As an extracellular parasite, T. vaginalis
binds to human host epithelial cells to establish and maintain an infection. The differences in binding abilities of T. vaginalis
strains are therefore likely to play an important role in determining the outcome of an infection. Variations in the pathogenesis of T. vaginalis
strains are well established (35
To our knowledge, prior to this study, the adherence to and cytolysis of cells originating from the male urogenital tract by T. vaginalis
had not been examined, notwithstanding that as early as 1936, Drummond reported that T. vaginalis
can infect the prostate (62
). Recent reports link trichomoniasis to prostate cancer (20
), benign hyperplastic prostatic tissue (19
), and, at a high prevalence, male nongonococcal urethritis (NGU) (3
We have compared the attachment and cytolytic properties of 26 T. vaginalis stains on an ectocervical cell line, Ect1, and a benign prostate cell line, BPH-1. A high degree of variability in attachment to and cytolysis of both cell lines was observed, with no general trend associating higher levels of attachment or cytolysis for either cell line. However, adherence to and cytolysis of both prostate and ectocervical cells by T. vaginalis strains were found to be highly correlated. Interestingly, adherence correlated with cytolysis in weakly adhering strains but not in highly adhering strains. This indicates that while attachment is necessary for T. vaginalis cytolysis, greater levels of attachment do not necessarily lead to higher levels of cytolysis. We also demonstrated that cytolysis is contact dependent for all 26 strains. Finally, we found that cytolysis of epithelial cells did not correlate with lysis of red blood cells.
As T. vaginalis
is not internalized by the host cell and remains attached to its surface, the variation in attachment of different strains observed might be predicted to play a role in the variable documented infection outcomes (9
). Overall, the strains do not exhibit a preference for binding to ectocervical cells over prostate cells. This suggests that attachment to the urogenital tract of men is not the limiting factor in the decreased pathogenesis observed in the male host. However, as the single epithelial cell monolayers used in our assays are far simpler than the complex milieu of cells in the urogenital tract, these data may not fully represent the adherence and cytolysis levels that occur in situ
during an infection. Moreover, variations in adherence and cytolysis among strains indicate that different biological mechanisms may be employed by different T. vaginalis
strains for interacting with different host epithelial cells. Alternatively, common mechanisms may be used and the proteins or surface molecules mediating these interactions would vary greatly, either in abundance or type, between strains.
The observation that adherence and cytolysis correlate for weakly adhering strains led us to examine whether cytolysis is contact dependent by placing a 0.4-μm filter between parasites and host cells. This prevented contact while allowing secreted proteins and vesicles to diffuse freely. We found no cytolysis when parasite attachment to host cells was blocked. Previous analyses aimed at determining whether T. vaginalis
cytolysis is contact dependent have yielded mixed results (39
). Unlike the simple physical barrier used here, these studies used a variety of methods to prepare “cell-free filtrates” without controls to eliminate contamination with T. vaginalis
plasma membrane debris. Our finding that 26/26 T. vaginalis
strains required contact with host cells to induce cytolysis strongly argues this is a general property of this parasite.
hemolytic activity has been widely reported (11
); however, there are conflicting data on whether hemolysis correlates with virulence (45
) or cytopathology (56
). The levels of hemolysis reported also vary, which may be in part due to differences in methods and conditions used for measuring hemolysis. Analyses of the 26 strains studied here revealed that only 4 are capable of lysing >20% of RBCs and that the majority of strains (15) lyse fewer than 10% in assays using a 1:30 parasite/RBC ratio. Notably, 2 of the 3 more hemolytic strains exhibit low levels of adherence to and cytolysis of epithelial cells. Thus, no correlation between T. vaginalis
adherence to and cytolysis of epithelial cells and its hemolytic activity was observed. This is in agreement with the observations of Krieger et al. (41
) that the ability of T. vaginalis
strains to cause hemolysis correlates poorly with virulence. Finding that so few strains are capable of lysing RBCs raises the question of whether T. vaginalis
hemolysis is important in vivo
. T. vaginalis
allegedly lyses RBCs to gain nutrients and iron present during menses (40
). However, the nutrient-rich components of the menstrual flow are endometrial tissue, squamous epithelium debris, white blood cells, and carbohydrate-rich mucus, not RBCs (66
). Furthermore, menses happens only once a month and does not occur at all in postmenopausal women, who have a higher prevalence of T. vaginalis
infection than younger women (68
). Our results suggest that while some strains of T. vaginalis
can lyse RBCs, this is not a common attribute.
In summary, we show that T. vaginalis cytolysis of human epithelial cells is contact dependent. We found that to initiate cytolysis, attachment must reach a threshold, but higher levels of attachment do not necessarily induce greater cytolysis. We conclude that while the trigger for cytolysis is attachment, cytolysis mechanisms are independent of attachment. Adherence is likely to serve as a trigger for activation and/or secretion of additional critical factors immediately upon contact. It will be of interest to determine whether differences in expressed proteins and/or surface lipoglycans correlate with levels of adherence and cytolysis. Similarly, identifying differences between the three strains that displayed gender preference in adherence and cytolysis may be a first step toward dissecting the causes of gender-specific outcomes of infection.