Macrophage repolarization by immune checkpoint blockade drives T cell engagement in the tumor microenvironment

Immunotherapy combinations can improve patient outcomes, yet the interactions within the tumor microenvironment (TME) that drive therapeutic synergy are poorly understood. Tumor establishment drives monocyte recruitment and differentiation into tumor-associated macrophages (TAMs), which have essential roles in coordinating immune responses and are thus attractive targets for therapeutic modulation. In a murine model of combination anti-programmed cell death protein 1 (PD-1) and its ligand (anti-PD-L1) checkpoint blockade, tumor control was associated with increased infiltration of CD8⁺ T cells and M1-like repolarization of TAMs. Live-cell imaging of the tumor microenvironment revealed close contacts between tumor-infiltrating CD8⁺ T cells and TAMs, in which the extent of the contact interfaces increased with combination immunotherapy. Treatment with anti-PD-L1 was able to increase macrophage expression of pro-inflammatory factors and phagocytic activity, suggesting a role for TAMs in reactivating CD8⁺ T cells in the TME. However, co-treatment with anti-PD-1 was ultimately necessary for tumor control, indicating the need for combination targeting of the TME.