Researchers at the UCLA Health Jonsson Comprehensive Cancer Center used a CRISPR screen to identify a key molecular pathway that drives exhaustion in cancer-fighting immune cells, limiting their ability to keep tumors under control.
In the study , published in the journal Cancer Immunology Research, a journal of the American Association for Cancer Research, investigators found that blocking the pathway, known as the Cullin-5 RING E3 ligase complex (CRL5), allowed engineered immune cells to stay active longer, produce stronger anti-tumor responses and resist exhaustion in preclinical models.
The discovery could help researchers develop longer-lasting immunotherapies for solid tumors, including CAR T-cell therapies and other forms of adoptive cell therapy.
Why it matters
T-cell exhaustion is one of the main reasons many immunotherapies stop working or fail to produce lasting responses. T cells are the immune system's primary cancer-fighting cells, but in many cancers they are exposed to tumor signals for long periods, gradually losing their ability to effectively attack cancer.
This T cell exhaustion is especially problematic in solid tumors, where therapies such as CAR-T cells, tumor-infiltrating lymphocyte therapies and T-cell receptor-engineered therapies often struggle to remain effective over time.
"Immunotherapy depends on T cells staying active long enough to continuously recognize and kill cancer cells," said Justin Saco, a graduate student in the laboratory of Dr. Antoni Ribas at UCLA and first author of the study. "If they lose function too early, tumors can grow back or escape immune control. The longer T cells remain functional, the better the chance of durable cancer control."
What the study did
To identify the genes involved in T-cell exhaustion, the UCLA team used a genome-wide CRISPR screen in human CD8+ T cells engineered to recognize a melanoma antigen. The researchers systematically turned off thousands of genes and repeatedly exposed the CD8+ T cells to tumor cells in a laboratory system designed to mimic the chronic tumor exposure seen in patients with solid tumors.
"One unique aspect of the study was the repeated tumor rechallenge system," said Saco. "Instead of studying short-term T-cell activation, we modeled the chronic, repeated tumor exposure that T cells experience in solid tumors. That allowed us to identify regulators of long-term persistence and exhaustion more effectively."
The researchers then analyzed which genetic changes helped T cells stay functional, active and capable of killing cancer cells during repeated tumor exposure.
What they found
The screening process revealed a strong and consistent signal pointing to CRL5, a protein-regulating pathway that helps control how cells break down and recycle proteins. Importantly, it was not just a single gene that stood out. Several different components of the CRL5 machinery were independently enriched across experiments and time points, a pattern that often suggests a biologically important pathway.
When researchers disrupted CRL5-related genes, the modified T cells maintained stronger tumor-killing ability, produced higher levels of immune signaling molecules, showed fewer molecular signs of exhaustion and preserved gene programs linked to survival and long-term immune function.
The team also tested a drug called MLN4924, which blocks activation of Cullin-RING ligases, including CRL5. The drug partially reproduced the effects seen with genetic deletion, helping T cells maintain stronger long-term function in laboratory experiments. In mouse models of human melanoma, animals treated with CRL5-deficient T cells showed improved tumor control compared with those receiving standard engineered immune cells.
Impact of findings
The findings suggest that targeting the CRL5 pathway could potentially help improve the durability and effectiveness of T-cell–based immunotherapies for patients with solid tumors. By reducing a key biological mechanism that drives T-cell exhaustion, it may be possible to help immune cells stay active longer and maintain their cancer-fighting ability, potentially leading to stronger and more sustained treatment responses.
A companion research article by the research group of Dr. Julia Carnevale of UCSF provides additional support for the findings and the potential for targeting molecular mechanisms of T-cell exhaustion to improve cancer immunotherapy.
About the research team
The study's senior author is Dr. Antoni Ribas , professor of medicine at the David Geffen School of Medicine at UCLA and director of the UCLA Health Jonsson Comprehensive Cancer Center's Tumor Immunology & Immunotherapy Program. Ribas is also the director of the Parker Institute for Cancer Immunotherapy Center (PICI) at UCLA and a member of the Eli and Edythe Broad Center of Regenerative Medicine and Stem Cell Research. Other UCLA authors include Daniel Karin, J.J. Patten, Yichen Fan, Egmidio Medina, Nataly Naser Al Deen, Ameya Champhekar, Katie Campbell and Cristina Puig-Saus.
Funding
The study was funded in part by PICI, the National Institutes of Health and philanthropic support.
