Immune Cells Use Previously Unknown TCR-Dependent, MHC-Independent Pathway to Eliminate AML
Key Insights
Researchers at MD Anderson Cancer Center discovered a novel T-cell killing mechanism that eliminates AML cells without requiring MHC molecules, challenging decades of immunological understanding.
The alternative pathway is TCR-dependent but MHC-independent, relying on the CD64 (search) protein and interferon-gamma signaling, as identified through genome-wide CRISPR screening.
Activated T cells successfully eliminated AML cells across multiple cell lines and primary patient samples, including those with high-risk mutations and relapsed disease.
Researchers at The University of Texas MD Anderson Cancer Center have identified a previously unknown mechanism by which T cells attack and eliminate acute myeloid leukemia (search) (AML) cells, a discovery that challenges a fundamental assumption about how T cells recognize and kill tumor cells. The study, published in the Proceedings of the National Academy of Sciences (PNAS), was led by Cassian Yee, M.D., professor of Melanoma Medical Oncology and Director of the Program for T-Cell-Based Therapies (ProTCT), and Kapil Saxena, M.D., former member of the Yee laboratory.
The findings reveal that activated T cells can destroy AML cells through an alternative pathway that is MHC-independent yet TCR-dependent — meaning T cells still require an intact and activated T cell receptor (search) signaling system, but they are not using it in the conventional way scientists have understood for decades.
A contrarian observation drives discovery
T cells typically recognize cancer through their T cell receptor (search) (TCR), which binds to specific cancer-related peptides displayed on major histocompatibility complex (MHC) molecules. The research team expected that removing MHC from AML cells would prevent T cell attack. Instead, activated T cells were somehow still able to efficiently eliminate the AML cells.
“By chasing a simple, unexplained observation, we challenged a basic assumption about how T cells can recognize and kill tumor cells,” Yee said. “The freedom to explore things that aren’t conventional is what drives innovation, and this contrarian finding is an example of that, with broad implications for overcoming immune resistance in leukemia and possibly other cancers.”
This pattern of MHC-independent killing was observed in multiple AML cell lines and in primary samples from patients with AML, including those with high-risk mutations and relapsed disease.
Ruling out NK cell pathways
The researchers initially suspected that this mechanism might be driven by pathways associated with natural killer (NK) cells, which are not dependent on MHC molecules. However, disrupting several known NK cell pathways did not eliminate the effect. Since MHC molecules did not seem to be required, the team further hypothesized that TCRs also were unnecessary for this alternative killing pathway. Surprisingly, when the researchers removed TCRs, the T cells lost their ability to destroy the leukemia cells — confirming the pathway is TCR-dependent.
CRISPR screen identifies CD64 (search) as key driver
After testing several known immune pathways, the researchers performed a genome-wide CRISPR screen, repeatedly exposing genetically modified AML cells to T cells to search for genes that allowed the leukemia cells to survive. This approach highlighted CD64 (search), a receptor commonly expressed on early myeloid cells, as well as genes involved in interferon-gamma signaling.
Removing CD64 (search) made AML cells resistant to T cells, while adding CD64 into normally resistant AML cells made them more susceptible, establishing CD64 as a critical component of this noncanonical killing mechanism.
Clinical implications for AML treatment
AML is an aggressive blood cancer that originates in the bone marrow and can progress rapidly. The current standard of care includes stem cell transplantation with chemotherapy, yet not all patients experience positive results and many who do eventually relapse. A key challenge is that cancer cells can evade immune attack by reducing or losing MHC molecules, which normally help T cells identify their targets.
The newly discovered pathway may help explain why AML is particularly sensitive to immune-based therapies, including stem cell transplantation. The researchers continue to investigate exactly how CD64 (search) and the TCR signaling machinery interact, with future studies exploring how these mechanisms might be leveraged to improve T cell therapies for AML and possibly other cancer types.