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. 2021 Oct 12;101(3):348–356. doi: 10.1177/00220345211040729

Novel Preosteoclast Populations in Obesity-Associated Periodontal Disease

KH Kwack 1, L Zhang 1, J Sohn 1,2,3, V Maglaras 1, R Thiyagarajan 2,4, KL Kirkwood 1,5,✉
PMCID: PMC8982008  PMID: 34636272

Abstract

Although there is a clear relationship between the degree of obesity and periodontal disease incidence, the mechanisms that underpin the links between these conditions are not completely understood. Understanding that myeloid-derived suppressor cells (MDSCs) are expanded during obesity and operate in a context-defined manner, we addressed the potential role of MDSCs to contribute toward obesity-associated periodontal disease. Flow cytometry revealed that in the spleen of mice fed a high-fat diet (HFD), expansion in monocytic MDSCs (M-MDSCs) significantly increased when compared with mice fed a low-fat diet (LFD). In the osteoclast differentiation assay, M-MDSCs isolated from the bone marrow of HFD-fed mice showed a larger number and area of osteoclasts with a greater number of nuclei. In the M-MDSCs of HFD-fed mice, several osteoclast-related genes were significantly elevated when compared with LFD-fed mice according to a focused transcriptomic platform. In experimental periodontitis, the number and percentage of M-MDSCs were greater, with a significantly larger increase in HFD-fed mice versus LFD-fed mice. In the spleen, the percentage of M-MDSCs was significantly higher in HFD-fed periodontitis-induced (PI) mice than in LFD-PI mice. Alveolar bone volume fraction was significantly reduced in experimental periodontitis and was further decreased in HFD-PI mice as compared with LFD-PI mice. The inflammation score was significantly higher in HFD-PI mice versus LFD-PI mice, with a concomitant increase in TRAP staining for osteoclast number and area in HFD-PI mice over LFD-PI mice. These data support the concept that M-MDSC expansion during obesity to become osteoclasts during periodontitis is related to increased alveolar bone destruction, providing a more detailed mechanistic appreciation of the interconnection between obesity and periodontitis.

Keywords: periodontitis; inflammation; bone; myeloid-derived suppressor cells; osteoclasts, metabolic diseases

Introduction

Over the past 4 decades, the prevalence of obesity has tripled in the United States (Hales et al. 2018). The Western diet has greatly contributed to this surge with concomitant comorbidities associated with metabolic diseases, such as diabetes and cardiovascular disease, which affect chronic periodontitis severity. A diet rich in fats not only causes elevated levels of circulating fatty acids but also shifts gut microbiome to a more gram-negative state with increased lipopolysaccharide circulation (Gallagher et al. 2010). Additionally, obesity-related changes in adipose tissue raise the production of proinflammatory cytokines and adipokines, which induce inflammatory processes and oxidative stress disorders, leading to pathophysiologic changes. In terms of percentage composition, macrophages make up 5% to 10% of normal adipose tissue, but this increases to about 60% macrophage infiltration during obesity (Weisberg et al. 2003). In addition, obese individuals typically have high levels of circulating interleukin 6 (IL-6), tumor necrosis factor α (TNF-α), and prostaglandin E2. As macrophage infiltrations increase, so do inflammatory mediators, promoted further by TNF-α, IL-1β, and IL-6, which are additionally stimulated by leptin, creating a feedback loop that sustains the proinflammatory environment.

Inflammation is an immunologically appropriate response against various pathogens and tissue damage, but excessive inflammation secondary to obesity and hyperglycemia can lead to excessive production of inflammatory cytokines, as well as several chemokines, which ultimately can damage numerous tissues, including alveolar bone. Therefore, as a direct or indirect result of this excessive immune response, multiple chronic inflammatory bone-related diseases develop concurrently with obesity, such as arthritis, osteoporosis, and periodontal disease (Zhao et al. 2007; Suvan et al. 2018; de Resende Guimaraes et al. 2019).

