Cellular and functional characterization of aging-independent senescent-like cells that modulate tissue repair
Cells expressing features of cellular senescence, including upregulation of p21 and p16, appear transiently following tissue injury, yet the properties of these cells or how they contrast with age-induced senescent cells remains unclear. Using skeletal fracture as a model of acute injury, we identified rapidly-appearing senescent-like cells, marked by p21 expression, that negatively affected fracture healing. p21+ callus cells, which consisted predominantly of neutrophils and osteochondroprogenitors, existed as transient cells specific to injury and expressed high levels of senescence-associated factors known to impair bone formation and induce paracrine senescence. Targeted genetic clearance of p21+ cells suppressed senescence-associated signatures within the fracture callus and accelerated fracture healing. By contrast, p21+ cell clearance did not alter bone loss due to aging; conversely, p16+ cell clearance, known to alleviate skeletal aging, did not affect fracture healing. Together, our findings establish contextual roles of senescent/senescent-like cells that may be leveraged for therapeutic opportunities. Overall design: 2 Male, 2 Female p21-LOX-ATTAC mice were used and a  closed diaphyseal tibial fracture introduced. The callus was harvested on day 14. Subsequently, GFP+ and GFP- cells were FACS-sorted.