Several drugs currently in use in humans have been reported to have endothelial barrier protective function; all are in use as antihypertensive therapeutics, and some for other therapeutic purposes as well. We therefore tested four different drugs with different mechanisms of action for the ability to prevent the damage to endothelial layers in culture caused by L. interrogans. Lisinopril binds to and competitively inhibits angiotensin 1 binding to angiotensin converting enzyme (ACE), which is expressed by endothelial cells, while telmisartan competitively inhibits angio-tensin 2 binding to its receptor AT 1 . Dopamine is an antagonist of VEGF/VEGFR2-mediated cell layer permeability in treatment of human umbilical vein endothelial cells (HUVECs) in vitro at 10mM, as well as VEGF-mediated angiogenesis in vivo and proliferation of HUVECs at 1 mM in vitro [34, 35] . Furosemide is an anion transport blocker and is used as a diuretic but has antihypertensive activity as a consequence, and was used as a control not expected to preserve endothelial layer integrity. While telmisartan, furosemide, and dopamine did not protect the endothelial layers from the damage due to L. interrogans Copenhageni infection, lisinopril did at 100 nM, 1 mM and 10 mM ( Figure 5 , representing 3 independent experiments, and data not shown). There are several possible explanations for this, including: 1) lisinopril inhibits L. interrogans attachment to the cells, and 2) that attachment is unaffected but the interaction of the bacteria triggers activation of a signaling cascade or release of a mediator whose action or activation is inhibited by lisinopril. We therefore investigated the possibility that lisinopril might prevent Figure 5 . Effects of specific drugs that protect endothelial barrier function on damage caused by L. interrogans. Panels A and B: Ea.hy926 endothelial cell layers were infected with L. interrogans sv. Copenhageni or L. biflexa sv. Patoc as described in Materials and Methods, except that just prior to the addition of the bacteria the drugs lisinopril, telmisartan, dopamine, or furosemide were added to 1 mM. The micrographs shown in Panel A (representative of three experiments) were taken at the 6 hour time point; the graphs in Panel B show the transmigration of leptospires over the entire 72 hr. time course. Shown are the means and standard deviations of all data from three experiments. Statistical significance was determined using repeated measures ANOVA followed by Bonferroni's multiple comparison test. For wells with cells, L. interrogans vs. L. biflexa, p,0.001, L. interrogans with no additions vs. telmisartan p.0.05 (not significant), L. interrogans with no additions vs. lisinopril p,0.001, L. interrogans with telmisartan vs. lisinopril p,0.001. There were no significant differences in the absence of cells, and the drugs did not affect bacterial motility or attachment of 35 S-labeled leptospires to the cells (Panel C and data not shown). doi:10.1371/journal.pntd.0000918.g005 Figure 6 . Lisinopril concentrations effective in protection of endothelial cell layers from damage due to L. interrogans. Ea.hy926 endothelial cell layers were infected with L. interrogans sv. Copenhageni as described in Materials and Methods, except that just prior to the addition of the bacteria the drugs lisinopril or telmisartan were added to the concentrations indicated. The micrographs were taken at the 3 hour time point. Lisinopril at 10 mM, 1 mM and 100 nM blocked endothelial disruption by L. interrogans; lisinopril at 10 nM or below did not. doi:10.1371/journal.pntd.0000918.g006