Background: Bone loss is a common complication after allogeneic stem cell transplantation. Osteoprotegerin (OPG) plays a critical role in bone remodeling by neutralizing the effect of receptor activator of nuclear factor-kappaB ligand (RANKL) on differentiation and activation of osteoclasts. We investigated OPG and RANKL in serum and marrow plasma in transplanted patients.
Materials and methods: In 36 patients and 36 controls, the relationships among bone mineral density, circulating OPG, RANKL, interferon-gamma, and interleukin-6 levels were investigated; in addition, OPG and RANKL were measured in marrow plasma and in conditioned medium of long-term cultures of marrow mesenchymal-derived osteogenic cells.
Results: Lumbar and femoral bone mineral density were lower in patients than in controls (P<0.01). Serum OPG (sOPG) and interferon-gamma were significantly higher in patients than in controls (P<0.05). Patients' interferon-gamma correlated with sOPG levels (r=0.4; P=0.03). Interleukin-6 did not differ between patients and controls. By contrast, OPG levels were lower in patients than in controls in marrow plasma (P<0.001) and in conditioned media after one (P=0.035) and three months (P=0.003) of culture of marrow mesenchymal-derived osteogenic cells. RANKL was similar in patients and controls. The OPG/RANKL ratio "in situ" was significantly lower in patients than in controls (P<0.05). There was no correlation between sOPG and marrow OPG, RANKL levels, densitometric values, and chronic graft-versus-host disease.
Conclusion: Our findings suggest that after allogeneic stem cell transplantation: 1) sOPG bear no relationship with OPG in the bone marrow; 2) increased sOPG can be the result of its enhanced production in extra bone tissues triggered by inflammatory cytokines; 3) low bone marrow OPG levels may be partly related to the persistent quantitative and qualitative deficit of osteoblastic precursors; and 4) reduced OPG/RANKL ratio in bone microenvironment may increase bone remodeling by promoting bone resorption.