Osteoimmunology research in rheumatoid arthritis:From single-cell omics approach

Cellular immune responses as well as generalized and periarticular bone loss are the key pathogenic features of rheumatoid arthritis(RA).Under the pathological conditions of RA,dysregulated inflammation and immune processes tightly interact with skeletal system,resulting in pathological bone damage via inhibition of bone formation or induction of bone resorption.Singlecell omics technologies are revolutionary tools in the field of modern biological research.They enable the display of the state and function of cells in various environments from a single-cell resolution,thus making it conducive to identify the dysregulated molecular mechanisms of bone destruction in RA as well as the discovery of potential therapeutic targets and biomarkers.Here,we summarize the latest findings of single-cell omics technologies in osteoimmunology research in RA.These results suggest that single-cell omics have made significant contributions to transcriptomics and dynamics of specific cells involved in bone remodeling,providing a new direction for our understanding of cellular heterogeneity in the study of osteoimmunology in RA.

Osteoimmunology: memorandum for rheumatologists

Rapid progress has been made in exploring the connections between the skeletal system and the immune system over the past decade. Bone tissue forms developmental niches for hematopoietic stem cells, and activated immune cells are involved in bone metabolism regulation and are potent mediators of osteoporosis and bone erosion under pathological conditions. The interdisciplinary field of osteoimmunology has emerged to pool the knowledge of the interdependence of these two systems, including the shared ligands and receptors, their crosstalk and interaction, and common intracellular signaling pathways with bidirectional influence. The receptor activator of nuclear factor-kappa B(RANK)/RANK ligand(RANKL)/osteoprotegerin(OPG) triad is the key vinculum, with multifaceted potency, being not only essential for osteoclastogenesis but also critical for lymph node organogenesis and lymphopoiesis as well as for immune regulation. In this review, we summarize the progress in this area, focusing on those aspects of interest concerning rheumatic diseases.

Implantable blood clot loaded with BMP-2 for regulation of osteoimmunology and enhancement of bone repair

The treatment of large-area bone defects still faces many difficulties and challenges.Here,we developed a blood clot delivery platform loaded with BMP-2 protein(BMP-2@BC)for enhanced bone regeneration.Blood clot gel platform as natural biomaterials can be engineered from autologous blood.Once implanted into the large bone defect site,it can be used for BMP-2 local delivery,as well as modulating osteoimmunology by recruiting a great number of macrophages and regulating their polarization at different stages.Moreover,due to the deep-red color of blood clot gel,mild localized hyperthermia under laser irradiation further accelerated bone repair and regeneration.We find that the immune niche within the bone defect microenvironment can be modulated in a controllable manner by the blood clots implantation and laser treatment.We further demonstrate that the newly formed bone covered almost 95%of the skull defect area by our strategy in both mice and rat disease models.Due to the great biocompatibility,photothermal potential,and osteoimmunomodulation capacity,such technology shows great promise to be used in further clinical translation.

The Role of T Lymphocytes in Bone Remodeling

Bone remodeling is a tightly regulated resorption and formation of bone matrix for physiological processes or to maintain bone function. Bone remodeling involves the synchronized differentiation and activity of bone-related cell types including bone matrix-depositing osteoblasts, bone matrix-resorbing osteoclasts, collagen/extracellular matrix-producing chondrocytes, and the progenitors of these cell types. T and B cells are adaptive immune cells that can influence bone remodeling by directly regulating the function of bone-related cells under normal and pathophysiological conditions. The specific mechanisms through which T cells control remodeling are not well defined. Here, we review the impact and influence of T cells and their products on the differentiation and function of bone cells during bone remodeling. Synthesizing new connections and highlighting potential mechanisms may promote additional avenues of study to elucidate the full role that immune cells play in regulating bone homeostasis.

Degradation particles derived from high-purity magnesium inhibit osteogenic differentiation through regulation of macrophage polarization

Magnesium(Mg)and its alloys have great potential as orthopedic implant candidates,which could release various bioactive substances during degradation.Degradation particles(DPs)are one of the degradation products,but the osteoimmunology effects are still unclear.In this study,the effect of DPs on the polar-ization of macrophages and their release of cytokines was investigated.The results verify that excessive generation of DPs from biodegradable Mg can induce macrophages to realize polarization of proinflamma-tory M1 phenotype.Moreover,macrophages secrete proinflammatory cytokines to inhibit the osteogenic differentiation of rat bone marrow stem cells(rBMSCs).This suggests that the effects of DPs should be considered when evaluating Mg-based implants.DPs continuously produced with the degradation of Mg-based implants may influence osseointegration.

