Immune protein sharpens lung cell memory
Lung cells respond faster after a previous infection. Not through new genes, but via a component of the innate immune system. That finding could matter for how we protect older people from repeated respiratory infections.
When the body encounters a pathogen for the second time, it responds more quickly and forcefully than it did the first time. This is called trained immunity: the immune system retains a functional memory of earlier exposures, without relying on the adaptive immune response. Until now, it was unclear which molecules drive this process in lung cells specifically.
Complement protein C3 as the key player
Research published in eLife shows that the complement protein C3 plays a central role in trained immunity among alveolar macrophages (immune cells residing in the lung’s air sacs). The researchers found that C3 and its receptor C3aR1 increased in human alveolar macrophages following intranasal exposure to a training stimulus. In mouse experiments, trained alveolar macrophages lacking C3 responded substantially less vigorously to a second stimulus: they produced fewer inflammatory cytokines and showed reduced capacity to clear bacteria.
The mechanism was notable: alveolar macrophages take up C3 as an intact protein more rapidly than they take up the smaller breakdown product C3a. Restoring C3 from an external source largely rescued the blunted response in C3-deficient cells, but only when the C3aR receptor remained functional. That points to a specific signaling pathway.
Relevance for aging and repeated infections
Why does this matter for longevity? Pneumonia is among the most common causes of death in older adults. As people age, the function of the innate immune system, the fast, non-specific arm of immunity, gradually declines. If trained immunity depends on C3 signaling, understanding this pathway could open new directions for preventing or treating recurrent respiratory infections in older people. The researchers note that these are preliminary findings, largely from animal models, and that further human studies are needed.
Energy metabolism was also implicated: trained lung cells with intact C3 showed greater glycolytic activity, which correlates with a more active immune response. The link between cellular energy use and innate immune memory is an active area of investigation.
Want to research this yourself? Try these search terms: trained immunity alveolar macrophages, complement C3 signaling lung, glycolysis innate immune memory