Organ-specific immune setpoints underlie divergent immune profiles across metastatic sites in breast cancer.

Immune composition within the tumor microenvironment (TME) plays a central role in the propensity of cancer to metastasize and to respond to therapy. Previous studies suggested that the metastatic TME is immune suppressed. However, limited accessibility to multiple metastatic sites within patients has made assessment of the immune TME in the context of multi-organ metastases difficult. We utilized a rapid postmortem tissue collection protocol to assess immune composition in numerous sites of breast cancer metastasis and paired tumor-free tissues. Metastases were found to have comparable immune cell densities and composition to paired tumor-free tissues of the same organ type. In contrast, immune cell densities in both metastatic and tumor-free tissues were significantly different between organ types, with lung immune infiltration consistently greater than liver. These immune profiling results were consistent between both flow cytometry and multiplex immunofluorescence-based spatial analysis. Furthermore, we found granulocytes were a predominant tumor-infiltrating immune cell in both lung and liver metastases and these granulocytes made up the majority of PD-L1-expressing cells in many tissue sites. We also identified distinct potential mechanisms of immunosuppression in lung and liver metastases, with lung having increased expression of PD-L1+ antigen-presenting cells and liver having higher numbers of activated regulatory T cells and HLA-DRlow monocytes. Together these results demonstrate that immune contexture of metastases is dictated by organ type, and that immunotherapy strategies may benefit from unique tailoring to tissue-specific features of the immune TME.