How exercise in old age prevents the immune system from declining

Purely anecdotal: My grandfather is nearing 90 and still jumps on his indoor bike every day. Last winter he fell (due to ice) and got away with a few bruises. For other people his age this kind of fall could have meant a broken hip. He had two heart attacks when he was around 60 and a life long lung disease - far away from the immune system (or health) of a 20-year old.

Paper: https://doi.org/10.1111/acel.12750

This age of biotechnology is also an age of comparative indolence and comfort. As the research community measures specific biochemical aspects of aging, such as the decline of the cardiovascular system, or metrics relating to immunosenescence in the immune system, we might question the degree to which the results are peculiar to our era. How much of aging is the result of our choices - to eat more and exercise less than our ancestors - rather than the result of inexorable processes of biochemical damage that we, as yet, have little influence over? (Conversely, how much of past aging was due to infectious disease, malnutrition, and other adverse external circumstances that are controlled to a much greater degree today?) This topic crops up fairly often in research into the effects of exercise on health, and the research noted here is a particularly striking example of the type.

The study authors find that the age-related decline of new T cells maturing in the thymus is negligible in some people, those who exercise much more than the rest of us. This diminished supply of new T cells is thought to be an important component of immune system aging, and the failure of the immune system is very influential over many other aspects of aging: senescent cell accumulation, frailty, loss of regenerative capacity, chronic inflammation, cancer risk, and so on. Yet when we look at the demographic evidence for spread of life span based on exercise, it appears to be, at most, 6 or 7 years (with a much larger divergence when it comes to state of health over time). What does this tell us about the likely gains resulting from rejuvenation therapies seeking to regenerate the thymus? Less than we would like, I suspect, and not just because it is hard to evaluate any one contribution to aging in isolation of all of the others.

The thymus atrophies over adult life, with active tissue necessary for the production of T cells being replaced by fat. The first major loss of active thymic tissue occurs at the end of childhood, however, in a process known as involution. Immune cells are generated at a tremendous rate in children in comparison to young adults; evolution selected for a system that would be highly effective at the outset, at the cost of later issues. When it is observed that old people in their 60s and 70s who maintained a high level of fitness throughout life exhibit much the same thymic output as young people in their 20s, that tells us little regarding the outcome were the thymus restored to the same level of active tissue as is present in children. Only a mild restoration, to move thymic activity from typical aged to typical young adult, would be comparable - and why would we stop there?