93 The immune phenotype of response to neoadjuvant chemoradiation in locally advanced rectal cancer

Abstract Background Neoadjuvant chemoradiation is used to downstage locally advanced rectal tumours prior to surgery. Approximately 20% of patients exhibit a pathological complete response (pCR), which is associated with improved local and distant disease control and survival. The mechanisms underlying pCR are incompletely understood and there are no biomarkers currently used in clinical practice to predict response. Aims/methods To identify immune biomarkers associated with pCR, 20 pre-treatment biopsy samples from 7 patients with a pCR, and 23 samples from 8 patients with a poor response, underwent imaging mass cytometry with a panel designed to characterise the tumour microenvironment. Individual cells were segmented by StarDist, and classified using a random forest machine-learning model. Groups were compared with the Wilcoxon rank-sum test, and control of the false discovery rate (FDR) in multiple comparisons was performed by the Benjamini-Hochberg method (Q < 0.05). Results Included patients had a median age of 77 (range 43 – 82), and 11 (73%) were male. Enrichment of M1 macrophages and cytotoxic CD8+ lymphocytes (p < 0.001 and p < 0.01 respectively), but not M2 macrophages or CD4+ lymphocytes, within 10µm of tumour cells was associated with pCR. Tumour expression of HLA-DR was also associated with pCR (p = 5.20e-06). Conclusion Increased numbers of M1 macrophages and cytotoxic T-cells in the tumour microenvironment, and tumour HLA-DR expression should be prospectively validated as a predictive biomarker for pCR. Strategies to increase the intensity of tumour-directed immune responses could be explored to improve pCR rates.