A High Percentage of CD16+ Monocytes Correlates with the Extent of Bone Erosion in Chronic Lymphocytic Leukemia Patients: The Impact of Leukemic B Cells in Monocyte Differentiation and Osteoclast Maturation

SIMPLE SUMMARY: Bone is a dynamic tissue undergoing resorption and formation. We previously described bone derangement in CLL patients in the advanced stages of the disease and demonstrated the pivotal role of cytokines, released by CLL cells, in stimulating osteoclast differentiation in vitro. Aiming to clarify mechanisms favoring osteoclasto-genesis, we, here, first characterized circulating CLL monocytes and then addressed whether healthy monocytes, under the influence of leukemic cells, acquire phenotypic and functional features distinctive of osteoclast precursors, such as the previously proposed CD16 marker. We determined: (i) a direct correlation between circulating CLL monocytes expressing CD16 and the levels of bone erosion; (ii) that the treatment of healthy monocytes with CLL-conditioned medium up-regulated the expression of CD16, RANK and RANKL and increased the rate of osteoclast formation; (iii) that monocytes polarized toward the M2 phenotype showed higher CD16 expression than M1 and were more prone to differentiate into osteoclasts. ABSTRACT: Significant skeletal alterations are present in Chronic Lymphocytic Leukemia (CLL) patients; bone erosion, particularly evident in the long bone shaft, appeared increased in the progressive disease stage. Moreover, the partial colonization of the bone with reactive bone marrow we documented via PET-FDG imaging suggests that neoplastic cell overgrowth contributes to bone derangement. Indeed, cytokines released by leukemic B cells impair osteoblast differentiation and enhance osteoclast formation in vitro. CD16, Fcγ-RIIIa, has been previously indicated as a marker of osteoclast precursors. We demonstrate, here, that the percentage of circulating monocytes, CD16+, is significantly higher in CLL patients than in normal controls and directly correlated with the extent of bone erosion. When we assessed if healthy monocytes, treated with a CLL-conditioned medium, modulated RANK, RANKL and CD16, we observed that all these molecules were up-regulated and CD16 to a greater extent. Altogether, these findings suggest that leukemic cells facilitate osteoclast differentiation. Interestingly, the evidence that monocytes, polarized toward the M2 phenotype, were characterized by high CD16 expression and showed a striking propensity to differentiate toward osteoclasts may provide further explanations for the enhanced levels of bone erosion detected, in agreement with the high number of immunosuppressive-M2 cells present in these patients.