Predictive value of tumor-infiltrating lymphocyte (TIL) dynamics in the tumor micro-environment (TME) during preoperative chemoradiotherapy (CRT) on pathologic complete response (pCR) in microsatellite-stable (MSS) locally advanced rectal cancer (LARC)

3608 Background: Preoperative CRT followed by consolidation nivolumab before surgery showed a promising pCR rate in MSS LARC; both high PD-L1 expression and an elevated CD8 + T cell/effector regulatory T cell ratio analyzed by pre-CRT samples were independently related to high pCR rate (Bando H, Clin Cancer Res. 2022). Here, we applied AI-powered spatial TIL analysis to the VOLTAGE study to investigate whether dynamic change of TIL in TME may predict pCR in MSS LARC. Methods: The VOLTAGE study is a multicenter phase I/II study to evaluate the efficacy of CRT followed by 5 cycles of nivolumab and surgery in patients (pts) with LARC. In this study, tumor samples were obtained at multiple time points; pre-CRT (B1), post-CRT/pre-nivolumab (B2), post-3 cycles of nivolumab (B3), and surgery. In this analysis, a total of 86 H&E whole-slide images (WSI) harvested at B1 and B2 time points from all the pts enrolled in the VOLTAGE study were included in the analysis. Lunit SCOPE IO (Lunit, Republic of Korea), AI-powered H&E-WSI analyzer developed based on 16,443 WSI with pathologists' annotations, was applied to quantify TIL density in TME. Results: In a total of 43 pts, pCR rate of MSS and microsatellite instability-high (MSI-H) were 28.9% (11/38) and 60% (3/5), respectively. In MSS subgroup (n = 38), which has a higher unmet need for pCR prediction, tumor samples with pCR (n = 11) had increased TIL density in TME (tTIL) in post-CRT (B2, median [IQR] 726 [249-1607] /mm 2 ) compared to pre-CRT (B1, 598 [366-905] /mm 2 ), whereas those without pCR (n = 27) had decreased tTIL in post-CRT (405 [148-748] /mm 2 ), compared to pre-CRT (B1, 748 [460-1153] /mm 2 ). Intratumoral TIL density changes were more prominent than stromal TIL density changes in MSS tumor samples with pCR (mean fold change x17.7 vs. x1.5). MSS Pts with tTIL change from B1 to B2 (during CRT) more than x1.8 times achieved 75% (6/8) pCR rate, whereas those with tTIL change less than x1.8 had 16.7% (5/30) pCR rate (p = 0.0035). Conclusions: Change of TIL density in TME during CRT was significantly correlated with favorable clinical outcome of preoperative CRT and consolidation nivolumab, resulting in high pCR rate in MSS LARC.