1A). Etx mutant Y30A-Y196A was expressed and purified as described in Materials and Methods. Purified recombinant Y30A-Y196A prototoxin had an apparent molecular weight of ∼37 kDa as detected by SDS-PAGE C646 clinical trial (Fig. 1B, lane 2). Thermal stability assay  revealed that the melting temperature (Tm) of Y30A-Y196A was similar to that of Etx with H149A mutation, providing further evidence that
the double tyrosine mutant is folded correctly ( Fig. 1C). The H149A mutation has previously been shown not to have an effect on the prototoxin tertiary structure . The cytotoxic activity of trypsin activated Y30A-Y196A toward MDCK.2 and ACHN cells were measured by the LDH assay. The average dose of Y30A-Y196A required to kill 50% of MDCK.2 cells was determined to be 1.49 μM, corresponding to an approximately 430-fold reduction in cytotoxic activity relative to wild type Etx with a CT50 value of 3.47 nM (Fig. 2A). In contrast, the results of our cytotoxicity assay in ACHN cells revealed
that the cytotoxic activity of trypsin activated Y30A-Y196A was equivalent to that of wild type toxin (Fig. 2B). No LDH release could be measured when MDCK.2 or ACHN cells were treated with trypsin activated Etx mutant H106P , even at the maximum concentration of 10 μM tested. We also evaluated the effect of Y30A-Y196A prototoxin on its ability to bind to MDCK.2 and ACHN cells using the On-Cell Western assay. As Casein kinase 1 shown in Fig. 3, the fluorescent signal of MDCK.2 cells treated with Y30A-Y196A prototoxin was similar to that of TGF-beta inhibitor cells treated with PBS only. In contrast, ACHN cells treated with Y30A-Y196A prototoxin showed fluorescence
equivalent to that of cells treated with wild type toxin (Fig. 3). Etx mutant H106P showed similar binding to wild type toxin in both cell lines (Fig. 3). The mean IgG titre against purified Y30A-Y196A prototoxin was measured by indirect ELISA on day 107 of the immunisation schedule and determined to be 1:16,000 (Immune Systems Ltd., UK), indicating that immunisation of rabbits with Y30A-Y196A prototoxin induced a specific antibody response. To test the ability of the polyclonal antiserum raised in rabbits against Y30A-Y196A prototoxin to neutralise the cytotoxic activity of wild type Etx in MDCK.2 cells, we used the in vitro neutralisation assay as described in Materials and Methods. As shown in Fig. 4, the polyclonal antiserum raised against Y30A-Y196A prototoxin was able to protect MDCK.2 cells against wild type Etx-induced cytotoxicity in a dose-dependent manner (up to dilution 26, which corresponds to 0.2 μg/ml antibody concentration). In contrast, the negative control antibody did not inhibit Etx-induced cytotoxicity at any of the doses tested.