Although there is a clear relationship between the degree of obesity and periodontal disease incidence and severity (Pischon et al. 2007; Chaffee and Weston 2010; Suvan et al. 2011; Martinez-Herrera et al. 2017; Suvan et al. 2018), early studies examining the mechanistic underpinnings focused on the aspect of changes in immune function due to systemic inflammation with simultaneous increased bacterial susceptibility (Amar et al. 2007; Zelkha et al. 2010). Obesity is often associated with alterations in innate immune responses, including cytokine and chemokine expression, which appear to be secondary to chronic elevated hyperglycemia and circulating free fatty acids. Indeed, immune dysregulation seems to be an important component of the pathogenesis of obesity-related periodontal disease progression (Zhu and Nikolajczyk 2014).

Myeloid progenitor cells are the precursors of red blood cells, platelets, granulocytes, monocytes/macrophages, dendritic cells, mast cells, and osteoclasts. The balance between proliferation and differentiation of myeloid progenitor cells in the bone marrow is tightly controlled. When there is an abnormality, such as a disease state, the balance of the myeloid precursor population changes. Myeloid-derived suppressor cells (MDSCs) represent one of these myeloid precursor populations, well known to be expanded during obesity with potent immunosuppressive function (Singer et al. 2014; Ostrand-Rosenberg 2018; Pawelec et al. 2019).

Since the initial discovery of MDSCs, one of the controversial issues related with MDSC biology has been defining this immature myeloid cell population. The subtypes of MDSCs include monocytic MDSC (M-MDSC) and granulocytic or polymorphonuclear MDSC (PMN-MDSC), which are morphologically very similar to monocytes and granulocytes, respectively. The PMN-MDSC phenotype is identical to that of neutrophils, and the M-MDSC phenotype is the same as inflammatory monocytes. For this reason, MDSCs must be identified not only by specific cell surface markers but also by their immunosuppressive activity. In mice, a subset marker of MDSCs is defined as CD11b+Ly6G−Ly6Chigh for M-MDSC and CD11b+Ly6G+Ly6Clow for PMN-MDSC. Inhibition of T-cell proliferation has become the gold standard for defining MDSCs and appears to be sufficient to designate MDSC if T-cell proliferation is evident when phenotypic criteria are met (Bronte et al. 2016).

In chronic inflammation, MDSCs expand and activate, thereby contributing to immunosuppression and oxidative stress. Several reports support M-MDSC plasticity as osteoclast progenitors under various pathologic conditions associated with bone destruction (Sawant and Ponnazhagan 2013; Zhang et al. 2015; Kirkwood et al. 2018). However, just 1 study addressed MDSCs in relation to periodontal disease but not in the context of experimental periodontitis, and it lacked any mechanistic insights (Su et al. 2017). In this study, new knowledge is provided by showing that M-MDSCs are significantly expanded in obesity with experimental periodontal disease, with enhanced osteoclastogenic potential indicating that M-MDSCs can contribute to obesity-associated periodontal disease progression.

Materials and Methods

Mouse Obesity Model

Male C57Bl/6J mice were purchased from Jackson Laboratories at 3 wk of age, randomized into 2 groups (n = 4/group), and housed under specific pathogen–free animal facility conditions. One group was fed an irradiated chow low-fat diet (LFD; 10% energy from fat, D12450Hi [Research Diets]), and the other group was fed a high-fat diet (HFD; 45% energy from fat, D12451i [Research Diets]) that was micronutrient matched to the LFD. All animal studies were conducted according to the policies of the Institutional Animal Care and Use Committee and performed in accordance with the National Institutes of Health’s Guide for Care and Use of Laboratory Animals. The ARRIVE Guidelines Checklist was implemented for this study.

