In a pair of studies, investigators from the lab of Huiping Liu, MD, Ph.D., associate professor of Pharmacology and of Medicine in the Division of Hematology and Oncology, have uncovered how specific cellular interactions in the bloodstream may be fueling the spread of breast cancer.
Published in Nature Communications and The Journal of Clinical Investigation, the studies reveal how circulating tumor cells form clusters alongside immune cells, dramatically increasing their ability to spread.
Identifying drivers of tumor cell clustering
The first study, published in Nature Communications, identifies the protein Plexin-B2 (PLXNB2) as a critical player in breast cancer’s ability to metastasize, or spread to new parts of the body.
In the study, investigators employed a computational ranking algorithm to identify proteins associated with unfavorable patient outcomes. They found that PLXNB2 is highly expressed in multicellular circulating tumor cell (CTCs) clusters, which are up to 50 times more efficient at forming metastases than single CTCs.
“Tumor cells travel from one site to another in the blood and face a lot of physical challenges and immune cell attacks,” said Liu, who was senior author of the study. “By joining with these friendly immune cells, the cancer can promote seeding, avoid surveillance and have a kind of shield from hostile factors as it spreads.”
In mouse models, knocking out PLXNB2 significantly reduced lung metastases, suggesting it could be a promising therapeutic target, Liu said.
“These clinical sample analyses and preclinical tumor studies in mice demonstrated that PLXNB2 is not only a biomarker for poor outcomes, but it is a driver of tumor spreading,” said Liu, who is also a member of the Robert H. Lurie Comprehensive Cancer Center of Northwestern University.
Rare t-cells play a surprising role
The second study, published in the Journal of Clinical Investigation, shifts focus to the immune system’s role in metastasis.
“Tumor cells not only interact among themselves, but they also have crosstalk with immune cells,” said Liu, the senior author of the study. “But for the circulating tumor cells in the blood, there have not been many comprehensive studies. Our goal was to fill the gap and to understand better how tumor cells really talk to engage supportive immune cells, and then to find out how tumor cells escape from defensive immune cells in general.”
By analyzing blood samples from patients with advanced breast cancer, scientists discovered that more than 75% of CTC-positive samples contained clusters of tumor cells and white blood cells, the immune system’s fighter cells. Among these, a rare subset of T-cells—CD4 and CD8 double-positive T-cells (DPTs)—were disproportionately represented.
These DPT cells exhibited traits of immune exhaustion and suppression, which may help tumor cells evade immune detection, according to the study.
“Double-positive T-cells are very unique and rare. They have shown the capacity to promote cancer stem cells and suppress the immune response,” Liu said. “The more of these signals you have, the less other tumor-killing T-cells will fight, which leads to more tumors in the body. These cells protect the tumor cells from immune cell attacks.”
The study investigators then identified two molecules on the cell surface—VLA-4 in DPT cells and VCAM1 in tumor cells—as essential for cluster formation. In mice with cancer, blocking this interaction with antibodies disrupted clustering and significantly extended survival.
The pair of studies not only deepens scientific understanding of cancer biology but also underscores the importance of the immune system’s role in cancer progression—sometimes as an unwitting accomplice.
By targeting PLXNB2 or VLA-4/VCAM1, investigators may be able to develop therapies that prevent metastasis before it begins, Liu said.
“We’re approaching this from multiple angles with the same goal of identifying targets to help us control this tumor cell behavior,” Liu said. “By eliminating tumor cell clusters and inhibiting this devastating phenotype, we can hopefully stop the cancer from spreading, allowing patients to live longer and have a better quality of life.”
More information:
Emma Schuster et al, Computational ranking identifies Plexin-B2 in circulating tumor cell clustering with monocytes in breast cancer metastasis, Nature Communications (2025). DOI: 10.1038/s41467-025-62862-z
David Scholten et al, Double-positive T cells form heterotypic clusters with circulating tumor cells to foster cancer metastasis, Journal of Clinical Investigation (2025). DOI: 10.1172/jci193521
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