FIGURE 2.

Certhrax cytotoxicity. A, LD₅₀ curve of Certhrax with RAW264.7 cells in the presence (squares) and absence (circles) of 100 ng/ml PA and increasing concentrations of Certhrax. B, LD₅₀ experiment similar to that in A, using R3D cells lacking the protective antigen receptor in the presence (squares) and absence (circles) of 100 ng/ml PA and increasing concentrations of Certhrax. C, effect of Certhrax on mouse macrophage cell growth as indicated by percentage viability of cells 72 h following the addition of 100 ng/ml PA and increasing concentrations of wild-type Certhrax (dark blue) or QXE variant of Certhrax (light blue), where the catalytic Gln and Glu have each been substituted with Ala. In A and B, Certhrax concentrations ranged from 0 to 100,000 pg/ml, and in A–C, cell viability was measured using an MTT colorimetric assay (described under “Experimental Procedures”). Error bars, S.D.