Balla, K.M., Lugo-Villarino, G., Spitsbergen, J.M., Stachura, D.L., Hu, Y., Banuelos, K., Romo-Fewell, O., Aroian, R.V., and Traver, D. (2010) Eosinophils in the zebrafish: prospective isolation, characterization, and eosinophilia induction by helminth determinants. Blood. 116(19):3944-3954.
Eosinophils are granulocytic leukocytes implicated in numerous aspects of immunity and disease. The precise functions of eosinophils, however, remain enigmatic. Alternative models to study eosinophil biology may thus yield new information regarding their functions. Eosinophilic cells have been observed in zebrafish but have not been thoroughly characterized. We utilized a gata2:eGFP transgenic animal to enable prospective isolation and characterization of zebrafish eosinophils. We demonstrate that all gata2(hi) cells in adult hematopoietic tissues are eosinophils, and that their light-scatter characteristics are distinct from other leukocyte subsets. Although eosinophils are rare in most organs, they are readily isolated from whole kidney marrow (WKM) and abundant within the peritoneal cavity. Eosinophils within WKM displayed mononuclear morphological characteristics, in contrast to polymorphonuclear forms observed in the peritoneal cavity. Molecular analyses demonstrated that zebrafish eosinophils express genes important in the development and function of mammalian eosinophils. In addition, gata2(hi) cells degranulate in response to helminth extract as determined by colorimetric and ultrastructural analyses. Chronic exposure to helminth extract or papain resulted in profound eosinophilia over time, demonstrating that an eosinophilic response to helminth-related allergens has been conserved over evolution. Equally important, infection of adult zebrafish with Pseudocapillaria tomentosa, a natural capillarid nematode pathogen of teleosts, caused marked increases in eosinophil number within the intestine, the major site of colonization. Together, these observations support a conserved role for eosinophils in the zebrafish immune response to helminth antigens or infection, and provide a new model to better understand how parasitic worms activate, co-opt, or evade the vertebrate immune response.