Abstract 1370: Antibodies derived from patient tumors augment response to immune checkpoint blockade in cancer

Abstract Background: Immune checkpoint blockade (ICB) has revolutionized treatments for cancer; however, only a minority of patients achieve a durable response. Landmark studies have now shown that B-cells and Tertiary Lymphoid Structures (TLS) within the tumor typify exceptional response to ICB. However, the functional role of B-cells and TLS in cancer remains largely unexplored. Here, we produce and evaluate the therapeutic efficacy of tumor-derived antibodies alone or in combination with ICB-based regimen. Methods and Results: Tumors from high-risk resectable melanoma patients treated with neoadjuvant ICB were analyzed. Major pathologic response (MPR), defined as <10% viable tumor at the time of surgery, was used to evaluate response (R - responder; NR - non-responder). Formalin-fixed paraffin-embedded (FFPE) surgical resections were analyzed on NanoString GeoMx. TLS+ regions of interest (ROI) were selected based on melanoma (S100B) and immune markers (CD45 and CD20). Principal component (PC) analysis of ∼18, 000 probes revealed distinct transcriptional profiles between R and NR TLS, with PC1 strongly correlating with response status (r=0.88, p<0.0001). Differential immune analysis showed that R TLS were enriched for CD8+ T-cells (p<0.001), NK cells (p=0.004), γδ T-cells (p=0.017), and memory B-cells (p=0.026). Pathway analysis indicated an upregulation of NK-cell mediated cytotoxicity pathways in R TLS. Targeted BCR sequencing performed on fresh surgical resections from 12 high-risk, resectable melanoma patients (R=9, NR=3) yielded 1277 unique BCR amplicons and 546 unique heavy and light chain-matched antibodies. In silico analysis revealed that BCR clones from R tumors showed higher levels of somatic hypermutation (p=7.2e-9) and isotype switching (50.1% in R vs 32.6% in NR). Flow cytometry and immunofluorescence microscopy identified 73 unique antibodies (R=59, NR=14) that bound to A375 human melanoma cells in vitro. Next, an in vitro ADCC assay (NK:tumor ratio of 10:1) was used to identify that 10 of the 73 melanoma-binding antibodies induced significant cell death of both A375 human and BP (BrafV600EPten-/-) murine melanoma cells. In vivo experiments in BP melanoma bearing mice revealed that 4 ADCC+ antibodies (RAPC5-7-156, RAPC5-9-9, RAPC5-9-8, and RAPC5-9-98) homed to the TME within 24 hours of systemic administration. Remarkably, when combined with αPD-L1, RAPC5-7-156 significantly reduced BP melanoma tumor growth compared to mice treated with isotype control (p=0.0003) and α-PD-L1 alone (p=0.005). Importantly, melanoma derived RAPC5-7-156 did not demonstrate this synergistic effect in ICB-treated MC38 colon carcinoma or LLC1 Lewis lung carcinoma models. Conclusion: These findings suggest that tumor-resident antibodies are capable of augmenting response to ICB thus presenting new opportunities for target discovery and for improving clinical outcomes with ICB. Citation Format: Manoj Chelvanambi, Monika A. Zelazowska, Somnath Paul, Joshua B. Plummer, Brenda Melendez, Sarah B. Johnson, Elise F. Nassif Haddad, Laurence P. Diggs, Rossana N. Lazcano, Khalida M. Wani, Davis R. Ingram, Bharat B. Singh, Diana Shamsutdinova, Rebecca A. Soto, Florentia Dimitriou, Michael G. White, Matthew J. Lastrapes, Golnaz Morad, Melissa S. Simper, Fabiana J. Veguilla, Y David Seo, Beth A. Helmink, Russell G. Witt, Bin Liu, Nadim J. Ajami, Tullia C. Bruno, Wolf H. Fridman, James P. Allison, Padmanee Sharma, Emily Z. Keung, Tina Cascone, Christina L. Roland, Alexander J. Lazar, Kevin M. McBride, Jennifer A. Wargo. Antibodies derived from patient tumors augment response to immune checkpoint blockade in cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2025; Part 1 (Regular Abstracts); 2025 Apr 25-30; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2025;85(8_Suppl_1):Abstract nr 1370.