As your body ages, increasing amounts of your cells enter a state of cellular senescence. These cells no longer divide or support the tissues of which they are a part, and they emit a cocktail of harmful chemical signals that encourage other nearby cells to also enter the same senescent state.
The presence of senescent cells contributes to decreasing tissue function, increases chronic inflammation, and can even raise the risk of cancer and other age-related diseases.
Senescent cells normally destroy themselves via a programmed process called apoptosis and are removed by the immune system; however, the immune system declines with age, and increasing numbers of these senescent cells escape this disposal process and accumulate.
By the time people reach old age, significant numbers of these senescent cells have built up in their bodies, causing inflammation and damage to surrounding cells and tissues. These senescent cells are one of the hallmarks of aging and a key process in the progression of aging and age-related diseases[1-2].
One proposed way to deal with this problem is to remove these senescent cells by causing them to go into apoptosis using therapies known as senolytics.
Macrophages with markers of cellular senescence may not be senescent
Researchers commonly determine the presence of senescent cells through the use of the p16ink4a (p16) and β-galactosidase biomarkers. Senescent cells are known to exhibit both of these biomarkers, but it seems they are not the only cells that do this, and the concern is that therapies that target these biomarkers may also be removing cells that are not senescent.
A study was published recently, and it shows that some cells that are not necessarily senescent share these two biomarkers. This new study is by the same research team who investigated cellular senescence in the immune system last year with a focus on macrophages.
So, could this mean that older people have a partially senescent macrophage population, or does it mean that these cells are not harmful and just exhibit some of the same markers that senescent cells do?
This new study suggests that the latter may be the case, although it has not yet been determined if the macrophages exhibiting these commonly shared biomarkers are harmful.
So is this an issue for senolytics?
The good news is that we are aware of no research group developing senolytics that target β-galactosidase. The use of β-galactosidase is limited to assessing the presence and numbers of senescent cells but is not actually the target of therapies. So far, so good then.
However, p16 is a potential issue, as at least one company, Oisin Biotechnology (we interviewed their CEO here) does use this biomarker to target senescent cells. Their approach uses target genes and their level of expression in order to target cells for destruction. Oisin will no doubt be doing due diligence here and determining the effect of removing p16-positive macrophages as part of their ongoing development process.
That said, one of the first proof-of-concept studies used p16 as the target for senescent cell removal, and positive health results were seen. So, it is worth bearing in mind that, in this case, p16 cells, regardless of the source, were removed in a broad stroke in this study, and yet positive benefits were observed.
Therefore, it may be that case that removing them is not a significant problem, and certainly earlier studies strongly suggest that removing senescent cells is beneficial, even taking into account the risk of removing non-senescent cells.
The researchers in the paper conclude that the importance of senescent cells in the aging process and removing senescent cells is not in question, and their findings do not suggest that developing senolytics is a bad idea.
The points raised here really are that we need to better refine how we identify senescent cells and that our current understanding of if a cell is senescent is not as clear as it should be. This observation highlights the need for the development of better biomarkers to identify senescent cells.
Considering the high level of focus on senescent cells that the research community now has and the considerable funding behind developing senolytics, those gaps in our knowledge will likely not remain in place for long.
Given the pace of progress in this particular area, it is reasonable to believe that such issues may be resolved in the near future once more studies are concluded.
 López-Otín, C., Blasco, M. A., Partridge, L., Serrano, M., & Kroemer, G. (2013). The hallmarks of aging. Cell, 153(6), 1194-1217.
 van Deursen, J. M. (2014). The role of senescent cells in ageing. Nature, 509(7501), 439-446.
 Hall, B. M., Balan, V., Gleiberman, A. S., Strom, E., Krasnov, P., Virtuoso, L. P., … & Leonova, K. I. (2017). p16 (Ink4a) and senescence-associated β-galactosidase can be induced in macrophages as part of a reversible response to physiological stimuli. Aging.
 Hall, B. M., Balan, V., Gleiberman, A. S., Strom, E., Krasnov, P., Virtuoso, L. P., … & Leonova, K. (2016). Aging of mice is associated with p16 (Ink4a)-and β-galactosidase-positive macrophage accumulation that can be induced in young mice by senescent cells. Aging (Albany NY), 8(7), 1294.
 Baker, D. J., Wijshake, T., Tchkonia, T., LeBrasseur, N. K., Childs, B. G., Van De Sluis, B., … & Van Deursen, J. M. (2011). Clearance of p16Ink4a-positive senescent cells delays ageing-associated disorders. Nature, 479(7372), 232.