Thymic eosinophils: What are you doing here?

The thymus is a primary lymphoid organ where major types of T lymphocytes undergo essential developmental processes. Eosinophils are among the cell types present in microenvironments within the thymus, and perhaps surprisingly, the role of thymic eosinophils, especially during homeostatic conditions, remains unclear. Major physiological events impact thymic organization and function throughout life, including age-related involution, pregnancy, and exposure to chemotherapy or radiation. In this review, we summarize literature that has explored factors that regulate the accumulation, phenotype, and location of thymic eosinophils during homeostatic development and during conditions in which homeostasis is perturbed. Further, we discuss the current theories as to the function of thymic eosinophils and consider how the heterogeneity of thymic eosinophil populations may reflect a temporal, spatial, and situational multifunctionality of thymic eosinophils.