Composition and functional state of T and NK cells in the extramedullary myeloma tumor microenvironment

Abstract Extramedullary multiple myeloma (EMM) is a high-risk feature of multiple myeloma (MM) associated with increased resistance to treatments, including modern immunotherapies, and shorter survival. Composition and functional state of immune cells within the EMM tumor microenvironment (TME) remain poorly understood. Using single-cell RNA sequencing, flow cytometry, and spatial transcriptomics, we revealed significant differences in the EMM TME compared to MM bone marrow. T and NK cells were verified as the most abundant immune subsets in the EMM TME. Compared to the bone marrow counterparts, we found these tumors to have a significantly reduced effector-to-tumor cell ratio, significantly lower number of CD4⁺ T cells, and increased proportion of regulatory CD16⁻ NK cells. We observed a high proportion of exhausted, tumor-reactive CD8+ T cells in roughly half of EMM tumors. Furthermore, we identified elevated expression of immune checkpoints, such as PD-1 on CD8⁺ T cells and KLRC1 (NKG2A) on CD16⁻ NK cells.