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
In the early 1900s, Metchnikoff proposed that macrophages are the phagocytic cells involved in pathogen clearance . 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 . This situation was further supported in rabbits by the use of ear chambers in which extravascular development of the monocytes was observed . 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 . 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 . 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 . Increased levels of RANKL accumulation induces bone resorption, and osteoprotegerin (OPG) negatively regulates this event by limiting RANK-RANKL interaction . 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 . Sclerostin is mainly expressed by osteocytes, which negatively regulates bone formation . 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  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 . In addition to osteoclasts, bone microenvironment comprised of a resident macrophage population named osteomacs . The main difference between osteoclasts and osteomacs is the expression of specific markers in which osteoclasts express tartrate-resident acid phosphatase (TRAP) , cathepsin K  and calcitonin receptor  whereas osteomacs express F4/80, Mac3, CD115 and CD68 [13,28]. In vivo evidence shows that osteomacs involved in bone anabolism . On the other hand, recruited monocyte-derived macrophages participate in inflammatory trafficking via circulation, and infiltrate infected or damaged tissue to coordinate immune responses .
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 . 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 . 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 . 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 . Mast cells and basophils contribute to M2-like polarization through the release of IL-4 and IL-13 .
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 . On the other hand, conditioned medium consisting of M2-like macrophages increased mineralization and osteoblast formation indicating that bone formation was induced . 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 . 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 . The gingival epithelium, perivascular tissues, lamina propria, and the blood vessels comprised of activated macrophages in humans with advanced periodontal disease . 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 , and macrophage-derived pro-inflammatory cytokines such as IL-1β and TNF-α increased at bone resorption sites . 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 . 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 . 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 . During bone resorption, degradation of organic bone matrix occurs. Once macrophages are activated, they release soluble mediators for osteoclasts . Depletion of macrophages reduces the number of osteoclasts whose differentiation is dependent on M-CSF and RANKL . 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 .
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 . Besides, factor XIII‐A (FXIII‐A) -a marker for alternative pathway activation- and IL-4 were observed at higher concentrations in patients with periodontitis  indicating that M2-like polarization was enhanced in periodontitis. Yu et al. investigated M1 and M2 macrophage phenotype in a periodontitis mouse model . 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 . 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 . Indeed, this phenotypic switch to M1 has demonstrated in the inflammatory infiltrates of human gingival biopsies in experimental gingivitis  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 . Moreover, Garaicoa-Pazmino et al.  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.  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 . Recent research explored M1-like phenotype related molecular mechanisms on the alveolar bone destruction using mouse periodontitis model . 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 . On the other hand, Yamaguchi et al.  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 . One possible explanation for this unexpected finding is that Yamaguchi et al.  used RAW264.7 macrophage cell line more recently reported to be polymorphic  and might demonstrate its immunomodulatory activities in terms of altering macrophage polarization from M1 to M2-like phenotype . Secondly, the phenotypes of osteoclasts are interchangeable in which pre-osteoclasts can be converted either macrophage-like cells that perform phagocytosis  or Langhans-type giant cells when RANKL pre-treated macrophages incubated with IFN-γ and LPS . In the study of Yamaguchi et al. , 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.  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 . 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 , suggesting that cystatin-c might be increased to resolve periodontal inflammation. Zhuang et al.  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 . RA symptoms include synovial hypertrophy and inflammation that are leading to cartilage and bone destruction . 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 . 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 , 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 . 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 , 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 , but the exact role of STMs in this process is still uncertain. Patients with RA in remission also demonstrated STMs ; however, increased number of recruited-monocyte derived pro-inflammatory macrophages were shown in the inflamed synovium of active RA , indicating that different disease states have distinct macrophage characteristics. STMs from the healthy joints have been reported to express scavenger receptor CD163  and OPG  but rarely expressed RANKL , indicating phagocytic and bone protective role of STMs against inflammation and joint damage.
Fukui et al.  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 , 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.  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 , indicating preventive the role of Sema3A on RA and its connection to macrophage polarization and bone metabolism. Sultana et al.  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 , 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.  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 . Chang et al.  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 , 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 . In the other study, Ortiz et al.  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.  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 . 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 , 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 , suggesting that pro-inflammatory cytokines may have interdependency among each other. Catrina et al.  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 . The blockade of the IL-6 receptor with tocilizumab was also shown to be an efficacious therapeutic approach in the treatment of RA . Tocilizumab treatment was suggested to delay bone resorption in RA .
