Abstract 2762: Spatial distribution and immune cell infiltration at different sites of melanoma metastases (mets)

Abstract Purpose: The site of metastasis in advanced melanoma patients (pts) influences pt response to immunotherapy and overall prognosis. Liver mets are particularly resistant to immunotherapy and their presence often confers shorter survival and reduced response rates. This study characterised and compared the tumor immune microenvironment of melanoma mets at different anatomical sites. Methods: T-cell and myeloid cell markers were stained using multiplex IHC on FFPE samples from 136 untreated metastatic melanoma pts, from 5 anatomical sites (liver, lung, brain, subcutaneous, and lymph node (LN)). Cell densities, subpopulations and the spatial distribution of cells were compared between sites. Results: CD3+ T cell infiltration was less dense in the liver (med = 154.3 cells/mm2) and brain mets (med = 180.7 cells/mm2) v's lung (med = 434.7 cells/mm2) and LN mets (med = 504.8 cells/mm2) (p<0.05), and T-cells were further away from melanoma cells in liver v's lung mets (med distance = 83µm v's 57µm respectively; P<0.05). The liver displayed a unique T-cell profile, with a significantly lower proportion of CD3+ T cells expressing PD-1 (med = 0.7%) compared to all other sites (P<0.05) while brain mets expressed the highest proportion of PD-1+ T cells (med = 8.9%). There were higher proportions of Tim3+ T cells in liver (10.7%) and lung mets (12.9%) v's brain, subcut and LN mets (P<0.05). Specifically, in liver mets CD3+ T cells within 20um of a melanoma cell had a significantly higher expression of Tim3 (med = 18.57%) v's PD-1 (med = 2%). Brain mets had the lowest density of CD68+ macrophages (med = 33 cells/mm2) v's lung (med = 372 cells/mm2), liver (med = 290 cells/mm2) and Subcut mets (med = 154 cells/mm2). Lung mets also had the highest proportion of PD-L1+ macrophages (med = 18.73%) compared to liver (med = 5.2%) and brain mets (med = 0.65%). While the most highly expressed T cell population varied between sites, CD16+ macrophages were consistently the highest expressed macrophage subpopulation regardless of metastatic site. Conclusion: These data suggest that liver mets are less immunogenic, with fewer T cells and greater distances between CD3+ T cells and melanoma cells, and may account for the poor prognosis of melanoma patients with liver metastases and their reduced response rate to immunotherapy compared to other mets. While liver mets have reduced PD-1 expression, higher Tim3 expression in the liver may provide a therapeutic opportunity to improve outcomes for pts with liver mets by using anti-TIM3 immunotherapy. Overall, these data provide novel insights into the site-specific biology of melanoma mets, heterogeneity in the tumor immune microenvironment, and the need for site specific biomarkers to be considered when selecting treatment targets. Citation Format: Jordan W. Conway, Robert V. Rawson, James S. Wilmott, Georgina V. Long, Richard A. Scolyer, Inês Pires Da Silva. Spatial distribution and immune cell infiltration at different sites of melanoma metastases (mets) [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2021; 2021 Apr 10-15 and May 17-21. Philadelphia (PA): AACR; Cancer Res 2021;81(13_Suppl):Abstract nr 2762.