Further studies on the phagocytic capacity of chicken thrombocytes.

Thrombocytes, functional analog to mammalian platelets, have been described as the primary circulating phagocyte in chicken blood when challenged with bacteria (Chang and Hamilton, 1979). In order to determine if the phagocytic capacity could be extended to protozoa, interaction of chicken thrombocytes with tachyzoites of Toxoplasma gondii and bloodstream trypomastigotes of Trypanosoma cruzi was performed. Interaction with Staphylococcus aureus and Escherichia coli was also performed using fluoresceinated and living bacteria, to be examined by fluorescence microscopy (after ethidium bromide staining) and transmission electron microscopy (after ruthenium red fixation). Using these approaches it was possible to distinguish internalized from attached bacteria. T. cruzi was only found attached to the thrombocyte surface while T. gondii could be observed within the cell. To determine if T. gondii invasion was active or by phagocytosis, interaction was performed under conditions where active penetration and phagocytosis were inhibited by previous fixation of the parasites or treatment of thrombocytes with cytochalasin D, respectively. Interactions with fixed T. gondii showed only attached parasites. Cytochalasin D treated thrombocytes could still be found with internalized T. gondii. By fluorescence and transmission electron microscopy it was possible to observe a small number of bacteria internalized by thrombocytes. These findings show that T. gondii invade thrombocytes through an active penetration process and these blood cells cannot be considered as the primary circulating phagocyte in chicken.