Peroxisome proliferator-activated receptor-γ (PPAR-γ) induced monocyte/macrophage differentiation  and treatment of mouse macrophages with PPAR-γ ligand resulted in upregulation of M2-like markers and shift from M1- to M2-like phenotype . 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 . PPAR-γ agonist rosiglitazone was shown to reduce chronic inflammation  and suppress periodontal bone loss through the inhibition of RANKL-mediated osteoclastogenesis  in experimental animals. Moreover, rosiglitazone reduced TNF‐α‐induced Cyr61 expression -a factor related to chronic inflammation in RA- in human fibroblast-like synoviocytes . 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 . 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 . 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 . 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 . 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 . Moreover, Zheng et al.  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 . 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.
. Cavaillon J. The historical milestones in the understanding of leukocyte biology initiated by Elie Metchnikoff. J Leukoc Biol. 2011;90: 413–424.
2. Clark ER, Clark EL. Relation of monocytes of the blood to the tissue macrophages. Am J Anat. 1930;46: 149–185.
3. Ebert RH, Florey HW. The extravascular development of the monocyte observed in vivo. Br J Exp Pathol. 1939;20: 342.
4. Moonis M, Ahmad I, Bachhawat BK. Macrophages in host defence–an overview. Indian J Biochem Biophys. 1992;29: 115–122.
5. Epelman S, Lavine KJ, Randolph GJ. Origin and functions of tissue macrophages. Immunity. 2014;41: 21–35.
6. McInnes IB, Schett G. The pathogenesis of rheumatoid arthritis. N Engl J Med. 2011;365: 2205–2219.
7. Pussinen PJ, Vilkuna-Rautiainen T, Alfthan G, Palosuo T, Jauhiainen M, Sundvall J, et al. Severe periodontitis enhances macrophage activation via increased serum lipopolysaccharide. Arterioscler Thromb Vasc Biol. 2004;24: 2174–2180.
8. Sims NA, Martin TJ. Coupling the activities of bone formation and resorption: a multitude of signals within the basic multicellular unit. Bonekey Rep. 2014;3.
9. Hadjidakis DJ, Androulakis II. Bone remodeling. Ann N Y Acad Sci. 2006;1092: 385–396.
10. Cochran DL. Inflammation and bone loss in periodontal disease. J Periodontol. 2008;79: 1569–1576.
11. Gough AKS, Emery P, Holder RL, Lilley J, Eyre S. Generalised bone loss in patients with early rheumatoid arthritis. Lancet. 1994;344: 23–27.
12. Hume DA, Loutit JF, Gordon S. The mononuclear phagocyte system of the mouse defined by immunohistochemical localization of antigen F4/80: macrophages of bone and associated connective tissue. J Cell Sci. 1984;66: 189–194.
13. Alexander KA, Chang MK, Maylin ER, Kohler T, Müller R, Wu AC, et al. Osteal macrophages promote in vivo intramembranous bone healing in a mouse tibial injury model. J bone Miner Res. 2011;26: 1517–1532.
14. Viniegra A, Goldberg H, Çil Ç, Fine N, Sheikh Z, Galli M, et al. Resolving Macrophages Counter Osteolysis by Anabolic Actions on Bone Cells. J Dent Res. 2018; 0022034518777973. doi:10.1177/0022034518777973
15. Bord S, Ireland DC, Beavan SR, Compston JE. The effects of estrogen on osteoprotegerin, RANKL, and estrogen receptor expression in human osteoblasts. Bone. 2003;32: 136–141.
16. Zhao S, Kato Y, Zhang Y, Harris S, Ahuja SS, Bonewald LF. MLO‐Y4 osteocyte‐like cells support osteoclast formation and activation. J bone Miner Res. 2002;17: 2068–2079.
17. Takayanagi H, Iizuka H, Juji T, Nakagawa T, Yamamoto A, Miyazaki T, et al. Involvement of receptor activator of nuclear factor κB ligand/osteoclast differentiation factor in osteoclastogenesis from synoviocytes in rheumatoid arthritis. Arthritis Rheum Off J Am Coll Rheumatol. 2000;43: 259–269.
