Macrophages are central players of the innate immune system which demonstrate heterogeneity and plasticity within the tissue microenvironment. Bone metabolism is tightly regulated by the macrophages. In response to alterations within the microenvironment, both resident and circulating macrophages can switch their phenotype. While classically activated pro-inflammatory macrophages exhibit M1-like phenotype, alternatively activated M2-like macrophages exhibit an M2-like phenotype. On the other hand, inflammatory osteolytic diseases such as periodontitis and rheumatoid arthritis characterized by bone loss and phenotypic switch in macrophages was shown to play an important role in the pathogenesis of both diseases. Here, we report, discuss and speculate about several studies which include (a) the role of macrophages in periodontitis and alveolar bone loss, (b) the role of macrophages in rheumatoid arthritis and bone destruction and (c) targeting macrophages to reduce bone resorption. Understanding more about macrophage and bone biology will provide new insights and more efficacious therapeutic interventions in inflammatory osteolytic diseases such as periodontitis and rheumatoid arthritis.

Keywords: macrophages, bone loss, rheumatoid arthritis, periodontitis, macrophage polarization

1.   Introduction

In the early 1900s, Metchnikoff proposed that macrophages are the phagocytic cells involved in pathogen clearance [1]. Later in the 1930s, in the tails of amphibian larvae, it was observed that monocytes of the blood migrate into tissues and become tissue macrophages [2]. This situation was further supported in rabbits by the use of ear chambers in which extravascular development of the monocytes was observed [3]. Since the beginning of Metchnikoff’s proposal, studies investigated the important roles of macrophages in various fields, including host-defense and tissue homeostasis [4,5]. The significance of macrophages in the progressive inflammatory osteolytic diseases has been previously shown to control tissue homeostasis and change the immune responses [6,7] through the release of soluble mediators. Thus, macrophages are the central players of the innate immune system which orchestrate adaptive immune system cells and drive inflammatory signals to sense the microenvironment they involved in actively.

Skeleton has a dynamic nature in which bone is continually resorbed and formed. The remodeling process included coordination of osteoblasts and osteoclasts within basic multicellular units [8]. While osteoblasts form new bone material, osteoclasts break down bone and release minerals. There is a balance between bone formation and resorption, which is regulated by immune cells, osteocytes, and hormonal systems in response to biochemical and mechanical influences [9]. When bone resorption superseded bone formation, impairment in this balance leads to bone loss. Inflammatory osteolytic diseases, such as periodontitis and rheumatoid arthritis, are characterized by inflammation and bone loss [10,11]. On the other hand, changes in macrophage phenotype are important in the healing process, and anabolic actions of macrophages on bone formation were previously shown [12–14]. Thus, further understanding the impact of macrophage behavior on bone inflammation, especially anabolic actions of macrophages (e.g., bone formation, bone healing) on bone remodeling, is central to provide potential treatment strategies.

Receptor activator of NFKβ ligand (RANKL) is an essential cytokine for osteoclastogenesis produced by osteoblasts, osteocytes, and osteoclasts [15–17]. When RANKL is bound to receptor activator of NFKβ (RANK) receptor on pre-osteoclasts, osteoclastogenesis is promoted [18]. Increased levels of RANKL accumulation induces bone resorption, and osteoprotegerin (OPG) negatively regulates this event by limiting RANK-RANKL interaction [19]. Osteoclasts are considered resident macrophages of the bone in which its differentiation from hematopoietic progenitors is required for the production of macrophage colony-stimulating factor (M-CSF) and RANKL [20,21]. The mineralized bone matrix comprised of osteocytes in which bone metabolism is regulated through the cellular interactions and soluble mediator release [18]. Sclerostin is mainly expressed by osteocytes, which negatively regulates bone formation [22]. Therefore, different types of bone cells sense and control bone microenvironment (Figure 1).

Macrophages, cells of the innate immune system, undertake important roles in both anabolic [12–14] and catabolic [23] immune responses within the bone microenvironment. Resident macrophages can be present almost all tissues, and play important roles in tissue homeostasis and continuous immune surveillance within the specific tissue/niche [5]. In addition to osteoclasts, bone microenvironment comprised of a resident macrophage population named osteomacs [24]. The main difference between osteoclasts and osteomacs is the expression of specific markers in which osteoclasts express tartrate-resident acid phosphatase (TRAP) [25], cathepsin K [26] and calcitonin receptor [27] whereas osteomacs express F4/80, Mac3, CD115 and CD68 [13,28]. In vivo evidence shows that osteomacs involved in bone anabolism [13]. On the other hand, recruited monocyte-derived macrophages participate in inflammatory trafficking via circulation, and infiltrate infected or damaged tissue to coordinate immune responses [5].

