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The cytopathogenic mechanisms of Trichomonas vaginalis have been debated since the 1940s. We examined the following three proposed pathogenic mechanisms: contact-dependent extracellular killing, cytophagocytosis, and extracellular cytotoxins. Serial observations of Chinese hamster ovary (CHO) cell monolayers exposed to trichomonads revealed that (i) trichomonads form clumps, (ii) the clumps adhere to cells in culture, and (iii) monolayer destruction occurs only in areas of contact with T. vaginalis. Kinetic analysis of target cell killing by trichomonads revealed that the probability of CHO cell death was related to the probability of contact with T. vaginalis, supporting the observation by microscopy that trichomonads kill cells only by direct contact. Simultaneous studies of "lindium oxine label release from CHO cells and trypan blue dye exclusion demonstrated that T. vaginalis kills target cells without phagocytosis. Filtrates of trichomonad cultures or from media in which trichomonads were killing CHO cells had no effect on CHO cell monolayers, indicating that trichomonads do not kill cells by a cell-free or secreted cytotoxin. The microfilament inhibitor cytochalasin D (10 ,ug/ml) inhibited trichomonad killing of CHO cell monolayers by 80% (P < 0.0001). In contrast, the microtubule inhibitor vinblastine (10-6 M) caused only 17% inhibition of trichomonad destruction of CHO cell monolayers (P < 0.020), whereas colchicine (10-6 M) had no effect. T. vaginalis kills target cells by direct contact without phagocytosis. This event requires intact trichomonad microfilament function; microtubule function appears not to be essential.

Polymorphonuclear neutrophils (PMNs) were shown to kill Trichomonas vaginalis in vitro; 10(2)-10(3) trichomonads were incubated with 3 x 10(6) PMNs on tissue culture plates, and surviving organisms were enumerated in pour plates. After 60 min of aerobic incubation at 37 C, 100% (+/- 0) of the trichomonads had been killed, and nitroblue tetrazolium was reduced at the interface between the PMNs and trichomonads. The importance of oxidative microbicidal systems was confirmed by the observations that only 12% +/- 12% of trichomonads were killed under anaerobic conditions and that aerobic killing was eliminated by the addition of catalase or superoxide dismutase. PMNs killed trichomonads in fresh or absorbed serum but not in bovine serum albumin, in heat-inactivated serum, or in the presence of 1 mM trypan blue; this finding suggested a role for alternative pathway activation of complement. Phase-contrast cinemicrography and electron microscopy revealed the pursuit and surrounding of individual trichomonads by groups of PMNs that were able to fragment the large protozoa and to phagocytize the pieces.

Although Trichomonas vaginalis causes one of the most common sexually transmitted diseases, little is known about the antigenic variation of the parasite or about differences between strains in epidemiology or virulence. Variation among isolates of T. vaginalis was investigated by using a panel of monoclonal antibodies, each reactive with different antigens, to test 88 isolates from diverse geographic areas of North America. All isolates of T. vaginalis reacted with at least one of the nine monoclonal antibodies; the individual antibodies reacted with 22%-76% of the isolates. A pool of two broadly reactive antibodies identified all isolates in the study. Four of the most narrowly reactive, or "specific," antibodies demonstrated differences in the antigenic composition of trichomonads isolated from patients in Seattle, Baltimore, and Brooklyn, New York (P less than .005 by chi 2 test). Application of these and other monoclonal antibody probes may facilitate epidemiological studies and provide rapid, reliable methods for direct diagnosis of trichomonads in clinical specimens.

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