Periodontal diseases are highly prevalent and are linked to several systemic diseases. The goal of periodontal treatment is to halt the progression of the disease and regenerate the damaged tissue. However, achieving complete and functional periodontal regeneration is challenging because the periodontium is a complex apparatus composed of different tissues, including bone, cementum, and periodontal ligament. Stem cell-based regenerative therapy may represent an effective therapeutic tool for periodontal regeneration due to their plasticity and ability to differentiate into different cell lineages. This review presents and critically analyzes the available information on stem cell-based therapy for the regeneration of periodontal tissues and suggests new avenues for the development of more effective therapeutic protocols.

In response to a defined panel of stimuli, immature macrophages can be classified into two major phenotypes: proinflammatory (M1) and anti-inflammatory (M2). Although both phenotypes have been implicated in several chronic inflammatory diseases, their direct role in bone resorption remains unclear. The present study investigated the possible effects of M1 and M2 macrophages on RANKL-induced osteoclastogenesis. In osteoclastogenesis assays using RAW264.7 cells or bone marrow cells as osteoclast precursors, addition of M1 macrophages significantly suppressed RANKL-induced osteoclastogenesis compared to nonstimulated conditions (M0), addition of M2 macrophages, or no macrophage addition (P < 0.05), suggesting that M1 macrophages can downregulate osteoclastogenesis. This effect was maintained when direct contact between M1 and osteoclast precursors was interrupted by cell culture insertion, indicating engagement of soluble factors released from M1. Macrophages which originate from monocytes not only are the key effector cells in innate immunity but also play a pivotal role in the initiation of adaptive immunity (1). It is well documented that polarized macrophages can be classified mainly into two different phenotypes: proinflammatory (M1) and antiinflammatory (M2). The production of inflammatory cytokines, such as tumor necrosis factor alpha (TNF-␣) and interleukin-6 (IL-6), by M1 macrophages promotes inflammation in the context of innate immune response, whereas the production of antiinflammatory cytokines and arginase by M2 macrophages leads to the resolution of inflammation (2). On the other hand, it is also true that osteoclasts that are engaged in bone resorption also belong to monocyte-lineage cells. Although macrophages and osteoclasts share the same precursor, macrophage colony-stimulating factor (M-CSF)-stimulated monocytes, the possible influence of macrophages, and especially the difference between M1 and M2, on osteoclastogenesis is largely unknown.Bone is a unique mineralized tissue which constantly undergoes a physiological remodeling process, and its homeostasis is achieved by the tuned balance between osteoclasts and boneforming cells (osteoblasts). As such, aberrantly promoted osteoclastogenesis is attributed to the bone destruction found in bone lytic diseases such as periodontitis and rheumatoid arthritis, which affects more than 50 million people in the United States alone (3). Of importance to this study, recent research has revealed that osteoclastogenesis is regulated by the immune system. For instance, a recent study (4) revealed that ␥␦-T cells inhibit osteoclastogenesis by their production of interferon gamma (IFN-␥), whereas B and T cells can produce RANKL under inflammatory conditions, thus working toward the promotion of osteoclastogenesis (5). However, in the context of bone lytic diseases involving chronic inflammation, such as periodontitis and rheumatoid arthritis, infiltrations of not only B and T cells but also of macrophages are observed (6, 7). It is true that macrophages are t...

