Abstract
Smoldering Multiple Myeloma (SMM) is an asymptomatic plasma cell disorder with high risk of progression, characterized by clonal plasma cell expansion with no evidence of end-organ damage. In addition to tumor cells, evolving immune dysfunction and bone marrow (BM) microenvironmental changes are believed to contribute to progression to overt malignancy. Considering the changing treatment landscape for SMM patients, it is imperative to analyze tumor microenvironment (TME) imprinting in order to improve risk stratification and therapeutic strategies. However, the early immunological alterations and stromal remodeling events that distinguish stable from progressive disease remain poorly understood.Methods:We employed single-cell RNA sequencing (scRNA-seq) to comprehensively profile the immune and stromal compartments in over 125 samples, including 42 paired BM and peripheral blood (PB) specimens from 14 SMM patients collected longitudinally at diagnosis and follow-up, 32 BM samples from age-matched healthy donors, and 51 BM samples from newly diagnosed MM patients. Our analysis focused on immune cell composition, transcriptional states, and ligand–receptor interactions. We paid special attention to immune aging signatures and macrophage inhibitory factor (MIF)-mediated stromal reprogramming.Results:We observed evidence of increased immune aging in SMM, with T cells, B cells, and NK cells exhibiting a significant increase in exhaustion and senescence markers compared to healthy donors (p < 0.001). This phenotype was characterized by loss of naïve markers, activation of inhibitory receptors such as PD-1 and LAG-3, and a shift toward effector memory differentiation. These features are consistent with hallmarks of chronic antigen exposure and immune dysfunction. Interestingly, only CD4+ (R = 0.46, p = 0.1), CD8+ (R = 0.49, p = 0.1), and rare T cell (R = 0.64, p = 0.02) populations showed correlation between BM and PB. Our finding highlights the potential use of PB as an accessible biomarker of immune dysregulation.Our ligand–receptor interaction analyses revealed that clonal plasma cells in SMM actively engage the microenvironment through enhanced MIF signaling. This pathway engages both stromal and immune cells via CD74 and CXCR4 (p < 0.01). MIF-induced stromal reprogramming was associated with increased IL-6 production from stromal cells and macrophage polarization towards a tumor-supportive phenotype. Notably, these interactions were either absent or significantly attenuated in healthy BM. In parallel, communication between monocytes and plasma cells via the TACI–BCMA pathway was increased in both SMM and MM, suggesting that innate immune remodeling begins early in disease evolution.Conclusion:Our results demonstrate that immune aging and MIF-mediated stromal rewiring synergistically remodel the bone marrow niche and promote disease in SMM. The parallel changes as observed in PB T cells underscore their potential as accessible biomarkers for tracking immune dysregulation. Furthermore, our finding highlights MIF signaling as a candidate for further studies to evaluate its impact on disease progression. Immune aging also requires more attention as it reflects the tumor cell division and the ability of the microenvironment to handle SMM cells. Integrating these immune and microenvironmental insights with clinical and genomic data may enhance disease assessment and enable understanding of the disease.
