Studies of parabiosis, in which a young and an old mouse have their blood systems joined, show that altering the balance of circulating proteins in old tissue can restore stem cells to action and revert a range of measures that change with age. It is thought that frequent blood transfusions should capture at least some of this outcome, although it is unclear as to the degree to which the relevant proteins are short-lived in circulation, and transfusions are really just a stand-in for some yet to be established but more effective way of directly altering levels of the proteins of interest.
Stem cells decline and protein levels change as a reaction to rising levels of cellular damage, or at least that is the dominant view of aging as a process in the research community. In the case of stem cells this may be an evolved mechanism to suppress cancer risk, a balance between death by failing tissue maintenance versus death due to damaged cells running amok. Thus there is some concern that crude changes intended to bring stem cells back into a youthful mode of activity will produce high rates of cancer, but it is entirely possible that this can be avoided while still retaining benefits. First generation stem cell treatments came attached to much the same concern, and where that concern was professionally addressed these therapies are clearly producing meaningful benefits in older people.
In both mice and humans, GDF11 falls with age. We don't know why it declines, but we know it is involved in several mechanisms that control growth. It is also thought to mediate some age-related effects on the brain, in part by activation of another protein that is involved in neuronal growth and long-term memory. So the billion-dollar question is: would a GDF11 boost have the same effect in humans? [Researchers think] it will, having taken the next step of injecting young human blood plasma into old mice. His preliminary results suggest that human blood has similar rejuvenating benefits for old mice as young mouse blood does. "We saw these astounding effects. The human blood had beneficial effects on every organ we've studied so far."
Now, the final step - giving young human blood plasma to older people with a medical condition - is about to begin. Getting approval to perform the experiment in humans has been relatively simple, thanks to the long safety record of blood transfusions. So in early October, a [team] will give a transfusion of blood plasma donated by people under 30 to older volunteers with mild to moderate Alzheimer's. Following the impressive results in animal experiments, the team hopes to see immediate improvements in cognition, [but] cautions that it is still very experimental. "We will assess cognitive function immediately before and for several days after the transfusion, as well as tracking each person for a few months to see if any of their family or carers report any positive effects. The effects might be transient, but even if it's just for a day it is a proof of concept that is worth pursuing."
All researchers involved in the work agree that GDF11 is unlikely to be the only factor that keeps organs youthful. "It's too optimistic to think there would be just one factor. It's much more likely to be several factors that exert these effects in combination."