The discovery points to new drug targets that could prevent the spread of cancer

The discovery points to new drug targets that could prevent the spread of cancer

Any cancer cell that migrates from a tumor to colonize elsewhere in the body will face a brutal attack from an immune system programmed to seek out and destroy abnormal cells. But two recent studies from Stanford Medicine show that the brave few who manage to infiltrate nearby lymph nodes pull off an astonishing biological coup - convincing the body's defense system to accept them as part of their own tissue. This sophisticated rebranding gives tumor cells the opportunity to easily metastasize to any location in the body, significantly worsening cancer prognoses.

The studies, conducted on laboratory mice, human cells and human tissue samples from cancer patients, overturn the idea that lymph nodes - often the first site of metastasis - are simply passive downstream havens for circulating cancer cells that have broken away from nearby tumors.

"It's not just plumbing," said Edgar Engleman, MD, professor of pathology and medicine at Stanford School of Medicine, clarifying the dubious distinction of nodules as the first agents in the spread of cancer. "It's something more nefarious. Once in the node, the cancer cells reprogram the immune system to not only stop the attack, but directly support metastasis. This concept is fundamental to our understanding of how tumors spread and how we might intervene in this process."

Engleman is the lead author of one of the papers published online May 26 in Cell. Nathan Reticker-Flynn, PhD, Lecturer in Medicine, is the first author of the Cell article. The second paper was published June 2 in Nature Methods. Weiruo Zhang, PhD, research engineer, is the first author of the Nature Methods study, and Sylvia Plevritis, PhD, professor and chair of biomedical data science, is the senior author.

A closer look at the lymph nodes

The research combines work from Engleman's team showing how cancer cells gain the ability to metastasize using a new imaging platform developed in the Plevritis lab. The analysis platform can accurately identify individual cell types, including healthy and cancerous, in cross-sections of human tissue collected during cancer surgeries or biopsies.

When we started these studies, we really wanted to know if something was happening in the nodes to facilitate metastasis. Are there early changes that suppress the immune system response throughout the body? And are these changes dependent on the spatial organization of individual cell types in the nodes?”

Sylvia Plevritis, PhD, Professor and Chair of Biomedical Data Science

Lymph nodes are small chambers or glands in the armpits, neck, abdomen, and groin that are filled with immune cells. They are part of the lymphatic system, which collects excess fluid that naturally builds up in tissues as blood circulates. Lymphatic vessels deliver the fluid called lymph to the nodes, trapping bacteria, abnormal cells, and other detritus. The filtered lymph is then returned to the circulation via a vein near the heart. The nodes serve as a surveillance center, training ground and mobilization center for immune cells that monitor lymphatic contents and spring into action to attack cancer or infected cells at the first sign of danger.

Curiously, although the lymph nodes are often one of the first sites of metastasis for neighboring tumors, the exact reason was unclear. It was previously believed that they could serve as natural downstream havens for circulating cancer cells, which then gradually accumulate additional mutations that further increase their ability to metastasize. However, the latest studies suggest that once metastases have formed in the nodes, the nodes play a far more active role in allowing the cancer cells to metastasize to other organs.

Engleman and Reticker-Flynn asked themselves what changes a cancer cell must undergo in order to split off from the original tumor and successfully colonize a lymph node. To study this, they started with a skin cancer called melanoma, which doesn't often metastasize in mice. Reticker-Flynn implanted the cells into laboratory mice and observed whether cancer cells managed to reach the animals' lymph nodes. These cancer cells were removed from the nodes and transplanted into another animal, and the process was repeated. Over the course of nine iterations, the cells became increasingly adept at metastasizing—evolving into a supercharged version of their former selves.

The researchers then compared the original cells with their unattached progeny to learn how the cells escaped patrolling immune cells called natural killer, or NK, cells.

“It’s sneaky”

"It turns out that it's not an easy way for cancer cells to get to a node," Engleman said. "There is an intense immune system attack. To avoid this, the tumor cells increase the production of molecules on their surfaces that tell the NK cells 'Don't kill me.' It’s sneaky.”

Once in the node, the cancer cells switch from avoidance to active immune system intervention, the researchers found. In particular, the arrival of the cancer cells increases the number of another type of immune cell called regulatory T cells, or Tregs (pronounced tea-regs), in the node. Much like a school monitor roaming the halls to break up fights, Tregs suppress inappropriate or overactive immune responses in the node that would lead to an attack on the body's own cells and tissues.

As researchers in Engleman's lab continued their study of the types of proteins and cells in the nodules, Plevritis, Zhang and their colleagues mapped cellular neighborhoods in nodules in tissue samples from patients with head and neck cancer. Building on existing technology that provides an image of individual cells in tissue samples, they developed a machine learning tool called CELESTA that labels each cell with its cell type. Imagine zooming in on a Google Earth map that initially shows only place names, only to produce an expanded version that pinpoints the locations of individual cars, houses, pools, and grills. The results of the computer-based tool are much more meaningful and are generated faster than previously thought possible.

“Automatically labeling objects in images makes it easier for computers to further process an image to answer more specific questions, such as whether grills are more common in homes with pools,” Plevritis said. "Even for an experienced pathologist, such an analysis would take weeks or months. But CELESTA can provide an answer within minutes."

Because neighboring cells, such as human neighbors, are likely to talk to each other or share resources frequently, the cells' relative locations indicate their relationships. “The ability to use imaging and computational methods to infer cross-talk between cell types in health and disease allows us to better understand the fundamental biological properties of human tissue,” Plevritis said.

Disinformation campaign

CELESTA's analysis of the human nodules confirmed what Engleman and Reticker-Flynn found in their mouse studies. The cancer cells sneak up on Tregs in the nodes and begin a campaign of disinformation, asserting their harmlessness through a series of biological winks, whispered messages and elbow nudges. The deceived Tregs grant the cancer cells unrestricted access, lifting them from the immunological most wanted list to body-wide VIP status in one fell swoop.

“Immune cells communicate with each other throughout the body, and cancer cells use this communication, which begins in the lymph nodes,” Engleman said. "Essentially, the conditioned Tregs circulate to distant tissues and prepare them for the arrival of their new 'friends', the cancer cells. It is the induction of what we call metastatic tolerance, and it reflects naturally occurring immune tolerance."

Although piggybacking on a critical tolerance pathway is a clever way to avoid some potential cancer therapies, the researchers' discovery points to some new drug targets that could prevent cancer from spreading.

“Our studies have identified hundreds of genes that cancer uses to evade the immune system,” Engleman said. "These all represent potential targets, and I expect that many laboratories will now look for new ways to intervene in the metastasis process. Like viruses, cancers have learned to take advantage of what nature offers them, and the fact that tumors generally appear to be able to exploit this mechanism makes this process an attractive target for cancer researchers."

Researchers from UC San Francisco and Tel-Aviv University also contributed to the study.

Source:

Stanford Medicine

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