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 (programmed cell death protein 1) is an inhibitory receptor expressed on activated and chronically stimulated T cells. When tumour-expressed PD-L1 (CD274) engages PD-1, the ITSM motif in PD-1's cytoplasmic tail recruits SHP-2 phosphatase, which dephosphorylates TCR signalling intermediates (ZAP70, CD3ζ, LAT) and PI3K–AKT pathway components. The net effect is a cell-intrinsic switch from effector T-cell activation to immune exhaustion: reduced IFN-γ, IL-2, and TNF-α production, impaired cytotoxic granule release, and eventual T-cell apoptosis. Tumours that densely express PD-L1 create a contact-dependent immunosuppressive microenvironment that disables infiltrating T cells precisely at the moment of antigen recognition.

Tumours upregulate PD-L1 through two parallel mechanisms. Adaptive resistance: IFN-γ secreted by infiltrating T cells activates JAK1/JAK2–STAT1–IRF1 signalling in tumour cells, transcriptionally driving PD-L1 — a paradoxical feedback where anti-tumour immune activity induces the very checkpoint that suppresses it. Intrinsic oncogenic PD-L1 expression: KRAS G12D/V mutations, MYC amplification, constitutively active STAT3 signalling, and PI3K/AKT pathway hyperactivation each independently drive PD-L1 transcription independent of IFN-γ, making checkpoint evasion an inherent property of oncogenically driven cancer cells regardless of immune infiltration.

STAT3: The Oncogenic Driver of Immune Evasion

STAT3 (signal transducer and activator of transcription 3) is constitutively activated in many cancers downstream of JAK2, EGFR, IL-6, and other receptor tyrosine kinase and cytokine pathways. Constitutively active STAT3 drives PD-L1 transcription through direct STAT3 binding sites in the CD274 (PD-L1) promoter, suppresses MHC class I antigen presentation, promotes secretion of immunosuppressive cytokines (IL-10, IL-6, VEGFA), and recruits regulatory T cells and myeloid-derived suppressor cells to the tumour microenvironment. In this way, STAT3 functions as a master transcriptional regulator of tumour immune evasion that simultaneously promotes tumour cell survival and suppresses anti-tumour immunity.

The STAT3–PD-L1 axis creates a mechanistic rationale for combining checkpoint inhibitors with STAT3 inhibitors or JAK inhibitors. Ruxolitinib and other JAK1/2 inhibitors reduce STAT3-driven PD-L1 expression and potentially enhance checkpoint inhibitor response in STAT3-hyperactivated tumours — a hypothesis being evaluated in combination trials across multiple solid tumour types.

CTLA4 Checkpoint Signalling at the T-Cell Priming Phase

CTLA4 operates at an earlier phase of the immune response than PD-1 — in lymph nodes during antigen-specific T-cell priming rather than in the tumour microenvironment at the effector phase. CTLA4 is rapidly induced on T cells following TCR activation and competes with the costimulatory receptor CD28 for binding to B7 ligands (CD80/CD86) on antigen-presenting cells. Because CTLA4 has approximately 20-fold higher B7 affinity than CD28, its induction effectively outcompetes CD28-mediated costimulatory signalling, raising the TCR activation threshold and limiting clonal expansion of tumour-antigen-specific T cells. Regulatory T cells (Tregs), which constitutively express CTLA4, use this mechanism to suppress effector T-cell priming in lymph nodes draining tumour antigens.

Ipilimumab (anti-CTLA4) blocks CTLA4–B7 interaction, enabling more robust T-cell priming and Treg depletion. Combining ipilimumab with nivolumab (anti-PD-1) simultaneously releases priming-phase (CTLA4) and effector-phase (PD-1) suppression, producing synergistic immune activation. This dual checkpoint blockade achieves 58% ORR in melanoma (vs 40–45% single-agent PD-1) with superior complete response rates and long-term survival benefit across melanoma, NSCLC, RCC, MSI-high CRC, and mesothelioma — at the cost of substantially increased immune-related adverse events from broad T-cell activation.

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

Response to checkpoint inhibitors varies enormously across patients, driving intense biomarker research. Three primary predictors have emerged with distinct clinical utility. MSI-high (microsatellite instability-high)/dMMR (mismatch repair deficient) status is the most reliably predictive biomarker — frameshift mutations at microsatellite repeats generate large numbers of unique neoantigens, creating a highly immunogenic mutational landscape with ~40% ORR pan-tumour and highly durable responses. Pembrolizumab received the first tumour-agnostic FDA approval in cancer (2017) based on dMMR status alone. Tumour mutational burden (TMB-high ≥10 mutations/megabase), a surrogate for neoantigen load across all mutation types, received a separate tumour-agnostic pembrolizumab approval. PD-L1 IHC predicts response in some histologies (NSCLC: PD-L1 ≥50% predicts benefit from pembrolizumab monotherapy over chemotherapy) but not others (RCC, MSI-high CRC).

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 affects the majority of patients and reflects fundamental defects in anti-tumour immune activation. 'Cold' or 'immune-excluded' tumours — driven by WNT/β-catenin signalling that prevents T-cell trafficking into the tumour, low TMB providing insufficient neoantigens, or loss-of-function mutations in B2M (beta2-microglobulin, required for MHC class I surface expression) that prevent CD8+ T-cell recognition — fail to respond to checkpoint blockade regardless of PD-L1 expression. TP53-mutant tumours with high chromosomal instability can also suppress immunogenicity through cGAS–STING pathway suppression, reducing innate immune sensing of cytoplasmic DNA from micronuclei.

Acquired resistance after initial response commonly involves JAK1/JAK2 loss-of-function mutations that impair IFN-γ signalling — paradoxically reducing tumour PD-L1 expression but also eliminating IFN-γ-mediated tumour cell killing and MHC class I antigen presentation upregulation, producing immune escape despite theoretically lower checkpoint activity. Alternative checkpoint upregulation (TIM-3, LAG-3, TIGIT) provides parallel inhibitory mechanisms that substitute for PD-1 upon its blockade. Combinations with anti-VEGF therapy (atezolizumab+bevacizumab in hepatocellular carcinoma: IMbrave150 OS benefit) exploit the immunostimulatory effect of tumour vessel normalisation, which improves T-cell access to previously immune-excluded tumours.

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.
• ·KRAS mutations, MYC amplification, and constitutively active STAT3 drive intrinsic tumour PD-L1 expression independent of immune activity — making checkpoint evasion an oncogenic property, not merely an adaptive response to immune infiltration.
• ·MSI-high/dMMR status is the most reliably predictive checkpoint inhibitor biomarker (~40% ORR pan-tumour), earning the first tumour-agnostic FDA approval in cancer history (pembrolizumab, 2017) — driven by frameshift neoantigen load from mismatch repair deficiency.
• ·Dual PD-1 + CTLA4 blockade (nivolumab + ipilimumab) releases both priming-phase and effector-phase T-cell suppression simultaneously, achieving 58% ORR in melanoma and superior outcomes across multiple tumour types at the cost of increased immune-related adverse events.
• ·Primary resistance reflects 'cold' tumour biology (WNT/β-catenin T-cell exclusion, low TMB, B2M loss); acquired resistance commonly involves JAK1/2 loss-of-function — both motivating combinations with anti-VEGF, chemotherapy, and next-generation checkpoint targets (TIM-3, LAG-3, TIGIT).