(PA-246) LILRB4 Drives Myeloid Immunosuppressive Microenvironment in Multiple Myeloma and Facilitates the Inferior Outcome of Patients

Introduction: Recent advancements in the treatment of multiple myeloma (MM), including immunomodulatory agents, proteasome inhibitors, monoclonal antibodies, and T cell-redirecting therapies such as chimeric antigen receptor (CAR) T cells and bispecific antibodies (BsAbs), have significantly improved patient outcomes. Nevertheless, MM remains incurable. Our previous work demonstrated that expanded myeloid-derived suppressor cells (MDSCs), impaired dendritic cells, and protumoral osteoclasts in the bone marrow of MM patients play a major role in establishing a protumoral tumor immune microenvironment (TiME). However, the origins of tumor-associated myeloid cells in myeloma remain unclear.

Methods: Single-cell RNA sequencing was performed to investigate the biological characteristics of MM cells and the TiME of MM patients. Survival-associated immune regulatory molecules were identified. LILRB4-overexpressing MM cell lines and mouse models were constructed. Flow cytometric analysis was performed on myeloid suppressive cells, megakaryocyte–erythroid progenitors, CD45⁺ erythroid progenitor cells (EPCs), and erythroid-derived myeloid cells (EDMCs) in the bone marrow. Co-culture assays evaluated the immunosuppressive function of EDMCs/MDSCs. Cytokine arrays, RNA-seq, Western blotting, and functional analyses were conducted to elucidate the direct and indirect interactions between MDSCs and tumor cells that drive immune evasion and cancer progression.

Results: Our prior studies identified LILRB4 as a high-risk gene in MM, with its overexpression correlating with poor prognosis and promoting monocyte-to-MDSC differentiation, thereby contributing to the TiME-induced immunosuppressive phenotype in MM patients (Haematologica 2024 & 2025). This study further revealed that the proportion of cells with myeloid features within the erythroid differentiation lineage is increased in patients with high LILRB4 expression. Through in vivo and in vitro studies, we show that CD45⁺ EPCs in the myeloma microenvironment lose their erythroid maturation potential and instead undergo trans-differentiation into EDMCs, driven by high levels of CCL3 secreted by LILRB4⁺ MM cells. Our findings propose a novel hematopoietic pathway in which MM "hijacks" erythroid-committed CD45⁺ EPCs, diverting them toward a myeloid fate to facilitate tumor immune escape. EDMCs, as key immunosuppressive components, potently inhibit CD8⁺ T-cell responses and accelerate tumor progression. Anemia, a common complication in myeloma, is associated with poor prognosis. While  rhEPO and ESAs alleviate anemia, they fail to improve survival in cancer patients—possibly because they induce CD45⁺ erythroid progenitors that tumors exploit to expand immunosuppressive myeloid cells.

Conclusions:

Targeting LILRB4-mediated erythroid-to-myeloid diversion (EDMCs) may overcome immunotherapy resistance in MM and tumor-associated anemia.