Young blood reverses aging in old organs

The fountain of youth could come in the form of a familiar viscous fluid: blood. Studies dating back to the 1950s showed that the blood of a younger rodent rejuvenates the organs of an elderly one after scientists surgically joined the circulatory systems of two rodents in a procedure called parabiosis (1–3). 

“In a way, it’s nature’s cocktail or nature’s elixir,” said Tony Wyss-Coray, who studies brain aging and rejuvenation at Stanford University. “It's almost like you have a polypharmacy in the blood.”

Since blood is chock full of nutrients, proteins, enzymes, platelets, and helpful immune cells, Wyss-Coray and other scientists study the possibility that youthful blood — or specific components of it — could help fight the effects of aging throughout the body. “The advantage is that it's a natural composition,” said Wyss-Coray. “You're not making it artificially, but you would use it the way nature made it.”

The concept of using blood as a drug certainly has its fair share of skeptics and has caused ongoing disagreements among scientists. Steve Horvath, an aging researcher at Altos Labs, admitted that he was one of those skeptics until he saw the data himself. Horvath is well known in the field of aging for creating epigenetic clocks, which predict the biological age of tissues based on the epigenetic mechanism of DNA methylation. “I did many studies where I applied epigenetic clocks to this idea ... and I kept seeing the same results,” said Horvath. “By now, I'm kind of a believer. I do think there is a strong effect, especially for rodents, no debate about it.”

“The question is what about humans?” he said.

Joining the young and old

The first experiment to suture two animals together using parabiosis dates back to 1864 when physiologist Paul Bert performed experiments on albino rats. In the 1950s, Clive McCay, a biochemist at Cornell University, began using the procedure with mice of different ages to study the effects of aging. However, there are great risks for the two joined animals, who sometimes die from a mysterious condition called “parabiotic intoxication” just a few weeks after the surgery (4). 

In 2005, the technique finally gained prominence once again after the publication of a Nature  paper by researchers from the group of Thomas Rando, a neurologist and stem cell researcher now at the University of California, Los Angeles (2). In that work, Rando worked with two postdoctoral researchers in his lab, Irina and Michael Conboy, who are both now at the University of California, Berkeley, and showed that the young blood activated molecular signaling pathways in liver and muscle cells that led to regeneration of tissue in old mice paired with young mice.

“The main finding of that paper was a confirmation that the age of a cell appears to be malleable,” said Rando. “All cells can become younger when put in a younger environment.”

Their paper launched a wave of curiosity from other researchers who then used parabiosis or plasma transfusions to demonstrate similar rejuvenating effects in the brain, intestines, kidneys, and bones of rodents (5–8).

Plasma for the people

In humans, it won’t be feasible to surgically join two people who must suddenly do everything together. Instead, transfusions of young blood or plasma — the clear, yellow liquid part of blood that includes everything from proteins and antibodies but no blood cells — offer a more promising approach.

In 2014, Wyss-Coray cofounded Alkahest, a biotechnology company focused on developing plasma-based therapies to treat age-related diseases. In one of their therapeutic approaches, researchers at Alkahest infuse older adults with young fresh frozen plasma and plasma fractions that are enriched in albumin, the most abundant protein in blood. Albumin itself acts as a carrier for thousands of other proteins in the blood. So far, Alkahest has completed Phase 1 and 2 clinical trials to treat patients with Alzheimer’s disease and Parkinson’s disease with young plasma fractions (9–11). “What we were able to do is to show that, first of all, treating patients with Alzheimer's disease or Parkinson's disease with young plasma fractions is well tolerated, and it had promising effects,” said Wyss-Coray. The company is now planning Phase 3 trials, but Wyss-Coray noted challenges with finding the correct patient population and obtaining funding.

Grifols, the company that makes the plasma fractions used in Alkahest’s studies, completed a Phase 2b/3 clinical trial in patients with Alzheimer’s disease in 2020 (12). They used a process called plasmapheresis, in which patients have their blood drawn. It goes into a machine that separates out the plasma and administers the blood cells back into the patients. In the trial, they infused patients undergoing plasmapheresis with an albumin fraction from young plasma donations. (Wyss-Coray noted that the average donor age is around 30-35 years.) Their results suggested that plasmapheresis with albumin replacement may slow cognitive decline for severe Alzheimer’s disease patients.

