Harnessing Cellular Cannibalism for Cancer Treatment

Scientists have solved a cellular murder mystery nearly 25 years after the case went cold. Following a trail of evidence from fruit flies to mice to humans revealed that cannibalistic cells likely cause a rare human immunodeficiency. Now the discovery shows promise for enhancing an up-and-coming cancer treatment.

“This paper takes us from very fundamental cell biology in a fly, to explaining a human disease and harnessing that knowledge for a cancer therapy,” said UC Santa Barbara’s Denise Montell. “Each one of those steps feels like a major discovery, but here they are, all in one paper.”

Researchers in Montell’s lab published their findings in the Proceedings of the National Academy of Sciences and are now investigating the mechanisms and implications.

An ancient gene

The primary character in this story is a gene, Rac2, and the protein it encodes. Rac2 is one of three Rac genes in humans. “Rac is very ancient in evolution, so it must serve a fundamental function,” said senior author Montell, Duggan Professor and Distinguished Professor of Molecular, Cellular, and Developmental Biology.

Rac proteins help build a cell’s scaffolding, called the cytoskeleton. The cytoskeleton is made of dynamic filaments that allow cells to maintain their shape or deform, as needed. In 1996, while studying a small group of cells in the fruit fly ovary, Montell determined that Rac proteins are instrumental in cell movement. Since then, it has become clear that Rac is a nearly universal regulator of cell motility in animal cells.

Back in the ’90s, she noticed that a hyperactive form of the Rac1 protein, expressed in only a few cells in a fly’s egg chamber, destroyed the whole tissue. “Just expressing this active Rac in six to eight cells kills the entire tissue, which is composed of about 900 cells,” explained lead author Abhinava Mishra, a project scientist in Montell’s lab.

Why did this happen? How does it work? “This was our 25-year-old cold case,” Montell said.

A few years ago, evidence began to mount implicating cell eating, also known as cannibalism, in tissue destruction. There’s a step in normal fly egg development where certain cells similar to the border cells consume their neighbors because they are no longer needed. Indeed, cellular cannibalism is not as rare as you might expect: Millions of old red blood cells are eliminated from the human body this way every second.

Rac2 is one component of the complex eating process. Rac helps the eating cell to envelop its target. The team was curious if a hyperactive form of the protein was causing border cells to prematurely consume their neighbors.

For this to occur, the border cells need to recognize their targets, which requires a particular receptor. Indeed, when Mishra blocked this receptor, the border cells expressing activated Rac didn’t consume their neighbors, and the egg chamber remained alive and healthy.

“Our 25-year-old cold case was solved, and that was very satisfying for us,” Montell exclaimed. “But this is a fairly niche area of Drosophila egg development.” The implications would soon grow, though.

A mysterious immune condition

Around the time that her lab made their breakthrough, Montell caught wind of an intriguing study in the journal Blood. This paper found that three unrelated people suffering from recurrent infections had the exact same mutation, which hyperactivates Rac2, a Rac protein produced in blood cells. She suspected her lab’s recent revelation in fruit flies might shed light on this enigma.

The patients’ mutation was just mildly activating, and yet it was enough that they all suffered from multiple infections and ultimately needed bone marrow transplants. Blood tests revealed that these patients had nearly no T cells, a specialized kind of white blood cells crucial to the immune system. The team at the National Institutes of Health inserted the Rac2 mutation into mice and found the same mysterious loss of T cells. They also found that the T cells with hyperactive Rac developed normally in the animals’ bone marrow, and migrated to the thymus, where they continued to mature without incident. But then they just seemed to disappear. So, the paper ended with a mystery: what was causing the T cells to disappear?

The authors of that journal study had noticed that many of the patients’ neutrophils — another type of white blood cell — were enlarged. They seemed to be consuming quite a lot of material, unusual behavior in an otherwise healthy person.

Montell wondered if the patients’ T cells were disappearing because their innate immune cells like neutrophils with active Rac2 were eating them, much like the fruit fly border cells with active Rac were eating the egg chamber. Her team turned their attention to macrophages — the neutrophil’s more voracious counterpart — to investigate. Mishra cultured human macrophages with and without hyperactive Rac2 together with T cells. He observed that macrophages with hyperactive Rac consumed more cells, confirming the group’s hypothesis from their work with fruit flies.

Sources:

Abhinava K. Mishra, Melanie Rodriguez, Alba Yurani Torres, Morgan Smith, Anthony Rodriguez, Annalise Bond, Meghan A. Morrissey, Denise J. Montell. Hyperactive Rac stimulates cannibalism of living target cells and enhances CAR-M-mediated cancer cell killing. Proceedings of the National Academy of Sciences, 2023; 120 (52) DOI: 10.1073/pnas.2310221120

Materials provided by University of California – Santa Barbara. Original written by Harrison Tasoff. Note: Content may be edited for style and length.

University of California – Santa Barbara. “When bad cells go good: Harnessing cellular cannibalism for cancer treatment.” ScienceDaily. ScienceDaily, 8 January 2024. <www.sciencedaily.com/releases/2024/01/240105182106.htm>.

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