In response to changes within the microenvironment, both resident and recruited-monocyte derived macrophages demonstrate heterogeneity and plasticity. The bipolar M1/M2 model proposes a distinct form of polarization states in which the classical activation represents pro-inflammatory M1 phenotype, and alternative activation stands for anti-inflammatory M2 phenotype [29]. On the other hand, researchers extended the current M1/M2 polarization model of macrophage activation using transcriptome-based network analysis. Their findings exhibited that macrophages use specific transcriptional programming in response to distinct signals [30]. Thus, although bipolar M1/M2 terminology is still in use, intermediate macrophage phenotypes existing in the disease states and cellular interactions that change microenvironmental dynamics (e.g., the release of soluble mediators; transcription factors that macrophages exposed) should also be considered.

M1 macrophages secrete pro-inflammatory cytokines such as tumor necrosis factor (TNF)-α, interleukin (IL)-12, IL1β, superoxide anions whereas M2 macrophages secrete anti-inflammatory cytokines such as IL-4, IL-10, and tumor growth factor (TGF)-β [31]. Activation of M1 macrophages occurs through the release of interferon-γ (IFN-γ) from T helper 1 (TH1) cells and natural killer cells, or toll-like receptor (TLR) ligands [32]. Mast cells and basophils contribute to M2-like polarization through the release of IL-4 and IL-13 [32].

The impact of M1- and M2-like macrophages on bone remodeling has been previously demonstrated during bone inflammation. For instance, the M1-like phenotype contributed to orthodontic tooth movement and alveolar bone resorption and increased the expression of TNF-α [23]. On the other hand, conditioned medium consisting of M2-like macrophages increased mineralization and osteoblast formation indicating that bone formation was induced [33].  Thus, the bipolar M1/M2 model plays a crucial role in both bone remodeling and bone-related pathologies. In the present work, we will describe and discuss important roles of macrophages in inflammatory osteolytic diseases characterized by bone loss, such as periodontitis and rheumatoid arthritis. Moreover, we will provide some examples of how we could target macrophages to regulate bone metabolism as a therapeutic intervention.

Figure 1: Different types of bone cells sense and control bone microenvironment (self-drawing). Osteoclasts are derived from granulocyte-macrophage progenitor (CFU-GM) in the presence of M-CSF and RANKL. While osteoclasts involved in bone destruction, osteoblasts form new bone material. Osteomacs are located adjacent to bone lining cells and participate in osteoblast proliferation and differentiation. Osteocytes are derived from osteoblasts and inhibit bone formation. RANKL = receptor activator of NFKβ ligand; RANK = receptor activator of NFKβ; M-CSF = macrophage colony-stimulating factor.

2.   Macrophages in Periodontitis

Periodontitis is an oral inflammatory disease characterized by the resorption of alveolar bone and impairments in the structure of periodontal connective tissue, resulting in tooth loss [10]. Periodontitis is initiated by subgingival gram-negative anaerobic bacteria such as Porphyromonas gingivalis (Pg), Bacteroides forsythus, Prevotella intermedia, and Actinobacillus actinomycetemcomitans [34,35]. However, the host response that is leading to bone resorption and local tissue destruction mediates disease pathology [36]. The gingival epithelium, perivascular tissues, lamina propria, and the blood vessels comprised of activated macrophages in humans with advanced periodontal disease [37]. Moreover, elevated levels of pro-inflammatory cytokines such as IL-1α, IL-8, and IFN-α have been found in the gingival crevicular fluid of patients with periodontitis [38], and macrophage-derived pro-inflammatory cytokines such as IL-1β and TNF-α increased at bone resorption sites [39]. Thus, increased amounts of pro-inflammatory cytokine release are associated with periodontal disease progression [39,40]. Lipopolysaccharide (LPS) released from Pg activated both TLR-2 and TLR-4 responses [41,42] and induced cytokine secretion in macrophages through the TLR-2 pathway [43]. Moreover, in the presence of TNF-α and IL-1, Pg LPS promoted the release of prostaglandin E2 from cultured fibroblasts of the periodontium, which is responsible for the bone resorption that induces M1-like pro-inflammatory phenotype [34,44–46]. Although Pg intraoral infection alone resulted in the recruitment of Ml-like macrophages into gingival tissue and increased alveolar bone resorption, macrophage depletion led to a reduction in alveolar bone resorption and Pg infection in mice [47]. In the gingival inflamed sites, CD86+ M1 phenotype was more dominant than CD206+ M2 phenotype. Pro-inflammatory cytokines such as IL-1β, IL-6, TNF-α and nitric oxide (NO) productions were elevated in murine macrophages [47]. During bone resorption, degradation of organic bone matrix occurs. Once macrophages are activated, they release soluble mediators for osteoclasts [48]. Depletion of macrophages reduces the number of osteoclasts whose differentiation is dependent on M-CSF and RANKL [49]. Hence, bone resorption is reduced. On the other hand, Pg LPSweakly activated M1 and M2 macrophages in vitro, but it induced the release of especially TNF-α from M1-like and IL-10 from M2-like polarized macrophages [50].

