Osteoclast-Induced Immunosuppression Occurs through Dysregulation of Immune Checkpoint Axes in Multiple Myeloma

Osteolytic bone disease has a significant impact on both quality of life and overall survival of patients with multiple myeloma (MM). Given that bone and immune cells share the same microenvironment and interact with each other via cytokines, signaling molecules and regulatory proteins in the bone marrow (Tsukasaki M et al., Nat Rev Immunol 2019), we sought to elucidate the effect of osteoclasts (OCs) on the immune microenvironment. Specifically, T cells significantly suppress differentiation of OCs through signaling crosstalk between RANKL and IFN-γ (Takayanagi H et al., Nature 2000). However, little is known regarding the effect of OCs on the immune system.

Here we investigated the effect of OCs on immune cells, especially on T cells. Dysregulation of several checkpoint molecules has been shown in MM (Kwon M et al., J Immunol 2017). We, therefore, assessed the expression of various immune checkpoint receptors, such as PD-1, TIGIT, OX40 and CD137 on T cells in co-culture with or without autologous OCs by multi-color flow cytometry. OC co-culture significantly increased co-inhibitory checkpoint (PD-1 and TIGIT) through direct contact, while decreasing co-stimulatory checkpoint (OX40 and CD137) in CD3⁺ T cells. Expression of the checkpoint ligand, PD-L1 was significantly increased on MM cells in the presence of both T cells and OCs compared to the presence of T cells alone. Conversely, OCs in the absence of T cells did not induce significant increase of PD-L1 on MM cells. According to previous literature, PD-L1 expression on MM cells is induced in part by IFN-γ (Liu J et al., Blood 2007). Interestingly, our data demonstrated that OCs activated IFN-γ producing T cells in co-culture conditions. Furthermore, we observed that T cell and OC-mediated upregulation of PD-L1 on MM cells was partially reversed by using IFN-γ neutralizing antibody. This implies that OCs indirectly induce PD-L1 upregulation on MM cells by enhancing IFN-γ secretion from T cells. In addition, we found that naïve CD4⁺ T cells have a higher propensity to differentiate into Th17 lineage in the presence of Th17 differentiation cytokines when co-cultured with OCs. Moreover, we observed an increased expansion of Th17 cells in co-culture with OCs. Those Th17 cells also showed a similar pattern of dysregulation of immune checkpoint axes.

Our study demonstrates that OCs positively regulate co-inhibitory checkpoint molecules and negatively regulate co-stimulatory molecules on T cells. These findings indicate that OCs play an important role in inhibiting T cell-mediated antitumor immunity. Targeting OCs may help restore impaired immune surveillance in MM in addition to their critical role in preventing lytic bone lesions in MM. These data may support the role of antiresorptives in immune surveillance with indirect anti-tumor effects.