Elicit: Tumor Immune Evasion and Pembrolizumab

Tumor Immune Evasion and Pembrolizumab

How does the PD-1/PD-L1 pathway contribute to tumor immune evasion, and how does pembrolizumab interrupt it?

The PD-1/PD-L1 pathway enables tumors to evade immune destruction by suppressing T cell cytolytic activity, inducing T cell exhaustion, and creating adaptive immune resistance through interferon-driven PD-L1 upregulation, while pembrolizumab interrupts this pathway by blocking PD-1/PD-L1 interaction to restore T cell proliferation and effector function, resulting in intratumoral CD8+ T cell expansion and durable tumor regression in 18-76% of patients depending on tumor type and baseline immune infiltration.

Abstract

The PD-1/PD-L1 pathway enables tumor immune evasion through multiple mechanisms: direct suppression of T cell function by inhibiting cytokine production and cytolytic activity, induction of T cell exhaustion, and adaptive immune resistance whereby interferon production by tumor-infiltrating CD8+ cells upregulates PD-L1 expression. This interaction dampens T cell proliferation and activation, blocks effector functions, and creates an immunosuppressive tumor microenvironment through upregulation of genes involved in extracellular matrix remodeling and metabolic changes favoring immune evasion. Some CD8+ T cells enter an irreversible dysfunctional state that cannot be rescued by pathway blockade.

Pembrolizumab interrupts this pathway by blocking PD-1 interaction with PD-L1 and PD-L2, removing the suppressive effect on T-cell proliferation and immune response. This blockade restores T cell function, leading to increased proliferation of intratumoral CD8+ T cells that directly correlates with tumor size reduction, expansion of memory CD8+ T cells, enhanced effector function marked by granzyme B expression, and development of a more clonal TCR repertoire. Clinical evidence across 10 sources demonstrates objective response rates of 18-76% depending on tumor type and treatment regimen, with responses lasting >1 year in most responders. Pre-existing CD8+ T cells at the invasive tumor margin with close proximity between PD-1 and PD-L1 expressing cells predict response, while higher PD-L1 expression (≥50%) and high tumor mutational burden correlate with improved response rates, though responses occur in PD-L1-negative tumors.

Methods

We analyzed 10 sources from an initial pool of 200, using 9 screening criteria. Each paper was reviewed for 9 key aspects that mattered most to the research question.

Records from Elicit search

Data extraction

Study Design: Extract study type and basic methodology for research investigating PD-1/PD-L1 pathway immune evasion mechanisms and pembrolizumab interruption, including:

Cancer Types: Extract specific cancer types and patient populations studied in relation to PD-1/PD-L1 immune evasion and pembrolizumab treatment, including:

PD-1/PD-L1 Immune Evasion Mechanisms: Extract all described mechanisms of the PD-1/PD-L1 pathway contributing to tumor immune evasion including:

Pembrolizumab Interruption Mechanisms: Extract specific mechanisms by which pembrolizumab interrupts the PD-1/PD-L1 immune evasion pathway including:

Immune Cell Changes: Extract detailed changes in immune cell populations and function during pembrolizumab treatment that demonstrate PD-1/PD-L1 pathway interruption including:

Biomarker Evidence: Extract biomarker data demonstrating PD-1/PD-L1 pathway activity and its interruption by pembrolizumab including:

Clinical Evidence: Extract clinical outcome data that serves as evidence of successful PD-1/PD-L1 pathway interruption by pembrolizumab including:

Predictive Factors: Extract factors predicting response or resistance to pembrolizumab treatment in the context of PD-1/PD-L1 pathway interruption including:

Safety Profile: Extract safety and toxicity data related to pembrolizumab treatment and PD-1/PD-L1 pathway blockade including:

Results

Characteristics of Included Studies

The review included 10 sources published between 2012 and 2020, comprising:

