FAPα Macrophages Orchestrate Immune Evasion in Multiple Myeloma by Dual Regulation of PD-L1 and T Cell Senescence.

Multiple myeloma (MM) is a hematologic malignancy driven by clonal expansion of malignant plasma cells. Despite the long-term disease control achieved with immunotherapies in some patients, treatment resistance remains a major cause of disease relapse. Accumulating evidence highlights the tumor immune microenvironment, especially macrophages, as a key contributor to immunotherapy failure in MM. Herein, we identified a subset of MM-associated macrophages with high expression of fibroblast activation protein alpha (FAPα), defined as FAPα macrophages. Clinical data showed that FAPα macrophages were enriched in the bone marrow versus peripheral blood of MM patients, and their abundance positively correlated with tumor burden. In MM mouse models, depletion of FAPα macrophages significantly boosted the efficacy of anti-PD-1/PD-L1 antibody therapy but not anti-CTLA-4 therapy; this combinatorial strategy also exerted enhanced anti-tumor effects in EL4 lymphoma and CT26 colorectal carcinoma models. Mechanistically, FAPα stabilized PD-L1 expression by maintaining its N-glycosylation and inhibiting proteasomal degradation, and induced PD-L1 synthesis via promoting vimentin (VIM) phosphorylation at the S72 residue. Additionally, FAPα macrophages accelerated T cell senescence by secreting soluble FAPα. Collectively, our findings demonstrate that FAPα macrophages mediate MM immune evasion via dual mechanisms, positioning them as promising therapeutic targets to potentiate anti-tumor immunotherapies.