      Although clinical studies with human samples have consistently demonstrated the significance of M1-like phenotype in periodontitis and its impact on bone resorption, studies have conflicting results regarding the significance of M2-like phenotype [47,51–53]. For instance, arginase levels that are important for the polarization of the M2-like phenotype were increased in salivary samples of periodontitis patients [51]. Besides, factor XIII‐A (FXIII‐A) -a marker for alternative pathway activation- and IL-4 were observed at higher concentrations in patients with periodontitis [52] indicating that M2-like polarization was enhanced in periodontitis. Yu et al. investigated M1 and M2 macrophage phenotype in a periodontitis mouse model [54]. Their results demonstrated that in addition to an increase in the number of NOS2+ expressing M1 macrophages in the inflamed periodontium, the number of CD206+ expressing M2 macrophages were also increased [54]. Moreover, osteoclasts on the alveolar bone surfaces were significantly higher, and RANKL mRNA levels were increased in the periodontitis group. This study also showed an increased M1/M2 ratio in which macrophages phenotypically switch from M2 to M1 phenotype in the inflamed periodontium [54]. Indeed, this phenotypic switch to M1 has demonstrated in the inflammatory infiltrates of human gingival biopsies in experimental gingivitis [55] indicating the dominant role of M1-like phenotype in periodontal diseases. In parallel to previous studies [54,55], it has been suggested that M1 macrophages contribute to the initiation and continuation of pro-inflammatory stages during periodontitis [56]. Moreover, Garaicoa-Pazmino et al. [57] investigated the bipolar status of macrophage polarization from the gingival biopsies of healthy individuals and patients with gingivitis and periodontitis. Their findings indicated that macrophage polarization in healthy tissues shared similarities with periodontitis but not gingivitis. More importantly, although M1 and M2 polarization between healthy and periodontitis tissues did not significantly differ, gingivitis tissues showed significantly greater levels of M1 and M2 polarization. However, samples of gingivitis and periodontitis demonstrated higher levels of macrophages than healthy samples. If so, why high M1/M2 ratio drives periodontitis pathogenesis and alveolar bone resorption? The polarization of macrophages is a dynamic process in which cellular microenvironment is adapted to alterations (e.g., changes in the cytokine levels). While doing so, as an adaptation, resolving actions of M2-like macrophages may change polarization dynamics in the periodontal tissue, and thus, bipolar macrophage polarization within the tissue may represent healthy tissue. However, aberrant and continuous host response may determine the severity of alveolar bone destruction regulated by M1/M2 polarization. Besides, distinct stages of periodontal inflammation represent different macrophage polarization dynamics. Moreover, Zhou et al. [58] found a high M1/M2 ratio in chronic periodontitis samples with the increased expression of pro-inflammatory cytokines, and M1/M2 ratio was correlated with clinical probing depth, suggesting that phases of periodontal disease are important in the alveolar bone destruction [58]. Recent research explored M1-like phenotype related molecular mechanisms on the alveolar bone destruction using mouse periodontitis model [59]. In this study, the conditioned medium of the M1 macrophages (M1-CM) was used to analyze the impact of osteoblastogenesis. Findings demonstrated that M1-CM activates TLR4/AP1 signaling of pre-osteoblasts by the inhibition of osteoblastogenesis via paracrine and, thus, M1-like macrophages involved in alveolar bone loss in periodontitis [59]. On the other hand, Yamaguchi et al. [60] showed that iNOS+ M1 macrophages but not CD206+ M2 macrophages inhibited RANKL-induced osteoclastogenesis even in the presence of pro-inflammatory cytokine TNF-α. While the production of IFN-γ from M1 macrophages inhibited osteoclastogenesis by suppressing NFATc1 expression, the production of IL-12 from M1 macrophages inhibited osteoclastogenesis by promoting apoptosis in osteoclasts precursors [60]. One possible explanation for this unexpected finding is that Yamaguchi et al. [60] used RAW264.7 macrophage cell line more recently reported to be polymorphic [61] and might demonstrate its immunomodulatory activities in terms of altering macrophage polarization from M1 to M2-like phenotype [62]. Secondly, the phenotypes of osteoclasts are interchangeable in which pre-osteoclasts can be converted either macrophage-like cells that perform phagocytosis [63] or Langhans-type giant cells when RANKL pre-treated macrophages incubated with IFN-γ and LPS [64]. In the study of Yamaguchi et al. [60], it is difficult to predict whether M1 macrophages can behave like either osteoclast precursors directing their differentiation toward another type of cell (e.g., Langhans-type giant cells) or functional and phenotypic stability of RAW264.7 cell lines might have been changed in the cell culture system that they tested [61–64].

