Adhesion of Toxoplasma gondii tachyzoite-infected vehicle leukocytes to capillary endothelial cells triggers timely parasite egression.

Intracellular pathogens have numerous strategies for effective dissemination within the host. Many intracellular pathogens first infect leukocytes, which they use as a vehicle to transport them to target organs. Once at the target organ, intracellular parasite Toxoplasma gondii can cross the capillary wall in extracellular form by infecting endothelial cells. However, after egression from leukocytes, extracellular parasites face the risk of host immune attack. In this study, observation of infected mouse organs, using a method that renders tissue transparent, revealed that adhesion of tachyzoite-infected leukocytes to endothelial cells triggers immediate egression of the parasite. This signal enables the parasite to time egression from its vehicle leukocyte to coincide with arrival at a target organ, minimizing the opportunity for immune attack during the transition from a vehicle leukocyte to capillary endothelial cells.