# 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.