Immunotherapy Biomarkers: Tests That Help Guide Cancer Treatment

Immunotherapy biomarkers are measurable features within tumors that can offer crucial insights for oncologists when considering immune checkpoint inhibitor therapies. Among the most frequently assessed are PD-L1 expression, MSI/MMR status, and tumor mutational burden (TMB). However, it’s important to remember that none of these biomarkers, whether used alone or in combination, guarantee a reliable prediction of who will respond to immunotherapy.

Biomarker assessment is instrumental in identifying patients who are better suited for certain immune checkpoint inhibitors. For example, PD-L1 testing often precedes the administration of particular PD-1 inhibitors, while evaluating MSI-H/dMMR status is key in certain types of cancers. Even so, biomarker presence does not guarantee efficacy—patients with favorable markers sometimes fail to respond, while some cancers with seemingly unfavorable profiles can still benefit, depending on tumor type, therapy combinations, and individual circumstances.

PD-L1

PD-L1 is a protein that may be present on the surface of tumor or immune cells. It binds to PD-1 receptors on T cells, acting as a shield that allows tumor cells to escape immune destruction. PD-L1 levels are typically measured using immunohistochemistry (IHC) and results may be reported as tumor proportion scores (TPS), combined positive scores (CPS), or by assessing immune cell percentages, based on both the tissue type and the specific lab assay.

MSI and MMR Status

Microsatellite instability (MSI) and mismatch repair (MMR) status are interrelated markers reflecting how well a tumor corrects mistakes during DNA replication. MSI-H (high microsatellite instability) and dMMR (deficient MMR) tumors generally carry large numbers of mutations, making them potentially more visible to immune surveillance. Testing their status is clinically relevant across several cancers but is especially significant in colorectal cancers, where MSI-H/dMMR findings can heavily influence immunotherapy eligibility.

Tumor Mutational Burden (TMB)

Tumor mutational burden quantifies the total number of somatic mutations in a defined portion of the cancer genome, typically reported as mutations per megabase. High TMB can result in more tumor-specific (neoantigen) proteins, which might make the tumor more recognizable to the immune system. TMB is usually determined through sequencing-based assays, particularly next-generation sequencing (NGS), but its role varies according to cancer type, therapy used, and there are no universal thresholds that apply across all contexts.

PD-L1 testing involves IHC staining of tumor samples to determine the presence and quantity of PD-L1 protein. This information can guide the use of specific PD-1 or PD-L1 blocking agents. Importantly, PD-L1 scoring systems and positivity thresholds are often unique for different cancers (e.g., lung, head and neck, gastric, or triple-negative breast cancers). A positive or high PD-L1 outcome may support using certain immunotherapies, but it does not guarantee a favorable clinical response—and a negative result does not always preclude potential benefit. Interpretation always factors in the full breadth of clinical evidence.

MSI-H and dMMR are important immunotherapy biomarkers for select cancers, particularly because they signal defects in DNA repair, leading to a high burden of mutations and increased immune detection potential. MSI testing investigates instability in repeated DNA sequences, while MMR IHC focuses on the presence or absence of repair proteins like MLH1, MSH2, MSH6, and PMS2. Although they both examine DNA repair, the two tests measure different aspects of the pathway.

As an immunotherapy biomarker, TMB reflects the count of acquired (non-inherited) mutations in tumor DNA. High TMB may suggest increased likelihood of immunogenicity. TMB is often viewed alongside PD-L1 and MSI/MMR findings, as well as clinical features such as kind of cancer, staging, and treatment history. Still, its predictive power tends to be strongest in specific settings and isn’t a standalone metric for all cancers.

Methods depend on the biomarker being evaluated. PD-L1 is most often tested via IHC on tumor tissue, while MSI/MMR and TMB generally require molecular pathology techniques or NGS sequencing. Most testing relies on biopsied or surgical tumor samples, as these offer the most direct reflection of the tumor’s biology, though occasionally, blood-based (liquid biopsy) methods may be used when tissue is unavailable.