SN - Sentinel Lymph Node in Breast Cancer- revealing the interaction between tumor subtypes and host immune response. This project focus on the interaction between the host immunesystem and a primary tumor (immune response in regional lymhnodes related to features of the primary tumor in the breast) and the immunresponse within lymphnodes with metastatic tumor cells within the node. In collaboration with Myklebust group we used CyTOF on single-cell suspensions and identified a strong association between tumor metastasis size and the level of changes in T cell composition and functional. Lymph nodes with large metastases had changes suggesting T cell exhaustion and increased immunosuppression. This included increased frequency of TIGIT⁺ T cells with suppressed T cell receptor signaling, and increased frequency of regulatory T cells with an activated phenotype. Our study revealed increased immunosuppression even in some cases with micrometastases. Finally, our in-situ analyses suggested that T-cell alterations were caused by direct interaction between tumor cells and T cells. Now we are addressing the features of the primary tumor as well as expanding the study to 500 patient cases.