Single-cell analysis of plasma cell–centered inflammatory–immunosuppressive niches and response to daratumumab in primary light-chain amyloidosis.

7553 Background: Primary light-chain amyloidosis (pAL) is a plasma cell disease in which clonal plasma cells produce misfolded immunoglobulin light chains that lead to progressive organ dysfunction. Daratumumab (Dara), an anti-CD38 monoclonal antibody, is the backbone of first-line regimens for pAL; however, ~10–30% of treated patients fail to achieve ≥VGPR. Currently, there remains an unmet need to delineate the immune microenvironmental mechanisms underlying Dara resistance in pAL to enable the identification of predictive biomarkers and therapeutic targets, particularly for patients who lack prognostic cytogenetic markers such as 1q21 gain. Methods: We profiled 630,111 single cells obtained from a prospective observational cohort of 30 patients treated with a frontline Dara–bortezomib–dexamethasone regimen, including 11 paired samples collected pre-treatment and post-treatment. Overall, 37.0% of patients showed suboptimal responses (≤PR) before cycle 6 or initiation of next-line therapy. Plasma cells enriched with CD138⁺ magnetic beads and whole bone marrow were profiled with single-cell RNA-seq. Results: At cohort baseline, 93.3% of the patients were Mayo 2004 stage I–IIIa, 46.7% harbored the t(11;14) translocation, and 20.0% had 1q21 gain. Suboptimal responders showed two plasma cell-centered immune processes already upregulated at baseline: (i) an inflammatory axis with elevated PTGES2/PTGES3 (PGE₂)–EP2/EP4 (BH-adjusted Wilcoxon p < 0.05) signaling between plasma cells and myeloid, NK, and T cells, possibly driven by upstream PTGS2-expressing myeloid-derived suppressor cell-like CD38⁻ CD14⁺ monocytes that expanded during treatment; and (ii) an immunosuppressive axis in which non-classical MHC class I on plasma cells increased inhibitory interactions mainly with NK and T cells (including HLA-E–KLRK1, HLA-G–LILRB1, and HLA-F–LILRB1) (BH-adjusted Wilcoxon p < 0.05). Compared with patients with 1q21 gain, patients with t(11;14) were associated with higher non-classical MHC class I and PTGES3-dependent prostaglandin signaling and lower PTGES2-dependent prostaglandin signaling. Possibly driven by this microenvironment, myeloid cells downregulated phagocytosis and endocytosis, NK cells showed impaired effector functions, and T cells upregulated exhaustion, accompanied by an enhanced interferon response in suboptimal versus good responders (BH-adjusted Wilcoxon p < 0.05). Conclusions: This single-cell cohort revealed a baseline inflammatory–immunosuppressive niche in pAL that underpins suboptimal hematologic responses to Dara, characterized by up-regulated non-classical MHC class I and prostaglandin signaling between plasma cells and immune cells and associated with impaired immune effector functions during Dara treatment.