More on senescence and allograft rejection

Following up on yesterday’s post about the impact of cellular senescence in organ transplant failure: Another group, working in a mouse model, has shown that older donor tissues express higher levels of the senescence marker p16, which is negatively correlated with proliferative capacity.

But the old material doesn’t just start from behind, but also gets worse more rapidly: p16 levels rise dramatically in old grafts following transplantation, much faster than in young grafts, indicating that older cells are more sensitive to the stresses of undergoing transplantation and that they respond to this stress by undergoing senescence. (In contrast, young grafts only seem to express p16 if they are rejected). From Melk et al.:

Effects of Donor Age and Cell Senescence on Kidney Allograft Survival

The biological processes responsible for somatic cell senescence contribute to organ aging and progression of chronic diseases, and this may contribute to kidney transplant outcomes. We examined the effect of pre-existing donor aging on the performance of kidney transplants, comparing mouse kidney isografts and allografts from old versus young donors. Before transplantation, old kidneys were histologically normal, but displayed an increased expression of senescence marker p16INK4a. Old allografts at day 7 showed a more rapid emergence of epithelial changes and a further increase in the expression of p16INK4a. Similar but much milder changes occurred in old isografts. These changes were absent in young allografts at day 7, but emerged by day 21. The expression of p16INK4a remained low in young kidney allografts at day 7, but increased with severe rejection at day 21. Isografts from young donors showed no epithelial changes and no increase in p16INK4a. The measurements of the alloimmune response—infiltrate, cytology, expression of perforin, granzyme B, IFN-γ and MHC—were not increased in old allografts. Thus, old donor kidneys display abnormal parenchymal susceptibility to transplant stresses and enhanced induction of senescence marker p16INK4a, but were not more immunogenic. These data are compatible with a key role of somatic cell senescence mechanisms in kidney transplant outcomes by contributing to donor aging, being accelerated by transplant stresses, and imposing limits on the capacity of the tissue to proliferate.

In searching for a mechanistic relationship between aging and transplant failure, the authors focus on the loss of regenerative capacity rather than an active role for senescent cells. The latter type of hypothesis will likely gain credence as the field digests the idea that senescent cells secrete immunologically active and potentially deleterious molecules into the extracellular milieu.