SiglecF+Gr1hi eosinophils are a distinct subpopulation within the lungs of allergen-challenged mice.

Although eosinophils as a group are readily identified by their unique morphology and staining properties, flow cytometry provides an important means for identification of subgroups based on differential expression of distinct surface Ags. Here, we characterize an eosinophil subpopulation defined by high levels of expression of the neutrophil Ag Gr1 (CD45CD11cSiglecFGr1). SiglecFGr1 eosinophils, distinct from the canonical SiglecFGr1 eosinophil population, were detected in allergen-challenged wild-type and granule protein-deficient (EPX and MBP-1) mice, but not in the eosinophil-deficient ΔdblGATA strain. In contrast to Gr1 neutrophils, which express both cross-reacting Ags Ly6C and Ly6G, SiglecFGr1 eosinophils from allergen-challenged lung tissue are uniquely Ly6G Although indistinguishable from the more-numerous SiglecFGr1 eosinophils under light microscopy, FACS-isolated populations revealed prominent differences in cytokine contents. The lymphocyte-targeting cytokines CXCL13 and IL-27 were identified only in the SiglecFGr1 eosinophil population (at 3.9 and 4.8 pg/10 cells, respectively), as was the prominent proinflammatory mediator IL-13 (72 pg/10 cells). Interestingly, bone marrow-derived (SiglecF), cultured eosinophils include a more substantial Gr1 subpopulation (∼50%); Gr1 bmEos includes primarily a single Ly6C and a smaller, double-positive (Ly6CLy6G) population. Taken together, our findings characterize a distinct SiglecFGr1 eosinophil subset in lungs of allergen-challenged, wild-type and granule protein-deficient mice. SiglecFGr1 eosinophils from wild-type mice maintain a distinct subset of cytokines, including those active on B and T lymphocytes. These cytokines may facilitate eosinophil-mediated immunomodulatory responses in the allergen-challenged lung as well as in other distinct microenvironments.