Binding of fibronectin by Trichomonas vaginalis is influenced by iron and calcium

We have reported that Trichomonas vaginalis, the causative agent of the most common, non-viral sexually transmitted disease, bound to cover slips coated with fibronectin (FN) (Crouch & Alderete, Microbiol 1999 145: 2835-43). In this study, we extend that observation by showing that FN binding is specific, and we present data on the requirements of FN binding by T. vaginalis. Immunofluorescence and immuno-gold labelling readily detected FN throughout the trichomonal surface. Parasites bound to(125)I-labelled FN in a time- and concentration-dependent fashion. In the absence of protease inhibitor, iodinated FN was released from the trichomonad surface. Unlabelled FN specifically competed for binding in a concentration-dependent fashion with the(125)I-labelled FN. Interestingly, the amount of FN bound by T. vaginalis organisms was dependent on iron. High-iron-grown trichomonads acquired lower numbers of molecules but with 10-fold higher affinity than low-iron-grown organisms. Further, we show that for iron-replete organisms, calcium (Ca(2+)) at physiological levels increased amounts of bound FN. The increase in binding was rapid, occurring within 5 min of Ca(2+)addition, and required de novo protein synthesis. Co-incubation of live parasites with Ca(2+)in the presence of FN was necessary to increase the amount of FN bound. Treatment of trichomonads with okadaic acid, but not other phosphatase inhibitors, resulted in a 50% decrease in binding of FN, regardless of the presence of Ca(2+), suggesting a role for phosphatase in FN association. These results indicate that depending on the iron status of T. vaginalis organisms in vivo, Ca(2+)may influence trichomonad recognition and binding to FN during host parasitism.