Aged murine bone marrow myeloid and mesenchymal cells develop unique senescence phenotypes

Cellular senescence is a heterogeneous phenotype characterized primarily in mesenchymal cells, but the extent to which immune cells differ in their senescence phenotype, or "senotype", is unclear. Here, we applied single-cell approaches alongside both global and cell-specific genetic senolytic mouse models to evaluate the senotype of immune cells in the bone marrow of aging mice. We found that myeloid-lineage cells exhibited the highest expression of p16 and senescence-associated secretory phenotype markers among all immune cell types. In contrast to clearance of p16+ senescent mesenchymal cells, targeted clearance of p16+ myeloid cells in aged mice only had minor effects on age-related bone loss in male mice, with no effects in females. In more detailed analyses, p16+ myeloid cells were only acutely cleared, being repopulated back to basal levels within a short time period. This led to a lack of long-lasting reduction in senescent cell burden, unlike when targeting bone mesenchymal cells. In vitro, myeloid-lineage cells differed markedly from mesenchymal cells in the development of a senescent phenotype. Collectively, our findings indicate that aged bone marrow myeloid cells do not achieve the fully developed senescent phenotype originally described in mesenchymal cells, justifying further characterization of senotypes of immune cells across tissues.