The importance of B cells for osteogenic homeostasis.

Therapeutic interventions for diseases such as leukemia and autoimmune disorders are increasingly designed to selectively target and deplete specific immune cell subsets over prolonged periods. This can disrupt the homeostasis of the adaptive immune system, with consequences not only for peripheral blood but also for the "cradle of the immune system", the bone marrow. This study hypothesized that such immune imbalance due to immune cell depletion therapies impairs the differentiation capacity of mesenchymal stromal cells (MSCs), which are key progenitors of bone cells and are thus essential for maintaining bone health. To validate this hypothesis, a series of cell culture experiments were conducted in which MSCs were stimulated with immune-conditioned media derived from various immune cell subsets. A comprehensive analytical approach was employed to evaluate the differentiating MSCs, including their associated supernatants, deposited collagen, mineralized matrix, and lipid deposition at defined time points during their maturation. Therefore, fluorescence and nonfluorescent histological staining, enzyme-linked immunosorbent assays (ELISAs), and expression analyses with Ribonucleic acid (RNA) were performed. This study demonstrated that, compared with immune-conditioned media from T cells or peripheral blood mononuclear cells (PBMCs), B cell-immune-conditioned media enriched with interleukin 4, bone morphogenetic protein 2 and Dickkopf 1 enhanced the osteogenic differentiation of MSCs in vitro. This positive osteogenic effect is driven primarily by elevated cytokine secretion by stimulated B cells, which in vivo in turn potentially stimulate bone turnover and modulate immune system function. In summary, these data demonstrate an pro-osteogenic and pro-adipogenic influence of B cells on differentiating MSCs in vitro, suggesting that an imbalance in immune cells in the bone marrow perturbs bone homeostasis.