FIGURE 5.

Secretory IgA is more protease resistant than IgG. Human antibodies were tested for sensitivity for bacterial protease. A, human secretory IgA, serum IgA, and serum IgG at 0.8 mg/ml were combined with subtilisin protease solutions in PBS and incubated overnight at 37 °C. To observe differences in proteolytic cleavage, we used a dilution series of protease. As protease concentration is increased from 0 to 17 μg/ml, the differential sensitivity of HC bands can be observed. Although the majority of IgG is cleaved at 4 μg/ml [subtilisin], much of the IgA remains, and serum IgA is proteolyzed before secretory IgA. The data shown are representative of five independent experiments. B, antibody HC cleavage was quantified, showing the expected dependence on protease concentration. HC band intensities for the representative gel shown were integrated using ImageJ (National Institutes of Health) and normalized to the band intensity at 0 [subtilisin] using GraphPad Prism. C, vaginal proteases can cleave IgG and are found at significantly higher levels in BV specimens compared with normal controls, as determined by unpaired t test. *, p < 0.05.