Cancer Immunotherapy Boosted by Intercellular Transfer of Mitochondria to T Cells

Fighting cancer is exhausting for T cells. Hostile tumor microenvironments (TMEs) can drain their mitochondrial activity, leading to a condition known as T-cell exhaustion, which ultimately can result in poor antitumor responses. This phenomenon of T-cell exhaustion also hinders adoptive cell therapies, in which healthy, tumor-targeting T cells are infused into patients with cancer.

Investigators from Brigham and Women’s Hospital, a founding member of the Mass General Brigham healthcare system, in collaboration with colleagues at Leibniz Institute for Immunotherapy, have now developed a way to “supercharge” CD8⁺ T cells by supplying them with extra mitochondria from bone marrow stromal cells (BMSCs) that connect to the T cells via extensions known as tunneling nanotubes (TNTs). The researchers discovered that the T cells supercharged with extra mitochondria exhibited heightened antitumor activity and reduced signs of exhaustion in preclinical models of cancer, suggesting that the technique could help improve existing immunotherapies.

“These supercharged T cells overcome one of the fundamental barriers of immunotherapy by penetrating the tumor and overcoming immune barren state in the tumor,” said Shiladitya Sengupta, PhD, at Brigham’s Department of Medicine. “Mitochondria provide the fuel. It’s like we’re taking T cells to the fuel station and gassing them up. This transplanting of mitochondria is the dawn of organellar therapy where an organelle is delivered to a cell to make it more effective.”

Sengupta is corresponding author of the researchers’ published paper in Cell, titled, “Intercellular nanotube-mediated mitochondrial transfer enhances T cell metabolic fitness and antitumor efficacy.” In their paper, the investigators stated, “We found that TNTs enable effective mitochondrial transfer from BMSCs to T cells, providing the basis of a technology platform to potentiate the metabolic fitness and antitumor function of T cells for adoptive immunotherapy … Altogether, these findings establish intercellular mitochondrial transfer as a promising organelle-based technology platform to improve the outcomes of patients receiving next-generation cell therapies.

Adoptive T-cell therapies have been effective against hematologic malignancies, but less so against solid tumor types, the authors explained. One major obstacle faced by transferred T cells is the hostile tumor microenvironment, which disrupts normal mitochondrial activity, and drives T-cell exhaustion. “Ultimately, impaired mitochondrial fitness orchestrates transcriptional and epigenetic programs associated with terminal exhaustion, leading to defective antitumor T-cell responses and cancer immune evasion,” they stated. “Thus, strategies to boost mitochondrial function in infused T cells are highly sought after.”

Luca Gattinoni, MD, co-lead study author, further explained, “Previous efforts to enhance mitochondrial function in T cells have focused on targeting specific genes or pathways, but these methods fall short when the mitochondria are already damaged or dysfunctional.”

To develop their newly reported approach the researchers built upon their previous findings, which showed that cancer cells can acquire mitochondria from immune cells through intercellular nanotubes that the researchers describe as “tiny tentacles.” Drawing from these results, the group teamed up with scientists at the Leibniz Institute to investigate interactions between BMSCs and cytotoxic T cells. “We sought to leverage mitochondrial transfer from bone marrow stromal cells (BMSCs) to boost CD8⁺ T cell bioenergetic capacity, resistance to exhaustion, and antitumor efficacy,” they explained.

Using varied electron and fluorescent microscopy approaches, including field-emission scanning electron microscopy (FESEM), they observed that BMSCs co-cultured with CD8+ T cells extended nanotubes to the T cells, donating to them intact mitochondria. “We found that bone marrow stromal cells establish nanotubular connections with T cells and leverage these intercellular highways to transplant stromal cell mitochondria into CD8⁺ T cells,” they wrote. This helped to revive the T cells (the T cells receiving mitochondria were dubbed Mito⁺ cells), which showed increased respiratory capacity, a sign of enhanced metabolism. Gattinoni added, “This process is comparable to organ transplantation—like heart, liver, or kidney transplants—but conducted at a microscopic level.”

Tests showed that overall, Mito⁺ cells showed significantly higher basal respiration and spare respiratory capacity (SRC) when compared with either rCD8⁺ T cells that didn’t receive mitochondria from the BMSCs, or those that hadn’t been co-cultured with BMSCs. The research team then examined how supercharging T cells affected immune function. “We hypothesized that the higher SRC observed in Mito⁺ cells would provide the energetic advantage to thrive in harsh microenvironments, such as tumors, and additionally compensate for any loss of mitochondria to cancer cells that would have led to a loss of T cell viability,” they wrote. Their experiments showed that when infused into a mouse model of melanoma the Mito⁺ cells showed significantly higher antitumor responses and prolonged survival rates compared with T cells without any additional mitochondria. “Strikingly, Mito⁺ cells mediated a more robust tumor regression compared with Mito⁻ cells, significantly prolonging mouse survival,” the team stated.

Further tests revealed that Mito⁺ cells could easily penetrate tumors, multiply quickly, and pass on their extra mitochondria to daughter cells, where they persisted for a long time. “A compelling aspect of mitochondrial transfer technology is its capability to impart enduring benefits to recipient T cells,” the scientists pointed out. “We observed that the increased mitochondrial mass resulting from the acquired mitochondria can be maintained long-term across multiple divisions.”

In addition, the authors reported that Mito⁺ cells could survive and resist T-cell exhaustion within the tumor microenvironment. “Collectively, functional, proteomic, and transcriptomic findings indicate that the acquisition of donor mitochondria by CD8⁺ T cells provides significant advantages in terms of cell expansion, survival, tumor penetration, resistance to exhaustion, and differentiation into highly functional killers,” they stated.

The researchers found that supercharging human T cells helped the immune system fight tumors in multiple models of cancer. Of note, they demonstrated that tumor-infiltrating lymphocytes and CAR-Ts, which often develop damaged mitochondria within the tumor microenvironment, displayed augmented cancer-destroying properties when boosted with mitochondria from primary BMSCs from human donors. “The enhanced antitumor responses were observed across a variety of different T cell platforms (TCR, CAR, and TILs) in diverse in vitro and in vivo settings (syngeneic/human xenograft) against both liquid and solid tumors.”

The authors suggest that future applications could include using patient-matched BMSCs to supercharge T cells for adoptive transfer. “Our results provide proof of concept that BMSC mitochondrial transfer can be successfully utilized to potentiate the antitumor efficacy of both mouse and human CD8⁺ T cells,” they concluded. “Taken together, these findings highlight the translational potential of our mitochondrial transfer technology platform and provide initial proof of concept for the feasibility of using patient-matched BMSCs in a fully autologous co-culture system.”