Gut bacteria , gut flora, microbiome. Bacteria inside the small intestine, concept, representation. 3D illustration.
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Extensive research is providing new insights into the roles that our normal gut microbiota play on health and disease. A healthy gut microbiome contributes to skeletal health, but studies in mice have now shown that disrupting the commensal microbiome with antibiotics leads to a proinflammatory immune response that disrupts post-pubertal skeletal development. The research team, headed by scientists at the Medical University of South Carolina College of Dental Medicine, said their studies, reported in The American Journal of Pathology, are also the first to demonstrate sex-dependent differences in how antibiotic use changes gut bacterial composition and skeletal morphology.

“This report introduces antibiotics as a critical exogenous modulator of gut microbiota osteoimmune response during post-pubertal skeletal development,” said Chad M. Novince, DDS, PhD, assistant professor in both the Colleges of Medicine and Dental Medicine. “People have shown that antibiotics perturb the microbiota, but this is the first known study evaluating how that has downstream effects on immune cells that regulate bone cells and the overall skeletal phenotype. This work brings the whole story together.”

Novince and colleagues described their studies and results in a paper titled, “Antibiotic Perturbation of Gut Microbiota Dysregulates Osteoimmune Cross Talk in Postpubertal Skeletal Development.”

An accumulation of research has demonstrated that a healthy gut microbiome can influence the health and disease of organs very distant to the gut itself,  including the liver, heart, brain, and skeleton, the authors stated. And while studies have shown that immune cell interactions with bone cells regulate skeletal development and homeostasis, research in mice has also found that the immunomodulatory actions of gut microbiota act to “potently regulate and promote the development and homeostasis of skeletal tissues.” This indicates that disrupting the gut microbiome using antibiotics might dysregulate normal osteoimmunological processes. “Considering that antibiotic disruption of the indigenous gut microbiota has been reported to induce proinflammatory hyperimmune response states, antibiotic administration could notably have unintended pathophysiologic effects impairing the attainment and maintenance of peak skeletal bone mass,” the authors stated.

The post-pubertal phase of development represents a key period during which we develop about 40% of our peak bone mass. Antibiotic disruption of the normal gut microbiome during postnatal skeletal development has already been shown to influence the buildup of bone mass and bone mechanical properties, the team noted. What hasn’t been understood is the osteoimmune mechanisms that link the effects of antibiotics to skeletogenesis.

To investigate the association between antibiotics, micobiome, and skeletal development the researchers administered broad-spectrum antibiotics to male and female mice from the age of 6–12 weeks. Initial tests confirmed that the cocktail of antibiotics used caused major, sex-dependent changes to the composition of the animals’ gut microbiomes. When the team then examined the integrity of the skeletal system in the antibiotic-treated, compared with the control mice, they found that while antibiotic therapy didn’t impact on cortical bone, it was associated with significant changes to trabecular bone, which is the type of bone that undergoes high rates of bone metabolism. Again, the changes were more pronounced in male, compared with female mice. “Broad-spectrum antibiotics induce sex-dependent alterations in the gut microbiota composition, which result in a more profound suppression of trabecular bone parameters in male versus female mice,” they stated.

The control of bone metabolism is a balance between the activity of bone-resorbing osteoclast and bone-building osteoblast cells. The team’s studies found that while antibiotic therapy didn’t result in changes to the osteoblasts, osteoclast cell number, size, and activity were increased. “Antibiotics did not influence osteoblastogenesis or endochondral bone formation, but notably enhanced osteoclastogenesis,” the scientists wrote.

Further analyses found that blood levels of pro-osteoclastic signaling molecules were increased in the antibiotic-treated mice, indicating that upregulation of osteoclast activity resulted from specific immune responses triggered by alterations to the gut microbiota. “Unchanged Tnf or Ccl3 expression in marrow and elevated tumor necrosis factor-a and chemokine (C-C motif) ligand 3 in serum indicated that the pro-osteoclastic effects of the antibiotics are driven by increased systemic inflammation,” the team noted. “Antibiotic-induced broad changes in adaptive and innate immune cells in mesenteric lymph nodes and spleen demonstrated that the perturbation of gut microbiota drives a state of dysbiotic hyperimmune response at secondary lymphoid tissues draining local gut and systemic circulation.”

Examination of immune cell populations in the bone marrow of antibiotic-treated animals also—and surprisingly, the authors acknowledged—highlighted a significant increase in myeloid-derived suppressor cells (MDSCs) in antibiotic-treated animals. MDSCs play a role in regulating the innate and adaptive immune responses to various diseases, but they have not been extensively studied in health. The team’s analyses also found that antigen presentation and processing were suppressed in the bone marrow upon antibiotic treatment. “Intriguingly, antibiotic treatment up-regulated MDSCs and suppressed major histocompatibility complex (MHC) class II antigen processing/presentation in the bone marrow … Antibiotic-induced up-regulation of MDSCs and suppressed MHC class II antigen processing/presentation in the bone marrow implies that the antibiotic disruption of the gut microbiota critically alters osteoimmune cross talk.”

MDSCs are increased in the bone marrow of mice treated with antibiotics (ABX, right) compared to control treated animals (left). [Dr. Chad Novince of the Medical University of South Carolina]

The reported study used a broad-spectrum cocktail of antibiotics that was designed to have a major disruptive effect on the gut microbiome. Nevertheless, the overall findings that antibiotic-related changes to the normal commensal microbiota result in dysregulation of communication between immune cells and bone cells merits further studies involving a more clinically relevant antibiotic regimen. It may then be possible to carry out clinical trials to study the effects of different antibiotics on the gut microbiome, and potentially suggest microbiome-targeted approaches to preventing or treating skeletal diseases.

“This research demonstrates that antibiotic disruption of gut microbiota composition alters host immune response effects, which critically modulates normal osteoimmune processes in the postpubertal developing skeleton,” the authors concluded. “This is the first known report to elucidate antibiotic alteration of gut microbiota effects on bone cell (osteoclast and osteoblast) outcomes … More importantly, this report discerns that exogenous perturbation of gut microbiota immunomodulation impacts normal growth and development at sites distant to the gut.”

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