Viniegra et al. [14] evaluated phenotypic changes in macrophages and the impact of M2-like macrophages on bone resorption using a murine model of periodontitis. In the inflammation stage (day 21), CD80, TNF-α, and TGF-β significantly increased, whereas increased CD206 expression was demonstrated in the healing stage (day 28). Moreover, M2-like macrophage activation reduced alveolar bone resorption, both rosiglitazone treated and clodronate impaired experiments. Cystatin-c – a protease inhibitor- secreted only from IL-4 activated M2-like macrophages mediated anabolic actions of bone cells in comparison to M0 and M1-like macrophages. Furthermore, depletion of cystatin-c in M2-like pro-resolving macrophage supernatants restored TRAP activity in osteoclasts and removed matrix mineralization on calvarial osteoblasts [14]. This study extended our knowledge in terms of demonstrating the impact of cystatin-c that is released from M2-like cells played an important role in bone metabolism, especially inducing bone formation. Indeed, a previous report found that the salivary level of cystatin-c was increased in patients with periodontal diseases [65], suggesting that cystatin-c might be increased to resolve periodontal inflammation. Zhuang et al. [66] investigated the impact of C-C motif chemokine ligand 2 (CCL-2) on macrophage polarization and alveolar bone resorption using murine periodontitis model. Local delivery of CCL-2 releasing microparticles into the periodontal tissue induced CD206+ M2-like phenotype and inhibited alveolar bone resorption and reduced the number of osteoclasts. In conclusion, CCL-2 releasing microparticles should be a promising modulating agent in terms of switching macrophage phenotype from M1 to M2 and reducing bone resorption in periodontitis.

Figure 2: Oral microenvironment and increased M1/M2 ratio modulate bone resorption (self-drawing). Gram-negative anaerobic bacteria release LPS, which is responsible for the initiation of periodontitis and polarization toward the M1-like phenotype. While M1-like macrophages secrete pro-inflammatory cytokines such as TNF-α, IL-1β, and IL-6 that promotes alveolar bone resorption via RANKL-induced osteoclastogenesis, M2-like macrophages release IL-10, TGF-β, and cystatin-c which is thought to induce alveolar bone formation during periodontitis. Within the bone microenvironment, M1- and M2-like macrophages exhibited plasticity, and an increased M1/M2 ratio is associated with alveolar bone resorption. Lipopolysaccharide = LPS; RANKL = receptor activator of NFKβ ligand; Interleukin = IL Tumour necrosis factor-α = TNF-α; Transforming growth factor-β = TGF-β.

