MHCII+CD80+ thymic eosinophils increase in abundance during neonatal development in mice and their accumulation is microbiota-dependent.

Eosinophils are present in the thymus of mammals, yet their function at this site during homeostatic development is unknown. We used flow cytometry to determine the abundance and phenotype of eosinophils (here defined as SSchigh SiglecF+ CD11b+ CD45+ cells) in the thymus of mice during the neonatal period, the later postnatal period and into adulthood. We show that both the total number of thymic eosinophils and their frequency amongst leukocytes increase over the first two weeks of life, and that their accumulation in the thymus is dependent on the presence of an intact bacterial microbiota. We report that thymic eosinophils express the IL-5R (CD125), CD80 and indoleamine 2, 3-dioxygenase (IDO), and that subsets of thymic eosinophils express CD11c and MHCII. We found that the frequency of MHCII-expressing thymic eosinophils increases over the first two weeks of life, and that during this early life period the highest frequency of MHCII-expressing thymic eosinophils is located in the inner medullary region. These data suggest a temporal and microbiota-dependent regulation of eosinophil abundance and functional capabilities in the thymus.