The role of checkpoint inhibitor PD-1H/VISTA in Multiple Myeloma bone disease

Background Multiple myeloma (MM) bone disease remains one of the most devastating complications of this incurable cancer, causing bone fractures, pain, mobility issues, and neurological deficits. MM cells secret pro-osteoclastogenic factors which lead to osteoclast (OCL) activation. Our previous work showed that matrix metalloproteinase 13 (MMP-13) is a critical osteoclastogenic factor highly secreted by MM cells and induces OCL fusion and bone resorption independently of its proteolytic activity (JCI 2016). We recently reported that MMP-13 binds to checkpoint inhibitor programmed death-1 homolog (PD-1H/VISTA), a surface receptor that is expressed on OCLs and mediates MMP-13 induced OCL fusion and bone resorption activity (ASH 2019). Bone resorption activity is significantly impaired in Pd-1h-/- OCLs in vitro. However, the function of PD-1H in MM bone disease has not been defined. Methods The role of PD-1H in MM bone disease was investigated using the intratibial 5TGM1 Rag2-/- MM bone disease mice model. Pd-1h-/-Rag2-/- mice were generated by crossbreeding Pd-1h-/- with Rag2-/- mice. Firefly luciferase-expressing 5TGM1 cells were intratibially injected into age and sex-paired Rag2-/- or Pd-1h-/-Rag2-/- mice (N=10). 3 weeks later, tibiae were harvested for quantitative micro-CT followed by histological analysis. Results Morphological analyses of trabecular and cortical bones confirmed that Pd-1h-/- recipient mice exhibited significantly less 5TGM1-induced bone loss (P Conclusions Taken together, our study, for the first time, reveals that checkpoint inhibitor PD-1H/VISTA is the critical receptor for MMP-13 in osteoclasts, thereby mediating MMP-13-induced osteoclast fusion, activation, and bone resorption. MM-induced trabecular bone loss was significantly lower in Pd-1h-/- mice, demonstrating that PD-1H/VISTA plays a critical role in MMP-13-induced MM bone disease. Given the checkpoint role of PD-1H/VISTA in cancer immunosuppression, we further posit that targeting the interaction of MMP-13 and PD-1H may represent a novel therapeutic strategy to treat MM bone disease and modulate the MM immune environment.