Immune checkpoint blockades (ICBs) have transformed the treatment landscape for many advanced cancers. Nevertheless, their effectiveness has leveled off due to cancer cells developing resistance, which limits the ability of tumor-infiltrating lymphocytes (TILs) to fight tumor cells. Therefore, a critical objective for cancer specialists is to find methods to overcome this resistance and revitalize TILs to enhance their tumor-fighting capabilities. Any potential strategy must consider the unique challenges posed by the cancer microenvironment, which is often low in oxygen because of rapid tumor growth and abnormal blood vessel formation.
Immune checkpoint blockades (ICBs) have transformed the treatment landscape for many advanced cancers. Nevertheless, their effectiveness has leveled off due to cancer cells developing resistance, which limits the ability of tumor-infiltrating lymphocytes (TILs) to fight tumor cells. Therefore, a critical objective for cancer specialists is to find methods to overcome this resistance and revitalize TILs to enhance their tumor-fighting capabilities. However, any intervention must take into account the unique challenges presented by the cancer microenvironment, which is often low in oxygen due to the rapid growth of tumors and the abnormal formation of blood vessels.
A recent study published in Nature Communications reveals, for the first time, how HIF1α in T cells is essential for producing interferon gamma (IFN-γ) in low-oxygen circumstances. IFN-γ is a cytokine crucial for activating T cells to destroy tumors. The researchers also identified that glycolysis, a metabolic process that generates energy in human cells when oxygen is scarce, is necessary for the production of IFN-γ in T cells.
“Interestingly, in normal oxygen levels, referred to as normoxia, the induction of IFN-γ and glycolysis in T cells is regulated not by HIF1α, a main regulator of glycolysis, but rather by its commonly accepted downstream target LDHa, as previously reported by other researchers,” explained Shi, a professor in the UAB Department of Radiation Oncology. “However, it remained unclear how HIF1α contributes to IFN-γ production and glycolysis in T cells under low-oxygen conditions.”
The team at UAB discovered that HIF1α and glycolysis are vital for IFN-γ production in T cells exposed to low oxygen levels. HIF1α is a component of HIF, or hypoxia-inducible factor, crucial for managing how cells respond to low-oxygen scenarios.
Shi and his colleagues demonstrated this significant function of HIF1α in low-oxygen settings using a combination of genetic mouse models, metabolic tracing techniques with 13C-labeled glucose, and a Seahorse analyzer, alongside pharmacological methods.
In both human and mouse T cells under low-oxygen conditions, the deletion of HIF1α limited the shift in T cell metabolism from breaking down substances for energy to building them up, which includes anaerobic glycolysis. This deletion also inhibited the production of IFN-γ. Additionally, blocking glycolysis in T cells under hypoxia also hindered the production of IFN-γ. On the other hand, increasing HIF1α stability by removing a negative regulator enhanced IFN-γ levels in low-oxygen conditions.
The findings regarding cancer defense indicated that hypoxic T cells lacking HIF1α were less effective in killing cancer cells in laboratory settings. In live mice, those with HIF1α-deficient T cells did not respond to ICB therapy.
The researchers then found a method to counteract resistance to ICB therapy. By exploring the specific functions of HIF1α loss, they revealed that the absence of HIF1α significantly reduced glycolytic activity in low-oxygen T cells, leading to low levels of intracellular acetyl-CoA and decreased activation-induced cell death (AICD). Replenishing intracellular acetyl-CoA by adding acetate to the growth medium restored AICD and boosted IFN-γ production in the hypoxic T cells missing HIF1α.
Shi and his team showed in live mice that acetate supplementation was an effective way to overcome ICB resistance in tumor-bearing mice with HIF1α-deleted T cells. When these mice received acetate along with combined ICB therapy, they experienced notable improvements in treatment response, evident in marked tumor growth suppression and reduced tumor weights.
“TILs and tumor cells utilize similar metabolic pathways and inhabit challenging tumor microenvironments characterized by low oxygen and limited nutrients, which creates a tough metabolic competition,” said Shi. “Successfully directing this metabolic struggle to favor TILs is crucial, and our findings demonstrated that acetate supplementation reinstated IFN-γ production in TILs lacking HIF1α, effectively overcoming ICB resistance.”
“Our research, alongside earlier studies, strongly indicates that impaired HIF1α functionality in T cells is a major internal mechanism behind their resistance to ICB therapies, such as anti-CTLA-4 and anti-PD-1/L1,” Shi noted.
The study, titled “HIF1α-regulated glycolysis promotes activation-induced cell death and IFN-γ induction in hypoxic T cells,” includes co-authors Hongxing Shen, Oluwagbemiga A. Ojo, Haitao Ding, Chuan Xing, Abdelrahman Yassin, Vivian Y. Shi, Zach Lewis, Ewa Podgorska, and James A. Bonner from the UAB Department of Radiation Oncology, Logan J. Mullen from the University of Alaska Fairbanks, M. Iqbal Hossain, Shaida A. Andrabi from UAB Department of Pharmacology and Toxicology, and Maciek R. Antoniewicz from the University of Michigan, Ann Arbor.
Support for this research was provided by UAB; the O’Neal Comprehensive Cancer Center at UAB; NIH grants CA230475-01A1, CA25972101A1, CA279849-01A1; the V Foundation Scholar Award V2018-023; the Department of Defense’s Congressionally Directed Medical Research Programs grant ME210108; and the Cancer Research Institute’s CLIP Grant CRI4342.
UAB Radiation Oncology and Pharmacology and Toxicology are programs in the Marnix E. Heersink School of Medicine. Shi is a researcher at the O’Neal Comprehensive Cancer Center and is endowed with the Koikos-Petelos-Jones-Bragg ROAR Professorship for Cancer Research.
