HIV-infected individuals are at an increased risk of osteoporosis despite effective viral suppression. Observations that myostatin null mice have increased bone mass led us to hypothesize that simian immunodeficiency virus (SIV)-associated bone loss may be attenuated by blocking myostatin/TGF beta signaling. In this proof-of-concept study, pair-housed juvenile male rhesus macaques were inoculated with SIVmac239. Four weeks later, animals were treated with vehicle or Fc-conjugated soluble activin receptor IIB (ActR2B.Fc, iv. 10 mg * kg(-1) * week(-1)) an antagonist of myostatin and related members of TGF beta superfamily. Limb and trunk bone mineral content (BMC) and density (BMD) using dual-energy X-Ray absorptiometry, circulating markers of bone growth and turnover, and serum testosterone levels were measured at baseline and during the 12-week intervention period. The increase in BMC was significantly greater in the ActRIIB.Fc-treated group (+ 8 g) than in the placebo group (- 4 g) (p < 0.05). BMD also increased significantly more in the ActRIIB.Fc-treated macaques (+ 0.03 g/cm2) than in the placebo-treated animals (+ 0 g/cm2) (p < 0.005). Serum osteocalcin was about two-fold higher in the ActRIIB.Fc-treated group than in the placebo group (p < 0.05), but serum C-terminal telopeptide and testosterone levels did not differ significantly between groups. The expression levels of TNFalpha (p < 0.05), GADD45 (p < 0.005), and sclerostin (p < 0.038) in the bone-marrow were significantly lower in the ActRIIB.Fc-treated group than in the placebo group. Conclusion: The administration of ActRIIB.FC in SIV-infected juvenile macaques significantly increases BMC and BMD in association with reduced expression levels of markers of bone marrow inflammation. (C) 2017 Published by Elsevier Inc.