A Foreign Protein Incorporated on the Tip of T3 Pili in Lactococcus lactis Elicits Systemic and Mucosal Immunity

Abstract

The use of Lactococcus lactis to deliver a chosen antigen to the mucosal surface has been shown to elicit an immune response in mice and is a possible method of vaccination in humans. The recent discovery on Gram-positive bacteria of pili that are covalently attached to the bacterial surface and the elucidation of the residues linking the major and minor subunits of such pili suggests that the presentation of an antigen on the tip of pili external to the surface of L. lactis might constitute a successful vaccine strategy. As a proof of principle, we have fused a foreign protein (the Escherichia coli maltose-binding protein) to the C-terminal region of the native tip protein (Cpa) of the T3 pilus derived from Streptococcus pyogenes and expressed this fusion protein (MBP*) in L. lactis. We find that MBP* is incorporated into pili in this foreign host, as shown by Western blot analyses of cell wall proteins and by immunogold electron microscopy. Furthermore, since the MBP* on these pili retains its native biological activity, it appears to retain its native structure. Mucosal immunization of mice with this L. lactis strain expressing pilus-linked MBP* results in production of both a systemic and a mucosal response (IgG and IgA antibodies) against the MBP antigen. We suggest that this type of mucosal vaccine delivery system, which we term UPTOP (for unhindered presentation on tips of pili), may provide an inexpensive and stable alternative to current mechanisms of immunization for many serious human pathogens.Pili of Gram-positive bacteria are filamentous structures that extend outward from the bacterial surface and are covalently anchored to the bacterial cell wall. They are believed to be the primary means of attachment to the appropriate environmental receptor for the organism, which, for pathogens, is within the human host. The backbone of the pilus in Gram-positive bacteria is composed of multiple covalently linked identical subunits (major pilin), to which one or more minor pilin subunits are covalently attached. Pilin proteins are synthesized with an N-terminal Sec signal, which is cleaved during transit through the cytoplasmic membrane, and a C-terminal cell wall sorting signal (CWSS), which contains an LPXTG (or similar) amino acid motif, followed by a hydrophobic region and a positively charged C terminus. Pilus assembly is catalyzed by a pilus-specific sortase family transpeptidase, which cleaves the CWSS motif between the threonine (T) and glycine (G) residues and forms a covalent bond between this T and a conserved lysine (K) residue of another major pilin subunit. As this process repeats, the pilus is polymerized until it is covalently linked to the cell wall by either the "housekeeping" sortase, which is responsible for anchoring most surface proteins of Gram-positive bacteria to the cell wall, or the pilusspecific sortase (for reviews, see references 21, 35, and 38).We have been investigating assembly of T3 pili of Streptococcus pyogenes, an important human pathogen. In this organism, the T3 pilus lo...

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