Study: Cytomegalovirus Infection May Enhance Immune System

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In search of a way to rejuvenate the immune system of older adults, a team of scientists from the University of Arizona College of Medicine and elsewhere has found that the cytomegalovirus, a common viral infection affecting between 60% and 99% of adults worldwide, may not weaken, but actually boost our immune system.

This transmission electron microscopic image depicts numbers of cytomegalovirus virions. Image credit: Sylvia Whitfield / CDC.

This transmission electron microscopic image depicts numbers of cytomegalovirus virions. Image credit: Sylvia Whitfield / CDC.

“The cytomegalovirus is contracted, for most part, at a young age. Because there is no cure, this virus is carried for life, and is particularly prevalent in older adults,” said study first author Dr. Megan Smithey, a researcher in the Department of Immunobiology and the Arizona Center on Aging at the University of Arizona College of Medicine.

“The virus doesn’t usually cause outward symptoms, but we still have to live with it every day since there’s no cure.”

“Our immune system always will be busy in the background dealing with this virus.”

Dr. Smithey and co-authors wondered how this lifelong virus infection ultimately affects the immune system.

For the study, they infected aged mice with the murine cytomegalovirus.

“We assumed it would make mice more vulnerable to other infections because it was using up resources and keeping the immune system busy,” Dr. Smithey explained.

“But that’s not what happens. When infected with Listeria, old mice carrying the cytomegalovirus proved to be tougher than old mice without the virus.”

“We were completely surprised; we expected these mice to be worse off. But they had a more robust, effective response to the infection.”

The scientists are not certain how the cytomegalovirus 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,” Dr. Smithey said.

When Dr. Smithey and colleagues examined the mice’s T-cells, they found that both groups of older mice had a decent supply of diverse T-cells.

“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,” said study senior author Dr. Janko Nikolich-Zugich, co-director or the University of Arizona Center on Aging and chairman of the Department of Immunobiology at the University of Arizona College of Medicine.

For years, immunobiologists assumed that T-cell diversity decreased as we age. This was one of the reasons why older adults succumbed to disease more easily.

But the team’s new study shows that T-cells are almost as diverse in old mice as they are in young mice.

The problem is that diverse T-cells are not recruited to the battlefield in older mice — unless they are infected with the cytomegalovirus.

“It’s as if the cytomegalovirus is issuing a signal that gets the best defenses out onto the field,” Dr. Nikolich-Zugich said.

“This shows that the ability to generate a good immune response exists in old age — and the cytomegalovirus, or the body’s response to the virus, can help harness that ability,” Dr. Smithey added.

The study appears in the Proceedings of the National Academy of Sciences.

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Megan J. Smithey et al. Lifelong CMV infection improves immune defense in old mice by broadening the mobilized TCR repertoire against third-party infection. PNAS, published online July 2, 2018; doi: 10.1073/pnas.1719451115