Spatial Proximity and Relative Distribution of Tumor-Infiltrating Lymphocytes and Macrophages Predict Survival in Melanoma

: The tumor microenvironment (TME) plays a critical role in primary cutaneous melanoma (CM) progression. Although the role of tumor infiltrating lymphocytes (TILs) density has been known for a long time, their spatial distribution and their impact with or without tumor associated macrophages (TAMs) remains controversial. Herein, we investigated spatial proximity between tumor cells and immune cells in 113 primary CM and their correlation with disease-free (DFS) and overall survival (OS). The study cohort included clinical Stage II (n=79) and Stage III (n=34) primary CM with Breslow thickness >2 mm (median age 64 years, 72 male and 41 female). In univariate models, patients with SOX10+ melanoma cells with High proximity to CD8+ TILs in a 20 μm radius showed longer DFS (HR, 0.58, 95%CI 0.36-0.93, p=0.025) and OS (HR, 0.55, 95%CI 0.32-0.92, p=0.023). Furthermore, at multivariate combined analysis patients with SOX10+ melanoma cells with High proximity to CD8+ TILs/Low proximity to CD163+ TAMs in a 20 μm radius showed an increased OS (aHR, 0.37, 95%CI 0.14-0.96, p=0.04) compared to melanoma patients with Low proximity to CD8+ TILs/High proximity to CD163+ TAMs. In a subgroup analysis including 92 patients, a significant negative impact on DFS (aHR 4.49, 95%CI 1.73-11.64, p=0.002) and OS (aHR 3.97, 95%CI 1.37-11.49, p=0.01) was observed in SLN negative patients with a High proximity of CD163+ TAMs to CD8+ TILs. These findings could help to identify high risk patients in the context of thick melanoma and a negative SLN. Our study suggests the importance of quantifying not only the density of immune cells but the individual and combined relative spatial distributions of tumor cells and immune cells for clinical outcome in SLN negative primary CM patients.