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.
Paper search
We performed a semantic search across over 138 million academic papers from the Elicit search engine, which includes all of Semantic Scholar and OpenAlex.
We ran this query: "How does the PD-1/PD-L1 pathway contribute to tumor immune evasion, and how does pembrolizumab interrupt it?"
The search returned 200 total results from Elicit.
Screening
We screened in sources based on their abstracts that met certain criteria:
- PD-1/PD-L1 Pathway Focus: Does the study investigate PD-1/PD-L1 pathway mechanisms in tumor immune evasion OR examine pembrolizumab’s mechanism of action on the PD-1/PD-L1 pathway?
- Clinical Relevance: Does the study involve human subjects, human tissue samples, or clinically relevant animal models?
- Study Design: Is the study a randomized controlled trial, observational study, mechanistic study, systematic review, or meta-analysis?
- Relevant Outcomes: Does the study report on immune response biomarkers, tumor microenvironment changes, or clinical outcomes related to PD-1/PD-L1 modulation?
- Pembrolizumab or PD-1/PD-L1 Inclusion: If the study focuses on immune checkpoint inhibitors, does it include pembrolizumab or analyze the PD-1/PD-L1 pathway?
- Beyond Pharmacokinetics: Does the study examine mechanistic or clinical outcomes beyond only pharmacokinetics or drug metabolism?
- Adequate Sample Size: If the study is a case report or case series, does it include 10 or more patients?
- Human System Validation: If the study uses in vitro methods with non-human cell lines, does it include validation in human systems?
- Publication Type: Is the study a full peer-reviewed publication (not a conference abstract, editorial, or opinion piece)?
Results
Characteristics of Included Studies
The review included 10 sources published between 2012 and 2020, comprising 5 primary clinical studies, 4 review articles, and 1 literature review.
| 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 |
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.
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.
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.
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.
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%.
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.
Management Strategies
Management of immune-related toxicities involved glucocorticoids and replacement therapy.
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.