Correlation of enriched specific subset of immune cells nearby tumor associated macrophage (TAM) with pathologic complete response (pCR) of concurrent chemoradiotherapy followed by nivolumab in locally advanced rectal cancer (LARC).

58 Background: We previously reported tumor-infiltrating lymphocyte (TIL) dynamics predictive of pCR in microsatellite-stable (MSS) LARC. In the current analysis, we investigate whether an immune cell-to-cell spatial network may predict pCR in MSS LARC. Methods: VOLTAGE study is a phase I/II study to evaluate the efficacy of chemoradiotherapy (CRT) followed by nivolumab and subsequent surgery in LARC pts. H&E images and multiplex immunofluorescence (mIF) images containing a total of 35 markers for T cells, TAM, Myeloid-derived suppressor cells, and Dendritic cells were analyzed with the Lunit SCOPE platform (Lunit, Republic of Korea). The proportion of T cells among all DAPI-positive (-pos) cells in a 50-micrometer radius centered on the TAM cells was counted. Results: In the samples of MSS LARC pts gathered at baseline, pre-treatment (n=38), TIL density in tumor microenvironment (TME) analyzed by H&E images was significantly correlated with the proportions of CD8-pos, CD4-pos, and FOXP3-pos cells (coefficient [coeff] 0.635, 0.482, and 0.580, respectively), but loosely correlated with PDL1-pos, PD1-pos, and CTLA4-pos cells (coeff 0.255, 0.174, and 0.180, respectively). Interestingly, intratumoral TIL density was more strongly correlated with TAM markers such as CD68 (coeff 0.269), compared to stromal TIL density (coeff 0.086). pCR rate was 28.9% in all MSS LARC, with baseline TIL density in TME predicting pCR with area under the receiver operating characteristic curve (AUROC) of 0.636 (p = 0.101). Interestingly, the ratio of baseline PDL1-pos cell proportion nearby CD68-pos cells, over that in all TME area (hereafter referred to as 'TAM-PDL1-proximity score') showed the best predictive performance for pCR, with an AUROC of 0.768 (p = 0.005). At the optimal cutoff of 1.66 times TAM-PDL1-proximity score, pCR rates for pts ≥ the cutoff and < the cutoff were 44% (11/25) and 0% (0/13), respectively (p = 0.006). Conclusions: PDL1-expressing immune cells nearby tumor associated macrophages before treatment is a promising predictive biomarker for pCR of neoadjuvant CRT followed by nivolumab in MSS LARC.