Flow Cytometry and M-MDSC Characterization

Single-cell suspensions were isolated from the bone marrow and spleen of LFD- and HFD-fed male mice at 16 wk of age. Cytologic examination of bone marrow M-MDSCs was achieved through cytospin isolation and imaging. Live cells were treated with Fc block and stained for anti-mouse: anti-CD11b APC (M1/70.15.11.5), anti-Ly6G PE (REA 526), and anti-Ly6C FITC (REA 796). For T-cell proliferation assay, M-MDSCs were isolated from mouse bone marrow via the Myeloid-Derived Suppressor Cell Isolation Kit (Miltenyi Biotec) according to the manufacturer’s protocol. T cells were isolated from the spleen of C57Bl/6J mice and labeled with CTV (Invitrogen), according to the manufacturer’s instruction. CTV-labeled T cells were placed at 2 × 105 cells per well, activated with CD3/CD28 (Thermo Fisher Scientific), and cocultured with the isolated M-MDSCs for 3 d. The cells were washed and stained with CD3-APC (REA641; Miltenyi), CD4-pE-cy7 (GK1.5; e-Bioscience), and CD8-APC-cy7 (REA601; Miltenyi). Data were collected on a MACSQuant System (Miltenyi Biotec) and analyzed with FlowJo software (version 10.6).

In Vitro Osteoclastogenesis Assay

M-MDSCs were isolated from bone marrow and sorted by an AutoMACS Pro Separator (Miltenyi Biotec). For osteoclastogenic differentiation, M-MDSCs were initially seeded at 1 × 105 per well in the presence of monocyte colony-stimulating factor (25 ng/mL) and RANKL (50 ng/mL) from R&D Systems in α-MEM media containing 10% FBS and 1% penicillin/streptomycin. Osteoclast formation was assessed by tartrate-resistant acid phosphatase (TRAP) staining after 6 d.

NanoString Analyses

Following isolation of M-MDSCs from LFD- and HFD-fed mice and incubation in osteoclastogenic differentiation medium for 2 d, mRNA was isolated and expression profiling performed through the NanoString nCounter gene expression system (NanoString Technologies). An 85-gene subset was analyzed by the nCounter custom mouse Osteoclast_520 Profiling Panel (designed by Mouse Genome Informatics) and by using “osteoclast differentiation” in the Gene Ontogeny Browser. Statistical testing was performed via a 2-sample t test, and the results were visualized with RStudio 1.0.143 with the EnhancedVolcano 1.4.0 and pheatmap 1.0.12 packages (Kolde 2019; Blighe et al. 2020).

Experimental Periodontitis

Male C57Bl/6J mice were randomly divided into 2 groups and fed either LFD or HFD for 16 wk. Equal numbers of 16-wk-old LFD- and HFD-fed mice were used in an experimental periodontitis model. A 6-0 silk ligature (Hu-Friedy) was gently tied around the left maxillary second molar to prevent damage to surrounding periodontal tissues. The contralateral second molar of each mouse was mock ligated as a negative control. The ligatures remained intact in all mice during the 7-d experimental period. Mice were euthanized and the specimens collected for downstream analyses.

Microcomputed Tomography and Histologic Examination

Maxillae were fixed and scanned as previously described (Steinkamp et al. 2018). The examiner was blinded to treatment groups. Three-dimensional images were generated and rotated with a standard orientation. Bone volume fraction was measured as the percentage of bone volume within the total volume (region of interest) with AnalyzePro software (version 14.0) (AnalyzeDirect, Inc.). Maxillae were decalcified with 0.5M EDTA as previously described (Steinkamp et al. 2018). Inflammation was scored by dividing the maxillary periodontal tissue into 2 parts: the part around the ligation and the other part. The 2 parts were scored separately and then combined. The following scoring system was used for each part: 0 = 0% to 5% inflammatory cell (IC) infiltration; 1 = 5% to 25% ICs; 2 = 25% to 50% ICs; and 3 = >50% ICs. Sections of 7-µm-thick periodontal tissues were stained with TRAP and fast green (counterstain) to enumerate osteoclasts as shown previously (Steinkamp et al. 2018). TRAP-positive areas and eroded bone perimeters were quantified with Image J software. Data are reported according to a standardized nomenclature (Dempster et al. 2013).

Statistical Methods

Experiments were analyzed with Prism 8 (GraphPad) via unpaired t tests and 1- and 2-way analysis of variance with Tukey’s multiple-comparisons test.

