Cloning, expression, and affinity purification of recombinant Shigella flexneri invasion plasmid antigens IpaB and IpaC.

Shigella flexneri and related enteropathogenic bacteria are important agents of bacillary dysentery, a potentially life-threatening illness for children in underdeveloped regions of the world. Onset of shigellosis stems from S. flexneri invasion of colonic epithelial cells, leading to localized cell death and inflammation. Invasion plasmid antigens (Ipa) B, C, and D are three secreted proteins encoded by the large virulence plasmid of S. flexneri that have been implicated as essential effectors of this cell invasion process. These proteins are expressed as part of the ipa operon and are among the major targets of the host immune response to shigellosis. Biochemical characterization of the Ipa invasins has been complicated by the fact they have not been purified in the quantities needed for detailed in vitro analysis. Here we describe the first cloning, expression, and extensive purification of IpaB and IpaC fusion proteins from Escherichia coli for use in dissecting of the protein biochemistry of S. flexneri pathogenesis. A variety of approaches were used to prepare significant quantities of these proteins in their soluble forms, including the use of different host cell lines, modification of bacterial growth conditions, and the use of alternative plasmid expression vectors. Now that these Ipa proteins are available in a highly pure form, it will be possible to initiate studies on their important biological and immunological properties as well as their recruitment into high-molecular-weight protein complexes. Together with IpaD (purified as part of a previous study), these purified proteins will be useful for: (a) exploring properties of the host immune response to S. flexneri invasion, (b) elucidating the specific biochemical properties that lead to pathogen internalization, (c) analyzing the importance of specific Ipa protein complexes in host cell invasions, and (d) monitoring, or perhaps even augmenting, the efficacy of live oral vaccines in human trials.