Bone Abstracts

The influence of immune dysregulation on bone metabolism in children with inflammatory bowel disease: the potential for bone as a secondary lymphoid organ.

Background: Whilst their clinical relevance in terms of fracture may be questioned, systemic inflammatory disorders in children impacts on their bone metabolism and reduces bone mineral density. Similar observations in adults are in part explained by interactions between lymphocytes and osteoclasts via the receptor activator of nuclear factor kappa-B ligand/osteoprotegerin pathway, but in a child’s growing bone it is necessary to look at the effects of lymphocytes on osteoblasts.

Methods: A series of experiments were undertaken to investigate the effects of immune cells on the growth and alkaline phosphatase activity of the osteosarcoma cell line Saos-2. Peripheral blood mononuclear cells were isolated from healthy volunteers, and a CD4⁺ lymphocytes enriched population generated. These two populations were co-cultured with Saos-2 cells studying the effects of immune cell number, their activation status, and the role of cell contact.

Results: The experiments showed that increasing numbers of activated CD4⁺ lymphocytes reduced Saos-2 cell number by 48% (P < 0.001) with a clear dose response effect. In contrast, resting CD4⁺ lymphocytes increased Saos-2 cell number by 27% (P=0.013). The presence of a transwell insert increased the number of Saos-2 cells by 106% in the activated condition (P < 0.001) and by 7% in the resting condition (P=0.466). In addition, Saos-2 altered expression of activation markers by CD4⁺ lymphocytes, increasing expression of CD25 (3.5% vs 27.5%; P=0.002) and CD69 (0.7% vs 24%; P=0.029) by resting cells and decreasing their expression by activated cells. Introduction of immune cells after Saos-2 adhesion abrogated the observed effect on their growth. PBMC effects were similar to that of the CD4⁺ lymphocytes.

Conclusion: The findings outlined support the paradigm that activated CD4⁺ lymphocytes inhibit the growth of osteoblasts. The additional findings of immune cells supporting growth of Saos-2 cells and two-way signaling suggest a potentially more complex relationship. Observations in the published literature describe antigen experienced lymphocytes maintained within the bone, and osteoimmune interactions supporting both the immune system and bone metabolism. Therefore we need to consider bone not only as a primary, but also a secondary lymphoid organ.

Disclosure: The authors declared no competing interests.