According to the BMJ Best Practice resource, "Cytomegalovirus (CMV) is a ubiquitous beta-herpes virus that infects the majority of humans."
Infected individuals typically do not experience any symptoms. The virus can be transmitted by coming into direct contact with the bodily fluids, such as blood, of an already-infected individual.
Once acquired, it remains in a person's body for their entire life.
In a new study that was conducted in mice, Dr. Janko Nikolich-Žugich — of the University of Arizona College of Medicine in Tucson — and team decided to look into how, and under what conditions, aging individuals would mount a stronger immune response against viral infections.
Typically, young bodies have stronger defenses. But as we age, our immunity begins to decrease. "That's why older people are more susceptible to infections than younger people," explains Dr. Nikolich-Žugich.
The scientists involved with the new study were interested in finding out how the immune systems of aging individuals might be fortified and rendered more efficient once more.
Does CMV keep the immune system busy?
In this process, Dr. Nikolich-Žugich and team compared older mice infected with CMV with mice in the same age range but without the virus, expecting to see that the CMV-infected mice had a weaker immune system and thus mounted a poorer defense against other viruses.
"CMV doesn't usually cause outward symptoms," notes first study author Megan Smithey, "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," she adds.
"We assumed [therefore that] it would make mice more vulnerable to other infections because it was using up resources and keeping the immune system busy," Smithey goes on to explain.
However, the researchers were in for a surprise.
CMV determines 'more robust' defense
Working with a group of aging mice — some carrying CMV and others not — the team tried to infect them all with Listeria, a type of harmful bacteria that is usually found in contaminated foods. Listeria can cause a disease known as "listeriosis," characterized by fever, sickness, and diarrhea.
The researchers expected the CMV-infected mice to be more susceptible to the bacteria — in fact, they turned out to be more resilient than their CMV-free counterparts.
"We were completely surprised; we expected these mice to be worse off. But they had a more robust, effective response to the infection."
Although they are not yet sure how or why CMV enhances the immune response, the researchers are happy to have made an important discovery about the functioning of the immune system as it ages — namely, that it is capable of mounting a better defense against foreign agents that specialists had previously believed.
"This study shows us," says Smithey, "that there is more capacity in the immune system at an older age than we thought."
Specifically, the researchers found that both the CMV-infected and the CMV-free mice, though fairly advanced in age, had diverse population of T cells, which are specialized immune cells with various functions.
"Diversity is good," says Dr. Nikolich-Žugich. "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."
T cells no less diverse in old age
This revelation also took the scientists by surprise; for a long time, the assumption had been that T cell populations become less diverse with age, and this had been considered a main factor in the decreased effectiveness of immune responses.
Now, Dr. Nikolich-Žugich and team saw that the T cells in aging mice were no less diverse than in younger animals.
The problem was that the T cells were not typically recruited to defend the systems. However, older CMV-infected mice did not seem to encounter this issue, and their T cells were more active.
"It's as if CMV is issuing a signal that gets the best defenses out onto the field," Dr. Nikolich-Žugich observes.
"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 hypothesizes.
The scientists' findings are now reported in PNAS. And already, they are planning to deepen their understanding of CMV's effect on the immune system in further studies.
In the future, the team hopes to be able to replicate its recent results in another study with human participants.
Should this happen, Dr. Nikolich-Žugich and colleagues would aim to design a vaccine that could more effectively boost the immune systems of older adults.