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Study examines why newborns can’t fight colds

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By Jennifer Couzin-Frankel

ScienceNOW

One of newborns’ biggest vulnerabilities is largely invisible: In the weeks after birth, babies are especially susceptible to infection because their immune systems aren’t fully functional. There are a handful of theories to explain this liability, and now a research team has added a new one to the list: Immune suppression in early life might help prevent inflammation in the infants’ intestines as they become colonized by the helpful bacteria they need to stay healthy.

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Newborns are more likely than older babies to catch, and die from, serious infections. The reason is fuzzy — indeed, there may be more than one explanation.

Seeking new ways to better understand this process, Sing Sing Way, a pediatric infectious disease doctor at Cincinnati Children’s Hospital Medical Center, wondered whether transferring immune cells from adults might rev up their immune systems. Yet when he and his colleagues injected infection-fighting cells from the spleens of adult mice into 6-day-old pups, nothing happened: The pups were just as vulnerable to harmful bacteria as control animals. Probing more deeply into this surprise, they found the injected cells simply stopped functioning in the newborn animals. Then Way’s group did the reverse transplant — they gave adult mice newborn immune cells that were inactivated in the pups and found that they “turned on” in the mature animals. These experiments “told us it wasn’t a problem with the neonatal cells themselves,” Way says. Rather, he believes, the environment — either a newborn body, or an adult one — guided how the cells behaved.

Others in Way’s lab study the gut microbiome, the constellation of healthy bacteria that populates our intestines. Newborn mice, just like human babies, are born “clean,” with little intestinal bacteria. Very rapidly that changes. Way wondered whether there might be some connection between this colonization and what looked like a purposeful suppression of the immune system in his mice.

To find out, his group focused on immune cells that eventually develop into red blood cells and that express a surface receptor called CD71, which causes immune suppression of other cells. Way and his colleagues also found that, as the mice grew, fewer and fewer cells boasted CD71 receptors, suggesting the suppression wasn’t needed. He theorizes that that’s because the gut has been colonized by that point.

The work, reported online Wednesday in Nature, “adds a new and very important chapter” to the story of how the immune system develops, says Mike McCune, an immunologist at the University of California, San Francisco.

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