell-laden GelMA to mimic the cellular microenvironment. The optimum weight ratio of alginate/polyacrylamide:laponite XLS was 10:5 based on mechanical strength test results. The constructs yielded high porosity (91.50 m2/g) and mesoporous structure, owing to the diffusivity of CO2 at 200 bar (0.49 × 10-7 m2/s). Constructs were then treated with collagen to increase cell adhesion and ATDC5 cells were seeded in the cartilage layer, whereas hFOB cells to the bone layer. About 10-15 % higher cell viability was attained. The porous structure of the construct allowed infiltration of macrophages, promoted polarization and positively affected the behavior of macrophages, yielding a decrease in M1 markers, whereas an increase in M2 on day 4. The formulated tissue constructs would be of value in tissue engineering applications.

Interacción del microbioma intestinal y el hueso / Interaction between the intestinal microbiome and bone

El "microbioma" no solo está constituido por los microbios, sino por todos los componen-tes que viven en el mismo hábitat conforman-do un nicho ecológico. Es decir, está conformado por los microorganismos (bacterias, hongos, protozoos, etc.), todo el espectro de moléculas producidas por ellos tales como sus componentes estructurales (ácidos nucleicos, proteínas, lípidos y glúcidos), meta-bolitos, toxinas, etc., y las moléculas producidas por el huésped. El microbioma intestinal (MI) ha emergido como un factor que tiene un gran efecto sobre la cantidad, calidad y fuerza del hueso. Las investigaciones revelan que la homeostasis ósea está ligada al micro-bioma saludable, mientras que la disbiosis (alteración en la biodiversidad microbiana) puede exacerbar la actividad osteoclástica y promover la osteoporosis. Los mecanismos potenciales involucrados en la interacción del microbioma intestinal y el hueso son la influencia del metabolismo del huésped, el mantenimiento de la integridad intestinal y regulación de la absorción de nutrientes, la regulación del eje intestino-sistema inmune y la modulación del sistema endocrino. Es decir que hay múltiples vías por las cuales el MI influye sobre el hueso, pero estos y otros mecanismos deben profundizarse más aún. También es necesario que se identifiquen y caractericen mejor los microorganismos que están asociados a las enfermedades óseas. El conocimiento de estos aspectos podría ser útil para el desarrollo de herramientas terapéuticas basadas en el MI que puedan mejorar la eficacia de los distintos tratamientos existentes. (AU)

The microbiome is not only constituted by microbes, but by all the components that live in the same habitat forming an ecological niche. It is conformed by the microorganisms ( bacteria, fungi, protozoa, etc), the entire spectrum of molecules produced by them (nucleic acids, proteins, lipid and carbohydrates, metabolites, toxins, etc) and the molecules produced by the host. The intestinal microbiome (IM) has emerged as a factor with great effects on the quantity, quality and strength of bone. The investigations reveal that bone homeostasis is linked to the healthy microbiome, while the dysbiosis (alteration in the microbial biodiversity) can exacerbate the osteoclastic activity and promote osteoporosis. The potential mechanisms involved in the interaction between IM and bone are the influence of the host metabolism, the maintenance of the intestinal integrity and regulation of the nutrient absorption, the regulation of the intestine/ immune system axis and the modulation of the endocrine system. That is, there are multiple ways through which IM influences on bone, but these and other mechanisms need to be further studied. It is also necessary to identify and characterize the microorganisms associated with the bone diseases. Knowledge of these aspects could be useful to develop therapeutical tools based on the IM that could improve the efficacy of the current treatments. (AU)

The Role of the Immune Microenvironment in Bone Regeneration.

Bone is an active tissue, being constantly renewed in healthy individuals with participation of the immune system to a large extent. Any imbalance between the processes of bone formation and bone resorption is linked to various inflammatory bone diseases. The immune system plays an important role in tissue formation and bone resorption. Recently, many studies have demonstrated complex interactions between the immune and skeletal systems. Both of immune cells and cytokines contribute to the regulation of bone homeostasis, and bone cells, including osteoblasts, osteoclasts, osteocytes, also influence the cellular functions of immune cells. These crosstalk mechanisms between the bone and immune system finally emerged, forming a new field of research called osteoimmunology. Therefore, the immune microenvironment is crucial in determining the speed and outcome of bone healing, repair, and regeneration. In this review, we summarise the role of the immune microenvironment in bone regeneration from the aspects of immune cells and immune cytokines. The elucidation of immune mechanisms involved in the process of bone regeneration would provide new therapeutic targets for improving the curative effects of bone injury treatment.

The role of autophagy in the process of osseointegration around titanium implants with micro-nano topography promoted by osteoimmunity.

Osteoimmunity plays an important role in the process of implant osseointegration. Autophagy is a conservative metabolic pathway of eukaryotic cells, but whether the interaction between autophagy and osteoimmunity plays a key role in osseointegration remains unclear. In this study, we prepared smooth titanium disks and micro-nano topography titanium disks, to study the immune microenvironment of RAW264.7 cells, and prepared the conditioned medium to study the effect of immune microenvironment on the osteogenesis and autophagy of MC3T3-E1 cells. Autophagy inhibitor 3-MA was used to inhibit autophagy to observe the change of expression of osteogenic markers. The results showed that the micro-nano topography titanium disks could stimulate RAW264.7 cells to differentiate into M2 type, forming an anti-inflammatory immune microenvironment; compared with the control group, the anti-inflammatory immune microenvironment promoted the proliferation and differentiation of osteoblasts better. The anti-inflammatory immune environment activated the autophagy level of osteoblasts, while the expression of osteogenic markers was down-regulated after inhibition of autophagy. These results indicate that anti-inflammatory immune microenvironment can promote cell proliferation and osteogenic differentiation, autophagy plays an important role in this process. This study further explains the mechanism of implant osseointegration in osteoimmune microenvironment, and provides reference for improving implant osseointegration.

