More than 100 years ago Stephen Paget postulated the important role played by the microenvironment in cancer metastasis. He proposed studying both the “seed and the soil.” However, it has only been in the last 20 years that there has been an explosion in studying factors in the tumor and metastatic microenvironment, said David Lyden, MD, PhD, Weill Cornell Medicine.

Lyden and other experts on metastatic microenvironment discussed some of the recent discoveries during the Major Symposium Matrix, Exosomes, and TME Cells in the Metastatic Niche. This session and its related panel discussion are available on replay to registrants through June 21, 2021

According to Lyden, there is now knowledge of the extensive array of cells involved in this process, including fibroblasts, lymphocytes, neutrophils, and macrophages, as well as cytokines, growth factors, and exosomes. All of these influence the distant metastatic microenvironment, including the premetastatic niche, which is well established prior to the arrival of cancer cells, and the dormancy niche.

Tumor exosomes

After providing an introduction, Lyden opened the session with his presentation on tumor exosomes and exomeres, and how they promote organotropic metastasis and systemic disease.

Some years ago while studying the pre-metastatic niche, Lyden said, they noticed exosomes in these niches and discovered an unknown particle they called exomeres. Continued research was designed to determine if tissue- and plasma-derived exosome and exomere proteins could serve as biomarkers representing tumor formation—even in early stages—along with progression and metastasis.

For example, because of exosomes’ double lipid bilayer, Lyden and colleagues could label exosomes with a dye and follow them once injected intravenously. In murine models, they could track what cells would uptake exosome. Only 5 minutes after injection, they could see exosomes in a blood vessel in the lung, but similar to viruses they either get discarded in the urine or they are taken up in certain cells in specific organ sites. In a melanoma model, the exosomes were taken up by specific cells in the lung and fused with immune cells in the bone marrow.

The idea is that in patients with cancer, using tissue and blood samples, exosomes and exomeres and their proteins can be isolated and potentially used to predict who is going to get cancer early on, eventually using these proteins for prognosis and as biomarkers for treatment response.

This approach may also help people with tumors of unknown origin, Lyden said. Importantly, there are about 2,000 to 3,000 exosome proteins for each patient sample. Remarkably, there is always a set of proteins unique to each tumor type. If there is a sample from a patient with tumor of unknown origin, tumor identification may be able by using a small set of proteins to lead to a diagnosis of breast, colorectal, lung, or pancreatic cancer, for example.

Eventually, exosome and exomere proteomics may identify specific proteins and molecular pathways to target, and, ideally, exosome proteomic-based drug development can avoid unwanted side effects of targeting normal tissues.

During the panel discussion, Lyden spoke about the growing excitement of targeting the premetastatic niche with treatment and discussed some of his continued research into the role of exosomes and exomeres.

For example, researchers have found that there are more exomeres in the primary tumor and less exomeres in the metastatic tumor. They are examining possible reasons behind that, Lyden said.

Additionally, blood samples from patients could be exosome dominant or exomere dominant, and it is unclear what that means.

“When you are exosome dominant are you likely to have metastases and when you are exomere dominant are you likely to have metabolic changes in your liver and your muscle?” Lyden said. “We still have a lot of work to do there.”

Although much of Lyden’s work has been in early-stage disease, he said his team does have late-stage samples and is working to see if exosome proteins that help detect early-stage cancer persist in the late stage.

Myeloid cells

Dihua Yu, MD, PhD, University of Texas MD Anderson Cancer Center, presented new insights on myeloid cells’ functions in cancer metastases and therapeutic resistance.

Myeloid cells including neutrophils, monocytes, microphages, myeloid dendritic cells and mast cells, are largely responsible for innate defense against pathogens. However, in the tumor microenvironment myeloid cells or myeloid-derived suppressor cells are immunosuppressive, Yu said.

In the past few years her team has done experiments with microglia and immunosuppressive neutrophils in brain metastases and tumor-associated macrophages in immunotherapy. During her presentation she highlighted some of their findings.

Brain metastasis is an imposing clinical challenge that affects millions of cancer patients, Lu said. She and her team wanted to understand the biology of brain metastasis to guide the development of effective therapies. In the past few years, they have found that brain metastases coevolve with tumor microenvironment cells to develop. For example, they found that astrocyte-derived exosomal microRNA induces PTEN loss that primes brain metastases outgrowth via crosstalk between tumor cells and brain metastasis microenvironment.

More recently, their studies found that immunosuppressive neutrophils play a crucial role in brain metastases outgrowth. Specifically, they found that EZH2, a histone methyltransferase, is highly expressed in patients’ brain metastases. EZH2 promotes brain metastases but was not blocked by GSK126, a selective small-molecule inhibitor of EZH2 methyltransferase function. Instead, they discovered that EZH2 promoted brain metastases independent of methyltransferase.

Experiments showed that EZH2 is phosphorylated at pY696. Src induces pY696-EZH2, which increases G-CSF secretion that recruits immunosuppressive neutrophils into the brain to inhibit T-cell functions and promote brain metastasis.

In her presentation, Yu explained further how pY696-EZH2 increased G-CSF secretion and how pY696-EZH2-driven brain metastases could be deterred.

Inflammatory cells

Xuetao Cao, MD, PhD, Nankai University, spoke on innate molecules and inflammatory cells in premetastatic niche formation and cancer metastasis.

According to Cao, primary tumors create a premetastatic niche in secondary organs for subsequent metastasis. Up to now, he said, there has been a lot of work in the field to characterize the premetastatic niche and its significance in cancer metastases.

In mouse model experiments, Cao and colleagues have found that toll-like receptor (TLR) 3 deficiency reduces neutrophil recruitment to the lung in tumor-bearing mice. Further, they have showed that adenovirus-mediated in vivo transfection of TLR3 in the lung rescues neutrophil recruitment and inhibits lung metastasis.

RNA-sequencing analysis indicated that small nuclear RNAs are enriched in tumor exosomes and potential ligands of TLR3. To translate the finding to clinical practice, their analysis showed that high TLR3 in adjacent tissue of lung cancer closely correlates to the massive neutrophil infiltration. This massive TLR3 expression and neutrophil infiltration correlate with premetastatic niche formation and predict poor prognosis for lung cancer patients.

Cao also discussed further experiments looking for other tumor-promoting cell types in the remote organs of the tumor-bearing host, and continued studies involving the danger-associated molecular pattern (DAMP) derived from inflammatory cells in innate sensing and premetastatic niche formation.