Juxtacrine Interactions of Bone Marrow Cells Activates IL-6-mediated Events

Objectives: This study investigates interleukin-6 (IL-6) signaling in the juxtacrine interaction of macrophages and bone marrow stromal cells, and its influence on the proliferation and migration of bone marrow stromal cells.
Methods: M-CSF enriched primary macrophages and primary bone marrow stromal cells (BMSCs) from both wild-type and IL-6 knockout mice were co-cultured and analyzed for IL-6 mRNA by realtime-PCR, and for IL-6 protein levels (in culture supernatants) by ELISA. Functionally, cell migration of primary mouse BMSCs, treated with either conditioned medium generated from co-cultures or recombinant IL-6 protein, was measured by cell scratch assay and transwell assay and proliferation of BMSCs measured by cell enumeration. Activation of STAT3, a downstream regulator of the gp130/IL-6 family signaling pathway, was investigated by Western blot in the juxtacrine cultures, and in BMSC cultures treated with conditioned medium or recombinant IL-6 protein. Soluble gp130, an inhibitor of IL-6/gp130 signaling activation, was utilized to investigate whether the juxtacrine interaction influenced BMSC migration and proliferation via its IL-6/gp130 activation.
Results: Juxtacrine interaction of primary mouse macrophages and primary mouse BMSCs activated IL-6 signaling in the co-cultures, which subsequently enhanced BMSC migration and increased BMSC numbers. BMSCs and macrophages harvested from IL-6 knockout mice revealed that IL-6 signaling was essential for enhancement of BMSC migration and increased BMSC numbers via juxtacrine interaction. BMSCs were the main contributor of IL-6 signaling, and hence activation of the IL-6/gp130/STAT3 pathway. Meanwhile, macrophage derived IL-6 was still important for the overall production of IL-6 protein in the co-cultures. Taken together, these findings show the function of macrophages as co-inducers of migration and growth of BMSCs, which could directly influence bone formation and turnover.
Conclusions: Juxtacrine interaction of macrophages and bone marrow stromal cells induced interleukin-6 signaling and promoted BMSC migration and proliferation via activation of the IL-6/gp130/STAT3 pathway in vitro.