An immune army already in place, ready to be mobilised
Within the living tissue of melanoma tumors, a quiet cellular drama has long gone unwitnessed — until now. Researchers at the Garvan Institute of Medical Research have captured, for the first time on film, immune cells called macrophages actively consuming live cancer cells, revealing a defense mechanism the body has been running in silence. This discovery, confirmed in both mouse models and human tissue, challenges the T-cell-centric story of cancer immunity and opens a new chapter in the long human effort to turn the body's own forces against malignancy.
- Current immunotherapy fails roughly half of all melanoma patients because 'cold tumors' physically wall out the T cells that checkpoint drugs depend on.
- A specific subset of macrophages — marked by the protein CD169 — has been filmed in real time patrolling tumor edges and engulfing live cancer cells entirely on their own.
- When scientists selectively removed these CD169-positive macrophages, tumors grew larger, proving this overlooked population was actively suppressing cancer growth.
- Macrophages don't just consume threats — they display molecular fragments of what they've eaten, potentially acting as alarm signals that could summon T cells into otherwise hostile tumor environments.
- Future therapies may target macrophage numbers or their cancer-tagging ability, with implications stretching beyond melanoma to most solid tumors where macrophages are abundantly present.
For the first time, researchers at the Garvan Institute of Medical Research have filmed immune cells called macrophages actively engulfing live melanoma cancer cells — a discovery published in the Journal of Experimental Medicine that could fundamentally change how cancer is treated. Using intravital two-photon microscopy, a technique capable of observing cellular activity inside living organisms, Dr. Yuki Keith and her team recorded these encounters in mice. Human tissue samples examined by collaborators at Melanoma Institute Australia confirmed the same macrophages appear around the edges of human melanoma tumors, anchoring the finding in clinical reality.
Macrophages constitute roughly 30 percent of cells within a melanoma tumor, yet their role had remained contested. Previous research eliminated all macrophages at once — too blunt an approach to reveal the truth. Keith's team instead identified a specific subpopulation expressing a protein called CD169. Removing only these cells caused tumors to grow larger, demonstrating that this subset was actively holding cancer in check — and doing so independently of T cells and B cells, the immune actors most celebrated in oncology.
The discovery carries particular weight for patients with so-called cold tumors, malignancies that exclude T cells and therefore don't respond to immune checkpoint therapies — currently effective in only about half of melanoma cases. CD169-positive macrophages, which also display fragments of consumed cancer cells as biological warning signals, may hold the ability to tag tumors and recruit T cells into these resistant environments. Senior author Professor Tri Phan notes that macrophages are abundant across most solid tumors, suggesting the implications of this work extend well beyond melanoma. Researchers now aim to decode exactly how macrophages communicate with T cells — a mechanism that, once understood, could be amplified through new drugs and combined with existing therapies to reach patients currently left behind.
Researchers at the Garvan Institute of Medical Research have watched, for the first time, immune cells called macrophages actively consuming live melanoma cancer cells—a moment captured on film that could reshape how doctors treat one of Australia's most lethal and prevalent cancers.
The work, published in the Journal of Experimental Medicine, documents a population of immune cells that had been largely overlooked: macrophages stationed at the perimeter of melanoma tumors, methodically engulfing cancer cells and restraining tumor expansion. Dr. Yuki Keith, who led the research, describes the significance plainly: "This is the first time anyone has captured a macrophage attacking and engulfing a live cancer cell in real time." The team used intravital two-photon microscopy, an imaging technique sophisticated enough to observe cellular-level activity inside living organisms, to record these encounters in mice. When collaborators at Melanoma Institute Australia examined human tissue samples, they found the same macrophages present in healthy skin and concentrated around the edges of human melanoma tumors, confirming the clinical relevance of what the microscope had revealed.
Macrophages make up roughly 30 percent of the cells within a melanoma tumor, yet their role in cancer progression has remained murky. Scientists knew they were present but disagreed about whether they helped or hindered the disease. Previous attempts to understand their function involved eliminating all macrophages from the body—a blunt approach that obscured the real story. Keith's team discovered that not all macrophages are identical. They identified a specific subpopulation that produces a protein called CD169. When they selectively removed these CD169-positive macrophages, tumors grew larger, revealing that this particular subset was actually holding cancer in check.
What makes this discovery genuinely unexpected is that the macrophages appear to attack cancer cells independently, without the involvement of T cells and B cells—the immune soldiers most celebrated in cancer research. Professor Tri Phan, the senior author, explains the macrophage's dual nature: "Macrophages have always been known as the body's housekeepers—they clear away dead cells and debris. What Dr Keith caught on camera was these cells actively nibbling away and engulfing live cancer cells, constraining tumor growth." Beyond simply consuming cancer, macrophages also function as immune messengers. After they devour a threat, they display fragments of it on their surface like a biological warning signal, potentially alerting T cells to the presence of cancer.
The implications for cancer treatment are substantial. Immune checkpoint blockade therapy, which mobilizes T cells to hunt down and destroy cancer, has revolutionized melanoma care for advanced cases—but only about half of patients respond. A major obstacle is the "cold tumor," a malignancy that actively excludes T cells from its interior, rendering conventional immunotherapy ineffective. If CD169-positive macrophages can be harnessed to tag cancer cells and summon T cells into these hostile environments, an entirely new therapeutic avenue opens. Dr. Keith suggests the macrophages may hold "the key to calling the T cell cavalry into the tumor to finish the job."
The next phase of research focuses on understanding precisely how these macrophages communicate with T cells. If that mechanism can be decoded, future treatments might involve drugs designed to increase macrophage numbers, enhance their appetite for cancer cells, or improve their ability to mark tumors for destruction. Combining such approaches with existing therapies could extend immunotherapy's reach to patients who currently derive no benefit. Professor Phan notes that this strategy likely extends beyond melanoma: "Macrophages are highly abundant in most solid tumors," he says, suggesting the discovery could transform treatment for many cancer types. The researchers have identified an immune force already stationed within tumors, waiting to be mobilized.
Notable Quotes
This is the first time anyone has captured a macrophage attacking and engulfing a live cancer cell in real time.— Dr. Yuki Keith, first author of the research
If we can harness this population of macrophages, we potentially have an immune army already in place, ready to be mobilised.— Professor Tri Phan, senior author
The Hearth Conversation Another angle on the story
Why does it matter that they filmed this rather than just theorizing about it?
Because the immune system in a living body is vastly more complicated than what happens in a petri dish. You see interactions, timing, the actual physical mechanics of how a cell eats another cell. Theory gets you part of the way. Video gets you the rest.
So these macrophages were always there, doing this work, and nobody noticed?
Not quite. People knew macrophages existed in tumors, but they couldn't agree on whether they were helping or hurting. Some thought they were actually enabling cancer. The video proved they were actively fighting it—at least this particular type.
What's the CD169 protein? Why does that matter?
It's a marker—a flag on the surface of certain macrophages that distinguishes them from other macrophages. When the researchers removed only the cells with that flag, tumors grew. That told them: this specific population is the one doing the protective work.
And the T cells—why are those usually the stars of cancer treatment?
Because they're the assassins. They recognize cancer and kill it directly. But they can't do their job if the tumor walls them out. These macrophages might be the ones who open the door.
So the next step is figuring out how to make more of them, or make them work better?
Exactly. Or both. If you could boost their numbers or sharpen their ability to tag cancer cells, you'd have a weapon that works even in tumors that normally shut out T cells. That's the real prize.
Could this work for other cancers?
That's the hope. Macrophages are present in most solid tumors. If the mechanism works in melanoma, there's no reason it wouldn't apply elsewhere.