Characterisation of the green turtle’s leukocyte subpopulations by flow cytometry and evaluation of their phagocytic activity

Phagocytosis is a fundamental aspect of innate immunity that is conserved across many species making it a potentially useful health-assessment tool for wildlife. In non-mammalian vertebrates, heterophils, monocytes, macrophages, melanomacrophages, and thrombocytes all have phagocytic properties. Recently, B lymphocytes from fish, amphibians, and aquatic turtles have also showed phagocytic capacity. Phagocytes can be studied by flow cytometry; however, the use of this tool is complicated in reptiles partly because nucleated erythrocytes complicate the procedure. We separated green turtle leukocytes by density gradient centrifugation and identified subpopulations by flow cytometry and confocal microscopy. Additionally, we assessed their ability to phagocytize Fluorspheres and Ovoalbumin-Alexa. We found that heterophils and lymphocytes but not monocytes could be easily identified by flow cytometry. While heterophils from adults and juvenile turtles were equally able to phagocytize fluorspheres, adults had significantly more phagocytic ability for OVA-Alexa. Lymphocytes had a mild phagocytic activity with fluorospheres (27–38 %; 39–45 %) and OVA-Alexa (35–46 %; 14–22 %) in juvenile and adult green turtles, respectively. Confocal microscopy confirmed phagocytosis of fluorospheres in both heterophils and lymphocytes. This provides the first evidence that green turtle lymphocytes have phagocytic activity and that this assay could potentially be useful to measure one aspect of innate immunity in this species.