The OPG/TRAIL Interaction in an In Vitro Osteoclastogenesis Model Derived from Human Multiple Myeloma-Bone Disease.

The development of multiple myeloma (MM)-bone disease is mediated by increased recruitment and activity of osteoclasts (OCs). Osteoclastogenesis is regulated by a complex signaling system, that involves receptor activator of nuclear factor-kappa B (RANK), receptor activator of nuclear factor-kappa B ligand (RANKL) and osteoprotegerin (OPG), all belonging to the Tumor Necrosis Factor (TNF) family. The aims of our study were to investigate: the MM T cell involvement in osteoclastogenesis, and the expression of the major osteoclastogenic mediators. Unstimulated and unfractionated peripheral blood mononuclear cells (PBMCs) isolated from 32 MM patients with or without osteolytic bone lesions, and parallel T cell-depleted cultures were used as in vitro osteoclastogenesis models. In addition, unstimulated and unfractionated PBMC cultures from 32 controls with nonneoplastic disease without any skeletal involvement were also established. Our results showed that the OCs derived from MM-bone disease PBMCs spontaneously developed and displayed a longer survival in a T cell-dependent way. Differently in T cell-depleted MM PBMC cultures, the addition of macrophage-colony stimulating factor (M-CSF) and RANKL was necessary to promote the formation of OCs, that however did not exibit a longer survival. MM-bone disease T cells overexpressed RANKL, OPG and TNF-related apoptosis inducing ligand (TRAIL), also detected in large amounts in the culture media. Despite high OPG levels, the persistence of osteoclastogenesis in our system can be related to the interaction between OPG and TRAIL, that were coimmunoprecipitated by a monoclonal antibody (mAb) against TRAIL. The evidence that TRAIL binds to OPG blocking OPG anti-osteoclastogenic effect is also supported by the addition of different concentrations of functional anti-TRAIL mAb, significantly decreasing the OC formation. The OCs developed from MM-bone disease PBMCs expressed a T cell-modulated balance of death and decoy TRAIL receptors. In particular, we found these OCs overexpressed TRAIL decoy receptor DcR2 in the presence of T cells, and death receptor DR4 in the T cell-depleted cultures. In conclusion, our results highlight that MM-bone disease T cells support the spontaneous OC formation with longer survival, involving the OPG/TRAIL interaction and the unbalanced OC expression of TRAIL death and decoy receptors.