Mechanisms of Resistance to Immune Checkpoint Inhibitors and Strategies to Reverse Drug Resistance in Lung Cancer
Introduction
Lung cancer remains the leading cause of cancer-related deaths worldwide. In recent years, immune checkpoint inhibitors (ICIs) targeting the programmed cell death-1 (PD-1)/programmed cell death-ligand 1 (PD-L1) and cytotoxic T-lymphocyte antigen 4 (CTLA-4) axes have emerged as a promising treatment for various cancers, particularly lung cancer. Despite their success, not all patients benefit from these therapies, and some experience relapse after an initial response. Resistance to ICIs can be categorized into primary, adaptive, and acquired resistance. Primary resistance occurs when tumors do not respond to immunotherapy initially, while acquired resistance refers to cases where tumors initially respond but later progress. Understanding the mechanisms of resistance and developing strategies to overcome it are critical for improving patient outcomes.
Mechanisms of Resistance to ICIs
Primary and Adaptive Resistance
Antigen Presentation and Recognition
The inability of T cells to recognize tumor antigens is a primary cause of resistance. Tumor-specific or tumor-associated antigens are essential for T-cell recognition. Neoantigens, delivered through dendritic cells (DCs), have shown promise in inducing potent T-cell responses in cancers like melanoma and glioblastoma. Genetic instability, such as mutations in DNA repair genes like BRCA2, can increase tumor mutational burden (TMB), leading to neoantigen formation and improved ICI responses. Epigenetic modifications in tumor cells can also affect antigen processing and presentation. For example, histone deacetylase (HDAC) inhibitors enhance major histocompatibility complex (MHC) and tumor antigen expression, potentially improving immune recognition.
T Cell Priming and Activation
Abnormal Wnt/β-catenin signaling can reduce the infiltration of CD103+ DCs in the tumor microenvironment (TME), impairing T-cell activation. High levels of β-catenin in melanoma are associated with reduced CD103+ DCs and poor response to immunotherapy. Additionally, the interaction between CD28 and B7 is crucial for T-cell activation. Blocking this interaction or knocking out CD28 prevents T-cell responses to PD-1 inhibitors. Negative regulatory factors like IL-10, vascular endothelial growth factor (VEGF), and transforming growth factor-β (TGF-β) can disrupt dendritic cell (DC) maturation and function, further impairing T-cell priming.
T Cell Trafficking and Tumor Infiltration
Cancer-associated fibroblasts (CAFs) can remodel the extracellular matrix (ECM), creating a physical barrier that limits T-cell infiltration. Chemokines like CCL5 and CCL7 recruit myeloid-derived suppressor cells (MDSCs) and regulatory T cells (Tregs) into the TME, suppressing the immune response. VEGF, an immunosuppressive cytokine, is associated with resistance to ICIs. In mouse models, VEGF impedes T-cell lineage commitment and promotes Treg infiltration, contributing to immune evasion.
T Cell Killing Activity within the Tumor Microenvironment
The interferon-γ (IFN-γ) pathway plays a dual role in anti-tumor immunity. While IFN-γ can enhance MHC expression and recruit immune cells, prolonged exposure can lead to immune escape. Mutations in IFN-γ pathway components, such as Janus kinase (JAK) 1/2 and signal transducer and activator of transcription (STAT) proteins, can render tumor cells resistant to PD-1 blockade. Loss of phosphatase and tensin homolog (PTEN) upregulates PD-L1 expression, contributing to immune evasion. Additionally, indoleamine 2,3-dioxygenase (IDO) depletes tryptophan, impairing T-cell function and promoting Treg differentiation.
Tumor-associated macrophages (TAMs) and MDSCs are key immunosuppressive cells in the TME. M2-like TAMs suppress T-cell responses, while MDSCs inhibit T-cell proliferation and function. Targeting these cells, such as through colony-stimulating factor 1 receptor (CSF-1R) blockade, can enhance ICI efficacy. Tregs, which maintain immune tolerance, can suppress effector T-cell responses. Depleting Tregs in the TME has been shown to enhance anti-tumor immunity in preclinical models.
