Paper Details

ABSTRACT

Background: BCMA- and GPRC5D-directed bispecific antibodies (BsAbs) have transformed relapsed/refractory multiple myeloma (RRMM) treatment, but resistance limits durable clinical benefit. Although BsAbs directly engage T cells against target-expressing plasma cells, sustained anti-myeloma efficacy may also depend on additional immune interactions. Antigen-presenting monocytes can support CD4? T cells, which in turn promote CD8? effector function, whereas monocytic myeloid-derived suppressor cells (M-MDSCs) suppress T-cell activity. How the balance between antigen-presenting and suppressive monocyte programs evolves during BsAb therapy and relates to response and resistance remains undefined.

Methods: Thirty patients with RRMM receiving anti-BCMA (n=22) or anti-GPRC5D (n=8) BsAbs were classified by progression-free survival (PFS) as responders (n=16; median PFS not reached after a median follow-up of 17.4 months) or non-responders (n=14; median PFS, 2.5 months). A total of 85 longitudinal peripheral blood samples were collected at pre-treatment, best response, and at progression in non-responders or latest on-treatment follow-up in responders. Paired single-cell RNA (scRNA-seq) and T-cell receptor sequencing of peripheral blood mononuclear cells (PBMCs) profiled immune composition and phenotype, tracked T-cell clonotypes, and inferred ligand-receptor interactions. Whole-genome sequencing (WGS) was performed on bone marrow plasma cells from available cases.

Results: scRNA-seq profiled 621,647 PBMCs, including 18,788 circulating plasma cells (CPCs). WGS identified biallelic TNFRSF17 loss at progression in one non-responder after BCMA BsAb therapy, with no biallelic target loss in remaining patients. In patients without target loss, baseline and paired longitudinal profiles were evaluated to identify pre-existing and treatment-emergent features of resistance.

At baseline, cytotoxic CD8? T cells from non-responders had higher exhaustion-signature scores (PDCD1, LAG3, HAVCR2, TIGIT, and TOX; P=0.03). Classical monocytes in both groups exhibited a comparable immunosuppressive M-MDSC-like program, characterized by high S100A8/S100A9 and low HLA-DR expression. Although baseline CPC abundance was similar, non-responder CPCs were enriched for inflammatory and pro-survival pathways, including non-canonical NF-κB signaling.

Longitudinally, classical monocytes in responders reduced the M-MDSC-like suppressive signature (P<0.001) and increased MHC-II antigen presentation (P=0.004), whereas non-responder monocytes retained the suppressive program and failed to increase antigen presentation. This impaired monocyte activation in non-responders was accompanied by disrupted reciprocal monocyte-CD4? T-cell cross-talk during treatment, including reduced ICAM1-LFA-1-mediated adhesion between monocytes and Th1 and cytotoxic CD4? T cells, together with reduced Th1-derived IFNG signaling to monocytes relative to responders (P=0.01 and P=0.03). Non-responders also showed a decline in cytotoxic-effector programs in ZEB2? CD8? T cells, whereas responders maintained these programs (P=0.03). Despite similar expansion of baseline-detected clonotypes in both groups, only those from responders gained cytotoxic-effector programs, whereas non-responder clonotypes retained elevated exhaustion-associated programs and showed declining cytotoxic-effector signatures (P=0.02).

CPCs persisted longitudinally in non-responders but declined in responders from a median of 0.3% to 0.0% (P=0.002). Paired differential expression of residual responder CPCs (n=3) showed enhanced pro-apoptotic signaling (DAPK1, TRAF3 up; MCL1, BCL2 down). Persistent CPCs in non-responders showed reduced DAPK1, elevated CCND1, and attenuated immune-interaction programs, consistent with diminished immune responsiveness and greater proliferative potential.

Conclusion: Baseline CD8? T-cell exhaustion and CPC pro-survival signaling represent candidate blood-based biomarkers of poor response to BCMA- or GPRC5D-directed BsAbs. Longitudinally, resistance in patients without target-antigen loss was characterized by failure of monocytes to transition from a suppressive to an antigen-presenting program and disruption of monocyte-CD4? T-cell cross-talk. Despite clonal expansion, T-cell clonotypes remained exhausted rather than gaining cytotoxic-effector programs, while CPCs persisted. Together, these findings support a coordinated cellular mechanism of BsAb resistance without target-antigen loss.