Cambridge scientists found tumours release antioxidants that block immune cells from destroying cancer, and blocking that trick helps immunotherapy work.
Antioxidant supplements have long been sold on the promise that they mop up cell-damaging molecules and lower cancer risk. New research from the University of Cambridge and Oregon Health & Science University suggests the story is more complicated, and in one specific way, backwards.
The molecules in question are reactive oxygen species, sometimes called free radicals. They’ve traditionally been viewed purely as damaging byproducts of metabolism. But scientists have discovered that immune cells called T cells actually depend on small amounts of these molecules to switch on their cancer-killing ability. Tumours, it turns out, have learned to exploit that dependency.
T Cells Need the Very Molecules Antioxidants Remove
The immune system hunts down harmful material, including cancer cells, using several types of specialised cells. T cells are one of the most important: they seek out and destroy tumour cells directly. But T cells need small amounts of reactive oxygen species to activate before they can do their job.
A team from Cambridge and Oregon Health & Science University analysed the fluid surrounding cells within tumours grown in mice. They found that cancers chemically “smother” T cells, stopping them from activating and defending against the tumour.
Tumours do this by releasing large amounts of a natural antioxidant protein called Peroxiredoxin 1, or PRDX1, which soaks up reactive oxygen species and starves T cells of the signal they need to fight back.
Removing the Antioxidant Let the Immune System Fight Back
To test whether PRDX1 actually causes the problem, the team used CRISPR gene-editing technology to create mouse cancer cells that could no longer produce the antioxidant. Cancer cells without that ability triggered more immune activity, and tumour growth slowed.
The researchers then checked whether the same mechanism shows up in people. They examined published data on proteins released by human cancer cell lines, studied gene activity across thousands of human tumours, and analysed fluid taken from tumours removed from patients. All three approaches pointed the same way: human cancers also release PRDX1, stripping away the reactive oxygen species T cells rely on.
Dr Xan Wesolowski from Cambridge’s Department of Pathology, one of the study’s lead authors, said: “We tend to think of reactive oxygen species purely as damaging byproducts of metabolism. But we are increasingly understanding they have important functions within cells, and T cells need them to activate. Our study develops this picture, revealing that tumours can exploit this very dependency to evade elimination.”
A Possible Boost for Immunotherapy
Cancer treatment has increasingly turned toward harnessing the immune system to fight the disease, but immunotherapies only work for some patients. In the study, removing cancer cells’ ability to produce PRDX1 made tumours respond better to immunotherapy.
Professor Rahul Roychoudhuri, also from Cambridge’s Department of Pathology, said: “We’ve found a new way by which cancers shut down the immune system. When we block this process, tumours that don’t respond to some immunotherapies start responding to treatment. That tells us this is a pathway worth targeting therapeutically, and there are several ways we might be able to achieve this.”
Dr Robert Eil from Oregon Health & Science University’s School of Medicine suggested a specific route forward: “Our study suggests a possible way to boost T cell immunotherapies. Rather than using antioxidants, it could be that prooxidants can kickstart the T cells into action against tumour cells.”
Why Antioxidant Supplements May Not Help, and Could Even Backfire
The findings may also help explain a puzzling pattern seen in past research. Several large randomised clinical trials have found that antioxidant supplements fail to reduce cancer risk, and in some cases actually worsen outcomes. If T cells depend on reactive oxygen species to fight tumours, antioxidants could be inadvertently blunting the immune system’s own attack on cancer.
The researchers were careful to stress that these findings come from preclinical laboratory models, and that people living with cancer should not change their treatment or diet based on this study without speaking to their doctor first.
The research was largely funded by the Medical Research Council, the European Research Council, Wellcome, and the National Institutes of Health/National Cancer Institute, and was published on September 3, 2026, in the journal Science.