Study Characteristics

Study Full text retrieved? Study Type Cancer Types Sample Size Key Methods
A. Ribas et al., 2016 No Observational study Melanoma 102 tumor biopsies from 53 patients Multicolor flow cytometry with computational approaches
J. McDermott & A. Jimeno, 2015 No Review Melanoma, non-small cell lung cancer N/A Literature review of preclinical studies and early clinical trials
K. Mahoney et al., 2015 Yes Review of clinical trials Melanoma 10-418 patients across multiple trials Review of clinical trial data and abstracts
J. Gong et al., 2018 Yes Literature review Melanoma, NSCLC, urothelial carcinoma, HNSCC, classical Hodgkin lymphoma, MSI-H/dMMR solid tumors N/A MEDLINE and manual literature search
P. Tumeh et al., 2014 Yes Clinical trial and mechanistic study Metastatic melanoma 46 patients with metastatic melanoma; validation cohort of 16 patients Quantitative immunohistochemistry, multiplex immunofluorescence, next-generation TCR sequencing
J. Brahmer et al., 2012 Yes Phase 1 clinical trial (anti-PD-L1) Non-small-cell lung cancer, melanoma, colorectal cancer, renal-cell cancer, ovarian cancer, pancreatic cancer, gastric cancer, breast cancer 207 patients Intravenous anti-PD-L1 antibody administration, immunohistochemistry, flow cytometry
S. Topalian et al., 2012 Yes Phase 1 clinical trial (anti-PD-1) Advanced melanoma, non-small-cell lung cancer, castration-resistant prostate cancer, renal-cell cancer, colorectal cancer 296 patients Anti-PD-1 antibody administration, immunohistochemistry, flow cytometry
Z. Xu-Monette et al., 2017 No Review Hodgkin lymphoma, melanoma, advanced cancers N/A Review of functional and clinical studies
J. Miguel et al., 2015 No Phase 1 clinical trial Multiple myeloma (relapsed/refractory) 34 patients Modified 3+3 dose-escalation design, pembrolizumab with lenalidomide and dexamethasone
K. Hudson et al., 2020 Yes Review Melanoma, breast, gastric, ovarian, liver, kidney, pancreatic, bladder, NSCLC, urothelial, Merkel cell carcinomas N/A Review of oncology models and clinical data

Disease stages across studies were predominantly advanced or metastatic. Patient populations included heavily pretreated individuals, with 47% receiving at least three prior regimens in one study, and patients who had failed at least two prior therapies in another.

Mechanisms of PD-1/PD-L1 Pathway in Tumor Immune Evasion

Suppression of T Cell Function

The PD-1/PD-L1 pathway serves as a critical immune checkpoint that tumors exploit to evade host immunity. PD-1/PD-L1 interaction suppresses immune responses by reducing T-lymphocyte function and inhibiting cytokine production and cytolytic activity of PD-1-positive, tumor-infiltrating CD4+ and CD8+ T cells. This interaction dampens T cell function and proliferation, blocks effector functions, and reduces T cell killing capacity. At the molecular level, PD-1 signaling inhibits glucose metabolism in T lymphocytes and involves inhibitory signaling downstream of the T-cell receptor, including recruitment of SHP1/2 phosphatases that dephosphorylate signaling molecules necessary for T cell activation.

Adaptive Immune Resistance

A key mechanism of immune evasion involves adaptive immune resistance, where PD-L1 expression is upregulated in response to immune attack. Production of interferons by tumor-infiltrating CD8+ cells can induce PD-L1 expression, creating a negative feedback loop. This upregulation of PD-L1 in response to interferon-induced JAK-STAT signaling, along with expression of indoleamine 2,3-dioxygenase (IDO), represents an adaptive mechanism by which tumors respond to immune pressure. PD-1 expression is induced by T-cell activation and can lead to an “exhausted” T-cell phenotype if the immune response is unsuccessful, with some CD8+ T cells entering an irreversible dysfunctional state that cannot be rescued by PD-1/PD-L1 blockade.

Tumor Microenvironment Effects

Tumors create an immunosuppressive microenvironment through multiple mechanisms. PD-L1 is abnormally expressed by tumor cells and lymphocytes in the tumor microenvironment, and its overexpression enhances tumorigenesis and makes tumor cells less susceptible to T-cell mediated lysis. The tumor microenvironment undergoes changes including upregulation of genes involved in cell adhesion, extracellular matrix remodeling, mesenchymal transition, angiogenesis, and wound healing. Additionally, increased lactate production and hypoxia favor tumor growth and immune evasion. PD-L1 expression on both cancer cells and myeloid-derived cells contributes to the creation of a suppressive environment.

