Evaluation of a ricin vaccine candidate (RVEc) for human toxicity using an in vitro vascular leak assay

To protect against ricin intoxication, a genetically derived ricin A chain vaccine candidate (RVEc) was developed lacking the toxic N-glycosidase activity (Olson et al., 2004). The vaccine protects animals against an aerosolized ricin holotoxin (RT) challenge (Carra et al., 2007). In the current study, the RVEc vaccine was evaluated for its interaction and effect on human endothelial cells. RVEc was tested in an in vitro cellular-based bioassay, consisting of primary human endothelial cells cultured on collagen-coated inserts, to which concentrations of the vaccine candidate (0.6, 2, 2.5 or 9 μM) were added. RVEc showed no signs of adverse activity on the cells (e.g., cytotoxicty activity) as measured by changes in trans-endothelial electrical resistance (TEER). In contrast, ricin toxin (RT) cytotoxicity was observed at all concentrations tested. Under light microscopy, no cytotoxicity was visible at 24h with 0.6 or 9 μM of RVEc. However, cytotoxicity was observed for RT and to a lesser degree for RTA. Flow cytometric analysis showed binding of RT, slight binding of RTA, and no binding of the RVEc vaccine to endothelial cells. The presence of RTB as a contaminant contributing to the cytotoxicity in the RTA preparation was ruled out by a RTB-specific ELISA. In addition, RTA at 9 μM produced a cytotoxic activity that could not be explained exclusively by the presence of azide in the RTA buffer. In the current study, the model demonstrated no discernable adverse events of the RVEc vaccine on human endothelial cells, when compared to the toxicity caused by holotoxin or native RTA preparations.