Sunburn may be partly self-inflicted: the immune step that makes UV damage worse.

The part of sunburn that isn't the sun

Here is something most people have never thought about. When you get sunburned, the red, hot, tight, tender skin does not appear while you are lying on the beach. It shows up hours later, usually peaking somewhere between six and twenty-four hours after the exposure ended. The ultraviolet light was gone long before your back turned scarlet.

That delay is a clue. The redness is not the light. The redness is your immune system arriving at a scene it has decided is an emergency, and the question researchers have been circling for a while is a genuinely uncomfortable one: how much of the resulting damage is done by the photons, and how much is done by the responders?

A paper published in Aging Cell in 2026 by a dermatology group at the First Affiliated Hospital of Chongqing Medical University pushes hard at that question. Their answer, in the models they used, is that a meaningful chunk of the damage comes from the response — specifically from a defensive manoeuvre that white blood cells perform called a neutrophil extracellular trap.¹

Before we go further, the honest frame. This is preclinical work. It was done in mice and in a cultured line of human skin cells. The human tissue in the paper was looked at, not treated. Nothing in it tells you to change a single thing about how you handle sun exposure, and the version of this story that shows up in a caption reading "your immune system is what's really aging your skin" is overshooting by a wide margin. What it does offer is a different way to think about what sunburn is, and a different place to eventually aim a drug. Those are worth understanding on their own terms.

What a neutrophil trap actually is

Start with the cell. A neutrophil is the most common white blood cell in your blood and the first responder of the innate immune system. It is blunt, fast, short-lived and not remotely subtle. Its classic job is to swallow bacteria.

In 2004, a group at the Max Planck Institute for Infection Biology described a second, stranger tactic in Science. Under the right provocation, a neutrophil will decondense its own nuclear DNA, coat it in granule proteins, and fling the whole thing out of the cell as a sticky extracellular web.⁴ Bacteria get caught in it and killed. The researchers called these structures neutrophil extracellular traps — NETs for short — and the process of making one is called NETosis. It is a suicide attack. The cell generally dies producing the web.

The machinery matters for what follows. A key enzyme in the process is peptidyl-arginine deiminase 4, abbreviated PAD4. PAD4 chemically modifies the histone proteins that DNA is normally wound around, which lets the chromatin unspool so it can be thrown outward. Blocking PAD4 largely blocks the ability to make traps. There is a well-characterised laboratory inhibitor for this, GSK484, and it is the tool the Chongqing group reached for.¹

Now the other half of the mechanism: what receives the signal. In 2020, a team at Sun Yat-Sen University published a paper in Nature showing that the DNA in these webs is not just inert debris. Other cells can read it. They identified a transmembrane protein, coiled-coil domain-containing protein 25 — CCDC25 — as the receptor that senses extracellular trap DNA and passes a signal inward, and they showed that knocking out CCDC25 abolished the effect they were studying in cancer cells.³ That paper was about metastasis, not skin. But it established that trap DNA is a legible signal with a named receiver, and that is the hinge the 2026 skin work swings on.

There is one more piece worth having before the main event, and it comes from human cells. A group at Jagiellonian University in Kraków showed in 2019 that both bands of ultraviolet light reaching the ground — the longer-wave UVA and the shorter-wave UVB — are enough on their own to trigger trap release from human neutrophils in culture, and that the process runs faster under ultraviolet light than under the chemical and biological triggers normally used in the lab.¹⁰ That is in a dish, not a person. But it means the first step of the chain — light provokes traps — has been shown in human cells, not just mouse skin.

So the proposed chain, in plain terms: ultraviolet light hits skin → neutrophils show up and throw DNA webs → skin cells detect the web DNA through CCDC25 → an internal stress pathway switches on → the skin cell inflames, oxidises and, in enough cases, kills itself. The damage in the back half of that chain is not caused by the light. It is caused by the signal the light pulled.

What the Aging Cell paper actually did

The study has three parts, and they are worth separating because they carry very different evidential weight.

Part one: human tissue, observed only. The researchers took skin samples from patients with actinic dermatosis — a group of sun-triggered skin conditions — and stained them for the molecular fingerprints of neutrophil traps. They found the traps abundantly present.¹ This is the only human data in the paper and it establishes exactly one thing: the traps are there in sun-damaged human skin. It does not establish that they caused anything. Immune cells turn up at damaged tissue constantly. Presence is not proof of blame.

