Covid-19 and flu may reawaken dormant cancer cells in the lungs
Samples of mouse pulmonary tissues, showing the Nuceli of cells (blue), cancer cells (green) and proliferation markers (Magenta)
Bryan Johnson
Respiratory viruses can activate the growth of dormant cancer cells that have spread to the lungs in the body. Infections like flu seem to trigger an inflammatory response that helps the immune system to fight the microbial invader, but has negative consequences for the progression of cancer.
Cancer deaths often occur when tumor cells migrate from their original location in the body. It is difficult to detect when this happens because these cells can be dormant – and not by dividing – for years or even decades on their new site before training tumors.
What triggers these cells to ultimately proliferate is not clear, but previous studies suggest that when cancer cells reach the lungs, inflammation caused by respiratory viral infections can play a role. “But no one had really studied to really establish the cause and the effect,” said James Degregori at the University of Colorado.
To fill this gap, Degregori and his colleagues used mice genetically designed to grow tumors in their breast glands. At about 2 months old, each mouse had mammary gland tumors and less than 10 sleeping cancer cells in their lungs.
The team then infected half of the mice by the H1N1 strain of the influenza virus, also known as the swine flu, making them sick for about two weeks. Nine days after the infection, the number of cancer cells in their lungs had increased by 100 times, but barely changed in uninfected mice.
When the researchers repeated the experience but with the SARS-COV-2 virus, which causes COVID-19, they saw an increase of 10 times in the number of cancer cells in the lungs of the mice, but still no change in unmanned mice.
Researchers have hypothesized that such expansions have occurred because viral infections increase the levels of an inflammatory molecule called IL-6, which helps the immune system to destroy viruses, but can also promote tumor growth.
To validate this idea, they repeated the mouse experience that was genetically designed to miss IL-6, and found that they had much fewer cancer cells in their lungs compared to mice with typical IL-6 levels.
Another experience revealed that IL-6 seems to react from dormant cancer cells that had already migrated to the lungs, rather than increasing the propagation of these breast cells.
But IL-6 levels calm down once the infections have been eliminated. At this stage, the team found that cancer cells in the mouse lungs ceased to proliferate, but had acquired properties – such as gene expression changes – which were linked to the spread of tumors, explains Degregori.
The results could have implications for people with undetectable levels of cancer cells in their lungs when they are in remission, explains Ann Zeuner at the National Institute of Health in Rome, Italy.
To explore if the results could really apply to people, the researchers analyzed the health files of 36,800 women in the United States which were diagnosed before the COVVI-19 pandemic with breast cancer which was not supposed to spread.
Women with a positive test during the first three years of the epidemic were much more likely to have a diagnosis of secondary lung cancer during this period than those whose swabs came back negative or that have not been tested at all. But some women may have had asymptomatic infections, and therefore did not ask for a test, while others may not have had access to one, which would reduce the validity of this result, explains Degregori.
Other studies on people are necessary to verify research and explore how various respiratory viruses and cancer types interact, explains Zeuner. “Individual factors are [also] Likely to deeply affect the link between respiratory infections and relapse of cancer, “she says.
While the team only looked at the swine flu and the Sars-Cov-2, Degregori expects that a range of viruses act in the same way, because many induce increased levels of IL-6. He also thinks that the results support another reason to be vaccinated. “If I were a cancer survivor, I would make sure I was vaccinated against common respiratory viruses like the flu, wearing, so on,” explains Degregori.
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