TREM2 Deficiency Reprograms Gut Macrophages and Microbiota to Enhance Anti-PD-1 Tumor Immunotherapy Background Immune checkpoint inhibitors (CPIs), such as drugs that block programmed cell death protein-1 (PD-1), programmed death-ligand 1 (PD-L1), and cytotoxic T-lymphocyte-associated protein-4 (CTLA-4), have been successfully used to activate anti...