Immune checkpoint remodeling across disease progression in multiple myeloma.

Immune checkpoint dynamics within the bone marrow (BM) critically shape disease evolution and therapeutic responses in multiple myeloma (MM). To delineate immune remodeling in the BM during plasma cell malignancy evolution, we profiled inhibitory (PD-1, CTLA-4, LAG-3, TIM-3, TIGIT, BTLA, and 2B4) and co-stimulatory (ICOS, CD27, DNAM-1, 4-1BB, and OX40) checkpoints across adaptive and select innate compartments in healthy donors (HD, n=25), monoclonal gammopathy of undetermined significance (MGUS, n=17), newly diagnosed MM (NDMM, n=57), and relapsed/relapsed-refractory MM (MM, n=72; on-treatment n=27, off-treatment n=12). Progressive disease featured loss of mature, memory, and activated/proliferating B cell subsets and an NDMM-specific expansion of plasmablasts/plasma cells. B cell maturation was accompanied by broad remodeling of inhibitory receptors (notably reduced PD-1, TIM-3, TIGIT, and 2B4 on mature B cells; decreased CTLA-4 on activated B cells and plasmablasts/plasma cells; and reduced TIM-3, LAG-3, and 2B4 on plasmablasts/plasma cells) alongside selective co-stimulatory changes (OX40 decreased on mature B cells; CD27 loss on activated and plasmablast/plasma compartments; divergent 4-1BB regulation). T cell compartments showed early CD4+ expansion with CD8+ cytotoxic reduction and checkpoint shifts: broad PD-1 downregulation with subset-restricted increases in LAG-3, TIM-3, TIGIT, and BTLA; variable upregulation of CD27, DNAM-1, and ICOS; and consistent 4-1BB loss. NKT and γδ T cell frequencies were stable, but their checkpoints were reconfigured: NKT cells exhibited decreased PD-1 and 4-1BB and increased TIGIT, LAG-3, and DNAM-1, whereas γδ T cells showed reduced CTLA-4, BTLA, and the co-stimulatory receptor OX40. Innate NK cells demonstrated reduced frequency and phenotypic shifts, including decreased TIM-3 and PD-1, loss of 4-1BB and OX40, stage-specific increases in TIGIT and 2B4, and elevated DNAM-1. Checkpoint alterations, such as low TIGIT or CTLA-4 and elevated OX40 expression, were correlated with superior progression-free survival. MM progression entails extensive, stage- and subset-specific remodeling of inhibitory and activating immune checkpoints in the BM, with implications for immunotherapeutic targeting.