The nature and site of tumor-antigen presentation to immune T cells by bone-marrow-derived cells within the tumor microenvironment remains unresolved. We generated a fluorescent mouse model of spontaneous immunoevasive breast cancer and identified a subset of myeloid cells with significant similarity to dendritic cells and macrophages that constitutively ingest tumor-derived proteins and present processed tumor antigens to reactive T cells. Using intravital live imaging, we determined that infiltrating tumor-specific T cells engage in long-lived interactions with these cells, proximal to the tumor. In vitro, these cells capture cytotoxic T cells in signaling-competent conjugates but do not support full activation or sustain cytolysis. The spatiotemporal dynamics revealed here implicate nonproductive interactions between T cells and antigen-presenting cells on the tumor margin.
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