Cells that allow helpful bacteria to safely colonize the intestines of newborn infants also suppress their immune systems thus making them more vulnerable to infections, according to new research led by a team of scientists at Cincinnati Children’s.

These findings, published online Nov. 6, 2013 in Nature, could prompt a major shift in how medicine views the threat of neonatal infections – and how researchers go about looking for new strategies to stop it.

During the first few days after birth, a baby’s immune system must adapt to many new stimulants. This includes environmental microbes that are not present in the womb, but immediately colonize tissues such as the intestine and skin. Leading up to this study, the prevailing view has been that newborn infants are susceptible to infection because their immune system cells are immature or underdeveloped.

 “Our findings suggest that neonatal susceptibility to infection is caused by active immune suppression during this developmental period, as opposed to the immaturity of immune cells,” says  Sing Sing Way, MD, senior investigator and a physician in the Division of Infectious Diseases at Cincinnati Children’s.

The suppressive cells in this case are CD71+ precursors of mature red blood cells. The researchers found CD71+ precursor cells are enriched in newborn mice (and in human umbilical cord blood) to prevent an over-reactive immune response as infants adapt to their new microbe-filled world. CD71+ cells express an enzyme called arginase-2 that is essential to suppressing immune cells. This process plays a vital role in developing intestines by preventing an onslaught of inflammation in response to colonizing bacteria that help digestion and related functions.

In general, the benefits of healthy bacterial colonization of intestines outweigh the threat of other infections, Way says. But it may be possible to modulate the process to improve protection from infection while still allowing CD71+ cells to do their job. However, much more follow-up research is needed before these findings could be applied to human infants, Way says.