Inside a dendritic cell, something is being decided. The cell has just swallowed a fragment of debris from a dead tumour neighbour and is now, in a sense, reading it, translating molecular scraps into instructions for the immune system’s killer T cells. Whether that message gets through with enough clarity to trigger an effective response turns out to depend not on anything obviously immunological but on a protein complex tucked into the inner membrane of the cell’s mitochondria. Complex I, as it’s known, one of the workhorses of cellular respiration, is apparently running a checkpoint. And when it’s broken, or merely faltering, the whole chain of command that leads to T cell activation can quietly collapse.
That, in rough outline, is what a team led by David Sancho at Spain’s Centro Nacional de Investigaciones Cardiovasculares (CNIC) has found, in work published this week in Science Immunology. The implications stretch well beyond the biology of mitochondria: they touch on why some cancer immunotherapies fail, why tumour microenvironments are so effective at defanging the immune response, and perhaps on how to improve dendritic-cell-based vaccines.
Dendritic cells are the immune system’s most sophisticated sentinels. Rather than attacking pathogens directly, they function as intelligence agents, ingesting foreign material, processing it, and presenting peptide fragments on their surface to rally T lymphocytes. The process Sancho’s team focused on is called cross-presentation, and it’s distinctly demanding. It involves shuttling antigens from the endosomal compartment where they were captured out into the cell’s cytosol, loading them onto MHC class I molecules, and displaying the result to CD8 T cells, which are the killers capable of destroying tumour cells or virus-infected cells. The conventional dendritic cells responsible for this, the cDC1 subset, are already something of a rarity; now it seems their performance is exquisitely sensitive to the health of their mitochondria in a way nobody quite appreciated before.
The Redox Switch
“We discovered that mitochondrial complex I acts as a genuine metabolic switch. Without its proper function, dendritic cells lose much of their ability to activate T lymphocytes to fight threats such as tumours or viruses,” says Sancho. The mechanism the CNIC team traced is, at its heart, a redox story. Complex I’s main job is oxidising NADH, a form of the coenzyme nicotinamide adenine dinucleotide, to NAD+. When the complex is impaired, NADH accumulates while NAD+ drops. This shift in ratio, a kind of reductive stress, turns out to be far more consequential for dendritic cells than anyone had previously worked out.
Working with mice engineered to lack NDUFS4, a structural subunit essential for complex I stability, the researchers watched cross-presentation falter while direct antigen presentation remained largely intact. The two pathways clearly depend on different metabolic conditions. Phagocytosis, endosomal acidification, and T cell receptor expression on the surface of the deficient cells all looked roughly normal; what failed was the escape of ingested material from endosomal compartments into the cytosol, the critical handoff step that cross-presentation depends on. Less antigen making it through meant fewer peptide-MHC-I complexes reaching the surface, which meant fewer CD8 T cells receiving activation signals strong enough to matter.
Sofía C. Khouili, one of the study’s co-first authors, puts it plainly: “when complex I function is impaired, dendritic cells struggle to present sufficient antigen to T lymphocytes, reducing both T cell activation and the immune response against viruses or tumours.”
A Rescue, and a Surprise
The rescue experiment is perhaps the most striking part of the paper. When the researchers restored the NAD+/NADH balance in complex-I-deficient cells, whether by adding alpha-ketobutyrate (an alternative electron acceptor) or nicotinamide riboside (a direct NAD+ precursor), cross-presentation recovered. Substantially, and specifically: direct presentation was unaffected throughout. Elena Priego, the other co-first author, describes this as a key to unlocking what’s really going on: “the key lies in the increased NADH-to-NAD+ ratio that results from complex I deficiency. Rebalancing this ratio by pharmacological means restores the ability of dendritic cells to activate T lymphocytes during viral infections or antitumour responses.” The team also showed, in a separate line of evidence using genetically engineered mice that expressed a yeast enzyme called NDI1 capable of oxidising NADH independently, that restoring the ratio alone is sufficient. You don’t need complex I itself to be intact; you just need its product.
The lipid biology threw up a slight puzzle. Previous work had implicated lipid droplets and lipid peroxidation in endosomal antigen escape, and the NDUFS4-deficient cells did show altered lipid profiles and reduced peroxidation. But when the team blocked lipid peroxidation pharmacologically in otherwise healthy cells, cross-presentation barely budged. So peroxidation seems to be a downstream consequence of the redox shift, not its primary cause. The NAD+ imbalance appears to be undermining cross-presentation through several mechanisms simultaneously, among them reduced ATP supply, altered lipid composition, and presumably other redox-sensitive processes not yet mapped. A tidy single-mechanism story would have been satisfying; the reality is rather messier, and perhaps more credible for it.