Immunoporosis: Role of Innate Immune Cells in Osteoporosis.

Osteoporosis or porous bone disorder is the result of an imbalance in an otherwise highly balanced physiological process known as 'bone remodeling'. The immune system is intricately involved in bone physiology as well as pathologies. Inflammatory diseases are often correlated with osteoporosis. Inflammatory mediators such as reactive oxygen species (ROS), and pro-inflammatory cytokines and chemokines directly or indirectly act on the bone cells and play a role in the pathogenesis of osteoporosis. Recently, Srivastava et al. (Srivastava RK, Dar HY, Mishra PK. Immunoporosis: Immunology of Osteoporosis-Role of T Cells. Frontiers in immunology. 2018;9:657) have coined the term "immunoporosis" to emphasize the role of immune cells in the pathology of osteoporosis. Accumulated pieces of evidence suggest both innate and adaptive immune cells contribute to osteoporosis. However, innate cells are the major effectors of inflammation. They sense various triggers to inflammation such as pathogen-associated molecular patterns (PAMPs), damage-associated molecular patterns (DAMPs), cellular stress, etc., thus producing pro-inflammatory mediators that play a critical role in the pathogenesis of osteoporosis. In this review, we have discussed the role of the innate immune cells in great detail and divided these cells into different sections in a systemic manner. In the beginning, we talked about cells of the myeloid lineage, including macrophages, monocytes, and dendritic cells. This group of cells explicitly influences the skeletal system by the action of production of pro-inflammatory cytokines and can transdifferentiate into osteoclast. Other cells of the myeloid lineage, such as neutrophils, eosinophils, and mast cells, largely impact osteoporosis via the production of pro-inflammatory cytokines. Further, we talked about the cells of the lymphoid lineage, including natural killer cells and innate lymphoid cells, which share innate-like properties and play a role in osteoporosis. In addition to various innate immune cells, we also discussed the impact of classical pro-inflammatory cytokines on osteoporosis. We also highlighted the studies regarding the impact of physiological and metabolic changes in the body, which results in chronic inflammatory conditions such as ageing, ultimately triggering osteoporosis.

The Immune Landscape of Osteosarcoma: Implications for Prognosis and Treatment Response.

Osteosarcoma (OS) is a high-grade malignant stromal tumor composed of mesenchymal cells producing osteoid and immature bone, with a peak of incidence in the second decade of life. Hence, although relatively rare, the social impact of this neoplasm is particularly relevant. Differently from carcinomas, molecular genetics and the role of the tumor microenvironment in the development and progression of OS are mainly unknown. Indeed, while the tumor microenvironment has been widely studied in other solid tumor types and its contribution to tumor progression has been definitely established, tumor-stroma interaction in OS has been quite neglected for years. Only recently have new insights been gained, also thanks to the availability of new technologies and bioinformatics tools. A better understanding of the cross-talk between the bone microenvironment, including immune and stromal cells, and OS will be key not only for a deeper knowledge of osteosarcoma pathophysiology, but also for the development of novel therapeutic strategies. In this review, we summarize the current knowledge about the tumor microenvironment in OS, mainly focusing on immune cells, discussing their role and implication for disease prognosis and treatment response.

Fc Gamma Receptors as Regulators of Bone Destruction in Inflammatory Arthritis.

Bone erosion is one of the primary features of inflammatory arthritis and is caused by excessive differentiation and activation of osteoclasts. Fc gamma receptors (FcγRs) have been implicated in osteoclastogenesis. Our recent studies demonstrate that joint-deposited lupus IgG inhibited RANKL-induced osteoclastogenesis. FcγRI is required for RANKL-induced osteoclastogenesis and lupus IgG-induced signaling transduction. We reviewed the results of studies that analyzed the association between FcγRs and bone erosion in inflammatory arthritis. The analysis revealed the dual roles of FcγRs in bone destruction in inflammatory arthritis. Thus, IgG/FcγR signaling molecules may serve as potential therapeutic targets against bone erosion.

T-Cell Mediated Inflammation in Postmenopausal Osteoporosis.

Osteoporosis is the most prevalent metabolic bone disease that affects half the women in the sixth and seventh decade of life. Osteoporosis is characterized by uncoupled bone resorption that leads to low bone mass, compromised microarchitecture and structural deterioration that increases the likelihood of fracture with minimal trauma, known as fragility fractures. Several factors contribute to osteoporosis in men and women. In women, menopause - the cessation of ovarian function, is one of the leading causes of primary osteoporosis. Over the past three decades there has been growing appreciation that the adaptive immune system plays a fundamental role in the development of postmenopausal osteoporosis, both in humans and in mouse models. In this review, we highlight recent data on the interactions between T cells and the skeletal system in the context of postmenopausal osteoporosis. Finally, we review recent studies on the interventions to ameliorate osteoporosis.