Scientists are now trying to find ways to help treat patients by jumpstarting those lacklustre T cells. Much of the research in this field, known as cancer immunotherapy, has focused on finding ways to stimulate those T cells directly. MIT researchers have now uncovered a possible new way to indirectly activate those T cells, by recruiting a population of helper immune cells called dendritic cells. In a new study, the researchers identified a specific subset of dendritic cells that have a unique way of activating T cells. These dendritic cells can cloak themselves in tumour proteins, allowing them to impersonate cancer cells and trigger a strong T cell response.
"We knew that dendritic cells are incredibly important for the antitumor immune response, but we didn't know what really constitutes the optimal dendritic cell response to a tumour," said Stefani Spranger, the Howard S. and Linda B. Stern Career Development Professor at MIT and a member of MIT's Koch Institute for Integrative Cancer Research. The results suggested that finding ways to stimulate that specific population of dendritic cells could help to enhance the effectiveness of cancer immunotherapy, she said. In a study of mice, the researchers showed that stimulating these dendritic cells slowed the growth of melanoma and colon tumours.
Spontaneous regression When tumours begin to form, they produce cancerous proteins that T cells recognize as foreign. This sometimes allows T cells to eliminate tumours before they get very large. In other cases, tumours are able to secrete chemical signals that deactivate T cells, allowing the tumours to continue growing unchecked.
Dendritic cells are known to help activate tumour-fighting T cells, but there are many different subtypes of dendritic cells, and their individual roles in T cell activation are not fully characterized. In this study, the MIT team wanted to investigate which types of dendritic cells are involved in T cell responses that successfully eliminate tumours. To do that, they found a tumour cell line, from a type of muscle tumour that has been shown to spontaneously regress in mice. Such cell lines are difficult to find because researchers usually don't keep them around if they can't form tumours, Spranger said.
Studying mice, they compared tumours produced by that regressive cell line with a type of colon carcinoma, which forms tumours that grow larger after being implanted in the body. The researchers found that in the progressing tumours, the T cell response quickly became exhausting, while in the regressing tumours, T cells remained functional. The researchers then analyzed the dendritic cell populations that were present in each of these tumours. One of the main functions of dendritic cells is to take up debris from dying cells, such as cancer cells or cells infected with a pathogen, and then present the protein fragments to T cells, alerting them to the infection or tumour.
The best-known types of dendritic cells that are required for antitumour immunity are DC1 cells, which interact with T cells that are able to eliminate cancer cells. However, the researchers found that DC1 cells were not needed for tumour regression. Instead, using single-cell RNA sequencing technology, they identified a previously unknown activation state of DC2 cells, a different type of dendritic cell that was driving T cell activation in the regressing tumours. The MIT team found that instead of ingesting cellular debris, these dendritic cells swiped proteins called MHC complexes from tumour cells and displayed them on their own surfaces. When T cells encountered these dendritic cells masquerading as tumour cells, the T cells became strongly activated and began killing the tumour cells.
This specialized population of dendritic cells appeared to be activated by type one interferon, a signalling molecule that cells usually produced in response to viral infection. The researchers found a small population of these dendritic cells in colon and melanoma tumours that progressed, but they were not properly activated. However, if they treated those tumours with interferon, the dendritic cells began stimulating T cells to attack tumour cells. Targeted therapy Some types of interferon have been used to help treat cancer, but it can have widespread side effects when given systemically. The findings from this study suggested that it could be beneficial to deliver interferon in a much-targeted way to tumour cells or to use a drug that would provoke tumour cells to produce type I interferon, Spranger said. The researchers now plan to investigate just how much type I interferon is needed to generate a strong T cell response. Most tumour cells produced a small amount of type I interferon but not enough to activate the dendritic cell population that invigorates T cells. On the other hand, too much interferon can be toxic to cells.
"Our immune system is hardwired to respond to nuanced differences in type I interferon very dramatically, and that is something that is intriguing from an immunological perspective," Spranger said.