18. O’Brien CA, Nakashima T, Takayanagi H. Osteocyte control of osteoclastogenesis. Bone. 2013;54: 258–263.
19. Min H, Morony S, Sarosi I, Dunstan CR, Capparelli C, Scully S, et al. Osteoprotegerin reverses osteoporosis by inhibiting endosteal osteoclasts and prevents vascular calcification by blocking a process resembling osteoclastogenesis. J Exp Med. 2000;192: 463–474.
20. Kong Y-Y, Yoshida H, Sarosi I, Tan H-L, Timms E, Capparelli C, et al. OPGL is a key regulator of osteoclastogenesis, lymphocyte development and lymph-node organogenesis. Nature. 1999;397: 315.
21. Yoshida H, Hayashi S-I, Kunisada T, Ogawa M, Nishikawa S, Okamura H, et al. The murine mutation osteopetrosis is in the coding region of the macrophage colony stimulating factor gene. Nature. 1990;345: 442.
22. Van Bezooijen RL, Roelen BAJ, Visser A, Van Der Wee-pals L, De Wilt E, Karperien M, et al. Sclerostin is an osteocyte-expressed negative regulator of bone formation, but not a classical BMP antagonist. J Exp Med. 2004;199: 805–814.
23. He D, Kou X, Yang R, Liu D, Wang X, Luo Q, et al. M1-like macrophage polarization promotes orthodontic tooth movement. J Dent Res. 2015;94: 1286–1294.
24. Kaur S, Raggatt LJ, Batoon L, Hume DA, Levesque J-P, Pettit AR. Role of bone marrow macrophages in controlling homeostasis and repair in bone and bone marrow niches. Seminars in cell & developmental biology. Elsevier; 2017. pp. 12–21.
25. Hayman AR. Tartrate-resistant acid phosphatase (TRAP) and the osteoclast/immune cell dichotomy. Autoimmunity. 2008;41: 218–223.
26. Li Y, Chen W. Characterization of mouse cathepsin K gene, the gene promoter, and the gene expression. J Bone Miner Res. 1999;14: 487–499.
27. Lee SK, Goldring SR, Lorenzo JA. Expression of the calcitonin receptor in bone marrow cell cultures and in bone: a specific marker of the differentiated osteoclast that is regulated by calcitonin. Endocrinology. 1995;136: 4572–4581.
28. Chang MK, Raggatt L-J, Alexander KA, Kuliwaba JS, Fazzalari NL, Schroder K, et al. Osteal tissue macrophages are intercalated throughout human and mouse bone lining tissues and regulate osteoblast function in vitro and in vivo. J Immunol. 2008;181: 1232–1244.
29. Mohammadi A, Blesso CN, Barreto GE, Banach M, Majeed M, Sahebkar A. Macrophage plasticity, polarization and function in response to curcumin, a diet-derived polyphenol, as an immunomodulatory agent. J Nutr Biochem. 2019;66: 1–16.
30. Xue J, Schmidt S V, Sander J, Draffehn A, Krebs W, Quester I, et al. Transcriptome-based network analysis reveals a spectrum model of human macrophage activation. Immunity. 2014;40: 274–288.
31. Funes SC, Rios M, Escobar‐Vera J, Kalergis AM. Implications of macrophage polarization in autoimmunity. Immunology. 2018;154: 186–195.
32. Tan H-Y, Wang N, Li S, Hong M, Wang X, Feng Y. The reactive oxygen species in macrophage polarization: reflecting its dual role in progression and treatment of human diseases. Oxid Med Cell Longev. 2016;2016.
33. Fernandes TJ, Hodge JM, Singh PP, Eeles DG, Collier FM, Holten I, et al. Cord blood-derived macrophage-lineage cells rapidly stimulate osteoblastic maturation in mesenchymal stem cells in a glycoprotein-130 dependent manner. PLoS One. 2013;8: e73266.
34. Page RC, Kornman KS. The pathogenesis of human periodontitis: an introduction. Periodontol 2000. 1997;14: 9–11.
35. Haflajee AD, Socransky SS. Microbial etiological agents of destructive periodontal disease. Periodontol. 2000; 78–111.
