In mice, embryonic immune cells already fight like adult ones

A microscope image from the Turku team shows pale, weblike structures. They are the nets that neutrophils, the immune system's first responders, spin to trap and destroy pathogens. For most of medical history, such nets were seen in adults fighting infection. The new study, published in Nature Communications, shows they are also present in the embryo, long before any infection could arrive.

Neutrophils are white blood cells that rush from the bloodstream into tissues when inflammation starts, to eliminate harmful microbes. Their job is well documented in adults. In embryos it was not. Lintukorpi put the puzzle simply: the uterus is an almost sterile environment, so what would embryonic defense cells be defending against? That question led the team to look at developing tissues in mice, where they found large populations of neutrophils in several organs.

To find out whether these cells were truly working, the researchers used single-cell RNA sequencing, a method that reads which genes each individual cell is switching on. Embryonic and adult neutrophils turned out to differ slightly in their gene expression, but their main developmental stages were similar. Functional tests then showed that embryonic cells could ingest particles, produce toxic oxygen compounds and form weblike traps outside the cell. Hofmann's conclusion is that they are not merely immature versions of adult cells but functionally mature cells.

The most unexpected finding, according to Hofmann, concerns location. Many of these neutrophils sit outside the blood vessels, in direct contact with developing tissue cells. That suggests they may also help shape tissues and keep them in balance, not only fight microbes. Rantakari says the results show neutrophils play a much more active role in developing tissues than previously thought.

What changes, and for whom? For now, the gains are for research. Doctors treating fetuses and newborns could eventually benefit if the same cells are confirmed in human embryos, since the authors say the work could contribute to future treatments for prenatal and perinatal infections and inflammatory diseases. The study gives no timeline for that, and everything so far comes from experimental models in mice.