Researchers have found that neutrophils can travel with circulating tumor cells (CTC) and promote metastasis. In patients with breast cancer and in mouse models, the presence of CTC–neutrophil clusters correlates with shorter progression-free survival, the study revealed. Neutrophils may promote metastasis by releasing cytokines that promote cell-cycle progression, thus spurring the tumor cells to divide.

Circulating tumor cells (CTC) sometimes travel with neutrophils, which improve their odds of establishing metastases, a study has found (Nature 2019;566:553–7). The results suggest that treatments that break the connections between tumor cells and their neutrophil escorts might reduce metastasis formation.

Patients with CTCs are more likely to develop metastases and have a poorer prognosis. These tumor cells sometimes move through the bloodstream with nonmalignant cells, including white blood cells (WBC). However, the types of WBCs that ride along with CTCs and whether they play a role in metastasis have remained unknown.

To identify CTC-associated WBCs, Nicola Aceto, PhD, of the University of Basel in Switzerland, and colleagues analyzed blood samples from 70 patients with invasive breast cancer and from five tumor-bearing mouse models. Thirty-four of the patients had CTCs: 88% of the CTCs were traveling solo, 8.6% were in groups of tumor cells, and 3.4% were CTC–WBC clusters. Depending upon the mouse model, between 0.05% and 61% of CTCs carried WBCs.


Neutrophils and other cells captured as part of a blood test.

Using single-cell RNA sequencing, the researchers found that most of the WBCs in the clusters were neutrophils. These cells accounted for 85.5% to 91.7% of the WBCs escorting CTCs in the mouse models and up to 75% of the escorts in patients.

“CTCs are able to carry along white blood cells such as neutrophils. When this happens, the CTCs become very metastatic,” says Aceto. Compared with patients with single CTCs or clusters of CTCs, patients who had at least one CTC–neutrophil cluster per 7.5 ml of blood had much shorter progression-free survival, the researchers found. They saw similar results when they injected CTCs into tumor-free mice: Metastasis and death occurred more rapidly in mice that received neutrophil-carrying CTCs than in those injected with clusters of CTCs or individual tumor cells.

Aceto and colleagues probed how neutrophils changed CTC behavior. They discovered that in mice and patients, CTCs that partnered with neutrophils displayed increased expression of genes that promote cell-cycle progression and DNA replication. Neutrophils stimulate the cancer cells by expressing the cytokines IL1β and IL6, the scientists determined.

Cancer cells connect with neutrophils before they depart the tumor, Aceto says. He and his colleagues tested whether they could prevent these liaisons by using CRISPR/Cas9 to knock out four genes in mice—F11r, Itgb2, Icam1, and Vcam1—that code for cell adhesion proteins. They found that the loss of one gene was particularly effective: Vcam1. This result hints that its protein is necessary for neutrophil–CTC clusters to form. The researchers are now trying to identify new ways to block the protein and improve metastasis-free survival, Aceto says.

The study “has a tremendous possible impact,” says Michael Speicher, MD, of the Medical University of Graz in Austria, who wasn't connected to the research. “This paper gives us some direction about how a decrease in metastasis can be achieved.”

Richard Cote, MD, of the University of Miami Miller School of Medicine in Florida, who also wasn't connected to the study, agrees that the data convincingly show that neutrophils play a role in metastasis. However, he notes, CTCs can also travel with other types of WBCs and, as he and his colleagues discovered, with specialized fibroblasts (Cancer Res 2015;75:4681–7). Neutrophils “are part of the story, but I don't think they are the whole story.” –Mitch Leslie

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