Part two: mice, with an intervention. This is the part that actually moves the argument. The team built an acute photodamage model by irradiating mice with ultraviolet-B, then treated a group with GSK484 — the PAD4 inhibitor that prevents trap formation. Injection of GSK484 significantly alleviated the ultraviolet-B-induced skin damage, inflammation and cell death.¹ They then ran RNA sequencing on the tissue to ask what the traps were doing transcriptionally, and found that PAD4 deficiency blocked ultraviolet-B-induced activation of the mitogen-activated protein kinase pathway — a central inflammatory and stress-signalling relay in cells.¹

Part three: cultured human skin cells. They took HaCaT cells — an immortalised laboratory line of human keratinocytes, the main structural cell of the epidermis — and exposed them directly to isolated neutrophil traps, without any ultraviolet light at all. The traps alone drove the cells into apoptosis, suppressed their viability, triggered inflammatory cytokine release and disrupted their oxidative balance, raising reactive oxygen species and malondialdehyde while altering superoxide dismutase activity.¹ Trap exposure also increased phosphorylated Jun-N-terminal kinase activity — p-JNK, a specific arm of that stress pathway. When the researchers knocked down CCDC25, the receptor, the p-JNK signal was suppressed and the cells largely recovered.¹

That third experiment is the cleanest logic in the paper. It isolates the traps from the light entirely and shows they are sufficient, on their own, to injure skin cells — and it identifies the door they come through.

Crucially, this is not a single-lab result. In 2024, an entirely separate group at Suzuka University of Medical Science in Japan, working on ultraviolet-B skin inflammation in ICR mice, reported that irradiation drove a significant rise in the two classic trap markers — citrullinated histone H3 and PAD4 — and that administering DNase I, an enzyme that digests the DNA scaffold of the webs, attenuated the resulting skin inflammation.² Different country, different mouse strain, different blocking method, same direction of effect. The same Japanese group extended it in 2026, showing that a topical antioxidant reduced ultraviolet-B skin damage in mice dose-dependently while lowering cutaneous PAD4 and citrullinated histone H3 — a third route to the same result, though from the same lab rather than an independent one.¹¹ That cross-lab replication is why this claim earns an EMERGING grade rather than a WEAK one.

Where the evidence stops, precisely

Now the discipline. It is easy to read the section above and slide into "so my immune system is damaging my skin in the sun." Here is exactly how far the data goes and where it stops.

The intervention data is animal-only. Both the GSK484 result and the DNase I result are preliminary signals in mice.¹² Mouse skin is not human skin. It is thinner, differently pigmented, densely furred and has a different repair and immune architecture. Dermatology has a long, humbling history of mouse results that did not survive contact with human trials.

The human-cell data is a cell line, not skin. HaCaT cells are genuinely useful and genuinely human, but they are immortalised — they have been altered to divide indefinitely — and they sit alone in a dish with no dermis, no immune context, no blood supply and no barrier function.¹ Showing that isolated traps kill a HaCaT cell is a mechanism finding, not a physiology finding.

The human tissue data is observational. Traps were seen in the skin of actinic dermatosis patients.¹ No one gave a person a trap-blocking drug and measured whether their sun damage improved. That trial does not exist.

And this is acute damage, not photoaging. The mouse model was an acute ultraviolet-B insult.¹ Photoaging — the wrinkling, sagging, mottled pigmentation and loss of elasticity that accumulate over decades — is a chronic, low-dose, long-horizon process. Whether the trap mechanism contributes meaningfully to that, as opposed to acute sunburn, is a reasonable hypothesis and an unanswered question. The paper does not test it.

Put together: mechanism-plausible, replicated across two independent animal models, and not yet trial-tested in humans. That is a real finding. It is not a health claim.

The longevity angle: older traps, stickier mess

There is a reason this sits in the longevity file rather than the skincare one, and it is not the skin. It is the neutrophils.