Dobri Kiprov, a physician and coauthor of the Grifols paper, has been practicing plasmapheresis in his clinic for 35 years. He often uses it to treat blood disorders, cancers of the blood, and neurological diseases. Similar to the treatment in the trial, Kiprov’s team offers older patients a plasmapheresis procedure that replaces their plasma with the albumin fraction. Kiprov uses the albumin fraction instead of plasma from young donors because plasma infusions can have rare but severe side effects like anaphylactic shock and lung disease. “I'm a physician, and most of the people that deal in this field are not physicians. They are PhD scientists or biologists,” he said. “My main concern is I don't want to harm anybody.” 

The plasmapheresis office procedure is quite expensive since it’s not covered by insurance, but Kiprov hopes that will change with data from more randomized clinical trials. He recently presented unpublished data at the American Academy of Anti-Aging Medicine’s 2023 LongevityFest conference in collaboration with researchers at the Buck Institute for Research on Aging. Their work showed that a person’s epigenetic age decreased by about two to five years, and the procedure reduced circulating levels of apolipoprotein E proteins, amyloids, and lipids in the blood, which are often associated with cardiovascular risk. “We have human proof — not mice, not monkeys,” said Kiprov.

It comes back to in animals versus in humans ... So, I'm scratching my head a bit about that. 
– Steve Horvath, Altos Labs

Horvath looks forward to seeing more human data since he has not found the data from his own studies to be as convincing as the data in rodents. In 2022, he published work with James Clement of the nonprofit scientific research organization Betterhumans Inc, which injected umbilical cord plasma concentrate into 18 elderly patients for ten weeks (13). Although their study showed that the plasma treatment was safe, and that it improved several markers of health including kidney function, they found little effect on their measures of aging with Horvath’s epigenetic clocks. The only significant change was that GrimAge, an epigenetic clock that predicts mortality risk, decreased by an average of 0.82 years. “Imagine you're a 74-year-old man or woman and you say, ‘Oh, after all this effort and high-potency [plasma], I get a reduction of 0.8 years.’ You shrug your shoulders,” said Horvath. In contrast, Horvath noted that in rats, they found strong effects on every epigenetic clock they measured. “It comes back to in animals versus in humans,” said Horvath. “So, I'm scratching my head a bit about that.”

Still, Horvath said that it’s a promising avenue of research, and it’s worth investigating further to identify an effective approach in people. “The challenge for the field is to find these magic ingredients … and basically make it more potent,” he said. “These plasma transfusions [and] the concentrate, one would perhaps call it a biologic. But maybe if you find the targets, you can develop a small molecule or what have you and really crank it up.” 

At the same time, Horvath pointed out that one major advantage of using natural young blood or plasma is that the delivery problem inherent to most drugs is already solved since blood can easily travel along the built-in circulatory system and reach every organ. 

Unraveling the mechanisms of young blood

Researchers have been working to identify the possible “magic ingredients,” as Horvath called them, in young blood and plasma that rejuvenate aging tissues. The clinical trials using an albumin fraction suggest that it could be beneficial, but Wyss-Coray said that the effects can’t be isolated to albumin alone because thousands of other proteins bind to it. Instead, researchers are looking for individual proteins or growth factors that provide the antiaging benefits.

As one example, studies from Amy Wagers, Richard Lee, and Lee Rubin at Harvard University suggest that growth differentiation factor 11 (GDF11) has positive effects on the aging brain (6,14). “GDF11, as it turns out, does not get into the brain. We think it just targets vasculature and regrowth of vasculature in the old brain, and vascular cells in the brain secrete neurogenesis-enhancing factors,” said Rubin. Along with Wagers and Lee, Rubin started a company called Elevian with plans to do a Phase 1 clinical trial testing a GDF11 drug to revascularize damaged brain tissue after stroke.

Researchers believe that it’s unlikely for a single drug to have rejuvenating effects on the whole body. “I don't think there is a magic one factor that does it all,” said Wyss-Coray. 

In 2022, Wyss-Coray’s group published work using single-cell RNA sequencing on 20 different organs to reveal the molecular mechanisms behind the benefits from young blood (15). “What we can see is that different cells respond in different ways. They age in different ways, but they also respond in different ways to rejuvenation,” said Wyss-Coray. In particular, his team found that adipose mesenchymal stromal cells, hematopoietic stem cells, and liver cells were especially responsive to the rejuvenating effects of young blood. The effects of the young blood appeared to work by increasing gene expression in these cells that reversed the age-related decline in gene expression. 

