p16(Ink4a) and senescence-associated β-galactosidase can be induced in macrophages as part of a reversible response to physiological stimuli

Abstract

Constitutive <i>p16^Ink4a</i> expression, along with senescence-associated β-galactosidase (SAβG), are commonly accepted biomarkers of senescent cells (SCs). Recent reports attributed improvement of the healthspan of aged mice following <i>p16^Ink4a</i>-positive cell killing to the eradication of accumulated SCs. However, detection of <i>p16^Ink4a</i>/SAβG-positive macrophages in the adipose tissue of old mice and in the peritoneal cavity of young animals following injection of alginate-encapsulated SCs has raised concerns about the exclusivity of these markers for SCs. Here we report that expression of <i>p16^Ink4a</i> and SAβG in macrophages is acquired as part of a physiological response to immune stimuli rather than through senescence, consistent with reports that p16^Ink4a plays a role in macrophage polarization and response. Unlike SCs, <i>p16^Ink4a</i>/SAβG-positive macrophages can be induced in p53-null mice. Macrophages, but not mesenchymal SCs, lose both markers in response to M1- [LPS, IFN-α, Poly(I:C)] and increase their expression in response to M2-inducing stimuli (IL-4, IL-13). Moreover, interferon-inducing agent Poly(I:C) dramatically reduced <i>p16^Ink4a</i> expression <i>in vivo</i> in our alginate bead model and in the adipose tissue of aged mice. These observations suggest that the antiaging effects following eradication of <i>p16^Ink4a</i>-positive cells may not be solely attributed to SCs but also to non-senescent <i>p16^Ink4a</i>/SAβG-positive macrophages.