SummaryThe important human pathogen Streptococcus pyogenes (group A streptococcus, GAS) initiates infection by pilus-mediated attachment to host tissue. Thus, the pilus is an excellent target for design of anti-infective strategies. The T3 pilus of GAS is composed of multiple covalently linked subunits of the T3 protein to which the two minor pilins, Cpa and OrfB, are covalently attached. Because the proteins of GAS pili do not contain either of the motifs required for pilus polymerization in other Gram-positive bacteria, we investigated the residues involved in their linkage. We show that linkage of Cpa to T3 by the sortase family transpeptidase SrtC2 requires the VPPTG motif in the cell wall-sorting signal of Cpa. We also demonstrate that K173 of T3 is required both for T3 polymerization and for attachment of Cpa to T3. Therefore, attachment of Cpa to K173 of a T3 subunit would block further addition of T3 subunits to this end of the growing pilus. This implies that Cpa is located exclusively at the pilus tip, a location supported by immunogold electron microscopy, and suggests that, as for well-studied pili on Gram-negative bacteria, the role of the pilus is to present the adhesin external to the bacterial capsule.

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...

The induction of a potent humoral and cellular immune response in mucosal tissue is important for the development of an effective HIV vaccine. Most of the current HIV vaccines under development use the intramuscular route for immunization, which is relatively poor in generating potent and long-lived mucosal immune responses. Here, we explore the ability of an oral vaccination with a probiotic organism, Lactococcus lactis (L. lactis), to elicit HIV-specific immune responses in the mucosal and systemic compartments of BALB/c mice. We expressed the HIV-1 Gag-p24 on the tip of the T3 pilus of Streptococcus pyogenes as a fusion to the Cpa protein (LL-Gag). Following four monthly LL-Gag oral immunizations we observed strong Gag-specific IgG and IgA responses in serum, feces, and vaginal secretions. However, the Gag-specific CD8 T cell responses in the blood were at or below our detection limit. Following an intramuscular modified vaccinia Ankara (MVA)/Gag boost, we observed a robust Gag-specific CD8 T cell responses both in systemic and mucosal tissue, including intraepithelial and lamina propria lymphocytes of the small intestine, Peyer's patches, and mesenteric lymph nodes. Consistent with strong immunogenicity, the LL-Gag induced activation of CD11c+ CD11b+ dendritic cells in the Peyer's patches after oral immunization. Our results demonstrate that oral immunization with L. lactis expressing an antigen on the tip of the Group-A streptococcus pilus serves as an excellent vaccine platform to induce strong mucosal humoral and cellular immunity against HIV.

The E. coli cls open reading frame (ORF) predicts a 54.8 kDa polypeptide, whereas mature cardiolipin (CL) synthase is 46 kDa. The N-terminal region extending to residue 60 contains several conserved residues but is not essential for enzyme activity. A deletion mutant that is missing residues 2-60 produces a fully active protein. These findings raise the question of why several residues in a region that is not required for enzyme activity are conserved. Recombinant DNA technology was used to introduce an EYMPE epitope (EE) tag into the interior of CL synthase. The EE tagged polypeptide retained the biological properties of wild type CL synthase, including full enzymatic activity. Site-directed mutagenesis was used to alter conserved residues in the N-terminal region. An EE tagged CL synthase in which Leu-7 and Val-8 were both replaced by Ser residues retains in vitro activity but loses most of its in vivo activity. Furthermore, the mutant protein has a higher apparent molecular mass than its parent protein. Taken together, these findings suggest that conserved residues L7 and V8 play a role in polypeptide processing, topology, or both.