Cellular senescence is one of the causes of aging: rising numbers of cells fall into a harmful senescent state and then linger there. The activities of these cells directly contribute to loss of tissue function and the progression of many age-related diseases. You might recall last year's investigations into possible cellular senescence in the immune system, focused on macrophages that exhibit some of the markers used to identify senescent cells. Does this mean that part of the macrophage population is in fact senescent in older people, and they would benefit from the removal of those cells, as is the case for other senescent cell types, or does it mean something else entirely, and these cells may not be harmful? In the open access paper here, the author's of last year's study suggest that the latter situation is the case, though whether or not these cells are damaging to an individual remains to be determined.
The markers in question here are p16 (also known as p16ink4a) and senescence-associated β-galactosidase (SAβG). If we approach this from the point of view of concern that treatments might be destroying cells unnecessarily, then SAβG isn't all that relevant, as I'm not aware of any group that actually targets that signal, versus only using it for assessment purposes. The companies that are developing pharmaceuticals to destroy senescent cells are not doing so in a way that specifically targets raised expression of these genes: drug development starts with a drug found in the compound libraries that is somewhat useful in killing the cells you want it to kill, and then you try to improve upon whatever it turns out to do under the hood. That mechanism doesn't have any necessary connection to the markers that the research community has developed to identify senescent cells.
On the other hand p16 is one of the targets used by Oisin Biotechnologies, and their gene therapy absolutely does recognize specific genes and and their expression levels, and selects cells for destruction on that basis. The Oisin team will in the course of their development find out one way or another whether or not removal of p16-expressing macrophages is useful. We might also recall that the earlier studies of mice genetically engineered to clear senescent cells used p16 as the identifying marker for cell destruction. Clearly the benefits there were achieved with a clean sweep of p16-expressing macrophages as well as other senescent cells, even if it can be argued that those macrophages are not senescent in the way we'd consider other cell types to be senescent.
The broader point raised in the paper here is that a refinement is needed in the current taxonomy of cellular senescence, especially in how it relates to markers that are coming to be understood as perhaps less specific than was originally hoped. That seems a fair enough comment on the current state of the research. I think that this desired progress will arrive, and fairly quickly now that senescent cells - as defined by various measures and markers - can be destroyed reliably and effectively. The size of the effects on health and life span in rodents obtained so far are large enough that future animal studies should fairly conclusively settle whether or not certain cell populations are bad and should be removed.
The accumulation of p16Ink4a-positive cells is observed in aged mice, and their eradication has been linked to certain improvements in the health state of older animals consistent with rejuvenation. Even though p16Ink4a-positive cells in vivo have been assumed to be senescent, little evidence exists to directly support this assumption. Our previous work identifying macrophage subtypes that co-express markers conventionally assigned to senescent cells (SCs), p16Ink4a/SAβG, has prompted additional interpretations of previously published experimental data regarding the role of p16Ink4a-positive cells in aging and age-related diseases.
As such, defining the exact nature of p16Ink4a-positive cells is crucial for proper development of therapeutics for the prevention and treatment of aging and age-related diseases. Today, the field of aging is focused on the development of senolytic compounds that are isolated for their ability to selectively kill SCs generated in vitro. If these cells are different from p16Ink4a-positive cells accumulating in vivo with age, this could misdirect both academic studies of senescence as a phenomenon, as well as practical efforts to develop anti-aging therapeutics. These considerations motivated our present work, which was aimed at defining the nature of p16Ink4a-positive cells found in mouse tissues in vivo and their relation to the phenomenon of cellular senescence.
What is "cellular senescence"? Currently, all definitions agree that SCs cease to proliferate. However, this parameter is not sufficient to define SCs since this is also the property of terminally differentiated cells. One apparent difference is that terminal differentiation occurs in response to various physiological stimuli, while induction of senescence almost always occurs in response to genotoxic stress. Accordingly, the onset of senescence commonly involves p53, a major universal genotoxic stress response mechanism that triggers cell cycle arrest, the first step in conversion to senescence. Another intrinsic property of the senescent phenotype is that it is not reversible through known physiological stimuli, only occurring through the acquisition of genetic mutation or epigenetic modulations. Thus, a more precise definition of SCs should include those cells that irreversibly cease to proliferate following genotoxic stress. Currently, none of the other properties of SCs that are being used for their recognition, such as p16Ink4a- or SAβG-positivity, are sufficiently specific for SCs as to be essential components of this definition.
We previously demonstrated that a significant proportion of p16Ink4a/SAβG-positive cells in the fat tissue of older mice are of hematopoietic origin, express surface markers of macrophages and are capable of phagocytosis. Here, we demonstrate that these cells appear and accumulate independently of their p53 status. Furthermore, induction of p16Ink4a/SAβG markers can be significantly modulated (in both directions) by physiological stimuli known to polarize macrophages. In recent literature, a role for p16Ink4a has been implicated in macrophage physiology with no relation to other properties of senescence. For example, p16Ink4a expression is induced during monocyte differentiation into macrophages in vitro without affecting the cell cycle, and macrophages from p16Ink4a-deficient mice are skewed towards an M2 phenotype, exhibiting defects in M1 polarization response.
In summary, we conclude that a significant proportion of p16Ink4a/SAβG-positive cells accumulating in aging mice are macrophages that acquired this phenotype as part of their physiological reprogramming towards an M2-like phenotype. This interpretation is consistent with reports that tumor-associated macrophages (TAMs), which possess also an M2 phenotype, were shown to express p16Ink4a. It is highly unlikely that senolytic compounds isolated for their ability to eradicate bona fide SCs would be equally potent and selective against cells that simply resemble SCs by two unreliable biomarkers (p16Ink4a/SAβG) yet lack the most definitive properties of senescence. However, several molecules identified with anti-SC activities, including ruxolitinib, dasatinib, and quercetin, have documented anti-inflammatory effects on macrophages that may contribute to improvements in healthspan. We believe that the assumptions made in a series of recent works - that p16Ink4a/SAβG-positive cells are SCs - needs to be carefully re-evaluated, and that the effects of anti-SC therapies on macrophages needs to be evaluated.
Importantly, our results do not overthrow the significance of the SC's role in aging or disprove the rationale for the development of senolytic compounds. Nevertheless, they do question the accuracy of interpretation of the reasons for the improvement of the health of mice following the eradication of p16Ink4a-positive cells, raising the possibility that SCs may not be the only ones implicated in age-related frailty and that other players may be involved that could require different approaches to target.