36. Genco RJ. Host responses in periodontal diseases: current concepts. J Periodontol. 1992;63: 338–355.
37. Charon J, Toto PD, Gargiulo AW. Activated macrophages in human periodontitis. J Periodontol. 1981;52: 328–335.
38. Mathur A, Michalowicz B, Castillo M, Aeppll D. Interleukin‐1 alpha, interleukin‐8 and interferon‐alpha levels in gingival crevicular fluid. J Periodontal Res. 1996;31: 489–495.
39. Delima AJ, Oates T, Assuma R, Schwartz Z, Cochran D, Amar S, et al. Soluble antagonists to interleukin‐1 (IL‐1) and tumor necrosis factor (TNF) inhibits loss of tissue attachment in experimental periodontitis. J Clin Periodontol. 2001;28: 233–240.
40. Assuma R, Oates T, Cochran D, Amar S, Graves DT. IL-1 and TNF antagonists inhibit the inflammatory response and bone loss in experimental periodontitis. J Immunol. 1998;160: 403–409.
41. Darveau RP, Pham T-TT, Lemley K, Reife RA, Bainbridge BW, Coats SR, et al. Porphyromonas gingivalis lipopolysaccharide contains multiple lipid A species that functionally interact with both toll-like receptors 2 and 4. Infect Immun. 2004;72: 5041–5051.
42. Hirschfeld M, Ma Y, Weis JH, Vogel SN, Weis JJ. Cutting edge: repurification of lipopolysaccharide eliminates signaling through both human and murine toll-like receptor 2. J Immunol. 2000;165: 618–622.
43. Hirschfeld M, Weis JJ, Toshchakov V, Salkowski CA, Cody MJ, Ward DC, et al. Signaling by toll-like receptor 2 and 4 agonists results in differential gene expression in murine macrophages. Infect Immun. 2001;69: 1477–1482.
44. Richards D, Rutherford RB. The effects of interleukin 1 on collagenolytic activity and prostaglandin-E secretion by human periodontal-ligament and gingival fibroblast. Arch Oral Biol. 1988;33: 237–243.
45. Matsuki Y, Yamamoto T, Hara K. Detection of inflammatory cytokine messenger RNA (mRNA)-expressing cells in human inflamed gingiva by combined in situ hybridization and immunohistochemistry. Immunology. 1992;76: 42.
46. Offenbaceer S, Odle BM, Van Dyke TE. The use of crevicular fluid prostaglandin E2 levels as a predictor of periodontal attachment loss. J Periodontal Res. 1986;21: 101–112.
47. Lam RS, O’Brien-Simpson NM, Lenzo JC, Holden JA, Brammar GC, Walsh KA, et al. Macrophage depletion abates Porphyromonas gingivalis–induced alveolar bone resorption in mice. J Immunol. 2014;193: 2349–2362.
48. Lassus J, Salo J, Jiranek WA, Santavirta S, Nevalainen J, Matucci-Cerinic M, et al. Macrophage activation results in bone resorption. Clin Orthop Relat Res. 1998; 7–15.
49. Teitelbaum SL. Bone resorption by osteoclasts. Science (80- ). 2000;289: 1504–1508.
50. Holden JA, Attard TJ, Laughton KM, Mansell A, O’Brien-Simpson NM, Reynolds EC. Porphyromonas gingivalis lipopolysaccharide weakly activates M1 and M2 polarized mouse macrophages but induces inflammatory cytokines. Infect Immun. 2014;82: 4190–4203.
51. Gheren LW, Cortelli JR, Rodrigues E, Holzhausen M, Saad WA. Periodontal therapy reduces arginase activity in saliva of patients with chronic periodontitis. Clin Oral Investig. 2008;12: 67–72.
52. Navarrete M, García J, Dutzan N, Henríquez L, Puente J, Carvajal P, et al. Interferon‐γ, interleukins‐6 and‐4, and factor XIII‐A as indirect markers of the classical and alternative macrophage activation pathways in chronic periodontitis. J Periodontol. 2014;85: 751–760.
53. Güllü C, Ozmeric N, Tokman B, Elgün S, Balos K. Effectiveness of scaling and root planing versus modified Widman flap on nitric oxide synthase and arginase activity in patients with chronic periodontitis. J Periodontal Res. 2005;40: 168–175.
