FIG. 1.

In vitro neutralization of cytotoxicity. (A) C. difficile toxin A (6 ng/well) was mixed with either HuMAb CDA1, 3H2, or 1B11 at various antibody concentrations from 100 to 0.001 nM. IMR-90 cells were plated at 1 × 10⁵ cells/well in a 96-well microtiter plate, and toxin A-antibody mixtures were applied to the cells. IMR-90 cells with a toxin A-antibody mixture applied were incubated for 24 h and scored for cytopathic effect (CPE) visually on a scale of 0 to 4, where 0 represents no observed toxicity and 4 indicates that 100% of the monolayer was effected by toxin A. (B) The experiment was performed as described for panel A, with the exception that C. difficile toxin B (20 pg/well) was mixed with either HuMAb 2A11, MDX1388, 1G10, or 103-174 at various antibody concentrations from 1,000 to 0.0001 nM prior to being applied to IMR-90 cells.