Metastasis, when cancer spreads to distant sites around the body, is complex. Cancer cells go through several transformations as they push their way out from their original tumor, travel in the bloodstream to new sites, and then put down roots in their new homes, seeding new tumors from scratch.
Now, a new study using mice and tiny transparent fish as models of human cancer has shed light on the very first stages of metastasis in the skin cancer melanoma.
Led by Fred Hutchinson Cancer Research Center postdoctoral fellow Dr. Minna Roh-Johnson, the study revealed that certain migrating immune cells known as macrophages share their internal contents directly with cancer cells — and that cellular mind-meld transfers the immune cells’ ability to migrate through the body to the tumor itself. The team’s findings were published Monday in the journal Developmental Cell.
It’s long been appreciated that the “tumor microenvironment” and, specifically, the immune cells in and around the tumor play an important role in many aspects of cancer growth and spread. But this is the first time researchers have observed an immune cell directly and physically contacting and sharing its contents with a tumor cell, Roh-Johnson said. Previous studies had seen macrophages sharing their contents by shedding tiny packages of molecules into their environment, so there seem to be multiple ways of signaling between immune cells and tumor cells, the researchers said.
It’s neither clear what’s being transferred in the exchange nor why it happens. And the team’s laboratory findings don’t yet confirm that these same interactions occur in humans. What is clear is how the contact dramatically changes the tumor cells, the researchers said. The cellular mixing shifts the tumor cells from “meandering” to making a beeline in one direction and — in the fishes’ bodies — hastens cancer metastasis.
In these macrophage-tumor entanglements, which can last for hours, it’s as if the immune cells are teaching the cancer cells how to walk. And they are good teachers.
“Immune cells, they migrate,” said Roh-Johnson, who is completing her fellowship in the Basic Sciences Division lab of Fred Hutch developmental biologist Dr. Cecilia Moens. “That’s their function in the body, to actively migrate and look for foreign molecules to eat or attack.”
To examine the role of macrophages in melanoma spread, the researchers used both a mixture of cells in petri dishes as well as human melanoma cells transplanted into zebrafish, tiny fish that are transparent in the early stages of their lives. Using the see-through fish and fluorescent dyes to label individual cells, the researchers could follow the course of individual melanoma cells as they went from stationary to mobile to spreading through the fish’s body.
That macrophages are present in and interact with tumors is not news. That these immune cells are typically bad news in the context of cancer is also not a new finding — in many types of human cancers, researchers have seen that the higher the number of macrophages in the tumor, the more likely the tumor is to metastasize and the worse the patient’s prognosis.
Macrophages are a part of our innate immune system, the body’s first line of defense against foreign invaders. But in the context of cancer a switch seems to get flipped, said Moens, who is also one of the study authors.
“We think of macrophages as being good guys, the first responders,” she said. “In the tumor microenvironment, they can have the opposite effect.”
In their zebrafish study, the biologists saw macrophages interacting extensively with melanoma cells. Even though these immune cells are “very dynamic,” Roh-Johnson said, those cell-to-cell contacts were extensive, lasting upward of nine hours.
Roh-Johnson captured some of those interactions in a video in which the tendrilled macrophage, labeled red, skitters over plump, stationary tumor cells, labeled green.
The researchers found that fish depleted of macrophages had lower levels of metastasis, and those with an extra boost of the immune cells saw more cancer spread.
Intrigued by the direct interactions between the two cell types, the researchers then adapted a method previously used to study how tumor cells interact with each other to ask whether macrophages and tumor cells were sharing their cellular contents during their close encounters.
The technique engineers each cell type to produce one new gene: The macrophages produce a “trigger” protein, and the tumor cells are engineered to make a red fluorescent protein that changes to green only if the trigger protein is transferred from the macrophage to the cancer cell. Applying this technique in the zebrafish, Roh-Johnson and her colleagues found that’s indeed what happens — the trigger protein passed from macrophages to tumor cells, and their color switched from red to green.
In a separate experiment, they showed that direct contact between the two cell types is needed for that cellular transfer, because when the two cell types are put together in a petri dish but separated by a tiny screen so they can’t touch, the color switch doesn’t happen as frequently.
In videos made by their collaborator and co-author Dr. Melissa Wong, a stem cell biologist at Oregon Health & Science University in Portland, the researchers watched cells in petri dishes to see how the melanoma cells’ behavior changed after being contacted by macrophages and gaining some of their contents.
“When you track the tumor cell location over time, if you were to look at it before transfer, you would see it sort of meandering along,” Roh-Johnson said, tracing small circles in front of her to indicate a directionless tumor cell.
After the transfer, she said, the cells become what cell biologists term “directionally persistent, which means that once it went along a path, it just kept going. The speed of migration didn’t actually change, but the way it migrates changed.”
In the context of a tumor, cancer cells may be spurred by these contacts to migrate more effectively, Moens said, which would increase the likelihood of eventually hitting a blood vessel and getting into circulation. And those cells may be the ones that seed new tumors.
While the researchers don’t know if these same events take place in people with cancer, they did repeat some of their experiments in a mouse model of melanoma and saw that mouse macrophages also transfer some of their contents to cancer cells and that the transfer hastens metastasis. And they don’t yet know if this type of exchange happens in other cancer types besides melanoma, although there have been previous studies showing that macrophages are important for metastasis in a number of different types of solid tumors.
Roh-Johnson’s next goal — once she establishes the laboratory team she will lead at the University of Utah early next year — is to uncover the identity of the molecules being passed from immune cell to tumor. Because macrophages are so good at migrating, their innards are likely full of “motility-promoting factors,” she said.
If researchers can understand what those factors are and why they promote metastasis, there could be downstream possibilities to use those molecules as markers of macrophages-gone-bad, or even as targets for new therapies to stem some of cancer’s dangerous spread.
The study was funded by the National Institutes of Health, the Fred Hutch Cooperative Center for Excellence in Hematology, the American Heart Association, the Fondation ARC pour la Recherche sur le Cancer, the European Commission, an OHSU Center for Women’s Health Circle of Giving Grant, the Crohn's and Colitis Foundation of America and the Prospect Creek Foundation.
Rachel Tompa is a former staff writer at Fred Hutchinson Cancer Center. She has a Ph.D. in molecular biology from the University of California, San Francisco and a certificate in science writing from the University of California, Santa Cruz. Follow her on Twitter @Rachel_Tompa.