54. Yu T, Zhao L, Huang X, Ma C, Wang Y, Zhang J, et al. Enhanced activity of the macrophage M1/M2 phenotypes and phenotypic switch to M1 in periodontal infection. J Periodontol. 2016;87: 1092–1102.
55. Topoll HH, Zwadlo G, Lange DE, Sorg C. Phenotypic dynamics of macrophage subpopulations during human experimental gingivitis. J Periodontal Res. 1989;24: 106–112.
56. Lam RS, O’Brien-Simpson NM, Holden JA, Lenzo JC, Fong SB, Reynolds EC. Unprimed, M1 and M2 macrophages differentially interact with Porphyromonas gingivalis. PLoS One. 2016;11: e0158629.
57. Garaicoa‐Pazmino C, Fretwurst T, Squarize CH, Berglundh T, Giannobile W V, Larsson L, et al. Characterization of macrophage polarization in periodontal disease. J Clin Periodontol. 2019;46: 830–839.
58. Zhou L, Bi C, Gao L, An Y, Chen F, Chen F. Macrophage polarization in human gingival tissue in response to periodontal disease. Oral Dis. 2019;25: 265–273.
59. Zhu L, Li L, Wang X, Pan L, Mei Y, Fu Y, et al. M1 macrophages regulate TLR4/AP1 via paracrine to promote alveolar bone destruction in periodontitis. Oral Dis. 2019;25: 1972–1982.
60. Yamaguchi T, Movila A, Kataoka S, Wisitrasameewong W, Torruella MR, Murakoshi M, et al. Proinflammatory M1 macrophages inhibit RANKL-induced osteoclastogenesis. Infect Immun. 2016;84: 2802–2812.
61. Taciak B, Białasek M, Braniewska A, Sas Z, Sawicka P, Kiraga Ł, et al. Evaluation of phenotypic and functional stability of RAW 264.7 cell line through serial passages. PLoS One. 2018;13: e0198943.
62. Chen S, Lu Z, Wang F, Wang Y. Cathelicidin-WA polarizes E. coli K88-induced M1 macrophage to M2-like macrophage in RAW264. 7 cells. Int Immunopharmacol. 2018;54: 52–59.
63. Nishimura K, Shindo S, Movila A, Kayal R, Abdullah A, Savitri IJ, et al. TRAP-positive osteoclast precursors mediate ROS/NO-dependent bactericidal activity via TLR4. Free Radic Biol Med. 2016;97: 330–341.
64. Jeganathan S, Fiorino C, Naik U, song Sun H, Harrison RE. Modulation of osteoclastogenesis with macrophage M1-and M2-inducing stimuli. PLoS One. 2014;9: e104498.
65. Henskens YMC, Veerman ECI, Mantel MS, Van der Velden U, Nieuw Amerongen A V. Cystatins S and C in human whole saliva and in glandular salivas in periodontal health and disease. J Dent Res. 1994;73: 1606–1614.
66. Zhuang Z, Yoshizawa-Smith S, Glowacki A, Maltos K, Pacheco C, Shehabeldin M, et al. Induction of M2 macrophages prevents bone loss in murine periodontitis models. J Dent Res. 2019;98: 200–208.
67. Yeo L, Adlard N, Biehl M, Juarez M, Smallie T, Snow M, et al. Expression of chemokines CXCL4 and CXCL7 by synovial macrophages defines an early stage of rheumatoid arthritis. Ann Rheum Dis. 2016;75: 763–771.
68. Dimitroulas T, Nikas SN, Trontzas P, Kitas GD. Biologic therapies and systemic bone loss in rheumatoid arthritis. Autoimmun Rev. 2013;12: 958–966.
69. Wijbrandts CA, Vergunst CE, Haringman JJ, Gerlag DM, Smeets TJM, Tak PP. Absence of changes in the number of synovial sublining macrophages after ineffective treatment for rheumatoid arthritis: Implications for use of synovial sublining macrophages as a biomarker. Arthritis Rheum Off J Am Coll Rheumatol. 2007;56: 3869–3871.
70. Misharin A V, Cuda CM, Saber R, Turner JD, Gierut AK, Haines III GK, et al. Nonclassical Ly6C− monocytes drive the development of inflammatory arthritis in mice. Cell Rep. 2014;9: 591–604.