The Tumour Problem
What makes this more than an academic footnote about electron transport chain biochemistry is what happens inside tumours. Tumour microenvironments are hostile to immune cells in a variety of well-characterised ways, and one of the things the CNIC team found when they looked at publicly available gene-expression data was that cDC1s infiltrating tumours show systematically reduced expression of complex I subunits. That correlation was, Sancho and his colleague Michel Enamorado acknowledge, exactly the kind of signal you’d expect if metabolic suppression of dendritic cells is part of how tumours evade surveillance: “We identified mitochondrial complex I in dendritic cells as a key checkpoint and demonstrated that correcting the internal chemical imbalance associated with its dysfunction can restore immune responses in experimental models.”
The antitumour experiments were fairly decisive on this point. When the team loaded dendritic cells lacking NDUFS4 with tumour antigen and used them as vaccines in tumour-bearing mice, the cells essentially failed: lung metastases from B16 melanoma cells were unchecked. Wild-type dendritic cells vaccinated the same way provided robust protection. The difference, functionally, was the ability to cross-present, and that ability tracked with complex I status.
There’s an uncomfortable implication lurking here for cancer medicine. Several experimental cancer therapies use complex I inhibitors, reasoning that tumour cells tend to rely heavily on oxidative phosphorylation. But if those inhibitors also hit dendritic cells in the tumour microenvironment, they may be undermining the very immune response needed to clear the tumour. That’s not a small caveat. The CNIC findings suggest, at minimum, that any clinical strategy involving mitochondrial interference should be accounting for what it does to immune cells, not just to tumour cells.
Whether NAD+ precursors like nicotinamide riboside, already available as supplements and being explored for various metabolic conditions, might genuinely improve dendritic cell function in people is a question no one is quite ready to answer yet. The mouse-to-human translation is long and uncertain. But the basic logic, that dendritic cells need functional mitochondrial redox metabolism to cross-present antigens, and that restoring that balance can rescue their immune function, seems solid enough to be worth following up. The checkpoint turns out to sit, of all places, in the power plant.
https://doi.org/10.1126/sciimmunol.aef0098
Frequently Asked Questions
What is cross-presentation, and why does it matter for cancer immunity?
Cross-presentation is the process by which certain immune cells, particularly a subset of dendritic cells called cDC1s, take up material from outside the cell and display fragments of it on MHC class I molecules to CD8 T cells. This is the pathway that activates the immune system’s killer T cells against tumours and virally infected cells. Without effective cross-presentation, CD8 T cells don’t receive the alarm signals they need, and tumours can go undetected or unchecked.
What does mitochondrial complex I actually do?
Complex I is the first and largest of the protein complexes in the mitochondrial electron transport chain. Its main function is converting NADH to NAD+ while shuttling electrons into the chain; this drives the production of ATP, the cell’s energy currency. Crucially, it also regulates the cell’s redox balance, the ratio of oxidised to reduced forms of key molecules. When complex I fails, NADH accumulates and NAD+ drops, creating what the researchers call reductive stress.
Could this explain why some cancer immunotherapies don’t work?
It’s plausible. The study found reduced expression of complex I subunits in dendritic cells infiltrating tumours, suggesting that the tumour microenvironment may be metabolically suppressing these immune cells. If dendritic cells in the vicinity of a tumour can’t cross-present antigens properly, the CD8 T cell response will be weak regardless of other interventions. This could help explain some cases of immunotherapy failure, though more research is needed to establish a direct causal link in patients.
Is nicotinamide riboside a useful treatment for this problem?
In mouse experiments, nicotinamide riboside (a form of vitamin B3 and direct NAD+ precursor) successfully restored the NAD+/NADH ratio in complex-I-deficient dendritic cells, and cross-presentation recovered. Whether the same approach would meaningfully improve immune function in cancer patients is unknown. Clinical translation requires understanding dosing, tissue distribution, and whether tumour-infiltrating dendritic cells can actually access and use the supplement, none of which has been tested in humans for this specific purpose.
Does this mean complex I inhibitors used in cancer treatment are counterproductive?
The findings raise that concern. Some experimental cancer therapies target complex I in tumour cells, but this study shows that complex I activity in dendritic cells is critical for mounting an antitumour immune response. Inhibiting complex I across cell types could simultaneously weaken the immune system while targeting the tumour, potentially reducing overall treatment effectiveness. The researchers suggest that therapeutic strategies should account for effects on immune cells, not just on tumour cells themselves.
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