Translational Insights into Immunotherapy Resistance

Abstract

Immune checkpoint inhibitors (ICIs) -- monoclonal antibodies targeting PD-1, PD-L1, CTLA-4, LAG-3, and TIM-3 -- have fundamentally transformed the treatment landscape for multiple solid tumours, achieving durable responses and long-term survival in patient populations where previously no effective systemic therapy existed. Pembrolizumab demonstrated 5-year survival rates of 23-31% in previously treated advanced NSCLC; nivolumab achieved 5-year survival of 26% in advanced melanoma; and ipilimumab plus nivolumab produced 10-year survival rates exceeding 23% in advanced melanoma -- outcomes inconceivable in the pre-ICI era. Yet despite these landmark achievements, the majority of patients either fail to respond to ICI therapy (primary resistance; 40-70% depending on tumour type and biomarker selection) or achieve an initial response followed by inevitable disease progression (acquired resistance; occurring in the majority of initial responders within 1-3 years). Translating the molecular understanding of ICI resistance mechanisms into actionable clinical strategies -- combination therapies, sequential treatment approaches, and novel resistance-overcoming agents -- is the defining challenge of immuno-oncology in 2022. This review synthesises the mechanisms of primary and acquired ICI resistance across 247 clinical and translational studies, developing an Immunotherapy Resistance Classification (IRC) framework that maps resistance mechanisms to therapeutic vulnerabilities. The IRC framework identifies six resistance categories: tumour cell intrinsic (loss of antigen presentation; oncogenic bypass; metabolic reprogramming); tumour microenvironment-mediated (immunosuppressive cell infiltration; cytokine milieu; physical barriers); and systemic (microbiome composition; host immune competence; prior therapy effects). IRC-guided combination strategies including ICI plus VEGF inhibition, ICI plus MEK inhibition, and ICI plus HDAC inhibition are evaluated with clinical trial evidence, offering a translational roadmap for the next generation of resistance-overcoming immunotherapy approaches.