When ancient humans interbred with Neanderthals, they inherited DNA that may influence modern Europeans' immune systems to this day, a new study suggests.
The research found that inflammation and other immune responses work differently in Africans than they do in Europeans, in part because Europeans have inherited some of their genetic information from Neanderthals, which were at one time the closest living relatives of modern humans.
The study showed that people of African ancestry may have a stronger inflammatory response to certain infections than people of European ancestry. For example, in one experiment, researchers investigated how people's immune cells would respond to Salmonella or Listeria bacteria. They compared these immune-cell responses in 80 African individuals with the immune-cell responses in 95 European individuals. They found that African ancestry was linked with a stronger inflammatory response, which reduced bacterial growth by more than triple the amount compared with the European cells. [9 Healthy Habits You Can Do in 1 Minute (Or Less)]
But although a strong inflammatory response "can be life-saving in the face of infectious agents," when that response malfunctions or gets triggered at the wrong time, it can damage tissues or cause inflammatory diseases, said Luis Barreiro, a geneticist at the University of Montreal and a senior author of the new study.
"Our results suggest that the immune systems of African- and European-descended individuals have evolved to better respond to the specific needs imposed by their specific environments," Barreiro told Live Science. "What is advantageous in one context is likely to be detrimental in another."
Inflammation & health
Previous research found that, when compared to people of African ancestry, people of European descent may experience infectious diseases, such as tuberculosis, as well as autoimmune disorders, such as lupus and psoriasis, up to three times more often.
In the new study, two independent groups of researchers wanted to investigate these ancestry-related differences. The group led by Barreiro, which compared how immune cells from people of African descent and those from people of European descent responded to germs, found that Europeans' and Africans' susceptibility to disease may differ, in part because Europeans inherited genes from Neanderthals.
Barreiro and his colleagues also found that nearly 10 percent of the genes in immune cells called macrophages, which gobble up invaders in the body, responded differently to infections depending on people's ancestry. They also found that hundreds of these ancestry-related differences displayed signs that they evolved recently, suggesting that they helped Africans and Europeans adapt to their environments in some way.
One possible explanation for this weaker inflammatory response in Europeans is that after modern humans migrated out of Africa, they were exposed to lower levels of pathogens, or harmful germs, "which reduced the need for strong, costly pro-inflammatory signals," Barreiro said. The shift toward less inflammation may have been favored because of the negative effects of too much inflammation, he said.
Alternatively, Europeans may have a weaker inflammatory response because pathogens outside of Africa were generally less harmful than the ones in Africa, Barreiro said. [11 Surprising Facts About the Immune System]
The scientists also noted that a small fraction of the ancestry-related differences they found were linked to genetic variants from Neanderthals. Recent findings suggest that the modern humans who migrated out of Africa about 60,000 to 120,000 years ago interbred with Neanderthals and other archaic human lineages that had left Africa before modern humans did.
Prior work found that about 2 percent of DNA in people living outside Africa today is Neanderthal in origin. It remains uncertain how those Neanderthal genetic variants may influence the immune differences seen between Africans and Europeans, Barreiro said.
The other research group analyzed how immune cells derived from 200 people of self-reported African or European ancestry responded to flu viruses and to molecules that trigger antibacterial reactions. This group also found that Neanderthals introduced genetic variants influencing responses to viruses into European genomes, said Lluis Quintana-Murci, a population geneticist at the Pasteur Institute in Paris and a senior author of this study.
Quintana-Murci and his colleagues also found that a single gene variant seen in Europeans but not in Africans reduced activity in 81 inflammation genes. This finding suggests that Europeans evolved to have this weaker immune response because it had a survival advantage, Quintana-Murci said.
All in all, "these results have important medical implications, since they uncover mechanisms that might explain ethnic disparities" in immune disorders, and could lead to future treatments, Quintana-Murci told Live Science.
The scientists detailed their findings online Oct. 20 in two studies in the journal Cell.
Original on Live Science.