Mitochondrial Transfer and Immune Evasion Mechanisms in the Tumor Microenvironment

Academic Background

Tumor cells evade immune system attacks, particularly by T cells, through various mechanisms in the Tumor Microenvironment (TME). Although Immune Checkpoint Inhibitors (ICIs) have achieved significant progress in the treatment of various cancers, many patients do not respond to treatment or experience only transient responses. Studies have shown that metabolic reprogramming in the TME and mitochondrial dysfunction in Tumor-Infiltrating Lymphocytes (TILs) impair antitumor immune responses. However, the detailed mechanisms of these processes remain unclear. This study aims to reveal how tumor cells influence TIL function through mitochondrial transfer, thereby evading immune system attacks.

Source of the Paper

This paper was collaboratively completed by a team of scientists from multiple research institutions in Japan, including the Chiba Cancer Center Research Institute, Okayama University, and the University of Tokyo. The paper was published in Nature in 2024 under the title “Immune evasion through mitochondrial transfer in the tumour microenvironment.”

Research Process and Results

1. Discovery of Mitochondrial DNA (mtDNA) Mutations in TILs and Tumor Cells

The research team first sequenced the mtDNA of TILs from 12 patients with different types of cancer and found mtDNA mutations in the TILs of 5 patients. Further analysis revealed that these mutations were highly consistent with mtDNA mutations in tumor cells. Electron microscopy observations showed that TILs and tumor cells carrying mtDNA mutations exhibited abnormal mitochondrial morphology with reduced cristae, while TILs and tumor cells with wild-type mtDNA had normal morphology.

2. Mitochondrial Transfer from Tumor Cells to TILs

To verify the possibility of mitochondrial transfer, researchers introduced fluorescently labeled mitochondria (mitoDsRed) into tumor cells and co-cultured them with TILs. The results showed that mitochondria from tumor cells could transfer to TILs, with transfer efficiency significantly increasing after 24 hours. By inhibiting the formation of Tunneling Nanotubes (TNTs) and Extracellular Vesicles (EVs), the researchers found that these structures played important roles in the transfer process. Specifically, EVs smaller than 200 nm contained mitochondrial proteins, indicating that EVs could mediate mitochondrial transfer.

3. Mitochondrial Replacement to Homoplasmy

The researchers further investigated whether homoplasmic replacement would occur after mitochondrial transfer. Through single-cell sequencing and time-lapse imaging, they found that mitochondria in TILs were gradually replaced by mitochondria from tumor cells, eventually achieving homoplasmy. This process depended on Reactive Oxygen Species (ROS)-induced mitophagy produced by tumor cells. However, mitochondria from tumor cells, carrying mitophagy-inhibitory molecules such as USP30, could resist mitophagy, allowing them to survive in TILs and gradually replace the original mitochondria.

4. Impact of Mutant Mitochondria on T Cell Function

TILs carrying mtDNA mutations exhibited metabolic abnormalities and senescence characteristics, including reduced membrane potential, increased ROS levels, elevated β-galactosidase activity, and upregulated expression of senescence-related molecules (e.g., p16 and p53). Additionally, the effector functions and memory formation capabilities of these TILs were significantly impaired, as evidenced by reduced PD-1 and CD69 expression, increased apoptosis, and decreased proportions of central memory cells and long-lived cells.

5. In Vivo Experimental Validation

The researchers validated the impact of mitochondrial transfer on antitumor immunity in mouse models. The results showed that tumor cells carrying mtDNA mutations could impair TIL function through mitochondrial transfer, reducing the efficacy of PD-1 blockade therapy. By inhibiting EV release, the researchers successfully reversed this phenomenon, restoring TIL function and the efficacy of PD-1 blockade therapy.

Conclusions and Significance

This study reveals a novel mechanism by which tumor cells evade immune system attacks through mitochondrial transfer. Specifically, tumor cells transfer mitochondria carrying mutations to TILs via TNTs and EVs, leading to mitochondrial dysfunction and impaired immune responses in TILs. This discovery not only deepens our understanding of tumor immune evasion mechanisms but also provides potential targets for developing new cancer immunotherapies. In particular, inhibiting mitochondrial transfer or enhancing mitochondrial function in TILs may become a new strategy to improve the efficacy of immune checkpoint inhibitors.

Research Highlights

1. Discovery of a New Mechanism: The first to reveal the mechanism by which tumor cells evade immune system attacks through mitochondrial transfer.
2. Clinical Significance: The presence of mtDNA mutations is negatively correlated with the efficacy of PD-1 blockade therapy, providing a basis for patient stratification and treatment strategy optimization.
3. Innovative Methods: Precise tracking of mitochondrial transfer and replacement processes using fluorescently labeled mitochondria and single-cell sequencing technology.
4. Potential Therapeutic Targets: The discovery of mitophagy-inhibitory molecules such as USP30 provides direction for developing new anticancer drugs.

Additional Valuable Information

This study also found that mitochondrial transfer not only occurs between tumor cells and TILs but may also be widespread among other cell types. Future research could further explore the role of mitochondrial transfer in different tumor types and immune cells, as well as how to enhance antitumor immune responses by regulating mitochondrial function.