Results

HFD Expands the M-MDSC Population

HFD-fed mice exhibited diet-induced obesity, with elevated inflammation and adipokine markers at 16 wk (Appendix Fig. 1), so we used HFD-fed mice at 16 wk of age. Cytospin was performed to compare the morphology of M-MDSCs in LFD- and HFD-fed mice, and there was no difference in that isolated from bone marrow (Fig. 1A). Single-cell splenocyte suspensions were analyzed by flow cytometry to determine myeloid populations in LFD- and HFD-fed mice. In HFD-fed mice, M-MDSCs (CD11b+Ly6ChighLy6G−) are significantly increased by percentage and cell number (Fig. 1B). M-MDSCs of LFD- and HFD-fed mice suppressed CD4+ and CD8+ T-cell proliferation, confirming that this population of M-MDSCs functionally immunosuppressive (Fig. 1C).

Figure 1.

Figure 1.

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M-MDSCs are expanded in HFD-fed mice. (A) Representative cytospin images show morphology of M-MDSCs in LFD/HFD-fed mice. Scale bar = 10 μm. (B) Flow cytometric analysis of immune cell populations in LFD/HFD-fed mouse spleen. (C) T-cell proliferation assay of M-MDSCs isolated from LFD/HFD-fed mice. One-way analysis of variance with Tukey’s multiple-comparisons test and unpaired t test. Data are presented as mean ± SEM. *P < 0.05. **P < 0.01. HFD, high-fat diet; LFD, low-fat diet; M-MDSC, monocytic myeloid-derived suppressor cell.

HFD Enhances the Osteoclastogenesis of M-MDSCs

TRAP staining was performed to determine whether M-MDSCs of LFD- and HFD-fed mice differentiate into osteoclasts. Quantitative analysis of TRAP+ staining of osteoclast formation indicated that osteoclast number is higher in HFD-fed mice (Fig. 2). M-MDSCs of HFD-fed mice exhibited a greater capacity to form mature osteoclasts (≥3 nuclei per osteoclast) and formed significantly larger osteoclasts under the same in vitro experimental conditions as compared with LFD-fed mice. To compare the mRNA transcripts associated with osteoclast differentiation in each gene, NanoString osteoclast differentiation–focused transcriptomic analysis was performed with the mRNA of M-MDSCs from LFD- and HFD-fed mice. We utilized a heat map to visualize the gene expression profiles of LFD- and HFD-fed mice, which were clearly distinguished (Fig. 3A). A volcano plot showed 59 variables, identifying 27 upregulated genes and 3 downregulated genes, with the remaining mRNAs unchanged or not expressed (Fig. 3B, Appendix Table 1). mRNAs that are highly expressed in HFD-fed mice are schematically presented. An increase in mRNA expression related to osteoclast differentiation/maturation from HFD M-MDSCs is depicted in red (Fig. 3C), including transferrin (Tfrc), TRAF6 (Traf6), and calcitonin receptor (Calcr). In addition, genes related to osteoclast function (Fig. 3D), including carbonic anhydrase 2 (Ca2) and V-type proton ATPase (Tcirg1), were significantly elevated HFD-fed mice, indicating that obesity regulates M-MDSC preosteoclasts via selective transcriptomic mechanisms.

Figure 2.

Figure 2.

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M-MDSCs from HFD-fed mice showed stronger osteoclastogenic differentiation potential. (A) Representative TRAP stain in which M-MDSCs from LFD/HFD-fed mice were cultured in osteoclastogenic differentiated medium. (B) Osteoclast cellular endpoints were performed on TRAP-stained LFD/HFD-fed mice. Unpaired t test. Data are presented as mean ± SEM. *P < 0.05. ****P < 0.0001. HFD, high-fat diet; LFD, low-fat diet; M-MDSC, monocytic myeloid-derived suppressor cell; OC, osteoclast; TRAP, tartrate-resistant acid phosphate.

Figure 3.

Figure 3.

