Medicine
Tumor Mutational Burden and Response to Immune Checkpoint Inhibitors
Quick fact
Tumors with more than 10 mutations per megabase of DNA are often considered 'TMB-high' and are more likely to respond to checkpoint inhibitors, regardless of cancer type.
Why this is interesting
You’ve probably heard that immunotherapy can cure some cancers, but why does it fail in others? The answer may lie in the tumor’s mutation count.
Read the full explanation
Understanding Tumor Mutational Burden and Response to Immune Checkpoint Inhibitors
Every cell in your body constantly acquires small mutations when it divides. Most are harmless, but in cancer, they accumulate rapidly. The total number of mutations in a tumor’s DNA is called the tumor mutational burden (TMB). Think of it like a spelling mistake count in a long book: the more typos, the more unusual the story. In cancer, these mutations can produce abnormal proteins called neoantigens. When a cell displays these neoantigens on its surface, they act like red flags for the immune system. However, tumors often exploit 'checkpoint' pathways, like the PD-1/PD-L1 interaction, to turn off these immune attacks. Immune checkpoint inhibitors (ICIs) are drugs that block these brakes, allowing T cells to attack the tumor. If a tumor has many mutations, it’s more likely to display many neoantigens, making it a better target for the unleashed immune response. This is why TMB is measured through genomic sequencing of a tumor biopsy and is used as a predictive biomarker.
A deeper explanation
The mechanism linking TMB to immunotherapy response lies in the relationship between mutation load and neoantigen presentation. High TMB means more mutations, which statistically increases the number of neoantigens that can be presented by MHC molecules on the tumor cell surface. When ICIs block the PD-1/PD-L1 axis, they remove the inhibitory signal that prevents T cells from killing the tumor. With many neoantigens, the T cell receptor repertoire is more likely to recognize the tumor as foreign, triggering a strong anti-tumor response. This is why TMB-high tumors, such as melanoma, lung cancer, and those with microsatellite instability, often show impressive clinical responses to ICIs. Conversely, tumors with low TMB have fewer neoantigens, so the immune system may not recognize them even when checkpoints are blocked. TMB is not a perfect predictor—some low-TMB tumors respond and some high-TMB tumors don’t—because other factors (like the immune microenvironment and the type of mutations) also matter. Nevertheless, measuring TMB helps oncologists match patients to the most effective therapies, a cornerstone of precision medicine.