Macrophages impede CD8 T cells from reaching tumor cells and limit the efficacy of anti–PD-1 treatment

Significance Cancer immunotherapy is a promising therapeutic intervention. However, complete and durable responses are seen in only a fraction of cancer patients. A key factor that limits therapeutic success is the lack of T cells in tumor cell regions, a profile termed “immune-excluded.” Here, we provide evidence that tumor-associated macrophages (TAMs) are an important determinant of the establishment of a T cell-excluded tumor phenotype. In human and murine tumors, we found that CD8 T cells poorly migrate and invade tumor nests due to long-lasting interactions with TAMs. The depletion of TAMs restores T cell migration and infiltration into tumor islets and improves the efficacy of anti–PD-1 immunotherapy. This study highlights the rationale of combining approaches targeting TAM and immune checkpoint proteins.