Mechanisms by Which Pembrolizumab Interrupts the PD-1/PD-L1 Pathway

Antibody-Mediated Blockade

Pembrolizumab is a highly selective, humanized IgG4 monoclonal antibody designed to block the interaction of PD-1 with its ligands PD-L1 and PD-L2. By preventing PD-1 from binding to PD-L1, pembrolizumab removes the suppressive effect on T-cell proliferation and immune response. This blockade prevents PD-L1 from inhibiting T cell function and promotes immune responses against tumors. The antibody achieves high receptor occupancy, with median PD-L1 receptor occupancy exceeding 65% at doses of 1-10 mg/kg on CD3+ peripheral-blood mononuclear cells.

Restoration of Immune Cell Function

Following PD-1 blockade, significant changes occur in immune cell populations and function. CD8+ T cells show increased proliferation within tumors, with responding patients demonstrating increased Ki67 positivity indicating active proliferation and elevated granzyme B expression indicating enhanced effector function. Memory CD8+ T cells were the most prominent phenotype that expanded intratumorally on therapy, representing the major T-cell subset expanded in patients with a response to therapy. The frequency of intratumoral B cells and monocytic myeloid-derived suppressor cells significantly increased in patients’ biopsies taken on treatment.

Treatment leads to the proliferation of intratumoral CD8+ T cells that are directly correlated with radiographic reduction in tumor size. There is significant expansion of T cell clones post-treatment in responders, with development of a more clonal TCR repertoire within the T cell population directed against the tumor. Expression of pSTAT1, a marker of interferon signaling, increases in responders, indicating enhanced immune activation.

Spatial and Temporal Dynamics

Pre-treatment samples from responding patients showed higher numbers of CD8-, PD-1-, and PD-L1-expressing cells at the invasive tumor margin and inside tumors, with close proximity between PD-1 and PD-L1 cells at the invasive tumor margin. These pre-existing CD8+ T cells distinctly located at the invasive tumor margin are associated with expression of the PD-1/PD-L1 immune inhibitory axis and predict response to therapy. Following treatment, increased CD8+ T cell density occurs at the invasive margin and inside tumors, demonstrating changes in immune cell trafficking and infiltration.

Clinical Evidence of Pathway Interruption

Response Rates Across Cancer Types

Clinical trials demonstrated substantial objective response rates across multiple cancer types, providing evidence of successful PD-1/PD-L1 pathway interruption. In melanoma, response rates ranged from 26-45.2%, with one study reporting a 28% objective response rate and another reporting a 41% overall response with 9% complete responses. For non-small-cell lung cancer, objective response rates were 18-21%. Renal-cell cancer showed a 27% response rate, while pembrolizumab in combination with lenalidomide and dexamethasone for relapsed/refractory multiple myeloma achieved a 76% objective response rate.

The anti-PD-L1 antibody trial reported objective responses in 9 of 52 patients with melanoma, 2 of 17 with renal-cell cancer, 5 of 49 with non-small-cell lung cancer, and 1 of 17 with ovarian cancer. Disease control rates showed prolonged stabilization in 12-41% of patients at 24 weeks.

Duration and Durability of Responses

Responses to PD-1/PD-L1 blockade demonstrated remarkable durability. In the anti-PD-1 trial, 20 of 31 responses lasted 1 year or more in patients with 1 year or more of follow-up. Similarly, the anti-PD-L1 study found responses lasted for 1 year or more in 8 of 16 patients with adequate follow-up. Median progression-free survival exceeded 7 months in one pembrolizumab melanoma study, with overall survival not reached at the time of reporting. In gastric cancer, the median duration of response was 8.1 months, while overall survival reached 25.9 months in one KEYNOTE-001 cohort.

Biomarker Correlations

PD-L1 expression emerged as a predictive biomarker, though with limitations. Higher PD-L1 expression levels were associated with better response rates to pembrolizumab. In one study, 36% of patients with PD-L1-positive tumors had an objective response, compared to none with PD-L1-negative tumors (P=0.006). Pembrolizumab showed improved response rates and progression-free survival in PD-L1-positive patients, with a 45.2% objective response rate in patients with PD-L1 ≥50%. However, PD-L1 expression proved inadequate as a definitive biomarker, as responses occurred in some PD-L1-negative tumors.

