Primary and acquired resistance to immunotherapy in NSCLC.
Non-small cell lung cancer (NSCLC) is one of the leading causes of cancer incidence and mortality worldwide. In recent years, immune checkpoint inhibitors (ICIs), particularly those targeting the programmed cell death protein 1/programmed death-ligand 1 (PD-1/PD-L1) axis, have significantly improved survival outcomes in a subset of patients. However, the magnitude and durability of clinical benefit vary considerably according to PD-L1 expression, treatment setting, histological subtype, oncogenic driver status, and whether ICIs are administered as monotherapy or in combination regimens. A substantial proportion of patients therefore exhibit either primary resistance or acquired resistance after an initial response. This review systematically summarizes the key mechanisms underlying immune resistance in lung cancer. These include defects in antigen presentation, such as abnormalities in major histocompatibility complex class I (MHC-I), transporter associated with antigen processing 2 (TAP2), and β2-microglobulin (B2M), as well as dysregulation of the interferon-γ/Janus kinase-signal transducer and activator of transcription (IFN-γ/JAK-STAT) signaling pathway. Tumors frequently exhibit an immune-excluded or 'cold' phenotype, which further limits immune recognition and reduces responsiveness to immunotherapy. This review summarizes immune resistance in NSCLC through a framework that distinguishes primary resistance from acquired resistance. Primary resistance reflects failure of immune activation at treatment initiation, usually due to pre-existing tumor-intrinsic or microenvironmental barriers, including impaired antigen presentation, defective IFN-γ/JAK-STAT signaling, low tumor immunogenicity, immune-cold or immune-excluded phenotypes, and suppressive TME states. In contrast, acquired resistance reflects adaptive tumor and immune ecosystem evolution under therapeutic pressure, leading to neoantigen loss, HLA or B2M alterations, compensatory checkpoint activation, progressive T cell exhaustion, TME remodeling, and epigenetic stabilization of immune escape. We further discuss mechanism-based biomarkers, translational correlates, and rational therapeutic strategies for overcoming resistance.