(613b) Computational Modeling of the Gut-Bone Axis and Implications of Butyrate Treatment on Osteoimmunology

The interplay between gut microbiota and the immune system has a pivotal role in the maintenance of bone health. Recently, short-chain fatty acids (SCFAs) produced by gut microbiota have emerged as key regulatory participants in shaping the immune system. Butyrate, the most versatile among SCFAs, has been observed to have local and systemic effects including inducing the differentiation of peripheral regulatory T cells (T_regs) in the intestine, blood and bone marrow [1]. T_regs are the central actors of the negative feedback component of the immune system. The interaction between T_regs and cytotoxic CD8+ T cells suppress the inflammatory status and promote the production of Wnt10b to increase bone anabolism [2]. Studies in the mouse models shows that the ablation of butyrate in the intestine alter the bone marrow density. However, the therapeutic benefit of butyrate in bone anabolism remains poorly understood. Hence, we use computational modeling to describe the butyrate induced differentiation of T_regs in the intestine blood and bone marrow that promotes the change of Wnt10b expression in the bone marrow.

We are developing a multi-compartment physiologically based pharmacokinetic model to track and quantify effects of butyrate on T_regs in the gut, blood, and bone marrow. The model consists of five species butyrate, naïve CD4+ T cells, T_regs, TGF-β, and Wnt10b distributed across three compartments intestine, blood, and bone. We consider an open system with the processes of formation, excretion, differentiation, cell death, and migration to another compartment. We define mass balances on butyrate, TGF-β, and Wnt10b and population balances for the naïve CD4+ T cells, T_regs. The microbial fermentation of the dietary fiber modulates the constant production of butyrate in the intestine. The constant supply of butyrate in the intestine depends on the degradation and migration to the blood and bone marrow. The butyrate that absorbs into the blood and bone compartments differentiates the naive CD4+ T cells in the bloodstream and bone marrow. The differentiation rate constant is evaluated based on the dynamic in vitro differentiation of naive CD4+ T cells to T_regs in the presence of butyrate. The system is assumed to be at steady state to estimate the unknown parameters for the model based on the in vivo experimental butyrate concentration and percentage T_regs of total cells in each compartment. T_regs accumulate in the bone and induce upregulation of TGF-β, which suppresses an inflammatory state. T_regs also promote Wnt10b secretion that increases net bone formation. The variation of butyrate concentration from homeostasis value changes the percentage of T_regs, and the production of TGF-β, and Wnt10b. We estimate the kinetic parameters for the differentiation of T_regs, fold changes of TGF-β, and Wnt10b with change of butyrate concentration based on data from published in vivo experimental studies.

Using the model, we analyze experimental data reported in [2] to evaluate the expansion of T_regs, TGF-β, and Wnt10b in the bone marrow. The probiotic LGG increases butyrate concentration in intestine and serum blood by 0.18 μM and 0.29 μM respectively. We assume butyrate concentration in bone marrow is same as serum blood. Our simulation result shows 6% increase of T_regs, 3.4-fold increase of TGF-β, and 3-fold increase of Wnt10b in the bone marrow consistent with the net change in formation due to stimulus of a probiotic microcrobiota in the gut. We also analyze the pathological condition where butyrate concentration reduces due to dysbiosis. Li et al. [3] reduced the serum butyrate concentration 0.18 μM using antibiotic in mouse model and reported 5% reduction of T_regs in bone marrow from homeostasis condition. We set the change of intestine butyrate concentration to -0.11 μM in our model to get 0.18 μM reduction of butyrate concentration in serum blood and observe 5.5% reduction of T_regs in bone marrow consistent with the experimental result. The computational approach described here gives insight into the pharmacokinetics of butyrate, biodistribution of T_regs in the gut-bone axis, fold changes of Wnt10b in bone marrow, and their contributions to modulating bone formation. This deeper understanding will allow us to explore the potential of modifying butyrate through dietary interventions as a treatment for different pathological conditions related to bone loss such as estrogen deficiency, hyperthyroidism, and vitamin D deficiency.

References

1. Furusawa, Y. et al. Nature.7480:446-450, 2013.
2. Tyagi, A. M. et al. Immunity.6:1116-1131, 2018.
3. Li, J. Y. et al. The Journal of Clinical Investigation. 130(4):1767-1781,

Acknowledgements

Research reported in this abstract was supported by the National Institute of General Medical Sciences of the National Institutes of Health under award number R35GM133763. The content is solely the responsibility of the authors and does not necessarily represent the official views of the National Institutes of Health.