Zonal Differences In Pd-1 Expression In Centre Of Tumour Versus Periphery In Microsatellite Stable And Unstable Colorectal Cancer

3574 Background: Colorectal cancers that show evidence of microsatellite instability (MSI-H) are marked by a high tumour infiltrating lymphocyte population (TiL) which is thought to be prognostic. Programmed Death-1(PD-1) is a negative regulator of the immune system and targeting the interaction with its ligand PD-L1 offers a potential therapeutic target in many malignancies. We aimed to characterize CD8 and PD-1 expression in both the tumour centre and tumour periphery of microsatellite stable (MSS) and unstable colorectal cancers. Methods: Paraffin-embedded tumour blocks were cut at 5um, prepared and stained using specific antibodies for CD8 and PD-1. The tumour periphery was defined as the area within a 400x high power field (HPF) from the outline of the tumor. The tumour centre was defined as the area at least one 400x HPF apart from the tumour outline toward the centre of the tumour. Images were taken at 40x, 100x, 200x and 400x. Positive cells were averaged across 3 high power fields and classified ...