Figure 5. Anti-TcdA neutralizing activity measured against HT-29 cells. (A) Light microscopic images of HT-29 cells with various treatments: “Cell control” lacks treatment by both diluted bacteria culture filtrate and rabbit immune sera; “Toxin control” is the addition of diluted bacteria culture filtrate without any rabbit immune sera; “TcdA-C,” “tPA-TcdA-C” or “Vector” indicates that the cells were cultured with diluted bacteria culture filtrate pre-incubated with sera from one of the following rabbit groups: TcdA-C, tPA-TcdA-C or empty vector. Sera were diluted at 1:200 except for vector control group sera, which was diluted at 1:100 dilution. (B) Summary of anti-TcdA neutralizing antibody titers (NAb) in individual rabbit sera two weeks after the 4th DNA immunization.