How Tumours Suppress T-Cell Activation: PD-1, PD-L1, and CTLA4 Checkpoint Signalling

How Tumours Exploit PD-1/PD-L1 Signalling to Silence Effector T Cells

PD-1 is an inhibitory receptor induced on activated T cells and other immune cells. PD-L1 or PD-L2 engagement recruits phosphatase activity to the PD-1 cytoplasmic motifs and reduces selected T-cell-receptor and costimulatory signals. Chronic stimulation can contribute to a spectrum of dysfunctional states; this is more nuanced than one switch from activation to exhaustion or inevitable T-cell apoptosis.

PD-L1 can be induced adaptively through IFN-γ–JAK–STAT–IRF1 signalling and can also be influenced by oncogenic and stress pathways. KRAS, MYC, STAT3 and PI3K-pathway associations differ by tumour and co-alteration. A driver alteration does not guarantee PD-L1 expression or make immune evasion independent of the surrounding immune infiltrate.

STAT3: The Oncogenic Driver of Immune Evasion

STAT3 can be activated downstream of cytokine receptors and oncogenic kinases and can shape tumour, stromal and immune-cell transcription. In selected models it affects PD-L1, antigen presentation, cytokines and myeloid-cell states. Calling STAT3 the master regulator obscures parallel pathways and the fact that STAT3 can have cell-type-specific effects.

STAT3- or JAK-directed combinations require care because JAK signalling also supports interferon responses needed for antigen presentation and tumour-cell killing. Lowering PD-L1 expression is not automatically immunostimulatory. Combination hypotheses therefore require agent-, dose-, tumour- and immune-context evidence.

CTLA4 Checkpoint Signalling at the T-Cell Priming Phase

CTLA4 competes with CD28 for CD80/CD86 and influences priming, expansion and regulatory-T-cell function. Its effects are prominent in lymphoid tissue but are not confined there, and a single affinity ratio does not capture ligand removal, trafficking and cellular context.

CTLA4 and PD-1 antibodies can be combined in specific diseases because they alter partly distinct inhibitory processes. Efficacy and immune-related toxicity differ substantially by tumour, dose and schedule. Treg depletion is antibody- and tissue-context-dependent, and response percentages from melanoma should not be transferred across cancers.

Predictive Biomarkers: MSI-High, Tumour Mutational Burden, and PD-L1

MSI-high/dMMR, PD-L1 immunohistochemistry and tumour mutational burden can support patient selection in specified settings. Their definitions are not interchangeable: MSI can be measured by molecular assays, dMMR by protein or genomic approaches, PD-L1 uses product- and disease-specific scoring, and TMB depends on panel and cut-off. The FDA companion-diagnostic list links particular tests, biomarkers and products.

The imperfect correlation between PD-L1, TMB, and MSI-high reflects that checkpoint response requires multiple conditions to align: sufficient neoantigen load, functional antigen presentation machinery (intact MHC class I), T-cell infiltration into the tumour, and absence of alternative immune suppression pathways (TIM-3, LAG-3, TIGIT upregulation; IDO1-mediated metabolic immunosuppression; TGF-β-driven T-cell exclusion). No single biomarker captures this multifactorial response determinant — motivating composite biomarker panels and tumour immune phenotyping.

Primary and Acquired Resistance to Checkpoint Inhibitors

Primary resistance can involve low antigenicity, defective presentation, exclusion of T cells, suppressive myeloid or stromal states and ineffective effector function. WNT–β-catenin, B2M loss and chromosome-instability-associated programmes are examples observed in selected contexts, not deterministic rules that override every other biomarker.

Acquired resistance can involve antigen loss, B2M or JAK-pathway changes, alternative inhibitory receptors, immune editing and cell-state change. Anti-VEGF combinations can alter vasculature and immunity in specific diseases, but vessel normalisation and synergy are conditional rather than universal explanations.

Key Takeaways

• ·Tumours actively suppress T-cell activation through two mechanistically distinct checkpoint pathways: PD-1/PD-L1 silences effector T cells in the tumour microenvironment via SHP-2-mediated TCR dephosphorylation; CTLA4 blocks T-cell priming in lymph nodes by outcompeting CD28 for B7 ligands.
• ·Oncogenic and inflammatory signals can both influence PD-L1, but no single driver determines expression or response across tumour types.
• ·MSI-high/dMMR, PD-L1 and TMB are distinct biomarkers with test-, disease- and product-specific roles.
• ·Dual checkpoint blockade can improve outcomes in defined populations while increasing immune-related toxicity; results should not be transferred between diseases.
• ·Primary and acquired resistance are heterogeneous and can involve antigenicity, presentation, trafficking, immune-cell state and tumour evolution.