Do gut activated immune cells mediate low bone mineral density in paediatric Crohn’s disease?

Background:
There has been an increasing awareness of the negative impact of inflammatory bowel disease (IBD) on bone metabolism and bone mineral density (BMD). A number of potential mechanisms have been suggested, including malnutrition and the use of steroids. Although it has been known for some time that the immune system can influence the cells of bone, the specific contribution of interactions between osteoblasts and T cells in the pathogenesis of IBD-associated osteoporosis in children has yet to be clearly demonstrated. This body of work consists of two parts: an in vitro study investigating the influence of T cells and their activation products on the growth and alkaline phosphatase activity of the Saos-2 osteosarcoma cell line, and an in vivo study comparing the phenotype of lymphocyte subpopulations in the gut and peripheral circulation of children with and without IBD, relating these to measures of bone metabolism and mineralisation.
Methods: For the in vitro studies, Saos-2 cells were incubated with increasing numbers of resting and polyclonally activated immune cells, with and without the presence of transwell inserts. The expansion of the Saos-2 cells and their alkaline phosphatase activity were measured, and flow cytometry was used to establish the activation status of T cell subpopulations in the immune cell populations.
The in vivo study recruited 30 children undergoing lower gastrointestinal (GI) endoscopy for investigation of GI symptoms aiming to identify 15 patients newly diagnosed with Crohn’s disease and 15 healthy controls. Immune cells were isolated from blood and gut biopsy samples, and analysed by flow cytometry. Samples of serum and urine were analysed for bone turnover markers. Confirmed cases of IBD had their bone mineral density measured by dual X-ray absorptiometry (DXA).
Results:
The in vitro study found that populations of PBMCs and CD4+ lymphocytes had similar effects on the growth of Saos-2 cells, and whilst activated immune cells significantly inhibited Saos-2 cell growth it was also observed that resting immune cells increased Saos-2 cell growth. These effects were lost when immune cells were added after saos-2 had become adherent. There was a reciprocal effect of Saos-2 cells on the activation status of CD4+ lymphocytes, increasing the activation of resting cells and potentially limiting the effects of polyclonal activation.
The in vivo study found that children with Crohn’s disease had significant reductions in BMD, reductions in both bone resorption and formation, lower total lymphocyte counts and increased expression of CD25. Linear regression showed a positive correlation between the numbers of circulating CD4+ lymphocytes and BMD, and a negative correlation with their expression of CD25. These correlations were not statistically significant, but the power of the study was only 20%
Conclusion:
The findings support the hypothesis that gut activated immune cells mediate low bone mineral density in paediatric Crohn’s disease. However, the findings also raise the possibility of osteoimmune interactions being important for the immune response and bone metabolism, in both health and disease. Therefore, further study of cellular responses in the bone are necessary to better understand the relationships between these two important systems.