The changes in gene expression may relate to the mechanism that Rando and Horvath now believe is at the heart of young blood’s antiaging effects: the epigenome. “What I think this field really is all about is how do you rejuvenate a cell through the epigenome,” said Rando. 

In 2020, Horvath published a preprint with Harold Katcher, a biochemist at Yuvan Research, showing for the first time that a plasma fraction directly influenced the epigenome and reversed the epigenetic age of rat organs (16).  “Coming back to why was I nervous about the whole field, it was not so much that I didn't believe the literature on functional rejuvenation. My concern was that it wouldn't affect the epigenome,” said Horvath, adding that he expected it to affect the proteome. “The real surprise is that it affects the [epigenome] … How is that even possible?”

Good young blood or bad old blood?

Researchers may never reveal all the molecular mechanisms by which young blood refreshes old organs. “Blood is a very, very complex tissue. And there are many kinds of molecules that are proteins and lipids and peptides and metabolites,” said Rando. “There’s a combinatorial effect of the environment of young blood and old blood that is communicating and influencing the cells and tissues throughout the body.”

However, not everyone agrees that the beneficial effects come from young blood in the first place. After their initial work on parabiosis with Rando, Irina and Michael Conboy continued to investigate why parabiosis between old and young pairs works in mice. They wanted to tease out whether the rejuvenating results come from adding beneficial factors from young plasma or removing harmful factors in old plasma. In 2015, they collaborated with David Schaffer, a stem cell and gene therapy researcher at the University of California, Berkeley to show that reducing levels of the transforming growth factor β (TGF-β) protein in the blood of old mice leads to the birth of new neurons and the regeneration of muscle (17). 

More recently, they decided to infuse old mice with an “ageless” substance that is neither young nor old. They chose to replace half of the plasma in older mice with a saline solution containing five percent albumin.

“Nobody believed that it was going to do anything,” said Michael Conboy. But then they saw rejuvenating effects in muscle tissue, liver tissue, and the birth of new neurons and less inflammation in the brain. They published their results in 2020 and 2021 and concluded that diluting the old blood had even stronger rejuvenating effects than infusing young blood in paired mice, and therefore dilution is all that’s needed for rejuvenation (18,19). Irina Conboy said that the old mice in parabiosis experiments likely did not become younger due to the young blood, but instead may have shown rejuvenating effects because the young mice removed or neutralized age-elevated proteins.

The Conboys interpret their results only to mean that diluting old blood helps, not that albumin is the beneficial factor at play. Irina Conboy emphasized that they did not add extra albumin, but rather “we removed 50 percent of the albumin when we threw out the old plasma and replaced it with the commercial albumin." She added, “[The albumin] is a commercial powder which is stored on the shelf and obviously does not hold the secret to life.” 

Horvath, Wyss-Coray, Rando, and Rubin all agreed that removing harmful factors from old blood through dilution or other means can show positive effects. However, they said that the Conboys’ results don’t take away from the many studies showing the rejuvenating effects of young blood. “To me, the statement that simply diluting is enough, I wouldn't go that far. Biology is so complex, it's really both. Yes, dilute away the old stuff. But also, conversely, add some of the young stuff,” said Horvath. 

“There's no reason why these both can't be true,” said Rando. “The fun part is where you realize how all these data can come together in a kind of unified theory and really help us to understand aging in a new way.”

Young blood on trial

Most evidence for the use of young blood comes from rodents, but that hasn’t stopped many older people around the world from trying young blood as a rejuvenation therapy — as long as they have the means to pay for it. Numerous Silicon Valley biotech companies have promised antiaging benefits from young blood transfusions for a hefty price. Going even further, Bryan Johnson, a billionaire tech entrepreneur, made headlines for receiving blood plasma transfusions from his teenage son for several years. In 2019, the FDA issued a warning to consumers that young plasma transfusions “have not gone through the rigorous testing that the FDA normally requires in order to confirm the therapeutic benefit of a product and to ensure its safety” (20).

“There are lots of rich people who treat themselves with young plasma infusions, not just in this country,” said Wyss-Coray. “At Alkahest, we always try to be very, very clear that we're trying something. We're doing clinical studies. We're trying to see whether this has clinical benefit, but we never made any claims that this is going to make people younger or things like that.” 