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Gene expression profile of the osteoclast differentiation was increased in M-MDSCs of HFD-fed mice. (A–D) NanoString analysis of M-MDSCs in HFD- and LFD-fed mice. (A) Heat map shows the 30 significant genes that were differentially expressed between HFD- and LFD-fed mice. (B) Volcano plots show the 59-variable distribution of the fold changes in gene expression. (C, D) Schematic diagram of osteoclastogenesis for NanoString analysis. High expression levels in HFD-fed mice are indicated in red. Schematic diagram of gene expression involved in (C) osteoclast differentiation and maturation and (D) osteoclast function. Comparisons were analyzed with unpaired t test. HFD, high-fat diet; LFD, low-fat diet; M-MDSC, monocytic myeloid-derived suppressor cell.

Obesity-Related Periodontal Disease Expands M-MDSCs and Accelerates Alveolar Bone Loss

To determine the change in M-MDSCs in periodontal disease associated with obesity, cell population changes in bone marrow and spleen were confirmed. As shown in Figure 4A, M-MDSCs expanded in bone marrow when periodontitis was induced, with a larger increase in HFD-fed mice when compared with LFD-fed mice. The spleen showed a similar trend and was significantly higher in HFD-PI than LFD-PI mice (Fig. 4B). These results indicate that HFD-PI significantly raises M-MDSC bone marrow and spleen when compared with LFD-PI. Static histomorphometric analysis of HFD-fed mice at 32 wk indicated only trabecular bone thickness changed with obesity (Appendix Fig. 2). Alveolar bone volume fraction was also significantly decreased during experimental periodontitis where significantly more bone loss occurred in HFD-PI mice than in LFD-PI mice (Fig. 4C, D).

Figure 4.

Figure 4.

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M-MDSCs are significantly expanded with concomitant increased bone loss in obesity-associated periodontal disease. (A, B) Flow cytometry analysis of immune cell populations. CD11b+Ly6ChighLy6G− M-MDSCs with percentage of total live lymphocyte population and total cell number in (A) bone marrow and (B) spleen displayed. (C) Representative µCT images show quantitative bone loss in obesity-associated periodontal disease. (D) µCT analysis of bone volume/total volume (BV/TV) of alveolar bone. One-way analysis of variance with Tukey’s multiple comparisons test. Data are presented as mean ± SEM. *P < 0.05. **P < 0.01. ***P < 0.001. HFD, high-fat diet; LFD, low-fat diet; M-MDSC, monocytic myeloid-derived suppressor cell; µCT, microcomputed tomography.

Obesity-Related Periodontal Disease Increases Periodontal Inflammation and Osteoclastogenesis Potential

Histomorphometric analysis was performed to examine the proinflammatory endpoints in obesity-associated periodontal disease. Inflammation was most severe in the ligation site and significantly higher in HFD-PI mice over LFD-PI (Fig. 5A, B). To determine the role of obesity-related periodontal disease in osteoclast formation, a TRAP stain was performed (Fig. 5C, D). The number and area of osteoclasts were significantly greater in the maxillae in which periodontitis was induced and in HFD-PI versus LFD-PI mice.

Figure 5.

Figure 5.

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Alveolar bone inflammation and osteoclasts were increased in obesity-associated periodontal disease. (A) Representative hematoxylin and eosin–stained maxillae from each group, with insert displaying inflammation at the part around the ligation (top) and the other part (bottom) of the second molar (scale bar = 500 µm). (B) Inflammation scoring of connective tissue around the second molar of maxillae and all other regions. (C) Representative TRAP stain of maxillae with insert displaying root outlined (scale bar = 500 µm at 4×, 100 µm at 20×). (D) Osteoclast number and area were calculated on TRAP and fast green–counterstained maxillae in each group. One-way analysis of variance with Tukey’s multiple comparisons test. Data are presented as mean ± SEM. *P < 0.05. **P < 0.01. ***P < 0.001. ****P < 0.0001. HFD, high-fat diet; LFD, low-fat diet; OC, osteoclast; TRAP, tartrate-resistant acid phosphate.

