A virus called cytomegalovirus, or CMV, may strengthen, rather than harm, the immune system in old age, according to new research with mice.
Our immune system is at its peak when we’re young, but after a certain age, it declines and it becomes more difficult for our bodies to fight off new infections.
“That’s why older people are more susceptible to infections than younger people,” explains Janko Nikolich-Žugich, co-director of the Center on Aging at the University of Arizona and chair of the immunobiology department at the university’s College of Medicine–Tucson.
In search of a way to rejuvenate the immune system of older adults, Nikolich-Žugich and Megan Smithey began investigating CMV. The virus, which people usually contract at a young age, affects more than half of all individuals. Because there is no cure, the virus is carried for life and is particularly prevalent in older adults.
“CMV doesn’t usually cause outward symptoms, but we still have to live with it every day since there’s no cure,” says Smithey, a research assistant professor specializing in immunobiology. “Our immune system always will be busy in the background dealing with this virus.”
Smithey and Nikolich-Žugich wondered how this lifelong virus ultimately affects the immune system. To study the effects of CMV, researchers infected mice with the virus.
“We assumed it would make mice more vulnerable to other infections because it was using up resources and keeping the immune system busy,” Smithey says.
But that’s not what happened. When infected with listeria, old mice carrying CMV proved to be tougher than old mice without CMV.
“We were completely surprised; we expected these mice to be worse off,” Smithey says. “But they had a more robust, effective response to the infection.”
Researchers are not certain how CMV strengthens the immune system—they are investigating that in a separate study—but they do believe they have gained new insight into the aging immune system.
“This study shows us that there is more capacity in the immune system at an older age than we thought,” Smithey says.
For years, immunobiologists thought T-cells—the army of defenders that fights off infection—decreased in diversity as people aged, leaving older adults more susceptible to diseases. But when researchers examined the mice’s T-cells, they found both groups of older mice had a decent supply of diverse T-cells.
“Diversity is good,” Nikolich-Žugich says. “Different types of T-cells respond to different types of infections; the more diverse T-cells you have, the more likely you’ll be able to fight off infections.”
Smithey and Nikolich-Žugich’s study shows that T-cells are almost as diverse in old mice as they are in young mice. The problem is diverse T-cells are not recruited to the battlefield in older mice unless they are infected with CMV.
“It’s as if CMV is issuing a signal that gets the best defenses out onto the field,” Nikolich-Žugich explained.
“This shows that the ability to generate a good immune response exists in old age—and CMV, or the body’s response to CMV, can help harness that ability,” Smithey adds.
The researchers plan to continue to study CMV and hope to see similar results in human studies. Their ultimate goal is to create a vaccine that can improve the immune systems of older adults and protect against infection.
The study appears in the Proceedings of the National Academy of Sciences. Additional researchers from the University of New South Wales Australia; the University of Arizona; and the University of North Carolina at Chapel Hill contributed to the work.
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