Researchers discover a new mechanism for activating cancer-killing T cells
Researchers have made significant progress in the discovery and administration of cancer immunotherapies, which employ the body’s own immune system to treat disease, during the last decade. The medicines, however, do not work for everyone or with every form of cancer, and gaps in our knowledge of how the body develops an anti-cancer immune response have hampered progress toward making them universally successful.
In a recent study, researchers from the University of Chicago Medicine Comprehensive Cancer Center and the University of Amsterdam shed light on a critical phase in the anti-cancer immune response process: T cell priming.
Previous research suggested that a single mechanism — antigen cross presentation — is responsible for preparing T cells, the immune system’s disease fighters, to recognize and destroy cancer cells. The new study discovered that a second mechanism known as MHC-I cross-dressing is also effective in inducing a T cell response.
“What’s significant is that we identified a totally unique pathway whereby tumors and the immune system talk to each other,” said Justin Kline, MD, an associate professor of medicine at UChicago Medicine and an author of the study. “Knowing that this pathway exists might have implications for how we think about vaccine design in the future or how we predict which tumor antigens might be best to target.”
The study was recently published in the journal Immunity.
‘Dressing’ in molecules
Kline and his colleagues investigated the function of dendritic cells in the immunological response to cancer. These cells alert the immune system to antigens – toxins and other foreign substances in the body – and activate T cells.
As previously stated, researchers assumed that antigen cross presentation was the only method by which dendritic cells communicated with T cells. Cross presentation occurs when a dendritic cell “eats” a cancer cell and then shows what it has eaten so that T cells may determine whether any antigens are present.
The mechanism Kline and his team identified simply requires dendritic cells to “dress themselves” with a tumor cell’s molecules to alert T cells to the disease.
The researchers realized the potential existence of this second mechanism by an unexpected observation in the lab.
“The initial discovery came about sort of serendipitously when we knocked out a specific MHC class 1 molecule on mouse tumor cell lines and found that the immune response against those tumors was significantly and negatively impacted despite the fact that cross presentation was fully intact,” said Kline. “It indicated to us that a second mechanism was operational, and we wanted to investigate it further.”
Identifying a new mechanism
To do this, the researchers genetically engineered dendritic cells in mouse models to suppress the expression of MHC-1 molecules, whose major function is to display tumor-associated antigens. They then injected the models with one of two types of cancer tumors. The first type, melanoma, has very low levels of MHC-1 molecules. The second type, leukemia, has very high levels.
Once injected, they used flow cytometry to measure the presence of MHC-1 molecules on the dendritic cells and found that the cells had taken on – or dressed themselves – in the molecules. Furthermore, they discovered that the dendritic cells of the models injected with leukemia showed significant amounts of the molecule, while the ones injected with melanoma had somewhat less.
“We knew that any MHC-1 molecule on a dendritic cell had to come from the tumor because there’s no other MHC-1 molecules in the system,” said Kline. “It showed definitively that cross-dressing was taking place.”
It was also significant that the amount of MHC-1 on the dendritic cells differed based on tumor type, Kline said. It indicates that this pathway may be more important in cancer tumors with high levels of class 1 molecules, and less important in those with low levels.
To realize any practical implications of this work, more research is necessary. Next, the team plans to investigate how MHC-1 cross-dressing happens – the molecular mechanisms behind it – and the extent to which cross-dressing can impact dendritic cells’ ability to spur T cells into action.
The research was funded by the National Institutes of Health, the University of Chicago, and the American Association of Immunologists.
Reference: “Dendritic cells can prime anti-tumor CD8+ T cell responses through major histocompatibility complex cross-dressing” by Brendan W. MacNabb, Sravya Tumuluru, Xiufen Chen, James Godfrey, Darshan N. Kasal, Jovian Yu, Marlieke L.M. Jongsma, Robbert M. Spaapen, Douglas E. Kline and Justin Kline, 25 May 2022, Immunity.