Cell fusion-mediated formation of multinuclear osteoclasts (OCs) plays a key role in bone resorption. It is reported that 2 unique OC-specific fusogens [ i.e., OC-stimulatory transmembrane protein (OC-STAMP) and dendritic cell-specific transmembrane protein (DC-STAMP)], and permissive fusogen CD9, are involved in OC fusion. In contrast to DC-STAMP-knockout (KO) mice, which show the osteopetrotic phenotype, OC-STAMP-KO mice show no difference in systemic bone mineral density. Nonetheless, according to the ligature-induced periodontitis model, significantly lower level of bone resorption was found in OC-STAMP-KO mice compared to WT mice. Anti-OC-STAMP-neutralizing mAb down-modulated in vitro: 1) the emergence of large multinuclear tartrate-resistant acid phosphatase-positive cells, 2) pit formation, and 3) mRNA and protein expression of CD9, but not DC-STAMP, in receptor activator of NF-κB ligand (RANKL)-stimulated OC precursor cells (OCps). While anti-DC-STAMP-mAb also down-regulated RANKL-induced osteoclastogenesis in vitro, it had no effect on CD9 expression. In our mouse model, systemic administration of anti-OC-STAMP-mAb suppressed the expression of CD9 mRNA, but not DC-STAMP mRNA, in periodontal tissue, along with diminished alveolar bone loss and reduced emergence of CD9 OCps and tartrate-resistant acid phosphatase-positive multinuclear OCs. The present study demonstrated that OC-STAMP partners CD9 to promote periodontal bone destruction by up-regulation of fusion during osteoclastogenesis, suggesting that anti-OC-STAMP-mAb may lead to the development of a novel therapeutic regimen for periodontitis.-Ishii, T., Ruiz-Torruella, M., Ikeda, A., Shindo, S., Movila, A., Mawardi, H., Albassam, A., Kayal, R. A., Al-Dharrab, A. A., Egashira, K., Wisitrasameewong, W., Yamamoto, K., Mira, A. I., Sueishi, K., Han, X., Taubman, M. A., Miyamoto, T., Kawai, T. OC-STAMP promotes osteoclast fusion for pathogenic bone resorption in periodontitis via up-regulation of permissive fusogen CD9.

By binding to its chemokine receptor CXCR4 on osteoclast precursor cells (OCPs), it is well known that stromal cell-derived factor-1 (SDF-1) promotes the chemotactic recruitment of circulating OCPs to the homeostatic bone remodeling site. However, the engagement of circulating OCPs in pathogenic bone resorption remains to be elucidated. The present study investigated a possible chemoattractant role of macrophage migration inhibitory factor (MIF), another ligand for C-X-C chemokine receptor type 4 (CXCR4), in the recruitment of circulating OCPs to the bone lytic lesion. To accomplish this, we used Csf1r-eGFP-knock-in (KI) mice to establish an animal model of polymethylmethacrylate (PMMA) particle-induced calvarial osteolysis. In the circulating Csf1r-eGFPþ cells of healthy Csf1r-eGFP-KI mice, Csf1rþ/CD11bþ cells showed a greater degree of RANKL-induced osteoclastogenesis compared to a subset of Csf1rþ/RANKþ cells in vitro. Therefore, Csf1r-eGFPþ/CD11bþ cells were targeted as functionally relevant OCPs in the present study. Although expression of the two cognate receptors for MIF, CXCR2 and CXCR4, was elevated on Csf1rþ/CD11bþ cells, transmigration of OCPs toward recombinant MIF in vitro was facilitated by ligation with CXCR4, but not CXCR2. Meanwhile, the level of PMMA-induced bone resorption in calvaria was markedly greater in wild-type (WT) mice compared to that detected in MIF-knockout (KO) mice. Interestingly, in contrast to the elevated MIF, diminished SDF-1 was detected in a particle-induced bone lytic lesion of WT mice in conjunction with an increased number of infiltrating CXCR4þ OCPs. However, such diminished SDF-1 was not found in the PMMA-injected calvaria of MIF-KO mice. Furthermore, stimulation of osteoblasts with MIF in vitro suppressed their production of SDF-1, suggesting that MIF can downmodulate SDF-1 production in bone tissue. Systemically administered anti-MIF neutralizing monoclonal antibody (mAb) inhibited the homing of CXCR4þ OCPs, as well as bone resorption, in the PMMA-injected calvaria, while increasing locally produced SDF-1. Collectively, these data suggest that locally produced MIF in the inflammatory bone lytic site is engaged in the chemoattraction of circulating CXCR4þ OCPs.