Abstract 5550: T cell transcriptome is altered by Talquetamab in presence of GPRC5D on MM cells

Abstract Background: G protein-coupled receptor class C group 5 member D (GPRC5D) has emerged as a promising immune target in multiple myeloma (MM). Talquetamab, a GPRC5D-directed bispecific antibody, has shown an efficacy of 70% in triple class refractory MM patients however, genomic loss mediated relapse is often observed. To better understand the interplay of talquetamab, GPRC5D loss and T cells within the immune microenvironment, we performed single cell sequencing on a coculture of GPRC5D loss models and healthy T cells in presence and absence of talquetamab. Methods: GPRC5D mono- and biallelic knockout models were generated using CRISPR-Cas9 technology. Cocultures were established using healthy T cells with or without talquetamab for 24 hours. CITE-seq was performed. Cell hashing with TotalSeq-C antibodies enabled multiplexing. Pooled cells underwent 10x Genomics 5’ GEM-X workflow and were sequenced on Illumina NovaSeq X Plus. Differential gene expression was assessed using the Wilcoxon rank-sum test, and interferon response scores were calculated using AddModuleScore. Results: Pronounced transcriptomic changes in MM cells were observed with both mono- and bi-alleic loss of GPRC5D via CITE-seq. Based on the expression of canonical markers (IL2RA, IRF4, TNFRSF4-18, TNF, IFNG), CD4 and CD8 activated T cells were only observed in presence of talquetamab and GPRC5D+ cells and completely disappeared when stimulated with GPRC5DDel/Del models. A significant induction of interferon stimulated genes (GBP2-4-5, IFI44, IFIT2, ISG15) in presence of talquetamab with GPRC5D on MM cells (p-value 0.001, absolute log2FC 2) was observed across T cell subpopulations. Interferon signaling genes (STAT1, IRF1) were found to be upregulated in T cells by talquetamab alone even in absence of GPRC5D+ cells in coculture. Additionally, we also identified genes only upregulated in presence of GPRC5D and talquetamab, including SOCS3, CD69, JUNB, BATF, CISH, and PRDM1, indicating a unique activation status of these T cells. Gene set enrichment analysis performed on T cell sub types indicated an enrichment of IFN-γ and IFNA-signaling (NES2.0, FDR 0,0) in CD4, CD8, Tcm and Treg populations. Additionally, IL2-STAT5 signaling and inflammatory response (NES2.0, FDR 0.0) pathways were found to be enriched only in presence of talquetamab and GPRC5D+ cells. Of note, in the absence of talquetamab, T cells transcriptomic profile did not change in response to GPRC5D presence or absence on MM cells. Conclusions: Our findings demonstrate that talquetamab upregulates the expression of interferon signaling genes in T cells even in the absence of GPRC5D and this response is enhanced when GPRC5D is present on MM cells. Moreover, talquetmab alters the activation profile across T cells subsets and both abundance and distribution of different T cells subpopulations is changed by GPRC5D status of the MM cells in coculture. Citation Format: Umair Munawar, Alexander Leipold, Seungbin Han, Silvia Nerreter, Shilpa Kurian, Emma Besant, Nina Rein, Johanna Lehmann, Max Koeppel, Xiang Zhou, Ondrej Slaby, Hermann Einsele, Leo Rasche, Emmanual Saliba, Johannes Waldschmidt, Martin Kortuem. T cell transcriptome is altered by Talquetamab in presence of GPRC5D on MM cells abstract. In: Proceedings of the American Association for Cancer Research Annual Meeting 2026; Part 1 (Regular Abstracts); 2026 Apr 17-22; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2026;86(7 Suppl):Abstract nr 5550.