Discussion

Epidemiologic data support a positive association between periodontal diseases and obesity; however, there is a considerable gap in knowledge regarding the mechanistic link that governs obesity-associated remodeling of alveolar bone. While it has been well established that bacterial components such as lipopolysaccharide can cause enhanced inflammatory alveolar bone loss during obesity-associated periodontitis (Jin, Machado, et al. 2014; Jin, Zhang, et al. 2014; Li et al. 2020), there are fewer mechanistic data addressing cell-intrinsic differences in osteoclast precursor populations that can contribute to oral bone loss in obese states. In this study, we found that M-MDSCs are expanded with enhanced osteoclastogenic capacity during obesity with greater concomitant alveolar bone loss during experimental periodontitis.

In the present study, only the trabecular bone microarchitecture (trabecular thickness) of the maxillae in 32-wk-old HFD-fed mice was reduced versus LFD-fed mice (Appendix Fig. 2). These data are consistent with published data where there was no change in the cortical bone mass in the tibia of HFD mice but just the trabecular thickness decreased (Cao et al. 2009). In addition, it is consistent with the results of previous studies showing that obesity negatively affects alveolar bone density and microarchitecture in rats and young growing mice (Pramojanee et al. 2013; Fujita and Maki 2015). Taken together, these data support the view that obesity raises the risk of periodontal bone loss by reducing oral bone integrity.

While there is a clear positive relationship between obesity and MDSC expansion (Xia et al. 2011; Clements et al. 2018), no studies have addressed the relationship of MDSC expansion and osteoclast differentiation, which could exacerbate obesity-associated periodontitis. Previous studies have shown that HFD increases osteoclastogenesis through extrinsic cellular mechanisms that indicate elevated levels of circulating proinflammatory cytokines, such as IL-1 and TNF-α (Fuentes et al. 2010), while others have shown higher RANKL expression in bone (Cao et al. 2009; Lorincz et al. 2010) and decreased antiosteoclastogenic cytokine IL-10 expression (Kyung et al. 2009). To date, no mechanistic data are available to understand intrinsic cell differences that govern HFD-induced M-MDSC differentiation into osteoclasts.

Although we have focused on immune cell changes, obesity also alters the microbiome, which may contribute to obesity-aggravated alveolar bone loss. A recent study highlights that obesity-related changes in the gut microbiome raise periodontal disease susceptibility through an increase in uric acid (purine degradation) and were inhibited by allopurinol (Sato et al. 2021). However, the authors did not show the changes in the immune cell populations that accompanied the oral microbiome shifts. Host immune changes and changes in the oral/gut microbiome clearly must work in concert to direct obesity-associated alveolar bone loss that occurs during periodontitis.

We demonstrated that M-MDSCs isolated from LFD- and HFD-fed mice can directly differentiate into osteoclasts. Furthermore, quantitative analysis of osteoclastogenesis with TRAP staining indicated that the number and area of osteoclasts were significantly higher from M-MDSCs isolated from HFD-fed mice than LFD-fed mice. These data support the view that M-MDSCs of HFD-fed mice may be primed within the bone marrow to support enhanced osteoclastogenesis capacity. Mechanistically, focused transcriptomic analyses of M-MDSCs during osteoclast differentiation from HFD-fed mice showed 27 of the 85 genes from the focus NanoString code set to have significantly higher expression than that in LFD-fed mice. Interestingly, only 3 genes were downregulated with 55 genes either unchanged or nonexpressed (see Appendix Table 1). Within the upregulated genes from the M-MDSCs derived from HFD-fed mice, some novel differences were observed. Several mRNAs were increased in M-MDSCs derived from HFD-fed mice within the RANKL-stimulated signaling pathway, including well-known osteoclastogenic signaling intermediates or definitive marker genes of osteoclast differentiation, such as Traf6 (tumor necrosis factor receptor–associated factor 6), Csf1r (colony-stimulating factor 1 receptor), Jun (transcription factor AP-1), Pparg (peroxisome proliferator–activated receptor gamma), and Calcr (calcitonin receptor), with the most significant increase in Calcr mRNA expression. Interestingly, Oscar (osteoclast-associated immunoglobulin-like receptor) was higher in HFD-derived M-MDSCs, indicating that canonical and noncanonical osteoclastogenic pathways are activated during obesity-induced osteoclastogenesis. In addition to the more traditional pathways engaged, we observed that Tfrc (transferrin receptor) was highly elevated in HFD-derived M-MDSCs when differentiating into osteoclasts. Iron, though used primarily for erythropoiesis in the bone marrow, can be vital to other cellular functions. Iron metabolism is known to be disturbed during obesity, resulting in iron deficiency and overload (Gonzalez-Dominguez et al. 2020). Osteoclasts deficient in TfR1 (transferrin receptor 1) have greater trabecular bone mass, suggesting that TfR1 is critical for osteoclastogenesis in bone remodeling (Balogh et al. 2018). Future studies will be needed to understand the role of obesity and iron metabolism with osteoclastogenesis. In addition, it was observed that although 3 mRNAs were downregulated in HFD-derived M-MDSC preosteoclast populations, Tnf (encoding for TNF-α) was the most significantly decreased, suggesting that potential compensatory mechanisms are in play during obese states to help reduce osteoclast formation. At this time, it is not understood if transcriptomic differences are unique to the M-MDSC preosteoclast populations. Forthcoming studies will compare M-MDSC osteoclast differentiation transcriptomic differences with other defined osteoclast precursor populations.

