Perforin Kills Cancer Cells Alone, Without Granzymes, Study Finds

For decades, immunologists have taught a tidy division of labor inside the immune system’s most lethal cells. T cells and natural killer (NK) cells, the soldiers of anti-tumor defense, carry a weapon called perforin, which punches holes in the membranes of target cells. Through those holes, the soldiers deliver their ammunition: a family of proteases known as granzymes, which enter the tumor cell and trigger its death. New work led by researchers at the University of Michigan Health Rogel Cancer Center, published in the journal Cell, upends that tidy picture. The study found that tumor cells can be killed even when the major granzymes are missing entirely, because perforin itself is enough to deliver a fatal blow.

The research team, led by co-senior authors Weiping Zou, MD, PhD, and Ilona Kryczek, PhD, of the University of Michigan, together with co-senior author Judy Lieberman, PhD, of Harvard Medical School, one of the international pioneers of cytotoxic lymphocyte research, set out to answer a question that had lingered beneath the classic model. Could perforin, stripped of its enzymatic partners, kill cancer cells on its own? And if so, how? The answer they report is a resounding yes, and the mechanism they uncovered is one of the most surprising in modern cell biology: ferroptosis, an iron-dependent form of cell death that was formally introduced only a little over a decade ago.

“If T cells and NK cells are the soldiers of the immune system, perforin is the weapon, and granzymes are the ammunition,” Zou said, describing the long-standing understanding his team has now revised. “At least, that’s what the classic understanding was.” The new study shows that the weapon can do damage without the ammunition. “Our findings show, for the first time, that perforin can independently induce ferroptosis in tumor cells,” said Kryczek. “This work changes our understanding of one of the longest held mechanisms of T cell and NK cell mediated tumor clearance.”

To appreciate why this is such a significant shift, it helps to understand the different ways a cell can die. Apoptosis, the pathway classically attributed to granzymes, is an orderly dismantling of the cell. Pyroptosis is a more inflammatory form of death, often triggered by bacterial infection. Ferroptosis, the newcomer among these, is an iron-dependent process driven by the accumulation of lipid peroxides in cellular and organelle membranes. Because ferroptosis is a relatively recent concept, scientists know less about it than about the more established forms of cell death, and it remains, as Zou put it, “a very hot area that needs a lot of research.” His own group was the first to report, in 2019, the immune system’s role in ferroptosis in tumors, highlighting the pathway’s potential as a target for immune checkpoint therapy.

The new study took a three-pronged approach to test whether perforin could act alone. The researchers paired in vitro experiments involving genetic and biochemical manipulations with in vivo mouse studies and with data from human cancer patients. Each line of evidence pointed in the same direction. In the laboratory experiments, perforin without granzymes was still deadly to cancer cells, and ferroptosis was the mechanism of death. In mice, an iron-rich diet slowed tumor progression in a manner related to the animals’ perforin expression. And in human patients, higher perforin expression was associated with improved survival and better responses to immune checkpoint blockade treatments.

“We wanted to know if perforin, by itself, can do the job,” Zou said. “The answer is yes.” The mechanistic details that emerged from the work are as striking as the headline result. Perforin, it turns out, is capable of more than simply puncturing the target cell membrane. The protein also interferes with the target cell’s iron uptake in a way that causes iron to accumulate problematically inside the cell, including inside the mitochondria. There, the excess iron rapidly oxidizes lipids. The cell’s whole system goes haywire, and the tumor cell dies. The study’s imaging work showed that perforin increases endosomal-mitochondrial contact sites in tumor cells, a detail that offers a visual signature of the cellular disruption the protein sets in motion.

“It is a new way of thinking about how immune cells kill tumors,” said Hannah Bell, PhD, the study’s first author and a recent postdoctoral researcher in Zou’s lab who is now an assistant professor in Molecular and Integrative Physiology at the University of Michigan. The discovery that a single pore-forming protein can simultaneously breach the membrane and hijack the target cell’s iron metabolism suggests that the cytotoxic lymphocyte arsenal is more versatile than the weapon-and-ammunition metaphor implied. It also means that the loss of a single molecule can be catastrophic: the researchers found that a loss of perforin causes a catastrophic failure of anti-tumor immunity, underscoring how central this protein is to the immune system’s ability to keep tumors in check.

The findings are likely to have ripple effects throughout immunology, because they revise one of the field’s longest-held mechanisms of tumor clearance. They may also open new therapeutic avenues. Previous work has found that immune checkpoint blockade can enhance T cell-mediated ferroptosis, and the new results suggest that understanding what makes a tumor cell susceptible to perforin could be a way to increase the effectiveness of immunotherapy. “If we can understand what makes a tumor cell susceptible to perforin, we may be able to increase the effectiveness of immunotherapy,” Zou said. The team’s future research will delve deeper into the mechanism of cell death, test whether multiple cell death pathways could be working together, and make inroads toward clinical applications.

Despite the tantalizing link between iron, perforin and tumor control, the researchers are emphatic that the study is not a dietary prescription. The takeaway is not to put all cancer patients on an iron-rich diet. “We need to be very cautious here,” Zou said. “That’s because some cancer types enjoy an iron-rich diet. For those tumors, and we don’t totally know which ones yet, an iron-rich diet may accelerate progression.” In the mouse experiments, the iron-rich diet slowed tumor progression in a perforin-related manner, but the team has not yet identified which human tumor types would respond favorably and which might be nourished by extra iron. Until that picture is clearer, the dietary implication remains firmly in the realm of research rather than recommendation.

It is mostly too soon to say what the clinical applications of the finding will be, the researchers caution, but the conceptual payoff is already substantial. “At the end of the day, the whole goal of different forms of cancer therapies — immunotherapy, chemotherapy, targeted therapy — is to kill tumor cells,” Zou said. “Improving our understanding of cell death, like we do in this study, is so fundamental and so important. That’s why we’re going to continue to push on this.” The study, titled “Perforin triggers iron-induced mitochondrial damage to drive ferroptosis and anti-tumor immunity,” was published in Cell on 23 October 2026, and was supported by funding from the National Cancer Institute and the National Science Foundation. Additional authors include Kavya Nair, Kole Tison, Peng Liao, Hongwei Lei, Marwa O. El-Derany, Ayijiang Yisimayi, Haiwei Zhang, Ryan Rebernick, Weichao Wang, Yoshitaka Adachi, Cristina Castillo, Nupur K. Das, Jinsu An, Vivien R. Sutton, Xun Lin, Heng Lin, Xinghao Wang, Yunfei Li, Linda Vatan, Amr Aboud, Shuang Wei, Miguel Gijon, Samantha Goodwin, Yatrik M. Shah, Joseph A. Trapani and Margarida Barroso.

Subject of Research: The ability of the immune protein perforin to independently kill tumor cells by triggering iron-dependent ferroptosis without granzymes

Article Title: Immune system protein is a cancer killer on its own, new study shows

Article References: Immune system protein is a cancer killer on its own, new study shows. Original publication (Bibliographic details unavailable.)

Image Credits: AI Generated

DOI: Not provided

Keywords: perforin, granzymes, ferroptosis, T cells, natural killer cells, tumor immunology, iron metabolism, mitochondria, immune checkpoint blockade, cell death, cancer immunotherapy, University of Michigan