Social status alters immune response in monkeys
Life at the bottom of the socioeconomic hierarchy can be stressful. People with lower social status have altered immune system function and are at increased risk for heart disease, stroke, and hypertension.
Like humans, monkeys establish a social hierarchy. A previous study with rhesus macaques revealed that social stress affects the expression of almost 1,000 genes, most of which are related to the immune system.
In a new study, a team of researchers led by Dr. Luis Barreiro from the University of Montreal, Canada, and Dr. Jenny Tung from Duke University further investigated the link between social status and the immune system in rhesus monkeys at the Yerkes National Primate Research Center in Atlanta. The research was funded in part by NIH's Office of the Director (OD), National Institute of General Medical Sciences (NIGMS), and National Institute on Aging (NIA). Results appeared in Science on Nov. 25.
The social hierarchy of female rhesus macaques in captivity can be carefully controlled. Groups form stable, linear social hierarchies, with those introduced first ranking higher. Each new introduction to a group, then, begins with the lowest social rank. The researchers assembled 45 unrelated captive adult female monkeys, one by one, into 9 groups of 5 monkeys each.
After a year, the researchers broke apart these groups and placed monkeys with the same rank together into new groups. The scientists established new social hierarchies by introducing the monkeys one at a time into the new groups.
To analyze changes in the monkeys' immune systems, the researchers isolated major immune cells from blood samples. They found that social rank altered the proportions of different immune cells. Social rank also affected the activity of genes within some of those cells, particularly natural killer (NK) cells and helper T cells. The changes seen in low-ranking females were consistent with increased inflammation. Chronic inflammation can play a role in numerous health problems, including heart disease, type 2 diabetes, and certain cancers.
As the social ranks of the monkeys changed during the second phase of the study, gene activity in NK and helper T cells changed as well. These findings suggest that social rank affects specific immune cells, and that these effects can be reversed. Specific behaviors modulated these effects as well. Grooming, a form of social support in macaques, affected the activity of 33 percent of the genes that responded to rank in NK cells. Harassment also affected activity of these genes in both helper T cells and NK cells.
Next, the researchers exposed the isolated immune cells to a bacterial compound that triggers a strong inflammatory response. They found that lower ranked females had a greater response to this immune challenge.
"A strong inflammatory response can be life-saving in the face of infectious agents," Barreiro says. But low social rank may lead these same self-defense mechanisms to become too active for too long, risking health. Further research will be needed to determine if social status in humans is linked to similar changes in the immune system.
--by Anita Ramanathan
For further information on this and other health topics, visit the web site of the National Institute of Health at www.nih.gov.