3.   Macrophages in Rheumatoid Arthritis

Rheumatoid arthritis (RA) is a chronic autoimmune disease in which synovial tissues, cartilage, and joints are mainly affected due to aberrant innate and adaptive immune responses [6]. RA symptoms include synovial hypertrophy and inflammation that are leading to cartilage and bone destruction [6]. Among the innate immunity cells, macrophages undertake key roles in terms of releasing pro-inflammatory cytokines and chemokines, including TNF-α, IL1-β, IL-6, CXCL4, and CXCL7, and induce synovial inflammation [67]. In addition, increased osteoclastogenesis leads to joint damage and bone loss [67,68]. In fact, early RA pathogenesis is characterized by an elevated number of sublining macrophages in the synovium [69], suggesting that macrophages are important players in the initiation of disease. Moreover, it was shown that circulating non-classical Ly6C monocytes, which polarize into M1-like macrophage phenotype drive initiation and pathogenesis of inflammatory arthritis in mice [70]. During the development of arthritis, phenotypic switch from M1 to M2 induced resolution of inflammation and, no numeric and phenotypic changes in tissue-resident synovial macrophages (STMs) were observed [70], indicating the active role of recruited-monocyte derived macrophages in RA. On the other hand, STMs were localized in the healthy synovium and were considered to play a role in joint homeostasis [71], but the exact role of STMs in this process is still uncertain. Patients with RA in remission also demonstrated STMs [72]; however, increased number of recruited-monocyte derived pro-inflammatory macrophages were shown in the inflamed synovium of active RA [6], indicating that different disease states have distinct macrophage characteristics. STMs from the healthy joints have been reported to express scavenger receptor CD163 [73] and OPG [74] but rarely expressed RANKL [74], indicating phagocytic and bone protective role of STMs against inflammation and joint damage.

Fukui et al. [75] investigated the relationship between M1/M2 monocyte subsets and osteoclastogenesis from the blood of patients with RA. Their findings indicated that an enhanced M1/M2 ratio was alone responsible for the increased number of osteoclasts and anticitrullinated protein antibody, rheumatoid factor, and erythrocyte sedimentation rate values did not contribute to enhanced osteoclasts numbers [75], indicating that M1 macrophages are direct inducers of osteoclastogenesis in RA. On the other hand, macrophage polarization and bone resorption could be affected by the impact of various molecules in RA. Teng et al. [76] investigated the impact of Sema3A on macrophage polarization and osteoclastogenesis in patients with RA. Their findings indicated that Sema3A induced IL-4 activated M2-like macrophage polarization and limited LPS/IFN-γ activated M1-like polarization. In addition, RANKL-induced osteoclastogenesis was reduced. Moreover, in vivo administration of Sema3A reduced joint damage and severity of experimental RA in mice [76], indicating preventive the role of Sema3A on RA and its connection to macrophage polarization and bone metabolism. Sultana et al. [77] investigated the impact of mannose decorated liposomes combined with withaferin-A (ML-WA) on bone resorption and macrophage polarization in adjuvant-induced arthritic rats. Their findings demonstrated that ML-WA treatment reduced osteoclastogenesis by increasing the amount of OPG and decreasing the amount of RANKL. Moreover, pro-inflammatory cytokines (TNF-α, IL-1β, IL-6) released from M1-like macrophages were reduced, and M1 macrophage surface marker -CD86 molecule- was downregulated. On the other hand, M2 macrophage surface marker CD163 mRNA was upregulated, and ML-WA treatment enhanced the production of the anti-inflammatory cytokine IL-10 [77], indicating that ML-WA treatment caused the phenotypic switch from M1 to M2 and reduced bone resorption and inflammation in arthritic rats. Hardy et al. [78] investigated the impact of 11 beta-hydroxysteroid dehydrogenase type 1 (11β-HSD1) enzyme on systemic bone loss, which was highly expressed at the active sites of inflammation in RA, using a mouse model of chronic polyarthritis. Their findings indicated that the global deletion of 11β-HSD1 resulted in exacerbated polyarthritis and increased joint destruction and bone loss. A significant increase in the number of M1-polarized macrophages in 11β-HSD1-null mice was observed [78]. Chang et al. [79] investigated the impact of CP-25 -an ester derivative of paeoniflorin- compound on inflammation and bone loss in rats with adjuvant-induced arthritis. CP-25 treatment reduced the M1-derived pro-inflammatory cytokine production (TNF-α, IL-1β, IL-6, and IL-23) and increased the M2-derived anti-inflammatory cytokine (TGF-β1) production. Moreover, CP-25 treated rats with arthritis demonstrated reduced RANKL production, reduced RANKL/OPG ratio, and decreased osteoclastogenic activity in the inflammatory synovium [79], indicating that CP-25 modulated inflammation and reduced bone resorption in rats with adjuvant-induced arthritis and could be a therapeutic agent for RA. Although the researches mentioned above [76–79] prove the impact of several compounds on macrophage polarization and bone anabolism in RA, the macrophage polarization and phenotypic switch should be affected in the different developmental stages of RA. Further studies should elucidate the potential macrophage polarization mechanisms and different roles of macrophages in the distinct stages of RA (e.g., active RA or RA in remission). Another limitation is that compounds proposed to be promising agents in terms of reducing RA symptomologies and systemic bone loss [77–79] were only tested in experimental animals and, their clinical significance in humans with RA requires further examination.

