Multimodal antigenic escape to GPRC5D-targeted T cell engagers in multiple myeloma.

Tumor-intrinsic adaptations with emerging resistant clones following T cell-targeted immunotherapies pose a major barrier to durable remissions in multiple myeloma. Through integrated genomic, transcriptomic and epigenomic interrogation of clonal plasma cells, we observed antigenic drift in 68.4% of relapsed cases following anti-GPRC5D T cell-engager (TCE) therapy (n = 21). These escape events were driven by three distinct mutational mechanisms involving (1) focal to large biallelic deletions at the GPRC5D gene locus; (2) monoallelic deletion coupled with GPRC5D single-nucleotide variants or insertions/deletions (indels) on the remaining allele; as well as (3) epigenetic GPRC5D promoter/enhancer silencing. Beyond biallelic deletions resulting in complete antigenic loss, we demonstrate that GPRC5D single-nucleotide variants and indels mutate anti-GPRC5D TCE-binding epitopes or more commonly affect G-protein-coupled receptor family conserved motifs critical for protein membrane trafficking resulting in endoplasmic reticulum GPRC5D trapping. Multiple subclones bearing distinct genomic alterations at GPRC5D locus co-emerged within individual cases, depicting their convergent evolutionary trajectories. Of note, anti-GPRC5D TCEs with varying epitope specificity, affinity and valency differentially targeted mutant subclones, underscoring their nonredundant functional roles in overcoming resistance.