Siglec-F-negative resident eosinophils represent an unexpectedly dominant population in homeostatic murine bone marrow

Abstract

Abstract Bone marrow, while known to be a hematopoietic site for eosinophils, also harbors resident populations of these cells. Identification of such resident eosinophils typically requires flow cytometry-assisted sorting, with Siglec-F as a key marker for identification of murine eosinophils. At baseline, we found that bone marrow resident eosinophils described as Siglec-F(+) constitute about 5% of CD45 cells. Although this identifies mature eosinophils, it may not be sufficient to represent all resident phenotypes. Using sequencing and FACS sorting approaches, we found that the Siglec-F(+) population represented only a minority of the resident eosinophil pool. Rather, we identified a lineage-positive population of IL-5Ra(+)Ly6G(+) but Siglec-F(−) cells distinct from lineage-negative EoPs (eosinophil progenitors). At 40% of all CD45(+) hematopoietic cells in adult mice, this bone marrow resident population dwarfs the Siglec-F(+) eosinophil pool. When sorted from bone marrow homogenates and treated with IL-5, these resident cells acquired all characteristics of mature eosinophils, including expression of Siglec-F and granular proteins (EPX, MPO). These tissue residents may play critical homeostatic roles in normal murine bone marrow and should be accounted for in studies of eosinophil biology.