Therapy-induced senescence as a driver of immune activation and myeloid plasticity in multiple myeloma

About This Grant

Abstract Multiple myeloma (MM) is an incurable plasma cell malignancy. Despite therapeutic advances, nearly all MM patients are expected to relapse and develop refractory disease, underscoring the need to improve the efficacy of first line therapy to prevent disease relapse. The current standard of care for newly diagnosed MM includes a myeloablative dose of the cytotoxic agent melphalan prior to autologous stem cell transplant (ASCT). Cytotoxic drugs are known to trigger therapy induced senescence (TIS) in normal cells. Several solid tumors have been shown to exhibit features of TIS following chemotherapy, and these markers in tumor cells are associated with superior progression free survival (PFS). One potential mechanism for this benefit is the activation of anti-tumor immune responses through the senescence associated secretory phenotype (SASP), which recruits and stimulates immune cells. Consistent with the findings in solid tumors, our analysis of longitudinal MM patient bone biopsies revealed a significant increase in senescence in MM cells in patients with PFS ≥2.5 years following ASCT. In contrast, patients that relapsed in <2.5 years did not exhibit increased tumor cell TIS. Additionally, patients that relapsed showed a positive correlation between MM burden post-ASCT and PFS, such that patients with the lowest plasma cell burden post-ASCT relapsed earliest. These data support that the ability of MM cells to evade apoptosis and enter a senescence-like, stable growth arrest in response to cytotoxic stress may protect against early relapse. To investigate the mechanisms underlying this phenomenon, we developed in vitro and in vivo models in which high-dose melphalan (HDM) consistently induces stable growth arrest and TIS features in human and mouse MM cell lines. Interestingly, HDM-MM cells also develop a myeloid gene signature. This myeloid gene expression was linked to functional plasticity, as HDM-MM cells cultured with osteoclast differentiation factors developed tartrate-resistant acid phosphatase (TRAcP5b) activity and multinucleation, two defining hallmarks of osteoclasts. Osteoclast-MM hybrids have previously been identified in MM patient biopsies, and our post-ASCT biopsy analysis revealed that nearly all patients had multinucleated CD138+ cells, including cells with osteoclast-like morphology on bone surfaces. Therefore, we hypothesize that TIS in MM cells activates anti-tumor immunity and drives MM-myeloid plasticity, enabling novel interactions with the bone niche that contribute to persistence and relapse. This hypothesis will be tested through the following specific aims: 1) Investigate the senescence-associated and immunological properties of MM cells that persist following cytotoxic therapy; 2) Define how TIS-myeloid plasticity supports MM cell survival and bone niche engagement. These studies will integrate our novel HDM in vitro and in vivo models with longitudinal patient biopsies. Because TIS and MM-myeloid plasticity are shaped by spatial and cellular interactions within the bone marrow microenvironment, in vivo models are essential to define how growth-arrested MM cells persist, engage the bone niche, and contribute to relapse. We expect these studies to reveal novel therapeutic strategies to target persistent, growth-arrested MM cells, ultimately preventing MM disease relapse.