A 2023 study in the Journal of Leukocyte Biology from Comenius University compared trap formation in neutrophils from patients grouped by age. Neutrophils from elderly patients responded to a sterile stimulus with enhanced trap formation, and the traps they made were more oxidised and showed higher resistance to being cleared by DNase I. Older participants also carried a higher concentration of residual traps circulating in their plasma, and that plasma was itself capable of priming other neutrophils toward further trap release through Toll-like receptor 9 signalling.⁶

Read those two literatures next to each other and a coherent, if still speculative, picture appears. If traps amplify ultraviolet injury, and if aging makes neutrophils both more eager to release traps and worse at clearing them, then the same afternoon of sun exposure would plausibly cost a seventy-year-old more than a twenty-year-old — not because their skin absorbs more light, but because their response to it runs longer and dirtier.

We want to be careful here, because this is the exact point where a story like this usually overreaches. Nobody has connected those two findings experimentally. One is a mouse-and-dish paper about skin;¹ the other is a human blood study about sterile inflammation.⁶ The bridge between them is inference, and inference is not evidence. But it is the kind of inference that tells you which experiment to run next, and it is why "sterile inflammation that never fully resolves" keeps appearing near the centre of aging biology.

What this changes for you today

Nothing. We want to be blunt about that, because it is the most useful sentence in this article.

There is no trap-blocking sunscreen. There is no PAD4 inhibitor you can buy, and you should be actively suspicious of any product that starts gesturing at "NET" language on a label in the next eighteen months. GSK484 is a laboratory reagent. DNase I exists as an approved drug, but as an inhaled therapy for cystic fibrosis — not as anything you put on skin.

Meanwhile, the thing that actually has randomized controlled trial evidence in humans remains exactly what it was last week. In the Nambour trial in Queensland, Australia, adults randomly assigned to daily broad-spectrum sunscreen showed 24% less skin aging by objective microtopography over 4.5 years than those using it at their own discretion.⁷ The same trial population found squamous cell carcinoma tumour incidence significantly lower in the daily-sunscreen group, though basal cell carcinoma was unaffected,⁸ and a ten-year follow-up found substantially fewer invasive melanomas in the daily-use group.⁹

One honest caveat on that, since we are being rigorous about everything else: those three results come from one randomized cohort, reported three ways. That is why the claim is graded MODERATE and not STRONG. It is still far and away the best human evidence in this entire article — and it is the only evidence here attached to an action you can take.

So the practical read is unchanged and unglamorous. Shade, clothing, timing and a sunscreen you will actually reapply. If you want the granular version of the product decision, we have covered how mineral and chemical filters actually compare and the vitamin D trade-off that gets raised every single summer separately. Neither of those pieces needs an edit because of this paper, which is itself a useful signal about how much has actually changed.

What does the trap mechanism add, practically? Only one soft, sensible implication: if part of the damage is the inflammatory response rather than the light, then not letting yourself burn in the first place matters for reasons beyond the burn itself. Avoiding the response is avoiding the second wave. That is not new advice. It is just a new reason for old advice, which is usually how mechanism papers land.

Grey areas: why "just block the traps" isn't free

This is the part that gets skipped in coverage of findings like this, and it is the most important thing here.

Traps exist because they work. They were discovered as an antimicrobial weapon — webs that catch and kill bacteria, described in experimental dysentery and in human appendicitis in the original 2004 report.⁴ An immune mechanism that costly is not evolutionary noise. Suppressing it systemically is not a neutral act.

And we have direct evidence of the cost. A 2024 study in Science Translational Medicine from the University of Cambridge found that traps in urine interact with uromodulin to form webs that entrap bacteria — and that inhibiting PAD4 in mice blocked trap formation and resulted in progression of simple bladder cystitis into full pyelonephritis, an ascending kidney infection.⁵ That is the same enzyme, the same class of blockade the skin paper used to get its protective effect, producing a serious harm in a different tissue.

So the honest framing of the therapeutic idea is not "block traps, save skin." It is: any future intervention would have to be topical, local, transient and precisely dosed, because the systemic version trades sunburn for infection risk. That is a hard pharmacological problem, not a formulation tweak. Anyone presenting trap suppression as an unambiguous win is not describing the literature.