4.   Targeting Macrophages to Regulate Bone Metabolism

Macrophage polarization could be targeted as a therapeutic intervention through the modulation of cytokines and macrophage polarization-modulating agents. The impact of anti-TNF-α therapy was tested in RA patients with periodontal diseases [80–82] in different randomized-controlled trials. Infliximab treatment reduced periodontal indices, and gingival crevicular fluid TNF‐α levels in patients with RA suggest that suppression of pro-inflammatory cytokine profiles in anti-TNF therapy may cease bone resorption and pathologies in relating periodontitis [80]. In the other study, Ortiz et al. [81] investigated the effect of periodontal and anti-TNF therapies in people diagnosed with RA and severe periodontitis. Although periodontal therapy had efficacious in terms of reducing RA symptomologies, anti-TNF therapy alone did not improve the periodontal condition. However, this study included a small sample size (N=40), and the parameters they used to investigate periodontal condition (e.g., scaling and root planing intervals) was relatively short to accurately describe the impact of anti-TNF on periodontal disease. Pers et al. [82] investigated the effect of infliximab on periodontal conditions in patients with RA.  RA patients who did not take infliximab exhibited more gingival inflammation than those who took infliximab and healthy subjects. In addition, infliximab treatment reduced attachment loss, indicating that TNF-α blockade may reduce bone resorption and help alleviate of periodontal symptomologies [82]. In the experimental model of periodontitis, therapeutic blockade of IL-1 and TNF-α activity decreased bone resorption and inflammation, and progression of periodontal disease [39,40]. Since pro-inflammatory cytokines such as IL1, TNF-α, IL-6 mainly released from M1-like macrophages previously reported to stimulate bone resorption and osteoclastogenesis [83–85], targeting these cytokines may help to regulate M1/M2 balance and reduce bone loss in periodontitis and RA. For instance, TNF-α dependent joint destruction and bone erosion were previously shown in experimental arthritis mice [86], and this event was regulated by the action of osteoclasts. Moreover, IL-1 was shown to induce RANKL expression and stimulated pre-osteoclast differentiation by mediating its osteoclastogenic effect through the induction of TNF-α [87], suggesting that pro-inflammatory cytokines may have interdependency among each other. Catrina et al. [88] investigated the impact of etanercept and infliximab on monocyte/macrophage population. Both etanercept and infliximab treatment decreased the number of monocyte/macrophages through the upregulation of synovial apoptosis [88]. The blockade of the IL-6 receptor with tocilizumab was also shown to be an efficacious therapeutic approach in the treatment of RA [89]. Tocilizumab treatment was suggested to delay bone resorption in RA [90].

            Peroxisome proliferator-activated receptor-γ (PPAR-γ) induced monocyte/macrophage differentiation [91] and treatment of mouse macrophages with PPAR-γ ligand resulted in upregulation of M2-like markers and shift from M1- to M2-like phenotype [92]. In parallel to this study, treatment of human circulating monocytes with PPAR-γ agonists led to shift from M1- to anti-inflammatory M2-like phenotype but this effect was not seen in resident macrophages [93]. PPAR-γ agonist rosiglitazone was shown to reduce chronic inflammation [94] and suppress periodontal bone loss through the inhibition of RANKL-mediated osteoclastogenesis [95] in experimental animals. Moreover, rosiglitazone reduced TNF‐α‐induced Cyr61 expression -a factor related to chronic inflammation in RA- in human fibroblast-like synoviocytes [96]. These findings are important in terms of understanding the impact of PPAR-γ agonists on M2-like polarization, reduction of inflammation and preventing bone loss. However, further clinical investigations in humans with RA and periodontitis are required that addressing overall role of rosiglitazone in macrophage and bone biology.