Beyond PD-L1 expression, baseline CD8+ T cell infiltration at the invasive tumor margin predicted response. Responding patients had higher numbers of CD8+, PD-1+, and PD-L1+ cells at the invasive margin and inside tumors, with close proximity between PD-1 and PD-L1 expressing cells. A more clonal TCR repertoire correlated with clinical response, and higher pSTAT1 expression was observed in responding patients both before and during treatment. High tumor mutational burden and POLE mutations predicted benefit from PD-1 blockade, with MSI-H or dMMR status associated with better responses.

Safety Profile

Immune-Related Adverse Events

PD-1/PD-L1 blockade demonstrated a more favorable safety profile compared to previous immunotherapies. Grade 3 or 4 drug-related adverse events occurred in 14% of anti-PD-1-treated patients, 9% of anti-PD-L1-treated patients, and 13% in one pembrolizumab melanoma study. In the multiple myeloma combination trial, 58% experienced grade 3/4 treatment-related adverse events, though no treatment discontinuation for toxicity occurred.

Common immune-related adverse events included pneumonitis (3%), with 1% experiencing Grade 3 or 4 severity. Other immune-related toxicities encompassed vitiligo, colitis, hepatitis, hypophysitis, and thyroiditis. In the anti-PD-L1 trial, 39% of patients experienced immune-related events such as rash, hypothyroidism, hepatitis, sarcoidosis, endophthalmitis, diabetes mellitus, and myasthenia gravis. The multiple myeloma study reported thrombocytopenia (47%), neutropenia (41%), fatigue (29%), anemia, hyperglycemia, and muscle spasms (23% each) as the most frequent treatment-related adverse events.

Three deaths from pulmonary toxicity occurred in the anti-PD-1 trial. Serious adverse events related to treatment occurred in 5% of anti-PD-L1-treated patients. Treatment discontinuation rates due to adverse events were 5% in the anti-PD-1 trial and 11% (6% treatment-related) in the anti-PD-L1 study.

Management Strategies

Management of immune-related toxicities involved glucocorticoids and replacement therapy. Events such as hypothyroidism and adrenal insufficiency were managed with replacement therapy, while hepatic or gastrointestinal events required treatment interruption and glucocorticoids. Education regarding recognition and management of immune-related effects proved essential for maximizing clinical benefit.

Synthesis

The body of evidence demonstrates a coherent mechanistic and clinical narrative for how pembrolizumab interrupts the PD-1/PD-L1 immune evasion pathway. The mechanistic studies provide direct cellular and molecular evidence linking antibody-mediated pathway blockade to restoration of anti-tumor immunity, while clinical trials confirm these mechanisms translate into durable clinical responses.

The apparent variability in response rates across studies (18-76%) reflects genuine biological heterogeneity rather than conflicting findings. Response rates correlate with specific tumor characteristics: Hodgkin lymphoma and melanoma show higher responses due to high PD-L1/L2 expression and high tumor mutational burden respectively, while most advanced cancers show approximately 20% response rates. The 76% response rate in multiple myeloma occurred with combination therapy (pembrolizumab plus lenalidomide and dexamethasone), not monotherapy, explaining the higher efficacy. Similarly, combination pembrolizumab with chemotherapy achieved a 55% response rate in NSCLC, demonstrating that combination approaches enhance efficacy beyond single-agent blockade.

PD-L1 expression serves as an imperfect but useful biomarker. While PD-L1-positive tumors show higher response rates (36-45.2%), responses occur in PD-L1-negative tumors, suggesting PD-L1 expression identifies a responsive subset but does not fully explain all responses. This paradox resolves when considering that pre-existing CD8+ T cells at the invasive margin—regardless of PD-L1 status—predict response, indicating that both adaptive immune resistance (PD-L1 upregulation) and pre-existing T cell infiltration contribute to responsiveness. The close proximity between PD-1 and PD-L1 expressing cells creates a microenvironment where blockade can immediately restore T cell function, explaining why spatial distribution matters as much as expression levels.

The durability of responses (>1 year in most responders) contrasts with the CD8+ T cell exhaustion described in the mechanistic studies, suggesting pathway blockade catches tumors before irreversible T cell dysfunction occurs. The observation that some CD8+ T cells enter an irreversible dysfunctional state explains primary resistance in non-responders, while the expansion of memory CD8+ T cells in responders provides a mechanism for durable immunity. The more clonal TCR repertoire in responders indicates that effective therapy amplifies pre-existing tumor-specific T cell clones rather than generating entirely new responses.