Probiotic bacterial strains such as Escherichia coli Nissle are potentially useful for the targeted delivery of therapeutic molecules to disease sites. However, current strategies for ensuring that these Gram-negative strains deliver their cargo are mostly limited to programmed lysis or display on the outer surface of the bacteria. To promote the release of therapeutic proteins from E. coli Nissle without the requirement for lysis, Lynch et al. co-opted a type III secretion apparatus (T3SA) from Shigella flexneri that was engineered to release its payload into the bacterial surroundings rather than injecting it directly into host cells. This platform, called PROT3EcT (probiotic type 3 secretion E. coli therapeutic), omits the tip complex from the Shigella T3SA. The tip complex would normally hold the secretion machinery in the ‘off’ configuration; in its absence, these bacterial cells secrete their cargo into the extracellular milieu without first invading host cells.
The secretion machinery of the PROT3EcT platform recognizes substrate proteins tagged at their N termini with motifs from Shigella effector proteins, and the authors demonstrated the use of PROT3EcT to produce and secrete nanobodies that targeted the pro-inflammatory cytokine TNF. In mice, delivery of these anti-TNF nanobodies via PROT3EcT prevented intestinal inflammation and offered protection against the development of colitis without disrupting the native gut microbiota. As the T3SA can process a variety of substrates, provided that they bear the proper tag, PROT3EcT provides a potentially general platform for the delivery of therapeutic proteins in the gut.
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