Immunosuppression with FK506 increases bone induction in demineralized isogeneic and xenogeneic bone matrix in the rat

The aim of the present study was to investigate a systemic induction of bon e formation in rats by immunosuppression with FK506 (1 mg/kg body weight in traperitoneally [ip]) in a model of osteoinduction of isogeneic and xenogen eic demineralized bone matrix (DBM) for a period of 28 days. In particular, alterations of in vitro cytokine synthesis and changes of lymphocyte subse ts were studied. DBM was implanted intramuscularly in the abdominal wad of Lewis rats (seven per group). Blood was sampled on days -7, 0, 7, and 28 fo r determination of in vitro tumor necrosis factor alpha (TNF-alpha) synthes is and lymphocyte subsets by flow cytometry (CD3+, CD4+, CD8+, CD45+, ED9+, and Ia+ antibodies). Ossicles of de novo formed bone and the tibias were r emoved on day 28 after double tetracycline labeling for histomorphometric a nalysis. Immunosuppression with FK506 significantly decreased lipopolysacch aride (LPS)-stimulated in vitro cytokine synthesis after 7 days and 28 days (p < 0.05). Compared with control animals FK506 treatment significantly in creased the volume of induced bone in isogeneic (2.1 +/- 0.3 mm(3) vs. 10.8 +/- 0.9 mm(3)) and xenogeneic (0 mm(3) vs. 4.7 +/- 0.8 mm(3)) DBM. Bone hi stomorphometry of the tibias revealed that immunosuppression increased both bone formation and bone resorption, accompanied by a significant reduction in the relative trabecular area (Tb.Ar). FK506 caused a decrease in the co unts of CD8+ T cells probably because of destruction or dislocation of thes e cells. This suggests that the amount of CD8+ cells and the degree of T ce ll activation in terms of mean fluorescence intensity (MFI) may be associat ed with bone metabolism. In support of this, statistical analysis revealed a significant positive correlation between parameters of bone formation as well as bone resorption and the CD4+/CD8+ ratio. There was a significant ne gative correlation between parameters of remodeling of the metaphysis of th e tibia and induced bone volume (BV), respectively, and MFI values of CD3+/ Ia+ cells. These findings suggest an important role of T lymphocytes in bon e formation and bone resorption in vivo. FK506 caused a marked increase of bone formation in DBM. However, the conclusion that immunosuppression incre ases fracture healing warrants further investigation.