One more, smaller grey area. The Aging Cell abstract describes CCDC25 as a "25-pass transmembrane protein." That is a misreading of the acronym — CCDC stands for coiled-coil domain-containing, and the protein was characterised in Nature in 2020 as a transmembrane trap-DNA receptor, not a 25-pass one.³ It is a trivial slip and does not affect the experiments. We flag it because it will propagate into every piece of secondary coverage that copies the abstract, and it is the kind of detail worth checking rather than repeating.

What we still don't know

The gaps are specific, and naming them is more useful than a general call for more research.

Does this hold in human skin? Not a cell line — actual human skin, ideally in an ex vivo explant or a controlled minimal erythema dose study with biopsies. Nobody has run it.

How much of the damage is the traps? The papers show the contribution is non-zero in mice.¹² They do not quantify what fraction of total ultraviolet injury it represents. If it is 5%, this is a curiosity. If it is 40%, it is a drug target. That number does not exist yet.

Does it apply to chronic photoaging or only acute burn? The models are acute.¹² The longevity-relevant question is the chronic one, and it is untested.

Does it differ by skin phototype? Darker skin has more melanin, a higher minimal erythema dose and a different inflammatory threshold. Whether trap involvement scales with that is entirely open.

Can it be blocked locally without the infection penalty? This is the whole translational question, and the Cambridge kidney data says it is the hard one.⁵

Our read: this is a genuinely good mechanism paper with a real independent replication behind its central claim, and it is roughly five to ten years and several unrun experiments away from meaning anything you can act on. Both halves of that sentence are true, and the story is only worth telling if you keep them attached.

References

1. Zou Y, Li J, Peng Y, Hu R, Li R, Chen A. Neutrophil extracellular traps exacerbate UVB-induced photodamage in HaCaT cells and mouse skin via CCDC25/MAPK pathway. Aging Cell. 2026;25(8):e70640. DOI · PMID 42492479
2. Inaba I, Hiramoto K, Yamate Y, Morita A, Tsutsumi T, Yasuda H, Sato EF. Inhibiting neutrophil extracellular traps protects against ultraviolet B-induced skin damage: effects of Hochu-ekki-to and DNase I. Int J Mol Sci. 2024;25(3):1723. DOI · PMID 38339001
3. Yang L, Liu Q, Zhang X, et al. DNA of neutrophil extracellular traps promotes cancer metastasis via CCDC25. Nature. 2020;583(7814):133–138. DOI · PMID 32528174
4. Brinkmann V, Reichard U, Goosmann C, et al. Neutrophil extracellular traps kill bacteria. Science. 2004;303(5663):1532–1535. DOI · PMID 15001782
5. Stewart AP, Loudon KW, Routledge M, et al. Neutrophil extracellular traps protect the kidney from ascending infection and are required for a positive leukocyte dipstick test. Sci Transl Med. 2024;16(766):eadh5090. DOI · PMID 39321268
6. Pastorek M, Konečná B, Janko J, et al. Mitochondria-induced formation of neutrophil extracellular traps is enhanced in the elderly via Toll-like receptor 9. J Leukoc Biol. 2023;114(6):651–665. DOI · PMID 37648664
7. Hughes MCB, Williams GM, Baker P, Green AC. Sunscreen and prevention of skin aging: a randomized trial. Ann Intern Med. 2013;158(11):781–790. DOI · PMID 23732711
8. Green A, Williams G, Neale R, et al. Daily sunscreen application and betacarotene supplementation in prevention of basal-cell and squamous-cell carcinomas of the skin: a randomised controlled trial. Lancet. 1999;354(9180):723–729. DOI · PMID 10475183
9. Green AC, Williams GM, Logan V, Strutton GM. Reduced melanoma after regular sunscreen use: randomized trial follow-up. J Clin Oncol. 2011;29(3):257–263. DOI · PMID 21135266
10. Zawrotniak M, Bartnicka D, Rapala-Kozik M. UVA and UVB radiation induce the formation of neutrophil extracellular traps by human polymorphonuclear cells. J Photochem Photobiol B. 2019;196:111511. DOI · PMID 31129510
11. Arakaki Y, Tominaga K, Hiramoto K, et al. Hispidin ameliorates acute ultraviolet B-induced skin inflammation by targeting reactive oxygen species-dependent neutrophil extracellular trap formation. Int J Mol Sci. 2026;27(8):3667. DOI · PMID 42074303