As every parent knows, kids spend their early years exploring the world with their mouths, gumming every germ-riddled object within reach and sampling their ever-sticky fingers. If left to their own devices, it seems likely they would taste-test door knobs and lick the floors of public bathrooms.

However horrifying, their slobbery ways have an upside—building up immune defenses. Their daily buffet of germs provides their immune systems with thorough intel on countless microscopic enemies. The dirt on the germs is enough to train immune cells to produce Y-shaped blood proteins called antibodies that can detect individual foes based on unique molecular patterns. From there, armies of antibodies act like security guards, surveilling the body for specific, pre-identified threats. Any time they recognize an invader, they can sound the alarm and lead a strike.

Thus, the drool-based defense system arms kids with a bulwark against a wide range of bugs that commonly float around daycares, schools, and beyond. Of course, for some particularly nasty diseases, vaccines do the work of a grimy mitt—safely. They prime the immune system to make antibodies that lead to longstanding protection, sans severe infections.

Forgotten foes

Such is the case for measles, which has made a comeback in recent years and has been the target of anti-vaccination misinformation . In kids left unvaccinated, measles storms the immune system, leading to a dangerous infection with a tell-tale rash and dramatic fevers. But that’s not the end of it.

According to two new studies, the highly-contagious virus also destroys kids’ immune defenses, undoing years of their mouthy recon and leaving them vulnerable to countless germs. In one study published in Science Immunology, an international team of researchers found that the deadly virus knocks out specific subsets of “memory” immune cells that make antibodies. In a related study appearing in Science, another group of researchers found that the virus can annihilate up to 73% of a kids’ arsenal of antibodies.

Together, the research “highlights the importance of [measles] vaccination not only to protect against measles but also for the maintenance of immunity to a range of other pathogens, which can be compromised after [measles] infection,” the team publishing in Science Immunology concluded.

For more than a century, researchers have noted that the measles virus seems to leave kids more vulnerable to other infections for months to years. Recent surveys have found that some children show signs of a suppressed immune responses five years after a measles infection, leading to other infections—some of which can be deadly. Epidemiological data from the pre-vaccine era linked the measles virus to up to 50% of all childhood deaths from infectious illnesses in that time period. Most of the deaths were from infections other than measles in the years following a measles infection.

Researchers hypothesized that the virus—which is known to directly infect immune cells—was wiping out preexisting immunities, a potential phenomenon called “immunological amnesia.” But, until now, there was little molecular evidence to explain how it might do that. More puzzling, while the measles virus seems to knock back immunity to other infections, kids who recover from the virus have strong, long-lasting protection against measles itself.

For the new studies, researchers drew upon a group of unvaccinated Dutch kids (aged 4 to 17) in doctrinally conservative Dutch Reformed communities with low vaccination rates. Researchers drew blood and examined their immune cells before and after a measles outbreak swept through in 2013.

Know thyself

In the Science Immunology study, researchers focused on immune cells called B cells, which can pump out antibodies, among other things. The researchers found that after measles swept through, the kids had different populations of what’s called B memory cells. These are a subtype of B cell that have relatively long lifespans and can swiftly fire off a large number of antibodies when a familiar germ reappears—making for an intense defensive that can quickly eliminate threats. The researchers found that in the wake of measles, the kids had depleted stores of B memory clones, meaning they wouldn’t be able to mount their normally fierce response to certain known enemies.

In the second study in Science, a team led by researchers at Harvard sorted through the kids’ collections of antibodies before and after measles. They did so using a new tool called VirScan that allows them to survey blood antibodies that might respond to thousands of pathogens. The researchers found that the kids lost 11% to 73% of their individual types of antibodies after recovering from measles. That is, they lost the ability to detect 11% to 73% of the molecular signatures on germs that they were able to detect before measles hit.

While the findings add data to support the old idea of immune amnesia, there are still lingering questions about what measles does—and how our own immune systems work. For instance, after researchers found depleted stores of B memory cells in the wake of a measles infection, they found that the pools of memory cells became more diverse. It’s not clear what that means for the immune system or how the immune system is able to maintain strong immunity to measles with the altered cell population.

In an accompanying commentary, Harvard immunologist Duane R. Wesemann notes that “the measles holds a special place in immunology,” given that humans are its only victims and it has complex interactions with our immune systems. The new studies, he concludes, “represent an opportunity to ex­amine the measles virus against the Delphic maxim to ‘know thyself’—because the unique relationship measles has with the human im­mune system can illuminate aspects of its in­ner workings.”

Science Immunology, 2019. DOI: 10.1126/sciimmunol.aay6125 (About DOIs).

Science, 2019. DOI: 10.1126/science.aay6485

Science Immunology, 2019. DOI: 10.1126/sciimmunol.aaz4195