Commensal gut bacterium critically regulates alveolar bone homeostasis.

The alveolar bone is a unique osseous tissue due to the presence of the teeth and the proximity of commensal oral microbes. Commensal microbe effects on alveolar bone homeostasis have been attributed to the oral microbiota, yet the impact of commensal gut microbes is unknown. Study purpose was to elucidate whether commensal gut microbes regulate osteoimmune mechanisms and skeletal homeostasis in alveolar bone. Male C57BL/6T germfree (GF) littermate mice were maintained as GF or monoassociated with segmented filamentous bacteria (SFB), a commensal gut bacterium. SFB has been shown to elicit broad immune response effects, including the induction of TH17/IL17A immunity, which impacts the development and homeostasis of host tissues. SFB colonized the gut, but not oral cavity, and increased IL17A levels in the ileum and serum. SFB had catabolic effects on alveolar bone and non-oral skeletal sites, which was attributed to enhanced osteoclastogenesis. The alveolar bone marrow of SFB vs. GF mice had increased dendritic cells, activated helper T-cells, TH1 cells, TH17 cells, and upregulated Tnf. Primary osteoblast cultures from SFB and GF mice were stimulated with vehicle-control, IL17A, or TNF to elucidate osteoblast-derived signaling factors contributing to the pro-osteoclastic phenotype in SFB mice. Treatment of RAW264.7 osteoclastic cells with supernatants from vehicle-stimulated SFB vs. GF osteoblasts recapitulated the osteoclast phenotype found in vivo. Supernatants from TNF-stimulated osteoblasts normalized RAW264.7 osteoclast endpoints across SFB and GF cultures, which was dependent on the induction of CXCL1 and CCL2. This report reveals that commensal gut microbes have the capacity to regulate osteoimmune processes in alveolar bone. Outcomes from this investigation challenge the current paradigm that alveolar bone health and homeostasis is strictly regulated by oral microbes.

Antimicrobial-induced oral dysbiosis exacerbates naturally occurring alveolar bone loss.

Periodontitis-mediated alveolar bone loss is caused by dysbiotic shifts in the commensal oral microbiota that upregulate proinflammatory osteoimmune responses. The study purpose was to determine whether antimicrobial-induced disruption of the commensal microbiota has deleterious effects on alveolar bone. We administered an antibiotic cocktail, minocycline, or vehicle-control to sex-matched C57BL/6T mice from age 6- to 12 weeks. Antibiotic cocktail and minocycline had catabolic effects on alveolar bone in specific-pathogen-free (SPF) mice. We then administered minocycline or vehicle-control to male mice reared under SPF and germ-free conditions, and we subjected minocycline-treated SPF mice to chlorhexidine oral antiseptic rinses. Alveolar bone loss was greater in vehicle-treated SPF versus germ-free mice, demonstrating that the commensal microbiota drives naturally occurring alveolar bone loss. Minocycline- versus vehicle-treated germ-free mice had similar alveolar bone loss outcomes, implying that antimicrobial-driven alveolar bone loss is microbiota dependent. Minocycline induced phylum-level shifts in the oral bacteriome and exacerbated naturally occurring alveolar bone loss in SPF mice. Chlorhexidine further disrupted the oral bacteriome and worsened alveolar bone loss in minocycline-treated SPF mice, validating that antimicrobial-induced oral dysbiosis has deleterious effects on alveolar bone. Minocycline enhanced osteoclast size and interface with alveolar bone in SPF mice. Neutrophils and plasmacytoid dendritic cells were upregulated in cervical lymph nodes of minocycline-treated SPF mice. Paralleling the upregulated proinflammatory innate immune cells, minocycline therapy increased TH 1 and TH 17 cells that have known pro-osteoclastic actions in the alveolar bone. This report reveals that antimicrobial perturbation of the commensal microbiota induces a proinflammatory oral dysbiotic state that exacerbates naturally occurring alveolar bone loss.

Specific Commensal Bacterium Critically Regulates Gut Microbiota Osteoimmunomodulatory Actions During Normal Postpubertal Skeletal Growth and Maturation.

The commensal gut microbiota critically regulates immunomodulatory processes that influence normal skeletal growth and maturation. However, the influence of specific microbes on commensal gut microbiota osteoimmunoregulatory actions is unknown. We have shown previously that the commensal gut microbiota enhances TH17/IL17A immune response effects in marrow and liver that have procatabolic/antianabolic actions in the skeleton. Segmented filamentous bacteria (SFB), a specific commensal gut bacterium within phylum Firmicutes, potently induces TH17/IL17A-mediated immunity. The study purpose was to delineate the influence of SFB on commensal gut microbiota immunomodulatory actions regulating normal postpubertal skeletal development. Two murine models were utilized: SFB-monoassociated mice versus germ-free (GF) mice and specific-pathogen-free (SPF) mice +/- SFB. SFB colonization was validated by 16S rDNA analysis, and SFB-induced TH17/IL17A immunity was confirmed by upregulation of Il17a in ileum and IL17A in serum. SFB-colonized mice had an osteopenic trabecular bone phenotype, which was attributed to SFB actions suppressing osteoblastogenesis and enhancing osteoclastogenesis. Intriguingly, SFB-colonized mice had increased expression of proinflammatory chemokines and acute-phase reactants in the liver. Lipocalin-2 (LCN2), an acute-phase reactant and antimicrobial peptide, was substantially elevated in the liver and serum of SFB-colonized mice, which supports the notion that SFB regulation of commensal gut microbiota osteoimmunomodulatory actions are mediated in part through a gut-liver-bone axis. Proinflammatory TH17 and TH1 cells were increased in liver-draining lymph nodes of SFB-colonized mice, which further substantiates that SFB osteoimmune-response effects may be mediated through the liver. SFB-induction of Il17a in the gut and Lcn2 in the liver resulted in increased circulating levels of IL17A and LCN2. Recognizing that IL17A and LCN2 support osteoclastogenesis/suppress osteoblastogenesis, SFB actions impairing postpubertal skeletal development appear to be mediated through immunomodulatory effects in both the gut and liver. This research reveals that specific microbes critically impact commensal gut microbiota immunomodulatory actions regulating normal postpubertal skeletal growth and maturation. © 2020 The Authors. JBMR Plus published by Wiley Periodicals, Inc. on behalf of American Society for Bone and Mineral Research.