Pop a question into a scientist’s head about thymic T-cell education and aging these days, and the result is akin to a slot machine spewing hypotheses.

The thymus has prominently entered longevity conversations in recent years amid new evidence that it is relevant in adulthood.

Now, the tiny and once written-off thymus is prompting another gee-whiz moment, because a new study in Nature Communications links its hormone-producing role as a gland to inflammaging and cancer immunotherapy effectiveness.

And it all happened because a surgical oncologist went searching for answers to why seemingly excellent preclinical foundations weren’t holding up in his immuno-oncology investigator-initiated phase 1 clinical trial. 

“I didn’t mean to find something that could reverse age-related chronic inflammation. I had no intention to do that,” said Fumito Ito, MD, PhD, surgical oncologist and professor at the University of Southern California Keck School of Medicine in Los Angeles. “This was motivated because I felt really disappointed to see that my first seven patients did not respond to the treatment. I wanted to get to the bottom of it.”

Through a well-designed succession of experimental models, Ito identified the thymic hormone thymulin as a potential causal link between thymic involution and inflammaging. It builds upon what are now known as the “big thymus, little thymus” papers in Nature from earlier this year that associated adulthood thymic function with longevity and cancer immunotherapy effectiveness.

Ito’s work reaches complementary conclusions and offers causal evidence.

“Maybe this is something we all need to think about now,” said Jarrod Dudakov, PhD, thymus researcher and professor at Fred Hutch Cancer Center in Seattle (he wasn’t involved in the study). “Maybe it’s not just about coming in with a thymus-boosting therapy. Do we need to tease apart the effects on T-cell development vs the effects on something like thymulin production? I honestly don’t know the answer to that, but it’s certainly provocative.” 

For Ito, the starting point was challenging a design decision in the preclinical work for his solid tumor clinical trial: Maybe the mice were too young.

A 50-Year-Old Woman (in Mouse Years)

The project felt high-risk at first, Ito admitted. Aged mice are expensive and difficult to acquire.

Richard Miller, MD, PhD, who specializes in aged mouse work, estimates that an aged research mouse costs $400 to $500 if you raise it yourself (for 2 years). That’s 8-10 times the cost of a young mouse.

“The market for aged mice is not great,” said Miller, professor of pathology and director of the Paul F. Glenn Center for Biology of Aging Research at the University of Michigan in Ann Arbor. His lab develops drugs to slow aging and wasn’t involved in Ito’s work.

In all of the phases of Ito’s study, the team used young mice (8-12 weeks old) and old mice (65-75 weeks old) and also tested both mouse and human samples. An adult mouse is approximately 48 weeks old, so Ito’s 65-week-old mice were comparable to female humans in their 50s.

Ito said he felt obligated to explore his hunch that aged mice would reveal important information following the lack of treatment response among his seven clinical trial patients. That sense of obligation transformed as the study progressed and eventually “gave me a totally different feeling. It felt like solving a mystery, solving a riddle, or doing puzzles. It ended up to be a fun project,” he said.

To start, Ito’s team put the same tumor cells in both young and old mice. The tumors grew much more aggressively in the older mice. They then used the immunotherapy from his clinical trial and observed a decreased immune response in the aged mice. Next, they identified the pro-inflammatory cytokines involved (they were interleukin (IL)-1a, IL-1b, IL-6, and TNF-alpha). The team traced the source of the cytokines to myeloid cells, which increased with age in both mice and in human samples. 

At this point, a question jumped into Ito’s mind: Is this inflammation reversible?

“We happened to have a postdoc, Dr Yuta Yamamoto, who has experience with the surgical joining of the mice,” Ito said.

Parabiosis and bone marrow chimeras revealed circulating factors present in young hosts could suppress age-associated myeloid cell inflammatory activation. Subsequent analyses identified thymulin as a potential suppressor, and finally the researchers demonstrated that thymulin boosted immune response against tumors, improved overall survival, and made older mice more responsive to anti-PD-1/PD-L1 therapy, a type of cancer immunotherapy.

“The anti-inflammatory properties of thymulin are not entirely new, but what is particularly interesting here is the biological context,” said Ailin Lepletier, PhD, immunologist and senior research fellow at Griffith University in Gold Coast, Australia (she was not involved in the study).

Beyond supporting T-cell development, Ito’s study shows thymulin can act at distant sites such as the tumor microenvironment and ultimately improve anti-tumor T-cell responses and responsiveness to checkpoint immunotherapy.

“The study suggests that thymic involution may affect immunity not only by reducing T-cell generation but also by reducing endocrine signals such as thymulin that help regulate systemic inflammation. I think that is a very interesting conceptual advance,” Lepletier said. “The most surprising aspect for me was the strength of the link between the thymus and innate immune regulation. As thymus researchers, we naturally tend to think first about T-cell development and adaptive immunity.”

A little-known fact about thymulin is that its activation depends on zinc binding, which is why zinc is often marketed as an immunity supplement, Lepletier noted. 

“I was also struck by how age-dependent the effect was,” she said. “Thymulin had relatively little effect in young mice but had a much stronger effect in aged animals. That suggests it may be correcting an age-associated deficit rather than acting as a nonspecific immune stimulant.”

Both Lepletier and Dudakov said they would like to see what happens when thymulin is selectively depleted or neutralized in a young animal.

A key takeaway from this study, though, is not about thymulin at all, Lepletier said. It’s also about the age of mice in preclinical work.

What’s Next

Just a couple of weeks after publishing his thymulin paper, Ito received $3.2 million for follow-up research from the National Institutes of Health. He plans to examine thymulin’s effect on cancer metastases and further explore underlying mechanisms of how thymulin regulates age-related chronic inflammation. The study is expected to take 5 years and will include aged mice with breast cancer or melanoma, plus in vitro work.

Ultimately, he hopes to progress to testing thymulin as a therapy in a clinical trial, he said.

As for his solid tumor clinical trial, Ito did find a way to improve the clinical regimen. It wasn’t thymulin therapy, he said, adding that the results will be published in a forthcoming paper.

What he really hopes others will take note of is that there is substantial remodeling of the immune system with age, and researchers shouldn’t underestimate the value of using aged mice in immunotherapy preclinical studies.

And despite the recent onslaught of advanced understanding of the thymus, its main mystery persists, Dudakov noted.

“This is the age-old question: Why does the thymus involute?” he said. “We know it happens across pretty much all invertebrate species. We have no idea why.”

Ito reported holding a consulting/advisory role with Otsuka Pharmaceutical Factory, Yakult Honsha Co., Ltd., and Hamamatsu Photonics. Dudakov disclosed that he is the scientific co-founder of Thymofox, a company working to develop thymus-boosting therapies. Miller and Lepletier reported having no conflicts. Disclosure information for study authors is available in the original study publications.