Researchers find switch turning on immune cell tumour clearing ability

The researchers hypothesised in the presence of mRNA let-7 molecules, T cells are quiet and the immune system is inactive, but when a threat is sensed, let-7 molecules disappear, which allows T cells to become functionally cytotoxic and able to clear pathogens, including tumour cells

Researchers at the University of Massachusetts Amherst have shown how a microRNA molecule known as Lethal-7 (let-7) serves as a molecular control hub to direct the function of cytotoxic T lymphocytes, cells involved in the immune system, by putting the brakes on the immune cells’ cell-killing activities.

When let-7 levels are low or absent, the body’s T cells, can potentially turn into “super killers” said Dr Pobezinsky, Molecular Biologist at the University of Massachusetts.

The discovery is a significant advance in adaptive immunotherapy, a therapy enlisting the body’s own immune defences to combat diesease. It is theorised to be a more effective approach to attacking disease-causing agents such as invasive cancer and chronic infections like HIV.

Dr Pobezinsky said: “We get cancer because CD8 T-cells are not always efficient and cancer can overcome them. Our lab looks at the molecular mechanisms that regulate the cytotoxic efficiency of T cells.”

He and colleagues report finding the control mechanism in a tiny strand of microRNA, only 20-30 nucleotides long, that determines how effective T cells will be in attacking disease.

Dr Pobezinksy continued: “This was very interesting because when microRNAs were discovered more than 20 years ago, people thought it was a product of RNA degradation. They were considered to be used fragments, like dust. They are so tiny, nobody paid attention to them. But since then, people have gradually been discovering what they really do. Our work is continuing that.”

He explained that normally RNA codes proteins, but microRNA does not. Instead, the tiny RNA snips found in mammals have regulatory activity on the whole genome.

Dr Pobezinsky said: “The specific microRNA known as Lethal 7 or let-7 is a very ancient RNA that existed in the first eukaryotes and has been conserved throughout evolution. Humans and animals have multiple genes that code for it instead of the usual one; only the most important genes are duplicated during evolution.”

This series of experiments was intiated by the observation that T-cells produce a lot of let-7 molecules and when T cells are in their inactive state, there are no pathogens present.

Our T cells are full of these let-7 cells. But the moment they see an antigen, suddenly the let-7s are gone. So the question is what do they regulate and why do they need to disappear?

The researchers hypothesised that when a threat is sensed, let-7 molecules, which were previously present when the immune system was inactive, disappear. This allows T cells to become functionally cytotoxic and able to clear pathogens, including tumour cells.

Dr Pobezinsky continued: “Our hypothesis turns out to be correct, in fact the microRNAs work as a brake on the cytotoxic T cells when there is no antigen present, so when we are healthy, they rest. As soon as they are gone, T cells initiate differentiation into cytotoxic T lymphocytes.”

If you keep let-7, T cells cannot become cytotoxic even in the presence of a tumour or virus. If you have none or almost none, function is enhanced. Nobody knew this before. We also figured out the molecular pathway using transcription factors that regulate the T cell differentiation and confirmed that let-7 microRNA is the most critical control.”

To kill invaders, T cells inject a toxic molecules or granzymes into a cancer or virally infected cell, this initiates apoptosis, or programmed cell death.

In experiments, the researchers found the complete absence of let-7 yielded the strongest differentiation of T cells to killer status.

The researchers hope this might lead to the ability to modulate immune responses that are regulated by CD8 cells and are testing it on mouse tumour models to try to enhance immune response against tumours using this technique.

Dr Pobezinsky concluded: “We would like to develop a way to suppress or enhance immune response. We might be able to combine this with adaptive immunotherapy to enhance immune function, so we would use a person’s own T cells, treat them in vitro, then to put back super killer T cells to boost their immune response.

"It is very promising, I feel it is a real possibility to go from this fundamental research and have an immediate application. For us it is very, very satisfying to do something for society.”

The research paper is available from the open access journal ELife.