The ecto-3′-nucleotidase activity of Acanthamoeba castellanii trophozoites increases their adhesion to host cells through the generation of extracellular adenosine

Abstract

Acanthamoeba castellanii, a free-living amoeba, can be pathogenic to humans causing a corneal infection named Acanthamoeba keratitis (AK). The mannose-binding protein (MBP) is well established as the major factor related to Acanthamoeba pathogenesis. However, additional factors that participate in the adhesion process and protect trophozoites from cytolytic effects caused by host immune responses remain unknown. Ectonucleotidases, including 3′-nucleotidase/nuclease (3′-NT/NU), a bifunctional enzyme that was recently reported in A. castellanii, are frequently related to the establishment of parasitic infections. We verified that trophozoites can hydrolyze 3′-AMP, and this activity is similar to that observed in other protists. The addition of 3′-AMP increases the adhesion of trophozoites to LLC-MK2 epithelial cells, and this stimulation is completely reversed by DTT, an inhibitor of ecto-3′-nucleotidase activity. Lesions in corneal cells caused by AK infection may elevate the extracellular level of 3′-AMP. We believe that ecto-3′-nucleotidase activity can modulate the host immune response, thus facilitating the establishment of parasitic infection. This activity results from the generation of extracellular adenosine, which can bind to purinergic receptors present in host immune cells. Positive feedback may occur in this cascade of events once the ecto-3′-nucleotidase activity of trophozoites is increased by the adhesion of trophozoites to LLC-MK2 cells.