Human Alternative Macrophages Populate Calcified Areas of Atherosclerotic Lesions and Display Impaired RANKL-Induced Osteoclastic Bone Resorption Activity

RATIONALE:Vascular calcification is a process similar to bone formation leading to an inappropriate deposition of calcium phosphate minerals in advanced atherosclerotic plaques. Monocyte-derived macrophages, located in atherosclerotic lesions and presenting heterogeneous phenotypes, from classical pro-inflammatory M1 to alternative anti-inflammatory M2 macrophages, could potentially display osteoclast-like functions. OBJECTIVE:To characterize the phenotype of macrophages located in areas surrounding the calcium deposits in human atherosclerotic plaques. METHODS AND RESULTS:Macrophages near calcium deposits display an alternative phenotype being both CD68 and mannose receptor (MR) positive (CD68MR), expressing carbonic anhydrase type II (CA2), but relatively low levels of cathepsin K (CTSK). In vitro IL-4-polarization of human primary monocytes into macrophages results in lower expression and activity of CTSK compared to resting unpolarized macrophages. Moreover, IL4-polarization lowers expression levels of the osteoclast transcriptional activator Nuclear Factor of Activated T cells type c-1 (NFATc-1), associated with increased gene promoter levels of the transcriptional repression mark H3K27me3. Despite higher expression of the RANK receptor, RANKL/MCSF induction of NFATc-1 and CTSK expression is defective in these macrophages due to reduced Erk/c-fos-mediated downstream signaling resulting in impaired bone resorption capacity. CONCLUSIONS:These results indicate that macrophages surrounding calcium deposits in human atherosclerotic plaques are phenotypically defective being unable to resorb calcification.