Removing a key gene could revive depleted immune cells to fight solid tumors

Removing a single gene can transform depleted anti-cancer immune cells called CD8⁺ T cells again become regenerated soldiers who can continue to fight malignant tumors, suggests a new study by researchers at UT Southwestern. The results, published online this week in the Journal of Cancer Immunotherapy, could offer a new way to harness the body’s immune system to attack cancers.

In 2017, the Food and Drug Administration approved treatments involving chimeric antigen T receptor (CAR-T) cells, which consist of immune cells called T cells that were designed to recognize specific proteins on the surface of cancer cells. When these cells are delivered by infusion, they develop a targeted immune response against cancer cells.

CAR-T cells have been used successfully to treat blood cancers such as leukemia and lymphoma, often with dramatic results, says Venuprasad Poojary, Ph.D., associate professor of internal medicine and immunology at the ‘UTSW. However, these treatments have had little success against solid tumors, such as those that develop in the colon, breast, and lungs.

When CAR-T cells enter these solid tumors, Poojary explains, they quickly become dysfunctional and lose their cancer-fighting abilities. This condition, called “depletion”, is accompanied by the addition of proteins, including PD1 and Tim3 to their surfaces and an inability to produce their usual immune molecules, such as interferon-gamma and tumor necrosis factor. Finding a way to prevent the depletion of CAR-T cells has become an important goal in cancer research, Poojary adds.

To that end, he and his colleagues scanned published research comparing gene activity in active and depleted T cells. Researchers quickly focused on a gene called Cbl-b, which is more active, or upregulated, in depleted cells.

Poojary and his colleagues have confirmed that Cbl-b was activated in T cells that infiltrated tumors in a mouse model of colon cancer. Not only did these cells lose their tumor-fighting abilities, they also developed a characteristic set of cell surface proteins and an inability to produce or express immune molecules characteristic of exhaustion. However, when scientists used the CRISPR gene editing tool to remove Cbl-b in these cells, they regained their ability to fight cancer and lost other characteristics of exhaustion.

Adding evidence to Cbl-bA key role in T cell depletion, the researchers used genetically engineered mice in which this gene was turned off. When they transplanted cancer cells into these animals, the cells developed significantly smaller tumors than in mouse cancer cells with Cbl-b.

Other experiments have shown that removing Cbl-b also prevented depletion specifically in CAR-T cells, adds Poojary. When he and his team suppressed this gene from CAR-T cells designed to recognize carcinoembryonic antigen, a cell surface protein commonly overexpressed in colon cancer, the cells effectively fought cancer in mice carrying these tumors, which considerably prolonged their survival. However, CAR-T cells functioning Cbl-b quickly became unnecessary, providing little anti-tumor effect in animal models.

Poojary and his colleagues are working to understand the molecular mechanisms behind Cbl-b causes depletion of T cells. Once researchers better understand this mechanism, he notes, using CAR-T cells to effectively treat solid tumors could be as simple as genetically modifying them without Cbl-b.

Our study is a major advance in the development of CAR-T cells to fight against solid tumors. This could overcome the limitations of some current immunotherapy strategies for cancer. “

Venuprasad Poojary, Ph.D., associate professor of internal medicine and immunology, UTSW