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 monocytes differentiation and osteoclasts maturation. 

Abstract

Background In Chronic Lymphocytic Leukemia (CLL) patients, skeletal alterations are more evident in progressive disease stages. The detection of reactive cells within Bone Marrow (BM), and the evidence of tissue loss in their long bone shafts, suggest that leukemic B cells overgrowth contribute to bone derangement. Indeed, CLL-cells-released-cytokines enhance osteoclasts formation. CD16, a potential marker of monocytes differentiating toward osteoclasts, categorizes three subtypes: “non-classical”, “intermediate” and “classical”. We aimed to clarify whether the expansion of a specific CD16 + population influence bone homeostasis deregulation. Methods Cytofluorimetric analysis and patients’ whole body X-ray CT scans were used to evaluate a possible correlation between the expansion of a monocytic subset and the entity of bone erosion. In healthy monocytes, the modulation of CD16, RANK and RANKL expression and the evaluation of osteoclasts differentiation, after CLL-conditioned-media treatment, were assessed by immunofluorescence and Tartrate-resistant alkaline phosphatases (TRAP)/bone-erosion assays, respectively. Osteoclastogenesis was also determined upon M1/M2 polarization or IL-10/TGFβ pre-treatment. Immuno-histochemistry on CLL-BM biopsies was performed to verify simultaneous TRAP/CD16 positivity in osteoclasts in vivo. Results Circulating CD16 + monocytes, significantly more abundant in patients than in controls, directly correlated with bone erosion levels. After CLL-cm treatment, CD16 was markedly up-regulated, together with RANK and RANKL, in healthy monocytes. M2-polarized monocytes further resulted CD16 + and showed a striking propensity to differentiate toward osteoclasts. The identification of TRAP+/CD16 + cells, in CLL bone marrow, keeps in line with in vitro data. Conclusions Microenvironment/leukemic cells interactions affect bone degradation: specific monocytic subsets (CD16+/M2), notably expanded in these patients, may contribute to this phenomenon.