Serum Amyloid A proteins reduce bone mass during mycobacterial infections

Osteopenia has been associated to several inflammatory conditions, including mycobacterial infections. How mycobacteria cause bone loss remains elusive, but direct bone infection may not be required. Using genetically engineered mice and morphometric, transcriptomic and functional analyses, we found that infection with Mycobacterium avium impacts bone turnover by decreasing bone formation and increasing bone resorption, in a IFNg- and TNFa-dependent manner. IFNg produced during infection enhanced macrophage TNFa secretion, which in turn increased the production of serum amyloid A (SAA) 3. Saa3 expression was upregulated in the bone of both M. avium- and Mycobacterium tuberculosis-infected mice and SAA proteins were increased in the serum of patients with active tuberculosis. Furthermore, the increased SAA levels seen in active tuberculosis patients correlated with altered serum bone turnover markers. Additionally, human SAA proteins impaired bone matrix deposition and increased osteoclastogenesis in vitro. Overall, we report a novel crosstalk between the cytokine network operating in macrophages and bone homeostasis and disclose SAA proteins as potential biomarkers of bone loss during infection by mycobacteria.