"The prevalence of osteoporosis is drastically increasing. Osteoporosis is characterized by low bone mineral density, which results in increased bone fragility and fractures. Current therapeutic drugs for osteoporosis are not effective for everyone, they are expensive, and long-term use can have unwanted and detrimental side effects. Therefore, new therapies that mitigate the current side effects are necessary. However, to further develop better treatments against osteoporosis it is imperative that we understand the mechanisms involved in this disease. In the first part of this thesis, we aimed to understand the role of interleukin 10 (IL-10) in type 1 diabetes (T1D) induced bone loss. Studies have shown that during diabetes there is a significant decrease in IL-10 levels. The role of this cytokine in bone physiology is well known. However, how IL-10 affects bone health during short- and long-term diabetes has not been studied. By using IL-10 deficient mice, we found that in the absence of IL-10, vertebral and femoral trabecular bone loss are significantly exacerbated during T1D, indicating that low levels of IL-10 during T1D can be detrimental to the bone. Interestingly, this effect was only seen at one month after diabetes onset and not at three months, suggesting that during the early stages of diabetes IL-10 is important in modulating bone density. We also identified that these effects were associated with corresponding changes in osteoblast mRNA expression in the bone, suggesting that IL-10 primarily affects bone anabolic events. In vitro data shows that the MAPK pathway is involved in IL-10 regulation of osteoblast gene expression in diabetic conditions. These results demonstrate that IL-10 is required for osteoblast regulation during early diabetes-induced bone loss and that low levels of IL-10 during diabetes could possibly exacerbate bone loss. In the second part of this thesis, we aimed to understand the role of T and B lymphocytes in Lactobacillus reuteri 6475 beneficial effects on bone. Our lab has shown that treatment with L. reuteri enhances bone density and prevents bone loss in several animal models. However, the mechanism by which L. reuteri increases bone density is not completely understood. This study demonstrates that T and B lymphocytes are required for the beneficial effects of L. reuteri on bone density in healthy male mice. We also reveal, in co-culture experiments, that L. reuteri regulates T-cell-expression of factors that increase osteoblast gene expression. Together, this study extends our knowledge in understanding the role of L. reuteri in regulating bone density and identify the T-lymphocytes as a possible mechanism. In the third part of this thesis, we aimed to understand the role of T and B lymphocytes on the effects of gut microbiota dysbiosis on bone density. Our lab recently demonstrated that 2-week oral treatment with broad spectrum antibiotics followed by 4-weeks of natural gut microbiota repopulation results in dysbiosis and femoral trabecular bone loss in male mice. By using T and B lymphocyte deficient mice (Rag-KO) we found that lymphocytes are required for dysbiosis-induced bone loss. We also show that post-antibiotic treated wild type and lymphocyte deficient mice express different microbiota profiles, characterized by low levels of Lactobacillales in wild type compared to Rag knockout mice. Furthermore, treatment with L. reuteri prevents dysbiosis-induced trabecular bone loss in wild type mice. Taken together, our studies demonstrate the role of T and B lymphocytes in dysbiosis-induced bone loss and identify L. reuteri as a possible treatment against bone loss."--Pages ii-iii.
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