Remodeling of the osteoimmune microenvironment after biomaterials implantation in murine tibia: Single-cell transcriptome analysis

Osseointegration seems to be a foreign body reaction equilibrium due to the complicated interactions between the immune and skeletal systems.The heterogeneity of the osteoimmune microenvironment in the osseointegration of implant materials remains elusive.Here,a single-cell study involving 40043 cells is conducted,and a total of 10 distinct cell clusters are identified from five different groups.A preliminary description of the osteoimmune microenvironment revealed the diverse cellular heterogeneity and dynamic changes modulated by implant properties.The increased immature neutrophils,Ly6C+CCR2hi monocytes,and S100a8hi macrophages induce an aggressive inflammatory response and eventually lead to the formation of fibrous capsule around the stainless steel implant.The enrichment of mature neutrophils,FcgR1hi and differentiated immunomodulatory macrophages around the titanium implant indicates favorable osseointegration under moderate immune response.Neutrophil-depletion mice are conducted to explore the role of neutrophils in osseointegration.Neutrophils may improve bone formation by enhancing the recruitment of BMSCs via the CXCL12/CXCR3 signal axis.These findings contribute to a better knowledge of osteoimmunology and are valuable for the design and modification of‘osteoimmune-smart’biomaterials in the bone regeneration field.

Exosome-functionalized polyetheretherketone-based implant with immunomodulatory property for enhancing osseointegration

The host immune response effecting on biomaterials is critical to determine implant fates and bone regeneration property.Bone marrow stem cells(BMSCs)derived exosomes(Exos)contain multiple biosignal molecules and have been demonstrated to exhibit immunomodulatory functions.Herein,we develop a BMSC-derived Exos-functionalized implant to accelerate bone integration by immunoregulation.BMSC-derived Exos were reversibly incorporated on tannic acid(TA)modified sulfonated polyetheretherketone(SPEEK)via the strong interaction of TA with biomacromolecules.The slowly released Exos from SPEEK can be phagocytosed by co-cultured cells,which could efficiently improve the biocompatibilities of SPEEK.In vitro results showed the Exos loaded SPEEK promoted macrophage M2 polarization via the NF-κB pathway to enhance BMSCs osteogenic differentiation.Further in vivo rat air-pouch model and rat femoral drilling model assessment of Exos loaded SPEEK revealed efficient macrophage M2 polarization,desirable new bone formation,and satisfactory osseointegration.Thus,BMSC-derived Exos-functionalized implant exerted osteoimmunomodulation effect to promote osteogenesis.

The role of dendritic cells derived osteoclasts in bone destruction diseases

The bone is previously considered as a dominant organ involved in the processes of locomotion.However,in the past two decades,a large number of studies have suggested that the skeletal system closely coordinated with the immune system so as to result in the emerging area of'osteoimmunology'.In the evolution of many kinds of bone destruction-related dis-eases,osteoclasts could differentiate from dendritic cells,which contributed to increased expression of osteoclast-related membrane receptors and relatively higher activity of bone destruction,inducing sewere bone destruction under inflammatory conditions.Numerous fac-tors could influence the interaction between osteoclasts and dendritic cells,contributing to the pathogenesis of several bone diseases in the context of inflammation,including both im-munocytes and a large number of cytokines.In addition,the products of osteoclasts released from bone destruction area serve as important signals for the differentiation and activation of immature dendritic cells.Therefore,the border between the dendritic cell-rela ted immune response and osteoclast-related bone destruction has gradually unravelled.Dendritic cells and osteoclasts cooperate with each other to mediate bone destruction and bone remodelling under inflammatory conditions.In this review,we will pay attention to the interactions be-tween dendritic cells and osteoclasts in physiological and pathological conditions to further understand the skeletal system and identifty potential new therapeutic targets for the future by summarizing their significant roles and molecular mechanisms in bone destruction.

Synergistic effects of immunoregulation and osteoinduction of ds-block elements on titanium surface

