FIGURE 6.

Impact of human plasma-derived IgA and IgM preparations on the secretion of virulence factors IpaB and IpaC by S. flexneri. Immune complexes between bacteria and human plasma-derived pIgA, SIgA, mIgA, IgG, IgM, SigM, or anti-Shigella LPS-specific SIgAC5 monoclonal Ab were formed for 1 h as described under “Experimental procedures,” and the expression of virulence factors was induced by Congo red. A and B, the secretion of IpaB (A) and IpaC (B) was examined by immunoblot analysis using monoclonal Abs directed against IpaB and IpaC, with immunodetection of OspF serving as a loading control. The images are representative of one individual experiment performed three times in duplicates. C, densitometric analysis of replicated immunoblots (n = 6) exposed for optimal times to avoid saturation of the photographic film. The intensity of the signals reached with S. flexneri alone was fixed arbitrarily at 100%. When obtained, significant statistical differences were calculated by comparison with the condition Sf. *, p < 0.02; **, p < 0.005.