Scientists have discovered a way to extend the lives of lab mice by 40 percent and suggest that it could one day be used as a treatment to boost human lifespans.

In a study published this week in the Proceedings of the National Academy of Sciences (PNAS), the researchers show that increasing levels of a hormone called FGF21 can protect against the loss of immune function that comes with age.

Lead study author Vishwa Deep Dixit, Ph.D., professor of comparative medicine and immunobiology at the Yale School of Medicine, said that while previous studies have shown the hormone enhances lifespan in mice and could be a potential treatment for type 2 diabetes, this research is the first to suggest that FGF21 can protect against age-related decline of immune function in mice -- a finding that could have big implications for humans.

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"Our research team is the first to show that FGF21 is a potential target for enhancing T cell function in the elderly and in cancer patients that undergo bone marrow transplantation," Dixit told CBS News.

FGF21 is predominantly produced in the liver, but is also found in the thymus, a small organ located between the lungs that is an important part of the body's immune system. When functioning normally, the thymus produces new infection-fighting T cells, but with age, it becomes fatty and loses this ability, leading to an increased risk of infections and certain cancers in the elderly.

Dixit and his team manipulated levels of FGF21 in lab mice and found that increasing amounts in older mice protected the thymus from age-related fatty degeneration and increased the ability of the thymus to produce new T cells. Meanwhile, a deficiency of the hormone led to accelerated degeneration of the thymus in old mice.

The overall lifespan of the mice with elevated levels of FGF21 was extended by 40 percent.

Though the findings are a long way off from being applicable to humans, Dixit entertains the possibilities of what this could mean for humans in terms of longevity.

"If the average lifespan is 80 years in humans, and if FGF21's effects from mice translate into humans, FGF21 may add approximately 30 years to life," he said. "But this is speculative at this time."

Perhaps more realistic than FGF21 being the key to immortality, Dixit said the hormone may one day help people fight off diseases as they age.

"The elderly have an increased risk of infection, vaccine failures, and certain cancers because the thymus does not enough make new T cells," he said. "If FGF21 can increase the T cell production from the thymus in humans, this can potentially be used to enhance immune function."

Additionally, increasing FGF21 to boost immune function is "also clinically relevant in cancer patients where the immune system needs to be re-established," Dixit said. "Currently, there is high risk of complications from infections in older patients that undergo this treatment because the thymus is unable to make T cells."

The researchers note that FGF21 is an endocrine hormone that can improve insulin sensitivity and induces weight loss, and it is also being studied for its therapeutic effects in type 2 diabetes and obesity. Its levels increase when calories are restricted to allow fat to be burned when glucose levels are low.

Dixit said future studies will look at understanding exactly how FGF21 protects the thymus from aging, whether an FGF21 drug therapy could be developed to extend human lifespan and protect against disease caused by age-related loss of immune function. Researchers are also interested in developing new ways to mimic calorie restriction without actually reducing the amount people eat.

When asked how far off these notions may be from reality, Dixit replied: "The early stage research discoveries require several years to be translated in humans. Substantial research funding and time is required to test the efficacy of FGF21 on immune function in the elderly and cancer patients. It is difficult to predict this at this time."