PD-L1+ exosomes as biomarkers and therapeutic targets in multiple myeloma: implications for CAR-T efficacy and combination immunotherapy

Abstract

Although chimeric antigen receptor T-cell (CAR-T) therapy has reshaped the treatment landscape for relapsed/refractory multiple myeloma (RRMM), durable remission remains challenging. T-cell dysfunction and exhaustion are key obstacles to long-term efficacy. Programmed death-ligand 1 (PD-L1)-positive exosomes may contribute to tumor-associated immunosuppression, but their clinical and functional relevance in multiple myeloma (MM) remains unclear. A total of 89 patients with MM were enrolled, including 34 patients with RRMM treated with CAR-T therapy. PD-L1 + exosome levels in peripheral blood (PB) and bone marrow (BM) were quantified by flow cytometry using exosome-coupled beads, and soluble PD-L1 (sPD-L1) was measured by enzyme-linked immunosorbent assay. Associations with clinicopathological features, treatment response, and progression-free survival (PFS) were analyzed. Effects of RRMM patient sample-derived PD-L1 + exosomes on CD8 + T-cell activation, cytokine production, and exhaustion phenotypes were assessed using in vitro experiments. The therapeutic potential of GW4869 combined with anti-programmed cell death protein 1 (PD-1) antibody to enhance CAR-T efficacy was further evaluated using in vitro and in vivo models. PD-L1 + exosome levels in PB and BM were significantly elevated in newly diagnosed MM and RRMM compared with healthy controls, and were closely associated with adverse prognostic features, including high tumor burden, extramedullary lesions, and high-risk cytogenetic features, whereas plasma sPD-L1 showed no significant clinical correlations. In RRMM patients receiving CAR-T therapy, pretreatment BM and PB PD-L1 + exosome levels discriminated best response (complete response versus less than complete response), with BM showing better predictive performance (area under the receiver operating characteristic curve, 0.846; optimal cutoff, 65.0%). High pretreatment PD‑L1 + exosome levels were associated with low peak levels of CAR transgene, interferon-γ, and tumor necrosis factor-α, as well as shorter PFS. In addition, elevated BM PD‑L1 + exosome level remained an independent adverse factor for PFS in multivariable analysis. In vitro, RRMM patient sample-derived PD-L1 + exosomes inhibited CD8 + T cell activation and effector cytokine production, and promoted terminal exhaustion, which was not fully reversed by anti-PD-1 antibody alone. In preclinical models, GW4869 plus anti-PD-1 antibody enhanced CAR-T-mediated antitumor activity in both cell-based assays and the MM xenograft model. PD‑L1 + exosomes may serve as biomarkers of disease aggressiveness and CAR‑T therapy outcomes in MM. By impairing CD8 + T cell function and promoting terminal exhaustion, they may contribute to immune resistance. Combined exosome-release pathway and PD‑1 blockade warrants further investigation as a strategy to enhance CAR‑T efficacy.