5.   Conclusion and Future Perspectives

Although aetiology of periodontitis and RA differs, the clinical characteristics and pathology of these two diseases share similarities [97]. For instance, increased inflammation and immune cells, elevated levels of pro-inflammatory mediators such as TNF-α, IL-1 and IL-6, increased osteoclast differentiation and bone resorption [6,98] were reported in periodontitis and RA. In addition, decreased level of anti-inflammatory M2-like cytokines such as IL-10 and TGF-β were also observed in both diseases [99]. Thus, imbalances in cytokine levels and impairment in the functional macrophage plasticity lead to pathologies, especially bone loss. On the other hand, macrophages and bone metabolism tightly connected each other in which alteration in cytokine levels regulates osteoblastogenesis and osteoclastogenesis. It is also important to mention that macrophages dynamically adapt themselves when there are alterations in the cytokine microenvironment [100]. This dynamic microenvironment is not only regulated by macrophages, but also other type immune cells undertake important roles in terms of releasing soluble mediators. In healthy tissues, macrophage activation and polarization occur as dynamic events in which pro-inflammatory M1 phenotype can be switched into anti-inflammatory M2 phenotype [101]. On the other hand, in the disease states such as RA and periodontitis, macrophages are mutually exposed to both pro-inflammatory and anti-inflammatory cytokines with the dominance of more pro-inflammatory one. This process may induce different epigenetic and transcriptomic profiles in macrophages that leading to irreversible impairment in macrophage polarization dynamics. Further research should focus on investigating epigenetic and transcriptomic macrophage polarization programs. Understanding more about macrophage dynamics may help us to reprogram macrophages into their healthy states. On the other hand, although current cytokine blockade agents used to treat RA manipulate macrophage dynamics,  they supress immune system and have various side effects [102]. Moreover, Zheng et al. [103] used mice model to investigate the impact of adoptive transfer of M2 macrophages on diabetic nephropathy. Their findings indicated that M2 macrophages reduced the alterations in kidney structure such as tubular atrophy and glomerular hypertrophy, and suppressed interstitial fibrosis occurrence [103]. Although the similar approach is not tested patients with RA and periodontitis, local delivery of ex-vivo polarized M2 macrophage transfusion therapy from individual’s own macrophages might be helpful to reduce RA and periodontitis related pathologies. Clinical data is required for further examinations.

In summary, bone remodelling is an important event tightly regulated by various type of cells. Of those, macrophages sense the microenvironment by the release of soluble mediators and change dynamically their phenotypes. Impairment in the macrophage plasticity (e.g., increased M1/M2 ratio) leads to inflammation and bone loss, as discussed in this review. As a limitation, current RA and periodontitis literature investigated more about circulating macrophages than tissue resident macrophages. Further research should also describe the distinct functions and phenotypes of tissue resident macrophages (e.g., osteomacs and STMs) in periodontitis and RA. While doing so, initiation, progression and remission stages of diseases should also be considered.

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Caglar Cil hakkında
Türkiye'nin kendimce en güzel şehirlerinden birinde, Denizli'de, dünyaya geldim. Liseyi Denizli Anadolu Lisesi'nde okudum. İzmir Yüksek Teknoloji Enstitüsü (İYTE) Moleküler Biyoloji ve Genetik mezunuyum. Şu an Glasgow Üniversitesi'nde İmmunoloji ve Enflamatuvar Hastalıklar üzerine yüksek lisans yapmaktayım. Lisans hayatım boyunca Lodz Üniversitesi, Göteborg Üniversitesi ve Toronto Üniversitesi'nde araştırmalara katıldım. Bu çalışmalar sonucunda Cardiovascular Research ve Journal of Dental Research'te yayınlanan çalışmalarımız var. Öykü yazmayı seviyorum. Öykü Fanzin'de yayınlanan öyküm ve İYTE'de almış olduğum bir "birincilik" bir de "ikincilik" ödülüm var. Almanca öğreniyorum, İngilizce konuşabiliyorum, az çok keman çalabiliyorum. Amacım Türkiye okuyucusuna bilimi sevdirmek, zaman buldukça eğlencesine bilimsel haberleri paylaşmak.

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