Acquired Resistance
Acquired resistance often involves changes in antigen presentation machinery. Loss of beta-2-microglobulin (β2M) or human leukocyte antigen (HLA) class I molecules prevents T-cell recognition. Mutations in JAK1/2 or PTEN can also contribute to acquired resistance. Upregulation of alternative immune checkpoints like T cell immunoglobulin mucin 3 (TIM-3), lymphocyte activation gene 3 (LAG-3), and V-domain immunoglobulin suppressor of T cell activation (VISTA) has been observed in resistant tumors. For example, TIM-3 expression on cytotoxic CD8+ T cells is associated with resistance to PD-1 blockade in non-small cell lung cancer (NSCLC).
Strategies to Reverse ICIs Resistance
Combining with Other ICIs
Combining CTLA-4 and PD-1 inhibitors has shown promise in enhancing T-cell responses. Other immune checkpoints, such as TIGIT and LAG-3, are being explored as potential targets. For instance, blocking both PD-1 and TIGIT has demonstrated enhanced anti-tumor immunity in preclinical models. Similarly, targeting NKG2A, CD39, CD73, or CD47 in combination with PD-1/PD-L1 inhibitors has shown synergistic effects.
Combining with Chemotherapy
Chemotherapy can enhance the immunogenicity of tumor cells by inducing immunogenic cell death. Drugs like anthracyclines and gemcitabine activate T cells and DCs, improving antigen presentation. Clinical trials like KEYNOTE-189 and KEYNOTE-407 have demonstrated the efficacy of combining pembrolizumab with chemotherapy in NSCLC, leading to FDA approval for first-line treatment.
Combining with Radiotherapy
Radiotherapy releases tumor antigens and enhances T-cell responses. Stereotactic body radiation therapy (SBRT) combined with PD-1 inhibitors has shown abscopal effects, where non-irradiated tumors also regress. The PEMBRO-RT study demonstrated improved progression-free survival (PFS) and objective response rates (ORR) in NSCLC patients treated with pembrolizumab and SBRT.
Combining with Targeted Therapy
Targeted therapies like VEGF inhibitors and EGFR tyrosine kinase inhibitors (TKIs) can enhance ICI efficacy. VEGF inhibitors reduce immunosuppressive cell populations and improve T-cell infiltration. The IMPOWER150 trial showed that combining atezolizumab with chemotherapy and bevacizumab improved PFS and overall survival (OS) in NSCLC. EGFR-TKIs upregulate IFN-γ and enhance T-cell activity, making them promising partners for PD-1/PD-L1 inhibitors.
Combining with Cytokines
IDO inhibitors, which block tryptophan metabolism, have shown potential in reversing immune suppression. Although early trials with IDO inhibitors were disappointing, combining them with ICIs remains an area of interest. Other cytokine targets, such as CD137 and CD40, are being explored to enhance anti-tumor immunity.
Combining with Epigenetic Modifiers
Epigenetic modifiers like HDAC inhibitors and hypomethylating agents can re-express immune-related genes and enhance antigen presentation. In preclinical models, HDAC inhibitors have been shown to increase MHC and tumor antigen expression, improving the efficacy of adoptive cell transfer therapy.
Combining with Cancer Vaccines
Personalized cancer vaccines, designed based on tumor-specific mutations, have shown promise in enhancing anti-tumor immunity. Combining these vaccines with ICIs has led to improved clinical outcomes in melanoma and other cancers.
Combining with CAR-T
Chimeric antigen receptor T-cell (CAR-T) therapy has been successful in hematologic malignancies. Overcoming challenges like the immunosuppressive TME and “on-target/off-tumor” toxicities is critical for extending CAR-T therapy to solid tumors. Combining CAR-T with PD-1/PD-L1 inhibitors or targeting VEGF and IL-2 pathways are strategies being explored to enhance efficacy.
Conclusion
Resistance to ICIs poses a significant challenge in the treatment of lung cancer. Understanding the mechanisms of resistance, including primary, adaptive, and acquired resistance, is essential for developing effective strategies to overcome it. Combining ICIs with other therapies, such as chemotherapy, radiotherapy, targeted therapy, and epigenetic modifiers, offers promising avenues for enhancing anti-tumor immunity. Personalized approaches, including cancer vaccines and CAR-T therapy, may further improve outcomes. Continued research into predictive biomarkers and novel combination strategies will be critical for advancing precision medicine in lung cancer treatment.
doi.org/10.1097/CM9.0000000000001124
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