With more randomized controlled clinical trials testing young blood, plasma, or drugs derived from beneficial blood factors, the answer will become clear in the coming years. Although he was skeptical earlier, Horvath said he will judge the rejuvenating effects of young blood treatments strictly by the data. The case is not yet closed.

References

1. McCay, C. M., Pope, F., Lunsford, W., Sperling, G. & Sambhavaphol, P. Parabiosis between Old and Young Rats. Gerontologia  1, 7–17 (1957).
2. Conboy, I. M. et al. Rejuvenation of aged progenitor cells by exposure to a young systemic environment. Nature  433, 760–764 (2005).
3. Zhang, B. et al. Multi-omic rejuvenation and lifespan extension on exposure to youthful circulation. Nat Aging  3, 948–964 (2023).
4. Leonard, L. Anderson, R.E., Ovitz, M.L. Parabiotic Intoxication in Germ-free Mice. Nature  215, 244-246 (1967).
5. Ceylani, T. et al. The rejuvenating influence of young plasma on aged intestine. J Cell Mol Med  27, 2804–2816 (2023).
6. Katsimpardi, L. et al. Vascular and Neurogenic Rejuvenation of the Aging Mouse Brain by Young Systemic Factors. Science  344, 630–634 (2014).
7. Baht, G. S. et al. Exposure to a youthful circulation rejuvenates bone repair through modulation of β-catenin. Nat Commun  6, 7131 (2015).
8. Huang, Q. et al. A Young Blood Environment Decreases Aging of Senile Mice Kidneys. J Gerontol A Biol Sci Med Sci  73, 421–428 (2018).
9. Sha, S. J. et al. Safety, Tolerability, and Feasibility of Young Plasma Infusion in the Plasma for Alzheimer Symptom Amelioration Study: A Randomized Clinical Trial. JAMA Neurology  76, 35–40 (2019).
10. Hannestad, J. et al. Safety and tolerability of GRF6019 in mild-to-moderate Alzheimer’s disease dementia. Alzheimer Dement  6, e12115 (2020).
11. Hannestad, J. et al. Safety and Tolerability of GRF6019 Infusions in Severe Alzheimer’s Disease: A Phase II Double-Blind Placebo-Controlled Trial. J Alzheimers Dis  81, 1649–1662 (2021).
12. Boada, M. et al. A randomized, controlled clinical trial of plasma exchange with albumin replacement for Alzheimer’s disease: Primary results of the AMBAR Study. Alzheimers Dement  16, 1412–1425 (2020).
13. Clement, J. et al. Umbilical cord plasma concentrate has beneficial effects on DNA methylation GrimAge and human clinical biomarkers. Aging Cell  21, e13696 (2022).
14. Driss, L. B. et al. GDF11 and aging biology - controversies resolved and pending. J Cardiovasc Aging  3, (2023).
15. Pálovics, R. et al. Molecular hallmarks of heterochronic parabiosis at single cell resolution. Nature  603, 309–314 (2022).
16. Horvath, S. et al. Reversing age: dual species measurement of epigenetic age with a single clock. 2020.05.07.082917 Preprint at https://doi.org/10.1101/2020.05.07.082917 (2020)
17. Yousef, H. et al. Systemic attenuation of the TGF-β pathway by a single drug simultaneously rejuvenates hippocampal neurogenesis and myogenesis in the same old mammal. Oncotarget  6, 11959–11978 (2015).
18. Mehdipour, M. et al. Rejuvenation of three germ layers tissues by exchanging old blood plasma with saline-albumin. Aging  12, 8790–8819 (2020).
19. Mehdipour, M. et al. Plasma dilution improves cognition and attenuates neuroinflammation in old mice. Geroscience  43, 1–18 (2021).
20. Commissioner, O. of the. Statement from FDA Commissioner Scott Gottlieb, M.D., and Director of FDA’s Center for Biologics Evaluation and Research Peter Marks, M.D., Ph.D., cautioning consumers against receiving young donor plasma infusions that are promoted as unproven treatment for varying conditions. FDA (2020). at <https://www.fda.gov/news-events/press-announcements/statement-fda-commissioner-scott-gottlieb-md-and-director-fdas-center-biologics-evaluation-and-0>