FIG 4.
In vitro neutralization activity of mouse serum antibodies against STa. Intracellular cyclic GMP levels (picomoles per milliliter) in T-84 cells incubated with 2 ng STa and serum samples of mice immunized s.c. with the 3×STaN12S-mnLTR192G/L211A, 3×STaL9A/N12S-mnLTR192G/L211A, 3×STaN12S/A14T-mnLTR192G/L211A, 3×STaL9A/N12S/A14H-mnLTR192G/L211A, or 3×STaL9A/A14H-mnLTR192G/L211A fusion protein were measured by using a cGMP EIA kit (Enzo Life). STa elevates intracellular cGMP levels in T-84 cells; neutralizing anti-STa antibodies prevent STa from stimulating cGMP production in T-84 cells. T-84 cells incubated with STa alone or STa with a control mouse serum sample were also included as controls. T-84 cells incubated with cell culture medium were used to show baseline cGMP levels in T-84 cells. ** indicates a P value of <0.01.