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, and blocks effector functions, creating an immunosuppressive tumor microenvironment. 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, restoring T cell function and leading to increased proliferation of intratumoral CD8+ T cells that correlates with tumor size reduction, enhanced effector function, 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. Higher PD-L1 expression and increased infiltration of CD8+ T cells at the invasive tumor margin predicts response, while 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.
- PD-1/PD-L1 Pathway Focus: Investigates PD-1/PD-L1 mechanisms in tumor immune evasion or examines pembrolizumab’s mechanism of action.
- Clinical Relevance: Involves human subjects, tissue samples, or clinically relevant animal models.
- Study Design: Randomized controlled trials, observational studies, mechanistic studies, systematic reviews, or meta-analyses.
- Relevant Outcomes: Reports on immune response biomarkers, tumor microenvironment changes, or clinical outcomes.
- Inclusion of Pembrolizumab or PD-1/PD-L1: Focus on pembrolizumab or analyze the PD-1/PD-L1 pathway.
- Beyond Pharmacokinetics: Examines mechanistic or clinical outcomes beyond pharmacokinetics.
- Adequate Sample Size: Case reports or series must include 10 or more patients.
- Human System Validation: Includes validation in human systems if using in vitro methods.
- Publication Type: Full peer-reviewed publications.
Results
Characteristics of Included Studies
- Study Types: The review included 10 sources: 5 primary clinical studies, 4 review articles, and 1 literature review. Four sources were available only as abstracts.
| 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 |
| J. McDermott & A. Jimeno, 2015 | No | Review | Melanoma, NSCLC | N/A | Literature review |
| K. Mahoney et al., 2015 | Yes | Review of clinical trials | Melanoma | 10-418 patients across multiple trials | Review of trial data |
| J. Gong et al., 2018 | Yes | Literature review | Multiple tumors | N/A | MEDLINE and manual search |
| P. Tumeh et al., 2014 | Yes | Clinical trial and mechanistic study | Metastatic melanoma | 46 patients | Immunohistochemistry, TCR sequencing |
| J. Brahmer et al., 2012 | Yes | Phase 1 clinical trial | Multiple cancers | 207 patients | Anti-PD-L1 antibody administration |
| S. Topalian et al., 2012 | Yes | Phase 1 clinical trial | Multiple cancers | 296 patients | Anti-PD-1 administration |
| Z. Xu-Monette et al., 2017 | No | Review | Multiple cancers | N/A | Review of studies |
| J. Miguel et al., 2015 | No | Phase 1 clinical trial | MM (relapsed/refractory) | 34 patients | Dose-escalation design |
| K. Hudson et al., 2020 | Yes | Review | Multiple cancers | N/A | Review of 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. PD-1/PD-L1 interaction suppresses T cell function by inhibiting cytokine production and cytolytic activity, leading to reduced T cell proliferation and activation. PD-1 signaling inhibits glucose metabolism in T lymphocytes.
Adaptive Immune Resistance
PD-L1 expression can be upregulated in response to immune attack via interferons from tumor-infiltrating CD8+ T cells, creating a negative feedback loop. PD-1 expression can lead to an exhausted T-cell phenotype if the immune response is unsuccessful, leading to irreversible dysfunction.
Tumor Microenvironment Effects
Tumors create an immunosuppressive microenvironment through PD-L1 overexpression, which enhances tumorigenesis and reduces susceptibility to T cell lysis. This includes upregulation of genes involved in cell adhesion, extracellular matrix remodeling, and angiogenesis, with increased lactate production favoring tumor growth.
Mechanisms by Which Pembrolizumab Interrupts the PD-1/PD-L1 Pathway
Antibody-Mediated Blockade
Pembrolizumab blocks PD-1 interaction with PD-L1 and PD-L2, removing suppression on T-cell proliferation, enabling immune responses against tumors.
Restoration of Immune Cell Function
Post-PD-1 blockade, CD8+ T cells demonstrate increased proliferation in tumors. Responding patients exhibit increased granzyme B expression and memory CD8+ T cells expansion.
Spatial and Temporal Dynamics
Pre-treatment samples from responders showed higher numbers of CD8+ T and PD-L1 expressing cells at the tumor’s invasive margin, which are predictors of response to therapy.
Clinical Evidence of Pathway Interruption
Response Rates Across Cancer Types
Clinical trials showed substantial objective response rates across various cancers, e.g. 26-45% in melanoma and 18-21% in non-small-cell lung cancer.
Duration and Durability of Responses
PD-1/PD-L1 blockade demonstrated durability, with responses lasting over a year in many patients.
Biomarker Correlations
Higher PD-L1 expression levels were linked to better responses. In one study, 36% of patients with PD-L1-positive tumors responded.
Safety Profile
Immune-Related Adverse Events
PD-1/PD-L1 blockade displayed favorable safety profiles compared to previous immunotherapies, with notable side effects like pneumonitis in 3% of patients.
Management Strategies
Management involved glucocorticoids and other therapies for immune-related toxicities.
Synthesis
Evidence demonstrates a coherent narrative supporting pembrolizumab's role in interrupting the PD-1/PD-L1 immune evasion pathway, translating into durable clinical responses. Variability in response rates across studies reflects genuine biological heterogeneity, and factors like PD-L1 expression and presence of pre-existing CD8+ T cells are critical in predicting responses.