Smuggling Mitochondria: A Trick Tumours Use to Evade Attack

Now that we know how tumours transfer defective mitochondria to immune cells, can we find ways to prevent it? Scientists believe so. 

T cells are a vital component of the body’s defence against cancer. The ability to harness the T cell response, either through activating T cells with antibody therapeutics (e.g. TCR engagers) or by genetically engineering the T cells to express a chimeric antigen receptor (CAR) have provided clinicians with a new repertoire of curative or life-extending therapies.  However, whilst these therapies show great promise, the vast array of mechanisms employed by cancer cells to escape anti-tumour immunity means we still need to understand these mechanisms better if we can expect to develop therapeutics that counteract tumour evasion.

When T cells are activated, they rapidly divide and migrate to the tumour in response to a chemokine gradient, the concentration of chemokine proteins that guides cells to move in a specific direction. To supply the large quantities of energy required for activation, T cells rely on mitochondria, small organelles within eukaryotic cells which act as sites for respiration. The energy requirement for T cell activation often outstrips the amount of energy generated by mitochondrial aerobic respiration (OxPHOS) and instead, T cells must switch to anaerobic respiration.

Anaerobic respiration is less efficient than OXPHOS but, vitally, does not require oxygen which can be in short supply, especially in the tumour microenvironment (TME). This occurs because tumour cells often expand aggressively outside the reach of capillaries, depriving cells of oxygenated blood. To survive in this hypoxic environment, tumour cells must also switch to anaerobic respiration – a process known as the Warburg effect. These metabolic changes are very similar to those seen in activated T cells, including those infiltrating the TME and highlights the importance of strict regulation of mitochondrial metabolism for both friends and foes.

Mitochondrial transfer: A double-edged sword

Mitochondrial transfer is a cellular process that allows cells to share mitochondria. Mitochondrial transfer can be beneficial as it provides a route for cells to rejuvenate metabolically active cells by giving them fresh mitochondria, but as Ikeda et al. found in their recent publication in Nature, it can also be detrimental to the immune response against cancer.

In their study Ikeda et al. found that tumour cells were able to transfer defective mitochondria into tumour-infiltrating T cells. The researchers began by sequencing mitochondrial DNA from tumour-specific T cells. They discovered that a proportion of the T cells had mutations in their mitochondrial DNA that matched those of the tumours they were isolated from. By staining the mitochondria in cultured tumour cells, and then co-culturing the tumour cells with healthy T cells, they found that the tumour cells were able to transfer their mutated mitochondria to the T cells both directly by contact-mediated mechanisms, and indirectly via extracellular vesicles.

Prolonged co-culture with tumour cells eventually resulted in the T cell mitochondria being entirely replaced by the mutated tumour cell mitochondria. Furthermore, these results were replicated in mouse models, indicating that a robust physiologically relevant mechanism by which tumours were impacting the function of T cells via mitochondria transfer was at play.

T cells in self-destruct mode

So why were the T cells losing their healthy mitochondria after exposure to tumour cells? Highly reactive molecules formed in the tumour environment, known as reactive oxygen species (ROS), can cause damage to DNA. Mitochondria, having no way to repair their DNA themselves, are highly susceptible to this damage. The researchers found that mitochondria native to the healthy T cells were being induced to self-destruct by the presence of ROS from the tumour, but this wasn’t occurring in the mutated mitochondria. The researchers found that USP30, a molecule which prevents mitochondrial self-destruction, was highly expressed in the cancer cells. USP30 was expressed at very low levels in healthy T cells too yet was moderately expressed in T cells containing cancer derived mitochondria. 

By using a combination of techniques including direct staining and siRNA based USP30 degradation, the researchers found that USP30 was being attached to the mutant tumour mitochondria before their transfer to healthy T cells, where it prevented the destruction of the mutated mitochondria, whilst the non-USP30-associated (healthy) mitochondria were destroyed.

Analysis of the T cell phenotype after culture with tumour cells demonstrated impaired mitochondrial OXPHOS, increased dependency on glycolysis and reduced ability of the T cells to differentiate into long term memory cells, as well as increased expression of markers associated with apoptosis (programmed cell death) and senescence (when cells age and die off). Reduced mitochondrial function prevented efficient activation of the T cells, thus severely limiting their ability to destroy tumour cells. Similar results were obtained in a mouse model, with T cells containing mutated mitochondria exhibiting markedly less tumour killing capacity alongside a significantly reduced ability to form memory T cells which were protective against tumour re-challenge. 

The researchers examined patient data to determine if correlation could be found between the in vitro/in vivo models and clinical data. Interestingly, patients with mtDNA mutations had significantly shorter progression-free survival and overall survival following initiation of PD-1 blockade therapy compared to individuals without mtDNA mutations, again supporting the observation that tumour-mediated transfer of mutated mtDNA within the TME is a novel mechanism facilitating tumour immune evasion.

Conclusion

So how can we use this information to generate therapies which overcome these resistance mechanisms? Ultimately, there is still a considerable amount we don’t know about the pathways involved, but one option may be to target USP30 association with tumour mitochondria to render them more prone to mitophagy; which could present a tractable option for drug hunters. Preventing transfer of the mutated mitochondria to tumour-specific CD8+ T cells would mean greater tumour control. In other words, we’d be catching the smugglers before they profit from their swag!

James Corbett, PhD and Robbie Evans, PhD are Senior Scientists with Charles River In Vitro Biology group. Louise Brackenbury, PhD, is Science Director of Advanced Modalities.