Genome editing of immune checkpoints: CRISPR-mediated PD-1 inhibition in cancer

The programmed cell death protein 1 (PD-1)/programmed death-ligand 1 (PD-L1) immune checkpoint is a primary mechanism by which tumors evade immune surveillance, limiting the efficacy of cytotoxic T lymphocytes (CTLs) and tumor-infiltrating lymphocytes (TILs). Although immune checkpoint blockade therapies have revolutionized cancer treatment, their efficacy is restricted by acquired resistance, T-cell exhaustion, and tumor heterogeneity. The advent of CRISPR-Cas9 genome editing provides a precise and versatile approach to disrupt PD-1 or PD-L1, directly enhancing anti-tumor immune responses. Preclinical studies demonstrate that ex vivo PD-1 knockout in primary human T cells or TILs enhances proliferation, cytokine production, and cytotoxicity, resulting in improved tumor clearance in xenograft and humanized mouse models. In chimeric antigen receptor (CAR) T cell therapy, CRISPR-mediated disruption of PD-1 improves effector function, persistence, and resistance to exhaustion, with universal and allogeneic CAR-T platforms benefiting from multiplex genome editing. Direct PD-L1 knockout in tumor cells, often facilitated via nanoparticle- or biomaterial-assisted delivery, reshapes the immunosuppressive tumor microenvironment, promotes T cell infiltration, and enhances the efficacy of adoptive cellular therapy. Combination approaches integrating PD-1 editing with viral antigen targeting, long noncoding RNA (lncRNA) modulation, or conventional checkpoint blockade demonstrate synergistic anti-tumor effects. Clinically, early-phase trials in non-small cell lung cancer, mesothelin-positive solid tumors, and hematological malignancies establish the feasibility, safety, and preliminary efficacy of PD-1-deficient T cells. Despite these promising outcomes, challenges such as off-target effects, delivery efficiency, immunogenicity, long-term persistence, and regulatory considerations remain. This review aims to comprehensively evaluate preclinical and clinical studies investigating CRISPR-mediated PD-1/PD-L1 inhibition across various cancers, summarize mechanistic insights, and highlight translational opportunities and challenges for clinical implementation.