Expression of IL-5 in Thymocytes/T Cells Leads to the Development of a Massive Eosinophilia, Extramedullary Eosinophilopoiesis, and Unique Histopathologies

Transgenic mice were generated using regulatory elements from the CD3δ gene to drive T cell expression of IL-5. Expression of this cytokine resulted in white blood cell counts that expand virtually unabated (∼400,000 cells/mm³). This expansion is characterized by a profound eosinophilia (>60%) and commensurate increases in the absolute numbers of all other white blood cell types. In particular, circulating B220⁺ B lymphocyte populations increased >30-fold over wild-type (+/+) levels. Cell differentials and expression studies using a marker for eosinophil precursor cells (major basic protein gene expression) suggest that the peripheral eosinophilia is induced primarily through the establishment of extramedullary sites of eosinophilopoiesis. These mice display a massive peritoneal cavity cell exudate (1-2 × 10⁸ cells) dominated by eosinophils (∼50%) and the infiltration of eosinophils in nearly all organ systems. Sudden unexplained death occurs in 70% of all transgenic animals by 12 mo of age. Surviving transgenic animals display severe inflammatory pathologies that include ulcerating skin lesions as well as lower bowel inflammation. These pathologies parallel clinical observations of patients with a profound eosinophilia and imply that IL-5 effector functions during some inflammatory responses may be contingent upon peripheral lymphohemopoietic expression.