Scientists reveal how cancer-causing p53 mutations evade immune detection

The Immune System Needs Visible Targets

T cells do not read tumor DNA directly. Instead, they inspect small protein fragments, called peptides, displayed on the cell surface by HLA molecules. The full set of these exposed fragments is known as the immunopeptidome, acting as a molecular shop window of the cell. If a cancer-specific fragment never reaches this window or disappears too quickly, even a highly capable T cell may have nothing durable to attack.

p53 Mutations Are Often Invisible

Mutations in the TP53 gene, which encodes p53, occur in about half of all human cancers. Because many TP53 mutations happen early in tumor development, they can be inherited by nearly every cancer cell as the tumor grows. These trunk mutations were traditionally considered highly attractive targets for immunotherapy. In principle, targeting a single such mutation could attack the entire tumor, not just a branch.

Mass Spectrometry Reveals Low Visibility

Using an ultrasensitive mass spectrometry platform that physically measures peptides actually displayed on tumor cells, researchers found that p53 is far less visible to T cells than DNA sequencing or simple computer predictions would suggest. For example, among 175 wild-type p53 peptide candidates tested on relevant HLA molecules, only five were robustly detected. Many common cancer-inducing hotspot p53 mutations were located in poorly processed regions, failing to generate detectable surface targets.

In some patients, tumors containing potentially strong p53 targets lacked the necessary HLA molecules to display them. In the case of the p53 I195F mutation, high activity in cancer cells of an enzyme called ERAP1 acted as an overly zealous molecular shredder, destroying an otherwise potent immunogenic p53 fragment before it could be presented to T cells. As a counterstrategy, deleting ERAP1 or blocking it with an inhibitor restored recognition of these cancer cells by p53-specific T cells in laboratory experiments.

Additional Escape Routes Identified

Another vulnerability emerged with the common p53 R175H mutation, already being tracked in clinical therapies based on engineered T cells. Researchers identified extremely sensitive T cell receptors capable of responding to exceedingly rare amounts of this target. However, the mutant p53 fragment bound so weakly to its HLA molecule that the resulting complex was unstable and short-lived. The immune receptor was capable, but the target itself was poor. Consequently, T cell activation and killing of cancer cells were much weaker than with a more stable p53 target.

Weak Binding Limits Immune Response

Ellis L. Reinherz, MD, chief of the Dana-Farber Laboratory of Immunotherapy and professor of medicine at Harvard Medical School, co-supervisor of the study, said: „Immunotherapy cannot attack what the immune system cannot see.”David A. Barbie, MD, director of the Lowe Center for Thoracic Oncology at Dana-Farber and professor of medicine at Harvard Medical School, also co-supervised the study. The lead author, Koji Haratani, MD, PhD, a postdoctoral researcher, was a major contributor, alongside scientists Bruce Reinhold, PhD, and Jonathan S. Duke-Cohan, PhD, both from Dana-Farber.

Quality of Target Matters More Than Immune Power

A tumor may carry an ideal mutation in every cancer cell, but if that mutation is not processed into a stable surface target, creating a stronger T cell is not enough. This conclusion underscores the need to focus not only on the power of immune cells but also on the quality of the targets they can see.