An Ancient Immune Protein Made Inside Tumors Could Boost Cancer Immunotherapy

An immune protein so ancient it predates blood circulation may hold a key to making cancer immunotherapy work in tumours that resist it.

The protein is complement C3, part of an evolutionarily old immune system found even in sponges and jellyfish. Scientists at Nagoya University found that what mattered was not C3 circulating in the blood, but C3 produced locally, inside the tumour.

“What determined the efficacy of the immunotherapy treatment was not the C3 in the blood, but the local C3 produced at the tumour site,” said researcher Yuki Miyai.

What the complement system is

Complement is a cascade of blood proteins that activate one another in sequence when triggered, and it is among the oldest components of immune defence.

Its classical functions are direct: punching holes in bacterial membranes, coating pathogens so phagocytes recognise them, and recruiting immune cells to sites of infection. It operates within minutes and requires no prior exposure, which is why it predates the adaptive immune system by a very long way.

C3 sits at the convergence point of the cascade. Every activation route passes through it, and its cleavage generates the fragments that carry out most downstream effects — which is why it is the component that matters most.

How it helps here

When C3 breaks down within tumour tissue it forms a fragment, iC3b, that blocks immune-suppressing myeloid cells from flooding into the tumour. With those suppressive cells kept out, checkpoint-inhibitor drugs (anti-PD-1 antibodies) can reach and attack the cancer more effectively.

The team showed the effect in mice with controlled C3 production and in human lung-cancer samples; the study appeared in Nature Communications in August 2026.

Why suppressive myeloid cells are the obstacle

Tumours actively recruit myeloid cells and reprogramme them into a suppressive state, and this is one of the principal reasons immunotherapy fails.

These cells release factors that inhibit T-cell function, deplete nutrients T cells require, and physically crowd the tumour margin. A T cell can be present, correctly targeted and rendered non-functional by the environment surrounding it.

Checkpoint inhibitors address a different problem — they release the brakes tumours apply to T cells directly. They do nothing about a suppressive environment, which is why they fail in tumours where myeloid suppression rather than checkpoint signalling is the limiting factor.

Why local versus circulating changes everything

The distinction Miyai draws has direct practical consequences.

Circulating C3 is abundant, easily measured and, on this evidence, uninformative — a blood test would tell clinicians nothing about whether a patient’s tumour will respond. What matters is production within the tumour itself, which requires tissue.

It also implies the relevant biology is compartmentalised. The tumour microenvironment is chemically and immunologically distinct from the bloodstream, and a protein doing one thing systemically can do something quite different a few millimetres inside a tumour.

The complication in the wider literature

Complement’s role in cancer has been genuinely contested, with prior work implicating it in promoting tumour growth through chronic inflammation and immune suppression.

This finding does not simply contradict that. It suggests the effect depends on which fragments are generated, where, and in what context — a cascade producing multiple active products can plausibly help in one configuration and harm in another.

Resolving that will matter before anyone attempts to manipulate complement therapeutically, because a crude intervention could easily worsen the situation.

What it might become

Immunotherapy transforms outcomes for some cancers and fails in many patients. Local tumour C3 could become a marker identifying who is most likely to benefit — and a lever to make resistant tumours respond.

The marker application is the nearer prospect, since it requires only measuring something rather than changing it. Existing biomarkers predict response imperfectly, so an additional one with independent information would be useful even short of a therapy.

The evolutionary framing, and what it actually implies

The observation that complement predates blood circulation is more than decoration — it says something about how the system is likely to behave.

A defence mechanism present in sponges evolved to work in tissue, locally, without a circulatory system to distribute it. Cells made these proteins where they were needed and the response operated in place.

Circulation came later, and complement was recruited into it. But the machinery for local tissue production was never lost, and cells throughout the body retain the capacity to make complement components themselves.

Read that way, the finding is less surprising than it first appears. Local complement production is the ancestral arrangement, and the tumour microenvironment — a disorganised tissue with its own chemistry, partly walled off from normal circulation — is exactly the kind of setting where a locally acting ancient system would operate on its own terms.

The findings are preclinical. Mouse tumour models have a poor record of predicting human immunotherapy results, though the human lung-cancer samples strengthen the case. Research news, not medical advice.