A new study revealed that melanoma cells released small extracellular packages containing the protein nerve growth factor receptor, which primed nearby lymph nodes for tumour metastases.

The study results, which were published in Nature Cancer, may one day help doctors determine which patients need more aggressive treatment and could help with the development of new therapies, said senior author, Dr David Lyden, the Stavros S. Niarchos Professor in Pediatric Cardiology and a professor of paediatrics and of cell and developmental biology at Weill Cornell Medicine.

Traditionally, scientists have had a tumour centric view of melanoma in which cells from the tumour break off and travel to nearby lymph nodes, as cancer metastasizes. "What our study shows is that the lymph node mechanistically prepares for future metastases," said Dr Lyden, who is also a member of the Gale and Ira Drukier Institute for Children's Health and the Sandra and Edward Meyer Cancer Center at Weill Cornell Medicine. "There are many changes taking place in the lymph node even before the tumour cell gets there. We call it a pre-metastatic lymph node."

Using mouse models, lead author Dr Hector Peinado, head of the Microenvironment and Metastasis Laboratory, Molecular Oncology Programme at the Spanish National Cancer Research Center (CNIO) in Madrid, Spain, examined the release of exosomes from melanoma tumour cells. Exosomes, also called extracellular vesicles, are small packages containing various particles and proteins that are secreted by cells and travel through lymph vessels and blood to distant sites where they can be taken up by other cells.

Dr Peinado, who was previously a postdoctoral associate in Dr Lyden's lab, where much of this work began, found that the protein NGFR is released in exosomes secreted by melanoma cells and channelled into the lymph nodes. The protein was then taken up by lymph endothelial cells, or cells lining the lymph vessels, where it reprogrammed the lymph node to form new lymph vessels. This process, called lymphangiogenesis, encouraged cancer spread. "The more vessels a lymph node has, the more tumour cells can come in," Dr Lyden said.

Lymphatic endothelial cells are the first encounter for exosomes containing nerve growth factor receptor (NGFR) and "open the gates to melanoma cells coming into the lymph node and setting up shop there," said one of the paper co-authors, Dr Irina Matei, assistant professor of immunology research in paediatrics at Weill Cornell Medicine.

Moreover, NGFR increased the secretion of the protein intracellular adhesion molecule 1, which encouraged tumour cells to bind to lymph vessels, also encouraging metastases, Dr Lyden said. Dr Peinado has also investigated how the presence of proteins called integrins on the surface of exosomes determined where metastatic melanoma cells would bind. In this paper, researchers found that integrin alpha is important for spread to the lymph nodes versus other organs such as the liver.

Overall, while the lymph nodes are supposed to be a site where the immune system is activated and ready to fight cancer cells, the processes outlined in the study demonstrated that lymph nodes are immunocompromised, which may have implications for prognosis, Dr Lyden said.

When someone has melanoma, doctors would examine the sentinel lymph node or the first node to which the tumour will likely spread, to see if it contains cancerous cells. "However, the examination of the sentinel lymph node for just tumour cells is not complete," Dr Lyden said. Additional information such as the presence of NGFR or lymphangiogenesis could indicate a poor prognosis for patients, meaning they likely need from aggressive treatment.

The research could also have implications for drug development. In a mouse model, Dr Peinado used a small molecule inhibitor called THX-B to block NGFR from binding to lymph endothelial cells, helping to deter lymph node metastases. "This drug needs a lot of testing," Dr Lyden said. "But eventually, I think an NGFR inhibitor could be used in the clinical setting."

The researchers followed up on the mouse model findings by studying lymph node tissue from 44 patients with stage III or IV melanoma and found that NGFR expression was significantly higher in metastatic lymph node tissues than in the original skin lesions. Evaluation of another 25 patients with stage III melanoma found a significant increase in the frequency of NGFR positive tumour cells in lymph node metastases compared with the originating melanoma tumour.

"Those with the metastatic disease actually have very high NGFR expression," Dr Lyden said. "So, at all stages, whether it's pre-metastatic or full-blown lymph node metastasis, it seems that molecule is very much up-regulated in the patients with worse prognosis."

The researchers aimed to validate their findings in larger numbers of patients. "We also would like to determine if we can use a simple blood test to see whether exosomes are carrying NGFR," Dr Lyden said. "This would help doctors to look at exosome NGFR over time during therapy to see how treatments should be adjusted."