(PA-262) Single-Cell Transcriptomic Profiling Reveals Immune Aging and Tumor Microenvironment Imprinting in Smoldering Multiple Myeloma

Introduction:  The tumor microenvironment (TME) is a key regulator of disease progression in Smoldering Multiple Myeloma (SMM). The interplay between malignant plasma cells and various immune components, such as myeloid cells and T cells, can affect tumor growth via cross-talk between these populations. Therefore, dissecting TME imprinting in SMM is crucial to better risk stratification based on the evolving paradigm of treating SMM patients. Here, we explore the intricate mechanisms of immune and SMM cell interactions and compare markers in peripheral blood that may be reflective of bone marrow status.

Methods:   We have sequenced 26 PBMC and 21 BM samples from 14 SMM patients using single-cell RNA sequencing. These include 21 paired BM and PB samples at the time of diagnosis and at later time points for disease monitoring. We have also sequenced 32 normal BM samples from healthy donors and 30 BM and 21 PB samples from MM patients.

Results:   We first looked at major cell populations (T, B, NK, monocytes, dendritic cells) representation in BM and PB and found that only CD4+ (R=0.46, p=0.1), CD8+ (R=49, p=0.1) and rare T (R=0.64, p=0.02) cell populations correlate between BM and PB. We did not observe any correlation between PB and BM for other populations (B, NK cells and monocytes). Cell signaling analysis in the BM showed that SMM cells release strong macrophage inhibitory factors (MIF) (CD74 and CXCR4) towards monocytes and T cells (p-value < 0.01). In contrast, higher TACI-BCMA interaction between monocytes and SMM cells was observed and was aberrant compared to normal plasma cells (p-value < 0.01). The SMM microenvironment was overall more similar to the MM microenvironment when compared to normal bone marrow. Accelerated immune aging significantly differed from healthy donors for B, NK, CD4+ T, and CD8+ T cells compared to SMM and MM (p-value = 0.00057).

Conclusions:   By combining large datasets processed very similarly, we observed that T cells in PB may reflect BM, and future studies on immune function may be restricted to PB. This correlation, however, is much weaker for other cell types such as B cells, monocytes, and macrophages. This may be consequent to these cells requiring further MIF signaling within the BM. MIF signal can stimulate BM stromal cells to produce IL-6, a pro-inflammatory cytokine that is important for myeloma cell survival and growth. MIF may also contribute to creating a pro-tumor microenvironment by influencing the polarization of macrophages. Accelerated immune aging in tumor samples, especially compared to normal donors, may be explained due to overstimulation of these cells. Our findings highlight the role of MIF-driven dysregulation in promoting immune aging as well as the potential of PB T cells as non-invasive biomarkers for immune monitoring in SMM. Further association studies with clinical features are ongoing and will be presented.