Understanding that M-MDSCs can be osteoclast progenitors and that the number of M-MDSCs increases significantly during obesity, their role in the other osteoclast-associated diseases has been considered. For example, in bone metastasis studies, MDSCs (Gr1+CD11b+) were primed to become osteoclast precursor cells, and the bone microenvironment triggered the differentiation of MDSCs into functional osteoclasts (Sawant and Ponnazhagan 2013). Importantly, MDSC populations differentiate into osteoclasts only when there is a bone marrow cell signal and bone metastasis signal. Given these data, we recently proposed a “2-hit” model in which 1) signals from the obese bone marrow prime the MDSCs in the bone marrow and 2) the periodontal microenvironment affects the fate of MDSCs in a context-dependent manner in which MDSCs differentiate into osteoclasts (Kwack et al. 2021). Future studies will need to address if obesity-expanded M-MDSCs traffic to the site of periodontal infection and directly differentiated into osteoclasts.

Author Contributions

K.H. Kwack, contributed to conception, design, data analysis, and interpretation, drafted and critically revised the manuscript; L. Zhang, contributed to design, data acquisition, and analysis, drafted and critically revised the manuscript; J. Sohn, contributed to design and data analysis, drafted and critically revised the manuscript; V. Maglaras, R. Thiyagarajan, contributed to design and data acquisition, drafted and critically revised the manuscript; K.L. Kirkwood, contributed to conception, design, and data interpretation, drafted and critically revised the manuscript. All authors gave final approval and agree to be accountable for all aspects of the work.

Supplemental Material

sj-pdf-1-jdr-10.1177_00220345211040729 – Supplemental material for Novel Preosteoclast Populations in Obesity-Associated Periodontal Disease
Click here for additional data file. (1.1MB, pdf)

Supplemental material, sj-pdf-1-jdr-10.1177_00220345211040729 for Novel Preosteoclast Populations in Obesity-Associated Periodontal Disease by K.H. Kwack, L. Zhang, J. Sohn, V. Maglaras, R. Thiyagarajan and K.L. Kirkwood in Journal of Dental Research

Footnotes

Declaration of Conflicting Interests: The authors declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding: The authors disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This work was supported by the National Institutes of Health grant 1K18DE029526 and School of Dental Medicine at the University at Buffalo.

A supplemental appendix to this article is available online.

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Supplementary Materials

sj-pdf-1-jdr-10.1177_00220345211040729 – Supplemental material for Novel Preosteoclast Populations in Obesity-Associated Periodontal Disease
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Supplemental material, sj-pdf-1-jdr-10.1177_00220345211040729 for Novel Preosteoclast Populations in Obesity-Associated Periodontal Disease by K.H. Kwack, L. Zhang, J. Sohn, V. Maglaras, R. Thiyagarajan and K.L. Kirkwood in Journal of Dental Research

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