Cowpea Mosaic Virus Nanoparticles May Help Prevent Metastatic Cancer

Nanoparticles obtained from a virus that infects black-eyed pea plants have demonstrated "widespread effectiveness" at helping to prevent several metastatic cancers in mice, according to a new study. The findings raise the prospect that the virus could offer similar benefits in humans.

Nanoparticles derived from the cowpea mosaic virus improved survival and suppressed tumour growth throughout the body in mice with several cancers, including breast, colon and ovarian cancer.

The University of California San Diego team also observed similar benefits in mice whose tumours had been surgically removed.

Cowpea mosaic virus and black-eyed pea plants

Cowpea mosaic virus is a plant pathogen that targets cowpeas (Vigna unguiculata), a legume species whose subspecies include the plants commonly called black-eyed peas.

Rather than attacking cancer cells directly in mice, the virus works as an immunotherapy: a treatment designed to help the body's immune system recognise and destroy cancer.

The latest research forms part of an ongoing programme led by Nicole Steinmetz's laboratory at the University of California San Diego. Steinmetz, a nanoengineer, and her team have spent years investigating cowpea mosaic virus nanoparticles as immunomodulators, which are substances able either to dampen or, in this instance, activate the immune system.

Earlier work found that the nanoparticles could encourage an immune response when injected straight into a tumour. Those results indicated that the approach might help stop cancer spreading and reduce its risk of returning.

Because cowpea mosaic virus is a plant virus, it cannot infect mammals. Even so, mouse immune systems generally recognise it as foreign, the researchers explain.

That recognition triggers a strong immune response, which is also directed towards the nearby tumour and, the team's research suggests, towards any tumours that later emerge.

Systemic treatment for metastatic cancer

The new study indicates that the nanoparticles need not be injected directly into tumours to work. They may instead be given systemically to hinder metastasis and improve survival across a broad range of cancers.

"Here, we do not treat established tumors or metastatic disease – we prevent them from forming. We are providing a systemic treatment to wake up the body's immune system to eliminate the disease before metastases even form and settle," Steinmetz says.

For the study, Steinmetz and her colleagues grew black-eyed pea plants in the laboratory before infecting them with cowpea mosaic virus. As the virus replicated, it produced millions of copies that the researchers could harvest.

Steinmetz says the resulting nanoparticles required no alteration before they could be administered to mice. They are "nature's powerful nanoparticles, as produced in black-eyed pea plants," she says.

The team injected mice with the nanoparticles and, a week later, exposed them to metastatic tumours. These included breast, colon and ovarian cancers, as well as melanoma.

Compared with untreated control mice, the animals given the nanoparticles experienced slower tumour growth and higher survival rates.

The pattern remained the same when the mice were exposed to fresh tumours again a month later. Once more, mice treated with cowpea mosaic virus nanoparticles performed significantly better than untreated animals.

Nanoparticles after tumour-removal surgery

The study also examined whether the nanoparticles could be effective following surgery to remove tumours. Treated mice again had lower tumour growth and better survival rates, a result the authors describe as particularly exciting.

"Even if you perform surgery to remove the tumors, no surgery is perfect and there is outgrowth of metastasis if no additional treatment is provided," Steinmetz says.

"Here, we use our plant virus nanoparticles after surgery to boost the immune system to reject any residual disease and prevent circulating tumor cells from metastatic seeding," she adds. "We found that it works really, really well!"

Whether the treatment will prove equally effective in people is still unknown, but the study represents an encouraging stage in the work and the researchers are already preparing further investigations.

Future studies will aim to identify the mechanism responsible for the effects observed in this research and assess the treatment's safety in other animals. This could ultimately prepare the way for clinical trials in humans.

The study was published in Advanced Science.