Abstract 3326: Putative value of HLA-neoantigen-TCR interactions as a response biomarker to neoadjuvant chemoradiotherapy in locally advanced rectal cancer

Abstract The standard initial treatment for locally advanced rectal cancer (LARC) is neoadjuvant chemoradiotherapy (nCRT), which aims to reduce tumor size before total mesorectal excision. A notable proportion of patients (10-30%) achieve a complete pathological response (pCR) post-nCRT, allowing organ-preserving approaches, such as the Watch and Wait protocol. However, predicting pCR and differentiating between responders (nCRT-R) and non-responders (nCRT-NR) remains challenging, as current clinical, radiological, and molecular tools lack sufficient accuracy. Identifying predictive biomarkers for nCRT response could thus transform LARC clinical management. Research consistently highlights that tumor-infiltrating lymphocytes (TILs), especially those enriched with cytotoxic CD8+ T-cells, correlate with improved outcomes across various cancers and treatments. Recent findings indicate that some LARC tumors exhibit “hot” immune microenvironments, which may be associated with enhanced nCRT responses. Furthermore, an immunohistochemistry-based scoring system that evaluates CD3+ and CD8+ T-cell enrichment in LARC TILs is being explored as a potential preoperative predictor of response. This study assesses whether elements of the anti-tumor immune response, such as the antigen-presentation pathway, might serve as viable molecular biomarkers for response. We conducted exome and T-cell receptor (TCR) sequencing on 69 LARC tumors (15 nCRT-R and 37 nCRT-NR) to investigate possible associations between HLA, neoantigens, and TCR genetic diversity and nCRT response. Results showed that the neoantigen load, a measure of tumor immunogenicity, was significantly higher in nCRT-NR patients (Wilcoxon test, p=0.03). HLA somatic mutations, however, had no significant impact on antigen presentation or drug resistance, with only 8 mutations identified in 8 patients (6 nCRT-NR and 2 nCRT-R; Pearson’s Chi-squared, p=0.76). HLA polymorphism did not reveal significant differences between response groups, showing no enrichment for specific HLA alleles or loss of heterozygosity (Pearson’s Chi-squared, p=0.64). TCR diversity also showed no association with response (Wilcoxon test, p=0.25), and no differences were detected in TCR gene composition between nCRT-NR and nCRT-R. Notably, we could not quantify the specific repertoire of anti-tumor CD8+ T-cells. Finally, we estimated the probability of HLA-neoantigen-TCR interactions through a machine-learning algorithm and observed that they are more frequent in nCRT-R patients (Wilcoxon test, p&lt0.001). This is the first time that the anti-tumor adaptative immune response was associated with nCRT response based on tumor and host immunogenetic data (tumor-specific neoantigen, HLA, and TCR sequences from TILs), corroborating the growing evidence of the predictive value of the basal immune repertoire. Citation Format: Ramon Torreglosa do Carmo, Vandeclécio Lira, Franciele Hinterholz Knebel, Fabiana Bettoni, Guilherme Pagin São Julião, Natália M. Felicio, Leonardo Corbi, Angelita Habr-Gama, Anamaria Aranha Camargo, Cibele Masotti, Rodrigo Oliva Perez. Putative value of HLA-neoantigen-TCR interactions as a response biomarker to neoadjuvant chemoradiotherapy in locally advanced rectal 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 3326.