The Role of LIGHT in Multiple Myeloma-Bone Disease

Background. The TNF superfamily member LIGHT, known to play a major role in T-cell homeostasis, has been reported in erosive bone disease associated with rheumatoid arthritis. Herein, we investigated whether LIGHT is implicated in the mechanisms leading to Multiple Myeloma (MM)-bone disease.

Methods. Peripheral blood (PB) and bone marrow (BM) aspirates were obtained from 40 patients (23M/17F, median age: 64 years), newly diagnosed as having symptomatic MM with or without bone disease, smoldering MM (sMM) or M.G.U.S. Bone disease assessment was performed by skeleton x-Ray, and spine and pelvis NMR or CT. The control group included PB and BM aspirates from 15 patients with non-neoplastic disease without any skeletal involvement, and PB from 25 healthy-donors matching for age and sex with the MM group. Patients and controls gave their written informed consent to the study, approved by Ethical Committee of University Hospital of Bari, and performed according to Declaration of Helsinki. By means of flow cytometry, western blotting, and real-time PCR, LIGHT expression was assessed in freshly purified CD14+ monocytes, CD2+ T-cells and neutrophils from PB and BM aspirates of patients and controls. Osteoclasts (OCs) were obtained from unfractionated PBMC cultures treated or not with an anti-LIGHT neutralizing monoclonal antibody (mAb). Mature OCs were identified as multinucleated tartrate-resistant acid phosphatase (TRAP) positive cells. In cultures from BM mononuclear cells (BMNCs), the formation of CFU-F and CFU-OB was evaluated in the presence or absence of anti-LIGHT neutralizing mAb. CFU-F and CFU-OB were identified with alkaline phosphatase (ALP) or Von Kossa staining, respectively. Further, in CFU-F and CFU-OB cultures, the expression of OB differentiation markers was analyzed by real-time PCR.

Results. We found overexpression of LIGHT on CD14+ monocytes, CD8+ T-cells and neutrophils of PB and BM from MM-bone disease patients, in whom LIGHT induced osteoclastogenesis and inhibited osteoblastogenesis, as we demonstrated by culture treatment with an anti-LIGHT antibody. Moreover, in cultures from healthy-donors, we found that LIGHT induced osteoclastogenesis in RANKL-dependent and –independent manners. In particular, in the presence of a sub-optimal RANKL concentration, LIGHT and RANKL showed synergic effects on osteoclast formation, associated to early and sustained activation of Akt, NFκB and JNK pathways. Otherwise in cultures of BM samples from patients without bone disease, LIGHT treatment inhibited the formation of CFU-F and CFU-OB, and the expression of osteoblastic markers such as collagen-I, osteocalcin and bone sialoprotein-II, supporting a LIGHT indirect inhibition of osteoblastogenesis, possibly and to some extent, through sclerostin expression by monocytes.

Conclusions. Our findings, for the first time, provide evidence that LIGHT plays a role in MM-bone disease development by increasing osteoclastogenesis and decreasing osteoblastogenesis.