Effect of Interleukin-6 Secreted by Engineered Human Stromal Cells on Osteoclasts in Human Bone

The effect of elevated human IL-6 (hIL-6) production by human bone marrow (Hu-BM) stromal cells on osteoclasts in human bone was examined. Human bone was implanted into nonobese diabetic mice with severe combined immunodeficiency (Hu-Bone-NOD/SCID mice). Immunohistochemistry of bone implants and mouse spleens (at 20 weeks), showed human CD451 cells, B cells, and macrophages in both tissues. Thus, Hu-BM cells survive human bone transplantation and infiltrate mouse tissue. Bone implants had 75 6 12% (mean 6 SD) human CD451 cells, and 9 6 4% mouse hematopoietic cells. A retrovirus vector containing the human IL-6 gene was used to transduce Hu-BM stromal cells (IL-6/stromal) and the PA317 cell line (IL-6/PA317). IL-6/ stromal cells (secreting, on average, 17 mg of hIL-6/106 cells per 24 h) were injected directly into human bone implants in Hu-Bone-NOD/SCID mice. IL-6/PA317 cells (secreting 16 mg/mL of hIL-6/106 cells per 24 h) were injected intraperitoneally into Hu-Bone-NOD/SCID mice. Analyses of sera from both groups of animals showed elevated levels of IL-6. However, only bone implants engrafted with IL-6/stromal cells had a statistically significant increase in osteoclast-lined mineralized trabecular bone surface (BS). Thus, a high concentration of serum hIL-6 in Hu-Bone-NOD/SCID mice alone does not increase osteoclast-lined BS in bone implants. Most importantly, it is the type of human BM cell that secretes the high levels of hIL-6 that is most critical.