The invasiveness and virulence of Shigella spp. are largely due to the expression of plasmid-encoded virulence factors, among which are the invasion plasmid antigens (IpaB, IpaC, and IpaD). Recent observations have indicated that the Ipa proteins (IpaB, IpaC, and possibly IpaD) form a multiprotein complex capable of inducing the phagocytic event which internalizes the bacterium. Researchers isolated a complex of invasins and LPS from water-extractable antigens of virulent shigellae. Western blot analysis of the complex indicates that all of the major virulence antigens of Shigella, including IpaB, IpaC, and IpaD, and LPS are components of this macromolecular complex. Guinea pigs (keratoconjunctivitis model) or mice (lethal lung model) immunized intranasally with purivied invasin complex (invaplex) on days 0, 14, and 28 and challenged 3 weeks later with virulent shigellae were protected from disease (P<0.01 for both animal models) (Turbyfill et al., 2000).
