Jeremy Baldwin, now a postdoctoral researcher at the Leibniz Institute for Immunotherapy, remembers when he first saw the cancer-treating potential of T cell-based therapies. “Tumors that were the size of maybe baseballs just dissolve away, just melt away like butter,” he said. It was enough to convince him to switch fields from regenerative medicine to cancer immunotherapy.
Now, Baldwin has teamed up with Luca Gattinoni, who has himself spent more than 20 years exploring strategies for enhancing T cells’ capabilities. In a recent Cell study, Baldwin and Gattinoni shared the results of their collaboration thus far: Mesenchymal stem cells (MSCs) in bone marrow can donate mitochondria to and consequently boost the energy levels of killer T cells (1).
The history of intercellular mitochondrial transfer research shares a trajectory with Baldwin’s professional journey. Almost 20 years ago, University of Vermont stem cell biologist Jeffery Spees and his collaborators discovered that MSCs can rescue damaged cells by giving them functional mitochondria, pointing to a new strategy for regenerative tissue repair (2). About a decade later, a team led by Malaghan Institute of Medical Research cancer biologist Mike Berridge found that metastatic tumor cells take advantage of this therapeutic strategy by stealing the organelles from their healthy neighbors (3).
Jeremy Baldwin (right) joined Luca Gattinoni (left) and his lab to improve T cell functionality with organelle transplantation.
credit: Jeremy Baldwin
Since Berridge published his work in 2015, evidence has emerged that tumors carry out these mitochondrial heists both for their own benefit and to the detriment of their marks. In 2022, two independent studies — one by Baldwin and Gattinoni — established that cancer cells connect with killer T cells via nanotubes and siphon out mitochondria, draining the immune cells of their energy (4,5).
“It’s like a tug-of-war,” said Washington University in St. Louis immunologist Jonathon Brestoff, who was not involved in the new study. “It’s really about which cell types have the best fitness advantage.”
Gattinoni’s research group had observed MSCs forming nanotubes with and donating mitochondria to T cells under a scanning electron microscope, but they brought in Baldwin because they needed his tissue engineering expertise to improve their culture system. “The problem was that their transfer rate was initially very low,” said Baldwin. “I had worked a lot with developing co-culture systems.” Within a few months, Baldwin had devised a membrane-separated well system that allowed MSCs and T cells to interact with each other even as the two cell types occupied distinct media suited to their own metabolic needs.
Baldwin explained that the mitochondrial transfer process boils down to three basic steps. The recipient cell first signals that it needs mitochondria. The two cells then build a nanotube bridge to connect with each other, and finally proteins shuttle the organelles from one cell to the other. He and Gattinoni’s team focused on the middle step and used RNA sequencing to see if there were any proteins that made MSCs and T cells more likely to build nanotubes. They found that T cells that failed to take in MSC-derived mitochondria had lower levels of the protein Talin-2, which helps assemble the cytoskeletal elements that enable cells to initiate protrusions from their membranes.
Baldwin and Gattinoni used CRISPR to delete Talin-2 from MSCs and T cells and found that losing its expression in either cell type was enough to gum up the works. “Basically, the transfer rate goes down a lot,” said Baldwin.
Brestoff was excited to see progress toward a more complete understanding of nanotube-mediated mitochondrial transfer. “They have identified a new protein that’s involved in that process,” he said.
Still, Baldwin and Gattinoni wanted to know if the donated mitochondria actually mattered in the specific context of T cell-based cancer therapy, so, they checked if the recipient T cells produced more energy after receiving the mitochondria. Using a respirometer, the researchers found that T cells that had received functional organelles consumed oxygen at higher rates than T cells that either had not received any mitochondria or had received damaged mitochondria from MSC donors.
Tumors that were the size of maybe baseballs just dissolve away, just melt away like butter.”
-Jeremy Baldwin, Leibniz Institute for Immunotherapy
Then, Baldwin and Gattinoni injected T cells with and without MSC-derived mitochondria into mice with tumors. Although both sets of T cells proliferated in the mice’s spleens over the course of a week, RNA sequencing and flow cytometry revealed that those mice with only their own mitochondria had higher levels of proteins associated with stress and cell death than their souped-up counterparts. Furthermore, T cells with MSC-derived mitochondria had more energy, as made evident by their ability to synthesize new proteins from amino acids. This latter effect was even more apparent for T cells found near a mouse’s tumor than for cells collected from the animal’s spleen.
Finally, Baldwin and Gattinoni tested their new method on two types of human immune cells that the Food and Drug Administration has approved for cancer treatment. After spending time in culture with MSCs, chimeric antigen receptor (CAR) T cells were better at eliminating leukemia in vitro and in mice, and tumor-infiltrating lymphocytes (TILs) were better at fighting melanoma in vitro.
Brestoff was impressed with Baldwin and Gattinoni’s results, and he was intrigued by the possibility that T cells and even other types of immune cells might naturally take advantage of mitochondrial transfer in the bone marrow. “Does it touch all lineages, or is it just the T cells?” he wondered. “There’s a lot more interesting biology that needs to be tackled.”
As a tissue engineer, Baldwin is more focused on refining the transfer protocol for applied medicine. Right now, he’s interested in modifying mitochondria to direct where the donated organelles end up in a recipient cell and in improving MSCs so that they deliver higher quality mitochondria. “We pre-treat them, and they’re basically supercharged,” said Baldwin. “There’s a lot of potential.”
References
- Baldwin, J.G. et al. Intercellular nanotube-mediated mitochondrial transfer enhances T cell metabolic fitness and antitumor efficiency. Cell 187, 6614-6630.E21 (2024).
- Spees, J.L. et al. Mitochondrial transfer between cells can rescue aerobic respiration. Proc Natl Acad Sci 103, 1283-1288 (2006).
- Tan, A. et al. Mitochondrial genome acquisition restores respiratory function and tumorigenic potential of cancer cells without mitochondrial DNA. Cell Metab 21, 81-94 (2015).
- Saha, T. et al. Intercellular nanotubes mediate mitochondrial trafficking between cancer and immune cells. Nat Nanotechnol 17, 98-106 (2022).
- Baldwin, J.G. and Gattinoni, L. Cancer cells hijack T cell mitochondria. Nat Nanotechnol 17, 3-4 (2022).