71. Smith MD. The normal synovium. Open Rheumatol J 5: 100–106. 2011.
72. Alivernini S, Tolusso B, Petricca L, Bui L, Di Sante G, Peluso G, et al. Synovial features of patients with rheumatoid arthritis and psoriatic arthritis in clinical and ultrasound remission differ under anti-TNF therapy: a clue to interpret different chances of relapse after clinical remission? Ann Rheum Dis. 2017;76: 1228–1236.
73. Singh JA, Arayssi T, Duray P, Schumacher HR. Immunohistochemistry of normal human knee synovium: a quantitative study. Ann Rheum Dis. 2004;63: 785–790.
74. Smith MD, Barg E, Weedon H, Papengelis V, Smeets T, Tak PP, et al. Microarchitecture and protective mechanisms in synovial tissue from clinically and arthroscopically normal knee joints. Ann Rheum Dis. 2003;62: 303–307.
75. Fukui S, Iwamoto N, Takatani A, Igawa T, Shimizu T, Umeda M, et al. M1 and M2 monocytes in rheumatoid arthritis: a contribution of imbalance of M1/M2 monocytes to osteoclastogenesis. Front Immunol. 2018;8: 1958.
76. Teng Y, Yin Z, Li J, Li K, Li X, Zhang Y. Adenovirus-mediated delivery of Sema3A alleviates rheumatoid arthritis in a serum-transfer induced mouse model. Oncotarget. 2017;8: 66270.
77. Sultana F, Neog MK, Rasool M. Withaferin-A, a steroidal lactone encapsulated mannose decorated liposomes ameliorates rheumatoid arthritis by intriguing the macrophage repolarization in adjuvant-induced arthritic rats. Colloids Surfaces B Biointerfaces. 2017;155: 349–365.
78. Hardy RS, Fenton C, Croft AP, Naylor AJ, Begum R, Desanti G, et al. 11 Beta-hydroxysteroid dehydrogenase type 1 regulates synovitis, joint destruction, and systemic bone loss in chronic polyarthritis. J Autoimmun. 2018;92: 104–113.
79. Chang Y, Jia X, Wei F, Wang C, Sun X, Xu S, et al. CP-25, a novel compound, protects against autoimmune arthritis by modulating immune mediators of inflammation and bone damage. Sci Rep. 2016;6: 26239.
80. Mayer Y, Balbir‐Gurman A, Machtei EE. Anti‐tumor necrosis factor‐alpha therapy and periodontal parameters in patients with rheumatoid arthritis. J Periodontol. 2009;80: 1414–1420.
81. Ortiz P, Bissada NF, Palomo L, Han YW, Al‐Zahrani MS, Panneerselvam A, et al. Periodontal therapy reduces the severity of active rheumatoid arthritis in patients treated with or without tumor necrosis factor inhibitors. J Periodontol. 2009;80: 535–540.
82. Pers J, Saraux A, Pierre R, Youinou P. Anti–TNF‐α Immunotherapy Is Associated With Increased Gingival Inflammation Without Clinical Attachment Loss in Subjects With Rheumatoid Arthritis. J Periodontol. 2008;79: 1645–1651.
83. Huynh NCN, Everts V, Pavasant P, Ampornaramveth RS. Interleukin-1β induces human cementoblasts to support osteoclastogenesis. Int J Oral Sci. 2017;9: e5–e5.
84. Graves DT, Oskoui M, Voleinikova S, Naguib G, Cai S, Desta T, et al. Tumor necrosis factor modulates fibroblast apoptosis, PMN recruitment, and osteoclast formation in response to P. gingivalis infection. J Dent Res. 2001;80: 1875–1879.
85. Steeve KT, Marc P, Sandrine T, Dominique H, Yannick F. IL-6, RANKL, TNF-alpha/IL-1: interrelations in bone resorption pathophysiology. Cytokine Growth Factor Rev. 2004;15: 49–60.
86. Redlich K, Hayer S, Ricci R, David J-P, Tohidast-Akrad M, Kollias G, et al. Osteoclasts are essential for TNF-α–mediated joint destruction. J Clin Invest. 2002;110: 1419–1427.
