Mitochondrial “swap” in the Tumor Microenvironment
Yosuke Togashi , Professor, Okayama University
Yosuke Togashi, M.D., Ph.D., is a Professor at Okayama University, specializing in tumor microenvironment research. He trained in respiratory medicine, earned his Ph.D. in genome biology, and began cancer immunology research in 2016.
Cancer cells evade immunity by transferring mutated mitochondria to tumor-infiltrating lymphocytes, disrupting metabolism and function. This transfer induces ROS production, senescence, and PD-1 blockade resistance. Blocking transfer restores TIL function, and tumor mtDNA mutations predict poor immune checkpoint inhibitor response.
1. How do cancer cells facilitate the transfer of mutated mitochondria to tumor-infiltrating lymphocytes (TILs), and what cellular mechanisms regulate this exchange?
Ans: The primary modes of transfer are extracellular vesicles (EVs) and tunneling nanotubes (TNTs). Regarding mitochondrial replacement, TILs have high mitophagy sensitivity due to ROS, causing their mitochondria to be rapidly degraded, whereas mitochondria derived from cancer cells are more resistant to mitophagy, leading to preferential replacement.
2. What specific mutations in mitochondrial DNA (mtDNA) contribute to the disruption of TIL metabolism, and how do they affect oxidative phosphorylation and glycolysis?
Ans: There are various mtDNA mutations, and we have not examined all of them. Our primary analysis focused on a tRNA mutation, which resulted in a widespread reduction in mtDNA-encoded protein expression.
3. How does the presence of mutated mitochondria in TILs lead to the production of reactive oxygen species (ROS), and what are the downstream effects on immune cell function?
Ans: Mitochondria are both a source of ROS and responsible for redox balance. When their function is impaired, ROS levels increase, leading to oxidative stress, which accelerates cellular senescence and reduces TIL functions.
4. What role does mitochondrial transfer play in promoting TIL senescence, and how does this impact the overall immune response against tumors?
Ans: Increased ROS due to mitochondrial dysfunction accelerates cellular senescence. TIL senescence results in decreased proliferation and a higher tendency toward apoptosis. Additionally, their abilities of activation and memory formation are impaired, reducing the overall immune response against tumors.
5. In what ways does mitochondrial dysfunction in TILs contribute to resistance against PD-1 blockade therapy, and what evidence supports this connection?
Ans: TIL dysfunction leads to inadequate activation upon PD-1 blockade, reducing therapeutic efficacy. Memory formation was particularly impaired. In a mouse model, rechallenged tumors after PD-1 blockade were less likely to be rejected. Additionally, clinical data showed that mtDNA mutations hindered long-term responses to PD-1 blockade.
6. Are there specific tumor-derived factors or exosomes that mediate mitochondrial transfer, and how can their inhibition improve TIL function?
Ans: We have not investigated the specific factors in detail. However, EV inhibition suppressed mitochondrial transfer, leading to improved TIL function.
7. What experimental models have been used to study mitochondrial transfer in the tumor microenvironment, and what are the key findings from these studies?
Ans: We used tumor cells engineered to express a mitochondria-specific reporter protein and analyzed the tumor microenvironment (TME). We demonstrated time-dependent mitochondrial transfer, homoplasmic replacement, and TIL dysfunction.
8. How do different types of cancer vary in their ability to transfer mitochondria to immune cells, and what factors influence these variations?
Ans: We have not extensively studied this, but it is likely to occur more frequently in cancers with strong ROS involvement.
9. What potential therapeutic strategies can be developed to block mitochondrial transfer, and how might they enhance the efficacy of immune checkpoint inhibitors?
Ans: Blocking mitochondrial transfer or suppressing ROS may improve TIL function and increase immune checkpoint inhibitor (ICI) efficacy.
10. Are there biomarkers that can predict which patients will experience mitochondrial transfer-related immune suppression, and how can they be integrated into clinical decision-making?
Ans: Detecting mtDNA mutations could serve as a useful biomarker and should be considered in treatment selection.
11. What are the implications of tumor-derived mitochondrial mutations on systemic immune responses beyond the tumor microenvironment?
Ans: This remains unclear and is a topic for future research. However, it is possible that this phenomenon is specific to the unique environment of tumors.
12. How do metabolic adaptations in tumor cells facilitate mitochondrial exchange, and can targeting these pathways disrupt the process?
Ans: Oxidative phosphorylation is impaired. Several mitochondrial-activating drugs that enhance oxidative phosphorylation exist, and we would like to explore their potential.
13. Could engineered mitochondria or mitochondrial replacement therapies be explored to restore TIL function, and what challenges exist in their implementation?
Ans: We are very interested in this approach, and it appears to have strong potential (i.e., mitochondrial transplantation).
14. Given the emerging understanding of mitochondrial transfer in cancer, how should future immunotherapy strategies be designed to account for this mechanism?
Ans: Similar to question 12, mitochondrial-activating drugs that enhance oxidative phosphorylation could be tested in combination with ICIs. Additionally, inhibiting ROS and mitochondrial transfer could be explored as combination strategies.
