Senescent neutrophils promote tumor survival in all cancer types

Renowned as first responders to threatening infections, neutrophils also happen to feature prominently in the tumor microenvironment, where they and other immune cells play opposing and often variable roles in promoting or resisting cancer progression. Although they have been linked to the development of multiple cancers, including those of the lung and breast, neutrophils can take on multiple functional states, only some of which have this effect. Locating these states proved, for technical reasons, quite difficult.

Now researchers led by Mikaël Pittet of Ludwig Lausanne have discovered a gene expression program executed by tumor-associated neutrophils (TANs) and a corresponding biomarker that uniformly support cancer cell survival and tumor progression in human and mouse tumors. The findings, detailed in the current issue Cancer Cell, identify TANs characterized by this conserved genetic program as a central tumor microenvironment (TME) variable linked to cancer progression. The researchers also describe how the associated marker—a protein called CCL3—functionally supports cancer growth.

We found that tumors induce in neutrophils a genetic program that sets them on a trajectory of continuous maturation, culminating in a final “senescence” state characterized by high CCL3 expression. These aged, “CCL3^HiNeutrophils preferentially occupy sites in the TME that lack oxygen, engage genetic subroutines that help them adapt to the harsh conditions there, and then activate a wide array of genes that promote tumor growth and survival.”

Mikaël Pittet, Ludwig Lausanne

Pittet and colleagues report that the gene expression of these aged TANs aligns with previous findings linking neutrophils to tumor development. Aged TANs, for example, have been found to fuel prostate cancer. “This conserved pro-tumor state in TANs represents a potential biomarker for predicting patient prognoses in many types of cancer,” added Pittet.

Although neutrophils are abundant in tumors, the technologies used to analyze cellular states fall short when applied to these cells. That’s because the technology—single-cell RNA sequencing (scRNAseq)—depends on reading gene transcripts in single cells. Due to some biological quirk, neutrophils tend to harbor extremely low levels of such RNA transcripts.

Pittet and his team developed a computational method—a probability classifier—to classify neutrophils into distinct functional states based on raw sequence data and applied it to more than 190 human and mouse tumors. These studies, which included retrospective analysis of existing data sets, revealed the terminal CCL3^Hi status assumed by TANs. Such TANs were observed across the entire range of tumor types examined.

But what exactly does CCL3 do? The researchers show that CCL3 produced by TANs engages a receptor on their surface (CCR1) to transmit signals that drive TANs toward terminal senescence, enhance their survival in the hypoxic microenvironment, and activate gene expression programs that drive tumor growth. Accordingly, mice lacking CCL3 in their neutrophils as well as mice whose neutrophils lacked CCR1 failed to support tumor growth, demonstrating that loss of any component of this signaling axis confers the same impairment in TAN-mediated tumor support.

“Our work demonstrates that tumors actively maintain pro-tumor neutrophils through CCL3 and identifies these cells as a conserved and clinically relevant cell compartment in the TME,” said Pittet. “This suggests that the genetic and biochemical circuits that ensure the survival of CCL3^Hi TANs may be targeted for cancer therapy.”

The findings complement a discovery reported by Pittet and colleagues Science in 2023 that the ratio of a pair of genes (CXCL9 and SPP1) expressed by macrophage-associated immune cells found in tumors – broadly predicts outcomes for cancer patients. He and his colleagues showed that genes are linked to a vast network of gene expression programs engaged by multiple cell types in the TME that control the progression of human cancers. When the ratio of CXCL9 to SPP1 is high, gene expression programs in other TME cells indicate a generally antitumor bias. A low CXCL9-to-SPP1 ratio, on the other hand, invariably accompanies pro-tumor gene expression signatures throughout the TME.

The current study suggests that CCL3^Hi TANs could be a second variable of similar unique prognostic significance for all tumor types and species.

In addition to being a regular member of Ludwig Lausanne, Mikaël Pittet is a professor at the Faculty of Medicine of the University of Geneva, where he is the Chair of the ISREC Foundation in Immuno-oncology.

This study was supported by the Ludwig Institute for Cancer Research, the Swiss National Science Foundation, the ISREC Foundation, Geneva Translational Oncology Program, the Fondation Privée of the University Hospitals of Geneva, the US National Institutes of Health, the Belgian American Educational Foundation, the Human Frontier Science Program, the European Union Horizon 2020 of HematcietCA MS.