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

• n = 200

• Papers screened using: PD-1/PD-L1 Pathway Focus, Clinical Relevance, Study Design, Relevant Outcomes, Pembrolizumab or PD-1/PD-L1 Inclusion, Beyond Pharmacokinetics, Adequate Sample Size, Human System Validation, Publication Type

• n = 200

• Papers screened out

• n = 190

• Papers included for extraction

• n = 10

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. Four sources were available only as abstracts.

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.

Treatment leads to proliferation of intratumoral CD8+ T cells that directly correlates 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 28% objective response rate and another reporting 41% overall response with 9% complete responses. For non-small-cell lung cancer, objective response rates were 18-21%. Renal-cell cancer showed 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 patients across various cancer types. 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, responses lasted 1 year or more in 20 of 31 patients. Similarly, the anti-PD-L1 study found that responses lasted for 1 year or more in 8 of 16 patients. Median progression-free survival exceeded 7 months in one pembrolizumab melanoma study.

Biomarker Correlations

Higher PD-L1 expression levels were associated with better response rates to pembrolizumab. Although PD-L1 emerged as a predictive biomarker, responses occurred in some PD-L1-negative tumors as well, indicating a more complex interaction between immune responsiveness and PD-L1 expression.

Safety Profile

Immune-Related Adverse Events

PD-1/PD-L1 blockade demonstrated a more favorable safety profile compared to previous immunotherapies. Common immune-related adverse events included pneumonitis and thyroiditis, with grade 3 or 4 drug-related adverse events occurring in a minority of patients, often related to the specific context of treatment.

Management Strategies

Management of immune-related toxicities involved glucocorticoids and replacement therapy, with education regarding recognition and management of these effects proving essential for maximizing clinical benefit.