A tiny protein called C3 appears very early in evolution, even in sponges. Most of it is made in the liver and travels in the blood to fight germs.
New research shows that when tumor cells make C3 right inside the tumor, it can keep bad immune cells out. Those bad cells usually stop the body from attacking cancer.
When C3 is made locally, it creates a fragment called iC3b. This fragment blocks the harmful cells from entering the tumor, giving the immune system a better chance to kill the cancer.
Scientists tested mice that had very little C3 coming from the liver. The cancer drug anti‑PD‑1 worked just as well as in normal mice. But when they stopped the tumor’s own cells from making C3, the drug stopped working well, even though blood C3 only dropped a little.
These results mean that the C3 inside the tumor, not the C3 in the blood, decides how well immunotherapy works.
To help cancers that usually resist treatment, the researchers tried a drug that copies C3’s action. The drug let immunotherapy work again and the mice lived much longer.
When doctors looked at lung‑cancer samples from people, they saw the same pattern. Patients whose tumors had lots of C3 lived longer and responded better to treatment. Patients with low C3 did not respond at all. Blood C3 levels did not matter.
The next steps are to find ways to raise C3 inside tumors and to figure out the best timing for treatment. Learning more about local C3 could also teach us about wound healing and inflammation.