87. Wei S, Kitaura H, Zhou P, Ross FP, Teitelbaum SL. IL-1 mediates TNF-induced osteoclastogenesis. J Clin Invest. 2005;115: 282–290.
88. Catrina AI, Trollmo C, af Klint E, Engstrom M, Lampa J, Hermansson Y, et al. Evidence that anti–tumor necrosis factor therapy with both etanercept and infliximab induces apoptosis in macrophages, but not lymphocytes, in rheumatoid arthritis joints. Arthritis Rheum Off J Am Coll Rheumatol. 2005;52: 61–72.
89. Smolen JS, Beaulieu A, Rubbert-Roth A, Ramos-Remus C, Rovensky J, Alecock E, et al. Effect of interleukin-6 receptor inhibition with tocilizumab in patients with rheumatoid arthritis (OPTION study): a double-blind, placebo-controlled, randomised trial. Lancet. 2008;371: 987–997.
90. Ash Z, Emery P. The role of tocilizumab in the management of rheumatoid arthritis. Expert Opin Biol Ther. 2012;12: 1277–1289.
91. Tontonoz P, Nagy L, Alvarez JGA, Thomazy VA, Evans RM. PPARγ promotes monocyte/macrophage differentiation and uptake of oxidized LDL. Cell. 1998;93: 241–252.
92. Penas F, Mirkin GA, Vera M, Cevey Á, González CD, Gómez MI, et al. Treatment in vitro with PPARα and PPARγ ligands drives M1-to-M2 polarization of macrophages from T. cruzi-infected mice. Biochim Biophys Acta (BBA)-Molecular Basis Dis. 2015;1852: 893–904.
93. Bouhlel MA, Derudas B, Rigamonti E, Dièvart R, Brozek J, Haulon S, et al. PPARγ activation primes human monocytes into alternative M2 macrophages with anti-inflammatory properties. Cell Metab. 2007;6: 137–143.
94. Di Paola R, Mazzon E, Maiere D, Zito D, Britti D, De Majo M, et al. Rosiglitazone reduces the evolution of experimental periodontitis in the rat. J Dent Res. 2006;85: 156–161.
95. Hassumi MY, Silva-Filho VJ, Campos-Júnior JC, Vieira SM, Cunha FQ, Alves PM, et al. PPAR-γ agonist rosiglitazone prevents inflammatory periodontal bone loss by inhibiting osteoclastogenesis. Int Immunopharmacol. 2009;9: 1150–1158.
96. Kwon E, Park E, Choi S, Kim S, Cho M, Kim J. PPARγ agonist rosiglitazone inhibits migration and invasion by downregulating Cyr61 in rheumatoid arthritis fibroblast‐like synoviocytes. Int J Rheum Dis. 2017;20: 1499–1509.
97. Mercado FB, Marshall RI, Bartold PM. Inter‐relationships between rheumatoid arthritis and periodontal disease: A review. J Clin Periodontol. 2003;30: 761–772.
98. De Pablo P, Chapple ILC, Buckley CD, Dietrich T. Periodontitis in systemic rheumatic diseases. Nat Rev Rheumatol. 2009;5: 218.
99. Bartold PM, Marshall RI, Haynes DR. Periodontitis and rheumatoid arthritis: a review. J Periodontol. 2005;76: 2066–2074.
100. Davis MJ, Tsang TM, Qiu Y, Dayrit JK, Freij JB, Huffnagle GB, et al. Macrophage M1/M2 polarization dynamically adapts to changes in cytokine microenvironments in Cryptococcus neoformans infection. MBio. 2013;4: e00264-13.
101. Piccolo V, Curina A, Genua M, Ghisletti S, Simonatto M, Sabò A, et al. Opposing macrophage polarization programs show extensive epigenomic and transcriptional cross-talk. Nat Immunol. 2017;18: 530.
102. Vassilopoulos D, Calabrese LH. Risks of immunosuppressive therapies including biologic agents in patients with rheumatic diseases and co-existing chronic viral infections. Curr Opin Rheumatol. 2007;19: 619–625.
103. Zheng D, Wang Y, Cao Q, Lee VWS, Zheng G, Sun Y, et al. Transfused macrophages ameliorate pancreatic and renal injury in murine diabetes mellitus. Nephron Exp Nephrol. 2011;118: e87–e99.