Ds-block elements have been gaining increasing attention in the field of biomaterials modification,owing to their excellent biological properties,such as antibiosis,osteogenesis,etc.However,their function mechanisms are not well understood and conflicting conclusions were drawn by previous studies on this issue,which are mainly resulted from the inconsistent experimental conditions.In this work,three most widely used ds-block elements,copper,zinc,and silver were introduced on titanium substrate by plasma immersion ion implantation method to investigate the rule of ds-block elements in the immune responses.Results showed that the implanted samples could decrease the inflammatory responses compared with Ti sample.The trend of anti-inflammatory effects of macrophages on samples was in correlation with cellular ROS levels,which was induced by the implanted biomaterials and positively correlated with the number of valence electrons of ds-block elements.The co-culture experiments of macrophages and bone marrow mesenchymal stem cells showed that these two kinds of cells could enhance the anti-inflammation and osteogenesis of samples by the paracrine manner of PGE2.In general,in their steady states on titanium substrate(Cu2+,Zn2+,Ag),the ds-block elements with more valence electrons exhibit better anti-inflammatory and osteogenic effects.Moreover,molecular biology experiments indicate that the PGE2-related signaling pathway may contribute to the desired immunoregulation and osteoinduction capability of ds-block elements.These findings suggest a correlation between the number of valence electrons of ds-block elements and the relevant biological responses,which provides new insight into the selection of implanted ions and surface design of biomaterials.

Engineered extracellular vesicles:Regulating the crosstalk between the skeleton and immune system

Osteoimmunology has gained momentum in recent years,focusing on the crosstalk between the skeleton and the immune system.Extracellular vesicles(EVs)are nanoscale vesicles that are potential candidates for cell-free tissue regeneration strategies.They may be used for repairing damaged tissues and regulating the body’s immune system and bone-related metabolic activities.Because of the ability of EVs to deliver bioactive signals and mediate intercellular communication,they can decipher the complex mechanisms of interaction within the“osteoimmune system”at the molecular level.To address the lack of targeting ability caused by vesicle heterogeneity in the clinical applications of EVs,these nanoscopical entities may be modified by bioengineering techniques to optimize the interaction between bone repair and immunomodulation for improving treatment efficacy,specificity and safety.In the present review,the endogenous properties that make EVs natural delivery agents are outlined.Properties that may be improved by bioengineering are highlighted.The therapeutic applications of EVs in the rehabilitation of bone defects are discussed.The opportunities and challenges that need to be addressed for translating this field of research into clinical practice are brought into perspectives.

Hydroxyapatite composited PEEK with 3D porous surface enhances osteoblast differentiation through mediating NO by macrophage

The adverse immune response mediated by macrophages is one of the main factors that are prone to lead poor osseointegration of polyetheretherketone(PEEK)implants in clinic.Hence,endowing PEEK with immunomodulatory ability to avoid the adverse immune response becomes a promising strategy to promote bone repair.In this work,sulfonation and hydrothermal treatment were used to fabricate a 3D porous surface on PEEK and hydroxyapatite(HA)composited PEEK.The HA composited PEEK with 3D porous surface inhibited macrophages polarizing to M1 phenotype and downregulated inducible nitric oxide synthase protein expression,which led to a nitric oxide concentration reduction in culture medium of mouse bone marrow mesenchymal stem cells(mBMSCs)under coculture condition.The decrease of nitric oxide concentration could help to increase bone formation-related OSX and ALP genes expressions and decrease bone resorption-related MMP-9 and MMP-13 genes expressions via cAMP–PKA–RUNX2 pathway in mBMSCs.In summary,the HA composited PEEK with 3D porous surface has the potential to promote osteogenesis of PEEK through immunomodulation,which provides a promising strategy to improve the bone repair ability of PEEK.

Macrophage exosomes modified by miR-365-2-5p promoted osteoblast osteogenic differentiation by targeting OLFML1

In the bone immune microenvironment,immune cells can regulate osteoblasts through a complex communication network.Macrophages play a central role in mediating immune osteogenesis,exosomes derived from them have osteogenic regulation and can be used as cariers in bone tissue engineering.However,there are problems with exosomal therapy alone,such as poor targeting,and the content of loaded molecules cannot reach the therapeutic concentration.In this study,macrophage-derived exosomes modified with miR-365-2-5p were developed to accelerate bone healing.MC3T3-E1 cells were incubated with the culture supermatants of Mo,M1 and M2 macrophages,and it was found that the culture medium of M2 macrophages had the most significant effects in contributing to osteogenesis.High-throughput sequencing identified that miR-365-2-5p was significantly expressed in exosomes derived from M2 macrophages.We incubated MC3T3-E1 with exosomes overexpressing or kmocking down miR-365-2-5p to examine the biological function of exosome miR-365-2-5p on MC3T3-E1 differentiation.These findings suggested that miR-365-2-5p secreted by exosomes increased the osteogenesis of MC3T3-E1.Moreover,miR-365-2-5p had a direct influence over osteogenesis for MC3T3-Ei.Sequencing analysis combined with dual luciferase detection indicated that miR-365-2-5p binded to the 3'-UTR of OLFML1.In summary,exosomes secreted by M2 macrophages targeted OLFML1 through miR-365-2-5p to facilitate osteogenesis.