Cellular interactions in the sentinel lymph node predict melanoma recurrence.

Melanoma outcomes have dramatically improved over the past decade, but some patients still experience disease recurrence, particularly those who present at later stage of disease. Prior studies identified an altered immune microenvironment in the sentinel lymph node (SLN) including the presence of dysfunctional CD8 T cells and CD4 T regulatory cells (Tregs), but the spatial organization of the SLN and the interactions of individual cell types have not been extensively studied. To understand how spatial organization of immune cells in the SLN is related to outcomes, we performed spatial proteomic profiling of Stage I and II melanoma SLN to generate an atlas of cell types. Following multiplexed immunofluorescence imaging, deep learning-based segmentation and clustering, we identified 33 subsets of T cells, B cells and Tumor cells. Lymphoid region analyses established a foundational spatial map of the melanoma SLN, revealing higher regional frequencies of activated and memory CD4 T cells in Stage I SLN and consistent positioning of T cells at functional spatial locations. To better understand cellular interactions, we evaluated the immediate neighbors around each index cell using Effect Size Interaction mapping (ESI-map), a novel computational toolkit for understanding spatial interactions. Nearest-neighbor analyses across biological replicates revealed rewiring in cell-cell interacting pairs across disease progression and patient outcomes, particularly with respect to exhausted TOX+ CD8 T cells. Stage II patients who experienced disease recurrence had notable enrichment of cellular interactions between Tregs and TOX+ CD8 T cells that was associated with reduced expression of granzyme B and interferon-γ that was most pronounced in exhausted CD8 T cells. Together, these data demonstrate that the specific spatial interactions between Tregs and exhausted CD8 T cells in the SLN provide a novel immune signature for understanding melanoma outcomes.